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
2011-12-06 T. Sakaguchi, Thermal photons and dileptons3
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-
2011-12-06 T. Sakaguchi, Thermal photons and dileptons4
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
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(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
7 2011-12-06 T. Sakaguchi, Thermal photons and dileptons
(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!
10 2011-12-06 T. Sakaguchi, Thermal photons and dileptons
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
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Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1
thermal
diluted by prompt
Chatterjee, Srivastava PRC79, 021901 (2009)
Hydro after t0
2011-12-06 T. Sakaguchi, Thermal photons and dileptons
Later thermalization gives larger v2 (QGP photons)
Large photon flow is not explained by models for QGP
12 2011-12-06 T. Sakaguchi, Thermal photons and dileptons
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?
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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
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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
2011-12-06 T. Sakaguchi, Thermal photons and dileptons15
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
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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
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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?
2011-12-0619 T. Sakaguchi, Thermal photons and dileptons
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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
21 2011-12-06 T. Sakaguchi, Thermal photons and dileptons
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)
22 2011-12-06 T. Sakaguchi, Thermal photons and dileptons
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
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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
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~7m
Charge VETO pad chamber
~7m
EMCal & (Hcal)
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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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Backup
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