takao sakaguchi bnl 2/9/2012 1t. sakaguchi, rbrc lunch meeting

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)

2/9/2012 T. Sakaguchi, RBRC lunch meeting 2

Electromagnetic probes (was challenging)

Photon Production: Yield s

g

*ge+

e-


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


First gdir in Au+Au (hard scattering)

Blue line: Ncoll scaled p+p cross-section


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


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


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)

2/9/2012

9T. Sakaguchi, RBRC lunch

meeting

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


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


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..


122/9/2012

A plate ~After cooking up ingredients~

132/9/2012 T. Sakaguchi, RBRC lunch meeting

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)

142/9/2012T. Sakaguchi, RBRC lunch

meeting

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


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

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System size dependence of g fraction

g fraction = Yielddirect / Yieldinclusive

Largest excess above pQCD is seen at Au+Au.◦ Moderately in Cu+Cu also.


meeting

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

2/9/2012T. Sakaguchi, RBRC lunch meeting

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

2/9/2012


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)


192/9/2012

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..


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

2/9/2012


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

2/9/2012


p0 v2


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

2/9/2012



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

T. Sakaguchi, RBRC lunch meeting 24

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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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…


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


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


Rapidity as a clock of system evolution

g2

g1

dz ~ 2R/100

dx ~

2R


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


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?


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)


meeting 32

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?


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


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


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


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


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.


Summary


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


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)

takao sakaguchi bnl 2/9/2012 1t. sakaguchi, rbrc lunch meeting

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