Studies for GMSB with photons

Shilei Zang

University of Colorado, Boulder

CU CMS Meeting, 18th Mar. 2008

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Outline

• GMSB with prompt photons• Trigger study• Background study

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G~

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~

01

~

p p

q

q

q

q ~…

…

jet

jet

jet

jet

G~

~

GMSB with photons

• Gauge Mediated Supersymmetry Breaking models

• NLSP (neutralino) LSP (gravitino) + photon

• Prompt decay (ctau=0)

• high pT photons• large MET due to

gravitinos• multi-jets

Experimental signature

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Trigger Study

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Efficiency and rate for default triggers

RD

RS

D

S

H

VH

SS, RSS, RS, DS, RS, D, RDS, RS, D, RD, Hall

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Problem: How to optimize the trigger thresholds with figures of Efficiency vs. Rate in an objective way ?

Optimize trigger thresholds

• Usually the cuts are determined bye eye to give reasonable values of efficiency and rate. Threshold,

how to set?

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Physics Analysis

• Selection criteria are optimized to maximize statistics (Optimize relative error of BR; Significance; 90% CL limit, etc)

• Selection criteria are optimized to minimize the mass uncertainty in mass measurement (e.g. top mass measurement)

• Artificially reduced the error of physical result!

• Not Really Blind !!

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• N events, the amount of information : log2 N.

• N is number of messengers;

physical results are the meaning of information taken by such N messengers.

• For BR, number of messengers is the meaning of info.;

• For width, mass, … , meaning of info. is taken by the messengers; depends on the kinematics (not just on the number of events).

• Good property: log (xy)= log(x) + log(y).

Information theory

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• Amount of information: log(NS ), log(NB )

• Signal efficiency ε and background efficiency b

• After the cut: log(NS ε), log(NB b)

• Reductions of information: -log(ε), -log(b)

• Ratio of the reductions: log(ε)/ log(b)

• the smaller log(ε)/ log(b), the better

• log(ε)/ log(b) <a ε > ba (0< ε, b, a ≤1).

We can use statistics log(ε)/ log(b) to optimize trigger thresholds!

Good property: Blind Analysis!

log(ε)/ log(b) depends on the amount of information; does not depend on the meaning of information.

log(ε)/ log(b) <a ε > ba .

a=1.0

a=0.7

a=0.5

a=0.3a=0.2a=0.1

a=0.05a=0.02

a=0.01

ε = ba

(1.,1.)

(0.,0.)

Trigger Study

MVA

b

ε ε

ε

b

1-b

K ID

ε

b

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How to deal such a problem in Physics Analysis?

Solution: log(ε)/ log(b) to optimize selections with final ε and b after the kinematics cut.

Our method will give worse physical results, but they are blind analysis and can be trusted.

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log(ε)/ log(b) vs. Cuts (default EMHighEt)

0.0350.129

Min=0.129

Min=0.101 Min=0.00470.0170.102

• Et>80; Iecal<5; Ihcal<12; Itrack<4

• Itrack is better than Iecal and Ihcal.

• Each figure is plotted with other cuts applied.

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log(ε)/ log(b) vs. Cuts (proposed EMHighEt)

• Et>60; Itrack<2

• Itrack is better only when the Itrack cut point <5

0.022Min=0.174

Min=0.190 0.068

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Et>40GeV

Et>40GeV Et>40GeV

Relaxed Single Photon candidates

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• Propose two new triggers:

p-EMHighEt (pH): Et>60GeV, Itrack<2

p-EMVeryHighEt (pVH): Et>120GeV

• Propose to use: pH, pVH, D for our physics.

Trigger H pH VH pVH H, VH pH, pVH

Rate (Hz) 0.63 2.01 0.13 0.97 0.753 2.888

Efficiency GM1b GM1c GM1e GM1f GM1g Rate (Hz)

RS, H, VH 81.31 85.68 89.58 90.52 91.10 3.510

pH, pVH, D 87.70 91.02 93.27 93.78 93.96 3.139

pH, pVH, RS 88.72 92.28 94.44 94.71 94.72 5.226

2.14 Hz

Results

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• Propose two new triggers:

p-EMHighEt (pH): Et>60GeV, Itrack<2

p-EMVeryHighEt (pVH): Et>120GeV

• Propose to use: pH, pVH, D for our physics.

I. Isolation is useful at low Et region to suppress bkg , but bad in high Et region for our signal.

II. Track isolation (cut position <6) is better than other isolaitons

III.GMSB points with small Lambda parameter (GM1b) have more events with two signal photons at generator level, so the Double trigger is helpful for them.

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efficiency GM1b GM1c GM1e GM1f GM1g Rate (Hz)

pH, pVH, D 87.70 91.02 93.27 93.78 93.96 3.139

pH, pVH, mD 89.77 92.24 93.81 94.14 94.22 3.762

• D: Et>20; Iecal<2.5; Ihcal<8 or 6; Itrack<3 (0.26 Hz)

• mD: Et>20; Itrack<3 (0.90 Hz)0.64 Hz

Further possible improvement

• With 3 photon triggers, for GM1e, it’s difficult to reach 95% efficiency within 5.5 Hz!

• For GM1e, it’s easy to reach 92% or 93% efficiency, but it’s difficult to reach 94% or 95% efficiency!!

• 98.6% events of GM1e have signal photons (>0) at generator level; after SusyAnalyzer, only 93.5% events have reconstructed photons.

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Efficiency and Rates for each group of triggers

0: H, VH1: H, VH, RS2: H, VH, RD3: H, VH, D

10: pH, pVH11: pH, pVH, RS12: pH, pVH, RD13: pH, pVH, D

15: pH, pVH, mD

pH, pVH, D

pH, pVH, mD

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Background Study

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a. Signal sample: selected data to yield physical results.

b. Signal MC: to get signal efficiency.

c. Bkg MC: to stduy the background if they can simulate well.

d. Control samples: Large data samples (then small errors) 1) to study the background; different cuts but same distribution shape with signal sample. 2) to extract good parameters (machine, condition, etc). 3) together with MC, to study efficiencies and the systematic errors of cuts.

CDF (2007) 1.2 fb-1

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CDF (2007) 1.2 fb-1 Analysis of Diphoton+MET

Diphoton signal sample:

Both photons are required to pass a standard set of photon ID cuts. (Iso-Ecal<2GeV; Iso-Track<2GeV ; Et>13GeV)

Diphoton control sample:

Both photons pass loose photon ID cuts and at least one photon fails the standard cuts. (Iso-Ecal<3GeV; Iso-Track<5GeV; Et>13GeV)

• Other cuts for both of diphoton signal and control sample:

vertex; suppress cosmic rays;

reject beam halo; MET and jets directions;

…..

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There are 3 major sources of background:

1) Inclusive e+gamma events with real intrinsic MET. Electron-photon mis-identification: W(enu)+gamma; W(enu)+jet (jet mis-identified as a photon); e+gamma events

scaled by the Et dependent probability of an electron faking photon

2) Non-collision events : cosmis rays: out-of-time gamma+gamma events; beam halo: beam halo events.

3) QCD events with fake MET: gamma+gamma, gamma+jet, jet+jet events; MET due to energy mis-measurement in the calorimeter. 1) Soft unclustered energy (underlying event, multiple interactions): diphoton control sample and Zee samples with no jets. 2) Jet’s contribution to MET: Get a jet energy resolution from gamma+jet or Z+jet samples; for diphoton signal events, predict the MET distribution due to jets: MET(i)=Et –Et(smear).

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DZero (2004) 0.26fb-1 GMSB in Diphoton+MET

Diphoton signal sample:

Both photons are required to pass: EM ID (shower shape, isolaiton); Et>20GeV.

1) QCD background: same cuts as diphoton signal, except the shower shape. Use the number of evnets with MET<15 GeV in QCD and diphoton sample to predict QCD bkg rates for high MET. (assume all MET <15GeV is contributed from QCD)

2) Electron background. Electron-photon mis-identification: W(enu)+gamma; W(enu)+jet (jet mis-identified as a photon); e+gamma events scaled by the probability of electron faking photon.

3) QCD to diphoton; QCD to e+gamma; e+gamma total; e+gamma to diphoton;

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DZero (2007) 1.1fb-1 GMSB in Diphoton+MET

Diphoton signal sample:

Both photons are required to pass: EM ID (shower shape, isolaiton); Et>25GeV.

1) QCD background (hh sample): same cuts as diphoton signal, except the shower shape and isolation. Use the number of evnets with MET<12 GeV in QCD and diphoton sample to predict QCD bkg rates for high MET. (assume all MET <12GeV is contributed from QCD)

2) Electron background. Electron-photon mis-identification: W(enu)+gamma; W(enu)+jet (jet mis-identified as a photon); e+gamma events scaled by the probability of electron faking photon.

3) QCD to diphoton; QCD to e+gamma; e+gamma total; e+gamma to diphoton;

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• Photon identificaiton: Virginia

• QCD control sample: Shilei

• EW control sample: Rome + Bernadette

• Beam halo and cosmic rays:

Efforts for GMSB photons analysis for the moment:

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• To use the CDF methods: 1) use gamma+jet or Z+jet MC to get the jet energy resolution function; 2) use QCD MC to test that the MET model works. (Ben’s suggestion)

(Difficulty: it’s better not use jet resolution for the early CMS data; the only thing we can ask of jets is a cut on the scalar sum of all jets’ Et.) (Yuri’s opinion)

• To use Dzero methods: CMS do not have shower shape cut, do we have enough isolation space between trigger and analysis to get enough control events? (need MC to test yes or no)

If yes Does/How the MET shape depents on the isolation cuts?

If no any other cuts (photon Et, number of jets, ..) to get the control sample? Does/how the MET shape depends on these cuts?

• CDF methods in 2004?

QCD bkg study for GMSB is in progress.

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• We propose 3 triggers for GMSB photons.

• We find a new method for trigger study, and physics analysis.

Summary CMS note of trigger study is being approved.

QCD bkg study for GMSB is in progress.• Further understand Dzero and CDF methods.

• Test these possible methods with MC samples at CMS with the common GMSB analysis codes.

Thank you!