FEASIBILITY STUDY OF DIRECT PHOTON MEASUREMENT VIA
INTERNAL CONVERSION IN ALICE
Feb. 6 2009High-pT Physics at LHC09 @
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T. HoraguchiHiroshima University
[email protected]. 4 for the 4th International Workshop High-pT
Physics at LHC09 @ Prague
Contents Introduction Direct Photon Measurement in ALICE Low pT Photon Virtual Photon Measurement Background Study
Background Sources Combinatorial Background Hadron Decay Combinatorial + Hadron PT Sliced Mass Spectra
Evaluation the Statistics of First Year Summary & Future Plan
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What dose mean the measurement of direct photons ?
Direct photons in p+p collisionsTest of pQCD calculationObtain the gluon distribution functionReference data of the heavy ion
collisionsDirect photons in heavy ion collisions
Jet quenchingThermal photons
Direct photons are a clear probe to investigate the characteristics of evolution of the matter created by heavy ion collisions.
Penetrate the created matter without the strong interaction
Emitted from every stage of collisions Hard photons (High pT)
– Initial hard scattering, Pre-equilibrium
Thermal photons (Low pT)– Carry the thermodynamic
information from QGP and hadron gas
Introduction
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Direct Photon Measurement in ALICE
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Hard photon Strong suppression of high pT
hadrons will help to improve the S/N ratio
High pT photons can be found Thermal photon Direct evidence of thermal
equilibrationCreated matter in LHC will have
high temperature, high density and long life time matter comparison with RHIC, so we can expect large thermal photon component in ALICE
Primary contributor in low pT regionThermal photon measurement is
very challenging because it is very hard due to a large background from hadron decays.
Low pT Photons
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In ‘real’ photon measurement Measured yield with a large systematic errorDifficulty on measuring low pT “real” direct photons
1. Finite energy resolution of the EMCal
2. Large hadron background
Advantages on measuring ‘virtual’ photons
1. High momentum resolution of the TPC
2. Reliable estimation of the hadron decay components using Kroll-Wada formula
Experimental determination is very important since applicability of pQCD is doubtable in low pT region
Virtual Photon Measurement
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Case of Hadrons
Obviously S = 0 at Mee > Mhadron Case of g*
– If pT2>>Mee2
Possible to separate hadron decay components from virtual photon in the proper mass window.
Any source of real g can emit g* with very low mass. Convert direct g* fraction to real direct photon yield
S : Process dependent factor
3
2
222 1
hadron
eeee M
MMFS
1S
qg*
g q
e+
e-
g SdN
MMm
Mm
dMNd
eeee
e
ee
e
ee
1214132
2
2
2
22
*
*
inclusive
direct
inclusive
direct
gg
gg
Kroll-Wada formula
Background Sources
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Real signal di-electron continuum
Background sources1. Combinatorial background2. Material conversion pairs3. Additional correlated
background– Cross pairs from decays
with 4 electrons in the final state
– Pairs in same jet or back-to-back jets
Hadron decays 0, h, h’, w, f, r, J/y, y’
π0
π0
e+e-
e+
e-γ
γ
π0e-γ
e+
π0 γ
e+e-
e-
e+
Jet cross pair
Dalitz + conversion cross pair
Background Sources
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Real signal di-electron continuum
Background sources1. Combinatorial background2. Material conversion pairs3. Additional correlated
background– Cross pairs from decays
with 4 electrons in the final state
– Pairs in same jet or back-to-back jets
Hadron decays 0, h, h’, w, f, r, J/y, y’
π0
π0
e+e-
e+
e-γ
γ
π0e-γ
e+
π0 γ
e+e-
e-
e+
Jet cross pair
Dalitz + conversion cross pair
PYTHIA Simulation
PYTHIA MSEL=1 14TeV pp 10M event 4 coverage No Detector Simulation No virtual photon event
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Pi+-
Red : +
Blue : -
h distribution (+
e++e- from hadron decay
PT distribution
Hadron Decay
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The hadron decay background affects mainly in low mass region.
ee
eeee
eeee
ee
ee
ee
r
hf
w
gh
gh
g
&
& 0
0
Hadron Decay Mode
Combinatorial Background Combinatorial background is
evaluated using mixed event method.
Normalization is done using the like sign pair.
The normalized combinatorial background is good agreement with the unlike sign pair in high mass region.
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Black : unlike sign pairRed : Like sign pair (++)Blue : Like sign pair (--)
Black : unlike sign pairRed : Normalized combinatorialbackground
NNN 2
)cos1(2 eeee
comee PPM
e-’
e+e-e+’
Combinatorial pair
Combinatorial + Hadron Decay The combinatorial +
hadron decay background is very good agreement with mass spectrum !
The mass spectrum in Low mass region is mainly produced by hadron decay.
The mass spectrum in High mass region is mainly produced by combinatorial background.
The contribution of jet correlated pair seems to be negligible in this mass region.
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Black: Total unlike sign pairRed : Combinatorial + Hadron Decay
Good Agreement !
pT Sliced Mass Spectra
The combinatorial + hadron decay background is very good agreement with mass spectra for each pT bin !
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0<pT<0.5GeV/c
3.5<pT<5.5GeV/c
2.0<pT<3.5GeV/c
1.0<pT<2.0GeV/c
0.5<pT<1.0GeV/c
Total
Evaluation the Statistics in First Year
Evaluation from NLO pQCD calculation
Used INCNLO http://wwwlapp.in2p3.fr/lapth/PHOX_FAMILY/readme_inc.htm CTEQ6M, BFG √s : 14TeV pp μ : 0.5pT,1.0pT,2.0pT
Evaluation of the number of the virtual photon
Assumed DAQ rate :100KHz 1 Day：~2M 1 Month：~60M 3 Month: ~ 180M
Acceptance Correction Considered TRD
acceptance | h|< 0.9 f coverage: 8/18 x 2
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30 Days90 Days
Enough Statics !
Summary & Future Plan The e+e- pair mass spectrum is calculated in
p+p 14TeV collisions with PYTHIA simulation. The mass spectrum is very good agreement with
the combinatorial background + hadron decay background !
More precise background study will be needed with the detector simulation.
The statistics of virtual photon is evaluated for first year. The signal will be seen if we have more one month beam time. This result strongly encourages us to measure the direct photon via internal conversion !
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Backup
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Direct Photon Production
There are two processes: “Direct process” “Fragmentation process”
The direct photon production dominates two leading –order subprocesses Quark-gluon Compton scattering
(qg→gq) Quark-anti-quark annhilation (qq-bar→
gg)
Feb. 6 2009
Quark-gluon Compton scattering (qg→gq) at LO
Quark-anti-quark annihilation (qq-bar→ gg) at LO
Fragmentation Process
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Review of other experiments
The measurement of direct photon cross sections in proton-proton collisions Midrapidity varying from 19.4GeV to
63GeV was performed by E706 at Tevatron. E704 at FNAL. UA6, WA70 and NA24 at SPS. R110, R806 and R807 at ISR
of CERNFeb. 6 2009
s
The measurement of direct photon cross sections proton-anti-proton collisions Midrapidity varying from 24.3GeV to
1800GeV was performed by UA6 at SPS. UA1 and UA2 at CDF and D0 at Tevatron.
pp pp
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Review of the Direct Photon Measurement
Feb. 6 2009
