The article describes the selection of samples of charm and beauty mesons, exclusively or semi-exclusively reconstructed on data collected in 2010. These samples have been used to calibrate the flavour tagging algorithms (through the selection of pure, or heavily enriched, b-jets samples) and for measurements of direct physical interest (b-hadrons production cross section)

Ferretto Parodi, A.

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DOI 10.1393/ncc/i2011-11083-8Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Charm and beauty reconstruction in ATLASA. Ferretto Parodi(∗)INFN, Sezione di Genova - via Dodecaneso 33, IT-16146 Genova, Italy andDipartimento di Fisica, Universita` di Genova - via Dodecaneso 33, 16146 Genova, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 7 Dicembre 2011)Summary. — The article describes the selection of samples of charm and beautymesons, exclusively or semi-exclusively reconstructed on data collected in 2010.These samples have been used to calibrate the flavour tagging algorithms (throughthe selection of pure, or heavily enriched, b-jets samples) and for measurements ofdirect physical interest (b-hadrons production cross section).PACS 13.20.-v – Leptonic, semileptonic, and radiative decays of mesons.PACS 13.25.Hw – Decays of bottom mesons.PACS 13.25.-k – Hadronic decays of mesons.PACS 13.25.Ft – Decays of charmed mesons.1. – IntroductionA precise identification of jets coming from b quark fragmentations (b-tagging) repre-sents an important ingredient for the ATLAS experiment analysis strategies, in particularfor the impact that multi b-jet final states could have on LHC physics. After the startof LHC operations, a precise estimate of b-tagging algorithms performances on real datahas a great importance. Jets containing charmed mesons correlated to leptons representan almost pure sample of b-jets, and therefore they are good candidates to evaluate heavyflavour tagging efficiencies. Furthermore, the reconstruction of charmed mesons, corre-lated with a lepton, is a useful tool to measure beauty hadrons production cross sectionsat LHC. This is a useful test to check the validity of QCD predictions on heavy quarkproduction at high center-of-mass energy, and also useful in the framework of searchesfor new physics phenomena for which b quark pairs can be a significant background.2. – Flavour tagging calibrationThe method [1] to evaluate the b-tagging efficiency is based on opposite sign D+μ−(1)correlations inside jets, where D+ is fully reconstructed as D+ → π+D0(→ K−π+).(∗) E-mail: ferretto@ge.infn.it(1) Charge conjugate states are always implied.c© CERN on behalf of the ATLAS Collaboration under CC BY-NC 3.0 280CHARM AND BEAUTY RECONSTRUCTION IN ATLAS 281) [MeV]π)-M(KsππM(K140 145 150 155Entries / 0.5 MeV0200400600ATLAS = 7 TeVsPreliminary-1 L=35 pb∫ in jets-/+μ+/-D* in jets+/-μ+/-D* 51±fit: N(D*) = 1586 Fig. 1. – ΔM distribution of the opposite- and same-sign D+μ− combinations reconstructedwithin a jet.This sample comes mostly from b → D+μ−X processes, and therefore represents analmost pure sample of b-jets. Figure 1 shows the distributions of the D+ − D0 massdifference (ΔM) for the D+μ− pairs reconstructed within a jet.The b-tagging efficiency of any lifetime tagging algorithm can be calculated by com-paring the number of fitted D+μ− candidates before and after requiring the jet to betagged, obtaining the b-tagging efficiency for jets associated to any type of D+μ− com-binations (Dμ); to extract the efficiency b on jets associated to D+μ− issued fromdirect b semileptonic decays, one has to deconvolve the background contribution, whichmainly includes cc¯ (c → D+, c¯ → μ−) and other b decays (as b → D+, b¯ → μ−, orb → D+D¯, D¯ → μ−):(1) b =Dμ(1 + ncc/nb + nb′/nb)1 + αnb′/nb,where the sample composition ncc/nb and nb′/nb and the charm tagging efficiency ccare taken from Monte Carlo, and α = b′/b is assumed equal to 1. [GeV]TJet p20 30 40 50 60 70 80 90Scale factor00.20.40.60.811.21.4DataStat + Syst UncertaintySV050 ATLAS-1L=35pb∫ PreliminaryFig. 2. – Efficiency of the SV0 tagger for a selection at 50% for jets associated to Dμ, as afunction of the jet transverse momentum.282 A. FERRETTO PARODI(B)  [GeV]Tp10 20 30 40 50 60 70 80(B)  [nb/GeV]T(BX)/dpσd110210310ATLAS = 7 TeVsWork in progress-1 L = 3.26 pb∫data 2010, POWHEG+PYTHIAPOWHEG+HERWIGMC@NLOFig. 3. – b-hadron production cross section as a function of pT (B).Systematic uncertainties have been obtained by varying by a conservative amountthese different components (50% for α and the sample composition factor, 100% for cc).Figure 2 shows the efficiencies, computed in data and Monte Carlo, for the SV0 taggeralgorithm (a tagger based on the reconstruction of a secondary decay vertex, [2]) for aselection at 50% efficiency, as a function of the transverse momentum (pT ) of the jets.This method can be applied to any lifetime tagger algorithm, and any efficiency value.3. – Beauty cross section measurementThe same D+μ− sample, without the request of jet association, can be used toevaluate the b-hadrons production cross section in pp collisions at√s = 7TeV.This cross section can be expressed as(2) σ(pp → BX) = fbNDμ2αBL ,where fb is the Dμ fraction coming from a direct semileptonic b decay (as in the previousmeasurement, cc¯ and other b are the main background sources), NDμ is the number ofDμ candidates in the reconstructed sample, α is the decay acceptance correction,  is| (B)η|0 0.5 1 1.5 2 2.5]η|(B)  [nb/unit of η(BX)/d|σd500010000150002000025000ATLAS = 7 TeVsWork in progress-1 L = 3.26 pb∫data 2010, POWHEG+PYTHIAPOWHEG+HERWIGMC@NLOFig. 4. – b-hadron production cross section as a function of |η(B)|.CHARM AND BEAUTY RECONSTRUCTION IN ATLAS 283the signal reconstruction efficiency, B is total branching ratio that takes into account thedifferent cascade decay chains (B = B(b → D+μ−X) × B(D+ → D0π+) × B(D0 →K−π+) [3]) and L is the integrated luminosity of the collected data sample. The factor 2takes into account that the data sample contains both charge states D+μ− and D−μ+,while the measurement aims at the b-hadron cross section.The measured b-hadron integrated production cross section, for pT (B) > 9GeV andpseudorapidity |η(B)| < 2.5 turns out to beσ(pp → BX) = 33.1± 0.8(stat)+4.3−5.5(syst)± 2.5(B)± 1.1(L)μb .Figure 3 and 4 show the differential b hadron cross sections as a function of pT (B)and |η(B)|, compared with three NLO theoretical predictions (the shaded band showsthe theoretical uncertainty of one of the predictions).4. – ConclusionD+μ− correlations associated to a reconstructed jet have been studied in the 7TeVcollision data collected by the ATLAS experiment. It has been showed how this samplecan be used to determine the tagging efficiency on b-jets, which is crucial for the un-derstanding of the b-tagging performance. D+μ− correlations, without jet association,have been also used for measuring the production cross section of b-hadron in 7TeVpp collisions. The measured cross section value has been compared with the availablenext-to-leading order QCD predictions, and has been found to be slightly higher, butstill compatible with the theoretical uncertainties of the predictions.REFERENCES[1] ATLAS Collaboration, Calibrating the b-Tag Efficiency and Mistag Rate in 35 pb−1 ofData with the ATLAS Detector, ATLAS-CONF-2011-89 (2011).[2] ATLAS Collaboration, Calibrating the b-Tag Efficiency and Mistag Rate of the SV0 b-Tagging Algorithm in 3 pb−1 of Data with the ATLAS Detector, ATLAS-CONF-2010-099(2010).[3] Nakamura K. et al. (Particle Data Group), J. Phys. G, 37 (2010) 075021.

Charm and beauty reconstruction in ATLAS

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Charm and beauty reconstruction in ATLAS

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