The LHCb experiment is designed to perform flavour physics measurements at the Large Hadron Collider. Using data collected during the 2010 run, we reconstruct a sample of Hb → h+h− decays, where Hb can be either a B0 meson, a B0s meson or a Λb baryon, while h and h' stand for π, K or p.

We provide preliminary values of the direct CP asymmetries of the neutral B0 and B0s mesons ACP (B0 → K+π−) = −0.074 ± 0.033(stat.) ± 0.008(syst.) and ACP (B0s → π+K−) = 0.15 ± 0.19(stat.) ± 0.02(syst.)

Perazzini, S.

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DOI 10.1393/ncc/i2011-11072-yColloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Measurements of ACP (B0 → K+π−) and ACP (Bs → π+K−) at LHCbS. Perazzini on behalf of the LHCb CollaborationUniversita´ di Bologna and INFN, Sezione di Bologna - Bologna, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 6 Dicembre 2011)Summary. — The LHCb experiment is designed to perform flavour physics mea-surements at the Large Hadron Collider. Using data collected during the 2010run, we reconstruct a sample of Hb → h+h′− decays, where Hb can be eithera B0 meson, a B0s meson or a Λb baryon, while h and h′ stand for π, K or p.We provide preliminary values of the direct CP asymmetries of the neutral B0and B0s mesons ACP (B0 → K+π−) = −0.074 ± 0.033(stat.) ± 0.008(syst.) andACP (B0s → π+K−) = 0.15± 0.19(stat.)± 0.02(syst.).PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.PACS 12.15.Hh – Determination of Cabibbo-Kobayashi & Maskawa (CKM) matrixelements.PACS 13.25.Hw – Decays of bottom mesons.PACS 14.40.Nd – Bottom mesons (|B| > 0).1. – IntroductionThe family of Hb → h+h′− comprises a large set of decays, namely: B0 → π+π−,B0 → K+π−, B0s → K+K−, B0s → π+K−, Λb → pK−, Λb → pπ−, B0 → K+K−,B0s → π+π− plus their CP-conjugate states. Such decays are sensitive probes of theCabibbo-Kobayashi-Maskawa [1,2] matrix and have the potential to reveal the presenceof New Physics (NP) [3, 4]. NP may alter in a subtle but sizeable way the StandardModel (SM) prediction of the CP asymmetries in these decays. Exploiting the 37 pb−1of integrated luminosity collected by LHCb during 2010, at a centre of mass energy of7 TeV, we measure the direct CP asymmetries in the B0 → K+π− and B0s → π+K−decays.2. – Events selectionCandidates used in this analysis are obtained fitting into a common vertex, displacedfrom the proton-proton interaction region, two tracks characterized by high transversemomentum and large impact parameter with respect to the reconstructed primary vertex.c© CERN on behalf of the LHCb Collaboration under CC BY-NC 3.0 292MEASUREMENTS OF ACP (B0 → K+π−) AND ACP (Bs → π+K−) AT LHCB 293Fig. 1. – (Colour on-line) K+π− (plus charge conjugate) invariant mass spectrum for eventssurviving the event selection optimised for the best sensitivity on ACP (B0 → K+π−) (left) andACP (B0s → π+K−) (right). The result of the unbinned maximum likelihood fit is superimposed.The main components contributing to the fit model explained in the text are also visible: B0 →Kπ (red), wrong sign B0 → Kπ combination (dark red), B0 → π+π− (light blue), B0s →K+K− (dark yellow), B0s → πK (green), combinatorial background (grey), 3-body partiallyreconstructed decays (orange).Then the different final states hypothesis (K+π−, K−π+, π+π−, K+K−, pπ−, p¯π+, pK−and p¯K+) are separated using the capabilities of the two Ring Imaging Cherenkov (RICH)detectors of LHCb. The Particle Identification (PID) observables have been calibratedusing sample of large statistics and high purity of D∗+ → D0(K−π+)π+ and Λ→ pπ−,obtained without using any PID information. In order to achieve the best sensitivity onACP (B0 → K+π−) and ACP (B0s → π+K−) we optimise two different selections, one foreach measurement, characterized by the same set of cuts but with different values.3. – Fits to the Hb → h+h′− mass spectraWe perform unbinned maximum likelihood fits to the mass spectra of events pass-ing the optimised offline selections for the measurements of ACP (B0 → K+π−) orACP (B0s → K−π+). The fits are performed simultaneously on all the eight categoriesdefined by means of the PID selection criteria. The signals are parameterized with asingle Gaussian function convolved with a component accounting for final state QEDradiation [5]. The combinatorial background is modeled with an exponential function.The invariant mass shapes of cross-feed backgrounds are parameterized by means of fullsimulated events, while the normalization of each mis-identified channel is determinedfrom the calibrated PID efficiencies. For the K±π∓ and π+π− categories it is necessaryto model also a component due to partially reconstructed 3-body B-hadron decays.The results of the fits superimposed to the K±π∓ mass spectra (seperately for thesamples obtained using the two optimised selections) are shown in fig. 1. The asymmetriesobtained from the fits are, respectively, ARAWCP (B0 → K+π−) = −0.086±0.033(stat.) andARAWCP (B0s → K−π+) = 0.15±0.19(stat.). The systematic errors due to the fit model andPID calibration are estimated to be respectively 0.002 and 0.004 for ACP (B0 → K+π−)and 0.021 and 0.001 for ACP (B0s → K+π−).4. – Correction to the ARAWCPThe physical CP asymmetries we want to measure are related to the raw asymmetriesobtained from the invariant mass fit by ACP = ARAWCP −AD(Kπ)− κAP .294 S. PERAZZINI on behalf of the LHCB COLLABORATIONAD(Kπ) is the detector induced asymmetry in reconstructing K+π− and K−π+ finalstates; AP is the production asymmetry of B mesons defined in terms of the B and B¯production rates AP = (RB¯ − RB)/(RB¯ + RB); κ is a factor that takes into accountthe B − B¯ oscillation and depends also on the proper-time acceptance introduced bythe events selection. The detector induced asymmetry AD(Kπ) is determined usinghigh statistics samples of tagged D∗+ → D0(K−π+)π+, D∗+ → D0(K+K−)π+ andD∗+ → D0(π+π−)π+, and untagged D0 → K−π+ decays (plus their charge conjugates).Following the method shown in [6] we obtain: AD(Kπ) = −0.004± 0.004.The production asymmetry AP is determined measuring the integrated raw asymme-try of B± → J/ψ(μ+μ−)K± decays, correcting it by the world average of the direct CPasymmetry ACP (B+ → J/ψK+) [7], and taking into account the reconstruction asym-metry between K+ and K−. We measure AP (B0) = −0.024±0.013±0.010. The seconderror has been introduced to take into account possible differences between AP (B+) andAP (B0) [8]. The κ factors are evaluated by means of full simulated events, obtaining,respectively, for B0 and B0s , κd = 0.33 and κs = 0.015. The value of κs makes negligiblethe correction due production asymmetry in the case of B0s → K−π+ decay.The corrected central values of the direct CP asymmetries are ACP (B0 → K+π−) =−0.074 and ACP (B0s → K+π−) = 0.15. The errors of AD(Kπ) and κAP are consideredas systematic uncertainties contributing to ACP (B0 → K+π−) and ACP (B0s → K+π−).5. – Final resultUsing data collected by the LHCb detector during the 2010 run we provide preliminaryvalues of the direct CP asymmetries:ACP (B0 → K+π−) = −0.074± 0.033(stat.)± 0.008(syst.),(1)ACP (B0s → π+K−) = 0.15± 0.19(stat.)± 0.02(syst.).(2)The current HFAG average [9] ACP (B0 → K+π−) = −0.098+0.012−0.011 and the CDF mea-surement [10] ACP (B0s → K−π+) = 0.39±0.15(stat.)±0.08(syst.) are in agreement withour values.REFERENCES[1] Cabibbo N., Phys. Rev. Lett., 10 (1963) 531.[2] Kobayashi M. and Maskawa T., Prog. Theor. Phys., 49 (1973) 652.[3] Fleischer R., Eur. Phys. J. C, 52 (2007) 267.[4] Fleischer R., Phys. Lett. B, 459 (1999) 306.[5] Baracchini E. and Isidori G., Phys. Lett. B, 633 (2006) 309.[6] CDF Collaboration, CDF Public Note 10296.[7] Nakamura K. et al. (Particle Data Group), J. Phys. G, 37 (2010) 075021.[8] Lambert R. W., CERN-THESIS-2009-001.[9] Barberio E. et al. (Heavy Flavor Averaging Group), CP Asymmetries in CharmlessB Decays: ICHEP 2010 update, http://www.slac.stanford.edu/xorg/hfag/rare/ichep10/acp/.[10] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 106 (2011) 181802,arXiv:1103.5762v1 [hep-ex].

Measurements of ACP (B0 → K+π−) and ACP (Bs → π+K−) at LHCb

The LHCb experiment is designed to perform flavour physics measurements at the Large Hadron Collider. Using data collected during the 2010 run, we reconstruct a sample of $H_b\rightarrow h^+h'^-$ decays, where $H_b$ can be either a $B^0$ meson, a $B_s^0$ meson or a $\Lambda_b$ baryon, while $h$ and $h^\prime$ stand for $\pi$, $K$ or $p$. We provide preliminary values of the direct $\mathcal{CP}$ asymmetries of the neutral $B^0$ and $B_s^0$ mesons $A_{CP} (B^0\rightarrow K^+\pi^-) = -0.074 \pm 0.033\mathrm{(stat.)} \pm 0.008\mathrm{(syst.)}$ and $A_{CP}(B_s^0\rightarrow\pi^+K^-)=0.15 \pm 0.19\mathrm{(stat.)} \pm 0.02\mathrm{(syst.)}$.The LHCb experiment is designed to perform flavour physics measurements at the Large Hadron Collider. Using data collected during the 2010 run, we reconstruct a sample of $H_b\rightarrow h^+h'^-$ decays, where $H_b$ can be either a $B^0$ meson, a $B_s^0$ meson or a $\Lambda_b$ baryon, while $h$ and $h^\prime$ stand for $\pi$, $K$ or $p$. We provide preliminary values of the direct $\mathcal{CP}$ asymmetries of the neutral $B^0$ and $B_s^0$ mesons $A_{CP} (B^0\rightarrow K^+\pi^-) = -0.074 \pm 0.033\mathrm{(stat.)} \pm 0.008\mathrm{(syst.)}$ and $A_{CP}(B_s^0\rightarrow\pi^+K^-)=0.15 \pm 0.19\mathrm{(stat.)} \pm 0.02\mathrm{(syst.)}$

Perazzini, Stefano

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Measurements of $A_{CP}(B^0\rightarrow K^+ \pi^-)$ and $A_{CP}(B_s\rightarrow \pi^+ K^-)$ at LHCb

http://eprints.bice.rm.cnr.it/17581/1/ncc10100.pdf

Measurements of ACP (B0 → K+π−) and ACP (Bs → π+K−) at LHCb

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