The top quark, also known as the t quark (symbol:
t) or truth quark, is the most massive of
all observed elementary particles. Like all
quarks, the top quark is a fermion with spin
1/2, and experiences all four fundamental
interactions: gravitation, electromagnetism,
weak interactions, and strong interactions.
It has an electric charge of +2/3 e. It has
a mass of 172.44 ± 0.13 (stat) ± 0.47 (syst)GeV/c2,
which is about the same mass as an atom of
tungsten. The antiparticle of the top quark
is the top antiquark (symbol: t, sometimes
called antitop quark or simply antitop), which
differs from it only in that some of its properties
have equal magnitude but opposite sign.
The top quark interacts primarily by the strong
interaction, but can only decay through the
weak force. It decays to a W boson and either
a bottom quark (most frequently), a strange
quark, or, on the rarest of occasions, a down
quark. The Standard Model predicts its mean
lifetime to be roughly 5×10−25 s. This
is about a twentieth of the timescale for
strong interactions, and therefore it does
not form hadrons, giving physicists a unique
opportunity to study a "bare" quark (all other
quarks hadronize, meaning that they combine
with other quarks to form hadrons, and can
only be observed as such). Because it is so
massive, the properties of the top quark allow
predictions to be made of the mass of the
Higgs boson under certain extensions of the
Standard Model (see Mass and coupling to the
Higgs boson below). As such, it is extensively
studied as a means to discriminate between
competing theories.
Its existence (and that of the bottom quark)
was postulated in 1973 by Makoto Kobayashi
and Toshihide Maskawa to explain the observed
CP violations in kaon decay, and was discovered
in 1995 by the CDF and DØ experiments at
Fermilab. Kobayashi and Maskawa won the 2008
Nobel Prize in Physics for the prediction
of the top and bottom quark, which together
form the third generation of quarks.
== History ==
In 1973, Makoto Kobayashi and Toshihide Maskawa
predicted the existence of a third generation
of quarks to explain observed CP violations
in kaon decay. The names top and bottom were
introduced by Haim Harari in 1975,
to match the names of the first generation
of quarks (up and down) reflecting the fact
that the two were the 'up' and 'down' component
of a weak isospin doublet. The top quark was
sometimes called truth quark in the past,
but over time top quark became the predominant
use.The proposal of Kobayashi and Maskawa
heavily relied on the GIM mechanism put forward
by Sheldon Lee Glashow, John Iliopoulos and
Luciano Maiani, which predicted the existence
of the then still unobserved charm quark.
When in November 1974 teams at Brookhaven
National Laboratory (BNL) and the Stanford
Linear Accelerator Center (SLAC) simultaneously
announced the discovery of the J/ψ meson,
it was soon after identified as a bound state
of the missing charm quark with its antiquark.
This discovery allowed the GIM mechanism to
become part of the Standard Model. With the
acceptance of the GIM mechanism, Kobayashi
and Maskawa's prediction also gained in credibility.
Their case was further strengthened by the
discovery of the tau by Martin Lewis Perl's
team at SLAC between 1974 and 1978. This announced
a third generation of leptons, breaking the
new symmetry between leptons and quarks introduced
by the GIM mechanism. Restoration of the symmetry
implied the existence of a fifth and sixth
quark.
It was in fact not long until a fifth quark,
the bottom, was discovered by the E288 experiment
team, led by Leon Lederman at Fermilab in
1977. This strongly suggested that there must
also be a sixth quark, the top, to complete
the pair. It was known that this quark would
be heavier than the bottom, requiring more
energy to create in particle collisions, but
the general expectation was that the sixth
quark would soon be found. However, it took
another 18 years before the existence of the
top was confirmed.Early searches for the top
quark at SLAC and DESY (in Hamburg) came up
empty-handed. When, in the early eighties,
the Super Proton Synchrotron (SPS) at CERN
discovered the W boson and the Z boson, it
was again felt that the discovery of the top
was imminent. As the SPS gained competition
from the Tevatron at Fermilab there was still
no sign of the missing particle, and it was
announced by the group at CERN that the top
mass must be at least 41 GeV/c2. After a race
between CERN and Fermilab to discover the
top, the accelerator at CERN reached its limits
without creating a single top, pushing the
lower bound on its mass up to 77 GeV/c2.The
Tevatron was (until the start of LHC operation
at CERN in 2009) the only hadron collider
powerful enough to produce top quarks. In
order to be able to confirm a future discovery,
a second detector, the DØ detector, was added
to the complex (in addition to the Collider
Detector at Fermilab (CDF) already present).
In October 1992, the two groups found their
first hint of the top, with a single creation
event that appeared to contain the top. In
the following years, more evidence was collected
and on April 22, 1994, the CDF group submitted
their paper presenting tentative evidence
for the existence of a top quark with a mass
of about 175 GeV/c2. In the meantime, DØ
had found no more evidence than the suggestive
event in 1992. A year later, on March 2, 1995,
after having gathered more evidence and a
reanalysis of the DØ data (who had been searching
for a much lighter top), the two groups jointly
reported the discovery of the top at a mass
of 176±18 GeV/c2.In the years leading up
to the top quark discovery, it was realized
that certain precision measurements of the
electroweak vector boson masses and couplings
are very sensitive to the value of the top
quark mass. These effects become much larger
for higher values of the top mass and therefore
could indirectly see the top quark even if
it could not be directly detected in any experiment
at the time. The largest effect from the top
quark mass was on the T parameter and by 1994
the precision of these indirect measurements
had led to a prediction of the top quark mass
to be between 145 GeV/c2 and 185 GeV/c2. It
is the development of techniques that ultimately
allowed such precision calculations that led
to Gerardus 't Hooft and Martinus Veltman
winning the Nobel Prize in physics in 1999.
== Properties ==
At the final Tevatron energy of 1.96 TeV,
top–antitop pairs were produced with a cross
section of about 7 picobarns (pb). The Standard
Model prediction (at next-to-leading order
with mt = 175 GeV/c2) is 6.7–7.5 pb.
The W bosons from top quark decays carry polarization
from the parent particle, hence pose themselves
as a unique probe to top polarization.
In the Standard Model, the top quark is predicted
to have a spin quantum number of ​1⁄2
and electric charge +​2⁄3. A first measurement
of the top quark charge has been published,
resulting in approximately 90% confidence
limit that the top quark charge is indeed
+​2⁄3.
== Production ==
Because top quarks are very massive, large
amounts of energy are needed to create one.
The only way to achieve such high energies
is through high energy collisions. These occur
naturally in the Earth's upper atmosphere
as cosmic rays collide with particles in the
air, or can be created in a particle accelerator.
In 2011, after the Tevatron ceased operations,
the Large Hadron Collider at CERN became the
only accelerator that generates a beam of
sufficient energy to produce top quarks, with
a center-of-mass energy of 7 TeV. There are
multiple processes that can lead to the production
of top quarks, but they can be conceptually
divided in two categories.
=== Top-quark pairs ===
The most common is production of a top–antitop
pair via strong interactions. In a collision,
a highly energetic gluon is created, which
subsequently decays into a top and antitop.
This process was responsible for the majority
of the top events at Tevatron and was the
process observed when the top was first discovered
in 1995. It is also possible to produce pairs
of top–antitop through the decay of an intermediate
photon or Z-boson. However, these processes
are predicted to be much rarer and have a
virtually identical experimental signature
in a hadron collider like Tevatron.
=== Single top quarks ===
A distinctly different process is the production
of single top quarks via weak interaction.
This can happen in several ways (called channels):
either an intermediate W-boson decays into
a top and antibottom quark ("s-channel") or
a bottom quark (probably created in a pair
through the decay of a gluon) transforms to
a top quark by exchanging a W-boson with an
up or down quark ("t-channel"). A single top
quark can also be produced in association
with a W boson, requiring an initial state
bottom quark ("tW-channel"). The first evidence
for these processes was published by the DØ
collaboration in December 2006, and in March
2009 the CDF and DØ collaborations released
twin papers with the definitive observation
of these processes. The main significance
of measuring these production processes is
that their frequency is directly proportional
to the | Vtb |2 component of the CKM matrix.
== Decay ==
Because of its enormous mass, the top quark
is extremely short-lived with a predicted
lifetime of only 5×10−25 s. As a result,
top quarks do not have time before they decay
to form hadrons as other quarks do, which
provides physicists with the unique opportunity
to study the behavior of a "bare" quark. The
only known way the top quark can decay is
through the weak interaction producing a W-boson
and a down-type quark (down, strange, or bottom).
In particular, it is possible to directly
determine the branching ratio Γ(W+b) / Γ(W+q
(q = b,s,d)). The best current determination
of this ratio is 0.91±0.04. Since this ratio
is equal to | Vtb |2 according to the
Standard Model, this gives another way of
determining the CKM element | Vtb |, or
in combination with the determination of | Vtb |
from single top production provides tests
for the assumption that the CKM matrix is
unitary.The Standard Model also allows more
exotic decays, but only at one loop level,
meaning that they are extremely suppressed.
In particular, it is possible for a top quark
to decay into another up-type quark (an up
or a charm) by emitting a photon or a Z-boson.
Searches for these exotic decay modes have
provided no evidence for their existence in
accordance with expectations from the Standard
Model. The branching ratios for these decays
have been determined to be less than 5.9 in
1,000 for photonic decay and less than 2.1
in 1,000 for Z-boson decay at 95% confidence.
== Mass and coupling to the Higgs boson ==
The Standard Model describes fermion masses
through the Higgs mechanism. The Higgs boson
has a Yukawa coupling to the left- and right-handed
top quarks. After electroweak symmetry breaking
(when the Higgs acquires a vacuum expectation
value), the left- and right-handed components
mix, becoming a mass term.
L
=
y
t
h
q
u
c
→
y
t
v
2
(
1
+
h
0
/
v
)
u
u
c
{\displaystyle {\mathcal {L}}=y_{\text{t}}hqu^{c}\rightarrow
{\frac {y_{\text{t}}v}{\sqrt {2}}}(1+h^{0}/v)uu^{c}}
The top quark Yukawa coupling has a value
of
y
t
=
2
m
t
/
v
≃
1
{\displaystyle y_{\text{t}}={\sqrt {2}}m_{\text{t}}/v\simeq
1}
where v = 246 GeV is the value of the Higgs
vacuum expectation value.
=== Yukawa couplings ===
In the Standard Model, all of the quark and
lepton Yukawa couplings are small compared
to the top quark Yukawa coupling. Understanding
this hierarchy in the fermion masses is an
open problem in theoretical physics. Yukawa
couplings are not constants and their values
change depending on the energy scale (distance
scale) at which they are measured. The dynamics
of Yukawa couplings are determined by the
renormalization group equation.
One of the prevailing views in particle physics
is that the size of the top quark Yukawa coupling
is determined by the renormalization group,
leading to the "quasi-infrared fixed point."
The Yukawa couplings of the up, down, charm,
strange and bottom quarks, are hypothesized
to have small values at the extremely high
energy scale of grand unification, 1015 GeV.
They increase in value at lower energy scales,
at which the quark masses are generated by
the Higgs. The slight growth is due to corrections
from the QCD coupling. The corrections from
the Yukawa couplings are negligible for the
lower mass quarks.
If, however, a quark Yukawa coupling has a
large value at very high energies, its Yukawa
corrections will evolve and cancel against
the QCD corrections. This is known as a (quasi-)
infrared fixed point. No matter what the initial
starting value of the coupling is, if it is
sufficiently large it will reach this fixed
point value. The corresponding quark mass
is then predicted.
The top quark Yukawa coupling lies very near
the infrared fixed point of the Standard Model.
The renormalization group equation is:
μ
∂
∂
μ
y
t
≈
y
t
16
π
2
(
9
2
y
t
2
−
8
g
3
2
−
9
4
g
2
2
−
17
20
g
1
2
)
,
{\displaystyle \mu {\frac {\partial }{\partial
\mu }}y_{\text{t}}\approx {\frac {y_{\text{t}}}{16\pi
^{2}}}\left({\frac {9}{2}}y_{\text{t}}^{2}-8g_{3}^{2}-{\frac
{9}{4}}g_{2}^{2}-{\frac {17}{20}}g_{1}^{2}\right),}
where g3 is 
the 
color gauge coupling, g2 is the weak isospin
gauge coupling, and g1 is the weak hypercharge
gauge coupling. This equation describes how
the Yukawa coupling changes with energy scale
μ. Solutions to this equation for large initial
values yt cause the right-hand side of the
equation to quickly approach zero, locking
yt to the QCD coupling g3. The value of the
fixed point is fairly precisely determined
in the Standard Model, leading to a top quark
mass of 230 GeV. However, if there is more
than one Higgs doublet, the mass value will
be reduced by Higgs mixing angle effects in
an unpredicted way.
In the minimal supersymmetric extension of
the Standard Model (MSSM), there are two Higgs
doublets and the renormalization group equation
for the top quark Yukawa coupling is slightly
modified:
μ
∂
∂
μ
y
t
≈
y
t
16
π
2
(
6
y
t
2
+
y
b
2
−
16
3
g
3
2
−
3
g
2
2
−
13
15
g
1
2
)
,
{\displaystyle \mu {\frac {\partial }{\partial
\mu }}y_{\text{t}}\approx {\frac {y_{\text{t}}}{16\pi
^{2}}}\left(6y_{\text{t}}^{2}+y_{\text{b}}^{2}-{\frac
{16}{3}}g_{3}^{2}-3g_{2}^{2}-{\frac {13}{15}}g_{1}^{2}\right),}
where yb is the bottom quark Yukawa coupling.
This leads to a fixed point where the top
mass is smaller, 170–200 GeV. The uncertainty
in this prediction arises because the bottom
quark Yukawa coupling can be amplified in
the MSSM. Some theorists believe this is supporting
evidence for the MSSM.
The quasi-infrared fixed point has subsequently
formed the basis of top quark condensation
theories of electroweak symmetry breaking
in which the Higgs boson is composite at extremely
short distance scales, composed of a pair
of top and antitop quarks.
== See also ==
CDF experiment
Quark model
Top quark condensate
Topcolor
Topness
