After a first inconclusive sighting, the search for exotic particles that consist of five quarks has been hotly pursued in the past few years. But the weight of evidence is now shifting against their existence.
Correct perceptions differ from mistaken ones in that they become clearer when experimental accuracy is improved — Irving Langmuir's observation may have gained a new example with the latest report on the question of the ‘pentaquark’ particle. A high-statistics experiment at the Jefferson Laboratory in Virginia finds no evidence to support claims of the existence of this enigmatic object that have been made over the past three years. Final tests of the data remain to be completed, and a second independent experiment is still in progress. But this is a serious setback for the many who had hoped that this novel particle had revealed unexpected phenomena in quantum chromodynamics (QCD), the theory governing the ‘strong’ interactions of quarks — subatomic particles thought to be elemental and indivisible.
The story began in 1997 with the prediction1 that an analogue of the proton should exist with a mass of “about 1,530 MeV” (some 50% more massive than the proton) and with both positive electric charge and a positive value for another fundamental quantum number, strangeness.
Such a correlation was not possible within the simplest quark model, where most strongly interacting particles (known as hadrons) are either mesons, which contain a quark and an antiquark, or baryons, which comprise three quarks. To make such a correlation of charge and strangeness requires a particle consisting of four quarks and one antiquark (hence dubbed a ‘pentaquark’). Such combinations are allowed by QCD but are expected to be highly unstable, with ‘widths’ of many hundreds of MeV. (An inherent property of quantum mechanics is that short lifetimes are correlated with large uncertainties in energy. Thus the mass, or energy at rest, of a short-lived particle is actually a distribution with an intrinsic width — for strongly interacting unstable particles, such widths typically exceed 100 MeV.) No pentaquark had ever been seen with certainty, and their absence had been one of the planks upon which the standard quark model had been developed.
A surprising feature of the 1997 prediction was that the particle would have a width of the order of a few MeV and not the hundreds that might have been expected. Initially, the paper1 received little attention, but the LEPS collaboration at the SPring-8 laboratory in Japan was encouraged to mount an experiment to look for the particle. The first pentaquark sighting was announced by them2 in early 2003, exactly at the predicted mass and with a narrow width.
The experiment involved photon beams interacting with protons or neutrons in a carbon nucleus. There was some surprise that the first sighting of a particle with such a narrow width should have occurred in such a complex environment: the nuclear constituents are bound and have kinetic energy, which tends to smear any signals. However, this stimulated experimentalists elsewhere to look again at their data from earlier experiments to see if these contained evidence for the pentaquark.
Within a few months, teams from the Jefferson Lab, from Russia and from the SAPHIR collaboration at the Electron Stretcher Accelerator (ELSA) in Bonn, Germany, all announced that they, too, had spotted tantalizing hints of the particle in data taken in other experiments. For instance, the SAPHIR team's evidence of the pentaquark3 came from data they had obtained in 1997–98 and confirmed its mass of 1,540 MeV. None of these experiments on their own was very significant, but the broad agreement among them created huge excitement.
By the end of 2003, more than ten experiments worldwide had reported evidence for the pentaquark (see ref. 4 and references therein), mostly produced by photons interacting with protons or neutrons (‘photoproduction’). The pentaquark particle then decayed into a K meson and a proton or neutron. The targets included protons, and deuterium and heavier nuclei; the kinematics covered both low and high energy; and the narrow peak invariably occurred around 1,540 MeV. Hints of sibling pentaquarks also emerged, for example one with a positive value of another quantum number, charm, rather than strangeness. Well over 1,000 theoretical papers have addressed such phenomena.
In 2004 a series of theoretical criticisms emerged, centred around some anomalies. On closer inspection there seemed to be small but systematic differences in the mass, and in the width of the signals4; also, it was unclear how such a narrow width state was apparently produced so readily. Moreover, reports of negative experimental searches began to appear. The null results tended to come from experiments using nuclei, or hadrons such as π mesons or protons, rather than photons, and also included searches involving very-high-energy electron beams. Unlike some of the supposedly positive sightings, the common feature of the null results was that they tended to have rather large statistical samples.
One suggestion was that the pentaquark might have some unusual production mechanism, such that photoproduction at energies of a few GeV is especially favoured. This loophole could be closed by dedicated photoproduction experiments with high statistics, and two such experiments had already been designed at the Jefferson Lab.
The latest news from researchers in the Jefferson Lab's Large Acceptance Spectrometer (CLAS) collaboration, announced at the American Physical Society spring meeting on 16 April, adds to the concern about the reality of the pentaquark. The researchers have taken data from an experiment in which a photon beam hit a liquid-hydrogen target (Fig. 1), under conditions similar to those of the earlier experiment conducted by the SAPHIR collaboration. The CLAS team's data contain statistics that are improved by two orders of magnitude, and find no evidence of a pentaquark with mass 1,540 MeV.
The CLAS collaboration data show, at a level of precision at least 50 times higher than the published SAPHIR result, that this particular reaction produces no pentaquark. Researchers at the Jefferson Lab are currently undertaking dedicated hunts for the pentaquark, including an experiment that repeats their original pentaquark search with much higher statistics. Those data are being analysed, and the results are expected later this year. If they show a null result, the pentaquark story will probably have come to an end for physicists but will live on as a case-history for historians and philosophers of science.
About this article
Physical Review D (2018)
Physical Review D (2017)
Θ + baryon, N* (1685) resonance, and πN sigma term reexamined within the framework of a chiral soliton model
Progress of Theoretical and Experimental Physics (2013)
The European Physical Journal H (2012)
JETP Letters (2008)