Measurement of the b-jet to inclusive jet ratio in PbPb and pp collisions at 2.76 TeV with the CMS detector

Modification to jets in high-energy heavy-ion collisions is expected to depend on the flavor of the fragmenting parton. To disentangle this flavor dependence, jets from heavy quark fragmentation are identified for the first time in heavy ion collisions. Jets are first tagged by their secondary vertices and the contribution from bottom quarks is extracted using template fits to their secondary vertex mass distributions. The bottom quark jet to inclusive jet ratio is measured with the CMS detector from PbPb and pp collisions at a center-of-mass energy of 2.76 TeV per nucleon. This b-jet fraction is measured in the range of $80 < $ jet $p_{\mathrm{T}} < 200$ ${\ensuremath{{\,\text{Ge\hspace{-.08em}V\hspace{-0.16em}/\hspace{-0.08em}}c}}\xspace}$ in PbPb collisions and found to lie in the range of 2.9--3.5\% $\pm$ 0.6--1.1\% depending on jet $p_{\mathrm{T}}$. The measured values are comparable to those predicted by {\sc{pythia}} simulation. The PbPb b-jet fraction is also compatible with the pp b-jet fraction, within sizeable uncertainties. The measurement is sufficiently precise to demonstrate that b-jets are subject to jet quenching, although a precise comparison of light and b-jet quenching would require a reduction of the statistical and systematic uncertainties. %This note also describes customizations made to heavy-ion reconstruction chain to enhance the efficiency for secondary track reconstruction and the commissioning of the b-tagging tools for the heavy-ion environment.