A pentaquark is a subatomic particle consisting
of four quarks and one antiquark bound together.
As quarks have a baryon number of +1/3, and
antiquarks of −1/3, the pentaquark would
have a total baryon number of 1, and thus
would be a baryon.
Further, because it has five quarks instead
of the usual three found in regular baryons
(a.k.a. 'triquarks'), it is classified as
an exotic baryon.
The name pentaquark was coined by Claude Gignoux
et al. and Harry J. Lipkin in 1987; however,
the possibility of five-quark particles was
identified as early as 1964 when Murray Gell-Mann
first postulated the existence of quarks.
Although predicted for decades, pentaquarks
proved surprisingly difficult to discover
and some physicists were beginning to suspect
that an unknown law of nature prevented their
production.The first claim of pentaquark discovery
was recorded at LEPS in Japan in 2003, and
several experiments in the mid-2000s also
reported discoveries of other pentaquark states.
Others were not able to replicate the LEPS
results, however, and the other pentaquark
discoveries were not accepted because of poor
data and statistical analysis.
On 13 July 2015, the LHCb collaboration at
CERN reported results consistent with pentaquark
states in the decay of bottom Lambda baryons
(Λ0b).
On 26 March 2019, the LHCb collaboration announced
the discovery of a new pentaquark that had
not been previously observed.
The observations pass the 5-sigma threshold
required to claim the discovery of new particles.
Outside particle physics laboratories, pentaquarks
also could be produced naturally by supernovae
as part of the process of forming a neutron
star.
The scientific study of pentaquarks might
offer insights into how these stars form,
as well as allowing more thorough study of
particle interactions and the strong force.
== Background ==
A quark is a type of elementary particle that
has mass, electric charge, and colour charge,
as well as an additional property called flavour,
which describes what type of quark it is (up,
down, strange, charm, top, or bottom).
Due to an effect known as colour confinement,
quarks are never seen on their own.
Instead, they form composite particles known
as hadrons so that their colour charges cancel
out.
Hadrons made of one quark and one antiquark
are known as mesons, while those made of three
quarks are known as baryons.
These 'regular' hadrons are well documented
and characterized; however, there is nothing
in theory to prevent quarks from forming 'exotic'
hadrons such as tetraquarks with two quarks
and two antiquarks, or pentaquarks with four
quarks and one antiquark.
== Structure ==
A wide variety of pentaquarks are possible,
with different quark combinations producing
different particles.
To identify which quarks compose a given pentaquark,
physicists use the notation qqqqq, where q
and q respectively refer to any of the six
flavours of quarks and antiquarks.
The symbols u, d, s, c, b, and t stand for
the up, down, strange, charm, bottom, and
top quarks respectively, with the symbols
of u, d, s, c, b, t corresponding to the respective
antiquarks.
For instance a pentaquark made of two up quarks,
one down quark, one charm quark, and one charm
antiquark would be denoted uudcc.
The quarks are bound together by the strong
force, which acts in such a way as to cancel
the colour charges within the particle.
In a meson, this means a quark is partnered
with an antiquark with an opposite colour
charge – blue and antiblue, for example
– while in a baryon, the three quarks have
between them all three colour charges – red,
blue, and green.
In a pentaquark, the colours also need to
cancel out, and the only feasible combination
is to have one quark with one colour (e.g.
red), one quark with a second colour (e.g.
green), two quarks with the third colour (e.g.
blue), and one antiquark to counteract the
surplus colour (e.g. antiblue).The binding
mechanism for pentaquarks is not yet clear.
They may consist of five quarks tightly bound
together, but it is also possible that they
are more loosely bound and consist of a three-quark
baryon and a two-quark meson interacting relatively
weakly with each other via pion exchange (the
same force that binds atomic nuclei) in a
"meson-baryon molecule".
== History ==
=== Mid-2000s ===
The requirement to include an antiquark means
that many classes of pentaquark are hard to
identify experimentally – if the flavour
of the antiquark matches the flavour of any
other quark in the quintuplet, it will cancel
out and the particle will resemble its three-quark
hadron cousin.
For this reason, early pentaquark searches
looked for particles where the antiquark did
not cancel.
In the mid-2000s, several experiments claimed
to reveal pentaquark states.
In particular, a resonance with a mass of
1540 MeV/c2 (4.6 σ) was reported by LEPS
in 2003, the Θ+.
This coincided with a pentaquark state with
a mass of 1530 MeV/c2 predicted in 1997.The
proposed state was composed of two up quarks,
two down quarks, and one strange antiquark
(uudds).
Following this announcement, nine other independent
experiments reported seeing narrow peaks from
nK+ and pK0, with masses between 1522 MeV/c2
and 1555 MeV/c2, all above 4 σ.
While concerns existed about the validity
of these states, the Particle Data Group gave
the Θ+ a 3-star rating (out of 4) in the
2004 Review of Particle Physics.
Two other pentaquark states were reported
albeit with low statistical significance—the
Φ−− (ddssu), with a mass of 1860 MeV/c2
and the Θ0c (uuddc), with a mass of 3099
MeV/c2.
Both were later found to be statistical effects
rather than true resonances.Ten experiments
then looked for the Θ+, but came out empty-handed.
Two in particular (one at BELLE, and the other
at CLAS) had nearly the same conditions as
other experiments which claimed to have detected
the Θ+ (DIANA and SAPHIR respectively).
The 2006 Review of Particle Physics concluded:
[T]here has not been a high-statistics confirmation
of any of the original experiments that claimed
to see the Θ+; there have been two high-statistics
repeats from Jefferson Lab that have clearly
shown the original positive claims in those
two cases to be wrong; there have been a number
of other high-statistics experiments, none
of which have found any evidence for the Θ+;
and all attempts to confirm the two other
claimed pentaquark states have led to negative
results.
The conclusion that pentaquarks in general,
and the Θ+, in particular, do not exist,
appears compelling.
The 2008 Review of Particle Physics went even
further:
There are two or three recent experiments
that find weak evidence for signals near the
nominal masses, but there is simply no point
in tabulating them in view of the overwhelming
evidence that the claimed pentaquarks do not
exist...
The whole story—the discoveries themselves,
the tidal wave of papers by theorists and
phenomenologists that followed, and the eventual
"undiscovery"—is a curious episode in the
history of science.
Despite these null results, LEPS results continued
to show the existence of a narrow state with
a mass of 1524±4 MeV/c2, with a statistical
significance of 5.1 σ.
=== 2015 LHCb results ===
In July 2015, the LHCb collaboration at CERN
identified pentaquarks in the Λ0b→J/ψK−p
channel, which represents the decay of the
bottom lambda baryon (Λ0b) into a J/ψ meson
(J/ψ), a kaon (K−) and a proton (p).
The results showed that sometimes, instead
of decaying via intermediate lambda states,
the Λ0b decayed via intermediate pentaquark
states.
The two states, named P+c(4380) and P+c(4450),
had individual statistical significances of
9 σ and 12 σ, respectively, and a combined
significance of 15 σ – enough to claim
a formal discovery.
The analysis ruled out the possibility that
the effect was caused by conventional particles.
The two pentaquark states were both observed
decaying strongly to J/ψp, hence must have
a valence quark content of two up quarks,
a down quark, a charm quark, and an anti-charm
quark (uudcc), making them charmonium-pentaquarks.The
search for pentaquarks was not an objective
of the LHCb experiment (which is primarily
designed to investigate matter-antimatter
asymmetry) and the apparent discovery of pentaquarks
was described as an "accident" and "something
we’ve stumbled across" by the Physics Coordinator
for the experiment.
=== Studies of pentaquarks in other experiments
===
The production of pentaquarks from electroweak
decays of Λ0b baryons has extremely small
cross-section and yields very limited information
about internal structure of pentaquarks.
For this reason, there are several ongoing
and proposed initiatives to study pentaquark
production in other channels.
It is expected that pentaquarks will be studied
in electron-proton collisions in Hall B E2-16-007
and Hall C E12-12-001A experiments at JLAB.
The major challenge in these studies is a
heavy mass of the pentaquark, which will be
produced at the tail of photon-proton spectrum
in JLAB kinematics.
For this reason, the currently unknown branching
fractions of pentaquark should be sufficiently
large to allow pentaquark detection in JLAB
kinematics.
The proposed Electron Ion Collider which has
higher energies is much better suited for
this problem.
An interesting channel to study pentaquarks
in proton-nuclear collisions was suggested
in Schmidt and Siddikov (2016).
This process has a large cross-section due
to lack of electroweak intermediaries and
gives access to pentaquark wave function.
In the fixed-target experiments pentaquarks
will be produced with small rapidities in
laboratory frame and will be easily detected.
Besides, if there are neutral pentaquarks,
as suggested in several models based on flavour
symmetry, these might be also produced in
this mechanism.
This process might be studied at future high-luminosity
experiments like After@LHC and NICA.
=== 2019 LHCb results ===
On 26 March 2019 the LHCb collaboration announced
the discovery of a new pentaquark, based on
observations that passed the 5-sigma threshold,
using a dataset that was many times as large
as the 2015 dataset.Designated Pc(4312)+ (Pc+
identifies a charmonium-pentaquark while the
number between parenthesis indicates a mass
of about 4312 MeV), the pentaquark decays
to a proton and a J/ψ meson.
The analyses revealed additionally that the
earlier reported observations of the Pc(4450)+
pentaquark actually were the average of two
different resonances, designated Pc(4440)+
and Pc(4457)+.
Understanding this will require further study.
== Applications ==
The discovery of pentaquarks will allow physicists
to study the strong force in greater detail
and aid understanding of quantum chromodynamics.
In addition, current theories suggest that
some very large stars produce pentaquarks
as they collapse.
The study of pentaquarks might help shed light
on the physics of neutron stars.
== See also ==
Baryon (triquark)
Exotic matter
List of particles
Quark model
Tetraquark
== Footnotes
