The results of searches for the strange pentaquark states, {Theta}{sub 5}(1540){sup +}, {Xi}{sub 5}(1860){sup --} and {Xi}{sub 5}(1860){sup 0} in data recorded by the BABAR experiment are presented. We search for these three states inclusively in 123.4 fb{sup -1} of e{sup +}e{sup -} annihilation data produced at the PEP-II asymmetric storage rings; we find no evidence for their production in any physics process, and set limits on their production rates that are well below the measured rates for conventional baryons. We also search for {Theta}{sub 5}(1540){sup +} produced in interactions of electrons or hadrons in the material of the inner part of the detector. No evidence for this state is found in a sample with much higher statistics than similar electroproduction experiments that claim a signal

/Ruhr U., Bochum

Gotzen, K.

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SLAC-PUB-11527BABAR-TALK-05/078hep-ex/0510041October 2005Search for Pentaquarks at BABARK. GötzenInstitut für Experimentalphysik I, Ruhr-Universität BochumUniversitätsstr. 150, 44780 Bochum , Germany(for the BABAR Collaboration)AbstractThe results of searches for the strange pentaquark states, Θ5(1540)+, Ξ5(1860)−− and Ξ5(1860)0 indata recorded by the BABAR experiment are presented. We search for these three states inclusivelyin 123.4 fb−1 of e+e− annihilation data produced at the PEP-II asymmetric storage rings; we findno evidence for their production in any physics process, and set limits on their production rates thatare well below the measured rates for conventional baryons. We also search for Θ5(1540)+producedin interactions of electrons or hadrons in the material of the inner part of the detector. No evi-dence for this state is found in a sample with much higher statistics than similar electroproductionexperiments that claim a signal.Presented at QCD 05: High Energy Physics International Conferencein Quantum Chromodynamics, Montpellier, France, 4-9 July 2005.Submitted to Nuclear Physics B (Proceedings Supplement).Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309∗Work supported in part by Department of Energy contract DE-AC02-76SF00515.1 IntroductionSeveral experiments have recently claimed observations [1] of narrow baryonic resonances with ex-otic quantum numbers whose interpretations as quark bound states require a minimum contentof five quarks; these states are denoted as Θ5(1540)+ (uudds̄), Ξ5(1860)−− (ddssū) and its corre-sponding partner Ξ5(1860)0. On the other hand, null results have also been published and havenow outnumbered the positive claims. The interpretation and the comparison of the various exper-imental results is complicated by the problem of comparing different production mechanisms andenergy ranges. Therefore it is of interest to perform high statistics and high resolution searcheswhich encompass different production processes.This has been done by BABAR by measuring upper limits for the production of pentaquarkstates in e+e− annihilations on the one hand and on the other hand performing a study of theelectro- and hadro-prodution of the pentaquark state Θ5(1540)+ in the material of the inner partof the BABAR detector.2 The BABAR DetectorThe BABAR experiment is taking data at the PEP-II e+e− collider at center of mass energy 10.58GeV. The BABAR detector is described in detail elsewhere [2]. Charged particle track parameters aremeasured by a five-layer double-sided silicon vertex tracker and a 40-layer drift chamber located ina 1.5-T magnetic field. Charged particle identification is achieved with an internally reflecting ringimage Cherenkov detector (DIRC), and by making use of specific energy loss (dE/dx) measured inthe tracking devices. Photons and neutral pions are detected with an electromagnetic calorimeterconsisting of 6580 CsI(Tl) crystals. An intrumented flux return provides muon identification anddetection of long-lived neutral hadrons.3 Inclusive search in e+e− annihilationsWe search for the inclusive production of pentaquark states in e+e− → PX reactions with anyfinal state X recoiling against the pentaquark candidate P . The analysis is based on 123.4 fb−1 ofdata recorded at or slightly below the Υ (4S)-resonance [3]. Investigations of several of the possiblestrange antidecuplet members usually considered as pentaquark candidates have been performed[4], and in particular the final results for the states Θ5(1540)+, Ξ5(1860)−− and Ξ5(1860)0 arereported on here.We reconstruct Θ5(1540)+ in the pK0S decay mode, where K0S → π+π−. A sample of K0Scandidates is obtained from all pairs of oppositely-charged tracks identified as pions, that areconsistent with a common vertex, and yield an invariant mass and momentum vector in accordwith K0S production from the e+e− collision axis. The p and p̄ candidates are identified by makinguse of the dE/dx information measured with the tracking system and the Cherenkov angle θCdetermined with DIRC. Fig. 1 shows the distribution of the pK0S invariant-mass. No enhancementis seen in the region of the reported Θ5(1540)+ mass (inset in Fig.1), whereas a clear peak at 2285MeV/c2 with a mass resolution of about 6 MeV/c2 containing 98,000 entries is visible, originatingfrom Λc → pK0S decays. To be independent of any production mechanism model leading to aspecific pK0S momentum spectrum in CM frame (p∗) different from that of the background, wesplit the data into subsamples according to the value of p∗, where each p∗ interval is 500 MeV/cwide. No evidence for a signal is found in any of these subsamples.1.500 1.550 1.6003637383940Candidates / (2 MeV/c2 )       1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0pK0S Mass    (GeV/c2)0.00.51.01.5Candidates / (8 MeV/c2 ) x105x103~~Λ+cFigure 1: The distribution of the pK0S invariant mass for combinations satisfying the criteriadescribed in the text. The same data are plotted for the full kinematically-allowed pK0S mass rangeand, in the inset, with statistical uncertainties and a suppressed zero on the vertical scale, for themass range in which the Θ5(1540)+ has been reported.We quantify this null result for a Θ5(1540)+ mass of 1540 MeV/c2 by fitting a convolution ofa double-Gaussian (HWHM ≈ 2 MeV/c2)and a P -wave Breit-Wigner as the signal lineshape anda background polynomial to the invariant-mass distribution of each subsample to determine thedifferential production cross section dσ/dp∗ from the fitted yields.Since the intrinsic width of the Θ5(1540)+ has not been measured until now, we use Γ = 1MeV(for a narrow Θ5(1540)+) and Γ = 8MeV (best upper limit) and quote results for each assumedwidth. We determine the 95% confidence level upper limit for the number of produced pentaquarksper e+e− → hadrons event and compare it to the known production rates of conventional baryons,assuming B(Θ5(1540)+→ pK0S) = 25%. The extracted numbers 5.0× 10−5/event (Γ = 1MeV) and11 × 10−5/event (Γ = 8MeV) are between 8 and 15 times lower than expected for conventionalbaryons. The corresponding cross section upper limits are 171 fb and 363 fb.We search, as well, for the reported Ξ5(1860)−− and Ξ5(1860)0 states decaying into a Ξ−and a charged pion, where Ξ− → Λ0π− and Λ0 → pπ−. The Λ0 candidates are selected fromall pairs of oppositely-charged tracks satisfying proton and pion identification requirements, andare consistent with a common origin. Fig. 2 shows the invariant-mass distributions for Ξ−π−and for Ξ−π+ combinations. In the Ξ−π+ mass spectrum, we see clear signals for the Ξ(1530)0and Ξc(2470) baryons with 24,000 and 8,000 entries respectively, but no other structure is visible.There are no visible narrow structures in the Ξ−π− mass spectrum; the mass resolution at ≈ 1.86GeV/c2 is 8 MeV/c2. As in the previous analysis we assume two different intrinsic widths of thispentaquark state, namely Γ = 1MeV (narrow) and Γ = 18MeV (resolution of the signal claimed byNA49, considered as upper limit) to determine the 95% confidence upper limit of the productionrate in e+e− interactions. The results 0.74 × 10−5/event (for narrow width) and 1.1 × 10−5/eventare 4-6 times lower than those for conventional baryons. The corresponding cross section upperlimits in this case are 25 fb and 36 fb, respectively.1.800 1.850 1.900 1.950100012001400160018002000Candidates / (2 MeV/c2 )   1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0Ξ−π+ or Ξ−π− Mass    (GeV/c2)051015Candidates / (8 MeV/c2 )Ξ−π+Ξ−π−x103~~Ξ(1530)0ΞcFigure 2: The Ξ−π+ (black) and Ξ−π− (grey) invariant mass distributions for combinations satis-fying the criteria described in the text. The same data are plotted for the full kinematically-allowedΞ−π± range and, in the inset, with statistical uncertainties and a suppressed zero on the verticalscale, for the mass range in which the Ξ5(1860)−− and Ξ5(1860)0 have been reported.4 Search for Θ5(1540)+ in electro- and hadro-productionMost of the positive evidence for exotic pentaquark states has been found in experiments basedon photo-, electro- or hadro-production reactions on nuclear targets. Therefore a second analysisis performed by BABAR which extends the search for Θ5(1540)+ to the interactions of secondaryhadrons (tracks of every type) and beam-halo electrons and positrons in the material of the innerpart of the BABAR detector, leading to inclusive production of the pK0S system.To reconstruct the pK0S candidates, protons are identified by evaluating the specific energy lossinformation, dE/dx, measured with the tracking system. K0S candidates are selected from all pairsof oppositely-charged tracks, which have a maximum distance of closest approach (DOCA) of 3mm, a minimum flight length of 2 mm, and whose kinematic fit resulted in a chi-square probabilityP (χ2) > 0.001. A candidate (K0S , p) vertex is defined as the mid-point of the DOCA (required tobe < 3 mm) of the K0S flight path and the p track. The radius of this vertex with respect to thecollision axis had to be > 2 cm; this limit is inside the beampipe, but well away from the collisionaxis.The candidates are shown to reproduce the detector geometry to a high degree of accuracygiving confidence, that these events are due to interactions in the detector material. However theinclusive pK0S invariant-mass distribution shows no evidence for the Θ5(1540)+.To avoid a possible dilution of a tiny signal by complex nuclear breakup processes involvinga higher number of baryons, pK0S candidates with a baryon like a p, p̄, d or t passing the (K0S ,p) vertex within a DOCA < 3 mm have been rejected. In addition sub-samples with at least oneassociated non baryonic charged track are examined. The requirement of an associated π± or K±results in the observation of signals originating from K∗(892)+ → K0Sπ+, K̄∗(892)− → K0Sπ−,Λ(1115) → pπ−, Λ(1520) → pK− and indications of coupling to a0(980)− and Λ(1405). In eachcase the corresponding pK0S invariant-mass distribution shows no evidence for the Θ5(1540)+.1.450 1.500 1.550 1.600 1.650 1.700pKshort Mass             (GeV/c2)0102030405060Candidates / (8 MeV/c2 )      HERMESBaBarNormalization regione±De−Becandidates1171227174ECM10.6 GeV9.4Q2~0~0Figure 3: The HERMES pK0S mass distribution [5] compared to the corresponding BABAR distribu-tion for electroproduction in Be normalized to the HERMES data for the region above 1.58GeV/c2.The study was then restricted solely to those regions which can be interpreted as correspondingto electro-production in the beampipe, which consists mostly of Be. Again no signal is seen. Sincethere is no quantitative information on the flux of beam-halo electrons and positrons, it is not possi-ble to derive upper limits for the production cross section or production rates as was done in section3. Therefore the BABAR electro-production results are compared to those of the HERMES [5] andZEUS [6] experiments which study e+d and e±p reactions and thus have comparable experimentalsituations. Fig. 3 shows the superposition of the invariant-mass distributions measured by BABARand HERMES. The comparison seems to indicate a significant loss of acceptance for the HERMESexperiment in the pK0S mass region below ≈ 1.52GeV/c2 resulting in a possible overestimation ofthe significance of the Θ5(1540)+ signal. Compared to the mass distribution measured by the ZEUSexperiment in Fig. 4, the BABAR spectrum shows no evidence for the poorly established baryoncandidate Σ(1480) [7] and Θ5(1540)+ signal of the ZEUS analysis. This creates serious reservationsabout the significance of the Θ5(1540)+ observation claimed by this experiment.5 ConclusionsIn summary, we have performed a search for the reported pentaquark states Θ5(1540)+, Ξ5(1860)−−and Ξ5(1860)0 in e+e− annihilations. We find large signals for known baryon states but no excessat the measured mass values for the pentaquark states. The measured upper limits for theirproduction rates are 4-16 times lower than those expected for conventional baryons. These resultsdo not directly disprove the reported positive evidence due to the unknown production mechanismof possible pentaquark states, but are nevertheless highly suggestive.In addition we studied the production of Θ5(1540)+ in electro- and hadro-production eventswithin the inner part of the BABAR detector. Again no signal was observed. Furthermore, thecomparison of the BABAR results on electro-production in Be to those from the HERMES (e+d)1.450 1.500 1.550 1.600 1.650 1.700pKshort Mass             (GeV/c2)0100200300Candidates / (5 MeV/c2 )      ZEUSBaBarNormalizationregione±pe−Becandidates14622227174ECM300 GeV9.4Q2>20 GeV2~0Figure 4: The ZEUS pK0S mass distribution [6] compared to the corresponding BABAR distributionfor electroproduction in Be normalized to the ZEUS data for the region below 1.48GeV/c2.and ZEUS (e±p) experiments leads to the conclusion that the prior claims for the observation ofΘ5(1540)+ in electro-production are less than convincing.References[1] For a recent experimental review see K. Hicks, hep-ex/0504027[2] B. Aubert et al., BABAR Collaboration, Nucl. Instr. Meth. A 479, 1-116 (2002)[3] B. Aubert et al., BABAR Collaboration, Phys. Rev. Lett. 95, 042002 (2005)[4] B. Aubert et al., BABAR Collaboration, hep-ex/0408064 (2004)[5] A. Airapetian et al., HERMES Collaboration, Phys. Lett. B 585, 213 (2004)[6] S. Chekanov et al., ZEUS Collaboration, Phys. Lett. B 591, 7 (2004)[7] S. Eidelman et al., Particle Data Group, Phys. Lett. B 592, 1 (2004)

Search for Pentaquarks at BABAR

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