RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Heavy-quark measurement using distance of closest approach analysis
R. Akimoto,∗1,∗2 Y. Akiba,∗1 H. Asano,∗1,∗3 T. Hachiya,∗1 M. Kurosawa,∗1 M. Shimomura,∗4 and the PHENIXVTX group

Owing to the optimization, the purity of heavy-quark
electrons in inclusive electrons is increased to morethan 50% where pT is larger than 1 GeV/c. The
left panel of Fig. 1 shows the S/N ratio, which is defined as the yield of heavy-quark electrons divided bythat of the others. The solid and open circles represent the S/N ratios as a function of electron pT withand without the isolation cut, separately. The contribution from heavy quarkonia was evaluated by using
their cross sections2) . The right panel of Fig. 1 shows
the yield fractions of electron components in inclusiveelectrons. The brown points represent the contributions of heavy-quarkonium decay. The contributionof heavy quarkonia is ∼15% in inclusive electrons at
∗1
∗2
∗3
∗4a)
b)

RIKEN Nishina Center
Center for Nuclear Study, Graduate School of Science, University of Tokyo
Kyoto University
Iowa State UniversityElectrons and positrons from heavy-quark decay are called
heavy-quark electrons.
The XY plane is defined as the plane perpendicular to thebeam axis, and the Z direction is defined as the direction
along the beam axis.

- xxv -

fraction in total

4w/o isolation cut
with isolation cut

32

1.0

heavy quark
π0 Dalitz decay
photon conversion

heavy quarkonia

0.5

10
12

3

45
electron p [GeV/c]

0.0
12

3T

45
electron p [GeV/c]
TFig. 1. S/N ratio (left) and yield fractions of electron components in inclusive electrons (right). The black, red,green, and brown points in the right panel represent
the fractions of heavy-quark electrons, electrons fromπ 0 Dalitz decay, electrons from photon conversion, andelectrons from decay of heavy quarkonia, respectively.

Figure 2 shows a comparison of the bottom fractionsin heavy-quark electrons as a function of electron pT
beforeand after the updates for p+p collisions with√s = 200 GeV. The open and solid circles indicate
the fractions before and after the updates, respectively.
The result after the updates is consistent with that
before the updates. The increase in the systematicerror is due to the error from the uncertainties of pT
distributions of charm and bottom quarks.
b→ e/(c→ e+b→ e)• Optimization of cut parameters for the isolation
cut, which is explained in ref.1) .
• Evaluation of contributions of electrons fromheavy quarkonia such as J/ψ.
• Evaluation of a systematic error from uncertainties of transverse momentum (pT ) distributions ofcharm and bottom quarks.

pT > 3 GeV/c. The systematic error from the uncertainties of pT distributions of charm and bottomquarks was evaluated using the distributions obtained
from a PYTHIA simulation and FONLL calculation3)
and was found to be ∼50% for all pT ranges.
S/N ratio

Heavy quarks (charm and bottom) can be used asprobes to study the interaction between partons anda quark-gluon plasma (QGP). Heavy quarks are created by initial hard scattering, and thus, the changes
intheir properties when passing through the QGP canbe clearly extracted from their final states.
Separate measurements of modifications for charmand bottom quarks are informative because the dependence of the modification on the quark mass canbe evaluated. The separation of charm and bottom quarks can be achieved by the analysis of thedistances between the tracks and the beam collisionvertex (DCA). The DCA distributions of bottomed
hadrons are wider than those of charmed hadrons because the lifetimes of bottomed hadrons are considerably longer than those of charmed hadrons.
In this study, electrons and positrons from heavyquark decay were measureda) , and the yields of charmand bottom quarks were evaluated by fitting the DCA
distribution in the XY planeb) . We updated the following items from the previous results reported in ref.1) :

1.0
0.8
0.6

p+p at s=200 GeV
FONLL (y=0)
FONLL error (y=0)
After update
Before update

0.4
0.2
0.0246electron p [GeV/c]
TFig. 2. Bottom fractions in heavy-quark electrons. The
open and solid circles indicate the fractions before andafter the updates. The bars and squares represent statistical and systematic errors, respectively. The solidline indicates the result of FONLL calculation at rapidity y = 0, and the dashed lines indicate the boundariesof the error band for the calculation.3)

References
1) R. Akimoto et al.: RIKEN Accel. Prog. Rep. 46, 60(2012).
2) A. Adare et al.: Phys. Rev. D 85, 092004 (2012).
3) M. Cacciari et al.: Phys. Rev. Lett. 95, 122001 (2005).