Different analyses measuring final states with a vector boson and heavy quarks are presented, based on the data collected by the CMS experiment at the LHC in 2010 and 2011

Casasso, S.

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DOI 10.1393/ncc/i2013-11463-0Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Measurements of the associated production of vector bosonsand heavy flavours with the CMS detectorS. Casasso on behalf of the CMS CollaborationINFN, Sezione di Torino and Universita` di Torino - Torino, Italyricevuto il 31 Agosto 2012Summary. — Different analyses measuring final states with a vector boson andheavy quarks are presented, based on the data collected by the CMS experiment atthe LHC in 2010 and 2011.PACS 13.85.Lg – Total cross sections.PACS 13.38.Dg – Decays of Z bosons.PACS 13.87.Ce – Jets production.1. – Measurement of the Z/γ∗ + b jet inclusive cross section [1]The main motivation to study this channel is to confirm or constrain theoreticalpredictions for these processes. Moreover, it is a benchmark for the search for new physics,for example in the bb + Φ channel in the MSSM with large tanβ, and a background inthe search for the SM Higgs boson.The cross section formula used is the following:(1) σ(pp → Z/γ∗ + b, Z/γ∗ → ) = N+b × (P − ftt¯)A × Chadron ×  × b × L ,  = e, μ,where N+b is the number of selected events, P is the b-purity, ftt¯ the fraction of tt¯contamination, A is the lepton acceptance, Chadron a correction factor for reconstructioneffects, (b) the lepton (b-tagging) efficiency and L is the integrated luminosty (2.11 fb−1).The cross section is measured in the kinematical region in which pbT > 25GeV, |ηb| <2.1, 60GeV < m < 120GeV, ΔR(, jet) < 0.5, where ΔR =√(Δη)2 + (Δφ)2.Leptons and jets are selected accordingly to standard identification criteria and kine-matical cuts used in the CMS collaboration. The b-tagging algorithm looks for a sec-ondary vertex, built from at least 3 tracks, inside jets.The main backgrounds in this analysis are due to Z + light jets and tt¯. The first issubtracted by extracting the “per event” b-purity (P) from a template fit to the secondaryc© CERN on behalf of the CMS Collaboration under CC BY-NC 3.0 286MEASUREMENTS OF THE ASSOCIATED PRODUCTION OF VECTOR BOSONS ETC. 287RΔ/dσ dσ1/00.050.10.150.20.250.30.35  = 7 TeVsRΔ/dσ dσ1/-1CMS preliminary, 4.6 fbdataMadgraph R(B candidates)Δ0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5MCdata-MC-101MCdata-MCR Δ/dσ dσ1/00.050.10.150.20.250.30.35R Δ/dσ dσ1/ = 7 TeVs-1CMS preliminary, 4.6 fbdataaMC@NLO R(B candidates)Δ0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5MCdata-MC-101MCdata-MCFig. 1. – Z/γ∗+BB differential cross section as a function ΔR(BB) compared to LO (left) andNLO (right) Monte Carlo.vertex mass distribution, while the second is estimated from the upper sideband of them distribution and subtracted from the number of selected events (see eq. (1)).All the correction factors, except P and ftt¯ described above, are estimated from MC,after having reweighted it to match the lepton and b-tagging efficiencies measured indata and the pile-up (multiple proton-proton interactions in the same bunch crossing)conditions observed in the 2011 data taking.The result, combining the ee and the μμ channels, is 5.84 ± 0.08(stat.) ±0.72(syst.)+0.25−0.55(theory) pb. The MCFM [2] NLO prediction, corrected for non-perturbative effects, is 3.97 ± 0.47 pb, so the measured value is found to be larger thanthe predictions. The reason for this discrepancy is still under study.2. – Angular correlation between BB-pair with Z/γ∗+ secondary vertices [3]The angular correlation between the BB-pair may help to better understand themechanisms that lead to the production of a Z boson and a pair of B-hadrons at theLHC (b-quarks fusion, gluon splitting, multiple parton interactions).The analysis reported here relies on the identification of displaced secondary verticeswith no use of jets, that allows to study the BB production even at small angularseparations.The measured quantity is the differential cross section in ΔR(BB) bins that is com-pared to LO and NLO predictions. The result is shown in fig. 1 from which it is clearthat none of the two MC can describe the trend of the data in the full ΔR range.Further studies are needed to shed light on this.REFERENCES[1] The CMS Collaboration, CMS-EWK-11-012.[2] Campbell J. M., Ellis R. K., Maltoni F. et al., Phys. Rev. D, 69 (2004) 074021.[3] The CMS Collaboration, CMS-EWK-11-015.

Measurements of the associated production of vector bosons

and heavy flavours with the CMS detector

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Measurements of the associated production of vector bosons and heavy flavours with the CMS detector

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