We present recent measurements of hadronic cross-sections from the BaBar
experiment and report preliminary results on searches for pentaquark states.Comment: 4 pages, 9 postscript figues, contributed to the Proceedings of
  DAFNE0

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Berger, Nicolas

English

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We present recent measurements of hadronic cross-sections from the BaBar experiment and report preliminary results on searches for pentaquark states. Studies of hadronic cross-sections using initial-state radiation offer promising prospects at BaBar. Many exclusive channels have already been measured, and more are in progress. A fully inclusive analysis should also offer a precise measurement of {Delta}{alpha}{sub QED}{sup Had}. Searches for pentaquark states have so far been negative, but they have served to highlight the potential for the study of charmed and non-charmed baryons at high-luminosity e{sup +}e{sup -} colliders

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Work supported in part by Department of Energy contract DE-AC02-76SF00515Frascati Physics Series Vol. VVVVVV (xxxx), pp. 000-000DAΦNE 2004: Physics at meson factories – Frascati, June. 7-11 , 2004Selected Contribution in Plenary SessionINCLUSIVE HADRONIC RESULTS AT BaBar :ISR AND PENTAQUARK SEARCHESNicolas Berger ∗SLAC, Menlo Park, California, U.S.A.ABSTRACTWe present recent measurements of hadronic cross-sections from the BaBarexperiment and report preliminary results on searches for pentaquark states.1 Inclusive hadronic cross-section measurements using Initial StateRadiation1.1 Physics MotivationThe total cross-section σ(e+e− → hadrons) for the production of hadrons ine+e− annihilation is a crucial ingredient for the calculation of hadronic cor-rections for the running of the QED coupling constant ∆αHadQED and for themuon anomalous magnetic moment aHadµ . The hadronic contribution to the∗ On behalf of the BaBar collaborationSLAC-PUB-11414August 2005running of αQED, ∆αHadQED is an input into the global standard model fits1)which can provide an indirect measurement of the Higgs boson mass. In bothcases, the hadronic contributions can be expressed as integrals of the ratioR(s) = σ(e+e− → hadrons)/σ0(e+e− → µ+µ−), where σ0 denotes the Borncross-section. We have∆αHadQED = −α3π∫ ∞4m2πR(s)s(m2Zs−m2Z)(1)aHadµ =(αmµ3π)2 ∫ ∞4m2πR(s)s(K(s)s), (2)where K(s) is sharply peaked at s = 0. In the case of ∆αHadQED, theweight factor is almost independent of s for small values of s, so that the entirespectrum of R(s) contributes to the integral. In the case of aµ the integral isdominated by the low s region.The error on ∆αHadQED is dominated by the region 1 GeV <√s < 7 GeV.Below 1 GeV, CMD-2 and KLOE have measured 2) σ(e+e− → π+π−) to < 1%accuracy. BES 3) has measured R(s) in the range 2 GeV <√s < 5 GeV at 6%accuracy, but there are no recent measurements in the region 1 GeV <√s <2 GeV, leading to large uncertainties.1.2 Initial-state Radiation at Υ(4S) EnergiesThe BaBar experiment operates at the PEP-II asymmetric e+e− collider. WhilePEP-II is a fixed-energy machine, initial-state radiation (ISR), can be used tovary of the center-of-mass energy of hadron production. The full spectrum ofs′, the reduced center-of-mass energy, is accessible. The range 0 < s′ < 7 GeVcan be reached for ISR photon energies of 3−5.3 GeV in the center-of-mass sys-tem. The photon can be detected by the BaBar electro-magnetic calorimeter(EMC) to provide a clear signature for the event. In particular, the presence ofa hard photon can separate e+e− annihilation events from beam-gas processeswhich constitutes an important source of background for energy-scan exper-iments. The hadronic system is also collimated by its recoil against a hardphoton and the spectrum of the observed particles is also hardened, improvingdetection efficiency and reducing the dependence on the hadronization model.Requiring the ISR photon in the sensitive part of the detector further improvesthe fiducial containment of the hadronic system. Final-state radiation (FSR)effects are expected to be small and kinematically well-separated from ISR.The cross-section for hadronic ISR events was evaluated using the Monte-Carlo generators. The total cross-section for s′ < 8 GeV in the fiducial region15.3 < θγ < 137.3o is calculated to be 90 pb, corresponding to 18 million eventsin the current BaBar dataset of 200 fb−1. Of these we expect 5.7 million eventsfor 2 < s′ < 5 GeV, to be compared with approximately 250,000 events usedfor the latest BES measurement in this energy range.The main challenge of the method is the determination of the reducedcenter-of-mass energy√s′. This is addressed differently in the various analyses.1.3 e+e− → h+h−h+h−γBaBar performed a common analysis of the processes e+e− → π+π−π+π−γ,e+e− → K+K−π+π−γ and e+e− → K+K−K+K−γ. Events with at least4 tracks and a neutral cluster are subjected to 1C kinematic fits with theconstraint mγ = 0. A kaon identification procedure is performed on the tracks,using ionization measurements in the tracking detectors and information fromthe C̆erenkov detector. The cross-sections are normalized using the processe+e− → µ+µ−γ. Results are shown in fig. 1.The 4π and 2K2π results agree with existing results, but are consid-erably more precise and cover a larger energy range. The 4K result is thefirst measurement of this quantity. In all cases, the leading uncertaintiesare systematic, dominated by uncertainties on the luminosity, tracking ef-ficiency and acceptance losses. The J/ψ resonance is clearly visible in all3 cases, leading to branching fraction results of B(J/ψ → π+π−π+π−) =(3.70±0.27±0.36)×10−3, B(J/ψ → K+K−π+π−) = (6.25±0.50±0.62)×10−3,B(J/ψ → K+K−K+K−) = (6.9 ± 1.2 ± 1.1) × 10−3, assuming the PDGvalue for Γ(J/ψ → e+e−). These results agree with the PDG values butare significantly more precise. The 4π mode also provides a measurement ofB(ψ(2S) → J/ψ(µ+µ−)π+π− through the mis-identification of the muons aspions. Assuming the PDG values for Γ(J/ψ → µ+µ−), Γ(ψ(2S) → e+e−) andB(J/ψ → µ+µ−), we get B(ψ(2S) → J/ψπ+π−) = 36.1 ± 1.5 ± 3.7%.0102030401000 1500 2000NDOLYACMDSNDM3NDM1GG2DM2CMD2BaBarEC.M. (MeV)σ(e+e- → 2π+2π- ) (nb)02461 2 3 4- DM1- BaBar ISREC.M. (GeV)σ(K+K- π+π-) (nb)00.050.10.150.22 3 4EC.M. (GeV)σ(K+K- K+K- ) (nb)Figure 1: Cross sections for e+e− → π+π−π+π− (top), K+K−π+π− (center)and K+K−K+K− (bottom) using 89 fb−1 of data.1.4 e+e− → J/ψ(µ+µ−)γThe analysis of e+e− → J/ψ(µ+µ−)γ is done in similar fashion to that of thepreceding section. We require energy and momentum conservation and performa 1C kinematic fit with mγ = 0. To reject ISR background, both tracks arerequired to be identified as muons.The cross-section for J/ψ production is obtained from the ratio of peakto continuum production. Assuming PDG values for Bµµ and B(J/ψ → e+e−),we obtain Γ(J/ψ → e+e−) = 5.61 ± 0.20 keV and the full width of the J/ψ tobe ΓJ/ψ = 94.7 ± 4.4 keV.1.5 Inclusive AnalysisAlongside the exclusive analyses presented above, a fully inclusive analysis ofhadronic ISR processes is being performed, with the goal of extracting ∆αHadQEDwith 3 − 4% error. We select events with an ISR photon with center-of-massenergy greater than 3 GeV. The various efficiency terms can all be calibratedto 1% or below; we have a triggering efficiency of 98% and a fiducial photondetection efficiency of 90%. The s′ integrated luminosity spectrum can becomputed from the BaBar integrated luminosity, which is known to about 1%.The precision of this calculation is claimed to be less than 1%.Leading background sources, such as radiative Bhabha, e+e− → γγ andvirtual Compton scattering processes can be vetoed with minimal signal lossesand biases. Other modes such as e+e− → µ+µ−γ and τ+τ−γ can be subtractedusing theoretical predictions. Finally, e+e− → qq̄ events are a major source ofbackground for s′ > 5 GeV, mainly due to production of high-momentum π0and η. Event shape variables of the hadronic system can be used to suppressthis background.For this inclusive measurement, s′ is determined from the ISR photonenergy. Due to the EMC energy resolution of about 3% for the energies consid-ered here, the R(s)/s spectrum is distorted, especially at low s′. However, since∆αHadQED is expressed as an integral in R(s)/s with a weakly-varying weight fac-tor, distortions in the spectrum do not affect the measured value for ∆αHadQED.The energy resolution therefore has minimal impact on the ∆αHadQED measure-ment. The inclusive method cannot be applied to the measurement for aHadµsince the weight factor in this case is strongly peaked at s′ = 0.2 Searches for Pentaquark ResonancesSeveral experiments have recently claimed observations of exotic baryon res-onances which seem to be composed of 5 constituent quarks. The LEPS 4)experiment has claimed observation of a resonance Θ+ at a mass of about1540 MeV. The NA49 experiment 5) reports two degenerate states, Ξ05 andΞ−−5 with masses of 1862 MeV. These resonances have been interpreted asmembers of a 1̄0 + 8 multiplet of flavor SU(3), with the isospin-singlet Θ+associated with states denoted as N5, Σ5 and Xi5 in analogy with the usualbaryon multiplets.BaBar is well suited to search for these states, with excellent kaon andproton identification and excellent tracking resulting in good mass resolutions.Searches for the Θ+, Ξ05, Ξ−5 and Ξ−−5 states have been performed.A search for Θ+ was done for the decay mode Θ+ → pK0S. We expect aresolution of about 2 MeV on the Θ+ mass, which would be the most preciseto date. However as shown in fig. 2, no peak is seen at the expected mass andonly a large signal for ΛC → pK0S is observed.A search for the Ξ05 and Ξ−−5 resonances was performed using the decaychain Ξ0/−−5 → Ξ−π±, Ξ− → Λπ−, Λ → pπ−, with the proton identified asbefore. As shown in fig. 2, no peak is seen at the expected masses. In theΞ+π− spectrum, prominent peaks for the Ξ∗(1530) and Ξ0c(2250) are seen. Nostructure is observed in the exotic Ξ−π− spectrum.Searches for Ξ05 → ΛK0S, N05 → ΛK0S, N+5 → ΛK+ and Ξ−5 → ΛK−, werealso performed, using kaon identification and reconstructing Λ → pπ− andK0S → π+π− as above. As shown in fig. 2 no exotic resonances were observed,while sharp peaks for Ω− → ΛK−, Λ+c → ΛK+ and Ξ0c → ΛK0S are clearlyseen.3 ConclusionStudies of hadronic cross-sections using initial-state radiation offer promisingprospects at BaBar. Many exclusive channels have already been measured, andmore are in progress. A fully inclusive analysis should also offer a precise mea-surement of ∆αHadQED. Searches for pentaquark states have so far been negative,but they have served to highlight the potential for the study of charmed andnon-charmed baryons at high-luminosity e+e− colliders.1.5 2 2.5 3 3.5 4 4.5 5]-2Events/[2 MeVc0200040006000800010000120001400016000180002000022000 Λ+c(pK0S) Inv. Mass [GeV/c2]1 1.9 2.8 3.7 4.6 5.5Entries / 1 MeV090180270360450 Expected Ξ−−5(Ξ−π−) Inv. Mass [GeV/c2]1 1.9 2.8 3.7 4.6 5.5Entries / 1 MeV0360720108014401800Ξ∗(1530)Expected Ξ05Ξ0c(Ξ−π+) Inv. Mass [GeV/c2]1.6 1.8 2 2.2 2.4 2.6 2.8 3Entries / 2 MeV0100020003000400050006000700080009000+5NΛ+c feed-downΛ+c(ΛK+) Inv. Mass [GeV/c2]1.6 1.8 2 2.2 2.4 2.6 2.8 3Entries / 2 MeV0100020003000400050006000  Ω−Expected Ξ−5(ΛK−) Inv. Mass [GeV/c2]1.6 1.8 2 2.2 2.4 2.6 2.8 3Entries / 2 MeV02004006008001000120014001600180005N Expected Ξ05Ξ0c(ΛK0S) Inv. Mass [GeV/c2]Figure 2: Mass spectra for pK0S and Ξ−π− (top row), Ξ+π− and ΛK+ (mid-dle row) and ΛK− and ΛK0S (bottom row) using 123 fb−1 of data. For thethree latter plots, the upper and lower histograms correspond to ΛK center-of-mass momenta respectively smaller than and greater than 3 GeV, with thelower histogram scaled up by a factor of 10 for lisibility. The positions of knowresonances and expected Ξ5 and N5 pentaquarks are shown.References1. LEP Collaborations, CERN-EP/2003-091 and hep-ex/03120232. S.E. Muller et al., Nucl.Phys.Proc.Suppl. 126, 335-340 (2004)R.R. Akhmetshin et al., Phys. Lett. B527, 161-172 (2002)R.R. Akhmetshin et al., Phys. Lett. B578, 285-289 (2004)3. J. Z. Bai et al., Phys. Rev. Lett. 84, 594 (2000)J. Z. Bai et al., Phys. Rev. Lett. 88, 101802 (2002)4. T. Nakano et al., Phys. Rev. Lett. 91, 012002 (2003)5. C. Alt et al., Phys. Rev. Lett. 92, 042003 (2004)

Inclusive Hadronic Results at BaBar: ISR and Pentaquark Searches

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