backgrounds to the diphoton + met analysis

Backgrounds to the Diphoton + MET Analysis

Bruce Schumm, channeling Ben Auerbach (Argonne) and Osamu

Jinnouchi (Tokyo Tech)

UC Santa Cruz / SCIPP

04 June 2013 SUSY Background Forum Meeting

04 June 2013 2

Three background sources:

• QCD (gamma-gamma, gamma-jet, jet-jet; no primary MET)• Electroweak (W-gamma, ttbar; usually with e fake)• Irreducible (W, Z; with neutrinos in decay)

Start with QCD background

Recall: 2x2x2=8 considered control samples, defined by data with one pseudophoton, with• Zero (QCDg) or one (QCDtg) tight isolated photon• 50 or 75 GeV ET cut on pseudophoton• pseudophoton isolated or not

Nominal MET is standard MetRefFinal, but also look at LocHadTopo as cross-check

Which of all of these are useful?

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First: MC performance on tight-tight sample

LocHadTopo has slightly larger tails than MetRefFinal

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• Statistics are limited: must make use of 50 GeV ET cut, no-iso if possible• QCDtg+Iso close to tight-tight (signal) distribution proxy for high MET• QCDtg provides good representation of tight-tight MET distribution

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• For LocHadTopo, both QCDtg and QCDg seen to provide a good representation of the tight-tight MET distribution (again, using QCDtg-Iso as a proxy at high MET)

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Next: Signal Regions

We define five signal regions, for:

• Strong production, high and low bino mass (SP1,SP2)• Weak production, high and low bino mass (WP1,WP2)• Choose MET cut to suppress backgrounds (MIS)

04 June 2013 7

Direct Background Estimate Methodology

A CB

e.g. for signal region WP2

Estimate = C*(A/B)

Nominal control sample is QCDtg_50_noIso

This is METRefFinal; can also look at QCDtg and QCDg of LocHadTopo

Met

Ref

Fin

al

Met

Ref

Fin

al

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Direct Background Results and Systematics

LocHadTopo QCDtg and QCDg provide good systematic check

MetRefFinal QCDg expected to be a bit high; ignore

SP1, SP2 statistics a bit poor complement with extrapolation technique

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SP1 Meff

Extrapolations

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SP2 Meff

Extrapolations

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Combining all the above information yields the following overall result for

QCD background(See Note for justifications…)

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Electroweak Backgrounds (W, ttbar, etc.)

~75% involve e fake; much of remainder incorporatedin QCD backgrounds

Reconstruct e sample; scale by measured e fake rate

+/- 25% uncertainty from non e fake processes

+/- 10% uncertainty from fake rate measurement

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e Fake Rate Results

04 June 2013 14

e Sample Statistics and EW Background Estimates

04 June 2013 15

Irreducible Backgrounds

Z; Z

• Small contribution • NLO K-factor 2.0 +/- 0.3 (well understood) Estimate directly from MC

W; Wl

• Larger contribution• NLO K-factor 3.0 +/- 3.0• Dominant background systematic Constrain with new data-driven study

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W K Factor

http://arxiv.org/pdf/1103.4613v1.pdf

LO “radiation zero” eliminated at NLOGrows with hardness of radiation Rapidly-varying function of W system recoil

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Can we constrain the W K factorwith an lgg (l = e,) sample?

Ben Auerbach

04 June 2013 18

Choose study region to be • 50 < MET < 250 (leave MIS signal region blind)

• PT(l) > 100

Ben Auerbach

Nexpected = 7.4 (6.5 W)

Nobserved = 7.0

K factor of 3.0 +\- 1.2

04 June 2013 19

Irreducible background results

And then putting it all together…

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Wrap-Up

• Preliminary estimates of background completed

• Updating the Note with new background studies (nearly done)

• Addressing comments from prior review (before p1328/p1181 MET changes that threw us back)

• When end is in sight, will request Ed Board meeting to discuss unblinding

• In the mean time, are developing limit-setting approach, and beginning to evaluate signal systematics

04 June 2013 21

MET Issues

• Are latest (“post-Moriond”?) object definitions included in p3128 EGamma10NoTauLoose MET?

• We will need in any case to assemble our own “fluctuated” EGamma10NoTauLoose in order to do systematic studies

• But for now, background estimates largely insensitive to MET systematics (data-driven), so could use intrinsic p1328 variable if “approved”

• Will definitely need to be able to assemble EGamma10NoTauLoose from scratch soon though.