We report measurements of hadronic final states produced in $e^+e^-$
annihilations from the BaBar and Belle experiments. In particular, we present
cross sections measured in several different processes, including two-photon
physics, Initial-State Radiation, and exclusive hadron productions at
center-of-mass energies near 10.58 GeV. Results are compared with theoretical
predictions.Comment: Parallel talk at ICHEP08, Philadelphia, USA, July 2008. 4 pages,
  LaTeX, 12 eps figure

Li, Selina Z.

English

arXiv

We report measurements of hadronic final states produced in e{sup +}e{sup -} annihilations from the BABAR and Belle experiments. In particular, we present cross sections measured in several different processes, including two-photon physics, Initial-State Radiation, and exclusive hadron productions at center-of-mass energies near 10.58 GeV. Results are compared with theoretical predictions

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SLAC-PUB-13440BABAR-PROC-08/104arXiv:0810.0796 [hep-ex]October, 2008Light Hadron Physics at the B FactoriesSelina Z. LiStanford Linear Accelerator CenterMenlo Park, CA 94025, USA(from the BABAR Collaboration)AbstractWe report measurements of hadronic ﬁnal states produced in e+e− annihilations from the BABAR and Belle experi-ments. In particular, we present cross sections measured in several diﬀerent processes, including two-photon physics,Initial-State Radiation, and exclusive hadron productions at center-of-mass energies near 10.58 GeV. Results arecompared with theoretical predictions.Contributed to the Proceedings of the 34th International Conference on High Energy Physics,7/29/2008—8/5/2008, Philadelphia, USAStanford Linear Accelerator Center, Stanford University, Stanford, CA 94309Work supported in part by Department of Energy contract DE-AC02-76SF00515.34th International Conference on High Energy Physics, Philadelphia, 20081. INTRODUCTIONThe BABAR and Belle experiments are designed mainly to study CP -violation in B decays produced in e+e−collisions. However, as a by-product, they can also be used to study other hadronic ﬁnal states, some of which will becovered in this proceeding. We will present some of the experimental results obtained from analyses of two-photonphysics, Initial-State Radiation (ISR), and exclusive hadron production at center-of-mass (CM) energies at or near10.58 GeV. Results presented here are based on 95 − 427 fb−1 data samples collected by the BABAR and the Belledetectors, whose detailed descriptions can be found in Ref. [1] and [2], respectively.2. TWO-PHOTON PHYSICS: THE γγ → π0π0 REACTIONTwo-photon reactions resulting from the process e+e− → e+e−γγ can be used to study resonance structure andto test QCD models. Belle analyzed a 95 fb−1 data sample to study the process γγ → π0π0 in a no-tag analysis [3],where the e+e− scatter at very small angles and escape undetected. In the region 0.8 − 1.6 GeV, the productioncross sections (Fig. 1a) as a function of the CM energy (W ) of the π0π0 system are ﬁtted to obtain informationon the S and D waves where two resonant structures f0(980) and f2(1270) contribute. The angular dependenceof the diﬀerential cross sections shows a large-scattering-angle enhancement at low energies (W < 1.7 GeV) and aforward angle peak at high energies (W > 2.4 GeV). The ﬁts suggest that a G-wave becomes necessary at energiesabove 1.9 GeV. Belle recently has analyzed a 223 fb−1 data sample to study the higher energy region [4] where aFigure 1: Belle’s γγ → π0π0: (a) total cross section; (middle) diﬀerential cross section with SD-wave (solid) and SDG-wave(dashed) ﬁts; (c) ﬁts including χc0 with and without interference for χc2.χc0 charmonium is observed with a signiﬁcance of more than seven standard deviations (Fig. 1c). The ratio of crosssections for neutral to charged pion-pair production [5] is almost constant at W > 3.1 GeV and is signiﬁcantly largerthan the prediction of leading order QCD. This ratio is compared with QCD models for hadron pair production listedin Table I.Table I: Comparison of the ratio of cross sections for neutral to charged pion-pair productionLeading Term QCD [6] pQCD [7] “Handbag” Model [8] Belle preliminary resultsσ(π0π0)/σ(π+π−) 0.03− 0.07 0.1 0.5 0.32± 0.03± 0.053. STUDIES OF INITIAL-STATE RADIATION PHYSICS AT Υ (4S) ENERGIESIn an ISR event, either the electron or positron emits an ISR photon, lowering the eﬀective invariant mass of thehadronic system. This allows us to measure the hadronic cross section of e+e− → hadrons from threshold to CMenergy of ∼ 5 GeV. Therefore, ISR physics gives access to hadron production over a continuous, wide energy range ina single experiment. The high integrated luminosity allows BABAR to be competitive with e+e− experiments at lowerCM energy region such as CMD-2 and SND, and to extend the measurements to higher masses. Experimentally, we234th International Conference on High Energy Physics, Philadelphia, 2008require the event to have an energetic ISR photon with energy greater than 3 GeV, and then fully reconstruct thehadronic ﬁnal states. Selecting ISR photon within the ﬁducial region of the detector also forces the recoil hadronicsystem to be within the ﬁducial region. In the next subsections, we will discuss measurements done for ISR physicsbased on 232 fb−1 of BABAR data.3.1. ISR: The K+K−π0, K0sK±π∓, K+K−η Final StatesFrom a Dalitz plot analysis of the KKπ ﬁnal state [9], we can separate the isoscalar and isovector components andmeasure their cross sections (Fig. 2a,b). The isoscalar and isovector channels are dominated by resonances that areconsistent with φ(1680) and ρ(1450). In the K+K−η ﬁnal state [9], the main contribution is from φη whose cross02.557.5101.4 1.6 1.8 2 2.2Ec.m.(GeV)σ0(e+e-→KK*) (nb)(a)012341.4 1.6 1.8 2 2.2Ec.m.(GeV)σ1(e+e-→KK*) (nb)(b)Figure 2: The cross sections for (a) isoscalar and (b) isovector components of KKπ from BABAR data; (c) transition formfactors as a function of energy combined with other measurements to compare with theoretical predictions and the solid bandis the result of a ﬁt [13].section is converted to transition form factors as a function of energy (Fig. 2c). The result from this ISR measurementcombined with that of the exclusive production of e+e− → φη at 10.58 GeV [10] shows a strong preference for anenergy dependence 1/s4 of the cross section as predicted in Ref. [11] rather than the prediction of 1/s3 from Ref. [12].3.2. ISR: The π+π−π0π0 Final StateThe channels with four pions in the ﬁnal state dominate the hadron cross section in the 1 − 2 GeV range wherea precision measurement is needed for an improved determination of the hadronic contribution to the anomalousmagnetic moment of the muon aµ = (g−2)µ/2 and of the running ﬁne structure constant at the Z0-pole α(m2Z). Thereduction of uncertainty in this mode is therefore an important contribution to the overall precision of aµ [14]. Thecross section is consistent with previous results, especially with SND at low energy region (Fig. 3b). This is the ﬁrstFigure 3: BABARs´ π+π−π0π0: (a) cross section; (b) zoom-in at low energy to compare with other experiments; (c) scatteringplot shows the ρ+ρ− peak.measurement above 2.5 GeV. The resonant structure observed here is dominated by the ωπ0 and a1(1260)π channels,along with evidence of ρ+ρ− (Fig. 3c). The latter channel is also seen in exclusive production of e+e− → ρ+ρ− at10.58 GeV as discussed later in Section 4.2. The preliminary precision of 8% (estimated to eventually be ∼ 5% overthe peak region) will help improve the precision uncertainty of the hadronic contribution to aµ.334th International Conference on High Energy Physics, Philadelphia, 20084. EXCLUSIVE HADRON PRODUCTION AT 10.58 GEVExclusive hadron production from e+e− at 10.58 GeV without ISR attracts little attention due to the expectationof low rates, but there can be surprises. With the high integrated luminosity collected at BABAR the ﬁrst observationsof the ﬁnal states e+e− → ρ0ρ0 and φρ0 [15] with C = −1, have established a previous unobserved new source of rarehadronic events through the Two-Virtual-Photon-Annihilation (TVPA) process. This opened a new avenue for thestudy of hadron production mechanism. Many other rare exclusive modes have been observed since then, some fromTVPA process while others through single photon annihilation. In the next two subsections, we will concentrate onBaBar’s new results of ﬁnal states allowed via single-virtual-photon annihilation.4.1. The e+e− → pp¯pp¯ ReactionThe strategy in studying the exclusive hadron production such as e+e− → pp¯pp¯ is to fully reconstruct the hadronicﬁnal states and then to require the invariant mass of the hadronic system to have the nominal CM energy within reso-lution requirements. BABAR has made the ﬁrst observation of a four-proton exclusive ﬁnal state in e+e− annihilationsfrom 427 fb−1 of data (Fig. 4). In the scatter plot Fig. 4b, the pp¯ mass spectrum is peaking near threshold. Baryonproduction in PYTHIA (JETSET) shown in Fig. 4c actually models this quite well which appears to be anothersuccess of the string fragmentation model. We performed a full angular analysis on the signal events and comparethe results to several diﬀerent hadronization models. For these models we have, JETSET is the closest match to thedata, but there are still other features in the data pointing to imperfections of the hadronization models.)2 (GeV/c ppp pm9 9.5 10 10.5 11 11.5 12)2Events / (50 MeV/c020406080100120 (a) BABARPreliminary)2 (GeV/c2p1pm1 2 3 4 5 6 7 8 9)2 (GeV/c4p 3pm1234567890510152025(b) BABARPreliminary)2 (GeV/c2p1pm1 2 3 4 5 6 7 8 9)2 (GeV/c4p 3pm123456789020406080100120140160180200220(c) BABARPreliminaryFigure 4: BABARs´ pp¯pp¯: (a) invariant mass of pp¯pp¯ with vertical lines indicating the mass requirement; (b) data; (c) JETSETsignal Monte Carlo sample.4.2. The e+e− → ρ+ρ− ReactionBABAR has also made the ﬁrst observation of e+e− → ρ+ρ− [16] based on 379 fb−1 of data from which eventswith exactly two well reconstructed, oppositely charged tracks and two π0 candidates are selected. We require theinvariant mass of the four pions to be within the CM beam energy. The mass scatter plot shows strong peaks at theρ mass. A 2D ﬁt yields 329± 25 e+e− → ρ+ρ− events with a total cross section measured to be 19.5± 1.6(stat.)±3.2(sys.) fb. The large, clean sample of signal events allows us to perform an angular analysis to test QCD at theamplitude level. Assuming a one-photon production mechanism, this vector-vector ﬁnal state can be described bythree independent helicity amplitudes, F00, F10, and F11. Fits to the angular distributions with the normalizationconstraint |F00|2 + 4|F10|2 + 2|F11|2 = 1 reveal that |F00|2 = 0.51± 0.14(stat.)± 0.07(syst.) which deviates from theperturbative QCD prediction of one by more than three standard deviations. This signiﬁcant disagreement suggeststhat either the decay is not dominated by single-virtual-photon annihilation as expected, or the QCD prediction doesnot apply to data in this energy region. Because charged ρ’s are involved, ρ+ρ− cannot be produced via two-virtual-photon annihilation unless there is signiﬁcant ﬁnal state interaction. Given the possible relevance to potential similareﬀects in B0 → ρ+ρ− process, which is crucial for the determination of angle α through CP -violation in this mode,the understanding of the observed e+e− → ρ+ρ− decay amplitudes could have broader implications that should bekept in mind.5. CONCLUSIONOver the past nine years of operation, the high luminosity data collected at the B factories have reopened severalinteresting areas for hadron physics. Belle has provided a high statistics measurement of neutral π-pair production in434th International Conference on High Energy Physics, Philadelphia, 2008two-photon physics. BABAR has exploited initial-state radiation to study e+e− annihilations at ECM from thresholdto ∼ 5 GeV, to study the resonant substructure, and to contribute to the hadronic cross section measurementsthat are important in improving our theoretical understanding of aµ and α(MZ). BABAR also has measured thecross sections and studied angular amplitudes of pp¯pp¯ and ρ+ρ− in e+e− annihilations. The amplitude results ine+e− → ρ+ρ− are puzzling. More interesting results from light hadron production are yet to come from the Bfactories.References[1] B. Aubert et al. (BABAR Collaboration), Nucl. Instrum. Methods A479, 1 (2002).[2] A. Abashian et al. (Belle Collaboration), Nucl. Instrum. Methods A479, 117 (2002).[3] S. Uehara et al. (Belle Collaboration), Phys. Rev. D 78, 052004 (2008), arXiv:0805.3387 [hep-ex].[4] I. Adachi et al. (Belle Collaboration), arXiv:0810.0334 [hep-ex].[5] T. Mori et al. (Belle Collaboration), Phys. Rev. D 75, 051101(R) (2007).[6] M. Benayoun and V.L. Chernyak, Nucl. Phys. B329, 285 (1990).[7] S. J. Brodsky and G.P. Lepage, Phys. Rev. D24, 1808 (1981).[8] M. Diehl, P. Kroll and C. Vogt, Phys. Lett. B532, 99 (2002).[9] B. Aubert et al. (BABAR Collaboration), Phys. Rev. D 77, 092002 (2008).[10] B. Aubert et al. (BABAR Collaboration), Phys. Rev. D 74, 111103 (2006).[11] S. J. Brodsky and G.P. Lepage, Phys. Rev. D24, 2848 (1981).[12] J.M. Gerard and G. Lopez Castro, Phys. Lett. B425, 365 (1998).[13] S. Pacetti, Eur. Phys. J. A31, 665 (2007).[14] M. Davier, S. Eidelman, A. Hocker, and Z. Zhang, Eur. Phys. J. C31, 503 (2003).[15] B. Aubert et al. (BABAR Collaboration), PRL 97, 112002 (2006).[16] B. Aubert et al. (BABAR Collaboration), arXiv:0806.3893 [hep-ex], submitted to PRD-RC.5

Light Hadron Physics at the B Factories

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