We have measured the time-integrated charge asymmetries in dimuon events and semileptonic B{sub s} decays. These results are the most precise semileptonic charge asymmetries in B decays to date. We combine these results with measurements from the decay B{sub s} {yields} J/{psi}{phi} to determine the CP-violating phase {phi}{sub s}. They find {phi}{sub s} = -0.56{sub -0.41}{sup +0.44}

Cheu, E.

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FERMILAB Conf-06/397-ED0 CONF 06-5274Time-Integrated Charge Asymmetries at DØE. CheuDepartment of Physics, University of Arizona1118 E. 4th StreetTucson, AZ 85749, USAAbstractWe have measured the time-integrated charge asymmetries in dimuon events and semileptonic Bsdecays. These results are the most precise semileptonic charge asymmetries in B decays to date. Wecombine these results with measurements from the decay Bs → J/ψφ to determine the CP-violatingphase φs. We find φs = −0.56+0.44−0.41 .1 IntroductionIn the neutral Bs, mixing occurs through |∆B| = 2 transitions. These oscillations can be described bythree physical quantitites: |M12|, |Γ12| and φs, where |M12| and |Γ12| are the elements of the complexmass matrix, and φs is a CP violating phase:φs = arg(−M12Γ12). (1)Determining the value of φs is interesting because new physics can lead to an enhancement of φs. Also,because φs is predicted to be small in the Standard Model[1], any measurement of a sizeable value of φswould be a clear signal for new physics. In this paper we will discuss two measurements which are usedto determine the value of φs. These results are preliminary as of September 2006.2 Dimuon AsymmetryThe dimuon charge asymmetry measures the charge asymmetry in direct decays of BB¯ pairs. One candetermine the charge asymmetry, ASL, usingASL =N(bb¯→ µ+µ+X)−N(bb¯→ µ−µ−X)N(bb¯→ µ+µ+X) +N(bb¯→ µ−µ−X). (2)Using dimuon pairs has an advantage over single muon events because the dimuon events are relativelyclean. To first order there is no charge asymmetry in dimuon background processes.This analysis uses approximately 1 fb−1 of data collected during Fermilab RunII collider operations.This resulted in approximately 3 million dimuon events. Sources of background dimuon events includecosmic rays, charged pion and kaon decays, punch throughs and W± and Z0 decays. The majority ofthese backgrounds are removed by applying timing cuts and cuts on the pT of the muon. Imposing therequirement that muons traverse the DØ toroid significantly reduces the number of punch throughs.The DØ detector has two separate spectrometers used for muon reconstruction. The central trackingdevice includes a silicon microstrip tracker and a scintillating-fiber tracker located within a 2T supercon-ducting solenoid. The muon system consists of toroidal magnets with proportional/mini drift tubes andscintillation counters for muon detection and triggering. The magnets in the two spectrometers can bereversed independently which allows cancellation of many detector asymmetries, as shown in Fig. 1.1pT  [GeV/c]0 2 4 6 8 10 12 14020406080100310×-1, 1.0 fb∅Dch*torpol=+1eta-3 -2 -1 0 1 2 301000020000300004000050000ch*torpol=+1pT  [GeV/c]0 2 4 6 8 10 12 14020406080100310×ch*torpol=-1eta-3 -2 -1 0 1 2 30100002000030000400005000060000ch*torpol=-1Figure 1: Muon pT (left) and η (right) distributions. Each plot contains two separate histograms witheach histogram having the same product of muon charge×toroid polarity. The two histograms lie nearlyon top of each other indicating that the detector response for opposite charges are nearly the same. Thetop plots are for the positive charge×polarity while the bottom plots contain the negative charge×polarity.To determine the semileptonic charge asymmetry, we divide the sample into eight subsamples onthe basis of the toroid polarity (β), muon charge (q) and the sign of the pseudorapidity (γ). We thenuse the eight subsamples (nβγq ), keeping terms up to second order, to calculate ASL using the followingexpression:ASL/fB +AfbAns =[(n+++ + n+−+ − n++− − n+−− )+e(n−++ + n−−+ − n−+− − n−−− )]/[(n+++ + n+−+ + n++− + n+−− )+e(n−++ + n−−+ + n−+− + n−−− )].(3)In this expression Afb is the tendency of a µ± to travel in the proton/antiproton direction, Ans is thenorth/south detector asymmetry where north is defined as η < 0. The protons travel from north to southin the DØ detector. The variable e is the ratio of events with opposite toroid polarities, and fB is thefraction of the muons that result from B meson decays.The term Afb × Ans means that an apparent asymmetry can be induced if the positive (negative)muons tend to go along the direction of the protons (antiprotons) and the detector has a north/southasymmetry. The terms Afb and Ans can be estimated by forming the appropriate combinations of nβγq [2].We find that the product Afb × Ans is consistent with zero, and we use this product to estimate oursystematic uncertainty.The number of dimuon events is listed in Tab. 1. Using these events, we determine the dimuonsemileptonic charge asymmetry to be ASL = (−0.0044± 0.0040± 0.0028)[2]. This value is competitivewith the measurements with BB¯ pairs from the B-factories[3, 4, 5].The systematic errors associated with this measurement are listed in Tab. 2. The largest source ofsystematic error results from charged kaon decays. Because of the presence of S = −1 baryons, theinelastic interaction length for K+ is different from that of K−. This results in a charge asymmetry of−0.0028± 0.0007. We correct for this and apply the error on the correction as a systematic uncertainty.Other sources of systematic error are significantly smaller than the K± contribution.The dimuon semileptonic charge asymmetry can be used to determine the Bs semileptonic chargeasymmetry. This is because our sample contains decays from both Bd and Bs mesons. Our measured2Table 1: Number of dimuon events.Sample Relative PolaritiesOpposite SameN++ 177,950 156,183N−− 176,939 156,148N+− 1,175,547 1,209,605asymmetry value can be related to both AdSL and AsSL byASL(DØ) = AdSL +fsZsfdZdAsSL, (4)where Zq =11−y2q− 11+x2q, with xq =∆MqΓqand yq =∆Γq2Γq; q is either d or s. The variable fq is the fractionof the time a b quark produces a Bq meson. Using a value of AdSL = 0.0011± 0.0055 derived from theresults from BaBar, Belle and CLEO[6], we find AsSL = −0.0076± 0.0102.3 Bs Semileptonic AsymmetryThe Bs semileptonic charge asymmetry can be measured using the decay Bs → Dsµν. We do not performany initial state flavor tagging, and the untagged semileptonic charge asymmetry can be writtenAuntaggedSL =N(Bs → D−s µ+)−N(B¯s → D+s µ−)N(Bs → D−s µ+) +N(B¯s → D+s µ−). (5)Because the Bs oscillations occur rapidly, the untagged charge asymmetry can be related to AsSL, the Bscharge asymmetry, via the following relationship:AuntaggedSL =12x2s1 + x2sAsSL,AuntaggedSL ≃12AsSL.(6)Table 2: Dimuon charge asymmetry systematic errors.Source of Error ∆AK± Decay + prompt µ 0.00068Sample Normalization 0.00018Misreconstructed Charge 0.00015Detector 0.00015Cosmic Rays 0.00010Punch Through 0.00001Total 0.00074For this measurement we reconstruct the Ds using the decay D±s → φpi±. As in the dimuon analysis,we divide the sample into eight subsamples based upon the muon charge, sign of the pseudorapidityand the toroid polarity. The charge asymmetry is then determined in a similar manner to the dimuoncharge asymmetry analysis. Again, the ability to reverse the DØ magnet polarities significantly reduces3Figure 2: The invariant φpi± mass distributions. The peak on the left corresponds to D+ decays, whilethe peak on the right corresponds to Ds decays. The eight plots show the mass distributions for each ofthe eight subsamples defined in the text.any detector induced charge asymmetries. Fig. 2 shows the reconstructed Ds mass for all Bs → D−s µ+candidates. The two peaks in the plot correspond to D+ and Ds decays.The systematic uncertainties associated with this measurement are shown in Tab. 3. The two largestsources of uncertainty are the determination of the sample composition and the determination of thenumber of events. To obtain a systematic error for the sample composition, we varied each contributionby one sigma and examined the change in the charge asymmetry. The systematic error associated withthe mass fitting was arrived at by varying the slope of the background by one sigma and also by changingthe width of the mass peak by one sigma.Including the effects of the systematic uncertainty, we find AsSL = 0.0245± 0.0193± 0.0035[7]. Thisis the first direct measurement of AsSL, and we find no significant charge asymmetry.Table 3: Systematic errors in the Bs charge asymmetry.Source ∆AMass fitting 0.0027Sample composition 0.0022pi± interactions 0.0004Contribution from AdSL 0.0002Total 0.00354 Combined ResultsThe results from the DØ semileptonic charge asymmetries can be combined together to place a constrainton φs. The combined AsSL measurement from the two results presented in this paper is AsSL = −0.0007±4 (radians)φ δ-3 -2 -1 0 1 2 3 (1/ps) Γ ∆-0.2-0.100.10.20.30.40.50.60.7)φ ψ J/→ s (B∅Dsemileptonic chargeasymmetry band-10.16 ps±) =-0.01 φδ tan(× Γ∆ - combined∅Dφ ψ J/→ sB constraintslA{)|φδ |cos(× SMΓ∆ = Γ∆SMFigure 3: The DØ combined result on ∆Γs and φs. The blue curve shows the result solely from ourmeasurement of Bs → J/ψφ while the red curve shows the combined constraint including the semileptoniccharge asymmetry results.0.0090. Using the relationshipAsSL =∆Γs∆mstanφs, (7)one can extract a value for φs. In addition, the DØ experiment has determined φs and ∆Γs from thedecay Bs → J/ψφ. We find ∆Γs = 0.17 ± 0.09 ± 0.03 ps−1 and φs = −0.79± 0.56+0.13−0.01[8]. Combiningthe results from the semileptonic and purely hadronic analyses, we find ∆Γs = 0.15+0.09−0.08 ps−1 andφs = −0.56+0.44−0.41[9]. The constraints on φs and ∆Γs can be seen in Fig. 3.5 ConclusionsThe DØ experiment has made important contributions to charge asymmetry measurements in the Bmeson sector. We find the following two results:• ASL = −0.0044± 0.0040± 0.0028 (Dimuon),• AsSL = 0.0245± 0.0193± 0.0035 (Semileptonic Bs).These new measurements can be used in conjunction with our recent measurement of Bs → J/ψφ toplace constraints on the CP violating phase φs. We find φs = −0.56+0.44−0.41. This result on the CPphase is consistent with the Standard Model prediction that φs be small, although the precision of themeasurements are at present limited.References[1] M. Beneke, G. Buchalla, A. Lenz and U. Nierste, Phys. Lett B576 173 (2003).[2] DØ Collaboration, Conference note 5042. Submitted to Phys. Rev. D.[3] BaBar Collaboration, hep-ex/0603053.[4] Belle Collaboration, hep-ex/0505017.[5] D. Jaffe et al., Phys. Rev. Lett. 86, 5000 (2001).[6] Y. Grossman, Y. Nir, G. Raz, hep-ph/0605028.[7] DØ Collaboration, Conference note 5143.5[8] DØ Collaboration, Conference note 5144.[9] DØ Collaboration, Conference note 5189.6

Time-integrated charge asymmetries at D0

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Time-integrated charge asymmetries at D0

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