The authors search for a new light non-Standard Model CP-odd Higgs boson, A{sup 0}, decaying to {tau}{sup +}{tau}{sup -}, {mu}{sup +}{mu}{sup -}, and invisible in radiative decays of the {Upsilon}(2S) and {Upsilon}(3S). We search for the light scalar particle produced in single-photon decays of the {Upsilon}(3S) resonance through the process {Upsilon}(3S) {yields} {gamma}A{sup 0}, A{sup 0} {yields} invisible. We also search for evidence of an undetectable {Upsilon}(1S) decay recoiling against the dipion system. The data samples contain 99 x 10{sup 6} {Upsilon}(2S) and 122 x 10{sup 6} {Upsilon}(3S) decays collected by the BABAR detector at the SLAC PEP-II B factory

Godang, Romulus

U., /South Alabama

UNT Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515.Hot Topics From BABAR ExperimentRomulus Godang∗On Behalf of the BABAR CollaborationDepartment of PhysicsUniversity of South AlabamaILB 115, 307 University Blvd., N.Mobile, Alabama 36688AbstractWe search for a new light non-Standard Model CP -odd Higgs boson, A0, decaying toτ+τ−, µ+µ−, and invisible in radiative decays of the Υ (2S) and Υ (3S). We search forthe light scalar particle produced in single-photon decays of the Υ (3S) resonance throughthe process Υ (3S) → γA0, A0 → invisible. We also search for evidence of an undetectableΥ (1S) decay recoiling against the dipion system. The data samples contain 99× 106 Υ (2S)and 122×106 Υ (3S) decays collected by the BABAR detector at the SLAC PEP-II B factory.Contributed to the Proceedings of 16th International Seminar on High Energy PhysicsQUARKS-2010, Kolomna, Russia, 6-12 June, 2010.1 IntroductionThe search for the origin of mass of fundamental particle is a cornerstone of the Standard Model(SM). The Higgs mechanism is one of the great interests in the particle physics. In recent yearsmany motivations for searching a Higgs boson. Direct searches at the CERN e+ − e− Collider(LEP Experiment) via the process e+ − e− → ZH for the Standard Model Higgs boson, a CP -even scalar, has excluded a lower bound on its mass below 114.4 GeV [1]. The search for theSM Higgs particle is continued at the Tevatron (The CDF and D0 Collaborations) for a Higgsboson decaying to W+W− and resulting limits on Higgs boson production exclude a SM Higgsboson in the mass range 162-166 GeV at the 95% C.L [2]. The search is currently continued atLarge Hadron Collider (LHC) with the relative increase in charged-particle multiplicity from√s = 0.9 to 7 TeV [3, 4].In recent years a number of theoretical models predicted for the existence of a light CP -oddHiggs boson (A0) related to the Next-to-Minimal Supersymmetric Model (NMSSM). Directsearches constrain the mass of mA0 < 2mb, where mb is the b quark and the decay of A0 → bbis forbidden [5]. Of particular interest is to search the lightest CP -odd Higgs boson in Upsilondecays such as Υ (1S) and Υ (3S). In this paper we present a search of the CP -odd Higgs bosondecaying to τ+τ−, µ+µ−, and invisible. The large data sets available at BABAR experimentallow us to place stringent constraints on such theoretical models.2 Data SampleThe data used in this paper were collected with the BABAR detector at the PEP-II asymmetric-energy e+− e− storage ring at the SLAC National Accelerator Laboratory , operating at Υ (2S)∗e-mail: godang@usouthal.edu or godang@slac.stanford.edu1SLAC National Accelerator Laboratory, Menlo Park, CA 94025SLAC-PUB-14254and Υ (3S). For searching for A0 decays to either τ+τ− or µ+µ−, we use a data sample of122 × 106 Υ (3S) events that corresponds to an integrated luminosity of 28 fb−1. We also useda data sample of 79 fb−1 accumulated on the Υ (4S) resonance (ON-resonance) and 8 fb−1 49MeV below the Υ (4S) resonance (OFF-resonance) for studying the background.For searching an undetectable Υ (1S) decay recoiling against the dipion system we usedthe data were taken using an upgraded muon system, instrumented with both resistive platechambers and the limited streamer tubes between between steel absorbers. The data triggerswas modified to substantially increase the pion trigger efficiency. The data sample of 96.5×10−6Υ (3S) were used. The BABAR detector is described in detail elsewhere [6].3 Analysis Method3.1 Search for A0 in the decays of Υ (3S) → γA0, A0 → τ+τ−We search the light CP -odd Higgs boson, A0, via the decays Υ (3S) → γτ+τ− for a wider massrange, 4.03 < mτ+τ− < 10.10 GeV/c2 [7], over the mass ranges by the CLEO Collaboration [8]and the D0 Collaboration [9], respectively. We exclude the mass region of 9.52 < mτ+τ− < 9.61GeV/c2 because of the irreducible background photons produced in the decays of Υ (3S) →γχbJ(2P ), χbJ (2P ) → γΥ (1S), where J = 0, 1, 2. We scan for peaks in the distribution of thephoton energy, Eγ , corresponding to peaks in the ττ invariant mass as given bym2τ+τ− = m2Υ (3S) − 2mΥ (3S)Eγ , (1)where mΥ (3S) is the Υ (3S) mass (10.355 GeV/c2) and the Eγ is measured in the Υ (3S) center-of-mass frame.We select events in which both τ -leptons decay leptonically to either τ+ → e+νeν̄τ or τ+ →µ+νµν̄τ . The events are then required exactly two charged tracks to reduce the background andto contain at least one photon with energy > 100 MeV in the electromagnetic calorimeter. Inaddition both charged tracks are required to be identified as leptons either electron or muon.The residual background is mostly due to radiative decays of e+e− → γτ+τ− and higher orderof QED processes, including two-photon reactions such as e+e− → e+e−e+e− and e+e− →e+e−µ+µ−. However this residual background has smaller contributions compared to otherΥ (3S) decays and the continuum background from the non-resonance Υ (3S) decays of e+e− →γ∗ → qq̄, where q = u, d, s, c.The events Υ (3S) → γχbJ(2P ), χbJ (2P ) → γΥ (nS), and Υ (nS) decays to τ+τ−, whereJ = 0, 1, 2 and n = 1, 2 are expected to peak in the photon energy distribution when thephoton comes from the decays of χbJ(2P ) → γΥ (nS) is misidentified as the radiative photonfrom the Υ (3S) decays. We use a Crystal Ball function (CB) [10] to describes each of the peaksthat comes from to the decays of χbJ(2P ) → γΥ (1S) in the photon spectrum. The mean valuesfor the χb0(2P ) and the χb1(2P ) CB function are fixed to the PDG values [11], and the widthvalues are fixed to the Monte Carlo (MC) resolution, but the mean and the width of χb2(2P ) arefree parameters. The search for the signal Υ (3S) → γA0, A0 → τ+τ− is performed by scanningfor peaks in the photon energy distributions. Figure 1 shows the fits to the photon energy, Eγ ,distributions in the different τ+τ− decay modes fitted in the region 0.2 < Egamma < 2.0 GeV.No evidence for a signature of the light Higgs boson decays to τ pairs is observed. Wemeasure the branching fraction of B(Υ (3S) → γA0)×B(A0 → τ+τ−) < (1.5− 16)× 10−5 90%confidence level (C.L.). We also set a 90% C.L. upper limit on the τ+τ− decays of the ηb atB(ηb → τ+τ−).3.2 Search for A0 in the decays of Υ (3S) → γA0, A0 → µ+µ−We search for a resonance in the µ+µ− invariant mass distribution for the fully reconstructedfinal state Υ (2S, 3S) → γA0, A0 → µ+µ−. We assume the A0 resonance is a scalar (or pseu-2 (GeV)γ E0.5 1 1.5 2Events/10 MeV 1101001000 ee→ττ(a) (GeV)γ E0.5 1 1.5 2Pull-505 (b) (GeV)γ E0.5 1 1.5 2Events/10 MeV 1101001000eµ →ττ(c) (GeV)γ E0.5 1 1.5 2Pull-505 (d) (GeV)γ E0.5 1 1.5 2Events/10 MeV 1101001000µµ →ττ(e) (GeV)γ E0.5 1 1.5 2Pull-505 (f)Figure 1: The photon energy distributions for the different ττ -decay modes. The data is shownin filled circles, the dotted lines represent the contributions from Υ (3S) → γχbJ(2P ), χbJ (2P ) →γΥ (2S), dotted-dashed lines show the contributions from Υ (3S) → γχbJ(2P ), χbJ (2P ) →γΥ (1S), and the solid lines show the total background functions. The different between thebackground function and the data divided by the uncertainty in the data for each ττ -decaymodes is shown in b, c, and d.doscalar) particle and the width of the resonance is negligibly small compared with the experi-mental resolution where the mass of light Higgs boson, mA0 , is sufficiently far from the mass ofthe ηb [12].We select events with exactly two oppositely charged tracks and a single energetic photonwith a center-of-mass energy E∗γ ≤ 0.2 GeV, while allowing additional photons with the center-of-mass energies below 0.2 GeV. We assign a muon mass hypothesis to the two tracks andrequire that at least one is positively identified as a muon. We then perform a kinematic fit tothe Υ candidate from the two muon candidates and the energetic photon.The backgrounds are dominated by the QED processes: the continuum background e+e− →γµ+µ− and the initial-state- radiation (ISR) production of ρ0, φ, J/ψ, ψ(2S), and Υ (1S) vectormesons. We suppress the background contributions from the decays of ρ0 → π+π− final statein which the pion is misidentified as a muon by requiring both charged tracks are positivelyidentified as muons in the range 0.5 ≤ mA0 < 1.05 GeV. We also suppress the decays ofΥ (2S) → γ2χb(1P ), χb(1P ) → γ1Υ (1S) [Υ (3S) → γ2χb(2P ), χb(2P ) → γ1Υ (1S)], where γ2 isthe secondary photon by requiring that no secondary photon above a center-of mass energy of0.1 GeV [0.008 GeV] is present in the event.Since we do not observe a significant excess of events in the range of 0.212 < mA0 < 9.3GeV, we set upper limits on the branching fractions of B(Υ (2S) → γA0)×Bµµ and B(Υ (3S) →γA0)×Bµµ, where Bµµ is the branching fraction ofA0 → µµ. The limit vary from 0.26−8.3×10−63for B2S and 0.27 − 5.5× 10−6 for B3S . The 90% C.L. Bayesian upper limits are computed witha uniform prior and assuming a Gaussian likelihood function. We set an upper limit on thecoupling f 2Υ (mA0 = 0.214 GeV) < 1.6 × 10−6 at 90% C.L., by assuming Bµµ = 1. This resultis significantly smaller than the value required to explain the HyperCP events as light Higgsproduction [13, 14]. Figure 2 shows the upper limits on the branching fractions as a functionof the mass mA0 . (GeV)A0m0 1 2 3 4 5 6 7 8 9 )  -6 UL (10µµ B 2 Υf110(c)  (GeV)A0m0 1 2 3 4 5 6 7 8 9 )   -6BF UL (1012345 (b)  (GeV)A0m0 1 2 3 4 5 6 7 8 9 )   -6BF UL (10012345678(a)0Figure 2: The 90% C.L. upper limits on the branching fractions of (a) B(Υ (2S) → γA0)×Bµµ,(b) B(Υ (3S) → γA0) × Bµµ, and (c) effective coupling f 2Υ × Bµµ as a function of mA0 (coloronline). The shaded ares show the regions around the J/ψ and ψ(2S) resonances excluded fromthe search.We also measure the branching fractions of B(Υ (2S) → γηb) × B(ηb → µ+µ−) = (−0.4 ±3.9± 1.4) × 10−6 and B(Υ (3S) → γηb)×B(ηb → µ+µ−) = (−1.5± 2.9± 1.6) × 10−6, where thefirst uncertainty is statistical and the second is systematic, dominated by the the uncertainty inΓ(ηb). Using the BABAR measurement of B(Υ (2S) → γηb) and B(Υ (3S) → γηb) [12], we deriveB(ηb → µ+µ−) = (−0.25 ± 0.51 ± 0.33)% and B(ηb → µ+µ−) < 0.9% at 90% C.L. This limit isconsistent with the mesonic interpretation of the ηb state [15].3.3 Search for Υ (3S) → γA0, A0 → InvisibleWe search for the light scalar particle produced in single-photon decays of the Υ (3S) resonancethrough the process Υ (3S) → γA0, A0 → invisible. We split the data sets into two broads energyranges based on the energy of the highest-energy in the center-off mass in our electromagneticcalorimeter cluster. The high-energy region is 3.2 < E∗γ < 5.5 GeV, where E∗γ = Ecom/2. Thebackgrounds in the high-energy region are dominated by the QED process such as e+e− →4γγ, where the photon energy distribution for this process peaks. The low-energy region is2.2 < E∗γ < 3.7. The backgrounds in this region are dominated by the radiative Bhabha eventse+e− → γe+e− → in which both electron and positron are misidentified.We extract the yield of signal events using a likelihood fits to the distribution of the observ-able missing mass squared in the mass interval 0 < mA0 < 6 GeV:m2X = m2Υ (3S) − 2E∗γmΥ (3S), (2)where mX is the missing mass squared, mΥ (3S) is the Υ (3S) mass, and E∗γ is the photon energyin the Υ center-off-mass system. The E∗γ is given byE∗γ =m2Υ −m2A02mγ(3)The current precise limit on the measurement of the branching fraction of B(Υ → γX) withX → invisible is given by CLEO collaboration on Υ (1S) [16].In the high-energy range, the selection efficiency for signal is 10−11% depending on the massmA0 , and it is below 10−5 for e+e− → γγ events, whereas in the low-energy range the selectionefficiency for signal is about 20%. The signal PDF is described by the Crystal Ball function [10]that centered around the expected value of m2X = m2mA0. We compare the distributions of thesimulated and reconstructed e+e− → γγ events to determine the the uncertainty of the PDFparameters.We determine the PDF as a function of mass mA0 . In the low-statistics simulated samplesof signal events, we exclude 6 < mA0 < 7.8 GeV and in the high-statistics simulated samples ofsignal events, we excluding the values of mA0 < 6 GeV due to low statistics. The fit results giveus signal yields of Nsig = 119 ± 71 (1.7σ) and Nsig = 37 ± 15 (2.6σ) for the low-energy datasetand the high-energy dataset, respectively. The fit results on the low-energy range (mA0 = 7.275GeV) and the high-energy (mA0 = 5.2 GeV) are shown in Fig. 3.We do not observe a significant excess of events above the background in the mass range of0 < mA0 < 7.8 GeV., and we set 90% C.L. upper limit on the branching fraction of B(Υ (3S) →γA0)×B(A0 → invisible) at (0.7− 31)× 10−6 in the mass range of mA0 ≤ 7.8 GeV [17]. Theseresults are preliminary.3.4 Search for Υ (1S) → InvisibleThe nature of dark matter is one of the most challenging issues in the elementary particlephysics. Observation of the SM particles coupling to undetectable (invisible) particles mightprovide information on dark matter candidates. The dark matter couples to the SM throughthe mediating boson. This particle is expected to be a vector, scalar, or pseudoscalar. The darkmatter to show up dominantly in interactions with heavy fermions such as b quarks [18, 19].We select events in the invisible subsample by requiring exactly two tracks originating fromthe interaction point (IP) with opposite charge. These track have a center-off-mass momentumof p∗ < 0.8 GeV/c and they are consistent with pions from dipion transition with the invariantmass of 0.25 < Mππ < 0.95 GeV/c2. The dipion recoil mass is given byM2rec = s+M2ππ − 2√(s)E∗ππ, (4)where Mrec is the reconstructed dipion recoil mass, E∗ππ is the center-off-mass energy of thedipion system, and√(s) = 10.3552 GeV/c2. The overall efficiency of the selection events isabout 64%.We collect the invisible subsample by selecting tree-track and four-track events using thedipion system. We search high-momentum tracks from Υ (1S) decays by requiring only one ortwo additional tracks originating from the IP point, each with momentum p∗ > 2.0 GeV/c2. We5Figure 3: [Left] Sample fit to the low-energy dataset (83× 106 Υ (3S)) decays. The bottom plotshows the data (solid points) overlaid by the full PDF curve (solid blue line), signal contributionwith mA0 = 7.275 GeV (solid red line). [Right] Sample fit the high-energy dataset (122 ×106 Υ (3S)) decays. The bottom plot shows the data (solid points) overlaid by the full PDFcurve (solid blue line), signal contribution with mA0 = 5.2 GeV, e+e− → γγ (dot-dashed greenline), and the continuum background (black dashed line).then use our lepton identification to treat these tracks. If either both tracks passing electron-identification criteria the both tracks are treated as electron candidates; otherwise, both aretreated as muon candidates. Figure 4 shows the Mrec distribution from our invisible subsample.We determine the parameters of the PDF by fitting the Mrec distribution in the four-trackdata subsample. We extract the peaking contribution by using an extended unbinned maximumlikelihood fit and the non-peaking contribution using a first-order polynomial function. Thepeaking background is estimated using Monte Carlo subsamples of 1019 Υ (1S) → e+e− events,1007 Υ (1S) → µ+µ− events, 92 Υ (1S) → τ+τ− events, and 2.9 ± 1.3 Υ (1S) → hadrons events.We search for evidence of the decays of Υ (1S) into undetectable final states recoiling againstthe dipion system in the Υ (3S) → π+π−Υ (1S), using a sample of 9.14×106 Υ (3S) mesons. Wefind no evidence for the decays of Υ (1S) → invisible. We set an upper limit on the branchingfraction of B(Υ (1S) → invisible) < 3.0 × 10−4 at the 90% C.L. [20].4 SummaryWe search for a new light non-Standard Model CP -odd Higgs boson in the Υ (3S), Υ (2S),and Υ (1S) systems. We also search for invisible decays of the Υ (1S) system. BABAR recentmeasurements are important input to rule out much of the parameter space allowed by the lightnon-Standard Model CP -odd Higgs and other models. These measurements also provide newand stringent constraints on the light CP -odd Higgs boson. In addition the measurement ofthe branching fraction of Υ (1S) → invisible is extremely important as a sensitive probe of darkmatter.6Figure 4: The maximum likelihood fit to the dipion recoil mass for data in the invisible sub-sample. Dash-dotted line is the nonpeaking background and the solid fitted-line is the peakingbackground. The solid line is the total fit function.5 AcknowledgmentsThe author would like to thank the organizers of QUARKS 2010, 16th International Seminar onHigh Energy Physics, for their excellent program and kind hospitality. The supports from theBABAR Collaboration, the University of South Alabama, and the University of Mississippi aregratefully acknowledged. This work was supported by the U.S. Department of Energy undergrant No. DE-FG02-96ER-40970.References[1] R. Barate et al. (LEP Working Group), Phys. Lett. B 565, 61 (2003).[2] T. Aaltonen et al. (CDF and D0 Collaborations), Phys. Rev. Lett. 104, 061802 (2010).[3] V. Khachatryan et al. (CMS Collaboration), arXiv:1005.3299 (2010);V. Khachatryan et al. (CMS Collaboration), JHEP 1002:041 (2010).[4] G. Aad et al. (ATLAS Collaboration), CERN-PH-EP/2010-004, Phys. Lett. B 688 (2010).[5] R. Dermisek and J. F. Gunion, Phys. Rev. D 73, 111701(R) (2010).[6] B. Aubert et al. (BABAR Collaboration), Nucl. Instrum. Methods Phys. Res., Sect. A 476,1 (2002).[7] B. Aubert et al. (BABAR Collaboration), Phys. Rev. Lett. 103, 181801 (2009).7[8] W. Love et al. (CLEO Collaboration), Phys. Rev. Lett. 101, 151802 (2008).[9] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 103, 061801 (2009).[10] M. J. Oreglia, Ph.D. Thesis, SLAC-236 (1980);J. E. Geiser, Ph.D. Thesis, Stanford University, SLAC-R-255 (1982);T. Skwarnicki, Ph.D. Thesis, DESY F31-86-02 (1986).[11] C. Amsler et al. (PDG Collaboration), Phys. Lett. B 667, 1 (2008).[12] B. Aubert et al. (BABAR Collaboration), Phys. Rev. Lett. 101, 071901 (2008).[13] H. Park et al. (HyperCP Collaboration), Phys. Rev. Lett. 94, 021801 (2005).[14] X. G. He, J. Tandean, and G. Valencia, Phys. Rev. Lett. 98, 081802 (2007).[15] B. Aubert et al. (BABAR Collaboration), Phys. Rev. Lett. 103, 081803 (2009).[16] R. Ballest et al. (CLEO Collaboration), Phys. Rev. D 51, 2053 (1995).[17] B. Aubert et al. (BABAR Collaboration), hep-ex/arXiv:0808.0017 (2008).[18] B. McElrath, Phys. Rev. D 72, 103508 (2005).[19] R. Dermisek, J. F. Gunion, and B. McElrath, Phys. Rev. D 76, 051105(R) (2007).[20] B. Aubert et al. (BABAR Collaboration), Phys. Rev. Lett. 103, 251801 (2009).8

Hot Topics From BaBar Experiment

Abstract

Similar works

Full text

Available Versions

UNT Digital Library