takao sakaguchi bnl 2/9/2012 1t. sakaguchi, rbrc lunch meeting
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T. Sakaguchi, RBRC lunch meeting
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Direct photon physics in heavy ion collisions
~Current Status and Future~
Takao SakaguchiBNL
2/9/2012
Production Process◦ Compton and annihilation (LO,
direct)◦ Fragmentation (NLO)◦ Escape the system unscathed
Carry dynamical information of the state
Temperature, Degrees of freedom◦ Immune from hadronization
(fragmentation) process at leading order
◦ Initial state nuclear effect Cronin effect (kT broardening)
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Electromagnetic probes (was challenging)
Photon Production: Yield s
g
*ge+
e-
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Before RHIC In 1986, search for direct photon started in heavy ion collisions at CERN
◦ Upper limits published in 1996 from WA80(S+Au at 200GeV/u◦ Followed by WA93
Third generation experiment, WA98, showed the first significant result◦ Pb+Pb sNN=17.3GeV, PRL85, 3595(2000).
p+Pb data shows initial nuclear effect
Baumann, QM2008
Au+Au = p+p x TAB holds – pQCD factorization works
NLO pQCD works. Non-pert. QCD may work in Au+Au system
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First gdir in Au+Au (hard scattering)
Blue line: Ncoll scaled p+p cross-section
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How direct photons are measured
Real Photon measurement◦ EMCal(PbSc, PbGl): Energy
measurement and identification of photons
◦ Tracking(DC, PC): Veto to charged particles
Dilepton measurement◦ RICH: Identify electrons◦ EMCal(PbSc, PbGl): Identify
electrons◦ Tracking(DC, PC): Momentum
measurement of electrons
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PHENIX Detector
0
Invariant Mass(pT=4GeV, peripheral)
p0 efficiency
Statistically subtract photon contributions from p0/h/h’/w◦ Measure or estimate yield of these hadrons◦ Measure: Reconstruct hadrons via 2 g invariant mass in EMCal◦ Mass = (2E1E2(1-cosq))1/2
Or, tag photons that are likely from these hadrons event-by-event◦ Possible if density of produced particles is
low (p+p or d+Au)
Subtract remaining background contributions:◦ Photons that are not from collision vertex◦ Hadrons that are misidentified as photons
Correct for detection efficiency of photons
Signal is very small.◦ ~ 5% S/B in 1-3GeV/c◦ Extremely difficult
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How to extract direct photons
Direct hadron decay
Inclusive photon
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Then, we make this. Taking /g p0 ratio cancels out systematic errors on energy scale measurement
Double ratio (double /g p0 ratio)
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Possible sources of photons
(fm/c)
log t1 10 107
hadrondecays
sQGP
hard scatt
jet Brems.
jet-thermalparton-medium interaction
hadron gas
Eg
Rate
Hadron Gas
sQGP
Jet-Thermal
Jet Brems.Hard Scatt
See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005)
Thermal radiation from QGP (1<pT<3GeV)◦ S/B is ~5-10%◦ Spectrum is exponential. One can extract temperature,
dof, etc..
Hadron-gas interaction (pT<1GeV/c): () (), K* K
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Difficult objects! Photons from QGP~big challenge~
)0(Im23
Mfpd
dRE em
Bem
)0(Im324
MfMpd
dRem
Bemee
fB: Bose dist. em: photon self energy
photons
dileptons
Interesting, but S/B is small
54321
S/B ratio
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New production mechanism introduced
qg q Jet in-medium bremsstrahlung
Jet-photon conversion
Both are “thermal hard”
• Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung)
• Compton scattering of hard scattered and thermal partons (Jet-photon conversion)
Turbide et al., PRC72, 014906 (2005)R. Fries et al., PRC72, 041902 (2005)Turbide et al., PRC77, 024909 (2008)Liu et al., arXiv:0712.3619, etc..
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A plate ~After cooking up ingredients~
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Adding virtuality in photon measurement
(fm/c)
log t1 10 107
hadrondecays
hadron gas
sQGP
hard scatt
Mass(GeV/c2)
0.5
1
g* e+e-virtuality
jet Brems.
jet-thermalparton-medium interaction
By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2)
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Focus on the mass region where p0 contribution dies out
For M<<pT and M<300MeV/c2
◦ qq ->* contribution is small◦ Mainly from internal
conversion of photons
Can be converted to real photon yield using Kroll-Wada formula
◦ Known as the formula for Dalitz decay spectra
Low pT photons with very small mass
Comptonq g*
g q
e+
e-
Internal conv.
One parameter fit: (1-r)fc + r fd
fc: cocktail calc., fd: direct photon calc.
r*dir(m 0.15)*inc (m 0.15)
*dir(m 0)*inc (m 0)
dirinc
1
N
dNeedmee
23
14me
2
mee2(1
2me2
mee2)1
meeF(mee
2 )2(1
mee2
M 2)3
PRL104,132301(2010), arXiv:0804.4168
Reconstruct Mass and pT of e+e-◦ Same as real photons
◦ Identify conversion photons in beam pipe using and reject them
Subtract combinatorial background
Apply efficiency correction
Subtract additional correlated background:◦ Back-to-back jet contribution◦ well understood from MC
Compare with known hadronic sources
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Dilepton Analysis
π0
π0
e+e-
e+
e-γ
γ
π0e-
γ
e+
2N N N
( , ) 22
T T
BGFG m p FG FG
BG BG
16
System size dependence of g fraction
g fraction = Yielddirect / Yieldinclusive
Largest excess above pQCD is seen at Au+Au.◦ Moderately in Cu+Cu also.
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No excess in d+Au
(no medium)
Excess also in Cu+Cu
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d+Au Min. Bias
Inclusive photon × gdir/ginc
Fitted the spectra with p+p fit + exponential function◦ Tave = 221 19stat 19syst MeV (Minimum Bias)
Nuclear effect measured in d+Au does not explain the photons in Au+Au
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Low pT photons in Au+Au (thermal?)
PRL104,132301(2010), arXiv:0804.4168
Au+Au
Won Nishina memorial prize!
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Initial temperature at Au+Au
Initial temperature Ti
◦ 300 ~ 600 MeV (different assumptions)
◦ Depends on thermalization time t0
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T. Sakaguchi, RBRC lunch meeting
Tc~170MeV from lattice QCD
PHENIX, Phys. Rev. C 81, 034911 (2010)
Theory calculations:d’Enterria, Peressounko, EPJ46, 451Huovinen, Ruuskanen, Rasanen, PLB535, 109Srivastava, Sinha, PRC 64, 034902Turbide, Rapp, Gale, PRC69, 014903Liu et al., PRC79, 014905Alam et al., PRC63, 021901(R)
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Direct photon v2
Depending the process of photon production, angular distributions of direct photons may vary
Jet-photon conversion, in-medium bremsstrahlung (v2<0)◦ Turbide, et al., PRL96, 032303(2006), etc..
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Direct photon v2 ~a photon source detector~
Turbide et al., PRC77, 024909 (2008)
Thermal photons
Bremsstrahlung (energy loss)
jet
jet photonconversion
v2 > 0
v2 < 0
For prompt photons: v2~0
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Inclusive photon v2
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T. Sakaguchi, RBRC lunch meeting
Calculation of direct photon v2
= inclusive photon v2 - background photon v2(p0, , h etc)
inclusive photon v2
Au+Au@200 GeVminimum bias
preliminary
R comes from virtual photon measurement
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Inclusive photon and p0 v2
p0 v2 ◦ similar to inclusive photon v2
Two interpretations◦ There are no direct photons◦ Direct photon v2 is similar to inclusive
photon v2
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T. Sakaguchi, RBRC lunch meeting
p0 v2
Au+Au@200 GeVminimum bias
inclusive photon v2
preliminary
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Direct photon v2
Very large flow in low pT
v2 goes to 0 at high pT
◦ Hard scattered photons dominate
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T. Sakaguchi, RBRC lunch meeting
Au+Au@200 GeVminimum bias
preliminary
PHENIX, arXiv:1105.4126
Later thermalization gives larger v2 (QGP photons)
Large photon flow is not explained by models for QGP
2/9/2012
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Comparison with models. No success..
Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1
thermal
diluted by prompt
Chatterjee, Srivastava PRC79, 021901 (2009)
Hydro after t0
thermal + prim.
van Hees, Gale, Rapp, PRC84, 054906 (2011)
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T. Sakaguchi, RBRC lunch meeting
This fits to data well, but..
Large flow can not be produced in partonic phase, but could be in hadron gas phase
This model changed ingredients of photon spectra drastically!◦ We realized the importance of the data…
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New degree of freedom?
We might have found that the QGP is formed◦ High enough temperature to induce phase transition◦ Need even precise measurement with larger statistics
How does the system thermalize?◦ In ~0.3fm/c ? How?◦ A hypothesis says at 0.3fm/c, the system is not thermalized
What happens in the pre-equilibrium state?◦ Longitudinal expansion. Landau? Bjorken?◦ What it the initial state condition? Glasma?
Penetrating probe might shed light on the pre-equilibrium states and thermalization mechanism
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Next step in photons
Since the thermalization time is very fast, let’s base on Landau picture (extreme case)
Less thermal photons flying to higher rapidity (g1) may be produced than those to mid-rapidity (g2)◦ with refer to the QGP formation time.◦ dz ~ 2R/100, dx ~ 2R
One could see more photons produced in pre-equilibrium states◦ Rapidity dependence photon
measurement may play a role as a system clock
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Rapidity as a clock of system evolution
g2
g1
dz ~ 2R/100
dx ~
2R
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Landau and Bjorken expansion models
central collision of equal nuclei at 2 1/NN Ns m
differ mostly by initial conditions
proper time 2 2z t1 t + z
ln2 t - z
space-time rapidity
Forward direct photons shed light on time evolution scenario◦ Real photons, g*->ee, g*->mm
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Rapidity dependence ~system expansion~
T. Renk, PRC71, 064905(2005)
Strong gluon field (Glasma) preceded by CGC + fluctuation
Strong color-electric and magnetic field in a flux tube◦ extended in z-direction
May play an important role on rapid thermalization
Is there any way to detect Glasma state?◦ Photons from early stages, i.e., high rapidity?
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CGC -> Glasma -> QGP, how?
Singular point in phase diagram that separates 1st order phase transition (at small T) from smooth cross-over (at small b)
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Finding the QCD Critical Point
Quark-number scaling of V2
• saturation of flow vs collision energy• /s minimum from flow at critical point
Critical point may be observed via:• fluctuations in <pT> & multiplicity• K/π, π/p, pbar/p chemical equilibrium• RAA vs s, ….
VTX provides large azimuthal acceptance & identification of beam on beam-pipe backgrounds
Higher the rapidity goes, higher the baryon density we may be able to reach
BRAHMS plot. Another way to access to the critical point?
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High Rapidity as a high baryon system
BRAHMS, PRL90, 102301 (2003)
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By changing rapidity, we can cover the missing region of √sNN, with high statistics.
√sNN, T and mb..
NPA772(2006)167
2/9/2012
My eye fit
My rough stat calc.
Charged hadron results and some pion/proton ratio results
Might be an idea to extend our measurement to p0/direct photons/dileptons
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Review ~BRAHMS results
BRAHMS, PLB 684(2010)22.BRAHMS, PRL91, 072305(2003).
Genuine process that involves “quark”◦ Quark energy loss can be measured◦ Need a lot of help from model calculations
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Drell-Yan as an energy loss probe
Hot matter created in HICS. Turbide, C. Gale, D. Srivastava,R. Fries, PRC74, 014903 (2006)
Take Axel’s strawman’s design (in TPD workshop)◦ Cover’s rapidity range of y = 3-4
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How about measurement? ~Detector Plan~
~7m
Charge VETO pad chamber
~7m
EMCal & (Hcal)
Muon Piston Calorimeter extension (MPC-EX) (3.1<|h|<3.8)◦ Shower max detector in front of existing MPC. Now sits at ~1m from IP◦ Measure direct photons/p0 in forward rapidity region in p+p, p+A
Study of how high in centrality in A+A we can go is on-going◦ In the future, placing in a very far position (from Interaction Point) would be an
option
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How about measurement?~A technology choice: MPC-EX~
Interesting physics are explored by direct photon measurements in HI collisions◦ Hard photons, Thermal photons, elliptic flow of photons
Rapidity may be a new degree of freedom on photon measurement
I would like to see many predictions on direct photons and dileptons at high rapidity!◦ I’d be happy to be involved in the theory effort, also.
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Summary
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Backup
A calculation tells that even in low pT region(pT~2GeV/c), jet-photon conversion significantly contributes to total
What do we expect naively?◦ Jet-Photon conversions Ncoll Npart (s1/2)8 f(xT), “8” is xT-scaling power◦ Thermal Photons Npart (equilibrium duration) f( (s1/2)1/4 )◦ Bet: LHC sees huge Jet-photon conversion contribution over
thermal?
Together with v2 measurement, the “thermal region” would be a new probe of medium response to partons
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LHC is a good place for thermal photons/dileptons?
~15GeV?~6GeV?
Jet-photonconversion
Thermal
pQCD
LHC
Turbide et al., PRC77, 024909(2008)
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Direct photon spectra at d+Au and p+p
Excess in d+Au?◦ No exponential excess
High-pT direct photon results from PHENIX and STAR◦ d+Au
Agree with TAB scaled pQCD consistent with PHENIX and STAR
◦ p+p Agree with pQCD and PHENIX
Low-pT direct photon ◦ No publication data at STAR
2/9/2012T. Sakaguchi, RBRC lunch meeting
STAR, Phys.Rev.C81,064904(2010)
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