We present some of the most recent results from the analysis of the full dataset collected by the BaBar experiment. They include searches for rare decays in the charm sector, the first evidence for the hb(1S) bottomomium state and results from the analysis of the data collected above the Υ(4S) mass

Manoni, E.

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DOI 10.1393/ncc/i2011-11040-7Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Recent results from the BaBar experimentE. ManoniINFN, Sezione di Perugia - Perugia, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 4 Novembre 2011)Summary. — We present some of the most recent results from the analysis ofthe full dataset collected by the BaBar experiment. They include searches for raredecays in the charm sector, the first evidence for the hb(1S) bottomomium stateand results from the analysis of the data collected above the Υ(4S) mass.PACS 13.25.Ft – Decays of charmed mesons.PACS 13.20.Fc – Decays of charmed mesons.PACS 13.20.Gd – Decays of J/Ψ, Υ, and other quarkonia.PACS 13.20.He – Decays of bottom mesons.1. – BaBar datasetThe BaBar experiment has run at the PEP-II b factory from 1999 to 2008. Most ofthe data (430 fb−1) have been collected at a center-of-mass (CM) energy correspondingto the Υ(4S) resonance mass. This sample allows to study B physics but also charm andtau properties, given the high cross section for the cc¯ and τ+τ− production from the e+e−initial state. Other CM energy regions have been investigated, resulting in 30.2 fb−1 and14.5 fb−1 collected at the Υ(3S) and Υ(2S) mass, respectively. Those events, other thanincreasing the statistics of the cc¯ and τ+τ− samples, have been used for charmonium andbottomomium spettroscopy studies. Moreover, an energy scan above the Υ(4S) mass,has provided events in which also Bs mesons are produced.2. – Charm physics2.1. Search for D0 → γγ. – The D0 → γγ decay is mediated by a flavour changingneutral current which cannot happen through a tree level diagram in the Standard Model(SM). Moreover it is subject to an additional suppression due to the GIM mechanism.In the SM, short- and long-distance contributions sum up to give a branching fraction(B) of the order of 10−8 [1], where the long-distance terms are dominant. In the MinimalSupersymmetric Standard Model the B can be enhanced by up to a factor 200 withrespect to the SM prediction, thanks to gluino exchange [2]. From the experimental pointof view, BaBar has performed the analysis using 407.5 fb−1 [3]. Events with reconstructedc© Societa` Italiana di Fisica 28RECENT RESULTS FROM THE BABAR EXPERIMENT 29D∗ → D0π+ decays are selected and the D0 → γγ and D0 → KSπ0 final states aresearched for. The second is used as normalization channel in order to reduce systematiceffects in common with the signal channel. The signal yield is extracted from a fit tothe γγ invariant-mass distribution and three contributions are accounted for: correctlyreconstructed signal events, combinatorial γγ pairs surviving the selection criteria, andD0 → π0π0 final states in which part of the neutral energy is lost. The resulting fittedsignal yield is −6 ± 15 and is used to set an upper limit on the process under study.The main systematic uncertainties affecting the branching fraction upper limit are dueto differencies between Monte Carlo simulation and data on the modelling of the photonreconstruction efficiency and on the distribution of the selection variables. Consideringstatistical and systematic uncertainties, the resulting upper limit on B(D0 → γγ) is2.4 × 10−6, a factor of 10 improvement with respect to the pervious measurement [4].Moreover, given that the D0 → π0π0 is the main background contribution to the γγ finalstate, its B has also been measured: B(D0 → π0π0) = (8.4 ± 0.5) × 10−4 with a 40%improvement on the precision with respect to previous analyses [4].2.2. CP violation measurement in D+ → KSπ+. – Violation of the CP symmetryin the charm sector is expected to be small in the SM but can be enhanced in severalnew physics models. In the D+ → KSπ+ decay, the SM expected CP asymmetryis: ACP = Γ(D+→KSπ+)−Γ(D−→KSπ−)Γ(D+→KSπ+)+Γ(D−→KSπ−) = (−0.332 ± 0.006)% [4] and is mainly due toK0 − K¯0 mixing. The experimental measurement is very challenging: since the CPeffect is small, a high statistic sample and a good control of the systematic effects areneeded. The main contribution to those is related to the detector-induced asymmetrywhich is carefully studied using a very high statistic control sample from data. The BaBaranalysis is performed on 469 fb−1 [5]. A signal yield, consisting of (807± 1)× 103 events,is extracted from a maximum likelihood fit to the KSπ invariant-mass distribution. TheCP asymmetry is extracted from the signal yield asymmetry A = ND+−ND−ND++ND− , beingND+ (ND−) the fitted number of D+ (D−) candidates. The latter asymmetry includescontribution from the CP asymmetry ACP , the forward-backward asymmetry in thee+e− → cc¯ process AFB due to γ∗ − Z0 interference and high-order QED processes,and the detector-induced asymmetry. The third component is subtracted following adata-driven approach, the first two are determined simultaneously. They can be writtenas a function of the cosine of the polar angle of the D candidate in the CM frame cos θ∗Das:ACP (|cos θ∗D|) =A(+ |cos θ∗D|)−A(− |cos θ∗D|)2,(1a)AFB(|cos θ∗D|) =A(+ |cos θ∗D|) +A(− |cosθ∗D|)2.(1b)Splitting the cos θ∗D range in 10 bins, the forward-backward asymmetry as a function ofcos θ∗D is measured; the ACP central value, which does not depend upon this angularvariable, is computed using a χ2 minimization to a constant in the | cos θ∗D| range. Theresulting CP measurement is ACP = (−0.44 ± 0.13 ± 0.10)% where the first error isstatistic and the second systematic. The dominant contributions to the latter termarise from the contamination from K, e, μ in the control sample used to determinethe detector-induced asymmetry. This introduces a small bias due to different selectionand PID performancies for oppositely charged leptons and kaons. This measurementrepresents the most precise determination of a CP -violating quantity in the charm sector.30 E. MANONITable I. – Signal yields, branching fractions, and decay constants for the leptonic Ds decays.The first uncertainty is statistical and the second systematic.Decay Signal yield B(Ds → ν) fDs (MeV)D−s → e−ν¯e 6.1± 2.2± 5.2 < 2.3× 10−4D−s → μ−ν¯μ 275± 17 (6.02± 0.38± 0.34)× 10−3 265.7± 8.4± 7.7D−s → τ−ν¯τ (τ− → e−ν¯eντ ) 408± 42 (5.07± 0.52± 0.68)× 10−2 247± 13± 17D−s → τ−ν¯τ (τ− → μ−ν¯μντ ) 340± 32 (4.91± 0.47± 0.54)× 10−2 243± 12± 142.3. Leptonic Ds decays. – Leptonic Ds decays happen through an annihilation andtheir decay rate is given by: Γ(Ds → −ν) = G2F |Vcs|2mDs8π (1 − m2m2Ds)2m2f2Ds. Such pro-cesses may be used to measure the Ds decay constant fDs in order to test the SM pre-dictions. Prior to this measurement, the experimental estimation was 3 σ away from thetheoretical one. The analysis has been performed by BaBar using a sample of 469 fb−1 [6].An inclusive sample of Ds candidates is searched for in the recoil of a DKXγ systemfrom an e+e− → DKXD∗s process, with D∗s → Dsγ, and being X a system of additionalpions produced in the fragmentation. An ensamble of (67.2 ± 1.5) × 103 Ds candidatesis determined by a binned fit to the distribution of the recoil mass. The leptonic finalstates are then searched for requiring one additional charged tracks identified as electronor muon plus missing energy associated to the undetected neutrino(s). The amount ofmissing energy allows to distinguish between tauonic and lighter lepton final states. TheDs → τντ yield is determined from a fit to the neutral energy in the event not asso-ciated to the DKXγ system. This quantity is expected to peak at zero for correctlyreconstructed events since the only missing particle in the event should be a neutrino.The Ds → ν yields, being here  = e, μ, are determined from a fit to the missing massdefined as m2miss(DKXγ) = (pe+ −pe− − (pD +pK +pX +pγ +p))2. The results of theanalysis in terms of signal yield, branching fraction and fDs measurement are reported intable I. For the electronic channel, no evidence for signal is found and an upper limit onthe branching fraction at 90% confidence level is set. The combined measurement for theDs decay constant is fDs = (258.6±6.4±7.5)MeV. Using this measurement, the HFAGgroup computed an averaged experimental value of fDs = (257.3± 5.3)MeV [7] which is1.6 σ away from the latest theoretical estimation from the HPQCD Collaboration [8].3. – Bottomomium spettroscopy: first evidence for hb(1S)The hb(1S) bottomomium state is expected to be the axial vector partner of thealready measured χbJ(1P ) states [4]. An observation of the hb(1S) state allows to testthe spin dependence of the qq¯ potential which, in the non-relativistic approximation,predicts a mass splitting between the 3PJ and 1P1 hadrons close to zero. The hb(1S)should be produced in Υ(3S) decays, in association with a neutral pion, and then it isexpected to dominantly decay in a ηbγ final state. BaBar has reported the first evidencefor the hb(1S) state using 28 fb−1 collected at the Υ(3S) mass [9]. From the experimentalpoint of view a γγ pair compatible with coming from a π0 is searched for in e+e− → Υ(3S)events. An additional photon associated to a high multiplicity hadronic decay of the ηbis used to select hb candidates. Such photon is expected to be monochromatic in the hbRECENT RESULTS FROM THE BABAR EXPERIMENT 31Fig. 1. – (Colour on-line) Left: hb(1S) search, the mrecoil(π0) spectrum after subtracting back-ground; the shaded histogram represents the signal function resulting from the fit to the data(dots). Right: semileptonic Bs decays, fs as a function of the CM energy; each point is asso-ciated to two error bars in the y-axis: the small one (red) corresponds to the statistical erroronly, while the big one (blue) incorporates the systematic uncertainties also.rest frame and, given the hb mass as the spin-weighted center of gravity of the χbj(1P )states, should have an energy around 490MeV in the e+e− CM frame. Photons from π0decays are the main source of background for γ from the hb → ηbγ decay. To reject them,each monochromatic photon is paired to another γ in the event and if the invariant massof this pair is consistend with being produced in a π0 decay the photon is rejected. Afit to the distribution of the mass recoiling against the π0, mrecoil(π0), is used to extractthe signal yield. Figure 1 (left panel) shows the background subtracted distributionfor mrecoil(π0). The resulting fitted signal yield consists of Nsig = (9154± 2804± 1082)events, establishing a 3.0 σ significance for signal evidence. The signal excess correspondsto a mass of mhb = (9902± 4± 1)MeV/c2 and to a mass splitting between the 3PJ and1P1 of ΔMHF = (2±4±1)MeV/c2; both values are in agreement with the expectations.4. – Above Υ(4S): semileptonic Bs decaysWhile Bd semileptonic (SL) decays have been extensively studied, semileptonic Bsdecays are poorly known. A precise measurement of the branching ratio of such processeswould be also useful for LHCb studies of Bs decay modes, whose measurement may benormalized to the inclusive SL branching fraction [10]. BaBar has exploited a datasetof 4.1 fb−1 collected above the Υ(4S) resonance to simultaneously measure the B(Bs →Xν) and the Bs production rate fs [11]. This is done by estimating the inclusive yieldof φ mesons and φ meson + lepton and using the following relations:Nbb¯ = Rb [fsεBs + (1− fs)εBd ] ,(2a)Nφ = Rb[fsP(BsB¯s → φX)εBs,φ + (1− fs)P(BdB¯d → φX)εBd,φ],(2b)Nφ, = Rb[fsP(BsB¯s → φνX)εBs,φ + (1− fs)P(BdB¯d → φνX)εBd,φ],(2c)being Rb the ratio between the e+e− → bb¯ and e+e− → μ+μ− cross-sections; εBs(Bd)the efficiencies for selecting a Bs (Bd) candidate, εBs(Bd),φ the efficiencies for selectinga φ in a Bs (Bd) final state, and εBs(Bd),φ the efficiencies for selecting a φ pair ina Bs (Bd) final state; P the probability of having a BB¯ final state containing a φ or32 E. MANONIa φ pair. Combining eqs. (2b) and (2c), fs as a function of the beam energy canbe extracted; P(BsB¯s → φνX) depends on B(Bs → Xν), that can be extractedfrom eq. (2c), once fs is determined. The number of B hadron events in eq. (2a) iscomputed by subtracting the amount of e+e− → qq¯ (q = u, d, c, s) events estimatedfrom data control sample to the events passing the trigger. The inclusive φ (eq. (2b))and φ (eq. (2c)) yields are extracted from a fit to the KK and KK +  invariant-mass distributions, respectively. This procedure allows to measure the SL Bs branchingfraction as B(Bs → Xν) =(9.9+2.6 +1.3−2.1 −2.0)% where the first error is statistical and thesecond systematic. Figure 1 (right panel) shows the results for the fs measurements asa function of the CM energy which are in agreement with the results reported by theBelle [12] and Cleo [13] Collaborations.5. – ConclusionsEven if BaBar has ended its data taking in 2008, the data analysis is still very activeand final results on the full dataset and with the final version of the reconstruction codeare ongoing. Some of the most recent results have been discussed, showing improvementswith respect to the existing measurements in the charm and Bs sectors along with thefirst evidence for the bottomomium state hb(1S).REFERENCES[1] Burdman G., Golowich E., Hewett J. L. and Pakvasa S., Phys. Rev. D, 66 (2002)014009; Fajfer S., Singer P. and Zupan J., Phys. Rev. D, 64 (2001) 074008.[2] Prelovsek S. and Wyler D., Phys. Lett. B, 500 (2001) 304.[3] Lees J. P. et al. (The BABAR Collaboration), preliminary result.[4] Nakamura K. et al. (Particle Data Group), J. Phys. G, 37 (2010) 075021.[5] del Amo Sanchez P. et al., Phys. Rev. D, 83 (2011) 071103.[6] del Amo Sanchez P. et al., Phys. Rev. D, 82 (2010) 091103.[7] Heavy Flavor Averaging Group, http://www.slac.stanford.edu/xorg/hfag/index.html.[8] Davies C. T. H. et al., Phys. Rev. D, 82 (2010) 114504.[9] Lees J. P. et al. (The BABAR Collaboration), arXiv:1102.4565 [hep-ex] (2011).[10] Urquijo P., arXiv:1102.1160 [hep-ex] (2011).[11] BaBar Collaboration, preliminary result.[12] Drutskoy A. et al. (Belle Collaboration), Phys. Rev. D, 76 (2007) 012002.[13] Huang G. S. et al. (CLEO Collaboration), Phys. Rev. D, 75 (2007) 012002.

Recent results from the BaBar experiment

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Recent results from the BaBar experiment

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