CMS-HIN-16-016

CMS-HIN-16-016 ; CERN-EP-2018-264
Studies of beauty suppression via nonprompt ${\mathrm{D^0}}$ mesons in PbPb collisions a ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV
CMS Collaboration
25 October 2018
Phys. Rev. Lett. 123 (2019) 022001
Abstract: The transverse momentum spectra of ${\mathrm{D^0}}$ mesons from b hadron decays are measured at midrapidity ( $\| y \| <$ 1) in pp and PbPb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV with the CMS detector at the LHC. The ${\mathrm{D^0}}$ mesons from b hadron decays are distinguished from prompt ${\mathrm{D^0}}$ mesons by their decay topologies. In PbPb collisions, the ${\mathrm{B}\to{\mathrm{D^0}}}$ yield is found to be suppressed in the measured ${p_{\mathrm{T}}}$ range from 2 to 100 GeV/ $c$ as compared to pp collisions. The suppression is weaker than that of prompt ${\mathrm{D^0}}$ mesons and charged hadrons for ${p_{\mathrm{T}}}$ around 10 GeV/ $c$ . While theoretical calculations incorporating partonic energy loss in the quark-gluon plasma can successfully describe the measured ${\mathrm{B}\to{\mathrm{D^0}}}$ suppression at higher ${p_{\mathrm{T}}}$ , the data show an indication of larger suppression than the model predictions in the range of 2 $< {p_{\mathrm{T}}} <$ 5 GeV/ $c$ .
Links: e-print arXiv:1810.11102 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: a) Example of a three-component invariant mass fit of a ${{\mathrm {D^0}}}$ DCA bin for ${p_{\mathrm {T}}}$ of 6-7 GeV/ $c$ in PbPb collisions; b) DCA distributions for ${{\mathrm {D^0}}}$ candidates in the signal invariant mass region and in the sidebands (scaled by the mass range ratio of 0.5) for ${{\mathrm {D^0}}}$ ${p_{\mathrm {T}}}$ of 6-7 GeV/ $c$ in pp collisions; c) Signal DCA distribution obtained with the invariant mass fit for each DCA bin, and a prompt+nonprompt two-component fit to it, for ${{\mathrm {D^0}}}$ ${p_{\mathrm {T}}}$ of 6-7 GeV/ $c$ in PbPb collisions; d) Signal DCA distribution obtained with the sideband subtraction, and a prompt+nonprompt two-component fit to it, for ${{\mathrm {D^0}}}$ ${p_{\mathrm {T}}}$ of 6-7 GeV/ $c$ in pp collisions.
png pdf	Figure 2: Upper panel: ${{{\mathrm {B}}}\to {\mathrm {D^0}}}$ ${p_{\mathrm {T}}}$ -differential cross section in pp collisions and invariant yield in PbPb collisions normalized with ${T_{\mathrm {AA}}}$ , at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV. The vertical bands around the data points represent the bin-by-bin systematic uncertainties. Uncertainties are smaller than the symbols in most cases. The cross section in pp collisions is compared to FONLL calculations [33]. Lower panel: The $\text {data}/\mathrm {FONLL}$ ratio for the ${{{\mathrm {B}}}\to {\mathrm {D^0}}}$ ${p_{\mathrm {T}}}$ spectra in pp collisions.
png pdf	Figure 3: The ${{{\mathrm {B}}}\to {\mathrm {D^0}}}$ nuclear modification factor ${R_{\mathrm {AA}}}$ for PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV (red circles) compared to other particles [18,19,16,24] (upper panel), and to various theoretical predictions [34,35,36,37,38,39,40,41] (lower panel). The vertical bands around the data points and at unity represent the bin-by-bin and global systematic uncertainties, respectively.

Summary

In summary, this letter presents the transverse momentum spectra of

${\mathrm{D^0}}$ mesons from b hadron decays measured in pp and PbPb collisions at a center-of-mass energy

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV per nucleon pair with the CMS detector at the LHC. The

${\mathrm{D^0}}$ mesons from b hadron decays are distinguished from the prompt

${\mathrm{D^0}}$ mesons by the distance of closest approach of the

${\mathrm{D^0}}$ path relative to the primary vertex. The measured spectrum in pp collisions is close to the upper limit of a Fixed-Order Next-to-Leading Logarithm perturbative quantum chromodynamics calculation. In PbPb collisions, the

${\mathrm{B}\to{\mathrm{D^0}}}$ yield is suppressed in the measured transverse momentum (

${p_{\mathrm{T}}}$ ) range from 2 to 100 GeV/

$c$ . The

${\mathrm{B}\to{\mathrm{D^0}}}$ nuclear modification factor

$R_{\mathrm{AA}}$ is higher than for prompt

${\mathrm{D^0}}$ mesons and charged hadrons around 10 GeV/

$c$ , which is in line with a quark mass ordering of suppression. Compared to theoretical predictions, the measured

$R_{\mathrm{AA}}$ is consistent with some models at higher

${p_{\mathrm{T}}}$ , but shows a hint of stronger suppression than all of the available models at low

${p_{\mathrm{T}}}$ . This could indicate a stronger energy loss of b quarks in the quark-gluon plasma than predicted at low

${p_{\mathrm{T}}}$ , or could reflect an enhanced b baryon production due to quark coalescence in PbPb collisions.

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