In particle physics, preons are point particles,
conceived of as subcomponents of quarks and
leptons. The word was coined by Jogesh Pati
and Abdus Salam in 1974. Interest in preon
models peaked in the 1980s but has slowed
as the Standard Model of particle physics
continues to describe the physics mostly successfully,
and no direct experimental evidence for lepton
and quark compositeness has been found.
In the hadronic sector, some effects are considered
anomalies within the Standard Model. For example,
the proton spin puzzle, the EMC effect, the
distributions of electric charges inside the
nucleons as found by Hofstadter in 1956, and
the ad hoc CKM matrix elements.
When the term "preon" was coined, it was primarily
to explain the two families of spin-½ fermions:
leptons and quarks. More recent preon models
also account for spin-1 bosons, and are still
called "preons". Each of the preon models
postulates a set of fewer fundamental particles
than those of the Standard Model, together
with the rules governing how those fundamental
particles combine and interact. Based on these
rules, the preon models try to explain the
Standard Model, often predicting small discrepancies
with this model and generating new particles
and certain phenomena, which do not belong
to the Standard Model.
== Goals of preon models ==
Preon research is motivated by the desire
to:
Reduce the large number of particles, many
that differ only in charge, to a smaller number
of more fundamental particles. For example,
the electron and positron are identical except
for charge, and preon research is motivated
by explaining that electrons and positrons
are composed of similar preons with the relevant
difference accounting for charge. The hope
is to reproduce the reductionist strategy
that has worked for the periodic table of
elements.
Explain the three generations of fermions.
Calculate parameters that are currently unexplained
by the Standard Model, such as particle masses,
electric charges, and color charges, and reduce
the number of experimental input parameters
required by the Standard Model.
Provide reasons for the very large differences
in energy-masses observed in supposedly fundamental
particles, from the electron neutrino to the
top quark.
Provide alternative explanations for the electro-weak
symmetry breaking without invoking a Higgs
field, which in turn possibly needs a supersymmetry
to correct the theoretical problems involved
with the Higgs field. Supersymmetry itself
has theoretical problems.
Account for neutrino oscillation and mass.
Make new nontrivial predictions, such as cold
dark matter candidates.
Explain why there exists only the observed
variety of particle species reproduce only
these observed particles (since the prediction
of non-observed particles is one of the major
theoretical problems, as, for example, with
supersymmetry).
== Background ==
Before the Standard Model (SM) was developed
in the 1970s (the key elements of the Standard
Model known as quarks were proposed by Murray
Gell-Mann and George Zweig in 1964), physicists
observed hundreds of different kinds of particles
in particle accelerators. These were organized
into relationships on their physical properties
in a largely ad-hoc system of hierarchies,
not entirely unlike the way taxonomy grouped
animals based on their physical features.
Not surprisingly, the huge number of particles
was referred to as the "particle zoo".
The Standard Model, which is now the prevailing
model of particle physics, dramatically simplified
this picture by showing that most of the observed
particles were mesons, which are combinations
of two quarks, or baryons which are combinations
of three quarks, plus a handful of other particles.
The particles being seen in the ever-more-powerful
accelerators were, according to the theory,
typically nothing more than combinations of
these quarks.
=== Comparisons of quarks, leptons, and bosons
===
Within the Standard Model, there are several
classes of particles. One of these, the quarks,
has six types, of which there are three varieties
in each (dubbed "colors", red, green, and
blue, giving rise to quantum chromodynamics).
Additionally, there are six different types
of what are known as leptons. Of these six
leptons, there are three charged particles:
the electron, muon, and tau. The neutrinos
comprise the other three leptons, and for
each neutrino there is a corresponding member
from the other set of three leptons.
In the Standard Model, there are also bosons,
including the photons; W+, W−, and Z bosons;
gluons and the Higgs boson; and an open space
left for the graviton. Almost all of these
particles come in "left-handed" and "right-handed"
versions (see chirality). The quarks, leptons,
and W boson all have antiparticles with opposite
electric charge.
=== Unresolved problems with the Standard
Model ===
The Standard Model also has a number of problems
which have not been entirely solved. In particular,
no successful theory of gravitation based
on a particle theory has yet been proposed.
Although the Model assumes the existence of
a graviton, all attempts to produce a consistent
theory based on them have failed.
Kalman observes that, according to the concept
of atomism, fundamental building blocks of
nature are indivisible bits of matter that
are ungenerated and indestructible. Quarks
are not truly indestructible, since some can
decay into other quarks. Thus, on fundamental
grounds, quarks are not themselves fundamental
building blocks but must be composed of other,
fundamental quantities—preons. Although
the mass of each successive particle follows
certain patterns, predictions of the rest
mass of most particles cannot be made precisely,
except for the masses of almost all baryons
which have been recently described very well
by the model of de Souza.The Standard Model
also has problems predicting the large scale
structure of the universe. For instance, the
SM generally predicts equal amounts of matter
and antimatter in the universe. A number of
attempts have been made to "fix" this through
a variety of mechanisms, but to date none
have won widespread support. Likewise, basic
adaptations of the Model suggest the presence
of proton decay, which has not yet been observed.
=== Motivation for preon models ===
Preon theory is motivated by a desire to replicate
the achievements of the periodic table, which
reduced the elements to three building-blocks,
and the later Standard Model which tamed the
"particle zoo", by finding more fundamental
answers to the huge number of arbitrary constants
present in the Standard Model.
Several models have been proposed in an attempt
to provide a more fundamental explanation
of the results in experimental and theoretical
particle physics, using names such as "parton"
or "preon" for their basic particles. The
particular preon model discussed below has
attracted comparatively little interest to
date among the particle physics community,
in part because no evidence has been obtained
so far in collider experiments to show that
the fermions of the Standard Model are composite.
=== Attempts ===
A number of physicists have attempted to develop
a theory of "pre-quarks" (from which the name
preon derives) in an effort to justify theoretically
the many parts of the Standard Model that
are known only through experimental data.
Other names which have been used for these
proposed fundamental particles (or particles
intermediate between the most fundamental
particles and those observed in the Standard
Model) include prequarks, subquarks, maons,
alphons, quinks, rishons, tweedles, helons,
haplons, Y-particles, and primons. Preon is
the leading name in the physics community.
Efforts to develop a substructure date at
least as far back as 1974 with a paper by
Pati and Salam in Physical Review. Other attempts
include a 1977 paper by Terazawa, Chikashige
and Akama, similar, but independent, 1979
papers by Ne'eman, Harari, and Shupe, a 1981
paper by Fritzsch and Mandelbaum, and a 1992
book by D'Souza and Kalman. None of these
have gained wide acceptance in the physics
world. However, in a recent work de Souza
has shown that his model describes well all
weak decays of hadrons according to selection
rules dictated by a quantum number derived
from his compositeness model. In his model
leptons are elementary particles and each
quark is composed of two primons, and thus,
all quarks are described by four primons.
Therefore, there is no need for the Standard
Model Higgs boson and each quark mass is derived
from the interaction between each pair of
primons by means of three Higgs-like bosons.
In his 1989 Nobel Prize acceptance lecture,
Hans Dehmelt described a most fundamental
elementary particle, with definable properties,
which he called the cosmon, as the likely
end result of a long but finite chain of increasingly
more elementary particles.
=== Composite Higgs ===
Many preon models either do not account for
the Higgs boson or rule it out, and propose
that electro-weak symmetry is broken not by
a scalar Higgs field but by composite preons.
For example, Fredriksson preon theory does
not need the Higgs boson, and explains the
electro-weak breaking as the rearrangement
of preons, rather than a Higgs-mediated field.
In fact, the Fredriksson preon model and the
de Souza model predict that the Standard Model
Higgs boson does not exist.
== Rishon model ==
The rishon model (RM) is the earliest effort
to develop a preon model to explain the phenomenon
appearing in the Standard Model (SM) of particle
physics. It was first developed by Haim Harari
and Michael A. Shupe (independently of each
other), and later expanded by Harari and his
then-student Nathan Seiberg.The model has
two kinds of fundamental particles called
rishons (which means "primary" in Hebrew).
They are T ("Third" since it has an electric
charge of ⅓ e, or Tohu which means "unformed"
in Hebrew Genesis) and V ("Vanishes", since
it is electrically neutral, or Vohu. [Bohu
means "void" in the Hebrew Tanakh (the Old
Testament), though bohu may be pronounced
as vohu by modern Israelis when the "b" is
preceded by a vowel and thus lacks dagesh.]
All leptons and all flavours of quarks are
three-rishon ordered triplets. These groups
of three rishons have spin-½.
The Rishon model illustrates some of the typical
efforts in the field. Many of the preon models
theorize that the apparent imbalance of matter
and antimatter in the universe is in fact
illusory, with large quantities of preon-level
antimatter confined within more complex structures.
== Criticisms ==
=== The mass paradox ===
One preon model started as an internal paper
at the Collider Detector at Fermilab (CDF)
around 1994. The paper was written after an
unexpected and inexplicable excess of jets
with energies above 200 GeV were detected
in the 1992–1993 running period. However,
scattering experiments have shown that quarks
and leptons are "pointlike" down to distance
scales of less than 10−18 m (or ​1⁄1000
of a proton diameter). The momentum uncertainty
of a preon (of whatever mass) confined to
a box of this size is about 200 GeV/c, 50,000
times larger than the rest mass of an up-quark
and 400,000 times larger than the rest mass
of an electron.
Heisenberg's uncertainty principle states
that
Δ
⁡
x
⋅
Δ
⁡
p
≥
1
2
ℏ
{\displaystyle \operatorname {\Delta } x\cdot
\operatorname {\Delta } p\geq {\tfrac {1}{2}}\hbar
}
and thus anything confined to a box smaller
than
Δ
⁡
x
{\displaystyle \operatorname {\Delta } x}
would have a momentum uncertainty proportionally
greater. Thus, the preon model proposed particles
smaller than the elementary particles they
make up, since the momentum uncertainty
Δ
⁡
p
{\displaystyle \operatorname {\Delta } p}
should be greater than the particles themselves.
So the preon model represents a mass paradox:
How could quarks or electrons be made of smaller
particles that would have many orders of magnitude
greater mass-energies arising from their enormous
momenta? This paradox is resolved by postulating
a large binding force between preons cancelling
their mass-energies.
=== Conflicts with observed physics ===
Preon models propose additional unobserved
forces or dynamics to account for the observed
properties of elementary particles, which
may have implications in conflict with observation.
For example, now that the LHC's observation
of a Higgs boson is confirmed, the observation
contradicts the predictions of many preon
models that did not include it.
Preon theories require that quarks and leptons
should have a finite size. It is possible
that the Large Hadron Collider will observe
this when raised to higher energies.
== In popular culture ==
In the 1948 reprint/edit of his 1930 novel
Skylark Three, E. E. Smith postulated a series
of 'subelectrons of the first and second type'
with the latter being fundamental particles
that were associated with the gravitation
force. While this may not have been an element
of the original novel (the scientific basis
of some of the other novels in the series
was revised extensively due to the additional
eighteen years of scientific development),
even the edited publication may be the first,
or one of the first, mentions of the possibility
that electrons are not fundamental particles.In
the novelized version of the 1982 motion picture
Star Trek II: The Wrath of Khan, written by
Vonda McIntyre, two of Dr. Carol Marcus' Genesis
project team, Vance Madison and Delwyn March,
have studied sub-elementary particles they've
named "boojums" and "snarks", in a field they
jokingly call "kindergarten physics" because
it is lower than "elementary" (analogy to
school levels).James P. Hogan's 1982 novel
Voyage from Yesteryear discussed preons (called
tweedles), the physics of which became central
to the plot.
== See also ==
Technicolor (physics)
Preon star
Preon-degenerate matter
Rishon model
== 
Notes ==
== 
Further reading ==
Ball, P. (2007). "Splitting the quark". Nature.
doi:10.1038/news.2007.292.
"Have We Hit Bottom Yet? - an article about
preons and minuteness".
