1. International CCAST Summer School and Workshop on QCD and RHIC Physics
Where did the energy go?
– OUTLINE –
motivation
analysis
results
summary
Fuqiang Wang
Purdue University
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

2. Lattice QCD prediction:
Physics motivation
Quantum Chromodynamics predicts phase transition between hadrons and Quark-Gluon Plasma at high energy density.
The goal of RHIC is to create QGP –
a state of deconfined, thermalized quarks and gluons
Lattice QCD prediction:
F. Karsch, Nucl. Phys. A698, 199c (2002)
(1) energy density?
(2) thermalization?
TC ~ 170  8 MeV
eC ~ 0.5 GeV/fm3
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

3. To probe energy density
self-generated, penetrating probe: large pT hadrons, jets
Gluon bremsstrahlung
Jet quenching: X.-N. Wang et al. ?
they are generated early (by hard-scatterings);
they need time to escape the collision zone;
during this time, a QGP (or whatever medium) is formed;
they interact with the medium, losing energy, thus giving us information about the medium.
Can also come from initial state suppression?
e.g. PDF different in Au and deuteron
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

4. International CCAST Summer School and Workshop on QCD and RHIC Physics
High pT suppression
suppression of high pT hadron yield
suppression of back side angular correlation
no suppression in d+Au
 Final state interaction in Au+Au, consistent with energy loss by high pT particles – jet quenching.
strong absorption
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

5. Energy density inferred from models
X.-N. Wang, PLB 579 (04) 299
pQCD calculations:
x30 gluon density
x100 energy density
in central Au+Au collisions
Bjorken estimate:
GeV/fm3 (t=0.2-1 fm/c)
High pT particles (partons/hadrons)
lose energy.
Where did the energy do?
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

6. International CCAST Summer School and Workshop on QCD and RHIC Physics
To measure energy loss
… by going to low pT.
S. Pal, S. Pratt, PLB574 (2003) 21.
X.-N. Wang, PLB 579 (2004) 299, nucl-th/
C.A. Salgado, U.A. Wiedemann, hep-ph/
M. Gyulassy, I. Vitev, X.-N. Wang, B.-W. Zhang, nucl-th/ ……
Pal, Pratt, PLB 574 (2003) 21
How is energy distributed?
amount of energy loss?
contribution from medium?
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

7. Reconstructing low pT associated particlesDf
(1/Ntrig) dN/d(Df)
STAR Preliminary
background
Signal
p+p
Jet-like structures
Au+Au top 5%
Select a leading particle 4<pT<6 GeV/c, |h|<0.75.
Associate other particles (0.15<pT<4 GeV/c,|h|<1.1) with the leading particle. Form Df,Dh correlations.
Background from mix- events. v2 modulation on background. Normalize in <|Df|<1.3.
Efficiency corrections are applied to associated particles.
Take difference and normalize per trigger.
High pT particle
p+p
High pT particle
Au+Au
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

8. Azimuth angular correlations
High pT:
1/Ntrigger dN/d(Df)
D f (radians)
Low pT:
M.G. Albrow et al. NPB145, 305 (1978)
p+p, 53 GeV
near side: |Df|<1.1, |Dh|<1.4 away side: |Df-p|<2, |h|<1.1
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

9. International CCAST Summer School and Workshop on QCD and RHIC Physics
“Jet” sizes
STAR Preliminary
near: |Df|<1.1, |Dh|<1.4
away: |Df-p|<2, |h|<1.1
Au+Au top 5%
bkgd subt.
RMS
(1/Ntrig) dN/d(Df) s Df
near
(1/Ntrig) dN/d(Dh)
With increasing centrality:
Near side broadens in h but not f.
Away side modest increase in size. Dh
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

10. Longitudinal broadening
N.Armesto, C.A.Salgado, U.A.Wiedemann, hep-ph/
long.
flow
S.A. Voloshin, nucl-th/
p + p
radial flow
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

11. “Jet” charge multiplicity and “energy”
STAR Preliminary
p+p
With the same final leading particle,
we are selecting a larger energy jet
in central AA than in pp.
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

12. International CCAST Summer School and Workshop on QCD and RHIC Physics
Jet quenching model
X.-N. Wang, PLB 579 (2004) 299, nucl-th/
with energy loss
without energy loss
DE = 1.4 – 2.2 GeV }
Caution: cannot be readily compared to data yet
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

13. International CCAST Summer School and Workshop on QCD and RHIC Physics
Medium contribution?
Total scalar pT: Initial parton energy + medium contribution?
TPC acceptance of away side partner?
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

14. Thermal-shower recombination
Hwa, Yang, nucl-th/
In this model, the thermal-
shower recombination
is the largest contribution
to high pT particles.
One mechanism for energy
contribution from medium.
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

15. To probe thermalization
Put two sources of particles together:
one from jet fragmentation that are initially hard.
the other from bulk medium that are soft.
and see how they become at end of the day.
Leading
hadrons
Medium
jet
medium
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

16. Associated particles pT distributions
STAR Preliminary: Quark Matter 2004
Away side:
energy from the
initial parton is
redistributed to
low pT
energy loss in
medium!
Near side:
Overall
enhancement
from pp to AA
larger initial
parton energy
(and modest
energy loss)
Away
syst. error
Near
softening in spectra: partial equilibration with medium
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

17. International CCAST Summer School and Workshop on QCD and RHIC Physics
Raito of d+Au spectra
6<pT<10 GeV/c
4<pT<6 GeV/c
Associated pT (GeV/c)
d+Au spectra ratio
Associated pT (GeV/c)
Au+Au / p+p
STAR Preliminary
STAR Preliminary
Initial parton energy was larger in AA than pp  a hardening of spectra is expected.
No hardening is observed in AA  contribution of soft hadrons from energy loss.
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

18. Partial approach toward thermalization
STAR Preliminary: QM 2004
Leading hadrons
Medium
In central Au+Au, the balancing hadrons are greater in number, softer in pT, and distributed broadly in angle, relative to pp or peripheral Au+Au.
 away-side products seem to approach equilibration with bulk medium traversed, making thermalization of the bulk itself quite plausible.
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

19. Summary: where did the energy go?
High pT suppressed. High pT partons/particles lose energy.
The lost energy is redistributed at low pT.
high enough energy density? Models require x30 normal nuclear gluon density.
parton thermalization? Broader angular distribution, larger multiplicity, softer pT. Partial thermalization with bulk medium. High degree of thermalization in medium itself plausible.
A new form of matter created at RHIC:
dense, strongly interacting, opaque.
QGP?
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

20. International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

21. Are leading particles from jets?
PHENIX, PRL 91, (2003)
p / p ~ 0.9 in central
p / p ~ 0.3 in peripheral
ISR
QM’04
non-frag. p / p ~ 0.6
non-frag. p / Nch ~ 0.3
pT=3-4 GeV/c: ~30% are
probably from other sources.
B. Alper, NPB 87 (1975) 41
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

22. Coalescence / recombination models
Fries et al, PRC 68 (03) 44902
Greco et al, PRC 68 (03) 34904
Hwa et al, nucl-th/
Coalescence / recombination models predict a range of non-fragmentation contributions.
All predict a rapid drop of non-fragmentation contribution above 4 GeV/c.
pT>4 GeV/c: may mainly come from jets, or related to jets.
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

23. International CCAST Summer School and Workshop on QCD and RHIC Physics
Compare to pp, pA
pT (GeV/c)
dN/dpT2 [a.u.]
PRELIMINARY
p+p
STAR Preliminary
p+p
pT (GeV/c)
<ztrigger>
G. Boca et al. ZPC49, 543 (1991)
p+A
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

24. International CCAST Summer School and Workshop on QCD and RHIC Physics
AA/pp vs IAA
Peripheral:
AA/pp=1.01
IAA=0.84
- broadening in h
- pp reference
- v2
- model difference
Central:
AA/pp=1.66
IAA=1.25 Df Df
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

25. International CCAST Summer School and Workshop on QCD and RHIC Physics
IAA < 1 in peripheral ?
STAR Preliminary
pp reference
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019

26. International CCAST Summer School and Workshop on QCD and RHIC Physics
Hijing (no quenching)
4 < pTtrig < 6 GeV/c
2 < pT < 4 GeV/c
0.15 < pT < 4 GeV/c Df Df
International CCAST Summer School and Workshop on QCD and RHIC Physics
2/23/2019