Particle physicists, over the last 50 years, have discovered several hundred strongly interacting “elementary” particles. The list of such particles begins with the proton and the p meson and goes up from there. We now understand that all of these particles are actually composites, formed from various combinations of quarks. Such particles are normally classified in two types. Mesons (the name implies medium weight) are particles with masses that go up from 140 MeV/c2 and have an intrinsic angular momentum or “spin” that is an integer in units of ħ (Planck’s constant over 2p). Baryons (the name implies heavy weight) are particles with masses that go up from 938 MeV/c2 and have half-integer spin. This column is about the discovery of the pentaquark, a brand new form of matter that represents a new particle species, neither meson nor baryon, but a combination of both.
Our current standard model of particle physics, which is called quantum chromodynamics or QCD, describes all particles that interact through the strong force as being made in one of two ways. Either the particle is a meson, a combination of a matter quark and an antimatter quark (q q-bar), or else it is a baryon, a combination of three quarks(q q q). An anti-matter baryon (or anti-baryon) is an equivalent combination of three anti-matter quarks (q-bar q-bar q-bar). The lightest meson, the p-, is an example of a two-quark particle and is made of one up quark and one anti-down quark (u d-bar). The lightest baryon, the proton, is an example of a three-quark particle and is formed by two up quarks and one down quark (u u d). The Particle Data Group is a consortium of particle physicists that maintains a roster of all known strongly-interacting particles, with separate listings for mesons and baryons. They list 148 mesons and 130 baryons in their latest particle tables. Until the pentaquark appeared on the scene, we could confidently say that all of the strongly-interacting particles (with the possible exception of the hypothetical “glueball”, which we will ignore for now) were either 2-quark mesons or 3-quark baryons.
The tables of the Particle Data Group classify particles on the basis of their mass-energy (usually put in parenthesis in units of MeV/c2 after the particle symbol) and their “quantum number” characteristics. Quantum numbers represent conserved quantities (charge, angular momentum, quark flavor, …) represented by integers or half-integers. One important quantum number is “strangeness”, which is a count of the number of anti-strange minus strange quarks present in a given particle. Thus, the K+(494) meson (u s-bar) has a mass of 494 MeV/c2 and a strangeness S=+1, while the W-(1672) baryon (s s s) has a mass of 1,672 MeV/c2 and a strangeness S=-3. Mesons can contain at most one strange or anti-strange quark, and so can at most have strangeness S=±1. Matter baryons, composed of three matter quarks, cannot have positive strangeness, since they cannot contain any anti-strange quark constituents.
The strong-force equivalent of electric charge that is carried by quarks and gluons is called “color” and comes in three color-charge values, often called red, green, and blue (primary colors that are easy to draw on overhead transparencies). Antiparticles carry the negative equivalents of these colors. The “color confinement” aspect of QCD requires that quarks must combine so that no net color charge remains, so that any resulting particle is “color-neutral”. Color neutrality is achieved in mesons by canceling a given quark color with the corresponding “negative-color” of an antiquark in a quark-antiquark pair. It is achieved in baryons by having a quark of each of the three colors in the same particle to form a color-neutral three-quark triplet.
However, QCD suggests that there may be other possible ways to produce color-neutral quark combinations, and that particles using these schemes may have “exotic” quantum numbers that are impossible for normal mesons and baryons. For example, a particle could combine several quark-antiquark color-canceling pairs, it could combine several three-quark triplets, or it could mix quark-antiquark pairs with three-quark triplets. Thus a 6 quark combination might be either a “super-meson” made of matter/antimatter pairs of up, down, and strange quarks (charge=0, strangeness=0; never observed), or a double “strangelet-baryon” made of-2 triplets of up, down, and strange quarks (charge=0, strangeness=-2; never observed).
Another alternative is the pentaquark particle, which has 5 quarks (or more precisely, 4 quarks and 1 anti-quark), i.e., combining a quark pair with a triplet. Such a configuration, which could produce an “illegal” S=1 baryon-like particle, was suggested some decades ago, and there have been many experimental searches for such particles. However, as it turns out, the searchers were looking too high in mass-energy.
In 1997, a theoretical paper predicted that pentaquarks might exist as “chiral solitons”, and that the lowest-mass member of this set of pentaquarks, which the authors called the Z+(1530), should be a 5-quark combination having two up and two down quarks along with one anti-strange quark (u u d d s-bar). The anti-strange quark has a strangeness of +1, which therefore gives the Z+(1530) a net strangeness of S=1. This is a “forbidden” quantum number, because no 3-quark baryon can have positive strangeness.
The Z+ pentaquark is predicted to be the low-mass member of a 10-element set of particles (an anti-dectuplet), all with similar 5-quark compositions. Other particles of this group that have forbidden quantum numbers are the highest-mass, strangeness S=-2 members of the dectuplet, the X+3/2(2070) (u u s s d-bar) and the X=3/2(2070) (d d s s u-bar), which have electric charges, respectively, of +1 and -2. These exotic pentaquarks, which have not yet been seen experimentally, have forbidden quantum numbers because there can be no 3-quark baryon with a charge of less than -1 and no positively charged baryon with a strangeness of -2.
The predicted mass of the Z+, 1,530 GeV/c2, is very low, and it happens to fall in a region where there had been no experimental searches for S=1 baryons, primarily because kaon beams previously used in such searches did not go down to sufficiently low energies. Therefore, the new prediction provided virgin territory for a new particle search. Taking up the challenge of these new theoretical predictions last year, a Japanese experimental group used the gamma ray beam of the 8 GeV Spring-8 electron storage ring facility, located in Kamigori, Japan (not far from Kobe) to produce a reaction between 1.5-2.4 GeV gamma rays and the neutrons in carbon nuclei placed in the gamma-ray beam. They expected that if the hypothetical Z+ particle existed, it might be created in the gamma + carbon reaction, along with a K- meson.
I should emphasize that this experimental undertaking was a bit of a long shot. Many experimental groups in particle physics have conducted searches for hypothetical exotic particles predicted by theorists, and very few of these searches are ever successful. The Japanese experimenters were therefore amazed to find that their data contained strong evidence for an S=1 particle that broke up into a K+ meson and a neutron and that made a mass peak at 1,540±10 MeV/c2, essentially on top of the mass-energy that the theory had predicted for the Z+(1530) pentaquark. They triumphantly announced these results in a paper published in the journal Physical Review Letters on July 4, 2003.
Subsequently, an American experimental grout at the Jefferson Laboratory in Newport News, Virginia, used the tagged-bremsstrahlung gamma ray beam of that facility with the CLAS spectrometer to reproduce the Japanese measurements, finding the mass peak of an S=1 particle, assumed to be that of the Z+(1530), at a mass-energy of 1,543±5.0 MeV/c2. The pentaquark had been discovered, and the discovery had been verified independently.
The discovery of the pentaquark has started a veritable “gold rush” among particle-physics experimental groups, many of whom are now combing over the data from past measurements, looking for evidence of pentaquark particles among their old results. The prediction of the X=3/2(2070) should be particularly interesting in this onslaught of experimental searches, because it should decay into two negatively charged strange particles, a kind of breakup that no 3-quark baryon could have.
Followup note (11/21/2014): After much analysis and re-analysis of old and new data, the current consensus is that the pentaquark does not exist.
Tables of Particles:
“The 2002 Review of Particle Physics”, K. Hagiwara, et al. (Particle Data Group), Physical Review D 66, 010001 (2002), and at: http://pdg.lbl.gov.
Prediction of the Pentaquark:
“Exotic Anti-Dectuplet of Baryons: Predictions from Chiral Solitons”, D. Diakonov, V. Petrov, and M. Polyakov, Zeitschrift für Physik A359, 305-314 (1997), preprint hep-ph/9703373 .
Observation of the Pentaquark:
“Evidence for Narrow S=+1 Baryon Resonance in Photo-production from Neutrons”, T. Nakano, et. al, (the LEPS Collaboration), Phys.Rev.Lett. 91, 012002 (2003), preprint hep-ex/0301020 .
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