Ⅱ-4. Hadron Physics
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Centrality dependence of charm and bottom quark suppressionin Au + Au collisions at RHIC†
T. Hachiya∗1,∗2 for the PHENIX Collaboration
Charm and bottom quarks, collectively referred asheavy ﬂavor, are a clean probe to study the propertiesof quark-gluon plasma (QGP) produced in high-energy
heavy-ion collisions. Because of their large mass, heavy
quarks are predominantly produced at the initial stageof the collisions. Once produced, heavy ﬂavors lose theirenergy as they propagate through the QGP. The energy loss of heavy quarks is expected to be suppressedby “dead cone” eﬀect, where gluon radiation caused bybremsstrahlung is suppressed at an angle smaller thanthe mass-to-energy ratio of the quark.1) Thus, energy
loss is expected to follow the mass ordering of quarks andgluons, ∆Eg > ∆Eu,d > ∆Ec > ∆Eb . The PHENIX experiment previously observed the diﬀerent suppression ofelectrons from charm and bottom hadron decays in min√imum bias Au + Au collisions at sN N = 200 GeV.2)
This article reports the centrality dependence of charmand bottom quark suppression measured at PHENIX.
Using the high-statistics dataset recorded in 2014 andthe updated p + p reference from 2015,3) the nuclear
modiﬁcation factor, RAA (a ratio of yields in Au + Au top + p after normalized by number of binary collisions) of
the charm and bottom electrons in four centrality classes(0–10, 10–20, 20–40 and 40–60%) of Au + Au collisions
can be measured with improved precision compared withour previous results.2)
Heavy ﬂavor electrons are measured using the central
arm PHENIX detector. An inner silicon tracker, VTX,
measures the distance of closest approach of the electron
track, DCAT , to the collision vertex in the transverse
plane. Using the diﬀerent decay lengths of charm andbottom hadrons (cτ = 123 µm for D0 and 455 µm forB 0 ), electrons from these decays are statistically separated by the DCAT . The measured electron samples
contain not only heavy ﬂavors but background electrons.
The main background is photonic electrons which arephoton conversions and Dalitz decays of light neutralmesons. The background is mostly rejected by the analysis cut with the hit-pair on VTX as the electron pairs
from the photon conversions produce hits that are close
to each other. The remaining backgrounds are estimatedthrough the full GEANT detector simulation and subtracted from the measured electron samples. The electron samples are separated by the unfolding, which simultaneously ﬁts the transverse momentum pT spectrum
and DCAT distributions.
Figure 1 shows RAA in 0–10% central Au + Au collisions. The blue and green lines represent the bottomand charm electrons with 1-sigma uncertainty bands,Fig. 1. RAA for charm and bottom electrons compared withthe theoretical models.

respectively. Signiﬁcant suppression is seen for both
charms and bottoms at high pT . Charm suppression isstronger than bottom suppression for pT = 2–5 GeV/c.
The result was compared with the models that expect amass ordering of energy loss in QGP. All models reproduce the data reasonably within large uncertainty.
The centrality dependence of the suppression is studied using the number of nucleon participants in the collision (Npart ). Figure 2 shows RAA vs Npart for three
diﬀerent pT intervals. There is no suppression for bothcharms and bottoms in low pT . The mid-pT region showsa clear suppression of charm hadrons but no bottom suppression. The high-pT region shows an increasing suppression of both charms and bottoms.

Fig. 2. RAA charm and bottom electrons as a function ofNpart for three pT intervals.

In summary, PHENIX studied the centrality dependence of charm and bottom suppression. A clear diﬀerent suppression was observed at pT = 2.5 GeV/c. The
suppression pattern is consistent with the models that
expect mass ordering of energy loss.
References†∗1∗2

Condensed from the article in arXiv:.2023.17058
Department of Mathematical and Physical Sciences, Nara
Women’s University
RIKEN Nishina Center

1) Y. L. Dokshitzer et al., Phys. Lett. B 519, 199 (2001).
2) A. Adare et al., Phys. Rev. C 93, 034904 (2016).
3) C. Aidala et al., Phys. Rev. D 99, 10.1103 (2019).

- 51 -