m. djordjevic 1 open questions in heavy flavor physics at rhic magdalena djordjevic the ohio state...
Post on 19-Dec-2015
219 views
TRANSCRIPT
M. Djordjevic 1
Open questions in heavy flavor physics at RHIC
Magdalena Djordjevic
The Ohio State University
M. Djordjevic 2
Quark Gluon Plasma
Form, observe and understand Quark-Gluon Plasma (QGP).
Heavy quarks (charm and beauty, M>1 GeV) are widely recognized as the cleanest probes of QGP.
High Energy Heavy Ion Physics
Heavy mesons not yet available, but they are expected soon!
N. Brambilla et al., e-Print hep-ph/0412158 (2004).
M. Djordjevic 3
Significant reduction at high pT suggests sizeable heavy quark energy loss!
Indirect probe- single electron suppression – is available
V. Greene, S. Butsyk, QM2005 talks J. Dunlop, J. Bielcik; QM05 talks
Can this be explained by the energy loss in QGP?
M. Djordjevic 4
Outline
Discuss the heavy quark energy loss mechanisms:
Heavy meson and single electron suppression results that come from the above mechanisms.
Open questions that can be addressed in the future RHIC experiments.
Radiative energy loss. Collisional energy loss.
M. Djordjevic 5
1) Initial heavy quark pt distributions
2) Heavy quark energy loss
3) c and b fragmentation functions into D, B mesons
4) Decay of heavy mesons to single e-.
From production to decay
D, B
1)
production
2)
medium energy loss
3)
fragmentation
c, b e-
4)
decay
M. Djordjevic 6
D mesons
, ’,
A
B
Initial heavy quark pt distributions
200S GeV
M. Cacciari, P. Nason and R.Vogt, Phys.Rev.Lett.95:122001,2005;
MNR code (M. L. Mangano, P.Nason and G. Ridolfi,
Nucl.Phys.B373,295(1992)).
R.Vogt, Int.J.Mod.Phys.E 12,211(2003).
M. Djordjevic 7
c
Medium induced radiative energy loss
To compute medium induced radiative energy loss for heavy quarks we generalize GLV method, by introducing both
quark M and gluon mass mg.
Caused by the multiple interactions of partons in the medium.
M. Djordjevic and M. Gyulassy, Nucl. Phys. A 733, 265 (2004).
M. Djordjevic 8
This leads to the computation of the fallowing types of diagrams:
++
nz
,n nq a
,n nq a
nz
,n nq a
,n nq a
nznz
,n nq a
,n nq a
Final Result to Arbitrary Order in Opacity (L/) with MQ and mg> 0
M. Djordjevic 9
Thickness dependence is closer to linear Bethe-Heitler like form. This is different than the asymptotic energy
quadratic form characteristic for light quarks.
The numerical results for induced radiative energy loss are shown for first order in opacity, for L= 5 fm, =1 fm.
M. Djordjevic 10M. D., M. Gyulassy and S. Wicks, Phys. Rev. Lett. 94, 112301 (2005).
Pt distributions of charm and bottom before and after quenching at RHIC
Before quenching After quenching
M. Gyulassy, P.Levai and I. Vitev, Phys.Lett.B538:282-288 (2002).
M. Djordjevic 11
Panels show single e- from FONLL M. Cacciari, P. Nason and R. Vogt, Phys.Rev.Lett.95:122001,2005
M. D., M. Gyulassy, R. Vogt and S. Wicks, Phys.Lett.B632:81-86,2006
Single electrons pt distributionsB
efor
e q
uen
chin
g
Aft
er q
uen
chin
g
Bottom dominate the single e- spectrum above 4.5 GeV!
M. Djordjevic 12
Single electron suppression as a function of pt
At pt~5GeV, RAA(e-) 0.70.1 at RHIC.
M. Djordjevic 13
Radiative energy loss is not able to explain the single electron data as long as realistic parameter values are taken into account!
1000gdN
dy
M. D. et al., Phys. Lett. B 632, 81 (2006)
Can single electron suppression be explained by the radiative energy loss in QGP?
Radiative energy loss predictions
with dNg/dy=1000
Disagreement!
M. Djordjevic 14
E. Braaten and M. H. Thoma, Phys. Rev. D 44, 2625 (1991).
M. H. Thoma and M. Gyulassy, Nucl. Phys. B 351, 491 (1991).
Collisional energy loss is negligible!
Conclusion was based on outdated assumptions (i.e. they used =0.2),
and assumed that dE/dL<0.5 GeV/fm is negligible.
Early work: Recent work:
Is collisional energy loss also important?
Collisional and radiative energy losses are comparable!
M.G.Mustafa,Phys.Rev.C72:014905,2005
A. K. Dutt-Mazumder et al.,Phys.Rev.D71:094016,2005
Will collisional energy loss still be important once finite size
effects are included?
Above computations are done in an ideal infinite QCD medium.
M. Djordjevic 15
Radiative energy loss Collisional energy loss
Collisional energy loss comes from the processes which have the same number of incoming and outgoing particles:
Radiative energy loss comes from the processes which there are more outgoing than incoming particles:
0th order
1st order
0th order
M. Djordjevic 16
The main order collisional energy loss is determined from:
L
Collisional energy loss in a finite size QCD medium
The effective gluon propagator:
Consider a medium of size L in thermal equilibrium at temperature T.
M. Djordjevic 17
Comparison between computations of collisional energy loss in finite and infinite QCD medium
Finite size effects are not significant, except for very small path-lengths.
M.D., nucl-th/0603066
M. Djordjevic 18
Bottom quark collisional energy loss is significantly smaller than charm energy loss.
M.D., nucl-th/0603066
Comparison between charm and bottom collisional energy loss
M. Djordjevic 19
Collisional v.s. medium induced radiative energy loss
Collisional and radiative energy losses are comparable!
M.D., nucl-th/0603066
Complementary approach by A. Adil et al., nucl-th/0606010: consistent results obtained.
M. Djordjevic 20
Heavy quark suppression with the collisional energy loss
The collisional energy loss significantly changes the charm and bottom suppression!
CHARM
BOTTOM
(S. Wicks, W. Horowitz, M.D. and M. Gyulassy, nucl-th/0512076)
M. Djordjevic 21
Most up to date single electron prediction (collisional + radiative)
Inclusion of collisional energy loss leads to better agreement with single electron data, even
for dNg/dy=1000.
(S. Wicks, W. Horowitz, M.D. and M. Gyulassy, nucl-th/0512076)
Radiative energy loss alone is not able to explain the single
electron data, as long as realistic gluon rapidity density
dNg/dy=1000 is considered.
M. Djordjevic 22
The agreement between the theory and the single electron data
may still not be good enough!
However, theoretical predictions depend
on the underlying assumptions.
How good are these assumptions?
What are the open questions?
How can future RHIC experiments improve our
understanding of heavy flavor physics at RHIC?
M. Djordjevic 23
How well do we understand:
Are single electrons good probe of heavy quark energy loss?
Open questions:
1) charm and bottom production at RHIC?
2) charm and bottom contributions to the single electrons?
3) the energy loss at RHIC?
M. Djordjevic 24Need work by both theory and experiment to gain a better understanding!
How well do we understand charm and bottom production at RHIC?
Theoretical computations seem to notably underpredict the data.
Theoretically: Experimentally:
STAR and PHENIX data may be systematically off by factor of 2.
STAR (nucl-ex/0607012) Ralf Averbeck’s talk (QM2004)
M. Djordjevic 25
How well do we understand charm and bottom contributions to the single electrons?
Good agreement with the data if only charm contribution is
taken into account.
Is charm enhanced at RHIC?
Need direct D and B measurements to resolve a puzzle and make stronger conclusions!(S. Wicks, W. Horowitz, M.D. and M. Gyulassy,
nucl-th/0512076)
Current pQCD calculations ce/be Ο(1)
M. Djordjevic 26
How well we understand the energy loss at RHIC?
According to pQCD theory, clear hierarchy in the suppression patterns!
Theoretically:
Gluons are more suppressed than light quarks! Charm is more suppressed than bottom!
(S. Wicks, W. Horowitz, M.D. and M. Gyulassy, nucl-th/0512076)
M. Djordjevic 27
Potential absence of hierarchy would challenge the pQCD energy loss mechanisms!
Data may indicate the same energy loss for charm and bottom!
Data may indicate the same energy loss for gluons and light quarks!
However, experimentally:
Need: direct D and B mesons + high accuracy pbar/p measurements
STAR (nucl-ex/0606003)
M. Djordjevic 28
For example for RHIC we should include heavy quarks up to |ymax|=2.5.
Single electron distributions are very sensitive to the rapidity window (Ramona Vogt)
At high rapidity, nonperturbative effects may become important!
+
Single electron suppression could be influenced by nonpertutbative effects
Upcoming D and B meson measurements at mid rapidity should resolve this issue
Are single electrons good probe of heavy quark energy loss?
M. Djordjevic 29
How D’s and B’s should be measured in the upcoming
RHIC experiments?
• Measure (just) D mesons directly in mid
rapidity region.
• Subtract D’s from single electrons to get B’s.
• Problem: Instead of mid rapidity B’s, in this
way we would get a mixture of high rapidity
D’s and all rapidity B’s.
NO!
Measure both D and B mesons directly in
central rapidity region.YES!
M. Djordjevic 30
Summary
Radiative energy loss mechanisms alone are not able to explain the recent single electron data.
Collisional and radiative energy losses are comparable, and both contributions are important in the computations of jet
quenching.
Inclusion of the collisional energy loss lead to better agreement with the experimental results.
Future direct D and B measurements will be important to get a better understanding of heavy quark physics at RHIC.
M. Djordjevic 32
Most up to date pion and single electron predictions (collisional + radiative)
Inclusion of collisional energy loss leads to good agreement with pions and an improved agreement with single electron data at dNg/dy=1000.
(S. Wicks, W. Horowitz, M.D. and M. Gyulassy, nucl-th/0512076)
M. Djordjevic 33
Path length fluctuations
Important for gluons and consistency of electron and pion predictions.
•Realistic Woods-Saxon nuclear density
•Jets produced ~ TAA
•1+1D Bjorken expantion
Hierarchy of fixed lengths fit the full
geometrical calculations.
No a priori justification for any fixed length.
(S. Wicks, W. Horowitz, M.D. and M. Gyulassy, nucl-th/0512076)
M. Djordjevic 34
Transition & Ter-Mikayelian effects on 0th order radiative energy loss
Transition & Ter-Mikayelian effects approximately cancel each other for heavy quarks.
M.D., Phys.Rev.C73:044912,2006
CHARM BOTTOM
M. Djordjevic 36
Radiative heavy quark energy loss
Three important medium effects control the radiative energy loss:
1) Ter-Mikayelian effect (M.L.Ter-Mikayelian (1954); Kampfer-Pavlenko (2000);
Djordjevic-Gyulassy (2003)) 2) Transition radiation (Zakharov (2002); Djordjevic (2006)). 3) Energy loss due to the interaction with the medium
(Djordjevic-Gyulassy (2003); Zhang-Wang-Wang (2004); Armesto-Salgado-Wiedemann (2004))
c
L
c
1) 2) 3)
M. Djordjevic 37
The uncertainity band obtained by varying the quark mass and scale factors.
Domination of bottom in single electron spectra
M. D., M. Gyulassy, R. Vogt and S. Wicks, Phys.Lett.B632:81-86,2006
R. Vogt, talk given at QM2005
M. Djordjevic 38
Transition & Ter-Mikayelian for charm
Two effects approximately cancel each other for
heavy quarks.
Transition radiation lowers Ter-Mikayelian
effect from 30% to 15%.
M. Djordjevic 39
Why, according to pQCD, pions have to be at least two times more suppressed than single electrons?
Suppose that pions come from
light quarks only and single e-
from charm only.
Pion and single e- suppression would really be the same.
g
0
b
b+ce-
However,
1) Gluon contribution to pions increases the pion suppression, while
2) Bottom contribution to single e- decreases the single e- suppression
leading to at least factor of 2 difference between pion and single e- RAA.