Pentaquark Quest Quickens Particle Physics

Pentaquark Quest Quickens Particle Physics

by Simon Mitton

WHAT'S HOT IN PHYSICS
Rank	Paper	Citations This Period (Sep-Oct 04)	Rank Last Period (Jul-Aug 04)
1	D.N. Spergel, L. Verde, et al., *"First-year Wilkinson Microwave Anisotropy Probe* (WMAP) observations: Determination of cosmological parameters,"** Astrophys. J. Suppl. Ser., 148(1): 175-94, September 2003. [6 U.S. and Canadian institutions] *715BR	174	1
2	C.L. Bennett, et al., *"First-year Wilkinson Microwave Anisotropy Probe* (WMAP) observations: Preliminary maps and basic results,"** Astrophys. J. Suppl. Ser., 148(1): 1-27, September 2003. [7 U.S. and Canadian institutions] *715BR	84	2
3	T. Nakano, et al., "Evidence for a narrow S = +1 baryon resonance in photoproduction from the neutron," Phys. Rev. Lett., 91(1): 2002, 4 July 2003. [19 institutions worldwide] *696YP	47	4
4	K. Eguchi, et al., "First results from KamLAND: Evidence for reactor antineutrino disappearance," Phys. Rev. Lett., 90(2): 1802, 17 January 2003. [12 institutions worldwide] *636FP	43	3
5	V.V. Barmin, et al. (the DIANA Collaboration), "Observation of a baryon resonance with positive strangeness in K⁺ collisions with Xe nuclei," Phys. Atom. Nuclei., 66(9): 1715-8, September 2003. [Inst. Theor. Exp. Physics, Moscow, Russia; IFN, Italy] *730XH	36	9
6	R. Jaffe, F. Wilczek, "Diquarks and exotic spectroscopy," Phys. Rev. Lett., 91(23): 2003, 5 December 2003. [MIT, Cambridge] *750PG	35	†
7	S. Stepanyan, et al. (the CLAS Collaboration), "Observation of an exotic S = +1 baryon in exclusive photoproduction from the deuteron," Phys. Rev. Lett., 91(25): 2001, 19 December 2003. [37 institutions worldwide] *755UY	31	†
8	S.S. Adler, et al. (the PHENIX Collaboration), "Absence of suppression in particle production at large transverse momentum in root^SNN = 200 Ge V d + Au collisions," Phys. Rev. Lett., 91(7): 2303, 15 August 2003. {49 institutions worldwide] *712UE	26	†
9	S.C. Chapman, et al., "A median redshift of 2.4 for galaxies bright at submillimetre wavelengths," Nature, 422(6933): 695-9, 17 April 2003. [Caltech, Pasadena; Royal Observ., Edinburgh, U.K.; U. Durham, U.K.] *668CG	25	†
10	J. Barth, et al. (the SAPHIR Collaboration), "Evidence for the positive-strangeness pentaquark Q⁺ in photoproduction with the SAPHIR detector at ELSA," Phys. Lett. B, 572(3,4): 127-32, 23 October 2003. [Bonn U., Germany] *734YB	25	†
SOURCE: ISI’s Hot Papers Database. the Legend.

he structure of matter at the very deepest level remains a frontier of physics research, just as exciting and challenging in its own way as the cosmologists’ quest to unwrap the structure of the universe. The Physics Top Ten this period has three papers (#3, #7, #10) claiming sight of pentaquarks, a new class of elementary particle. This is the most exciting discovery in nuclear physics for at least a decade. About a dozen groups of experimenters are working to confirm (or refute) the existence of pentaquark baryons. The particle physics community is divided on the issue, and a lively debate is fueling the citation rates.

Three quarks make a baryon, whereas a quark and its antiquark make a meson. Are other stable configurations of quarks possible? About 30 years ago theorists conjectured that quark matter made of five quarks might exist. Quantum chromodynamics (QCD), the mathematical theory for the strong nuclear force, allows configurations made of four quarks and one anti-quark. Despite many searches no evidence existed for pentaquarks until 2002 when a group known as the LEPS collaboration broke the news of a discovery at the SPring-8 facility in Japan. Takashi Nakano’s discovery paper (#3, see Science Watch, 15[4]: 6, July/August 2004) has ignited a worldwide search.

Why should anyone care to look for five-quark bosons? A major concern for theorists is that in the Standard Model of particle physics their understanding of QCD is very incomplete. Many paradoxes in experimental hadron physics remain unexplained. We lack a detailed picture of the quark-gluon distribution inside the proton. These deficiencies make a compelling case for studying five-quark baryons to gain insight into the quark-gluon interaction.

The reaction studied in the Japanese experiment (#3) involved events created when a source of photons collides with neutrons inside a carbon nucleus. This reaction normally produces kaons, but the group observed a strong resonance at 1.54 GeV that they attribute to the formation of a five-quark state. The particle has strangeness +1, a value that cannot be formed by just three quarks. Confirmation came quickly from a team led by Ken Hicks (Ohio University) and known as the CLAS Collaboration. Their paper (#7) announces evidence from an accelerator at the Thomas Jefferson Lab (Newport News, Virginia). In this case the photons collided with a deuterium target to produce the pentaquarks, now dubbed the Q⁺ state. Their experiment is notable for its strong statistics and the careful elimination of background. A European collaboration (SAPHIR) using an accelerator at Bonn University (Germany) also found the Q⁺ by photoproduction (#10), and this result too has strong statistics.

The three experiments in the Top Ten are representative of about a dozen investigations, employing various beams and targets that have seen the 1.54 GeV resonance. Most of the new searches have come from mining existing data that were taken for other purposes; this is the case for our three Hot Papers. More recently, one collaboration (NA49) has found evidence of an exotic pentaquark, the X-- with a mass of 1.86 GeV. Another collaboration (H1) has used the HERA accelerator in Hamburg, the world’s only colliding-beam facility capable of electron-proton scattering. At 3.2 GeV they find a narrow resonance, which can be interpreted as the Q⁰pentaquark. By grouping the three discoveries it is perhaps possible to see a whole new spectroscopy of charmed pentaquarks emerging. If that is confirmed then it really will open up a new route for exploring the forces that bind quarks together.

There are, however, clouds on the horizon. Eight experiments have produced a null result. These are mainly high-energy reactions, so the explanation could be that pentaquarks are only produced in low-energy reactions. Some physicists are critical of the positive detections, citing subtle experimental artifacts as a possible explanation of the observed signals. These concerns can be confronted by results from second-generation experiments that have already improved the volume of data by an order of magnitude.

Dr. Simon Mitton is the Senior Fellow of
St Edmund’s College, University of Cambridge, U.K.


	View the top 10 scientists and/or top 3 Hot Papers in Physics; for the period of January 1, 1994-October 31, 2004.

Science Watch^®, March/April 2005, Vol. 16, No. 2
Citing URL: http://www.sciencewatch.com/march-april2005/sw_march-april2005_page6.htm