Prompt Photons at ATLAS
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Prompt Photons at ATLAS
Regina Caputo(Universität Mainz)
Phenomenology Symposium 2012 – Pittsburgh
On Behalf of the ATLAS collaboration6 May 2012
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Pheno 2012, R. Caputo 2
Outline
May 7, 2012
• Photons – A brief introduction from the
Standard Model• Photon ID in the ATLAS
detector• Cross section measurements– prompt photons– photon+jet– diphoton
• Summary and Conclusions– Standard Model and Beyond
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Pheno 2012, R. Caputo 3
Prompt Photons• Direct photons
– produced hard scatter– @LHC Compton dominate at
Leading Order (LO)
• Diphoton production• LO is not the whole story…
– QED radiation off quarks: ISR, FSR– Fragmentation
• +direct = prompt
• PHOX family and ResBos MC generation for Next-to-LO– parton level– to correct (PS Monte Carlo with
hadronization and UE) or additional systematic
May 7, 2012
Compton Scattering
Annihilation
€
γ
Box
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Pheno 2012, R. Caputo 4
Prompt Photons• Direct photons
– produced hard scatter– @LHC Compton dominate at
Leading Order (LO)
• Diphoton production• LO is not the whole story…
– QED radiation off quarks: ISR, FSR– Fragmentation
• +direct = prompt
• PHOX family and ResBos MC generation for Next-to-LO– parton level– to correct (PS Monte Carlo with
hadronization and UE) or additional systematic
May 7, 2012
Annihilation Fragmentation
ISR
FSR€
γ
€
γ
€
γ
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Pheno 2012, R. Caputo 5
Photons can tell usStandard Model• Constrain Gluon PDF
– sensitive to gluon content– high purity sample of quark
jets
• Test pQCD– colorless probe of hard
scattering
• Constrain photon fragmentation functions
• Calibrate jets
Beyond the Standard model• Background to Higgs
searches• Background to SUSY/Exotics
searches
May 7, 2012
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Pheno 2012, R. Caputo 6
Photons@ATLAS
May 7, 2012
• Particle requirements• Isolation
• Shower Shape Layer 2• width in η and ϕ• hadronic leakage
• Shower Shape Layer 1 (strips)• excellent η resolution (π0)
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Pheno 2012, R. Caputo 7
Isolated Photons
May 7, 2012
• Isolation• small amount of energy around core• out of core-leakage correction applied• pile-up contribution subtracted
prompt photons: deposit energy in small radius
€
ΔR = Δη 2 + Δφ2
€
ΔR < 0.4
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Pheno 2012, R. Caputo 8
Backgrounds to Photons
May 7, 2012
Background Modeling is difficult: • Use data driven methods whenever possible• Reverse photon ID• Sideband method
ISR, FSR, light neutral mesons
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Pheno 2012, R. Caputo 9
Photons@ATLAS
May 7, 2012
Photon Candidate π0 Candidate
• Hadronic background suppressed• shower shape variables, isolation• leakage variables
35 GeV 30 GeV
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Pheno 2012, R. Caputo 10
Photon Cross Sections
May 7, 2012
• Prompt Photons
• Photon+Jets
• Diphotons
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Pheno 2012, R. Caputo 11
Prompt Photons
May 7, 2012
U: Unfolding coefficients - evaluated using Pythia - bin-by-bin unfolding
- inversion + regularizationBayes or SVD
- ~1 (good resolution)
Phys.Lett. B706 (2011) 150-167
Ingredients to measure the cross section:Nyield: events after background
subtraction
ε: efficiencies ΔET
Υ : ET bin size
€
dσ
dETγ =
NyieldU
L∫ dt( )ΔETγ ε triggerε recoε ID
trigger IDreconstruction
45 GeV:
€
99.4−0.2+0.6
( )% ~85% barrel~75% end-cap
~90-97%
reconstructed, isolated andID photons
reconstructed & isolated photons
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Pheno 2012, R. Caputo 12
Prompt Photons
May 7, 2012 Phys.Lett. B706 (2011) 150-167
NLO pQCD calculationsJETPHOX using CTEQ 6.6 PDFs • Theoretical predictions
– Systematics from renormalization, factorization, fragmentation (~10%)
• Good in high ET, fair in low ET
– NNLO corrections
• Results used to constrain PDFs by 20%– Nucl. Phys. B 3 311-338
(2012)
ET,γ > 45 GeV, |ηγ| bins up to 2.37
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Pheno 2012, R. Caputo 13
Prompt Photons
May 7, 2012 Phys.Lett. B706 (2011) 150-167
NLO pQCD calculationsJETPHOX using CTEQ 6.6 PDFs
Comparing different PDF sets
Theoretical PredictionsSystematics -renormalization, factorization, fragmentation (~10%)
Results used to constrain PDFs by 20%Nucl. Phys. B 3 311-338 (2012)
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Pheno 2012, R. Caputo 14
Photon Cross Sections
May 7, 2012
• Prompt Photons
• Photon+Jets
• Diphotons
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Pheno 2012, R. Caputo 15
Photon+jet
May 7, 2012 Arxiv:1203.3161, Accepted PRD
• Photons produced with jets– 3 jet rapidity regions as a
function of Etγ
• Fiducial/reconstruction requirements– Jet Algorithm: anti-kT, R=0.4
– Jet: pT>25 GeV, |y|>4.4
– photon: ET>25 GeV, |η|>1.37– separated by ΔR > 1.0
• Direction of photons/jets– ηγ yjet ≥ or < 0
• Different composition of fragmentation component
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Pheno 2012, R. Caputo 16
Photon+jet
May 7, 2012 Arxiv:1203.3161, Accepted PRD
same sign
Fair agreement: Data consistently lower than prediction in low Et
γ region• hint for need of NNLO (consistent with prompt photon results)
opposite sign
Theoretical Predictions:Systematics from
scale, PDF, Isolation, hadronization and underlying event correction.
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Pheno 2012, R. Caputo 17
Photon Cross Sections
May 7, 2012
• Prompt Photons
• Photon+Jets
• Diphotons
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Pheno 2012, R. Caputo 18
Diphotons
May 7, 2012
dσ/dmγγ dσ/dpT,γγ dσ/dΔϕγγ
Phys.Rev. D85 (2012) 012003
ET,γ > 15 GeVDifferential Cross section mγγ, pT,γγ,Δϕγγ
DIPHOX and ResBos provide NLO prediction
Theoretical Systematics Isolation, PDF, and scales
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Pheno 2012, R. Caputo 19
Diphotons
May 7, 2012
dσ/dmγγ dσ/dpT,γγ dσ/dΔϕγγ
Phys.Rev. D85 (2012) 012003
Generally good agreement: Discrepancies in low Δmγγ and Δϕγγ
• same cause: fragmentation and 2 4 processes• argument NNLO: 2gNNLO – box diagram (DeFlorian et. al)
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Pheno 2012, R. Caputo 20
Summary and Conclusions
May 7, 2012
Insight into QCD PDFs– able to constrain PDFs with 2010 data– analyzing ~5 fb-1 - results in progress– more fragmentation, higher pT, understand pileup
Backgrounds to new physics– contributes to Higgs searches– SUSY, model independent searches– Also more exotic particles UED, graviton,
resonance searches
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Pheno 2012, R. Caputo 21
BACKUPS
May 7, 2012
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Pheno 2012, R. Caputo 22
The PHOX Family• What it does
– NLO FORTRAN codes allowing users to compute single and double inclusive large pT cross sections for reactions involving photons, hadrons and jets.
– DIPHOX, JETPHOX, EPHOX and TWINPHOX
– MRST99, MRST01, CTEQ5 and CTEQ6 PDFs for the proton
– photo-production programs also include the AFG and the new AFB04 PDFs for the photon.
– option to link any parton distribution from the PDFLIB is also provided.
– production of massive heavy quarks is not described by these codes in which a massless approximation is used
• Pros– flexible and allows the users to
impose almost any experimental cuts, jet definitions, cross section definition via a histogram package
• Warnings– The production of massive heavy
quarks is not described by these codes in which a massless approximation is used
– Not full event generators:– do not provide a full, exclusive
portrait of events which could for example be further processed through a detector simulation
– PHOX codes are not designed to be interfaced with parton showers and hadronisation models.
May 7, 2012
See: http://lapth.in2p3.fr/PHOX_FAMILY/
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Pheno 2012, R. Caputo 23
Dominant SystematicsTheoretical • Scale
– renormalization, factorization, fragmentation
• PDF– from CTEQ 6.6 eigenvalues
• non-perturbative correction– maximum spread in Pythia
(Perugia 2010) and Herwig (UE7000-2)
• Parton Isolation
Experimental• Electron energy scale• Photon Purity• Unfolding technique
– compare methods
• Central jet– same/opposite sign
May 7, 2012
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Pheno 2012, R. Caputo 24
Prompt Photons
May 7, 2012 Phys.Lett. B706 (2011) 150-167
NLO pQCD calculationsJETPHOX using CTEQ 6.6 PDFs
ET,γ > 45 GeV
NLO pQCD calculationsJETPHOX using MSTW 2008 PDFs
NLO pQCD calculationsJETPHOX using NNPDF 2.0 PDFs
Dominant SystematicsScale uncertainty ~10%renormalization, factorization, fragmentation
Results used in: Nucl. Phys. B 3 311-338 (2012)
show CTEQ and then ratios for all three in 1 eta bin
Comment on agreement…
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Pheno 2012, R. Caputo 25
Ratios to Theory
May 7, 2012 Phys.Lett. B706 (2011) 150-167
ET,γ > 45 GeV
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Pheno 2012, R. Caputo 26
Photon+jet Systematics
• Central jet– same/opposite sign
• Dominant Systematics – Scale
• renormalization, factorization, fragmentation
– PDF • vary 68% CL
– Isolation • 3/5 GeV
– non-perturbative correction• maximum spread in Pythia
(Perugia 2010) and Herwig (UE7000-2)
May 7, 2012
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Pheno 2012, R. Caputo 27
Diphotons
• Probe QCD– Δϕγγ sensitive to
fragmentation model– soft gluon emission
Δϕγγ~π and low pT,γγ
• Irreducible background Higgs and BSM
• Differential cross section– mγγ,pT,γγ, Δϕγγ
May 7, 2012 Phys.Rev. D85 (2012) 012003
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Pheno 2012, R. Caputo 28
Diphoton Systematics
May 7, 2012
dσ/dmγγ dσ/dpT,γγ dσ/dΔϕγγ
Phys.Rev. D85 (2012) 012003
Dominant Systematics • normalization, fragmentation and factorization scales• eigenvalues of PDFs • MSTW 2008 PDF (CTEQ6.6)
Labels wrong on y axis, no atlas label