Abstract
We present a measurement of b jet transverse momentum (pt) spectra in proton-lead (pPb) collisions using a dataset corresponding to about 35 inverse nanobarns collected with the CMS detector at the LHC. Jets from b quark fragmentation are found by exploiting the long lifetime of hadrons containing a b quark through tagging methods using distributions of the secondary vertex mass and displacement. Extracted cross sections for b jets are scaled by the effective number of nucleon-nucleon collisions and are compared to a reference obtained from PYTHIA simulations of pp collisions. The PYTHIA-based estimate of the nuclear modification factor is found to be 1.22 +/- 0.15 (stat + syst pPb) +/- 0.27 (syst PYTHIA) averaged over all jets with pt between 55 and 400 GeV/c and with abs(eta[lab]) < 2. We also compare this result to predictions from models using perturbative calculations in quantum chromodynamics.
Highlights
By colliding heavy nuclei at ultra-relativistic energies, sufficiently large energy densities are reached to form a quark–gluon plasma (QGP), a state which is characterized by an effective deconfinement of quarks and gluons [1,2]
The b jet pT spectra in pPb are shown on the left in Fig. 3 for several ηCM selections, along with cross sections from the pythia pp reference
1.22 ± 0.15 ± 0.27 provides the first direct evidence that cold nuclear matter effects do not play a major role in jet quenching in the PbPb system
Summary
By colliding heavy nuclei at ultra-relativistic energies, sufficiently large energy densities are reached to form a quark–gluon plasma (QGP), a state which is characterized by an effective deconfinement of quarks and gluons [1,2]. Hard-scattered partons have been predicted to suffer energy loss as they traverse the QGP, primarily via collisional and radiative processes [3,4]. This energy loss is commonly thought to be the mechanism responsible for the observed suppression of high transverse momentum (pT) hadrons and jets in nucleus–nucleus collisions relative to proton–proton (pp) collisions [5,6]. At high-pT, the CMS Collaboration has shown that b jet suppression in PbPb is consistent with that of light quark jets above 80 GeV/c [16]
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