The Higgs boson or Higgs particle is an elementary
particle initially theorised in 1964, whose
discovery was announced at CERN on 4 July
2012. The discovery has been called "monumental"
because it appears to confirm the existence
of the Higgs field, which is pivotal to the
Standard Model and other theories within particle
physics. It would explain why some fundamental
particles have mass when the symmetries controlling
their interactions should require them to
be massless, and why the weak force has a
much shorter range than the electromagnetic
force. The discovery of a Higgs boson should
allow physicists to finally validate the last
untested area of the Standard Model's approach
to fundamental particles and forces, guide
other theories and discoveries in particle
physics, and potentially lead to developments
in "new" physics.
This unanswered question in fundamental physics
is of such importance that it led to a search
of more than 40 years for the Higgs boson
and finally the construction of one of the
world's most expensive and complex experimental
facilities to date, the Large Hadron Collider,
able to create Higgs bosons and other particles
for observation and study. On 4 July 2012,
it was announced that a previously unknown
particle with a mass between 125 and 127 GeV/c2
had been detected; physicists suspected at
the time that it was the Higgs boson. By March
2013, the particle had been proven to behave,
interact and decay in many of the ways predicted
by the Standard Model, and was also tentatively
confirmed to have positive parity and zero
spin, two fundamental attributes of a Higgs
boson. This appears to be the first elementary
scalar particle discovered in nature. More
data are needed to determine whether the particle
discovered exactly matches the predictions
of the Standard Model, or whether, as predicted
by some theories, multiple Higgs bosons exist.
The Higgs boson is named after Peter Higgs,
one of six physicists who, in 1964, proposed
the mechanism that suggested the existence
of such a particle. Although Higgs's name
has come to be associated with this theory,
several researchers between about 1960 and
1972 each independently developed different
parts of it. In mainstream media the Higgs
boson has often been called the "God particle",
from a 1993 book on the topic; the nickname
is strongly disliked by many physicists, including
Higgs, who regard it as inappropriate sensationalism.
In 2013 two of the original researchers, Peter
Higgs and François Englert, were awarded
the Nobel Prize in Physics for their work
and prediction. Englert's co-researcher Robert
Brout had died in 2011 and the Nobel is not
given posthumously except in unusual circumstances.
In the Standard Model, the Higgs particle
is a boson with no spin, electric charge,
or color charge. It is also very unstable,
decaying into other particles almost immediately.
It is a quantum excitation of one of the four
components of the Higgs field. The latter
constitutes a scalar field, with two neutral
and two electrically charged components, and
forms a complex doublet of the weak isospin
SU(2) symmetry. The field has a "Mexican hat"
shaped potential with nonzero strength everywhere,
which in its vacuum state breaks the weak
isospin symmetry of the electroweak interaction.
When this happens, three components of the
Higgs field are "absorbed" by the SU(2) and
U(1) gauge bosons to become the longitudinal
components of the now-massive W and Z bosons
of the weak force. The remaining electrically
neutral component separately couples to other
particles known as fermions, causing these
to acquire mass as well. Some versions of
the theory predict more than one kind of Higgs
fields and bosons. Alternative "Higgsless"
models would have been considered if the Higgs
boson were not discovered.
A non-technical summary
"Higgs" terminology
Overview
In particle physics, elementary particles
and forces give rise to the world around us.
Nowadays, physicists explain the behaviour
of these particles and how they interact using
the Standard Model—a widely accepted and
"remarkably" accurate framework based on gauge
invariance and symmetries, believed to explain
almost everything in the world we see, other
than gravity.
But by around 1960 all attempts to create
a gauge invariant theory for two of the four
fundamental forces had consistently failed
at one crucial point: although gauge invariance
seemed extremely important, it seemed to make
any theory of electromagnetism and the weak
force go haywire, by demanding that either
many particles with mass were massless or
that non-existent forces and massless particles
had to exist. Scientists had no idea how to
get past this point.
Work done on superconductivity and "broken"
symmetries around 1960 led physicist Philip
Anderson to suggest in 1962 a new kind of
solution that might hold the key. In 1964
a theory was created by 3 different groups
of researchers, that showed the problems could
be resolved if an unusual kind of field existed
throughout the universe. It would cause existing
particles to acquire mass instead of new massless
particles being formed. By 1972 it had been
developed into a comprehensive theory and
proved capable of giving "sensible" results.
Although there was not yet any proof of such
a field, calculations consistently gave answers
and predictions that were confirmed by experiments,
including very accurate predictions of several
other particles, so scientists began to believe
this might be true and to search for proof
whether or not a Higgs field exists in nature.
If this field did exist, this would be a monumental
discovery for science and human knowledge,
and is expected to open doorways to new knowledge
in many fields. If not, then other more complicated
theories would need to be explored. The easiest
proof whether or not the field existed was
by searching for a new kind of particle it
would have to give off, known as "Higgs bosons"
or the "Higgs particle". These would be extremely
difficult to find, so it was only many years
later that experimental technology became
sophisticated enough to answer the question.
While several symmetries in nature are spontaneously
broken through a form of the Higgs mechanism,
in the context of the Standard Model the term
"Higgs mechanism" almost always means symmetry
breaking of the electroweak field. It is considered
proven, but the exact cause has been exceedingly
difficult to prove. After 50 years, the Higgs
boson's existence – apparently proven in
2013 – would finally confirm that the Standard
Model is essentially correct and allow further
development, while its non-existence would
mean that other theories are needed instead.
Various analogies have also been invented
to describe the Higgs field and boson, including
analogies with well-known symmetry breaking
effects such as the rainbow and prism, electric
fields, ripples, and resistance of macro objects
moving through media, like people moving through
crowds or some objects moving through syrup
or molasses. However, analogies based on simple
resistance to motion are inaccurate as the
Higgs field does not work by resisting motion.
Significance
Scientific impact
Evidence of the Higgs field and its properties
would be extremely significant scientifically,
for many reasons. The Higgs boson's importance
is largely that it is able to be examined
using existing knowledge and experimental
technology, as a way to confirm and study
the entire Higgs field theory. Conversely,
proof that the Higgs field and boson do not
exist would also be significant. In discussion
form, the relevance includes:
"Real world" impact
As yet, there are no known immediate technological
benefits of finding the Higgs particle. However,
observers in both media and science point
out that when fundamental discoveries are
made about our world, their practical uses
can take decades to emerge, but are often
world-changing when they do. A common pattern
for fundamental discoveries is for practical
applications to follow later, once the discovery
has been explored further, at which point
they become the basis for social change and
new technologies.
For example, in the first half of the 20th
century it was not expected that quantum mechanics
would make possible transistors and microchips,
mobile phones and computers, lasers and M.R.I.
scanners. Radio waves were described by their
co-discoverer in 1888 as "an interesting laboratory
experiment" with "no useful purpose" whatsoever,
and are now used in innumerable ways, positrons
are used in hospital tomography scans, and
special and general relativity, which explain
black holes also enable satellite-based GPS
and satellite navigation. Electric power generation
and transmission, motors, and lighting all
stemmed from previous theoretical work on
electricity and magnetism; air conditioning
and refrigeration resulted from thermodynamics.
It is impossible to predict how seemingly
esoteric knowledge may affect society in the
future.
Other observers highlight technological spin-offs
from this and related particle physics activities,
which have already brought major developments
to society. For example, the World Wide Web
as used today was created by physicists working
in global collaborations on particle experiments
at CERN to share their results, and the results
of massive amounts of data produced by the
Large Hadron Collider have already led to
significant advances in distributed and cloud
computing, now well established within mainstream
services.
History
Particle physicists study matter made from
fundamental particles whose interactions are
mediated by exchange particles - gauge bosons
- acting as force carriers. At the beginning
of the 1960s a number of these particles had
been discovered or proposed, along with theories
suggesting how they relate to each other,
some of which had already been reformulated
as field theories in which the objects of
study are not particles and forces, but quantum
fields and their symmetries. However, attempts
to unify known fundamental forces such as
the electromagnetic force and the weak nuclear
force were known to be incomplete. One known
omission was that gauge invariant approaches,
including non-abelian models such as Yang–Mills
theory, which held great promise for unified
theories, also seemed to predict known massive
particles as massless. Goldstone's theorem,
relating to continuous symmetries within some
theories, also appeared to rule out many obvious
solutions, since it appeared to show that
zero-mass particles would have to also exist
that were "simply not seen". According to
Guralnik, physicists had "no understanding"
how these problems could be overcome.
Particle physicist and mathematician Peter
Woit summarised the state of research at the
time:
"Yang and Mills work on non-abelian gauge
theory had one huge problem: in perturbation
theory it has massless particles which don’t
correspond to anything we see. One way of
getting rid of this problem is now fairly
well-understood, the phenomenon of confinement
realized in QCD, where the strong interactions
get rid of the massless “gluon” states
at long distances. By the very early sixties,
people had begun to understand another source
of massless particles: spontaneous symmetry
breaking of a continuous symmetry. What Philip
Anderson realized and worked out in the summer
of 1962 was that, when you have both gauge
symmetry and spontaneous symmetry breaking,
the Nambu–Goldstone massless mode can combine
with the massless gauge field modes to produce
a physical massive vector field. This is what
happens in superconductivity, a subject about
which Anderson was one of the leading experts."
[text condensed]
The Higgs mechanism is a process by which
vector bosons can get rest mass without explicitly
breaking gauge invariance, as a byproduct
of spontaneous symmetry breaking. The mathematical
theory behind spontaneous symmetry breaking
was initially conceived and published within
particle physics by Yoichiro Nambu in 1960,
the concept that such a mechanism could offer
a possible solution for the "mass problem"
was originally suggested in 1962 by Philip
Anderson, and Abraham Klein and Benjamin Lee
showed in March 1964 that Goldstone's theorem
could be avoided this way in at least some
non-relativistic cases and speculated it might
be possible in truly relativistic cases.
These approaches were quickly developed into
a full relativistic model, independently and
almost simultaneously, by three groups of
physicists: by François Englert and Robert
Brout in August 1964; by Peter Higgs in October
1964; and by Gerald Guralnik, Carl Hagen,
and Tom Kibble in November 1964. Higgs also
wrote a short but important response published
in September 1964 to an objection by Gilbert,
which showed that if calculating within the
radiation gauge, Goldstone's theorem and Gilbert's
objection would become inapplicable. Properties
of the model were further considered by Guralnik
in 1965, by Higgs in 1966, by Kibble in 1967,
and further by GHK in 1967. The original three
1964 papers showed that when a gauge theory
is combined with an additional field that
spontaneously breaks the symmetry, the gauge
bosons can consistently acquire a finite mass.
In 1967, Steven Weinberg and Abdus Salam independently
showed how a Higgs mechanism could be used
to break the electroweak symmetry of Sheldon
Glashow's unified model for the weak and electromagnetic
interactions, forming what became the Standard
Model of particle physics. Weinberg was the
first to observe that this would also provide
mass terms for the fermions. 
However, the seminal papers on spontaneous
breaking of gauge symmetries were at first
largely ignored, because it was widely believed
that the theories in question were a dead-end,
and in particular that they could not be renormalised.
In 1971–72, Martinus Veltman and Gerard
't Hooft proved renormalisation of Yang–Mills
was possible in two papers covering massless,
and then massive, fields. Their contribution,
and others' work on the renormalization group
- including "substantial" theoretical work
by Russian physicists - was eventually "enormously
profound and influential", but even with all
key elements of the eventual theory published
there was still almost no wider interest.
For example, Coleman found in a study that
"essentially no-one paid any attention" to
Weinberg's paper prior to 1971 – now the
most cited in particle physics – and even
in 1970 according to Politzer, Glashow's teaching
of the weak interaction contained no mention
of Weinberg's, Salam's, or Glashow's own work.
In practice, Politzer states, almost everyone
learned of the theory due to physicist Benjamin
Lee, who combined the work of Veltman and
't Hooft with insights by others, and popularised
the completed theory. In this way, from 1971,
interest and acceptance "exploded"  and the
ideas were quickly absorbed in the mainstream.
The resulting electroweak theory and Standard
Model have correctly predicted weak neutral
currents, three bosons, the top and charm
quarks, and with great precision, the mass
and other properties of some of these. Many
of those involved eventually won Nobel Prizes
or other renowned awards. A 1974 paper and
comprehensive review in Reviews of Modern
Physics commented that "while no one doubted
the [mathematical] correctness of these arguments,
no one quite believed that nature was diabolically
clever enough to take advantage of them",
adding that the theory had so far produced
meaningful answers that accorded with experiment,
but it was unknown whether the theory was
actually correct. By 1986 and again in the
1990s it became possible to write that understanding
and proving the Higgs sector of the Standard
Model was "the central problem today in particle
physics." 
Summary and impact of the PRL papers
The three papers written in 1964 were each
recognised as milestone papers during Physical
Review Letters's 50th anniversary celebration.
Their six authors were also awarded the 2010
J. J. Sakurai Prize for Theoretical Particle
Physics for this work. Two of the three PRL
papers contained equations for the hypothetical
field that eventually would become known as
the Higgs field and its hypothetical quantum,
the Higgs boson. Higgs' subsequent 1966 paper
showed the decay mechanism of the boson; only
a massive boson can decay and the decays can
prove the mechanism.
In the paper by Higgs the boson is massive,
and in a closing sentence Higgs writes that
"an essential feature" of the theory "is the
prediction of incomplete multiplets of scalar
and vector bosons". In the paper by GHK the
boson is massless and decoupled from the massive
states. In reviews dated 2009 and 2011, Guralnik
states that in the GHK model the boson is
massless only in a lowest-order approximation,
but it is not subject to any constraint and
acquires mass at higher orders, and adds that
the GHK paper was the only one to show that
there are no massless Goldstone bosons in
the model and to give a complete analysis
of the general Higgs mechanism. All three
reached similar conclusions, despite their
very different approaches: Higgs' paper essentially
used classical techniques, Englert and Brout's
involved calculating vacuum polarization in
perturbation theory around an assumed symmetry-breaking
vacuum state, and GHK used operator formalism
and conservation laws to explore in depth
the ways in which Goldstone's theorem may
be worked around.
In addition to explaining how mass is acquired
by vector bosons, the Higgs mechanism also
predicts the ratio between the W boson and
Z boson masses as well as their couplings
with each other and with the Standard Model
quarks and leptons. Subsequently, many of
these predictions have been verified by precise
measurements performed at the LEP and the
SLC colliders, thus overwhelmingly confirming
that some kind of Higgs mechanism does take
place in nature, but the exact manner by which
it happens has not yet been discovered. The
results of searching for the Higgs boson are
expected to provide evidence about how this
is realized in nature.
Theoretical properties
Theoretical need for the Higgs
Gauge invariance is an important property
of modern particle theories such as the Standard
Model, partly due to its success in other
areas of fundamental physics such as electromagnetism
and the strong interaction. However, there
were great difficulties in developing gauge
theories for the weak nuclear force or a possible
unified electroweak interaction. Fermions
with a mass term would violate gauge symmetry
and therefore cannot be gauge invariant. W
and Z bosons are observed to have mass, but
a boson mass term contains terms, which clearly
depend on the choice of gauge and therefore
these masses too cannot be gauge invariant.
Therefore it seems that none of the standard
model fermions or bosons could "begin" with
mass as an inbuilt property except by abandoning
gauge invariance. If gauge invariance were
to be retained, then these particles had to
be acquiring their mass by some other mechanism
or interaction. Additionally, whatever was
giving these particles their mass, had to
not "break" gauge invariance as the basis
for other parts of the theories where it worked
well, and had to not require or predict unexpected
massless particles and long-range forces which
did not actually seem to exist in nature.
A solution to all of these overlapping problems
came from the discovery of a previously unnoticed
borderline case hidden in the mathematics
of Goldstone's theorem, that under certain
conditions it might theoretically be possible
for a symmetry to be broken without disrupting
gauge invariance and without any new massless
particles or forces, and having "sensible"
results mathematically: this became known
as the Higgs mechanism.
The Standard Model hypothesizes a field which
is responsible for this effect, called the
Higgs field, which has the unusual property
of a non-zero amplitude in its ground state;
i.e., a non-zero vacuum expectation value.
It can have this effect because of its unusual
"Mexican hat" shaped potential whose lowest
"point" is not at its "centre". Below a certain
extremely high energy level the existence
of this non-zero vacuum expectation spontaneously
breaks electroweak gauge symmetry which in
turn gives rise to the Higgs mechanism and
triggers the acquisition of mass by those
particles interacting with the field. This
effect occurs because scalar field components
of the Higgs field are "absorbed" by the massive
bosons as degrees of freedom, and couple to
the fermions via Yukawa coupling, thereby
producing the expected mass terms. In effect
when symmetry breaks under these conditions,
the Goldstone bosons that arise interact with
the Higgs field instead of becoming new massless
particles, the intractable problems of both
underlying theories "neutralise" each other,
and the residual outcome is that elementary
particles acquire a consistent mass based
on how strongly they interact with the Higgs
field. It is the simplest known process capable
of giving mass to the gauge bosons while remaining
compatible with gauge theories. Its quantum
would be a scalar boson, known as the Higgs
boson.
Properties of the Standard Model Higgs
In the Standard Model, the Higgs field consists
of four components, two neutral ones and two
charged component fields. Both of the charged
components and one of the neutral fields are
Goldstone bosons, which act as the longitudinal
third-polarization components of the massive
W+, W–, and Z bosons. The quantum of the
remaining neutral component corresponds to
the massive Higgs boson. Since the Higgs field
is a scalar field, the Higgs boson has no
spin. The Higgs boson is also its own antiparticle
and is CP-even, and has zero electric and
colour charge.
The Minimal Standard Model does not predict
the mass of the Higgs boson. If that mass
is between 115 and 180 GeV/c2, then the Standard
Model can be valid at energy scales all the
way up to the Planck scale. Many theorists
expect new physics beyond the Standard Model
to emerge at the TeV-scale, based on unsatisfactory
properties of the Standard Model. The highest
possible mass scale allowed for the Higgs
boson is 1.4 TeV; beyond this point, the Standard
Model becomes inconsistent without such a
mechanism, because unitarity is violated in
certain scattering processes.
It is also possible, although experimentally
difficult, to estimate the mass of the Higgs
boson indirectly. In the Standard Model, the
Higgs boson has a number of indirect effects;
most notably, Higgs loops result in tiny corrections
to masses of W and Z bosons. Precision measurements
of electroweak parameters, such as the Fermi
constant and masses of W/Z bosons, can be
used to calculate constraints on the mass
of the Higgs. As of July 2011, the precision
electroweak measurements tell us that the
mass of the Higgs boson is likely to be less
than about 161 GeV/c2 at 95% confidence level.
These indirect constraints rely on the assumption
that the Standard Model is correct. It may
still be possible to discover a Higgs boson
above these masses if it is accompanied by
other particles beyond those predicted by
the Standard Model.
Production
If Higgs particle theories are correct, then
a Higgs particle can be produced much like
other particles that are studied, in a particle
collider. This involves accelerating a large
number of particles to extremely high energies
and extremely close to the speed of light,
then allowing them to smash together. Protons
and lead ions are used at the LHC. In the
extreme energies of these collisions, the
desired esoteric particles will occasionally
be produced and this can be detected and studied;
any absence or difference from theoretical
expectations can also be used to improve the
theory. The relevant particle theory will
determine the necessary kinds of collisions
and detectors. The Standard Model predicts
that Higgs bosons could be formed in a number
of ways, although the probability of producing
a Higgs boson in any collision is always expected
to be very small—for example, only 1 Higgs
boson per 10 billion collisions in the Large
Hadron Collider. The most common expected
processes for Higgs boson production are:
Gluon fusion. If the collided particles are
hadrons such as the proton or antiproton—as
is the case in the LHC and Tevatron—then
it is most likely that two of the gluons binding
the hadron together collide. The easiest way
to produce a Higgs particle is if the two
gluons combine to form a loop of virtual quarks.
Since the coupling of particles to the Higgs
boson is proportional to their mass, this
process is more likely for heavy particles.
In practice it is enough to consider the contributions
of virtual top and bottom quarks. This process
is the dominant contribution at the LHC and
Tevatron being about ten times more likely
than any of the other processes.
Higgs Strahlung. If an elementary fermion
collides with an anti-fermion—e.g., a quark
with an anti-quark or an electron with a positron—the
two can merge to form a virtual W or Z boson
which, if it carries sufficient energy, can
then emit a Higgs boson. This process was
the dominant production mode at the LEP, where
an electron and a positron collided to form
a virtual Z boson, and it was the second largest
contribution for Higgs production at the Tevatron.
At the LHC this process is only the third
largest, because the LHC collides protons
with protons, making a quark-antiquark collision
less likely than at the Tevatron. Higgs Strahlung
is also known as associated production.
Weak boson fusion. Another possibility when
twofermions collide is that the two exchange
a virtual W or Z boson, which emits a Higgs
boson. The colliding fermions do not need
to be the same type. So, for example, an up
quark may exchange a Z boson with an anti-down
quark. This process is the second most important
for the production of Higgs particle at the
LHC and LEP.
Top fusion. The final process that is commonly
considered is by far the least likely. This
process involves two colliding gluons, which
each decay into a heavy quark–antiquark
pair. A quark and antiquark from each pair
can then combine to form a Higgs particle.
Decay
Quantum mechanics predicts that if it is possible
for a particle to decay into a set of lighter
particles, then it will eventually do so.
This is also true for the Higgs boson. The
likelihood with which this happens depends
on a variety of factors including: the difference
in mass, the strength of the interactions,
etc. Most of these factors are fixed by the
Standard Model, except for the mass of the
Higgs boson itself. For a Higgs boson with
a mass of 126 GeV/c2 the SM predicts a mean
life time of about 1.6×10−22 s.
Since it interacts with all the massive elementary
particles of the SM, the Higgs boson has many
different processes through which it can decay.
Each of these possible processes has its own
probability, expressed as the branching ratio;
the fraction of the total number decays that
follows that process. The SM predicts these
branching ratios as a function of the Higgs
mass.
One way that the Higgs can decay is by splitting
into a fermion–antifermion pair. As general
rule, the Higgs is more likely to decay into
heavy fermions than light fermions, because
the mass of a fermion is proportional to the
strength of its interaction with the Higgs.
By this logic the most common decay should
be into a top–antitop quark pair. However,
such a decay is only possible if the Higgs
is heavier than ~346 GeV/c2, twice the mass
of the top quark. For a Higgs mass of 126 GeV/c2
the SM predicts that the most common decay
is into a bottom–antibottom quark pair,
which happens 56.1% of the time. The second
most common fermion decay at that mass is
a tau–antitau pair, which happens only about
6% of the time.
Another possibility is for the Higgs to split
into a pair of massive gauge bosons. The most
likely possibility is for the Higgs to decay
into a pair of W bosons, which happens about
23.1% of the time for a Higgs boson with a
mass of 126 GeV/c2. The W bosons can subsequently
decay either into a quark and an antiquark
or into a charged lepton and a neutrino. However,
the decays of W bosons into quarks are difficult
to distinguish from the background, and the
decays into leptons cannot be fully reconstructed.
A cleaner signal is given by decay into a
pair of Z-bosons, if each of the bosons subsequently
decays into a pair of easy-to-detect charged
leptons.
Decay into massless gauge bosons is also possible,
but requires intermediate loop of virtual
heavy quarks or massive gauge bosons. The
most common such process is the decay into
a pair of gluons through a loop of virtual
heavy quarks. This process, which is the reverse
of the gluon fusion process mentioned above,
happens approximately 8.5% of the time for
a Higgs boson with a mass of 126 GeV/c2.
Much rarer is the decay into a pair of photons
mediated by a loop of W bosons or heavy quarks,
which happens only twice for every thousand
decays. However, this process is very relevant
for experimental searches for the Higgs boson,
because the energy and momentum of the photons
can be measured very precisely, giving an
accurate reconstruction of the mass of the
decaying particle.
Alternative models
The Minimal Standard Model as described above
is the simplest known model for the Higgs
mechanism with just one Higgs field. However,
an extended Higgs sector with additional Higgs
particle doublets or triplets is also possible,
and many extensions of the Standard Model
have this feature. The non-minimal Higgs sector
favoured by theory are the two-Higgs-doublet
models, which predict the existence of a quintet
of scalar particles: two CP-even neutral Higgs
bosons h0 and H0, a CP-odd neutral Higgs boson
A0, and two charged Higgs particles H±. Supersymmetry
also predicts relations between the Higgs-boson
masses and the masses of the gauge bosons,
and could accommodate a 125 GeV/c2 neutral
Higgs boson.
The key method to distinguish between these
different models involves study of the particles'
interactions and exact decay processes, which
can be measured and tested experimentally
in particle collisions. In the Type-I 2HDM
model one Higgs doublet couples to up and
down quarks, while the second doublet does
not couple to quarks. This model has two interesting
limits, in which the lightest Higgs couples
to just fermions or just gauge bosons, but
not both. In the Type-II 2HDM model, one Higgs
doublet only couples to up-type quarks, the
other only couples to down-type quarks. The
heavily researched Minimal Supersymmetric
Standard Model includes a Type-II 2HDM Higgs
sector, so it could be disproven by evidence
of a Type-I 2HDM Higgs.
In other models the Higgs scalar is a composite
particle. For example, in technicolor the
role of the Higgs field is played by strongly
bound pairs of fermions called techniquarks.
Other models, feature pairs of top quarks.
In yet other models, there is no Higgs field
at all and the electroweak symmetry is broken
using extra dimensions.
Further theoretical issues and hierarchy problem
The Standard Model leaves the mass of the
Higgs boson as a parameter to be measured,
rather than a value to be calculated. This
is seen as theoretically unsatisfactory, particularly
as quantum corrections should apparently cause
the Higgs particle to have a mass immensely
higher than that observed, but at the same
time the Standard Model requires a mass of
the order of 100 to 1000 GeV to ensure unitarity.
Reconciling these points appears to require
explaining why there is an almost-perfect
cancellation resulting in the visible mass
of ~ 125 GeV, and it is not clear how to do
this. Because the weak force is about 1032
times stronger than gravity, and the Higgs
boson's mass is so much less than the Planck
mass or the grand unification energy, it appears
that either there is some underlying connection
or reason for these observations which is
unknown and not described by the Standard
Model, or some unexplained and extremely precise
fine-tuning of parameters – however at present
neither of these explanations is proven. This
is known as a hierarchy problem. More broadly,
the hierarchy problem amounts to the worry
that a future theory of fundamental particles
and interactions should not have excessive
fine-tunings or unduly delicate cancellations,
and should allow masses of particles such
as the Higgs boson to be calculable. The problem
is in some ways unique to spin-0 particles,
which can give rise to issues related to quantum
corrections that do not affect particles with
spin. A number of solutions have been proposed,
including supersymmetry, conformal solutions
and solutions via extra dimensions such as
braneworld models.
Experimental search
To produce Higgs bosons, two beams of particles
are accelerated to very high energies and
allowed to collide within a particle detector.
Occasionally, although rarely, a Higgs boson
will be created fleetingly as part of the
collision byproducts. Because the Higgs boson
decays very quickly, particle detectors cannot
detect it directly. Instead the detectors
register all the decay products and from the
data the decay process is reconstructed. If
the observed decay products match a possible
decay process of a Higgs boson, this indicates
that a Higgs boson may have been created.
In practice, many processes may produce similar
decay signatures. Fortunately, the Standard
Model precisely predicts the likelihood of
each of these, and each known process, occurring.
So, if the detector detects more decay signatures
consistently matching a Higgs boson than would
otherwise be expected if Higgs bosons did
not exist, then this would be strong evidence
that the Higgs boson exists.
Because Higgs boson production in a particle
collision is likely to be very rare, and many
other possible collision events can have similar
decay signatures, the data of hundreds of
trillions of collisions needs to be analysed
and must "show the same picture" before a
conclusion about the existence of the Higgs
boson can be reached. To conclude that a new
particle has been found, particle physicists
require that the statistical analysis of two
independent particle detectors each indicate
that there is lesser than a one-in-a-million
chance that the observed decay signatures
are due to just background random Standard
Model events—i.e., that the observed number
of events is more than 5 standard deviations
different from that expected if there was
no new particle. More collision data allows
better confirmation of the physical properties
of any new particle observed, and allows physicists
to decide whether it is indeed a Higgs boson
as described by the Standard Model or some
other hypothetical new particle.
To find the Higgs boson, a powerful particle
accelerator was needed, because Higgs bosons
might not be seen in lower-energy experiments.
The collider needed to have a high luminosity
in order to ensure enough collisions were
seen for conclusions to be drawn. Finally,
advanced computing facilities were needed
to process the vast amount of data produced
by the collisions. For the announcement of
4 July 2012, a new collider known as the Large
Hadron Collider was constructed at CERN with
a planned eventual collision energy of 14
TeV—over seven times any previous collider—and
over 300 trillion LHC proton–proton collisions
were analysed by the LHC Computing Grid, the
world's largest computing grid, comprising
over 170 computing facilities in a worldwide
network across 36 countries.
Search prior to 4 July 2012
The first extensive search for the Higgs boson
was conducted at the Large Electron–Positron
Collider at CERN in the 1990s. At the end
of its service in 2000, LEP had found no conclusive
evidence for the Higgs. This implied that
if the Higgs boson were to exist it would
have to be heavier than 114.4 GeV/c2.
The search continued at Fermilab in the United
States, where the Tevatron—the collider
that discovered the top quark in 1995—had
been upgraded for this purpose. There was
no guarantee that the Tevatron would be able
to find the Higgs, but it was the only supercollider
that was operational since the Large Hadron
Collider was still under construction and
the planned Superconducting Super Collider
had been cancelled in 1993 and never completed.
The Tevatron was only able to exclude further
ranges for the Higgs mass, and was shut down
on 30 September 2011 because it no longer
could keep up with the LHC. The final analysis
of the data excluded the possibility of a
Higgs boson with a mass between 147 GeV/c2
and 180 GeV/c2. In addition, there was a
small excess of events possibly indicating
a Higgs boson with a mass between 115 GeV/c2
and 140 GeV/c2.
The Large Hadron Collider at CERN in Switzerland,
was designed specifically to be able to either
confirm or exclude the existence of the Higgs
boson. Built in a 27 km tunnel under the
ground near Geneva originally inhabited by
LEP, it was designed to collide two beams
of protons, initially at energies of 3.5 TeV
per beam, or almost 3.6 times that of the
Tevatron, and upgradeable to 2 × 7 TeV in
future. Theory suggested if the Higgs boson
existed, collisions at these energy levels
should be able to reveal it. As one of the
most complicated scientific instruments ever
built, its operational readiness was delayed
for 14 months by a magnet quench event nine
days after its inaugural tests, caused by
a faulty electrical connection that damaged
over 50 superconducting magnets and contaminated
the vacuum system.
Data collection at the LHC finally commenced
in March 2010. By December 2011 the two main
particle detectors at the LHC, ATLAS and CMS,
had narrowed down the mass range where the
Higgs could exist to around 116-130 GeV and
115-127 GeV. There had also already been a
number of promising event excesses that had
"evaporated" and proven to be nothing but
random fluctuations. However from around May
2011, both experiments had seen among their
results, the slow emergence of a small yet
consistent excess of gamma and 4-lepton decay
signatures and several other particle decays,
all hinting at a new particle at a mass around
125 GeV. By around November 2011, the anomalous
data at 125 GeV was becoming "too large to
ignore", and the team leaders at both ATLAS
and CMS each privately suspected they might
have found the Higgs. On November 28, 2011,
at an internal meeting of the two team leaders
and the director general of CERN, the latest
analyses were discussed outside their teams
for the first time, suggesting both ATLAS
and CMS might be converging on a possible
shared result at 125 GeV, and initial preparations
commenced in case of a successful finding.
While this information was not known publicly
at the time, the narrowing of the possible
Higgs range to around 115–130 GeV and the
repeated observation of small but consistent
event excesses across multiple channels at
both ATLAS and CMS in the 124-126 GeV region
were public knowledge with "a lot of interest".
It was therefore widely anticipated around
the end of 2011, that the LHC would provide
sufficient data to either exclude or confirm
the finding of a Higgs boson by the end of
2012, when their 2012 collision data had been
examined.
Discovery of candidate boson at CERN
On 22 June 2012 CERN announced an upcoming
seminar covering tentative findings for 2012,
and shortly afterwards rumours began to spread
in the media that this would include a major
announcement, but it was unclear whether this
would be a stronger signal or a formal discovery.
Speculation escalated to a "fevered" pitch
when reports emerged that Peter Higgs, who
proposed the particle, was to be attending
the seminar, and that "five leading physicists"
had been invited – generally believed to
signify the five living 1964 authors – with
Higgs, Englert, Guralnik, Hagen attending
and Kibble confirming his invitation.)
On 4 July 2012 both of the CERN experiments
announced they had independently made the
same discovery: CMS of a previously unknown
boson with mass 125.3 ± 0.6 GeV/c2 and ATLAS
of a boson with mass 126.5 GeV/c2. Using the
combined analysis of two interaction types,
both experiments reached a local significance
of 5-sigma—or less than a 1 in one million
chance of error. When additional channels
were taken into account, the CMS significance
was reduced to 4.9-sigma.
The two teams had been working 'blinded' from
each other from around late 2011 or early
2012, meaning they did not discuss their results
with each other, providing additional certainty
that any common finding was genuine validation
of a particle. This level of evidence, confirmed
independently by two separate teams and experiments,
meets the formal level of proof required to
announce a confirmed discovery.
On 31 July 2012, the ATLAS collaboration presented
additional data analysis on the "observation
of a new particle", including data from a
third channel, which improved the significance
to 5.9-sigma and mass 126.0 ± 0.4 ± 0.4
GeV/c2, and CMS improved the significance
to 5-sigma and mass 125.3 ± 0.4 ± 0.5 GeV/c2.
The new particle tested as a possible Higgs
boson
Following the 2012 discovery, it was still
unconfirmed whether or not the 125 GeV/c2
particle was a Higgs boson. On one hand, observations
remained consistent with the observed particle
being the Standard Model Higgs boson, and
the particle decayed into at least some of
the predicted channels. Moreover, the production
rates and branching ratios for the observed
channels broadly matched the predictions by
the Standard Model within the experimental
uncertainties. However, the experimental uncertainties
currently still left room for alternative
explanations, meaning an announcement of the
discovery of a Higgs boson would have been
premature. To allow more opportunity for data
collection, the LHC's proposed 2012 shutdown
and 2013–14 upgrade were postponed by 7
weeks into 2013.
In November 2012, in a conference in Kyoto
researchers said evidence gathered since July
was falling into line with the basic Standard
Model more than its alternatives, with a range
of results for several interactions matching
that theory's predictions. Physicist Matt
Strassler highlighted "considerable" evidence
that the new particle is not a pseudoscalar
negative parity particle, "evaporation" or
lack of increased significance for previous
hints of non-Standard Model findings, expected
Standard Model interactions with W and Z bosons,
absence of "significant new implications"
for or against supersymmetry, and in general
no significant deviations to date from the
results expected of a Standard Model Higgs
boson. However some kinds of extensions to
the Standard Model would also show very similar
results; so commentators noted that based
on other particles that are still being understood
long after their discovery, it may take years
to be sure, and decades to fully understand
the particle that has been found.
These findings meant that as of January 2013,
scientists were very sure they had found an
unknown particle of mass ~ 125 GeV/c2, and
had not been misled by experimental error
or a chance result. They were also sure, from
initial observations, that the new particle
was some kind of boson. The behaviours and
properties of the particle, so far as examined
since July 2012, also seemed quite close to
the behaviours expected of a Higgs boson.
Even so, it could still have been a Higgs
boson or some other unknown boson, since future
tests could show behaviours that do not match
a Higgs boson, so as of December 2012 CERN
still only stated that the new particle was
"consistent with" the Higgs boson, and scientists
did not yet positively say it was the Higgs
boson. Despite this, in late 2012, widespread
media reports announced that a Higgs boson
had been confirmed during the year.
In January 2013, CERN director-general Rolf-Dieter
Heuer stated that based on data analysis to
date, an answer could be possible 'towards'
mid-2013, and the deputy chair of physics
at Brookhaven National Laboratory stated in
February 2013 that a "definitive" answer might
require "another few years" after the collider's
2015 restart. In early March 2013, CERN Research
Director Sergio Bertolucci stated that confirming
spin-0 was the major remaining requirement
to determine whether the particle is at least
some kind of Higgs boson.
Confirmation of new particle as a Higgs boson,
and current status
On 14 March 2013 CERN confirmed that:
"CMS and ATLAS have compared a number of options
for the spin-parity of this particle, and
these all prefer no spin and positive parity
[two fundamental criteria of a Higgs boson
consistent with the Standard Model]. This,
coupled with the measured interactions of
the new particle with other particles, strongly
indicates that it is a Higgs boson." 
This also makes the particle the first elementary
scalar particle to be discovered in nature.
Examples of tests used to validate whether
the 125 GeV particle is a Higgs boson:
Public discussion
Naming
Names used by physicists
The name most strongly associated with the
particle and field is the Higgs boson and
Higgs field. For some time the particle was
known by a combination of its PRL author names,
for example the Brout–Englert–Higgs particle,
the Anderson-Higgs particle, or the Englert–Brout–Higgs–Guralnik–Hagen–Kibble
mechanism, and these are still used at times.
Fueled in part by the issue of recognition
and a potential shared Nobel Prize, the most
appropriate name is still occasionally a topic
of debate as at 2012.
A considerable amount has been written on
how Higgs' name came to be exclusively used.
Two main explanations are offered.
Nickname
The Higgs boson is often referred to as the
"God particle" in popular media outside the
scientific community. The nickname comes from
the title of a 1993 book on the Higgs boson
and particle physics by Nobel Physics prizewinner
and Fermilab director Leon Lederman. Lederman
wrote it in the context of failing US government
support for the Superconducting Super Collider,
a part-constructed titanic competitor to the
Large Hadron Collider with planned collision
energies of 2 × 20 TeV that was championed
by Lederman since its 1983 inception and shut
down in 1993. The book sought in part to promote
awareness of the significance and need for
such a project in the face of its possible
loss of funding.
While media use of this term may have contributed
to wider awareness and interest, many scientists
feel the name is inappropriate since it is
sensational hyperbole and misleads readers;
the particle also has nothing to do with God,
leaves open numerous questions in fundamental
physics, and does not explain the ultimate
origin of the universe. Higgs, an atheist,
was reported to be displeased and stated in
a 2008 interview that he found it "embarrassing"
because it was "the kind of misuse... which
I think might offend some people". Science
writer Ian Sample stated in his 2010 book
on the search that the nickname is "universally
hate[d]" by physicists and perhaps the "worst
derided" in the history of physics, but that
the publisher rejected all titles mentioning
"Higgs" as unimaginative and too unknown.
Lederman begins with a review of the long
human search for knowledge, and explains that
his tongue-in-cheek title draws an analogy
between the impact of the Higgs field on the
fundamental symmetries at the Big Bang, and
the apparent chaos of structures, particles,
forces and interactions that resulted and
shaped our present universe, with the biblical
story of Babel in which the primordial single
language of early Genesis was fragmented into
many disparate languages and cultures.
Today ... we have the standard model, which
reduces all of reality to a dozen or so particles
and four forces. ... It's a hard-won simplicity
[...and...] remarkably accurate. But it is
also incomplete and, in fact, internally inconsistent...
This boson is so central to the state of physics
today, so crucial to our final understanding
of the structure of matter, yet so elusive,
that I have given it a nickname: the God Particle.
Why God Particle? Two reasons. One, the publisher
wouldn't let us call it the Goddamn Particle,
though that might be a more appropriate title,
given its villainous nature and the expense
it is causing. And two, there is a connection,
of sorts, to another book, a much older one...
Lederman whimsically asks whether the Higgs
boson was added just to perplex and confound
those seeking knowledge of the universe, and
whether physicists will be confounded by it
as recounted in that story, or ultimately
surmount the challenge and understand "how
beautiful is the universe [God has] made".
Other proposals
A renaming competition by British newspaper
The Guardian in 2009 resulted in their science
correspondent choosing the name "the champagne
bottle boson" as the best submission: "The
bottom of a champagne bottle is in the shape
of the Higgs potential and is often used as
an illustration in physics lectures. So it's
not an embarrassingly grandiose name, it is
memorable, and [it] has some physics connection
too." The name Higgson was suggested as well,
in an opinion piece in the Institute of Physics'
online publication physicsworld.com.
Media explanations and analogies
There has been considerable public discussion
of analogies and explanations for the Higgs
particle and how the field creates mass, including
coverage of explanatory attempts in their
own right and a competition in 1993 for the
best popular explanation by then-UK Minister
for Science Sir William Waldegrave and articles
in newspapers worldwide.
An educational collaboration involving an
LHC physicist and a High School Teachers at
CERN educator suggests that dispersion of
light – responsible for the rainbow and
dispersive prism – is a useful analogy for
the Higgs field's symmetry breaking and mass-causing
effect.
Matt Strassler uses electric fields as an
analogy:
Some particles interact with the Higgs field
while others don’t. Those particles that
feel the Higgs field act as if they have mass.
Something similar happens in an electric field
– charged objects are pulled around and
neutral objects can sail through unaffected.
So you can think of the Higgs search as an
attempt to make waves in the Higgs field [create
Higgs bosons] to prove it’s really there.
A similar explanation was offered by The Guardian:
The Higgs boson is essentially a ripple in
a field said to have emerged at the birth
of the universe and to span the cosmos to
this day ... The particle is crucial however:
it is the smoking gun, the evidence required
to show the theory is right.
The Higgs field's effect on particles was
famously described by physicist David Miller
as akin to a room full of political party
workers spread evenly throughout a room: the
crowd gravitates to and slows down famous
people but does not slow down others. He also
drew attention to well-known effects in solid
state physics where an electron's effective
mass can be much greater than usual in the
presence of a crystal lattice.
Analogies based on drag effects, including
analogies of "syrup" or "molasses" are also
well known, but can be somewhat misleading
since they may be understood as saying that
the Higgs field simply resists some particles'
motion but not others' – a simple resistive
effect could also conflict with Newton's third
law.
Recognition and awards
There has been considerable discussion of
how to allocate the credit if the Higgs boson
is proven, made more pointed as a Nobel prize
had been expected, and the very wide basis
of people entitled to consideration. These
include a range of theoreticians who made
the Higgs mechanism theory possible, the theoreticians
of the 1964 PRL papers, the theoreticians
who derived from these, a working electroweak
theory and the Standard Model itself, and
also the experimentalists at CERN and other
institutions who made possible the proof of
the Higgs field and boson in reality. The
Nobel prize has a limit of 3 persons to share
an award, and some possible winners are already
prize holders for other work, or are deceased.
Existing prizes for works relating to the
Higgs field, boson, or mechanism include:
Nobel Prize in Physics – Weinberg and Salam,
for contributions to the theory of the unified
weak and electromagnetic interaction between
elementary particles 
Nobel Prize in Physics – 't Hooft and Veltman,
for elucidating the quantum structure of electroweak
interactions in physics 
Nobel Prize in Physics – Nambu, for the
discovery of the mechanism of spontaneous
broken symmetry in subatomic physics 
J. J. Sakurai Prize for Theoretical Particle
Physics – Hagen, Englert, Guralnik, Higgs,
Brout, and Kibble, for elucidation of the
properties of spontaneous symmetry breaking
in four-dimensional relativistic gauge theory
and of the mechanism for the consistent generation
of vector boson masses
Wolf Prize – Englert, Brout, and Higgs
Nobel Prize in Physics - Peter Higgs and François
Englert, for the theoretical discovery of
a mechanism that contributes to our understanding
of the origin of mass of subatomic particles,
and which recently was confirmed through the
discovery of the predicted fundamental particle,
by the ATLAS and CMS experiments at CERN's
Large Hadron Collider 
Additionally Physical Review Letters' 50-year
review recognized the 1964 PRL papers and
Weinberg's 1967 paper A model of Leptons "milestone
Letters".
Following reported observation of the Higgs-like
particle in July 2012, several Indian media
outlets reported on the supposed neglect of
credit to Indian physicist Satyendra Nath
Bose after whose work in the 1920s the class
of particles "bosons" is named.
Technical aspects and mathematical formulation
In the Standard Model, the Higgs field is
a four-component scalar field that forms a
complex doublet of the weak isospin SU(2)
symmetry:
while the field has charge +1/2 under the
weak hypercharge U(1) symmetry.
The Higgs part of the Lagrangian is
where and are the gauge bosons of the SU(2)
and U(1) symmetries, and their respective
coupling constants, a complete set generators
of the SU(2) symmetry, and and , so that the
ground state breaks the SU(2) symmetry. The
ground state of the Higgs field is degenerate
with different ground states related to each
other by a SU(2) gauge transformation. It
is always possible to pick a gauge such that
in the ground state . The expectation value
of in the ground state is then , where . The
measured value of this parameter is ~246 GeV/c2.
It has units of mass, and is the only free
parameter of the Standard Model that is not
a dimensionless number. Quadratic terms in
and arise, which give masses to the W and
Z bosons:
with their ratio determining the Weinberg
angle, , and leave a massless U(1) photon,
.
The quarks and the leptons interact with the
Higgs field through Yukawa interaction terms:
where are left-handed and right-handed quarks
and leptons of the ith generation, are matrices
of Yukawa couplings where h.c. denotes the
hermitian conjugate terms. In the symmetry
breaking ground state, only the terms containing
remain, giving rise to mass terms for the
fermions. Rotating the quark and lepton fields
to the basis where the matrices of Yukawa
couplings are diagonal, one gets
where the masses of the fermions are , and
denote the eigenvalues of the Yukawa matrices.
See also
Standard Model
Quantum gauge theory
Introduction to quantum mechanics
Noncommutative standard model and noncommutative
geometry generally
Standard Model
Other
Bose–Einstein statistics
Dalitz plot
Higgs boson in fiction
Quantum triviality
ZZ diboson
Scalar boson
Stueckelberg action
Notes
References
Further reading
External links
Popular science, mass media, and general coverage
Hunting the Higgs Boson at C.M.S. Experiment,
at CERN
The Higgs Boson" by the CERN exploratorium.
"Particle Fever", documentary film about the
search for the Higgs Boson.
"The Atom Smashers", documentary film about
the search for the Higgs Boson at Fermilab.
Collected Articles at the Guardian
Video – CERN Announcement on 4 July 2012,
of the discovery of a particle which is suspected
will be a Higgs Boson.
Video1 + Video2 – Higgs Boson Explained
by CERN Physicist, Dr. Daniel Whiteson.
HowStuffWorks: What exactly is the Higgs Boson?
Carroll, Sean. "Higgs Boson with Sean Carroll".
Sixty Symbols. University of Nottingham. 
Overbye, Dennis. "Chasing the Higgs Boson:
How 2 teams of rivals at CERN searched for
physics' most elusive particle". New York
Times Science pages. Retrieved 22 July 2013. 
- New York Times "behind the scenes" style
article on the Higgs' search at ATLAS and
CMS
The story of the Higgs theory by the authors
of the PRL papers and others closely associated:
Higgs, Peter. "My Life as a Boson". Talk given
at Kings College, London, Nov 24 2010. Retrieved
17 January 2013. 
Kibble, Tom. "Englert–Brout–Higgs–Guralnik–Hagen–Kibble
mechanism". Scholarpedia. Retrieved 17 January
2013. 
Guralnik, Gerald. "The History of the Guralnik,
Hagen and Kibble development of the Theory
of Spontaneous Symmetry Breaking and Gauge
Particles". International Journal of Modern
Physics 
A 24: 2601–2627. arXiv:0907.3466. Bibcode:2009IJMPA..24.2601G.
doi:10.1142/S0217751X09045431. , Guralnik,
Gerald. "The Beginnings of Spontaneous Symmetry
Breaking in Particle Physics. Proceedings
of the DPF-2011 Conference, Providence, RI,
8–13 August 2011". arXiv:1110.2253v1 [physics.hist-ph].,
and Guralnik, Gerald. "Heretical Ideas that
Provided the Cornerstone for the Standard
Model of Particle Physics". SPG MITTEILUNGEN
March 2013, No. 39,, and Talk at Brown University
about the 1964 PRL papers
Philip Anderson on symmetry breaking in superconductivity
and its migration into particle physics and
the PRL papers
Cartoon about the search
Cham, Jorge. "True Tales from the Road: The
Higgs Boson Re-Explained". Piled Higher and
Deeper. Retrieved 2014-02-25. 
Significant papers and other
Observation of a new particle in the search
for the Standard Model Higgs Boson with the
ATLAS detector at the LHC
Observation of a new Boson at a mass of 125
GeV with the CMS experiment at the LHC
Particle Data Group: Review of searches for
Higgs Bosons.
2001, a spacetime odyssey: proceedings of
the Inaugural Conference of the Michigan Center
for Theoretical Physics : Michigan, USA,
21–25 May 2001,, ed. Michael J. Duff, James
T. Liu, ISBN 978-981-238-231-3, containing
Higgs' story of the Higgs Boson.
A.A. Migdal & A.M. Polyakov, Spontaneous Breakdown
of Strong Interaction Symmetry and the Absence
of Massless Particles, Sov.J.-JETP 24,91 - example
of a 1966 Russian paper on the subject.
Introductions to the field
Spontaneous symmetry breaking, gauge theories,
the Higgs mechanism and all that - an introduction
of 47 pages covering the development, history
and mathematics of Higgs theories from around
1950 to 1974.
