New degree of freedom in thermal photon measurement

New degree of freedom in thermal photon measurement

Takao SakaguchiBrookhaven National Laboratory

Serious Preface Discovery and quantification are both important and should

go along– Mandatory condition as far as I think, in order to expand a field (and

get budget)

Challenging and promising measurements are therefore important and should go along

We have promising future measurements, but not (many) challenging measurements

Let’s think about challenging measurement.– Make the case successful, we need both good theoretical predictions

and experimental feasibility study

2011-12-06 T. Sakaguchi, Thermal photons and dileptons2


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)

Electromagnetic probes (was challenging)Photon Production: Yield s

g

g*e+

e-


First gdir in Au+Au (hard scattering)

Blue line: Ncoll scaled p+p cross-section

Au+Au = p+p x TAB holds – pQCD factorization works

NLO pQCD works. Non-pert. QCD may work in Au+Au system

5 2011-12-06 T. Sakaguchi, Thermal photons and dileptons

(fm/c)log t1 10 107

hadrondecays

sQGP

hard scatt

Possible sources of photons

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)

2009/08/04 T. Sakaguchi, Physics Colloquium6

Difficult objects! Photons from QGP~big challenge~ 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): () g(), K* Kg

)0(Im23 Mfpd

dRE em

Bem

g

)0(Im324 MfMpd

dRem

Bemee

fB: Bose dist. em: photon self energy

photons

dileptons

Interesting, but S/B is small

54321

S/B ratio


(fm/c)log t1 10 107

hadrondecays

hadron gas

sQGP

hard scatt

Adding virtuality in photon measurement

Mass(GeV/c2)

0.51

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)

T. Sakaguchi, Thermal photons and dileptons8 2011-12-06

Comptonq g*

g q

e+

e-

Internal conv.

One parameter fit: (1-r)fc + r fd

fc: cocktail calc., fd: direct photon calc.

rg*dir(m 0.15)g*inc (m 0.15)

g*dir(m 0)g*inc (m 0)

gdirginc

1Ng

dNeedmee

23

1 4me2

mee2 (1

2me2

mee2 )

1mee

F(mee2 )

2(1 mee

2

M 2 )3

Focus on the mass region where 0 contribution dies out

For M<<pT and M<300MeV/c2

– qq ->g* 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

PRL104,132301(2010), arXiv:0804.4168

Low pT photons with very small mass

d+Au Min. Bias

Low pT photons in Au+Au (thermal?)

9

PRL104,132301(2010), arXiv:0804.4168

2011-12-06 T. Sakaguchi, Thermal photons and dileptons

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

Au+Au

Won Nishina memorial prize!


Adding collision geometry dependence

annihilationcomptonscattering

Bremsstrahlung (energy loss)

jet

jet fragment photon

v2 > 0

v2 < 0

Depending the process of photon production, path length dependence of direct photon yield varies

v2 of the direct photons will become a source detector

Later thermalization gives larger v2

For prompt photons: v2~0

Results on path-length dependence

11

Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1

thermal

diluted by prompt

Chatterjee, Srivastava PRC79, 021901 (2009)

Hydro after t0


Later thermalization gives larger v2 (QGP photons)

Large photon flow is not explained by models for QGP


LHC is a good place for thermal photons/dileptons? 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

~15GeV?~6GeV?

Jet-photonconversion

Thermal

pQCD

LHC

Turbide et al.,arXiv:0712.0732

New degree of freedom?


One step forward with electromagnetic probes..

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


Rapidity as a clock of system evolution 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


g2

g1

dz ~ 2R/100

dx ~

2R

Landau and Bjorken expansion modelscentral collision of equal nuclei at 2 1/NN Ns mg

differ mostly by initial conditions

proper time 2 2zt t1 t + zln2 t - z

space-time rapidity 2011-12-0616 T. Sakaguchi, Thermal photons and dileptons


Rapidity dependence ~system expansion~ Forward direct photons shed light on time evolution scenario

– Real photons, g*->ee, g*->mm

T. Renk, PRC71, 064905(2005)

Rapidity dependence ~probing initial condition~

Color Glass Condensate

Glasma


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?


T. Sakaguchi, Thermal photons and dileptons20

Singular point in phase diagram that separates 1st order phase transition (at small T) from smooth cross-over (at small mb)

2011-12-06

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

Finding the QCD Critical Point


High Rapidity as a high baryon system

Higher the rapidity goes, higher the baryon density we may be able to reach

BRAHMS plot. Another way to access to the critical point?

BRAHMS, PRL90, 102301 (2003)


Review ~BRAHMS results Charged hadron results and some pion/proton ratio results

Might be an idea to extend our measurement to 0/direct photons/dileptonsBRAHMS, PLB 684(2010)22.BRAHMS, PRL91, 072305(2003).

Drell-Yan as an energy loss probe Genuine process that involves “quark”

– Quark energy loss can be measured– Need a lot of help from model calculations


Hot matter created in HIC S. Turbide, C. Gale, D. Srivastava,R. Fries, PRC74, 014903 (2006)

How about measurement? ~Detector Plan~ Take Axel’s strawman’s design

– Cover’s rapidity range of y = 3-4


~7m

Charge VETO pad chamber

~7m

EMCal & (Hcal)


How about measurement?~A technology choice: MPC-EX~ Muon Piston Calorimeter extension (MPC-EX) (3.1<||<3.8)

– Shower max detector in front of existing MPC. Now sits at ~3m from IP– Measure direct photons/0 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

Summary

Rapidity may be a new degree of freedom on photon/dilepton measurement

Higher rapidity may shed light to the pre-equilibrium state as well as time evolution of the system

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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New degree of freedom in thermal photon measurement