1. Photon Physics with PHOS
International Workshop
on Heavy Ion Physics at LHC
Photon Physics with PHOS
at Center China of Normal University
Institute of Particle Physics
May 21-24, 2008, Wuhan, China
Toru Sugitate / Hiroshima Univ.

2. Outline of the Talk
Photon sources and physics
Lessons from RHIC
Reality and strategy for the 1st years
Conclusion

3. Time Line of the Universe
Photons: the important probe to explore the nature

4. Photons in pp & AA collisions
pQCD photons
Precise calc. w/ pQCD
Isolated photons
Only little abundance
hadronic photons
Calc. w/ pQCD, eg. PYTHIA
meson decays in jets
Parton Distribution Function (PDF)
Subprocess cross section
calculated with pQCD
Fragmentation
Function (FF)
prompt photons in AA
Calc. w/ Lattice QCD
Thermal photons: QGP and HG
p~T~GeV

5. Photon Physics in AA collisions
Hadronization
(Freeze-out) +
Expansion
Pre-equilibrium
Thermalization
QGP phase?
Mixed phase
decay photons
direct thermal photons
direct pQCD photons
Global observables: Multiplicities,  distributions
Degrees of freedom as a function of T: hadron ratios and spectra, dilepton continuum, direct thermal photons
Early state manifestation of collective effects: elliptic flow
Energy loss of partons in quark gluon plasma: jet quenching, high pt spectra, open charm and open beauty
Deconfinement: charmonium and bottonium spectroscopy
Chiral symmetry restoration: neutral to charged ratios, res. decays
Fluctuation phenomena - critical behavior: event-by-event particle comp. and spectra
Geometry of the emitting source: HBT, impact parameter via zero-degree energy flow
pp collisions in a new energy domain
Global observables: Multiplicities,  distributions
Degrees of freedom as a function of T: hadron ratios and spectra, dilepton continuum, direct thermal photons
Early state manifestation of collective effects: elliptic flow
Energy loss of partons in quark gluon plasma: jet quenching, high pt spectra, open charm and open beauty
Deconfinement: charmonium and bottonium spectroscopy
Chiral symmetry restoration: neutral to charged ratios, res. decays
Fluctuation phenomena - critical behavior: event-by-event particle comp. and spectra
Geometry of the emitting source: HBT, impact parameter via zero-degree energy flow
pp collisions in a new energy domain
Experimental advantages of Photon Measurement
a single calorimeter measures photons and neutral mesons.
a calorimeter identifies particles up to very high momentum
Important physics outcome on DAY-1
Most-cited single results from RHIC; 422 cited as of May 2008
Suppression of hadrons with large transverse momentum in central Au+Au collisions at s(NN)**(1/2) = 130-GeV.
By PHENIX Collaboration (K. Adcox et al.). Sep pp. Published in Phys.Rev.Lett.88:022301,2002 / e-Print Archive: nucl-ex/

6. Lesson-1 from RHIC p+p->p0 + X KKP Kretzer h++h- p0
data vs pQCD
p+p->p0 + X
hep-ex/ S.S. Adler et al.
h++h- p0
Find discrepancies in Au+Au from elementary processes at high pT.
The low pT feature has been known at SPS and understood as nuclear effects.

7. The Jet Quenching at RHIC
Both neutral mesons and photons are essential probes for the jet quenching.
陽子＋陽子衝突におけるジェット生成
高運動量粒子
ｸｵｰｸ
ハドロン粒子多重生成
（ジェット） 真空
原子核衝突におけるジェット抑制
Suppression is very strong (RAA=0.2!) and flat up to 20 GeV/c
Common suppression for p0 and h; it is at partonic level
e > 15 GeV/fm3; dNg/dy > 1100
RAA at higher pT reveals new features.

8. Systematic error is dominant.
Lesson-2 from RHIC
Systematic error is dominant.

9. Direct g via g* measurements
First direct photon excess seen at PHENIX
Compare direct g and g* at LHC
PHENIX preliminary
decay photons
thermal photons:
Schematic spectrum
T0max ~ MeV !?
T0ave ~ MeV !?
The first promising result of direct photon measurement at low pT from low-mass electron pair analysis.
Are these thermal photons? The rate is above pQCD calculation. The method can be used in p+p collisions.
If it is due to thermal radiation, the data can provide the first direct measurement of the initial temperature of the matter.
pQCD photons
schematic purpose only

10. Another Ion Collider at CERN
s = 14 TeV for proton + proton
sNN = 5.5 TeV for Pb + Pb
sNN at LHC = 28 x RHIC =320 x SPS = 1000 x AGS
ＣＭＳ実験
ＬＨＣ-ｂ実験
ＡＴＬＡＳ実験
ＡＬＩＣＥ実験

11. Thermo-dynamic feature
“Expected” Features at LHC
QGP formation
X 2 TRHIC
X RHIC
X 3-5 VFORHIC
X 3-5 QGPRHIC
dominant hard process
heavy quark production
X 2000
~2% at SPS
~50% at RHIC
~98% at LHC
Thermo-dynamic feature
p~T~GeV
Thermal photon physics
High pT jet physics
Heavy flavor physics

12. Photon Detectors at LHC
Exp.
ATLAS
CMS
ALICE
Name
LAr Barrel
LAr Endcap
ECAL(EB)
ECAL(EE)
PHOS
EMCal
Structure
Liquid Ar
PWO + APD
~80,000ch
~18,000ch
Pb + APD
Coverage
0<|h|<1.4, 2p
1.4<|h|<3.2,
0<|h|<1.5,
1.5<|h|<3.0,
0<|h|<0.12,
0.6p
0<|h|<0.7,
Dynamic Range
20MeV-2TeV
upto 4TeV
5MeV-80GeV
16MeV-250GeV
Granularity x
0.003x x x0.050
0.025x x x0.050
0.0174x0.0174 to
0.05x0.05
0.004x0.004
0.0143x0.0143
Res.
10%/E
0.5%
2.7%/E
0.55%
5.7%/E
3.3%/E
 1.1%
7%/E
1.5%

13. Simulation Studies Event display with AliRoot
Background photon source map

14. Lots of PCB/frame/pipes there
ITS+TPC+TRD+TOF
X/X0~”43%”80%

15. Direct Photon Sensitivity
Direct photon sensitivity (sig/noise) along two scenarios; with and without jet quenching.
Thermal photons
gall/gdec
thermal g enhanced range
signal strength w/o quenching
Systematic error with a TRD/TOF hole
gall/gdec
signal strength with quenching
A hole in TRD/TOF for 3 central PHOS modules, reducing X/Xo=80% down to ~20%, open the thermal photon sensitive window down to 3-4 GeV.
Thermal photon sensitive window

16. PHOS Strategy in 1st LHC year
Photon physics with PHOS is very promising from the 1st year, but
There are some issues to be cleared for the success:
single warm PHOS in 2008
small acceptance; less yield, higher mgg cutoff, and calibration strategy
low LY/gain; larger missing energy, higher trigger threshold, and increase non-linearity
poor mgg resolution; increase sys. errors 0
See Yuri’s TF list 
Mgg[GeV] 0
1st Module as of 15 May, 2008
Mgg[GeV]
3 PHOS modules 
geometrical acceptance
1 PHOS module
pT[GeV]
0 &  acceptance by Takashi Iwasaki
pT[GeV]
* students’ working version

17. PHOS Strategy in 2nd LHC year
p+p at 14TeV and 1st Pb+Pb run expected
Install 3 cold PHOS modules for the 1st Pb+Pb runs
assemble two modules by this fall
build the air-tight shells
integrate photon triggers
Learn the spectrometer from p+p runs
Tune the spectrometer for the best energy and spatial performances to minimize the systematic uncertainties
Photon analysis in reality is not easy but fruitful output guaranteed
Subgroups are now being formed in PWG4 under Yves;
~1000 p0
in 1-2 days
500k p0/109 events w/ warm PHOS
by HT
“You are very welcome to join the p0 team.” said by Hisa Torii, the convener.

18. Conclusion
ALICE is a versatile detector and PHOS is optimized for measurements of thermal photons and neutral mesons up to moderate energies.
Physics scope with PHOS in the 1st LHC years;
pT spectra of neutral mesons in pp and AA
Seek new physics at the energy frontier!
Promising outcome comparing with pQCD
RAA of neutral mesons & photons up to mod. pT
Promising outcome from 1st years
RAA in d+A
Indispensable info. planned in 3rd year.
Thermal photons from QGP/HG
Need good understanding of apparatus for accurate all photon and meson yields, and good AA runs
pion yield from p+p in 30days by LB

19. Thank you for your attention.