1. Leptonic signatures in Heavy Ion Collisions
E.Kistenev
Brookhaven National Laboratory
1/17/2019Monday, February 15, 2010

2. BNL-RHIC Facility
Also: BNL-AGS, CERN-SPS, CERN-LHC

3. From pp to HI Collisions
PHENIX Au+Au central
STAR Au+Au central
STAR-Jet event in pp
High pT particle
Au+Au
High pT particle
p+p 4%
0.5%
c0 Bj =5.40.6 GeV fm-2
PRC 71 (2005)
per unit
velocity || to beam

4. Why all of it is of interest to taxpayer
D.Froidevaux in his first lecture at this school : LHC was built to shed the light on Early Universe;
Our current assumption : The Early Universe was flooded with matter in a state known under code name of Quark-Gluon Plasma (E.Shurjak);
Creationism : Our world is the only allowed product of Quark-Gluon Plasma thermalization;
What we do not know : if it will ever be experimentally verified.

5. Astrophysics – experiment on the scale of the Universe – let’s look around and/or ask our neighbors, they’ve probably been here before us … HI - attempt to recreate early Universe conditions on the scale of physics laboratory (no proof that we are actually doing this)
There is some hope and some sadness in both approaches ….
- a galaxy like ours should host hundreds of intelligent civilizations; the bad news is that the time between when such a civilization becomes technologically advanced enough and when it is wiped out by homesun going red giant is too short on galactic scale – at all times there are, in most simulations, no other such civilizations (or if there are, they are too far away) … we are likely to be alone – noone will probably ever answer;
numerical modeling of this type is generally a shadow of the entity it attempts to model, in this case the Big Bang and its constituents like Milky Way, stars, planets and other objects…..
30 years of attempts on recreating after BigBang conditions in the lab ended with Pandora box slightly opened but yet no proofs that reaches our original goal.

6. It took 80 years to learn that we are 1010 years old
The original Hubble Diagram
“A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae” E.Hubble (1929)
It took 80 years to learn that we are 1010 years old
Freedman, et al. Astrophys. J. 553, 47 (2001)
W. Freedman Canadian
Modern Hubble constant (2001)
Edwin Hubble Galaxies outside Milky Way
Original Hubble diagram
Henrietta Leavitt
Distances via variable stars
1929: H0 ~500 km/sec/Mpc
2001: H0 = 727 km/sec/Mpc

7. Observational astrophysics
Acceleration, return cosmological constant and/or vacuum energy.
Matter-dominated, structure forms
a(t)
a(t)t1/2
Radiation-dominated thermal equilibrium
Inflation, dominated by “inflaton field” vacuum energy t

8. Density of hadron mass states dN/dM increases exponentially with mass.
g*S
1 Billion oK
1 Trillion oK
All entropy is in relativistic species
Density of hadron mass states dN/dM increases exponentially with mass.
“…a veil, obscuring our view of the very beginning.” Steven Weinberg, The First Three Minutes (1977)
Keep adding more hadrons….

9. QCD to the rescue! Replace Hadrons (messy and numerous)
D. Gross
H.D. Politzer
F. Wilczek
Replace Hadrons (messy and numerous)
by Quarks and Gluons (simple and few)
QCD Asymptotic Freedom (1973)
e/T4
 g*S
Thermal QCD ”QGP” (Lattice)
“In 1972 the early universe seemed hopelessly opaque…conditions of ultrahigh temperatures…produce a theoretically intractable mess. But asymptotic freedom renders ultrahigh temperatures friendly…” Frank Wilczek, Nobel Lecture (RMP 05)
Hadron gas
Karsch, Redlich, Tawfik, Eur.Phys.J.C29: ,2003

10. “Before [QCD] we could not go back further than 200,000 years after the Big Bang. Today…since QCD simplifies at high energy, we can extrapolate to very early times when nucleons melted…to form a quark-gluon plasma.” David Gross, Nobel Lecture (RMP 05)
g*S
Thermal QCD -- i.e. quarks and gluons -- makes the very early universe tractable; but where is the experimental proof?
n Decoupling
Nucleosynthesis
e+e- Annihilation
Heavy quarks and bosons freeze out
QCD Transition
Mesons freeze out
Kolb & Turner, “The Early Universe”

11. Electromagentic probes (photon and lepton pairs) – measure of temperature
Photons and lepton pairs are cleanest probes of the dense matter formed at RHIC
These probes have little interaction with the matter so they carry information from deep inside of the matter
Temperature?
Matter properties?
Hadrons inside the matter? e- g* g
Ya. Akiba

12. Thermal photon from hot matter
Hot matter emits thermal radiation
Temperature can be measured from the emission spectrum
Ya. Akiba 12

13. Photons: More Sources, More Theory
Rate
Hadron Gas Thermal Tf
QGP Thermal Ti
“Pre-Equilibrium”?
Turbide, Rapp, Gale
Jet Re-interaction √(Tix√s)
Final-state photons are the sum of emissions from the entire history of a nuclear collision.
pQCD Prompt x√s Eg
Ya. Akiba

14. “Direct” vs “hadronic”g p r
Thermal Radiation
QGP / Hadron Gas
Fragmentation p0
Induced
Prompt
EM & Weak Decay
High-energy counts these
High-energy nuclear counts these

15. Thermal photons (theory prediction)g p r q g
High pT (pT>3 GeV/c) pQCD photon
Low pT (pT<1 GeV/c)
photons from hadronic Gas
Themal photons from QGP is the dominant source of direct photons for 1<pT<3 GeV/c
Recently, other sources, such as jet-medium interaction are discussed
Measurement is difficult since the expected signal is only 1/10 of photons from hadron decays
S.Turbide et al PRC 15

16. Direct Photons in Au+Au
Blue line: Ncoll scaled p+p cross-section
Direct photon is measured as “excess” above hadron decay photons
Measurement at low pT difficult since the yield of thermal photons is only 1/10 of that of hadron decay photons
PRL 94, (2005)
Au+Au data consistent with pQCD calculation scaled by Ncoll
Ya. Akiba 16

17. Alternative method --- measure virtual photon
Source of real photon should also be able to emit virtual photon
At m0, the yield of virtual photons is the same as real photon
Real photon yield can be measured from virtual photon yield, which is observed as low mass e+e- pairs
Advantage: hadron decay background can be substantially reduced. For m>mp, p0 decay photons (~80% of background) are removed
S/B is improved by a factor of five
Other advantages: photon ID, energy resolution, etc
Ya. Akiba 17

18. Not exactly a new idea
J.H.Cobb, et al, PL 78B, 519 (1978)
C. Albajar, et al, PLB209, 397 (1988)
The idea of measuring direct photon via low mass lepton pair is not new one. It is as old as the concept of direct photon.
This method is first tried at CERN ISR in search for direct photon in p+p at s1/2=55GeV. They look for e+e- pairs for 200<m<500 MeV, and set one of the most stringent limit on direct photon production at low pT
Later, UA1 measured low mass muon pairs and deduced the direct photon cross section.
g/p0 = 10%
Dalitz
g/p0 = 0.53 ±0.92%
(2< pT < 3 GeV/c)
Ya. Akiba 18

19. Relation between dilepton and virtual photon
Emission rate of (virtual) photon
e.g. Rapp, Wambach Adv.Nucl.Phys 25 (2000)
Boltzmann factor
temperature
EM correlator
Matter property
Emission rate of dilepton
Relation between them
Prob. g*l+l-
This relation holds for the yield after space-time integral
Dilepton
virtual photon
Virtual photon emission rate can be determined from dilepton emission rate
M ×dNee/dM gives virtual photon yield
For M0, ng*  ng(real); real photon emission rate can also be determined 19

20. Theory prediction of (Virtual) photon emission
Theory calculation by Ralf Rapp
at y=0, pt=1.025 GeV/c
Real photon yield
Turbide, Rapp, Gale PRC69,014903(2004)
The calculation is shown as the virtual photon emission rate. The virtual photon emission rate is a smooth function of mass.
When extrapolated to M=0, the real photon emission rate can be determined.
q+gq+g* is not in the calculation; it should be similar size as HMBT at this pT
Vaccuum EM correlator
Hadronic Many Body theory
Dropping Mass Scenario
q+q annihilaiton (HTL improved)
q+g  q+g*
qqg*
≈M2e-E/T 20
Ya. Akiba

21. Electron pair measurement in PHENIX
designed to measure rare probes: + high rate capability & granularity
+ good mass resolution and particle ID
- limited acceptance
Au-Au & p-p spin
PC1
PC3 DC
magnetic field &
tracking detectors e+ e- p g
2 central arms:
electrons, photons, hadrons
charmonium J/, ’ -> e+e-
vector meson r, w,  -> e+e-
high pT po, p+, p-
direct photons
open charm
hadron physics 21

22. e+e- mass spectra in pT slices
p+p
Au+Au
arXiv:
p+p in agreement with cocktail
Au+Au low mass enhancement concentrated at low pT 22

23. Enhancement of almost real photon
arXiv: pp
Au+Au (MB) Mp Mp
Low mass e+e- pairs (m<300 MeV) for 1<pT<5 GeV/c
p+p:
Good agreement of p+p data and hadronic decay cocktail
Small excess above mp at large mee and high pT
Au+Au:
Clear enhancement visible above mp =135 MeV for all pT
Excess  Emission of almost real photon
1 < pT < 2 GeV
2 < pT < 3 GeV
3 < pT < 4 GeV
4 < pT < 5 GeV
Ya. Akiba 23

24. Virtual Photon Measurement
Any source of real g can emit g* with very low mass.
Relation between the g* yield and real photon yield is known.
Process dependent factor
Case of hadrons (p0, h) (Kroll-Wada)
S = 0 at Mee > Mhadron
Case of direct g*
If pT2>>Mee2 S = 1
For m>mp, p0 background (~80% of background) is removed
 S/B is improved by a factor of five
Direct g p0 h 24

25. Determination of g* fraction, r
Direct g*/inclusive g* is determined by fitting the following function
fdirect : direct photon shape with S = 1.
r = direct g*/inclusive g*
Fit in MeV/c2 (insensitive to p0 yield)
The mass spectrum follows the expectation for m > 300 MeV
 S(m) ~ 1
arXiv:
arXiv: 25

26. Fraction of direct photons
Compared to direct photons from pQCD
p+p
Consistent with NLO pQCD
Au+Au
Clear excess above pQCD
arXiv:
arXiv:
p+p
Au+Au (MB)
μ = 0.5pT
μ = 1.0pT
μ = 2.0pT
NLO pQCD calculation by Werner Vogelsang
Ya. Akiba 26

27. Direct photon spectra Direct photon measurements
exp + TAA scaled pp
arXiv:
arXiv:
Direct photon measurements
real (pT>4GeV)
virtual (1<pT<5GeV)
pQCD consistent with p+p down to pT=1GeV/c
Au+Au data are above Ncoll scaled p+p for pT < 2.5 GeV/c
Au+Au = scaled p+p + exp: Tave = 221  19stat  19syst MeV
Fit to pp
NLO pQCD (W. Vogelsang) 27

28. The inverse slope TAuAu = 221±19±19 MeV (>Tc ~ 170 MeV)
Summary of the fit
Significant yield of the exponential component (excess over the scaled p+p)
The inverse slope TAuAu = 221±19±19 MeV (>Tc ~ 170 MeV)
p+p fit funciton: App(1+pt2/b)-n
If power-law fit is used for the p+p spectrum, TAuAu = 240±21 MeV
Ya. Akiba 28

29. Theory comparison Hydrodynamical models are compared with the data
D.d’Enterria &D.Peressounko
T=590MeV, t0=0.15fm/c
S. Rasanen et al.
T=580MeV, t0=0.17fm/c
D. K. Srivastava
T= MeV, t0=0.2fm/c
S. Turbide et al.
T=370MeV, t0=0.33fm/c
J. Alam et al.
T=300MeV, t0=0.5fm/c
F.M. Liu et al.
T=370MeV, t0=0.6 fm/c
Hydrodynamical models are in qualitative agreement with the data 29

30. Initial temperature From data: Tini > Tave = 220 MeV
TC from Lattice QCD ~ 170 MeV
Tave(fit) = 221 MeV
From data: Tini > Tave = 220 MeV
From models: Tini = 300 to 600 MeV
t0 = 0.15 to 0.6 fm/c
Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV
Ya. Akiba 30

31. Thermal emission from QGP: Summary
e+e- pairs for m<300MeV and 1<pT<5 GeV/c were measured
Excess above hadronic background is observed
Excess is much greater in Au+Au than in p+p
Treating the excess as internal conversion of direct photons, the yield of direct photon is deduced.
Direct photon yield in pp is consistent with a NLO pQCD
Direct photon yield in Au+Au is much larger.
Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±19(stat)±19(sys) MeV
Hydrodynamical models with Tinit= MeV at t0= fm/c are in qualitative agreement with the data.
Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV 31

32. Quarkonia & Deconfinement

33. Quarkonia & Deconfinement
For the hot-dense medium (QGP) created in A+A collisions at RHIC:
Large quark energy loss in the medium implies high densities
Flow scales with number of quarks
Is there deconfinement?  look for Quarkonia screening
Debye screening predicted to destroy J/ψ’s in a QGP with other states “melting” at different temperatures due to different sizes or binding energies.
Mocsy, WWND08
RHIC: T/TC ~ 1.9 or higher
Different lattice calculations do not agree on whether the J/ is screened or not – measurements will have to tell!
Satz, hep-ph/
5/25/2009
Mike Leitch 33

34. PHENIX A+A Data and Features
PHENIX Au+Au data shows suppression at mid-rapidity about the same as seen at the SPS at lower energy
but stronger suppression at forward rapidity.
Forward/Mid RAA ratio looks flat above a centrality with Npart = 100
Several scenarios may contribute:
Cold nuclear matter (CNM) effects
important, need better constraint
Sequential suppression
QGP screening only of C & ’- removing their feed-down contribution to J/ at both SPS & RHIC
Regeneration models
give enhancement that compensates for screening
Centrality (Npart)
5/25/2009
Mike Leitch 34

35. Reaching Higher pT for J/ - probing for the “hot wind”?
New PHENIX RCuCu out to pT = 9 GeV/c !
shows large suppression that looks roughly constant up to high pT
STAR points with their huge uncertainties were misleading
AdS/CFT (“hot wind”) - more suppression at high pT:
Liu, Rajagopal,Wiedemann
PRL 98, (2007)
Regeneration (2-compenent):
Zhao, Rapp
hep-ph/ & private communication
Equilibrating Parton Plasma:
Xu, Kharzeev, Satz, Wang,
hep-ph/
Gluonic dissoc. & flow:
Patra, Menon, nucl-th/
Cronin – less suppression at higher pT:
use d+Au data as a guide
Mike Leitch 35
5/25/2009

36. New CNM fits using 2008 PHENIX d+Au Rcp (Tony Frawley, Ramona Vogt, …)
similar to before, use models with shadowing & absorption/breakup
but allow effective breakup cross section to vary with rapidity
to obtain good description of data for projections to A+A
get “breakup(y)”; compare to E866/NuSea & HERA-B
Lourenco, Vogt, Woehri - arXiv:
common trend, with large increasing effective breakup cross section at large positive rapidity
need additional physics in CNM model – e.g. initial-state dE/dx
with EKS shadowing
with NDSG shadowing
5/25/2009
Mike Leitch 36

37. Upsilons at RHIC

38. Quarkonia Production & Suppression – Upsilons in p+p
Cross section follows world trend
Baseline for Au+Au
5/25/2009
Mike Leitch 38

39. Quarkonia – Upsilons Suppressed in Au+Au
10.5(+3.7/-3.6)
11.7(+4.7/-4.6)
NJ/Ψ
2653 ±70±345
4166 ±442(+187/-304)
RAA(J/Ψ)
---
0.425 ±0.025±0.072
RAuAu [8.5,11.5] < 0.64 at 90% C.L.
--- Includes 1S+2S+3S ---
5/25/2009
Mike Leitch 39

40. Leptons signals & heavy quarks
c, b quark
D, B e
Study medium effect in open charm and bottom production
Ideally, D or B meson should be measured, but for technical reason most of the measurement so far is done through electron decay channel.
From RAA and v2 of the electrons from heavy quark decays, the energy loss and the flow of heavy quarks are indirectly measured.
So far, ce and be are not separated

41. Large energy loss and flow of heavy quarks
v2 of b,c e
RAA of b,c e
Strong suppression of electron from c and b
Large energy loss of heavy quark
Large elliptic flow of electrons from c and b!
Heavy quark flows in the medium
These results require very strong interaction between the dense matter and heavy quarks.
Since the observed electron is mixture of ce (dominant) and be, we cannot determine the suppression or flow of be.
Theoretical expectation is that the medium-quark interaction becomes weaker for heavier quark. Large energy loss and/or flow are not expected. 41

42. Heavy flavor electron RAA and flow
PRL98, (2007)
Two models describes strong
suppression and large v2
Rapp and Van Hee
Moore and Teaney
From model comparison,
viscosity to entropy ratio h/s
can be estimated
DHQ × 2πT = 4 - 6
DHQ ~ 6 x h/(e+p) = 6 x h/Ts
 h/s ~ (4/3 – 2)/4p
The estimate of h/s is close
to the conjectured bound
1/4p from AdS/CFT 42

43. Comparison with other estimates
S. Gavin and M. Abdel-Aziz:
PRL 97:162302, 2006
pTfluctuations STAR
R. Lacey et al.: PRL 98:092301, 2007
v2 PHENIX
& STAR
H.-J. Drescher et al.: arXiv:
v2 PHOBOS
Estimates of h/s based on flow and fluctuation data indicate small value as well close to conjectured limit significantly below h/s of helium (4ph/s ~ 9)
conjectured quantum limit 43

44. Closing comments
Density > 5 GeV/fm3 (transverse energy measurements);
Temperature ~220 MeV (thermal photons);
Preserves flow (h/s ~0.4)
Interact strongly with non e/m probes (jet suppression);
Quarkonia data are still inconclusive – interplay of CME and QGP screening;
Unexpected scale of the heavy quark’s energy loss.

45. Heavy quark (charm and bottom) probe
Study medium effect in open charm and bottom production
Ideally, D or B meson should be measured, but for technical reason most of the measurement so far is done through electron decay channel.
From RAA and v2 of the electrons from heavy quark decays, the energy loss and the flow of heavy quarks are indirectly measured.
So far, ce and be are not separated
c, b quark
D, B e 45

46. BACKUP

47. Heavy flavor production in pp (base line)
Single electrons from heavy flavor (charm/bottom) decay are measured and compared with pQCD theory (FONLL)
The new data extends the pT reach to 9 GeV/c
FONLL pQCD calculation agree with the data
c e dominant in low pT
be is expected to be dominant in high pT
Phys. Rev. Lett 97, (2006) 47

48. Bottom Measurement
p+p 200 GeV Charm and bottom extracted via e-h mass analysis
Charm and bottom spectra are both by a factor  above FONLL pQCD calculations (but within the uncertainty)
STAR studied be/ce ratios in pp and obtained similar b/c ratios 48

49. Basic Thermodynamics Isentropic Adiabatic
Hot
Sudden expansion, fluid fills empty space without loss of energy.
dE = 0 PdV > 0 therefore dS > 0
Hot
Hot
Gradual expansion (equilibrium maintained), fluid loses energy through PdV work.
dE = -PdV therefore dS = 0
Hot
Cool
Isentropic Adiabatic

50. Depending on a taste the history began either 10*10 or 80 years ago
The original Hubble Diagram
“A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae” E.Hubble (1929)
Velocity
Distance
Edwin Hubble Galaxies outside Milky Way
Henrietta Leavitt
Distances via variable stars