1. Strangeness in Quark Matter 2007
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.

2. Outline Reminder of two major results from RHIC: The new contribution:
non-photonic electrons and the heavy quark energy loss ”puzzle”;
enhancement of baryons/mesons
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

3. Quark energy loss observed @RHIC
High pT suppression has been one of the most clear observables to sign the dense medium in Au+Au collisions.
PHENIX (PRL )
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

4. Non-photonic electron RAA @ RHIC
Charm and beauty hadrons are not yet One way out: the non-photonic electrons RAA !
PHENIX (PRL )
STAR (PRL )
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);
Sébastien Gadrat, SQM07

5. 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/ , van Hees & Rapp, PRC 73(2006)034913)
heavy quark fragmentation and dissociation in the medium → strong suppression for charm and bottom (Adil & Vitev, hep-ph/ )
R. Averbeck, exp. summary
Sébastien Gadrat, SQM07 5
Ralf Averbeck,

6. 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 coalescence is expected to dominate up to pT ~ 4-6 GeV/c Greco et al., Fries et al, PRL 90 (‘03).
Sébastien Gadrat, SQM07

7. 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 ?
Sébastien Gadrat, SQM07

8. Consequences of a c/D enhanced ratio on the RAA
D0, D0
D+, D-
Ds+, Ds-
c+, c-
BR (Xe) in %
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 !
Sébastien Gadrat, SQM07

9. First study of this effect done by P. Sorensen and X. Dong
PRC 74 (2006) , 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:
enhancement effect for low pT: ≾ pT ≾ 5 GeV/c;
high value for the max c/D ratio required (~ 1.7 taken from the /Ks0);
suppression less than 20%.
New RAA including c/D effect
~20%
Sébastien Gadrat, SQM07

10. 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
from Pythia
Maximum of the c/D ratio
~ 1.7 (from the /Ks0) 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/
Beauty contribution No
Yes
*qualitative agreement with coalescence models for HQ (Greco, Quenching Day, INFN, 2005).
Sébastien Gadrat, SQM07

11. The Pythia simulation
Simulation of charm and beauty using PYTHIA and the PHENIX settings from PRL ;
Relatively good agreement with the data over all the spectrum.
Sébastien Gadrat, SQM07

12. Pythia simulation compared to FONLL
Pythia Charm
FONLL
Pythia Beauty
FONLL
Pythia charm slope is softer than expectation from FONLL but still compatible within its uncertainties (M. Cacciari, et al. PRL 95 (2005) );
Pythia beauty slope is in good agreement with FONLL predictions.
Sébastien Gadrat, SQM07

13. 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 !
Sébastien Gadrat, SQM07

14. 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).
Sébastien Gadrat, SQM07

15. Electrons suppression from a c/D enhanced ratio
A c/D ~ 1 gives a suppression of 40% for pT between 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 GeV/c in this simple case.
Sébastien Gadrat, SQM07

16. Including charm energy loss (rad. and coll. ) from S. Wicks et al
Including charm energy loss (rad. and coll.) from S. Wicks et al., nucl-th/
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.
Sébastien Gadrat, SQM07

17. The beauty contribution effect on the RAA
b+c  e-
Due to the “dead cone” effect, beauty quarks are less suppressed and thus increase the final RAA, M. Djordjevic, SQM06.
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)
b/c crossing point
GeV/c2
Sébastien Gadrat, SQM07

18. Beauty contribution to the RAA
c+b, EnLoss+EnFactor, cp 10.5 GeV/c
c+b, EnLoss+EnFactor, cp 4.5 GeV/c
2 hypothesis studied :
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.
Sébastien Gadrat, SQM07

19. 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, beauty mesons and c separate measurements.
Sébastien Gadrat, SQM07

20. BackUp Slides
Sébastien Gadrat, SQM07

21. V. Greco, Quenching day, INFN, 2005
Baryon/meson
from coalescence
Quarkonia v2
from regeneration
pQCD
statistical
Contamination in single e : v2Lc > v2D
BR to single electrons 4.5%
-> contamination negligible
cut if one can verify those prediction …
Sébastien Gadrat, SQM07

22. Yield of charm hadrons from Pythia
D0, D0
D+, D-
Ds+, Ds-
c+, c-
Yield in %
67.5 21 11
>> 05
Sébastien Gadrat, SQM07