The experimental search for the Θ+ in the γd → Θ+Λ reaction withCLAS

P. Rossi a

for the Jefferson Lab CLAS Collaboration

aLaboratori Nazionali di Frascati - INFN,Via Enrico Fermi 40, 00044 Frascati, Italy

Since the LEPS collaboration reported the first evidence for a S=+1 baryon resonancein early 2003, dubbed Θ+, twelve other experimental groups have confirmed this exoticstate. At the same time, there are a number of experiments, mostly at high energies,that report null results. To find a definite answer to the question of the existence or non-existence of the Θ+ and of the other 5-quark baryons and, if they do exist, to determinetheir intrinsic properties, second generation experimental programs have been undertakenin different laboratories, among these at the Jefferson Laboratory (JLab). Here the statusof the analysis of the γd → Θ+Λ reaction measured with the CLAS detector in the Hall Bof JLab is reported.

1. PHYSICS MOTIVATION

Although all the well-established particles can be categorized using the constituentquark model which describes light mesons as bound states of qq̄ pairs and baryons as bound3-quark states, the idea of exotics, i.e. particles with more complex quark configurations,has been proposed since the early 70’s. The lack of clear evidence for them did notstop the theoretical work on this subject and in 1997 Diakonov, Petrov and Polyakov, inthe framework of the Chiral Quark Soliton Model, predicted an antidecuplet of 5-quarkbaryons with Jπ = 1/2+ [1]. The lowest mass member is an isosinglet state, the Θ+, withthe quark configuration (uudds̄) giving S=+1, mass ∼ 1.54 GeV and width ∼ 15 MeV.Experimental evidence for a S=+1 baryon resonance has been reported for the first time bythe LEPS Collaboration [2]. Immediately after, several other groups analyzing previouslyobtained data, have found this exotic baryon in both his decaying channels pK0 and nK+

[3]. The reported masses in some cases vary by more than the uncertainties given forthe individual experiments, ranging from 1522 to 1555 MeV, with the masses obtainedfrom processes involving nK+ signals in the initial or final states giving on average 10-15MeV higher values than those in the pK0 channel. The observed widths are found inthe interval ∼ (10 − 30) MeV, but in all cases they are dominated by the experimentalresolution. The relatively small statistical significance of every measurement, possiblyexplained by the fact that all the results come from the analysis of data taken for otherpurposes, and the discrepancy in mass determination, are not the only problems to faceto overcome the reticence to accept the existence of the pentaquark. In fact, a major

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problem is that several experiments at high statistics and high energies reported negativeresults in searches for the Θ+ [4]. To find a definite answer to these questions secondgeneration experimental programs have been undertaken in different laboratories, amongthese at Jefferson Lab.

2. EXPERIMENTAL PROGRAM IN HALL B AT JLAB:PHOTOPRODUCTION OF THE Θ+ ON A DEUTERIUM TARGET

The superconducting electron accelerator at Jefferson Lab delivers a 100% duty-cycleelectron beam to three different experimental Halls A, B and C, simultaneously. Themaximum energy is 5.8 GeV and the maximum current is 200 µA. Hall B is equippedwith a tagged photon facility [5] and the CLAS detector [6]. The tagger system is ableto tag photons in the energy range (0.20 − 0.95)E0 with an energy resolution of 0.1E0 %.The CLAS spectrometer is built around six superconducting coils producing a toroidalmagnetic field. The coils naturally separate the detector into six sectors each instrumentedwith 3 sets of multi-wire drift chambers for track reconstruction, one layer of scintilla-tor counters for time-of-flight measurements, Cherenkov counters and electromagneticcalorimeters for particle identification. During the year 2004, new dedicated experimentshave been performed in Hall B with the aim of improving the statistical accuracy, byat least one order of magnitude, of the two published positive results on the pentaquarkobtained from existing CLAS data. The experiment using a deuterium target ran duringthe spring 2004. It used a 24 cm length target and tagged photons in the energy range(0.8 - 3.59) GeV. An integrated luminosity of 50 pb−1 was achieved. This value is a factor20 greater than the luminosity available for the published data. By the end of 2004 thefinal data processing will be finished and the files will be ready for physics analysis. Thereaction channels which are the object of the analysis are reported in Table 1.

Table 1Reaction channels under study.

REACTION CHANNEL Θ+ DECAY MODE DETECTED FINAL STATEγd → pK+K−n nK+ pK+K−

γd → ΛK+n nK+ pπ−K+

γd → ΛK0p pK0s ppπ−π−π+ (or only 4, 3, 2 particles)

γd → ppK0s K

− pK0s pπ+π−K−

γ“n′′ → K+K−n nK+ K+K−

2.1. The γd → Θ+Λ Reaction ChannelThe γd → Θ+Λ reaction channel has very interesting features: i) the strangeness

content of the final state is well defined thanks to the presence of the Λ particle havingstrangeness S = −1; ii) the presence of only one K+ and no K− in the final state allowsto identify it without the need of cutting on competing channels (φ, Λ(1520)); iii) for thesame reason kinematical reflections in the NK invariant mass spectrum are excluded. Themain reaction mechanism can be pictured as the two-step process of Fig. 1.

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Figure 1. Production mechanism for the ΛΘ+ in deuterium.

The following decay channels are detected in CLAS:i) Λ → pπ−, ii) Θ+ → pK0 and Θ+ → nK+, iii) K0 → π+π−.

The final state therefore is K+npπ− for the nK+ decay mode and π+π−ppπ− for thepK0 decay mode.

2.1.1. K+npπ− final stateTo analyze this channel, the p,K and π are identified using momentum and time of

flight information and after removing the background under the kaon mass peak due top or π incorrectly identified as K. Exclusive K+nπ−p events are selected computing themissing mass of the K+nπ−X system and choosing the events around the neutron peak.In Fig. 2 we show the resulting missing mass spectrum from a preliminary analysis of thedata. The peak of the missing neutron is clearly visible at the mass of 930.8 MeV witha σ = 11.3 MeV. After that, the invariant mass of the pπ− system is computed and theΛ events are chosen within ±3σ around the Λ mass peak (1116 MeV). This spectrum isshown in Fig. 3. Finally the Θ+ signal will be looked for in the K + n invariant mass ofthe K + nΛ events.

2.1.2. π+π−ppπ− final stateIn this case, having many particles in the final state, many different topologies are

possible according to the number of detected particles. The cases taken into account forthis analysis are summarized in Table 2. After selecting the K0 and Λ from all the possiblecombinations of all topologies, the search of the Θ+ signal is made in the K0p invariantmass of the ΛK0p system. The policy of the CLAS Collaboration is that the Θ+ resultswill be presented only when final. So no preliminary results have been presented in thistalk. Final results of the high-statistics runs are expected soon.

3. CONCLUSION

To give a definite answer to the question of the existence of the pentaquarks, a broadexperimental program has been undertaken by the CLAS Collaboration at JLab. Highstatistics searches for exotic baryons on hydrogen and deuterium targets and in variousfinal states have been started in March 2004. One of the reaction channels under studyis γd → Θ+Λ. Here the analysis guidelines and the status of the analysis have beenreported.

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Table 2Topologies analyzed for the π+π−ppπ− final state.

N. OF DETECTED PARTICLES FINAL STATE MISSING PARTICLE5 π+π−ppπ−X MX = 04 π+π−ppX MX = π−

4 π−π−ppX MX = π+

4 π+π−π−pX MX = p3 π−ppX MX = K0

3 π+π−pX MX = Λ

Figure 2. Missing mass spectrum forthe γd → K+pπ−X reaction.

Figure 3. Invariant mass spectrum ofthe pπ− system.

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