Flavour Physics Results from ATLAS

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 241–242 (2013) 55–61 www.elsevier.com/locate/npbps

Flavour Physics Results from ATLAS M. Bona1 Queen Mary, University of London

Abstract We present here a selection of B-physics results from the ATLAS experiment taking data at the Large Hadron Collider (LHC). The ATLAS data collected in pp collisions during 2010 and 2011 allowed a variety of B-physics measurements, reproducing essential B-hadron properties, such as masses and lifetimes, and demonstrating a good performance of the detector within an increasing instantaneous luminosity of the LHC machine. In this report we focus on the search for the decay B0s → μ+ μ− using 2.4 fb−1 of data collected in the first half of 2011. The observed number of events is in agreement with the background expectation. The resulting upper limit on the branching fraction is BR(B0s → μ+ μ− ) < 2.2 × 10−8 at 95% confidence level. Keywords: ATLAS, B physics, rare B decays

1. Introduction The ATLAS experiment taking data at the LHC has a rich heavy flavour program including measurements of b-quark production, studies of b-hadron decays, measurements of quarkonium and production of exotic states. ATLAS is now also performing indirect searches for new physics, such as the rare decays of B0s → μ+ μ− , and measurements of the CP-violating phase in the B0s → J/ψφ decay: these provide important constraints to the Standard Model (SM) and are complementary to direct searches for new physics. In 2010 − 2011, ATLAS collected almost 5 fb−1 of data with a peak instantaneous luminosity of 3.65 × 1033 cm−2 s−1 . 2. Trigger and Tracking Performances The ATLAS detector is described elsewhere [1]. The ATLAS B-physics trigger strategy is based on muon signatures. Due to bandwidth limitations the trigger menus

Email address: [email protected] (M. Bona) behalf of the ATLAS Collaboration

1 On

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in 2011 were mainly based on di-muon signatures at the first-level trigger, and then combined at the higher trigger levels with precise tracking and vertex reconstruction capabilities. A good track-reconstruction performance with increasing instantaneous luminosity is crucial. For b hadrons, decay trajectories of secondary particles are displaced from the primary vertex and the reconstruction of their shortest distances from the primary vertex (impact parameters) is of key importance. A precise test of these capabilities was made in ATLAS by reconstructing transverse impact parameters of tracks originating (mostly) from the primary vertex. This has been done for different values of average number of interactions per beam crossing, during 2011 data taking. In Figure 1 left plot shows the estimated vertex resolution as a function of the number of tracks per vertex, while the right plot presents the impact parameter distributions at medium and high pile-up in 2011 data [2, 3]. It was demonstrated that the resolution is preserved with increasing instantaneous luminosity and pile-up. The tails of the impact parameter distribution are potentially sensitive to the rate of secondaries and fakes and the results show no significant increase in the fake rate.

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3. b-hadron Masses and Lifetimes Using 2010 − 2011 data, most b-hadron species have been observed in ATLAS using exclusive decays with J/ψ → μ+ μ− in the final states. Their masses and lifetimes have been measured. These measurements showed consistency with the world averages [4] and thus provided a precise test of pT -scale calibration in the low pT region and a validation of detector alignment and vertexing algorithms. An overview of b-hadron mass measurements using decay channels with J/ψ → μ+ μ− in the final states is given in Figure 2. High precision lifetime measurements performed by ATLAS using 2010 data were an important milestone on the way towards high precision time-dependent CP violation measurements. An overview of lifetime measurements using decay channels with J/ψ → μ+ μ− in the final states is given in Figure 3. 4. Rare B Decay B0s → μ+ μ− The rare decays B0s → μ+ μ− and B0 → μ+ μ− offer a profound probe into the effects of physics beyond the SM. The decays are flavour-changing neutralcurrent processes which are forbidden in the SM at tree level, occurring only via higher order diagrams. In the SM, these di-muonic B decays have been calculated with high precision and with minimal nonperturbative uncertainties. These decays are also helicity suppressed, resulting in expected branching ratios (BR) of (3.5 ± 0.3) × 10−9 and (1.0 ± 0.1) × 10−10 respectively [11, 12, 13, 14]. Larger branching fractions would potentially indicate contributions from New Physics. Until now neither an enhancement of the branching

fractions have been observed nor the theoretical limits have been reached experimentally [15, 16, 17, 18]. The first search for B0s → μ+ μ− in ATLAS is using 2.4 fb −1 of the 2011 data [19]. A di-muon trigger with pT > 4 GeV for each of the two muon candidates was used to select events. The trigger conditions remained unchanged for the data sample used in the analysis; on average, 6 interactions per crossing have been observed. 4.1. Analysis Strategy To be independent of the uncertainties of the luminosity and bb¯ production cross-section, the branching fraction measurement is performed with respect to an abundant reference channel B± → J/ψK ± (with J/ψ → μ+ μ− ): BR(B0s → μ+ μ− )

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Figure 2: Invariant mass distributions. Candidates in the plots pass all selections except for the proper decay time cut. The points with error bars are data. The solid line is the projection of the result of the unbinned maximum likelihood fit. The dashed lines are the projections of the background or signal components of the same fit. Top-left: Distribution of the invariant mass of B0d candidates reconstructed in the J/ψ(μ+ μ− )KS0 (π+ π− ) final state in 2010 data [5]. Top-right: Invariant mass distribution of J/ψK ± candidates in 3.4 pb−1 of the 2010 data (no distinction made between charges of the combinations) [6]. Middle-left: Invariant mass distribution of reconstructed candidates of B0d → J/ψK 0∗ and B¯ 0d → J/ψK 0∗ in 2010 data [7]. Middle-right: Invariant mass distribution of reconstructed B0s → J/ψφ candidates in 2010 data [7]. Bottom-left: Distribution of the invariant mass of Λb candidates in 1.2 fb−1 of the 2011 data [8]. The χ2 /Ndo f value shows the goodness of the agreement between data and the projection of the PDF onto the mass axis. Bottom-right: Invariant mass distribution of reconstructed B±c → J/ψπ± candidates in 4.3 fb−1 of the 2011 data [9].

0.021 [20] assuming fu = fd [21] and no pT or η dependence of the ratio. The yield N J/ψK ± is measured from data. The ratios of the efficiencies and the acceptances are estimated from MC samples. The num-

ber of signal candidates Nμ+ μ− is counted after unblinding. The CLs method [22] is used for the upper limit calculation. The mass resolution and therefore the signal/background separation power of the ATLAS detec-

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Figure 3: Distributions of proper decay time [10]: the points with error bars are data. The solid line is the lifetime projection of the simultaneous mass and lifetime fit. The figure shows the proper decay time functions of the individual background components and of the signal extracted from the fit. The promptly produced J/ψ background is shown by the dashed-dotted line, which also includes the symmetric exponential background component. The background from non-prompt J/ψ production is denoted by the dashed line. Finally the time function of the B0 signal is represented by the lower solid line. Left: Distribution of proper decay time of reconstructed B0d → J/ψK 0∗ decay candidates. Right: Distribution of proper decay time of reconstructed B0s → J/ψφ decay candidates.

tor depends strongly on the pseudo-rapidity η of the reconstructed particles. Therefore data was split into 3 mass resolution categories separated by |η|max of the two decay muons: 0 − 1 (60 MeV resolution), 1 − 1.5 (80 MeV resolution) and 1.5 − 2.5 (110 MeV resolution). The event selection was optimised separately for each category and the results are combined in the CLs method. 4.2. Background Composition and Signal Selection The background comprises continuous and resonant components. The continuous background component originates from the random combination of muon tracks created in qq¯ annihilation processes. These processes could be of prompt (e.g. Drell-Yan) or of non-prompt (dominated by bb¯ → μ+ μ− X decays) origin. The resonant background comes from the decays of neutral B mesons with one or two hadrons (h, h ) in the final state. For such decays one or both hadrons can be mis-identified as muons, mainly due to the punchthrough of hadrons to the ATLAS Muon Spectrometer or due to in-flight decays. The B → hh background mimics the signal topology and therefore is hard to suppress. In the analysis, the contributions from the resonant decays were estimated from the dedicated MC sample using data-driven mis-identification rates. These contributions were then accounted for in the limit extraction.

For the selection of the B0s candidates out of the continuum background, several discriminating variables related to the primary and secondary vertices and to the decay topology of the two muons, have been calculated. The agreement between the distributions of discriminating variables in data and MC has been verified on the B± data (sideband-subtracted) and MC samples. The distributions of B-meson kinematic variables pTB and ηB in MC were matched to the ones in data by applying a reweighting technique. 14 variables with the highest discriminating power, not correlated to the di-muon mass and not strongly correlated to each other, were chosen as inputs for the multi-variate analysis. The variables are listed elsewhere [19]. The 14 variables are used to train a Boosted Decision Trees (BDT) in the TMVA package [23]: the BDT output variable separates well the signal and the background, as seen in Figure 4 (top). A crosscheck on the B± reference channel (by applying the same BDT weights) shows good agreement between the MC signal sample and the sideband-subtracted data (Figure 4, bottom). The BDT selection and the size of the mass search window Δm have been optimised in each mass resolutioncategory by maximising the estimator P = ε sig /(1 + Nbkg ) for 95% CL [24], where ε sig is the signal selection efficiency and Nbkg is the number of background events in the search region obtained by interpolation from the sidebands.

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4.3. Results The N J/ψK ± yield was computed by performing a binned maximum likelihood fit on the selected events (see Figure 5). The systematic uncertainties on this yield were assessed by varying the bin size, the signal and background fit models, and by inclusion of the perevent mass resolution into the fit. The unblinded di-muon invariant mass distributions are shown in Figure 6: the observed numbers of events in the 3 resolution regions are 2/1/0, respectively. The events observed in the half of the sideband region events used for the interpolation are 5/0/2, respectively, in the 3 resolution regions. The continuous background interpolation gives 6.1 events expected in the signal region. The resonant background contributes 0.24 events. The expected limit is calculated prior to unblinding by setting the number of events in the blinded region to the sum of the expected background events ob-

tained by interpolation of the sidebands and the resonant background contribution. The expected limit is −8 at 95% CL (in the given range, 68% of the 2.3+1.0 −0.5 × 10 pseudo-experiments are found) and the observed limit is 2.2 × 10−8 at 95% CL [19]: these limits are shown in Figure 7. 5. Conclusions The ATLAS experiment has a rich program in the field of b-hadron decays. Precise measurements of lifetimes and masses of b-hadrons demonstrated that the experiment is well equipped for forthcoming CP violation measurements. A search for the rare decay B0s → μ+ μ− has been conducted, setting stringent constraints on extensions to the SM. [1] ATLAS Collaboration, The ATLAS Experiment at the CERN Large Hadron Collider, JINST 3 (2008) S08003. [2] ATLAS Collaboration, Approved Plots of the Inner Tracking Combined Performance Group: Vertex performance, ATL-COM-PHYS-2011-1312, September 2011, URL https://twiki.cern.ch/twiki/bin/view/AtlasPublic/ InDetTrackingPerformanceApprovedPlots#Figures. [3] ATLAS Collaboration, Performance of the ATLAS Inner Detector Track and Vertex Reconstruction in the High Pile-Up LHC Environment, ATLAS-CONF-2012-042, March 2012. URL https://cdsweb.cern.ch/record/1435196 [4] J. Beringer, et al., The Review of Particle Physics, Phys.Rev. D86 (2012) 010001. [5] ATLAS Collaboration, Observation of the decay B0d → J/ψ(μ+ μ− )KS0 (π+ π− ) with the ATLAS experiment at the LHC, ATLAS-CONF-2011-105, July 2011. URL https://cdsweb.cern.ch/record/1369830

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Figure 7: Observed CLs (circles) as a function of BR(B0s → μ+μ− ) [19]. The 95% CL limit is indicated by the horizontal (red) line. The dark (green) and light (yellow) bands correspond to ±1σ and ±2σ fluctuations on the expectation (dashed line), based on the number of observed events in the signal and sideband regions.

[10]

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Figure 6: Invariant mass distribution of candidates in data [19]. For each mass-resolution category (top to bottom) each plot shows the invariant mass distribution for the selected candidates in data (dots), the signal (continuous line) as predicted by MC assuming BR(B0s → μ+ μ− ) = 3.5 · 10−8 , and two dashed vertical lines corresponding to the optimised Δm cut. The grey areas correspond to the sidebands used in the analysis.

[15] [16]

[17] [18]

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