Abstract
The decay Z→bb¯ is reconstructed in pp collision data, corresponding to 2 fb−1 of integrated luminosity, collected by the LHCb experiment at a centre-of-mass energy of s=8 TeV. The product of the Z production cross-section and the Z→bb¯ branching fraction is measured for candidates in the fiducial region defined by two particle-level b-quark jets with pseudorapidities in the range 2.2<η<4.2, with transverse momenta pT>20 GeV and dijet invariant mass in the range 45<mjj<165 GeV. From a signal yield of 5462±763Z→bb¯ events, where the uncertainty is statistical, a production cross-section times branching fraction of 332±46±59 pb is obtained, where the first uncertainty is statistical and the second systematic. The measured significance of the signal yield is 6.0 standard deviations. This measurement represents the first observation of the Z→bb¯ production in the forward region of pp collisions.
Highlights
Measurements of Z -boson production in pp collisions constitute an important test of the Standard Model (SM), since they allow the electroweak sector to be precisely probed [1,2,3]
The inclusive search for the SM Higgs decay to two b quarks at the LHC is of great interest, since the measurement of the Higgs boson coupling to b quarks is an important test of the SM [4]
A sizeable Z → bbevent sample will enable the measurement of the bbforward-central asymmetry at the Z pole, which could be enhanced by the contributions from new physics processes [8]
Summary
Measurements of Z -boson production in pp collisions constitute an important test of the Standard Model (SM), since they allow the electroweak sector to be precisely probed [1,2,3]. The Z → bbdecay is important to determine the so-called b-jet energy scale This is the factor that has to be applied to the reconstructed b-jet energy in simulated events in order to reproduce the actual detector response. The CDF collaboration reconstructed the Z → bbdecay in p pcollisions at 1.96 TeV and determined the b-jet energy scale, obtaining a relative uncertainty on the product of the cross-section and the branching fraction of 29%. The analysis of the ATLAS collaboration reconstructed boosted Z → bbcandidates in the central region of pp collisions at 8 TeV, with pseudorapidity |η| < 2.5, and determined the cross-section with a relative uncertainty of 16%. The low trigger thresholds on the particle energies that are employed at LHCb and the excellent b-jet identification performance make it possible to select candidates within a large invariant mass range, including those with masses below the Z -boson pole. The interaction of the generated particles with the detector, and its response, are implemented using the Geant toolkit [20] as described in Ref. [21]
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