1/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section studies in ALICE experiment
Lucile Ronflette
QGP France 2014 - 2014/09/17
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
2/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Physics motivation
Prompt photon production
Isolation: a way to discriminate photon origin
Physics motivation
3/21
Challenge: study parton energy loss in the QCD medium
Photons as observables are not affected by hadronic matter
Tools: photon from hard scattering
Correlations
pp collisions
Cross section
test pQCD
contrain models (PDFs,
pp collisions) (nPDFs,
heavy ion collisions)
reference for heavy ion
collisions
PbPb, pPb collisions
gamma jet (cf. Denise
talk)
γ-hadron correlations (cf.
Nicolas Arbor thesis)
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Physics motivation
Prompt photon production
Isolation: a way to discriminate photon origin
Prompt photon production
4/21
Prompt photon
Figure : Direct photons
Figure : Fragmentation
photon
Prompt photon are produced in two partons hard
scattering.
Direct ones are not affected by the created medium.
Whereas in heavy ion collisions fragmentation ones
are affected by parton energy loss.
Cannot discriminate fragmentation from direct
photon by their kinematic properties
R.Ichou and al.arXiv:1005.4529
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Physics motivation
Prompt photon production
Isolation: a way to discriminate photon origin
Isolation: a way to discriminate photon origin
5/21
Necessity to distinguish prompt photon origin to access direct photon
Direct photon isolated from hadronic activity
R=
cone
P
cone
P
⇒ Isolation criteria
q
(Φ − Φphoton )2 + (η − ηphoton )2
pT ≤ pTthreshold
pT ≤ ǫpTphoton
Computation done with JETPHOX CTEQ6.6 and BFGII
R.Ichou and al.arXiv:1005.4529
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
ALICE experiment
EMCAL
Photon candidate
ALICE experiment
6/21
Main involved dectectors in isolated photon studies:
Charged particles tracks studies: ITS and TPC (|η| < 0.9 and φ = 2π)
Electromagnetic particles clusters studies: EMCAL |η| < 0.7 and 1.4 < φ < 3.3
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
ALICE experiment
EMCAL
Photon candidate
EMCAL
7/21
Pb/scintillator calorimeter
10 supermodules used in analysis (2011-2013)
11 520 cells
∆φ × ∆η = 0.014 × 0.014
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
ALICE experiment
EMCAL
Photon candidate
Photon candidate
8/21
Charged particle veto
Neutral clusters selection
|Φtrack − Φcluster | > 0.03 and |ηtrack − ηcluster | > 0.02
Shower shape
0
pTπ 10-60 GeV: merged π O decay photon clusters
into a high pT cluster
λ20 = 0.5 × (dηη + dΦΦ ) +
q
2
0.25 × (dηη + dΦΦ )2 + dηφ
Photon candidate cluster selection: 0.1 < λ20 < 0.3
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon in pp collisions @7 TeV
9/21
(Alexis Mas and al.)
2011 data set for pp collisions @ 7 TeV
L0 EMCAL trigger with pTthreshold = 5.5GeV
N evts ∼ 8M
Lint = 430nb−1
Efficiency estimation
Efficiency is computed using MC
simulations.
Purity estimation
Two existing data-driven methods to estimate
purity. One is presented here.
p(ET ) = 1 − (
N 6=iso (ET ) 0.1<λ2 <0.3
0
)
N iso (ET )
×(
N iso (ET ) 0.4<λ2 <1
0
)
N 6=iso (ET )
Lucile Ronflette
ǫ(ET ) =
id
dNγ
iso
dETreco
/
gen
dNγ
iso
gen
dET
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon in pp collisions @7 TeV
(Alexis Mas and al.)
Isolation criteria:
R = 0.4 and pTthreshold = 2GeV
|η| < 0.27
Jetphox computation with CTEQ6.6 PDF
and BFG II
Work ongoing on systematics
Results expected before the end of the year
Complementary to CMS (CMS PRL 106 (2011) 082001) and ATLAS (Phys.Lett.
B706(2011)150)
10/21
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Isolated photon cross section in pPb collisions @5.02 TeV
11/21
2013 data set for pPb collisions
2013 data set for Pbp collisions
L1 EMCAL trigger with 2 thresholds:
◮
◮
Rapidity shift
7GeV (N evts ∼ 700k)
11 GeV (N evts ∼ 1.2M)
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Isolation
12/21
Next Step: Isolation criteria determination
Energy contributions in isolation cone:
Photon candidate energy
Isolation cone: R = 0.4
Hadronic activity
Underlying events
Uderlying events come from all processes except the hard one. Its contribution is
especially significant in nucleus collisions
Substract underlying events contribution in isolation cone to apply a discriminant
isolation criteria
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Underlying events study
13/21
Charged and neutral UE: 2 implemented
methods
Only charged UE: 2 methods
Preliminary work:
Charged UE contribution: ∼ GeV /dΦdη
Neutral UE contribution: ∼ GeV /dΦdη
(To compare, in pp collisions, Charged and Neutral UE contribution: ∼ GeV /dΦdη)
In a next step:
Neutral UE contribution wouldn’t be needed if isolation only on charged particle.
It would enable to study isolation in all EMCAL acceptance improving statistics
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Normalisation: trigger rejection factor
Step: Trigger rejection factor is needed in order to determine the luminosity to
normalise the isolated photon spectra
TRJ L1 =
dN L1 /dENevt
dN MB /dENevt
Trigger rejection factors
are similar the three 2013
data set used in pPb
study.
TRJ 7GeV =
TRJ 11GeV =
14/21
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Outlook
Determination of isolated photon cross section in pPb collisions with 2013 data sets
isolation criteria: underlying event study ⇒ ongoing
isolated cluster raw spectra ⇒ to be determined using isolation on charged
particles only
purity ⇒ to be determined
efficiency ⇒ to be determined with MC simulations
normalisation ⇒ luminosity to be determined thanks to TRJ
Comparison with pQCD using EPS09 theoretical model
Further outlook: RpA
15/21
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
16/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Thanks!
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
17/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Back up
Lucile Ronflette
Isolated photon cross section studies in ALICE experiment
18/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Lucile Ronflette
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Isolated photon cross section studies in ALICE experiment
19/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Lucile Ronflette
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Isolated photon cross section studies in ALICE experiment
20/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Lucile Ronflette
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Isolated photon cross section studies in ALICE experiment
21/21
Physics motivation
Photon measurement in ALICE-EMCal
Isolated photon in pp collisions @7 TeV
Analysis strategy in pPb collisions
Lucile Ronflette
Isolated photon cross section in pPb@ 5.02 TeV
Isolation criteria
Normalisation
Outlook
Isolated photon cross section studies in ALICE experiment