Pentaquarks: HERA-B finds no evidence for the existence of pentaquarks, exotic bound states of four quarks and an antiquark

In an analysis of about 200 million proton-nucleus collisions, the HERA-B collaboration, in which the Ljubljana team is an active member, set stringent upper limits on the probability for possible production of Θ⁺ (read: Theta plus) and Ξ^{- -} (read: Xi minus minus) pentaquarks. Since their recent discovery [1-3], pentaquarks have been a hot topic of particle physics. Half of the 30 most cited publications in 2003 and 2004 are on the subject of exotic bound states of quarks.

QCD, the very successful theory of strong interactions, does not forbid the existence of five-quark bound states. Although such particles have been searched for in the past, they have not been found. All the known strongly interacting particles have up to now been classified either as three quark, or as quark-antiquark combinations. Other possible colour neutral stuctures, such as tetraquarks (two quarks and two antiquarks), pentaquarks (four quarks and one antiquark) or dibarions (six quarks) have therefore been named exotic particles.

This latest excitement in particle physics has been triggered by a personal contact between Russian theorist Dmitri Diakonov and Japanese experimentalist Takashi Nakano. Diakonov persuaded Nakano to analyze his data on high energy photon-nucleus scattering and search for the decay of the predicted pentaquark Θ⁺ into a neutron and a positive kaon. A very narrow resonance was indeed found at the predicted mass of 1.54 GeV/c² [1]. The discovery was followed by an enormous activity in both experimental and theoretical particle physics. Many of the leading world newspapers reported on the discovery. It's importance may be illustrated by the comment of this year's Nobel laureate, Frank Wilczek: "This is really a milestone in physics".

Soon after the discovery of Θ⁺ by the LEPS collaboration [1] another pentaquark particle, the Ξ^{- -}(1862) was found by the NA49 collaboration at CERN [2]. This was followed by the discovery of yet another exotic state, a narrow resonance in the D*^- p mass spectrum at 3.1 GeV/c², found by the H1 Collaboration at DESY [3]. Many old bubble chamber data sets have been reexamined and have confirmed the LEPS result for Θ⁺. The existence of this pentaquark state has therefore been declared as "very likely to certain" by the 2004 Review of Particle Physics.

The HERA-B collaboration also undertook a search for Θ⁺ and Ξ^{- -}. Since a common weak point of many of the above mentioned experiments is a low number of reconstructed events, the large data set of HERA-B could improve the situation and substantially help in further investigation of pentaquarks. The spectrometer, with its superior vertex detector, valuable tracking system and its excellent particle identification, provides good quality information, allowing accurate reconstruction of events. A group of physicists from the Department of Experimental Particle Physics of the Jozef Stefan Institute was involved in the design, the construction and operation of the spectrometer. In particular, they were responsible for one of the essential components of the detector, the ring imaging Cherenkov counter (RICH). They also take part in the analysis of data, with Tomi Zivko as a member of the HERA-B pentaquark search team. This team has thoroughly analysed about 200 million events and found no sign of either Θ⁺ or Ξ^{- -}, as can be seen from the plots below. They have set stringent upper limits on the cross section for production of these pentaquarks in proton-nucleus collisions. The preliminary results have been presented at the 2004 Quark Matter Conference [4], and the contribution has been the most cited among papers reporting on non-observation of pentaquark signals. The final results of HERA-B, which have recently been published in the prestigious journal Physical Review Letters, suggest that if these pentaquark states exist, they must have very peculiar production mechanisms. Or, as Robert Jaffe from the Center of Theoretical Physics at MIT says: "Either the Θ⁺ will go away, or it will force us to rewrite several chapters of the book on QCD".

Figure description: The upper plot corresponds to the search for Θ⁺ in decay channel to a proton and neutral kaon, while the lower plot corresponds to the search for pentaquarks in decay channels of the Ξ^{- -} family. Pentaquarks should appear as high and narrow peaks at positions denoted with arrows in histograms a) and b). See [5] for more details.

[1] T. Nakano et al. : Evidence for narrow S = +1 Baryon Resonance in Photoproduction from Neutron, Phys. Rev. Lett. 91, 012002 (2003), http://arXiv.org/ps/hep-ex/0301020
[2] C. Alt et al. (NA49 Collaboration),: Observation of an Exotic S=-2, Q=-2 Baryon Resonance in Proton-Proton Collisions at the CERN SPS, Phys. Rev. Lett. 92, 042003 (2004), http://arXiv.org/ps/hep-ex/0310014
[3] H1 Collaboration: Evidence for a Narrow Anti-Charmed Baryon State, Phys. Lett. B 588, 17 (2004), http://arXiv.org/ps/hep-ex/0403017
[4] K.T. Knopfle, M. Zavertyaev and T. Zivko for the HERA-B Collaboration: Search for Θ⁺ and Ξ^{- -} pentaquarks in HERA-B, J. Phys. G 30, S1363 (2004), http://arXiv.org/ps/hep-ex/0403020
[5] I. Abt et al. (HERA-B Collaboration): Limits for the central production of Θ⁺ and Ξ^{- -} pentaquarks in 920 GeV pA collisions, Phys. Rev. Lett. 93, 212003 (2004), http://arXiv.org/ps/hep-ex/0408048