Update from the Photons + MET Group

Bruce Schumm

UC Santa Cruz / SCIPP

26 August 2010 SUSY/MET Meeting

26 August 2010 2

This presentation is largely informal and unofficial

• Vacation/conferences: no official strategizing session yet

• Some opinions/thoughts will be presented, but are only from a subset of the group

• Intended to prompt discussion, or at least show the issues we’re considering as we re-group

26 August 2010 3

305 nb-1 Analysis: not approved!Single-

Two-

Statistics dictated that we concentrate on a single-photon analysis, even though we expect to require two photons in the photons+MET analysis to reject SM backgrounds (e.g. setting GMSB limits).

26 August 2010 4

Why not approved?

Main (QCD) background scaling not understood– K factors and ad-hoc scaling factors– Mis-id rates– Dijet vs. +jet contributions– MET spectrum too hard?

Could not benchmark against direct photon analysis at low MET

Non-standard analysis steps (isolation)?No independent constraints on mis-ID rates (jet

)

26 August 2010 5

Some Developments Since Late July

Photon Purity for GMSB Photon Sample

Benchmark analysis against Direct Photon study

Daniel Damiani

26 August 2010 7

Direct Photon Note Purity Method

• Method for photon purity calculation was taken from ATLAS-CONF-2010-077

• A two dimensional side-band background subtraction was used to measure the purity

• Two sets of cuts were reversed:– Tight Photon Strip Isem cuts

• DeltaE• Fracm• Wetalc• DEmaxs1

– Isolation• Etcone40 – (underlying event correction) < 3 GeV (> 5 GeV used as

reversed cut• Underlying event correction was on average about 500 MeV

26 August 2010 8

Direct Photon Note Purity Method

26 August 2010 9

Direct Photon Note Purity Method

• Formula used to calculate the purity in direct photon note:

• Data set used: 15.8nb-1

• Extracted purity 0.72±0.07 for tight photons with pT > 20 GeV

26 August 2010 10

Purity Measurement – GMSB Sample

• Used 305 nb-1 of 7 TeV data• Same shower shape cuts were reversed

• Direct photon note-like isolation cut– Etcone40<3 GeV and Etcone40>5GeV– NA: 20226 NB: 5023 MA: 460 MB: 213– Purity – 0.64 Compare to 0.72

• GMSB note isolation cut– Etcone20/et <0.1 and Etcone20/et>0.125– NA: 12355 NB: 9081 MA: 201 MB: 405– Purity – 0.46

Breakdown of Background Sources

Data-driven background estimation strategy may be source-dependent

Andrea Bangert

26 August 2010 12

selection cuts• event must fire EF_g10_loose trigger• all jets must pass jet quality cuts• event must produce primary vertex with at least four

tracks• event must produce at least one reconstructed photon

• pseudorapidity of cluster in 2nd sampling is |η| < 2.47• cluster is not in the crack region, 1.37 < |η| < 1.52• ET(ΔR < 0.2) < 10 GeV• photon passes isEM tight criteria• pT > 20 GeV

• no cut was placed on MET • no cut was placed on number of jets

• matching was performed using ΔR = √[(Δη)2 + (Δϕ)2]; may want to update?

26 August 2010 13

samples• JF17

• 10 million initial events• 27852 selected reconstructed photons

• JF35• 5 million initial events• 23929 selected reconstructed photons

• gamma + jets• pythia sample number 108087• 10,000 initial events• 42714 selected reconstructed photons

• diphoton• pythia sample number 105964• 100,000 initial events• 72931 selected reconstructed photons

• ttbar • mc@nlo sample number 105200• 165,000 initial events• 6943 selected reconstructed photons• negative event weights have not been taken into account (12% effect)

26 August 2010 14

fraction of selected reconstructed photons attributed to various truth objects

object matched to reco photon

JF17 JF35 gamma + jet

diphoton ttbar

photon from hard scatter 0.104 0.098 0.573 0.591 0

photon radiated by quark or gluon

0.191 0.239 0.380 0.365 0.043

hadrons 0.660 0.599 0.001 (56 photons)

0.000 (6 photons)

0.043

bremsstrahlung photon 0.033 0.048 0.044 0.044 0.137

photon radiated by W or Z 0.000 (2 photons)

0.000(4 photons)

0 0 0.105

photon radiated by heavy lepton 0.000(5 photons)

0 0 0 0.003 (23 photons)

electron from W -> ev or Z -> ee 0.002(52 photons)

0.005(135 photons)

0 0 0.531

electron from decay of B or D 0.002(77 photons)

0.003(73 photons)

0 0 0.004 (34 photons)

unknown object 0.004 0.005(124 photons)

0 0.000(1 photon)

0.037 (muons, taus, ν)

26 August 2010 15

fraction of reconstructed photons matched to various stable hadronsJF17 JF35 gamma + jets diphoton ttbar

neutral pion 0.827 0.848 0.875 (49 photons)

0.333(2 photons)

0.932

eta or eta’ 0.119 0.091 0.089 (5 photons)

0.166(1 photon)

0.028 (27 photons)

omega 0.018 0.015 0.017 (1 photon)

0 0.002 (2 photons)

neutral kaon 0.024 0.019 (285 photons)

0.017 (1 photon)

0 0.015 (15 photons)

charged pion or charged kaon

0.009 0.021 (315 photons)

0 0.333(2 photons)

0.019 (19 photons)

sigma 0 0.0002 (4 photons)

0 0 0

proton 0.0002(5 photons)

0.0011 (16 photons)

0 0 0.001 (1 photon)

neutron 0.0006(12 photons)

0.0011 (16 photons)

0 0.166(1 photon)

0

heavy ions 0 0 0 0 0

• Denominator is all reconstructed photons that were matched to true hadrons.

26 August 2010 16

Conclusions from Background Component Study

QCD processes that do not produce photons in the hard scatter are modeled by JF17 and JF35• (Note that the ~10% of events with photons in the hard

scatter were not removed, they were merely counted.)• 60% - 70% of the selected reco photons are attributed to

hadrons• 80% of these are attributed to neutral pions• 10% are attributed to eta or eta’

• 20% of the selected reco photons match a photon radiated by a quark or a gluon

• 4% of the selected reco photons match a bremsstrahlung photon

26 August 2010 17

Where from Here?

One thought: 10 pb-1 prototype analysis (i.e. 2010 data set)

• 10 pb-1 not enough to improve, e.g. on Tevatron GMSB (SPS8) limits

• Should be enough to– Develop analysis (background estimation)

techniques– Prepare for publishable limit– [Generate PhD theses]

26 August 2010 18

10 pb-1 “proto”-Analysis

• Scaling up, would have 50-100 diphoton events

• Perform analysis optimized for scaled-back limit (MET, HT cuts); fast-MC-based re-optimization underway

• Would likely circumvent issues encountered in late July (second photon likely to greatly suppress QCD backgrounds)

26 August 2010 19

Summary

• The strong and weak production mechanisms contribute as we had come to suspect

• There is a third intermediary type of events, but its cross section is very small

• Strong production component is already pretty small at Λ=90 for 7 TeV and drops off quite quickly from there

• No real revelations here but at least it conforms what we had suspected was going on

26 August 2010 20

definitions• truth objects were required to enter the fiducial area of the detector

• |η| < 3.0

• truth objects were required to have E > 10 GeV

• “photons” include both photons and electrons from pair production• “electrons” include both electrons and bremsstrahlung photons

• “hadrons” include hadrons deemed stable by the generator • π0, π±, η, ϖ, K, Σ, p, n, heavy ions

• “hadrons” include • hadrons, • photons from hadron decays• electrons from pair production

• for example, “neutral pions” include • π0, • photons from π0 -> γγ• e± from π0 -> γγ followed by γ -> e+e-

26 August 2010 21

fraction of reconstructed photons matched to hadron, photon, electron

JF17 JF35 gamma _ jets

diphoton ttbar

π0 0.394 0.326 0.346 (17 photons)

0.5(1 photon)

0.334

γ from π0 -> γγ 0.337 0.358 0.346 (17 photons)

0 0.388

e from γ ->ee 0.268 0.314 0.306 (15 photons)

0.5(1 photon)

0.277

JF17 JF35 gamma _ jets

diphoton ttbar

η0 0..340 0.353 0.2 (1 photon)

0 0.296 (8 photons)

γ from η0 -> γγ 0.376 0.317 0.2 (1 photon)

1 (1 photon)

0.555 (15 photons)

e from γ ->ee 0.282 0.328 0.6 (3 photons)

0 0.148 (4 photons)

• The denominator includes all reco photons matched to neutral pion (or photon or electron).

• The denominator includes all reconstructed photons matched to an eta, a photon from decay of an eta, or a conversion electron.

26 August 2010 22

fraction of reconstructed photons attributed to radiation from quark or

gluon that convertJF17 JF35 gamma + jet diphoton ttbar

q -> q gamma

0.442 0.420 0.001 (12 photons)

0.0003(3 photons)

0.652

q -> q gammafollowed by gamma -> ee

0.557 0.579 0.999 (8843 photons)

0.9997 0.347

• The denominator is the number of reconstructed photons attributed to radiation from a gluon (or a quark).• The numerator is the number of those reconstructed photons that were matched to a true electron from a conversion.

JF17 JF35 gamma + jet diphoton ttbar

g -> g gamma

0.214 0.253 0 0.0002(4 photons)

0.466 (7 photons)

g -> g gammafollowed by gamma -> ee

0.785 0.746 1 (7410 photons) 0.9997 0.533 (8 photons)