Heavy flavor azimuthal correlations in cold nuclear matter

Abstract

Background: It has been proposed that the azimuthal distributions of heavy flavor quarkantiquark pairs may be modified in the medium of a heavy-ion collision. Purpose: This work tests this proposition through next-to-leading order (NLO) calculations of the azimuthal distribution, dσ/dφ, including transverse momentum broadening, employing k 2 T and fragmentation in exclusive QQ pair production. While these studies were done for p+p, p+p and p+Pb collisions, understanding azimuthal angle correlations between heavy quarks in these smaller, colder systems is important for their interpretation in heavy-ion collisions. Methods: First, single inclusive pT distributions calculated with the exclusive HVQMNR code are compared to those calculated in the fixed-order next-to-leading logarithm approach. Next the azimuthal distributions are calculated and sensitivities to k 2 T , pT cut, and rapidity are studied at √ s = 7 TeV. Finally, calculations are compared to QQ data in elementary p + p and p + p collisions at √ s = 7 TeV and 1.96 TeV as well as to the nuclear modification factor R pPb (pT ) in p+Pb collisions at √ sNN = 5.02 TeV measured by ALICE. Results: The low pT (pT < 10 GeV) azimuthal distributions are very sensitive to the kT broadening and rather insensitive to the fragmentation function. The NLO contributions can result in an enhancement at φ ∼ 0 absent any other effects. Agreement with the data was found to be good. Conclusions: The NLO calculations, assuming collinear factorization and introducing kT broadening, result in significant modifications of the azimuthal distribution at low pT which must be taken into account in calculations of these distributions in heavy-ion collisions.

bb¯ kinematic correlations in cold nuclear matter

“…These findings are in accord with the hadron-level studies of bb correlations in Ref. [1]. ATLAS compared their final-state J/ψ+lepton results with several event generators, finding good agreement between the simulations and the data [6].…”

“…As discussed in Ref. [1], the bottom quark mass, m b , factorization scale, µ F , and renormalization scale µ R and their uncertainties were set by comparison to the bb total cross section data with m b = 4.65 ± 0.09 GeV, µ F /m = 1.40 +0.77 −0.49 , and µ R /m = 1.10 +0. 22 −0.20 .…”

“…The value of ǫ P , the Peterson function parameter, was set by comparison to the FONLL B meson p T distribution in Ref. [1] while k 2 T , the average broadening, was fixed previously by comparing the measured Υ p T distributions to calculations of Υ production in the color evaporation model. Here, for bb production, ǫ p = 0.0004 ,…”

“…While such a measurement is less direct than reconstructed D or B mesons, as discussed in Ref. [1], along with comparison to DD pairs measured by CDF at √ s = 1.96 TeV [16] and LHCb at √ s = 7 TeV [17] and B hadron-b jet pairs measusred by CMS at √ s = 7 TeV [18], it allows a more straightforward comparison to calculations than the dilepton decay channel.…”

“…Methods: The calculations, employing the exclusive HVQMNR code, assume the same intrinsic kT -broadening and fragmentation as in Ref. [1]. The pair distributions presented by LHCb are calculated in this approach, both for the parent bb and the J/ψJ/ψ pairs produced in their decay.…”