In what follows the calibration and performances of the flavour tagging algorithms using the decays B+ → J/ψK+, B0 → J/ψK∗0 and B0s → D−s π+ are reported. The data sample used correspond to 1.0 fb−1 of data collected by the LHCb experiment during 2011 (√s = 7TeV). The measured effective tagging efficiency is found to be 2.35 ± 0.06% for the opposite side tagging algorithms combination, while it is 1.5 ± 0.4% for same side kaon tagging algorithm

Falabella, A.

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DOI 10.1393/ncc/i2014-11683-8Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014Optimization and calibration of the flavour tagging algorithmsin the LHCb experimentA. FalabellaUniversita` di Ferrara - Ferrara and INFN, Sezione di Bologna - Bologna, Italyricevuto l’1 Ottobre 2013Summary. — In what follows the calibration and performances of the flavour tag-ging algorithms using the decays B+ → J/ψK+, B0 → J/ψK∗0 and B0s → D−s π+are reported. The data sample used correspond to 1.0 fb−1 of data collected bythe LHCb experiment during 2011 (√s = 7TeV). The measured effective taggingefficiency is found to be 2.35± 0.06% for the opposite side tagging algorithms com-bination, while it is 1.5± 0.4% for same side kaon tagging algorithm.PACS 13.85.Ni – Inclusive production with identified hadrons.PACS 12.15.-y – Electroweak interactions.PACS 14.40.Nd – Bottom mesons (|B| > 0).1. – Flavour TaggingOne of the main goals of the LHCb experiment is to perform precise measurement ofB meson decays, for instance studying CP violation and very rare decays. For these pur-poses the LHCb spectrometer is designed to provide a very selective and efficient trigger,excellent track and vertex resolutions, good mass resolution and excellent identificationof particles (pions, protons, kaons, muons and electrons) in a wide range of momenta.For CP violation and time dependent asymmetry measurements it is necessary to knowwhether the reconstructed B meson initially contain a b or a b¯ quark (flavour tagging).In LHCb several flavour tagging algorithms are used: same side (SS) and opposite side(OS) algorithms. The same side algorithms infer the flavour of the B using particles fromthe fragmentation of the b quark that produce the signal B. In LHCb proton-proton col-lisions produce b/b¯ pairs, that hadronize independently giving the signal B meson and anopposite B. The decay products of this second B can be used to assert the flavour of thesignal B. OS algorithms use leptons from semileptonic B decays(μ/e), a kaon from theb → c → s chain and Qvtx (weighted mean of track charges of the secondary inclusivevertex) of the opposite side B meson decay.Each algorithm provide a tagging decision (d = +1/1) for the B containing a b¯ or a bquark based on the charge of the tagging particle [1].c© CERN on behalf of the LHCb CollaborationCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 169170 A. FALABELLAThe tagging decision can be wrong if the particle selected to tag is not correct. More-over OS algorithms decision can be wrong due to the oscillation if the opposite B isneutral. The sensitivity to a CP asymmetry is directly related to the effective taggingefficiency eff = tag(1−2ω)2 which depends on the mistag fraction ω, which is the frac-tion of tagged events with a wrong decision, and the tagging efficiency tag which is thefraction of tagged events. The tagging algorithms have been developed using simulatedevents (MC). Their performance have been optimised on real data with the purpose ofmaximising eff .Each algorithm provides also an estimate of the per-event mistag probability (η)based on the output of a neural network that is trained on simulated events to identifythe correct flavour of the signal B. The neural network uses as input geometrical andkinematic variables of the tagger particle and of the event as input. Due to differencesbetween data and simulated events the neural network output must be calibrated in datain order to represent a correct mistag probability. The calibrated mistag estimation (ηc)can be used as a per-event mistag probability. The use of the per-event value is foundto improve the effective tagging efficiency with respect to the mean value. Moreover theper-event value can be used in CP measurements to weigh each event.The mistag ω can be measured on data using flavour-specific control channels, wherethe signal b-quark content is uniquely defined by its final state. The control channel usedare B+ → J/ψK+, B0 → J/ψK∗0 and B0s → D−s π+.For charged channels the mistag can be measured comparing the charge of the signalB with the tag decision. The mistag ω can then be measured from the number of rightlyand wrongly tagged events. For neutral control channels the mistag is determined froma fit to the time-dependent mixing asymmetry.2. – Optimization and calibrationThe tagging performances were optimized using 1.0 fb−1 of data collected by LHCbduring 2011 data taking. The aim of the optimization is to tune the cuts used to selectthe tagging particles in order to maximize the effective tagging efficiency.The opposite side algorithms have been optimized using the decay channel B+ →J/ψK+. The optimization of the same side kaon algorithm is performed studying theB0s → D−s π+ decay.The effective tagging efficiency for the opposite side algorithms combination and sameside kaon is found to be respectively 2.35± 0.06% and 1.5± 0.4%.The tagging optimization requires the calibration of the mistag probability (η) to themeasured mistag fraction (ω). In fact, when more than one tagging decision is available,these are combined according to η to provide a single combined decision and probability.Due to possible correlations among taggers, the combined mistag probability η has to becalibrated on data to match the measured mistag fraction.The calibration model used is ω(η) = p0 + p1(η − 〈η〉), where p0 and p1 are freeparameters and 〈η〉 is the average mistag probability. Deviations from p0 = 〈η〉 andp1 = 1 indicate that η must be corrected. The calibration is performed for each taggerand than for the combination.The B0 → J/ψK∗0 is used to cross-check the validity of the calibration. In table Ithe calibration results for B+ → J/ψK+ and B0s → D−s π+ decay channels are reported.The errors are statistical and systematic.The results show that for both channels the mistag is correctly calibrated withinerrors. The systematic uncertainty in the calibration parameters has been computedOPTIMIZATION AND CALIBRATION OF THE FLAVOUR TAGGING ALGORITHMS ETC. 171Table I. – Calibration results for B+ → J/ψK+ and B0s → D−s π+ channels. Statistical andsystematic errors are reported.Algorithm Channel p0 p1 ηcOS [2] B+ → J/ψK+ 0.392± 0.002± 0.009 1.035± 0.021± 0.012 0.391SS [3] B0s → D−s π+ 0.349± 0.015± 0.012 1.00± 0.30± 0.02 0.350taking into account of the possible differences of the parameters related to the datataking conditions (run period, magnet polarity), to the initial flavour of the B signal andto the control channel.3. – ConclusionsThe optimized and calibrated performances of flavour tagging algorithms with 1 fb−1of data collected in 2011 have been reported. For the OS algorithms the B+ → J/ψK+decay channel has been used. In this channel the effective tagging efficiency is 2.35 ±0.06%. The SS kaon algorithm has been optimized and calibrated on the B0s → D−s π+channel and the effective tagging efficiency is 1.5± 0.4%.Tagging algorithms have been used in several LHCb published results such as themeasurement of the CP -violating phase φs in B0s → J/ψK+K− and B0s → J/ψπ+π−decays [4] where the effective tagging efficiency using OS and SS kaon algorithms is(3.13 ± 0.12 ± 0.20)%. For the measurement of B0s − B¯0s oscillation frequency Δms [5]the OS and SS kaon algorithms have been used and the effective tagging efficiency is(3.5±0.5)%. The measurement of time dependent CP -violation in B0s → D∓s K± [6] usedOS algorithms and the effective tagging efficiency is 1.9%. Flavour tagging algorithmshave been used also in charmless two-body B decays studies [7] where the effective taggingefficiency with OS algorithms is 2.3± 0.1%.REFERENCES[1] Calvi M. et al. (LHCb Collaboration), Flavour Tagging Algorithms and Performancesin LHCb, [CERN-LHCB-2007-058].[2] LHCb Collaboration, Opposite-side flavour tagging of B mesons at the LHCbPerformance of flavour tagging algorithms optimised for the analysis of B0s → J/ψφ,[LHCB-CONF-2012-026].[3] LHCb Collaboration, Optimization and calibration of the same-side kaon taggingalgorithm using Bs hadronic decays in 2011 data, [LHCB-CONF-2012-033].[4] LHCb Collaboration, Measurement of CP -violation and the B0s decay width differencewith B0s → J/ψK+K− and B0s → J/ψπ+π− decays, [LHCB-PAPER-2013-002].[5] LHCb Collaboration, Measurement of the B0s − Bˆ0s oscillation frequency Δms in B0s →D−s π+, [LHCB-PAPER-2013-006-001].[6] LHCb Collaboration, Measurement of time-dependent CP-violation observables in B0s →D∓s K±, [LHCB-CONF-2012-029].[7] LHCb Collaboration, Measurement of time-dependent CP violation in charmless two-body B decays, [LHCB-CONF-2012-007].

Optimization and calibration of the flavour tagging algorithms in the LHCb experiment

For precise CP violation and time-dependent asymmetries measurements it is necessary to determine the flavour of B mesons at production (flavour tagging). In what follows the flavour tagging performances of the LHCb experiment are summarized

DOI 10.1393/ncc/i2013-11450-5Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Optimization and calibration of the flavour tagging algorithmsin the LHCb experimentA. FalabellaUniversita` degli Studi di Ferrara - Ferrara, Italy andINFN, Sezione di Bologna - Bologna, Italyricevuto il 31 Agosto 2012Summary. — For precise CP violation and time-dependent asymmetries measure-ments it is necessary to determine the flavour of B mesons at production (flavourtagging). In what follows the flavour tagging performances of the LHCb experimentare summarized.PACS 13.85 – Ni Inclusive production with identified hadrons.PACS 12.15 – y Electroweak interactions.PACS 14.40 – Nd Bottom mesons.1. – Flavour taggingOne of the main goals of the LHCb experiment is the measurement of CP violation inB mesons decays and the study of some of their rare decays. To do this the LHCb spec-trometer is designed to provide a very selective and efficient trigger, excellent track andvertex resolutions, good mass resolution and excellent identification of particles (pions,protons, kaons, and muons, electrons) in a large momenta interval.For CP violation and time-dependent asymmetries measurements it is necessary toknow if the reconstructed B meson contain a b or a b quark (flavour tagging).LHCb flavour tagging algorithms can use particles that comes from the fragmentationof the b quark that produce signal B meson (Same Side (SS) algorithms) or particlesfrom the decay chain of the accompayining B meson (Opposite Side (OS) algorithms).SS algorithms use π/K as tagger particle, while OS use μ/e/K and Qvtx (weighted meanof track charges of the secondary inclusive vertex) the charge of the tagger is correlatedto the flavour of the B meson [1].The performances of each algorithm are measured in term of eff = tag(1 − 2ω)2where ω and tag are calculated in terms of right(R), wrong(W ) and untagged events(U) according to(1) ω =WR + W, ω = mistag, tag =R + WR + W + U, tag = efficiency.c© Societa` Italiana di Fisica 257258 A. FALABELLATable I. – Performance results.B+ → J/ΨK+ B0 → J/ΨK∗0 B0 → D∗−μ+νμ(2.10± 0.08± 0.24)% (2.09± 0.09± 0.24)± (2.53± 0.10± 0.27)%These quantities can be measured for each tagger directly on data using flavour spe-cific control channels that are: B+ → J/ΨK+, B0 → J/ΨK∗0, B0 → D∗−μ+νμ. Thedevelopment of each algorithm has been done studying MC events but they have beenoptimized on real data to maximize eff .The tagging algorithms provide also an estimate of the mistag probability event byevent (η) based on the output of a neural network that is trained on MC to identify thecorrect response.It uses geometrical and kinematical properties of the tagger particle as input to de-termine the tagging decision and the probability of wrong decision.In case more than one tagging decision is available, these are combined according tothe predicted mistag probability (η) to provide a single combined decision and probability.For a correct combination the predicted mistag has to be calibrated on data to match themeasured mistag fraction. For this purpose it is used a control channel (B+ → J/ΨK+)that allows to directly measure ω. Using the hypothesis of linear calibration modelω(η) = p0 + p1(η − 〈η〉).ω is calculated as a function of η. The results of the calibration are p0 = 0.392±0.002,p1 = 1.035± 0.021 and 〈η〉 = 0.391.2. – ResultsThe results for the per-event eff of the OS combined algorithms measured with thefirst data collected by LHCb in 2011 (0.37 fb−1) are reported in table I for the threecontrol channels.Some relevant phyisics measurements that use flavour tagging has been made in 2011such as the mixing phase in B0 −B0 [2-4] and the Δms [5].REFERENCES[1] Aaij R. et al. (LHCb Collaboration), the LHCb experiment, Eur. Phys. J. C, 72(2012) 2022; doi:10.1140/epjc/s10052-012-2022-1 (e-print arXiv:1202.4979 [hep-ex], LHCb-PAPER-2011-027, CERN-PH-EP-2012-039).[2] LHCb Collaboration, Tagged time-dependent angular analysis of B0s → J/Ψφ decays atLHCb, LHCb-CONF-2012-002.[3] Aaij R. et al. (LHCb Collaboration), Phys. Lett. B, 707 (2012) 497;doi:10.1016/j.physletb.2012.01.017 (e-print arXiv:1112.3056 [hep-ex], LHCb-PAPER-2011-031, CERN-PH-EP-2011-205).[4] LHCb Collaboration, Measurement of the CP-violating phase φs in B0s → J/Ψπ+π−decays (LHCb-PAPER-2012-06).[5] Aaij R. et al. (LHCb Collaboration), Measurement of the B0s −B0s oscillation frequencyΔms in B0s → D−s (3)ππ decays (LHCb-PAPER-2011-010, CERN-PH-EP-2011-194).

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Optimization and Calibration of the Flavour Tagging algorithms in the LHCb experiment

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Optimization and calibration of the flavour tagging algorithms in the LHCb experiment

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