A new contribution to the nuclear modification
factor of non-photonic electrons
Strangeness in Quark Matter 2007
Levoča, Slovakia
24 – 29 June 2007
Sébastien Gadrat, Subatech, Nantes.
Outline
 Reminder of two major results from RHIC:
 non-photonic electrons RAA @RHIC and the heavy quark energy loss
”puzzle”;
 enhancement of baryons/mesons ratio @RHIC.
 The new contribution:
 enhancement of the c/D ratio;
 effect on the non-photonic electrons RAA.
 Including all ingredients to the non-photonic electrons RAA:
 c/D enhancement effect;
 quark energy loss;
 beauty contribution.
 Summary
Sébastien Gadrat, SQM07
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Quark energy loss observed @RHIC
High pT suppression has been one of the most clear observables to sign the
dense medium created @RHIC in Au+Au collisions.
PHENIX (PRL 91 072303)
The suppression is indeed
attributed to final state effect:
partonic energy loss in a dense
medium.
What can we expect for heavy quarks ?
 heavy quarks are also expected to lose energy at the partonic level but
should lose less energy than light quarks due to the “dead cone” effect (Phys.
Lett. B 519, 199 (2001)) .
Sébastien Gadrat, SQM07
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Non-photonic electron RAA @ RHIC
Charm and beauty hadrons are not yet measured @RHIC.
One way out: the non-photonic electrons RAA !
PHENIX (PRL 96 032301)
STAR (PRL 98 192301)
Suppression observed is much larger than expected one:
observed RAA ~ 0.2 (for pT ≳ 4-5 GeV/c), compatible with hadrons suppression !
 expected RAA from 0.5 to 0.2 depending of models and parameters used (the
description of the suppression is difficult);
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RAA of electrons from heavy flavor decays
 PHENIX & STAR: rough agreement
→ disagreement is common to p+p & Au+Au, cancels in
the nuclear modification factor RAA
A. Suaide
 describing the suppression is
difficult for models
– radiative energy loss with typical gluon
densities is not enough
(Djordjevic et al., PLB 632(2006)81)
– models involving a very opaque medium
agree better
(Armesto et al., PLB 637(2006)362)
– collisional energy loss / resonant elastic
scattering
(Wicks et al., nucl-th/0512076,
van Hees & Rapp, PRC 73(2006)034913)
R. Averbeck, exp. summary
– heavy quark fragmentation and
dissociation in the medium
→ strong
talk@QM06
suppression for charm and bottom
(Adil & Vitev, hep-ph/0611109)
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Ralf Averbeck,
Fragmentation vs Coalescence
In vacuum, hadrons are produced via parton fragmentation mechanism.
In medium, a new effect contribute to hadron production: coalescence/recombinaison.
 Parton fragmentation
 requires energetic parton;
 B/M controlled by the
fragmentation function.
 coalescence/reco
 requires “near-by”
partons;
 B/M depends on the
phase space density.
For Au+Au collisions @RHIC, coalescence is expected to dominate up to
pT ~ 4-6 GeV/c Greco et al., Fries et al, PRL 90 (‘03).
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Enhanced baryon/meson ratio @RHIC
Sarah Blyth, QM06
 ratio B/M become larger at intermediate pT;
 enhancement effect also seen for heavier
hadrons (like Ω/Φ);
 maximum of the enhancement is shifted in
higher pT for heavier hadrons.
 quark coalescence models qualitavely
describe the data (PRC65, PRL90, PRC68,
PRC67, JPG30, PRC70).
 Quark coalescence might be the
dominant production mechanism at
intermediate pT in A+A collisions.
What if B/M for charm is enhanced, then
is there any consequence on the RAA ?
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Consequences of a
BR (Xe)
in %
c/D enhanced ratio on the RAA
D0, D0
D+, D-
Ds+, Ds-
c+, c-
17.2  1.9
6.71  0.29
8 +6-5
4.5  1.7
BR(c  e anything) is smaller than any BR(D  e anything)
 This would lead to a « natural » single electrons suppression
with respect to p+p scaling !
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First study of this effect done by
P. Sorensen and X. Dong
PRC 74 (2006) 024902, SQM06 and HQ06.
Assumptions:
 use /Ks0 measured shapes as a
reference for c/D ones;
 the charm RAA is similar to light
hadrons RAA.
Results:
New RAA including
c/D effect
~20%
 enhancement effect for low pT:
2 ≾ pT ≾ 5 GeV/c;
 high value for the max c/D ratio
required (~ 1.7 taken from the /Ks0);
 suppression less than 20%.
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A more detailed study of the charm enhanced
B/M effect on the non-photonic RAA
Sorensen and Dong
Our study
c/D shape in Au+Au
as /Ks0
Gaussian*
c/D shape in p+p
as /Ks0
from Pythia
Maximum of the c/D ~ 1.7 (from the /Ks0)
ratio
for pT ~ 3 GeV/c
~ 1 for pT ~ 6 GeV/c
Energy loss
From the scaling of
the hadrons shape
From S. Wicks et al.,
nucl-th/0512076
Beauty contribution
No
Yes
*qualitative agreement with coalescence models for HQ (Greco, Quenching Day, INFN, 2005).
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The Pythia simulation
 Simulation of charm and beauty
using PYTHIA and the PHENIX
settings from PRL 88 192303;
 Relatively good agreement with
the data over all the spectrum.
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Pythia simulation compared to FONLL
Pythia Charm
Pythia Beauty
FONLL
FONLL
 Pythia charm slope is softer than expectation from FONLL but still
compatible within its uncertainties (M. Cacciari, et al. PRL 95 (2005) 122010);
 Pythia beauty slope is in good agreement with FONLL predictions.
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Electron spectra from c and D
c/D
(e  c)/(e  D)
Electron spectrum from c is softer than that from D.
 this will increase the electrons suppression for
intermediate and high pT when applying an enhancement of the
c/D ratio !
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Building the RAA with c/D enhancement
effect
2 different hypothesis considered for
the enhancement:
 constant (flat) over pT;
 gaussian (V. Greco, Quenching Day,
INFN, 2005).
Enhancement factor of 12  c/D ~ 1.
How to build the non-photonic electron RAA ?
RAA = (dN/dpT with nuclear effect) / (dN/dpT w/o any nuclear effect)
 hadrons and electrons pT spectra will be taken from the PYTHIA simulation;
 add nuclear effects (c/D enhancement, energy loss) and BR (from PDG).
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Electrons suppression from a c/D enhanced ratio
A c/D ~ 1 gives a suppression
of 40% for pT between 2 - 4
GeV/c in both:
- flat enhanced ratio;
- gaussian enhanced ratio.
A gaussian c/D ratio gives a
suppression max about 3 – 4
GeV/c  small suppression at
high pT.
 Suppression of 40% for pT 2 - 4 GeV/c in this simple case.
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Including charm energy loss (rad. and coll.) from
S. Wicks et al., nucl-th/0512076.
As already noticed, inluding
radiative and collisional charm
energy loss can reproduce the
PHENIX RAA (red symbols).
Finally, Adding an (gaussian)
enh. effect decreases even
more the final RAA (blue
symbols).
 Suppression from enhancement ratio is similar to that from
collisional energy loss.
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The beauty contribution effect on the RAA
Due to the “dead cone” effect,
beauty quarks are less suppressed
and thus increase the final RAA,
b+c 
eb
M. Djordjevic, SQM06.
b/c crossing point
p+p @200 GeV/c2
Nevertheless, the contribution of
the beauty is not well known and
the crossing point c/b in FONLL is
expected to occur somewhere in
between 2.5 GeV/c and ~10
GeV/c, M. Cacciari, et al. PRL 95
(2005) 122010.
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Beauty contribution to the RAA
2 hypothesis studied :
c+b,
c+b,EnLoss+EnFactor,
EnLoss+EnFactor,cpcp10.5
4.5 GeV/c
GeV/c
a crossing point c/b at 4.5 GeV/c
(central value predicted by FONLL)
RAA : 0.5  0.45 (10%)
a crossing point at 10.5 GeV/c
(highest value from FONLL)
RAA : 0.4  0.3 (25%)
 Smaller effect when taking into account the beauty contribution
but still significant, especially for a c/b crossing point of 10.5 GeV/c.
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Summary
 A c/D enhancement, as it is observed for p/+, /Ks0 and /, would decrease the
non-photonic electron RAA at intermediate pT.
 RAA : 1  0.6, which represents 40% of suppression.
 Considering rad. and coll. energy loss and c/D enhancement effect for charm
results in a huge electrons suppression (for pT 3 GeV/c).
 RAA : 0.3  0.2, which still represents 33% of the total suppression.
 Adding the beauty contribution increases the RAA but the c/D enhancement effect
(with c/D ~ 1) remains visible:
 RAA : 0.5  0.45 with a c/b cross. at 4.5 GeV/c, 10% of the suppression,
 RAA : 0.4  0.3 with a c/b cross. at 10.5 GeV/c, 25% of the suppression.
 It might also affect the electrons v2 because v2(c) ≻ v2(D) and might help to explain
the discrepancy between data and models.
 Detailed understanding definitively needs charm mesons,
Sébastien Gadrat,
SQM07
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beauty mesons and c separate
measurements.
BackUp Slides
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V. Greco, Quenching day, INFN, 2005
Baryon/meson
from coalescence
Quarkonia v2
from regeneration
pQCD
statistical
• Contamination in single e : v2c > v2D
• BR to single electrons 4.5%
-> contamination negligibleSébastien Gadrat, SQM07
cut if one can verify those prediction …
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Yield of charm hadrons from Pythia
Yield in %
D0, D0
D+, D-
Ds+, Ds-
c+, c-
67.5
21
11
>> 05
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