Measurements of
Non-photonic Electrons
at STAR
Workshop on Heavy Flavor Production in HighEnergy Nuclear Collisions
June 17-18, 2012
University of Illinois at Chicago
Mustafa Mustafa
Purdue University
What are & why Non-photonic Electrons?
D0
Non-photonic Electrons (NPE) are
“single” electrons/positrons from open
heavy flavor semileptonic decay
channels (produced with
(anti)neutrinos in weak decays).
K
+
e-/-
l
K
-
D0
Non-photonic
electron

Open heavy flavor
NPE can be used as proxies for studying heavy quarks. Although, one does not have
direct access to heavy quarks kinematics through NPE, the higher branching ratios and
ability to trigger on high pT electrons make NPE a very good tool to studying heavy
quarks production in different collision systems and at different centralities.
•
•
•
𝐵 0/𝐵+ → 𝑒+ + 𝑋: 𝐵𝑅: ~10%
𝐷 0 → 𝑒 + + 𝑋:
𝐵𝑅: ~7%
+
+
𝐷 → 𝑒 + 𝑋:
𝐵𝑅: ~17%
•
•
•
•
𝐵 + → 𝐾 + + 𝐽 𝜓 → 𝑒𝑒 :
𝐵 0 → 𝐾𝜋:
𝐷 0 → 𝐾𝜋:
𝐷 + → 𝐾𝜋𝜋:
UIC HF Workshop - Mustafa Mustafa
𝐵𝑅: ~6 × 10−5
𝐵𝑅: ~5 × 10−6
𝐵𝑅: ~4%
𝐵𝑅: ~9.4% (lower acceptance)
2
Analysis Technique – Background Sources
The trick is to find all electrons and then subtract all the uninteresting ones.
Most of the “uninteresting” electrons are coming from photonic sources (Photonic
Electrons).

0
   e
    e


  e e

 e
 e


BR: 1.2%.
BR: 0.7%.
Same for all experiments

•Mostly from  0 ( )    
•Conversion probability: 7/9* X0
Depends on material budget
Other sources of background include:
ρ,ω,Φ Dalitz decays (Photonic).
Drell-Yan, Charmonium, etc… (Non-photonic).
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Analysis Technique – Background Subtraction
At STAR, we use the “Reconstruction
Method” to find out the contribution of
Photonic electrons.
Unlike
Like
Unlike - Like
In this method we reconstruct the
invariant mass of e+e- pairs. The invariant
mass from photon conversion and Dalitz
decays has a maximum near 0 mass.
N ( npe )  
purity
)
* N ( inclusive
N ( photonic
)
 reco
εpurity : purity of inclusive electrons
sample. Calculated from data.
εreco :
photonic electrons reconstruction
efficiency. Calculated from embedding.
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Analysis Technique – Experimental Setup
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Analysis Technique – Electrons Identification
Low pT ( 0.2-2.0 GeV/c):
TPC dEdx + TOF
The combination of TPC dEdx and β
from TOF provides +95% purity down to
the lowest reachable pT at STAR
(0.2GeV/c) .
High pT ( > 1 GeV/c):
TPC dEdx + BEMC
1- Associating TPC tracks with BTOW
and BSMD clusters.
Unlike
Like
Unlike - Like
2- P/E0 cuts. (Due to their negligible
mass, electrons have P/E0 ~ 1).
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NPE Measurements – Spectra in p + p at √s = 200 GeV
• NPE measurements in p+p serve as a
baseline for the interpretation of the
modification of heavy quarks
production in the presence of the hot
dense partonic medium in Au+Au
collisions.
• Comparison of measurements in p+p to
theoretical predictions is important to
validate the current theoretical
frameworks and its phenomenological
inputs. Cacciari, et. al.PRL.95.122001
STAR Phys. Rev. D 83 (2011) 052006
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NPE-Hadron Correlation in p + p at √s = 200 GeV
Due to the decay kinematics and difference in
mass, the azimuthal correlations between the
daughter electron and daughter hadron are
different for bottom meson decays and charm
meson decays. This has been exploited to obtain
the ratio of the bottom electron contribution to the
NPE yield.
STAR PRL 105 (2010) 202301
The result from NPE-Hadron correlation has been
combined with the NPE spectra measurement to
disentangle the contribution of charm and bottom
contribution to NPE spectra.
STAR Phys. Rev. D 83 (2011) 052006
As will be discussed later in this presentation.
Separating the bottom and charm contributions to
NPE is a crucial measurement for understanding
the energy loss of heavy quarks in the hot dense
partonic QCD medium in Au+Au collisions.
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Measurements – Spectra in Au + Au at √s = 200 GeV
Published results. Measurements in most
central to peripheral collisions at
midrapidity and mid-high pT.
STAR PRL 106 (2011) 159902
Using TOF electron identification power,
final results from ongoing analysis will
extend the spectra to low pT in addition to a
highly improved measurement extending
the spectra to higher pT .
arXiv:1108.5467v1
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Nuclear Modification Factor (RAA)
in Au + Au at √s = 200 GeV
Nuclear Modification Factor (RAA)
measurement:
STAR PRL 106 (2011) 159902
PHENIX arXiv:1005.1627v2
is used to compare the production of heavy
quarks in the presence of the hot dense
partonic medium to that in vacuum (p+p
collisions).
• Binary scaling is observed which
confirms that charm is mostly produced
though initial parton hard scattering and
that thermal and pre-thermal charm
production are insignificant at RHIC
energies and collisions systems.
Lévai et. al. PRC 56 (1997) 2707
• Heavy suppression of NPE is
observed. The suppression is
comparable to that of charged hadrons.
More on this in the following slides.
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Suppression and Energy Loss Mechanisms
K+
Hot/Dense Medium
u
c quark
e-/-
c
l
D0
Gluon radiation and the dead
cone effect. Suppressed at
θ < MQ/EQ
radiative energy loss
(Baier et al, Kharzeev et al, Djordjevic et al, Wiedemann et al . )
K+
Hot/Dense Medium
u
c quark
e-/-
c
l
D0
Collisional energy loss. Heavy
quarks lose energy through
elastic collisions with other
partons.
collision energy loss
(Teaney et al, Rapp et al, Molnar et al, Gossiaux et al.)
K+
Hot/Dense Medium
c quark
u
e-/-
c
D0
meson energy loss
Vitev, et al
D0
l
Collisional Dissociation.
Medium induced dissociation
of heavy mesons.
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Suppression and Energy Loss Mechanisms
DGLV:
Djordjevic, PLB632, 81
(2006)
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Suppression and Energy Loss Mechanisms
DGLV:
Djordjevic, PLB632, 81
(2006)
BDMPS:
Armesto, et al.,PLB637, 362
(2006)
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Suppression and Energy Loss Mechanisms
DGLV:
Djordjevic, PLB632, 81
(2006)
BDMPS:
Armesto, et al.,PLB637, 362
(2006)
T-Matrix:
Van Hees et al.,
PRL100,192301(2008).
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Suppression and Energy Loss Mechanisms
DGLV:
Djordjevic, PLB632, 81
(2006)
BDMPS:
Armesto, et al.,PLB637, 362
(2006)
T-Matrix:
Van Hees et al.,
PRL100,192301(2008).
Coll. Dissoc.
R. Sharma et al., PRC 80,
054902(2009).
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Suppression and Energy Loss Mechanisms
DGLV:
Djordjevic, PLB632, 81
(2006)
BDMPS:
Armesto, et al.,PLB637, 362
(2006)
T-Matrix:
Van Hees et al.,
PRL100,192301(2008).
Coll. Dissoc.
R. Sharma et al., PRC 80,
054902(2009).
Ads/CFT:
W. Horowitz Ph.D thesis.
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NPE Elliptic Flow in Au+Au √s = 200 GeV
Flow of Non-photonic electrons is an important tool to study the degree of
thermalization of heavy quarks in the hot dense partonic medium.
Non-Photonic Electron angular
azimuthal correlations with the
event plane for 10 40% central
collisions.
The preliminary results of NPE
v2 in various centrality bins.
arXiv:1108.5467v1
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Summary
• The study of Non-photonic Electrons in p+p and Au+Au collisions provide
indirect access to heavy quarks.
• NPE spectra measurements in p+p validated and enriched the current heavy quarks
production theoretical framework and its phenomenological inputs.
• Measurement of NPE-hadron correlation in p+p allowed a first attempt at
disentangling charm and bottom mesons contribution to NPE, and extracting their
separated cross-sections.
• Measurements of NPE spectra validated parton hard-scattering as the main
mechanism for heavy quarks production in Au+Au at RHIC energies.
• Measurement of NPE RAA shed light on flavor dependence of energy loss in hot
dense QCD medium. It also posed serious challenges the proposed energy loss
mechanisms.
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