Photon Production at Ecm= 7 TeV
Suzanne Gascon-Shotkin
Institut de Physique Nucléaire de Lyon
Université Claude Bernard Lyon 1
IN2P3-CNRS
on behalf of the CMS and ATLAS Collaborations
Rencontres de Moriond QCD/High-Energy Interactions
March 10-17 2012 La Thuile Valle d’Aosta
S. Gascon-Shotkin Moriond QCD March 15 2012
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Outline
What direct photon measurements at the LHC can tell us
Anatomy of a differential direct photon cross section
measurement
Signal fraction measurement techniques
Differential cross sections, isolated inclusive photons
Comparison with Tevatron measurements
Differential cross sections, isolated photon + jets (ATLAS)
Differential cross sections, isolated inclusive diphoton pairs
Comparison with theoretical predictions and open questions
Conclusions and perspectives
Inclusive Photon/
Photon + Jet
Compton
Diphoton
Born
Box
HO
Box
Annihilation
2-Frag
Frag
1-Frag
S. Gascon-Shotkin Moriond QCD March 15 2012
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What direct photon measurements at the
LHC can tell us
Compared to the Tevatron, fraction of Compton
scattering remains ~80% with increasing
Et_gamma (isolated photons) [Ichou, d’Enterria 10]
Can probe gluon PDF at unprecedented Et
Can reduce gluon NLO PDF error by ~20%
Leads to ~20% reduction in s (gg → H) PDF
uncertainty (D’Enterria & Rojo 12)
Direct measurement of background processes to
Higgs boson production
D.d'Enterria & J.Rojo, arXiv:1202.1762
R.Ichou & D.d'Enterria,
PRD82 (2010) 014015
D.d'Enterria & J.Rojo, arXiv:1202.1762
S. Gascon-Shotkin Moriond QCD March 15 2012
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Anatomy of a differential direct photon cross
section measurement
d ns Isolated
n

i 1
dqi

1
n

i 1
qi
N
U  L 
Experimental measurement:
We measure the differential isolated cross section in bins of observables qi of width
qi
From a set of identified isolated photon candidates we estimate N, the number of
signal photons or photon pairs in each bin
To arrive at the true value of N in each bin:
We account for bin-to-bin migrations arising from detector resolution (unfolding
factors U)
We correct N for for =Trigger, reconstruction and photon identification
efficiencies
Comparison with (usually parton-level) theoretical predictions:
Impose a parton-level isolation
Correct for effect on isolation efficiency due to underlying event and hadronisation
(or assign a systematic error)
All measurements presented here were performed on the 2010 dataset with ~36 pb-1
of integrated luminosity
S. Gascon-Shotkin Moriond QCD March 15 2012
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Signal Fraction Measurement techniques: g + X
Phys. Rev. D 84, 052011
CMS: Combined photon Isolation
and Converted photon Et/pt
templates (MC w/ data validation),
2-comp. (un)binned EML fit, final
results combined with Best Linear
Unbiased Estimator method (Lyons
et al)
ATLAS: 'ABCD' double-sideband counting method
in EM shape and Isolation energy sum (cross-checked
with calorimeter isolation template) :
Phys. Lett. B 706 (2011) 150-167
Correlation between
2 variables, set to 1 and
systematic assigned
S. Gascon-Shotkin Moriond QCD March 15 2012
Signal Leakage 5
Signal Fraction Measurement techniques: g g + X
d 4s Isolated
dmgg dqTgg dgg d cosgg
*
Phys. Rev. D 85 (2012) 012003

1
mgg qTgg ( gg ) cosgg*
N gg unfolded
L 
ATLAS: ‘Event-weighting’ method (generalisation of that
used at Tevatron, see for example Phys. Rev. Lett.107,
102003 (2011).) Signal fraction :
where j={PP,PF,FP,FF} wrt
selection criterion on EtIso. , ni the observed outcome of the ith
event,  a matrix expressing efficiency for an event in true
state j’{gg, gj, jg, jj} to yield observed state j
Matrix coefficients obtained
from data-driven isolation PDFs
Xcheck: ABCD + 2d template
CMS: Electromagnetic Calorimeter Isolation template,
signal sample (random cones) and background sample
(photon candidates with a single clean track in the isolation
cone) entirely from data, validated with additional data
samples (W, Z and 2 clean-track sample [shown]). Binned
maximum likelihood fit to the isolation distributions of the two
photons
S. Gascon-Shotkin Moriond QCD March 15 2012
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Inclusive Differential Cross Sections: g + X
Comparison to pQCD fixed-order calculation: JETPHOX (Aurenche, Belghobsi, Catani,
Fontannaz, Guillet, Heinrich, Pilon, Werlen)
ds/dET for |h|<2.5 (CMS) 2.37 (ATLAS), 15(ATLAS) 25(CMS) GeV<ET<400 GeV
Correction to theory prediction from underlying event/hadronisation: ~2.5% (CMS)
Phys. Lett. B 706 (2011) 150-167,
Systematics-limited for ET<120 GeV, exp: ~7-20%
ATL-PHYS-PUB-2011-013
theoretical: dominated by scale (~10%)
Phys. Rev. D 84, 052011
S. Gascon-Shotkin Moriond QCD March 15 2012
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Inclusive Differential Cross Sections: g + X
Slight overprediction at ET<50 GeV
for both experiments
Phys. Lett. B 706 (2011) 150-167,
ATL-PHYS-PUB-2011-013
Phys. Rev. D 84, 052011
S. Gascon-Shotkin Moriond QCD March 15 2012
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Comparison with Tevatron Measurements
Opposite trends at low ET
values (<~50 GeV), possible
explanations?
Different regimes of
xT=2pT/√s ~0.003 (Tevatron)
~0.0015 (LHC)
Different maximum partonlevel Isolation ET : 2GeV
CDF, 4-5 GeV LHC
Phys. Rev. D 84, 052011
Regime dominated by
fragmentation
Reasonable for such a
rapid ET dependence to come
from physics?
Phys. Lett. B 706 (2011) 150-167,
ATL-PHYS-PUB-2011-013
S. Gascon-Shotkin Moriond QCD March 15 2012
An open question for
theorists and
experimentalists
9
Inclusive Differential Cross Sections: g + X
World data summary, obtain power slope of n=4.5 when scale by xT, supports pQCD
hypothesis (similar plot shown by K. Okada Tuesday)
D.d'Enterria & R.Ichou,
HEP-EPS'11 Proceeds.
S. Gascon-Shotkin Moriond QCD March 15 2012
xT=2pT/√s
10
Differential Cross Sections: ATLAS g + Jets
New! http://arxiv.org/abs/1203.3161
Motivation: Different photon-jet angular configurations correspond to regions with
different relative fragmentation contribution, also different values of {x,Q2}
Comparison to pQCD fixed-order calculation: JETPHOX, PYTHIA (Sjostrand et al)
SHERPA (Krauss et al)
ds/dETg for three different rapidity ranges of the leading-pT jet (anti-kt, radius parameter=
0.4), in same- and opposite-sign configurations of hg X yjet, |hg|<1.37, ETg>25 GeV, |yjet|<4.4,
pTjet>20 GeV, Rg,jet > 1.0
S. Gascon-Shotkin Moriond QCD March 15 2012
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Differential Cross Sections: ATLAS g + Jets
http://arxiv.org/abs/1203.3161
Same 'ABCD' double-sideband method for signal fraction extraction as for g + X
Correction to theory prediction from underlying event/pileup/hadronisation: ~0.85-0.92, 1
for high ET
Systematics-limited, typical errors, exp: ~3-40% , depending on rapidity/configuration,
highest for lowest- ETg photons; theoretical: dominated by scale (~20-40%)
Results consistent with those for g + X (data overprediction for ET<~40 GeV). Important test
of pQCD: Good data-theory agreement even in high-fragmentation (opp-sign) configurations
S. Gascon-Shotkin Moriond QCD March 15 2012
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Differential Cross Sections: gg + X
Comparison to pQCD predictions to order asa2: DIPHOX (Binoth, Guillet, Pilon, Werlen)
with gamma2MC (Bern,Dixon, Schmidt) [CMS], DIPHOX and ResBos (Balazs, Berger,
Mrenna, Nadolsky, Schmidt, Yuan) [ATLAS]. Correction for UE/hadronisation: ~5% (CMS)
ds/dQ for Q={mgg, ptgg, gg, cosq* (CMS)}, ET >{20, 23 GeV}, |h|<1.44 and |h|<2.5,
R(gg)>0.45 (CMS), ET >{16, 16 GeV}, |h|<1.37 and |h|<2.37, R(gg)>0.4
mgg : Slight underprediction for ATLAS at lowest values (low gg ,24 NNLO [see later]…
Phys. Rev. D 85 (2012) 012003
)
JHEP 01 (2012) 133
S. Gascon-Shotkin Moriond QCD March 15 2012
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Differential Cross Sections: gg + X
gg : Significant underprediction for all but highest values (<~2.8), corresponds to
collinear regime where no LO terms contribute, very sensitive to predictions of HO terms
Phys. Rev. D 85 (2012) 012003
JHEP 01 (2012) 133
S. Gascon-Shotkin Moriond QCD March 15 2012
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Differential Cross Sections: gg + X
ptgg, : Familiar ‘shoulder’ (underprediction) seen by both experiments as well as in prior
Tevatron measurements, combination of Et threshholds with non-back-to back topology
Phys. Rev. D 85 (2012) 012003
JHEP 01 (2012) 133
S. Gascon-Shotkin Moriond QCD March 15 2012
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Differential Cross Sections: gg + X
cosq*=tanh( ygg)/2
gg + X Uncertainties:
Experimental:
Statistical error ~20%
Systematics typically order of 10%
dominated by knowledge of
signal/background template shapes
(CMS), definition of control sample
(ATLAS)
Theoretical: 10-20% dominated by
scale uncertainties
JHEP 01 (2012) 133
Underprediction for high values seen
JHEP 01 (2012) 133
also by Tevatron experiments S. Gascon-Shotkin Moriond QCD March 15 2012
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Comparison with Theory: Open question
(JHEP 01 (2012) 133)
D. de Florian, L. Cieri et al
arXiv:1110.2375
Phys. Rev. D 85 (2012) 012003
D0: PLB690 (2010) 108
LHC-Higgs
XS WG
YR2
arXiv
1201.3084v1
First theoretical prediction of NNLO direct
contributions (de Florian, Cieri, et al 11, cf talk
Sunday), to be made public (2gNNLO)
Remarkable improvement in CMS data-theory
comparison for gg , stay tuned for other
observables and addition of box corrections
Caveat: Smooth (Frixione) not solid cone
isolation,
no frag.
S. Gascon-Shotkin Moriond QCD March 15 2012
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Conclusion and Perspectives
Direct photon measurements at the LHC performed with the 2010 dataset
corresponding to ~36 pb-1 of integrated luminosity are starting to be used to constrain
gluon and gluon fusion Higgs boson production PDF uncertainties
Differential cross sections, isolated inclusive photons
Measurements are already systematics-limited
Data in agreement with theory predictions and Tevatron measurements except for ET
<~50 GeV, where there are opposite tendencies, an open question
New ATLAS photon + jet differential cross-section measurement in three different
ranges of leading-pT jet y, results consistent with inclusive measurement, agreement
with theory in all g-jet configurations, even those with larger fragmentation contributions.
Differential cross sections, isolated inclusive diphoton pairs
Measurements consistent with those of the Tevatron experiments
Becoming systematics-limited
New NNLO calculation of direct contributions seems to resolve a large part of
collinear-regime discrepancies, need to take into account fragmentation and HO box
Perspectives: 2011 ~5fb-1 additional, 2012 ~15fb-1 at ECM =8 TeV
Possible measurements: Differential cross sections: Isolated inclusive, photon +
jets, inclusive diphoton pair , diphoton + jets, photon + hf, all with additional
observables. Ratios between cross-sections at ECM = 7 and 8 TeV .
Reexamine coherence of parton- and reconstruction-level isolation criteria, progress
with theorists in understanding of isolation/fragmentation relationship.
S. Gascon-Shotkin
Moriond QCD
March 15 2012
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Exploit direct measurement
of background
processes
for Hgg search
Acknowledgements
Thanks to:
L. Carminati , V. Cerulli , V. Chetluru, L. Cieri , D. D'Enterria, D. de Florian, P. Gras , M.
Grazzini, R. Ichou, J.-P. Guillet, K. Kousouris, S. Linn, D. Lopez Mateos, Y. Ma, L.
Malgeri, G. Marchiori, J. Owens, E. Pilon, A. de Roeck, J. Rojo, S.-S. Yu
S. Gascon-Shotkin Moriond QCD March 15 2012
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Generators/calculators of SM gg+X processes
DIPHOX
RESBOS
gamma2MC, NLO
Bern, Dixon, Schmidt,
Balazs, Berger, Mrenna,
Binoth, Guillet, Pilon, Werlen,
Yuan, hep-ph/9712471, 1997 hep-ph/0211216, 2002
hep-ph/9911340, 2000
2gammaNNLO
Catani et al,
hep-ph/11102375, 2011
qT SUB : NNLO
FIXED ORDER : NLO
NLO with NNLL
Resummation
FIXED ORDER : NLO
BORN + FRAG (and NLO corrections)
BOX (and NLO corrections)
Resbos only
BORN (up to NNLO corrections)
1-frag :
- LO, effectively in Resbos
- NLO in Diphox
2-frag :
DIPHOX only (NLO)
S. Gascon-Shotkin Moriond QCD March 15 2012
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