Abstract
Measurements of differential production cross-sections of a $Z$ boson in association with $b$-jets in $pp$ collisions at $\sqrt{s}=7$ TeV are reported. The data analysed correspond to an integrated luminosity of 4.6 fb$^{-1}$ recorded with the ATLAS detector at the Large Hadron Collider. Particle-level cross-sections are determined for events with a $Z$ boson decaying into an electron or muon pair, and containing $b$-jets. For events with at least one $b$-jet, the cross-section is presented as a function of the $Z$ boson transverse momentum and rapidity, together with the inclusive $b$-jet cross-section as a function of $b$-jet transverse momentum, rapidity and angular separations between the $b$-jet and the $Z$ boson. For events with at least two $b$-jets, the cross-section is determined as a function of the invariant mass and angular separation of the two highest transverse momentum $b$-jets, and as a function of the $Z$ boson transverse momentum and rapidity. Results are compared to leading-order and next-to-leading-order perturbative QCD calculations.
Highlights
Background estimation and reductionSelected events in data contain the signal of interest as well as various background processes with either real or fake leptons and real or fake b-jets
The b-jet, c-jet and light-jet Z+jets yields are allowed to float in the fit, while all non-Z+jets backgrounds are combined into a single template whose normalisation is determined from the sum of their predicted contributions and fixed in the fit; the multijet yields and shapes in this sample are extracted in a fashion analogous to that used for the 1-tag events
The prediction obtained with CT10 is lower, due primarily to the default choice of αS(m(Z)) in this parton density function (PDF) (0.118) compared to MSTW2008 and NNPDF2.3 (0.120 in each)
Summary
The ATLAS experiment [16] is a multi-purpose particle detector with large solid angle coverage around one of the interaction regions of the LHC. It consists of an inner tracking detector surrounded by a superconducting solenoid providing a 2 T axial magnetic field, followed by electromagnetic and hadronic calorimeters and a muon spectrometer with three superconducting toroid magnets. The inner detector (ID) is made up of a high-granularity silicon pixel detector, a silicon microstrip tracker, and a straw-tube transition radiation tracker These provide measurements of charged particles in the region |η| < 2.5. Two software-based trigger levels follow, which reduce the event rate to about 300 Hz, for offline analysis
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