Prompt Photon Production at ATLAS - Physics Division · Dj = 0.0245 Dh = 0.025 37.5mm/8 = 4.69 mm...

Prompt Photon Production at ATLAS

Mike HanceUniversity of Pennsylvania16 December 2010

Photons

GeVScale

10−15 → 10−18 m

Photons were the focus of our first(built-in) particle detectors....

One of the basic objectsreconstructed at moderndetectorsPresent in many interestingfinal statesProbes QCD and new physics

M. Hance 2 / 56 LBNL RPM- 16 December 2010

Outline

Outline of today’s seminar:

Prompt Photons at ATLASSM Prompt Photon Physics (Overview)The LHC and ATLASPhoton IDBackground EstimatesInclusive Isolated Cross Section

Future Work With PhotonsMore Inclusive StudiesSM DiphotonsHiggsExotics


Prompt Photon Physics

Inclusive Isolated Prompt Photons

How are photons produced in pp collisions?

q

q

qg

γ

Compton Scattering

q

q

q

g

γ

Annihilation

At leading order, the dominant mechanism at the LHC is the Compton-likeprocess.

This is true for all EγT (at Tevatron, annihilation process dominates athigh EγT )Allows us to probe the gluon content of the protonWe call these direct photons


Inclusive Isolated Prompt Photons

But the story goes beyond leading order....

q

q

qg

g

γ

ISR

q

q

qg

g

γ

FSR

q

q

g

g

γ

Fragmentation

At NLO, photons are produced via QED radiation off quark lines (brems) orvia fragmentation

At low EγT , these processes are very importantAt higher EγT , the LO diagrams are dominantWe call these fragmentation photons


Isolation?

Why only consider isolated prompt photons?

vs

Isolation helps suppress backgrounds (e.g. π0 → γγ), also helps suppressfragmentation photons.


Prompt Photons at NLOState of the art for theoretical predictions:

Fully NLO calculators available - most commonly used is JETPHOXAllows for different isolation prescriptionsBenchmark for most recent Tevatron analysesSister package to DIPHOX, for diphoton production

Some recent work at NNLL by Becher et al, but still relies onJETPHOX for isolationAll rely on unphysical scales to make finite predictions at NLO

Renormalization, factorization, fragmentation scales, all set to ≈EγT

[GeV]TE

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Tp=µσ)

/ dTp=µσ

- d

µσ(d

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Renormalization scale

Initial state factorization scale

Final state factorization scale

All scales

NLO pQCD JETPHOX 1.2.2

= 7TeVs

CTEQ6.6

| < 2.37, iso < 4 GeVη|

T = 2.0pµ and

T = 0.5pµ

[GeV]TE

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= 7TeVs

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T = 0.25pfM

T = 0.33pfM

T = 0.5pfM

T = 1.0pfM

T = 2.0pfM

T = 3.0pfM

T = 4.0pfM


Measuring the Cross Section

One of the first questions we can ask (and answer): how many do we see?

dσdEγ

T= N · P · U

∆EγT · ε · L

N: Number of candidatesP: Purity of candidates∆pT : Size of pT binε: Efficiency (trigger, reconstruction, selection)U: Unfolding factors (detector response)L: Integrated luminosity

Fortunately, we have just the detector for the job....


The LHC and ATLAS

First beam in 2009

First 7 TeV collisions in 2010

Luminosity

After all of the initial fits and starts... the machine performance has beenexcellent:

Day in 2010

24/03 21/0419/05 16/06 14/07 11/0808/0906/10 03/11

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LHC Delivered

ATLAS Recorded

-1Total Delivered: 48.9 pb-1Total Recorded: 45.0 pb

Day in 2010

24/03 19/05 14/07 08/09 03/11P

eak

<E

vent

s/B

X>

0

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2.5

3

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4

4.5 = 7 TeVs ATLAS Online

LHC Delivered

Absolute luminosity calibration using van der Meer scans:Many methods within ATLAS - all agree within a few percent11% total uncertainty on luminosity - dominated by bunch intensity


A Toroidal LHC ApparatuS

Inner Detector

Transition Radiation Tracker

350k channel tracker4mm (diameter) strawsTR detection: e/π±

discrimination≈36 hits on track≈130µm resolution

Semi-Conductor Tracker

6.3M channels4 cylinders, 8 hits/track≈17µm resolution

Pixel Tracker

80M channels, 3 layers≈10µm resolution


Calorimetry

EM CalorimeterPB-LAr Accordion∆E/E =

(10%/

√E)⊕ .7%

.025×.025 cells (η × φ)Angular res.: 50 mrad /

√E

Hadronic Calorimeter

Fe-scintillator for |η| < 1.7∆E/E =

(50%/

√E)⊕ 6%

Cu-LAr for 1.5 < |η| < 3.2∆E/E =

(50%/

√E)⊕ 3%


Detector Operations

Several years worth ofcosmics runs paid off forfirst collisions!

Good operationalefficiency from allsubsystems - will improve.


Photon Reconstruction and ID

Calorimeter Clusters

Sliding Window cluster finding (5×5cells)Clusters of different sizes for photonsand electrons:

Electrons: 3×7 cellsUnconverted photons: 3×5 cellsConverted photons: 3×7 cells

Electrons identified by associated track∆ϕ = 0.0245

∆η = 0.02537.5mm/8 = 4.69 mm ∆η = 0.0031

∆ϕ=0.0245x4 36.8mmx4 =147.3mm

Trigger Tower

TriggerTower∆ϕ = 0.0982

∆η = 0.1

16X0

4.3X0

2X0

1500

mm

470

mm

η

ϕ

η = 0

Strip towers in Sampling 1

Square towers in Sampling 2

1.7X0

Towers in Sampling 3 ∆ϕ×�∆η = 0.0245×�0.05

Clusters are fully calibrated offline:Simulation tuned using Test Beam dataEnergy resolution: 3% in TB, better than that with Z → ee


Photon/π0 Discrimination

Single Photon π0 Candidate


Shower Evolution - Layer 2

The layer 2 shower shape cuts require compact clusters consistent withsingle photons:

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Data 2010 candidates)γSimulation (all

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ATLAS PreliminaryUnconverted photons

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Unconverted photons



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Converted photons



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610ATLAS Preliminary

-1Ldt = 15.8 nb∫ = 7 TeV, s

Converted photons



Width in ηWidth in φLeakage into hadronic calorimeter


Shower Evolution - Strips

The layer 1 (strips) provide excellent eta resolution, and allow increaseddiscrimination of single photons from π0’s

ratioE

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ATLAS Preliminary-1

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|<2.37η|≤1.8

sideF

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ATLAS Preliminary-1

Ldt = 15.8 nb∫ = 7 TeV, s

|<2.37η|≤1.8

Look for two local maxima, or wider showers in η or φUsually measured over the equivalent of a few cells at layer 2

⇒ Largely uncorrelated with isolation variables


Conversion Finding

All that ID material comes at a price....

|η|0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

) 0Ra

diat

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leng

th (X

0

0.5

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leng

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1.5

2

2.5ServicesTRTSCTPixelBeam-pipe

Conversion reconstruction is critical, especially outside of central barrel:Look for secondary vertices consistent with pair productionAlso a clean source of low ET electrons


Conversion FindingDedicated algorithms reconstruct conversion vertices with high efficiency upto R ≈ 800 mm:

Back-tracking, from TRT into Si detectors, for vertex findingCluster-seeded vertex matching to ’recover’ photons tagged as electrons

x [mm]

-400-300-200-100 0 100 200 300 400

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Data

MC conversion candidates

MC true conversions

Conversion finding is also a powerful way to map the detector material:Material mapping is critical for precision measurements (W mass)


Measured Efficiencies

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|<1.81γη|≤1.52

< 3 GeVisoTE

Trigger Efficiency: 99.5%Reconstruction efficiency: 82%

Including recoverable acceptancelosses

ID efficiency determined from MC:Shift shower-shapes in MC to matchdataSeparately for converted/unconvertedVerified using W → eνSystematics from:

Material effectsPileupConversionsMany more....

Overall systematics ≈15% (relative)Will improve with Z → llγ (several inverse femtobarns)

Preliminary


Calorimeter Isolation

.1

.1

η

φ

Isolation [GeV]

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eV210

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Isolation [GeV]

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Data 2010)γSimulation (fake

)γSimulation (isolated prompt )γSimulation (non-iso prompt

ATLAS-1Ldt = 878 nb∫ = 7 TeV, s

(Uncalibrated) sum of cells outside of 5×7 central core:In this case: ∆R =

√∆φ2 + ∆η2 < .4

Need to correct for out-of-core leakage (Molière radius ≈ .1)Also need to account for non-perturbative effects....

Preliminary


Corrections for Underlying Event/Pileup

To correct for ambient energy not associated with the hard scatter, there areseveral different techniques on the market:

“Look at 90◦” technique, AKA TransMin/TransMax methods,AKA ...

Event-by-event correction, using transverse regions to estimate ambientenergy from underlying event

Counting primary verticesAssume a certain amount of energy contributed by each min-biasinteractionCan combine this with tracking information to get the charged particlecontribution to the cone of interest, making it more event-driven

Others?

A relatively new method, different from the above, uses low pT jets (andtheir areas) to estimate the ambient energy:

Method proposed by Cacciari, Salam, and Sapeta(http://arxiv.org/abs/0912.4926)Developed for ATLAS by Brian Martin and Joey Huston


http://arxiv.org/abs/0912.4926

Subtracting the Non-perturbative ContributionsBasic procedure of the jet area correction method:

Bin the detector in strips of ηIn our case: 0.00, 1.50, 3.00, 4.00, 5.00If bins are too small, results are notstable

Run jet findingkT algorithm, to avoid overly smoothedjet shapesMinimum pT at 0, to allow for very softobjects

Courtesy of Wikipedia

Compute Voronoi areas of jets (partitioning the (η, φ) space intoregions defined by nearest jet)From the jets and their areas, find the median energy density for the ηbin

Median helps to avoid any scale effects from setting an upper bound on jetpT

For events with low multiplicity and hard interactions, can remove n mostenergetic jets from event (where n ≈ 2)

Correction to isolation variables made based on the cone sizeM. Hance 29 / 56 LBNL RPM- 16 December 2010

Ambient Energy Corrections

Ambient Transverse Energy Density [GeV/Unit Area]

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Ent

ries

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MeV

0

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L dt = 880 nb∫ = 7 TeVsData 2010,

Tight Photons

> 15 GeVγTE

Number of Primary Vertices

1 2 3 4 5 6

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[MeV

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a]

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JF17 - RobustTight

JF17 - True Photons

ATLAS Preliminary = 7 TeVs

Tight Photons| < 1.37η| > 20 GeVγ

Tp

Corrections appear to be well behaved, and consistent with MC. Averagecorrection for 1 PV:

PYTHIA: 440 MeVHERWIG: 550 MeVData: 540 MeV

Preliminary Work in Progress


Background Estimation

Background Estimates

To estimate the residual background: use isolation.

vs

Main challenge: modeling signal and background isolation profiles:Stay data-driven as much as possibleAvoid biases from untuned MC


Background Estimates

To model the background - reversesome photon ID cuts:

Cuts on the strip variables aregood candidatesNot strongly correlated withisolation

Corrected EtCone40 [MeV]

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Filtered MinBias 6 GeV

Truth BackgroundReversed Cuts

< 20000T

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0.09± = 1.05 MCR


-5000 0 5000 10000 15000 20000 25000

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Filtered DiJets 17 GeV


< 25000T

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ATLAS“Work in Progress”

Isolation [GeV]

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fails tight ID cutsγData,

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γT E≤15 GeV

ATLAS

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fails tight ID cutsγData,

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γT E≤60 GeV

ATLASPreliminary Preliminary


2-D Sidebands

[GeV]isoTE

-5 0 5 10 15 20 25 30 35

IDγ

pass tight cuts

fail tight cuts

A

C

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Photon p

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NsigA = NA − NB

NC

ND

Assumes “reverse cuts” not correlatedwith isolation

→ systematic uncertainty

Also assumes NsigX � NX for X 6= A

Correct NX to account for signal leakage

Additional uncertainties:Definition of control regionsStatistical uncertainties

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ATLAS Simulation“Work in Progress”

Preliminary


Isolation Templates

Transverse Isolation Energy [GeV]

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= 7 TeVs2010 Data, Signal TemplateBackground TemplateFit Result

Work in ProgressATLAS-1 L dt = 880 nb∫


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< 40 GeVγT E≤35

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= 7 TeVs2010 Data, Signal TemplateBackground TemplateFit Result

Work in ProgressATLAS-1 L dt = 880 nb∫

A full template fit is also possibleSignal from Z → ee and W → eν

Background from reverse cutsMLL fit for signal yield forEiso

T < 3 GeVResults compatible with ABCDmethod

Agree within uncorrelateduncertainties

[GeV]T

Photon p

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ATLAS Simulation“Work in Progress”


Cross Section Measurement


We now have most of the ingredients for the cross section measurement:

dσdEγ

T= Nyield U

(∫ Ldt) ∆EγT εtrigger εreco εID

Nyield (= N · P) extracted from purity measurements, ε from efficiencymeasurements.

Unfolding coefficients (U) evaluated using PYTHIA signal MC:Ratio of true:reconstructed ET distributionsOverall small bin-to-bin migrations because of good resolution incalorimeter

A few percent, always larger than 1.0, approaching 1.0 at high ET

Depend on |η| (material effects) and ET(resolution + bin size effects)



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luminosity uncertainty

JETPHOX NLO pQCD

γT=E

Rµ=

Fµ=

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JETPHOX systematic uncertainty

= 7 TeVs

|<0.6γη|

< 3 GeVisoTE

ATLAS

[GeV]γTE

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Preliminary


Cross Section: Higher |η|

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JETPHOX NLO pQCD

γT=E

Rµ=

Fµ=

fµCTEQ 6.6,


= 7 TeVs

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η|≤0.6

< 3 GeVisoTE

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/theo

ry

0.60.8

11.21.4 20 30 40 50 60 70 80 90 100

]-1

[p

b G

eVγ T

/dE

σd

1

10

210

310

410

20 30 40 50 60 70 80 90 100

]-1

[p

b G

eVγ T

/dE

σd

1

10

210

310

410-1 Ldt = 880 nb∫Data 2010,


JETPHOX NLO pQCD

γT=E

Rµ=

Fµ=

fµCTEQ 6.6,


= 7 TeVs

|<1.81γ

η|≤1.52

< 3 GeVisoTE

ATLAS

[GeV]γTE

20 30 40 50 60 70 80 90 100

data

/theo

ry

0.60.8

11.21.4

[GeV]γTE

20 30 40 50 60 70 80 90 100

data

/theo

ry

0.60.8

11.21.4

Results compared with theoretical predictions from JETPHOX

Systematically limited across the full ET rangeGood agreement at high ET, where the systematics on both experimentand theory are smallest

Preliminary Preliminary


Future Photon Studies with ATLAS

To higher energies....

November 29, 2010 – 18 : 09 DRAFT 2

[GeV]TE100 200 300 400 500 600

[1/G

eV]

T1/

N d

N/d

E

-710

-610

-510

-410

-310

-210

-110ATLAS Preliminary

-1 Ldt = 37 pb = 7 TeV, s

)(tight, isolated Data 2010

Figure 1: Inclusive ET spectrum of isolated photon candidates passing the robust tight selection. Themarker positions correspond to the bin weighted averages.

A factor of 40 more data has been ac-cumulated since the last analysis wasfrozen....

Can extend the ET-reach to≈400 GeVTight, isolated photons above100 GeV are very pure(> 90%)

Compton process still dominant at high ET → constrain gluon content ofproton for PDFs.

In addition to the inclusive analysis, we plan to measure the γ+jet crosssection separately:

Event kinematics provide more informationAngular separation sensitive to fragmentation component


Isolation StudiesStefano Frixione proposed an isolation prescription for reducing thefragmentation component in the inclusive analysis:

EisolationT (R) < (εs · EγT) ·

(1− cos(R)1− cos(R0)

)n

Apply progressively tighter cuts on smaller and smaller conesTerminates at R = 0 with a cut at 0Eliminates collinear fragmentation component, leaving only the directcomponent

Theoretically attractive, as the fragmentation component is less wellunderstood

We worked with Frixione and the JETPHOX authors to modify theprescription to take into account experimental constraints:

Discrete calorimeter granularity→ discrete cone sizesMolière radius not zero→ terminate at R ≈ .1Needs ’corrections’ to to reconstructed isolation to properly removenon-perturbative contributions to the isolation cone


Frixione Isolation

A discrete, generalized form of this prescription will be used in the nextanalysis:

EisolationT (R) <

((ER0

T

)m + (εs · EγT)m)1/m

·(

1−cos(R)1−cos(R0)

)n

[GeV]T

Photon p

0 20 40 60 80 100 120 140 160 180 200

[GeV

]0

Isol

atio

n C

ut a

t R=

R

0

1

2

3

4

5

6

7

8

9

10

m=1

m=2

m=4

m=10

Cone Radius

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

)T

p∈Is

olat

ion

Cut

[GeV

] / (

0

0.2

0.4

0.6

0.8

1

1.2

1.4

=0.4, n=0.20R=0.4, n=0.50R=0.4, n=1.00R


Diphoton Measurements

Born Brem Box

q

q

q

γ

γ

g

q

q

q

γ

γ

g

g

γ

γ

(a) (b) (c)

Around 1 nb for ET > 15 GeVLargest (irreducible) background to H → γγ

Biggest challenge is extending the analysis to low ET

A critical step towards finding a low mass Higgs!


Higgs Physics

[GeV]Hm

100 120 140 160 180 200

NN

LOS

Mσ/σ

95%

CL

Upp

er B

ound

on

-210

-110

1

10

WW→H 4l→H

llbb→ ZZ→Hνν ll→ ZZ→H

γγ→H

b b→VH, Hττ→H

Combinedσ 1±σ 2±

-1 L dt=1 fb∫

ATLAS Preliminary

(Simulation)

=7 TeV Projections

ATLAS

-11 fb = 7 TeVs

(Simulation)

Signal x10

(Born & Brem)γ γ

(Box)γ γ

-jetγ

Di-jet

Drell Yan

Preliminary

[GeV]γγM100 105 110 115 120 125 130 135 140 145 150

Num

ber

of E

vent

s/G

eV

0

50

100

150

200

250

300

350

400

)-1 Toy sample (1 fb

= 120 GeV)H

(m γγ →H

H → γγ remains one of the more powerful channels at low mH .Critical for probing near the LEP limitThese plots from 2009/2010 MC studies: roughly 50% gains insensitivity since then (analysis improvements, pessimistic MC WRTdata)


SUSY and Exotic Searches

Getting a bit more exotic....Most recent work in ATLAS: UED searchesin γγ + Emiss

T

Probing TeV−1 sized UED2× (γ∗ → γ + G) = γγ + Emiss

TΛ: UV cutoff for radiative correctionsR: compactification radiusSet ΛR = 20

D0 limit: 1/R > 477 GeV (6.3 fb−1)ATLAS: 1/R > 728 GeV (3 pb−1)

4

TABLE I. The number of observed γγ candidates, as well as the SM backgrounds estimated from data and expected UEDsignal for 1/R values of 500 and 700 GeV, given in various Emiss

T ranges. The uncertainties are statistical only. The first row,for Emiss

T < 20 GeV, is the control region used to normalize the QCD background to the number of observed γγ candidates.

EmissT range Data Predicted background events Expected UED signal events(GeV) events Total QCD W (→ eν) + jets/γ 1/R = 500 GeV 1/R = 700 GeV0 - 20 465 465.0 ± 9.1 465.0 ± 9.1 - 0.28 ± 0.06 0.02± 0.01

20 - 30 45 40.5 ± 2.2 40.41 ± 2.17 0.11± 0.07 0.45 ± 0.07 0.03± 0.0130 - 50 9 10.3 ± 1.3 10.13 ± 1.30 0.16± 0.10 1.60 ± 0.12 0.08± 0.0150 - 75 1 0.93 ± 0.23 0.85 ± 0.23 0.08± 0.05 2.84 ± 0.16 0.14± 0.01

> 75 0 0.32 ± 0.16 0.28 ± 0.15 0.04± 0.03 40.45 ± 0.62 4.21± 0.06

[GeV]missTE

0 10 20 30 40 50 75 150 600

Ent

ries

/ 5 G

eV

-310

-210

-110

1

10

210 = 7 TeV )sData 2010 ( Total backgroundUED 1/R = 500 GeVUED 1/R = 700 GeV

-1Ldt = 3.1 pb∫

ATLAS Preliminary

FIG. 3. EmissT spectrum for the γγ candidates, superimposed

on the total SM background prediction. Also shown are theexpected UED signals for 1/R = 500 GeV and 700 GeV.Variable sized bins are used, and the vertical error bars andshaded bands show the statistical errors.

trigger requirement is essentially fully efficient for sig-267

nal events satisfying the offline analysis cuts. The var-268

ious signal systematic uncertainties are summarized in269

Table II, including the dominant 11% uncertainty on the270

integrated luminosity [17]. Uncertainties in the efficiency271

for reconstructing and identifying the γγ pair arise due272

to differences between MC and data in the distributions273

of the photon identification variables, the need to extrap-274

olate these studies to the higher ET values (see Fig. 1)275

typical of the UED photons, the impact of the photon276

quality cuts, varying the scale of the photon ET cut,277

and uncertainties in the detailed material composition278

of the detector. Together these provide a systematic un-279

certainty of 4.1% (3.7%) for 1/R = 500 (700) GeV. The280

influence of pileup gives a systematic uncertainty of 1.6%.281

Systematic effects on the EmissT reconstruction, including282

pileup, varying the cluster energies within the current un-283

certainties, and varying the expected EmissT resolution be-284

tween the measured performance and MC expectations,285

combine to give a 1% uncertainty on the signal efficiency286

for 1/R = 500 GeV, decreasing to 0.2% for 700 GeV.287

Finally, the 1% statistical error on the signal efficiency288

as determined by MC is treated as a systematic uncer-289

tainty on the result. Adding in quadrature, the total290

systematic uncertainty on the signal is 11.9% (11.8%)291

for 1/R = 500 (700) GeV.292

TABLE II. Systematic uncertainties for the 1/R = 700 GeVUED signal. For more details, see the text.

Source of uncertainty UncertaintyIntegrated luminosity 11%Photon reconstruction and identification 3.7%Effect of pileup 1.6%Emiss

T reconstruction and scale 0.2%Signal MC statistics 1%Total 11.8%

Given the good agreement between the measured EmissT293

spectrum and the expected background, a limit was set294

on 1/R in the specific UED model considered here. A295

Bayesian approach was used to calculate a limit based296

on the number of observed and expected events with297

EmissT > 75 GeV. A Poisson distribution was used as298

the likelihood function for the expected number of sig-299

nal events, and a flat prior was used for the signal cross300

section. Gaussian priors were used for the various sources301

of uncertainty, which were treated as nuisance parame-302

ters. It was verified that the result is not very sensitive303

to the detailed form of the assumed priors. Fig. 4 depicts304

the resulting 95% CL upper limit within the context of305

the UED model considered, together with the LO UED306

cross section as a function of 1/R. The LO cross section307

was used since higher order corrections have not been308

calculated for the UED model. An uncertainty on the309

signal cross section due to parton distribution functions310

(pdf) was determined by comparing the predictions us-311

ing MRST2007 [18] pdf’s with those from the full set of312

error pdf’s of CTEQ6.6 [19]. The resultant uncertainty,313

namely 8% essentially independent of 1/R, is shown by314

Along these same lines:Some SUSY searches (GMSB) have identical signaturesγγ resonances (Direct RS-graviton search)Model-independent resonance searchesγ + l + Emiss

T (sleptons) and many other combinationsM. Hance 46 / 56 LBNL RPM- 16 December 2010

Conclusion

Conclusion

First ATLAS measurement of prompt photon production

Photons are characterized for the first time by ATLASGood efficiency for very high purity, especially at high ET

Cross-section measurement up to 100 GeV, in three η regionsExtending to ≈ 500 GeV with all 2010 data

Good agreement with theory for EγT > 30 GeVSome things to be understood at lower EγT

Lots of interesting γ physics to come

UED searches (currently)Di-photon cross section, resonance searchesHiggs searches at low massSUSY, exotics, model independent searches...


Single Photon Trigger Efficiency

[GeV]γTE

0 2 4 6 8 10 12 14 16 18 20

trig

ger

effic

ienc

y

0

0.2

0.4

0.6

0.8

1

-1 Ldt = 880 nb∫Data 2010,

Minimum Bias MC

ATLAS

= 7 TeVs

Preliminary


Higgs

[GeV]HM

110 115 120 125 130 135 140

)/S

M @

95%

CL

γ γ →

x B

R (

H

σ

0

2

4

6

8

10

12

14

16

18

20

22

Median

Median (no systematics)

σ 1±Median

σ 2±Median

γγ →H =7TeVs

-11 fb

ATLAS Preliminary(Simulation)

[GeV]Hm

100 110 120 130 140 150 160 170 180 190 200

SM

σ/σ95

% C

L Li

mit

on

-110

1

10-10.5fb

-11fb-12fb-15fb

TeVatronLEP

ATLAS Preliminary (Simulation)

=7 TeVsProjections,


Previous Measurements7

[GeV]γTE

50 100 150 200 250 300 350 400

Ph

oto

n F

ract

ion

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

-1CDF Data, L=2.5 fb < 2 GeV)

isoTSignal Fraction (E

Systematic Uncertainty

[GeV]γTE

50 100 150 200 250 300 350 400

Ph

oto

n F

ract

ion

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

FIG. 2: Fraction of isolated prompt photons as a function ofEγ

T . The systematic uncertainty band is discussed in the text.

EγT

dσ/dEγT

dηγ Syst. Unc.(GeV) (pb/GeV) (%)30–34 (1.23±0.01)×102 +15.5,−14.534–39 (6.21±0.03)×101 +10.8, −9.839–44 (3.10±0.02)×101 +9.8, −8.444–50 (1.72±0.02)×101 +10.2, −8.150–60 (7.93±0.08)×100 +10.1, −8.460–70 (3.54±0.05)×100 +9.8, −8.570–80 (1.76±0.03)×100 +10.0, −9.180–90 (9.08±0.14)×10−1 +9.3, −7.990–110 (4.41±0.05)×10−1 +8.8, −8.7110–130 (1.68±0.03)×10−1 +8.6, −8.7130–150 (7.25±0.16)×10−2 +7.8, −8.0150–170 (3.41±0.08)×10−2 +8.8,−10.0170–200 (1.46±0.04)×10−2 +8.8, −9.1200–230 (5.66±0.24)×10−3 +9.0,−10.6230–300 (1.38±0.08)×10−3 +10.0,−10.7300–400 (1.49±0.21)×10−4 +15.2,−13.4

TABLE I: Measured inclusive isolated prompt photon crosssection for photons in the pseudorapidity region |ηγ | < 1.0and 30 < Eγ

T < 400 GeV. The uncertainties in the central col-umn are statistical. The additional 6% luminosity uncertaintyis not included in the table. A parton-to-hadron correction(Chad = 0.91 ± 0.03) is applied to the pQCD predictions.

M Bonesini et al., (WA70 Collaboration), Z. Phys. C 38,371 (1988); C. Alba et al., (UA1 Collaboration), Phys.Lett. B 209, 385 (1988); A.L.S. Angelis et al., (R110Collaboration), Nuc. Phys. B 327, 541 (1989); E. Anas-sontzis et al., (R807/AFS Collaboration), Sov. J. Nuc.Phys. 51, 5 (1990); J. Alitti et al., (UA2 Collaboration),Phys. Lett. B 263 544 (1991); G. Ballocchi et al., (UA6Collaboration), Phys. Lett. B 436, 222 (1998).

[4] V.M. Abazov et al., (DO Collaboration), Phys. Lett. B639, 151 (2006).

[5] A cylindrical coordinate system with the z axis along theproton direction is used, in which θ is the polar angle.We define ET = E sin θ, pT = p sin θ, and pseudorapid-ity η = − ln(tan(θ/2)). The missing transverse energy isdefined by E/T = −∑

iEi

T ni, where i is the calorimetertower number and ni is a unit vector perpendicular to thebeam axis and pointing at the ith calorimeter tower. Themissing transverse energy as measured by the calorime-ters is corrected for jets and muons.

[p

b/G

eV]

γ ηdγ T/d

Eσd

-410

-310

-210

-110

1

10

210 -1CDF Data, L=2.5 fbsystematic uncertaintyNLO pQCD JETPHOXCTEQ6.1M / BFG II

γT=ERµ=fµ=Fµ

CTEQ6.1M PDF uncertainties

scale dependenceγT=2Eµ and γ

T=0.5Eµ

| < 1.0γη|

< 2.0 GeVisoTE

(a)

[p

b/G

eV]

γ ηdγ T/d

Eσd

-410

-310

-210

-110

1

10

210

[GeV]γTE

50 100 150 200 250 300 350 400

dat

a/th

eory

0.80.91.01.11.21.31.41.51.6 (b)

[GeV]γTE

50 100 150 200 250 300 350 400

dat

a/th

eory

0.80.91.01.11.21.31.41.51.6

FIG. 3: (a) Measured inclusive isolated prompt-photon crosssection as a function of Eγ

T compared to NLO pQCD pre-dictions. (b) Ratio data/theory as a function of Eγ

T . Theshaded band includes the total systematic uncertainty on themeasurement except for the 6% luminosity uncertainty. Thedashed and dotted lines indicate the PDF uncertainty andthe variation with NLO pQCD predictions, respectively. Aparton-to-hadron correction, Chad = 0.91±0.03, is applied tothe theoretical predictions.

[6] F. Abe et al., Nucl. Instrum. Methods Phys. Res. A 271,387 (1988); D. Amidei et al., Nucl. Instum. MethodsPhys. Res. A 350, 73 (1994); F. Abe et al., Phys. Rev.D 52, 4784 (1995); P. Azzi et al., Nucl. Instrum. Meth-ods Phys. Res. A 360, 137 (1995); The CDFII DetectorTechnical Design Report, Fermilab-Pub-96/390-E.

[7] Cosmic rays may produce bremsstrahlung photons.Muons from beam-halo interactions with the beam pipemay in turn interact with the detector material produc-ing photons. Electrons from W decays may fake photonsif the associated track is not reconstructed. All these con-tributions lead to large missing transverse energy.

[8] F. Abe et al., (CDF Collaboration), Phys. Rev. D 48,2998 (1993).

[9] T. Sjostrand et al., Comput. Phys. Commun., 135, 238(2001).

[10] R. Field, FERMILAB-CONF-06-408-E, FNAL, 2005. T.Affolder et al., (CDF Collaboration), Phys. Rev. D 65,092002 (2002).

[11] R. Brun and F. Carminati, CERN, Programming LibraryLong Writeup W5013 (1993).

6

(GeV)γTp

0 50 100 150 200 250 300

(p

b/G

eV)

η/d

T/ d

pσ2

d

-310

-210

-110

1

10

210

310 data

NLO QCD

)γT=pfµ=Fµ=Rµ(

CTEQ6.1M

-1L = 326 pb

DØ

FIG. 4: The inclusive cross section for the production ofisolated photons as a function of pγ

T . The results from theNLO pQCD calculation with jetphox are shown as solid line.

than 0.3% for pγT > 70 GeV. The measured cross section,

together with statistical and systematic uncertainties, ispresented in Fig. 4 and Table I. (The data points areplotted at the pT value for which a smooth function de-scribing the cross section is equal to the average crosssection in the bin [22].) Sources of systematic uncer-tainty include luminosity (6.5%), event vertex determi-nation (3.6%− 5.0%), energy calibration (9.6%− 5.5%),the fragmentation model (7.3%− 1.0%), photon conver-sions (3%), and the photon purity fit uncertainty (shownin Fig. 3) as well as statistical uncertainties on the de-termination of geometrical acceptance (1.5%), trigger ef-ficiency (11% − 1%), selection efficiency (5.4% − 3.8%)and unsmearing (1.5%). The uncertainty ranges aboveare quoted with the uncertainty at low pγ

T first and theuncertainty at high pγ

T second. Most of these systematicuncertainties have large (> 80%) bin-to-bin correlationsin pγ

T . Varying the choice of NN cut from 0.3 to 0.7changed the measured cross section by less than 5%.

Results from a next-to-leading order (NLO) pQCD cal-culation (jetphox [23, 24]) are compared to our mea-sured cross section in Fig. 4. These results were derivedusing the CTEQ6.1M [25] PDFs and the BFG [26] frag-mentation functions (FFs). The renormalization, fac-torization, and fragmentation scales were chosen to beµR =µF =µf =pγ

T . Another NLO pQCD calculation [27],based on the small-cone approximation and utilizing dif-ferent FFs [28], gave consistent results (within 4%). Asshown in Fig. 5, the calculation agrees, within uncertain-ties, with the measured cross section. The scale depen-dence in the NLO pQCD theory, estimated by varyingscales by factors of two, are displayed in Fig. 5 as dashed

(GeV)γTp

0 50 100 150 200 250 D

ata

/ Th

eory

0.4

0.6

0.8

1

1.2

1.4

(GeV)γTp

0 50 100 150 200 250 D

ata

/ Th

eory

0.4

0.6

0.8

1

1.2

1.4

ratio of data to theory (JETPHOX) CTEQ6.1M PDF uncertainty scale dependence

)γT and 2p

γT=0.5pfµ=Fµ=Rµ(

DØ

300

-1L = 326 pb

FIG. 5: The ratio of the measured cross section to the theo-retical predictions from jetphox. The full vertical lines cor-respond to the overall uncertainty while the internal line indi-cates just the statistical uncertainty. Dashed lines representsthe change in the cross section when varying the theoreticalscales by factors of two. The shaded region indicates the un-certainty in the cross section estimated with CTEQ6.1 PDFs.

TABLE I: The measured differential cross section for the pro-duction of isolated photons, averaged over |η| < 0.9, in binsof pγ

T . 〈pγT 〉 is the average pγ

T within each bin. The columnsδσstat and δσsyst represent the statistical and systematic un-certainties respectively. (Five events with pγ

T > 300 GeV,including one with pγ

T = 442 GeV, were not considered inthis analysis.)

pγT 〈pγ

T 〉 d2σ/dpγT dη δσstat δσsyst

(GeV) (GeV) (pb/GeV) (%) (%)

23−25 23.9 4.14×102 0.1 2325−30 26.9 2.21×102 0.1 1930−34 31.7 1.01×102 0.2 1634−39 36.0 5.37×101 0.2 1539−44 41.1 2.88×101 0.3 1444−50 46.5 1.58×101 0.4 1350−60 53.8 7.90×100 0.4 1360−70 63.9 3.39×100 0.6 1370−80 74.1 1.68×100 0.9 1280−90 84.1 9.34×10−1 1.3 1290−110 97.2 4.38×10−1 1.4 12

110−130 118 1.66×10−1 2.3 12130−150 138 7.61×10−2 3.5 13150−170 158 3.20×10−2 5.6 13170−200 181 1.59×10−2 6.5 14200−230 212 7.36×10−3 9.8 14230−300 256 1.81×10−3 13 15


Inner Tracker Endcap


Inner Detector Alignment

Local x residual [mm]

−0.2 −0.1 0 0.1 0.2

m

µH

its o

n tr

acks

/ 4

20

40

60

80

100

120

140

310×

PreliminaryATLASPixel barrel

= 7 TeVs

Post−Collisions AlignmentmµFWHM/2.35=25

Pre−Collisions AlignmentmµFWHM/2.35=34

Monte CarlomµFWHM/2.35=18


−0.2 −0.1 0 0.1 0.2m

µ

Hits

on

trac

ks /

4

20

40

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80

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PreliminaryATLASSCT barrel

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Residual [mm]

−1 −0.5 0 0.5 1

m

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50

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= 7 TeVs





−0.2 −0.1 0 0.1 0.2

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= 7 TeVs





−0.2 −0.1 0 0.1 0.2

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PreliminaryATLASTRT end−caps

= 7 TeVs





Purity Estimates

20 30 40 50 60 70 80 90 100

Pho

ton

frac

tion

0.30.40.50.60.70.80.9

1

20 30 40 50 60 70 80 90 100

Pho

ton

frac

tion

0.30.40.50.60.70.80.9

1

-1 Ldt = 880 nb∫ = 7 TeV, sData 2010,


ATLAS


TE

20 30 40 50 60 70 80 90 100

Pho

ton

frac

tion

0.30.40.50.60.70.80.9

1

20 30 40 50 60 70 80 90 100

Pho

ton

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tion

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|<1.37γη|≤0.6

< 3 GeVisoTE

[GeV]γTE

20 30 40 50 60 70 80 90 100

Pho

ton

frac

tion

0.30.40.50.60.70.80.9

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[GeV]γTE

20 30 40 50 60 70 80 90 100

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ton

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tion

0.30.40.50.60.70.80.9

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|<1.81γη|≤1.52

< 3 GeVisoTE

20 30 40 50 60 70 80 90 100

Pho

ton

frac

tion

0.30.40.50.60.70.80.9

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20 30 40 50 60 70 80 90 100

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ton

frac

tion

0.30.40.50.60.70.80.9

1

-1 Ldt = 880 nb∫ = 7 TeV, sData 2010,


ATLAS


TE

20 30 40 50 60 70 80 90 100

Pho

ton

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tion

0.30.40.50.60.70.80.9

1

20 30 40 50 60 70 80 90 100

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< 3 GeVisoTE

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20 30 40 50 60 70 80 90 100

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|<1.81γη|≤1.52

< 3 GeVisoTE



Non-Collision Backgrounds

[GeV]γTE

20 30 40 50 60 70 80 90 100

Non

-col

lisio

n ba

ckgr

ound

frac

tion

-510

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-210

ATLAS

-1 Ldt = 880 nb∫= 7 TeV, sData 2010,

γBeam-induced background for loose

[GeV]γTE

20 30 40 50 60 70 80 90 100

Non

-col

lisio

n ba

ckgr

ound

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tion

-510

-410

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ATLAS

-1 Ldt = 880 nb∫= 7 TeV, sData 2010,

γCosmic-ray-induced background for loose

Non-collision backgrounds not an issue for this analysis:Will become more critical when extending past 100 GeVAlso more serious issue when requiring Emiss

T



Prompt Photon Production at ATLAS - Physics Division · Dj = 0.0245 Dh = 0.025 37.5mm/8 = 4.69 mm...

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Transcript of Prompt Photon Production at ATLAS - Physics Division · Dj = 0.0245 Dh = 0.025 37.5mm/8 = 4.69 mm...