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Dijet and Multijet SUSY search without Missing EtRishiraj Pravahan (For the UTA group)

The University Of Texas at Arlington, 502 Yates St. SH 108, Arlington Tx USA.

Introduction The Analysis : Initial Cuts

Kinematic and Topological Variables

References

Object Selection and Constraints

Monte Carlo for Signal and Background

Initial Look at the Distributions

Point m0(GeV ) m 12(GeV ) A0(GeV ) tan(β) Sign(µ) σ(NLO)(pb) σ(10TeV )(pb)

Coannahilation SU1 70 350 0 10 + 10.86 2.42Bulk SU3 100 300 -300 6 + 27.68 5.47

Low Mass SU4 200 160 -400 10 + 402.19 107.6

Cut Efficiencies

Background Estimation

Further Investigations

Variable ExpressionαT

HT−∆HT2MT

HT∑

i pji

T"Hmiss

T −∑

i "pji

T

∆HT Epj1

T − Epj2

T

∆φ(j1, j2) j1 and j2 are effective jets for N(j) > 2R(Hmiss

T ) HmissT >50GeV

HmissT >30GeV

m2T2(µN ) minp1

T +p2T =pmiss

T

[max{m2

T (p1T , pa

T ; µN ), m2T (p2

T , pbT ; µN )}

]

A perfectly balanced di/multi-jet eventwill have !Hmiss

T = −∑

i !pji

T = 0. How-ever, events with an undetected particlewill contribute to !Hmiss

T . The magnitudeof !Hmiss

T is the expected missing trans-verse energy for multi-jet events. No cutis made on | !Hmiss

T | for this analysis.

Ideally QCD dijet events are ‘back-to-back’ and have equal pT ’s. Thus for well measured QCD-dijetevents, ∆φ(j1, j2) = π and αT = 1

2 by construction Ref[1]. Also note, αT and ∆φ are dimensionlessvariables and thus independent of the energy scale of the calorimeter.The concept of the dijet αT

variable can also be applies to events like pp→ ss̄→ qq̄XX̄ where two effective jets are constructedby combining all jets in the event such that Epj1

T − Epj2

T is minimised.

The HT variable denotes the scalar sumof the transverse momentum of all jets inthe event. By chosing a high cut for HT

one can eliminate a lot of dominant lowpT background.

The MT2 variable was constructed usingthe bisection algorightm with a low µN

value for all the samples. Even though,the endpoints are not clear the variablestill can be used for discriminating signalversus background.

∆φ(j1, j2) is a dimensionless variable thatis ideally equal to π for QCD dijet eventsand is flat in SUSY like events. It is alsoscale independent.

Background (Combined) Sample σ(pb)AlpJets 1.7 x 106

TTbar 202.85W/Z+Jets 8.1 x 104

WW,WZ,ZZ 48.83

Di-Jet Cut Efficiencies in percentageAllJets T T̄ W/Z+Jets SU1 SU3 SU4

P jT 1 > 150GeV 0.31 18.04 0.33 80.89 81.77 53.48

HT > 300GeV 31.57 33.33 34.99 93.02 95.77 64.87αT > 0.55 0.00 4.16 0.00 32.50 23.52 23.99

Multi-Jet Cut Efficiencies in percentageN [j(PT > 50Gev)] > 2 2.51 49.65 0.03 59.15 61.89 59.24

HT > 300GeV 1.76 17.09 0.00 92.30 75.86 47.39αT > 0.55 0.00 1.01 0.00 37.5 40.90 13.65

Comments

Surviving Events SignificanceDi-Jet ChannelBackground 1.68

SU1 0.31SU3 0.87SU4 4.95 3.81

Multi-Jet ChannelBackground 2.78

SU1 0.21SU3 0.49SU4 3.55 2.1

The efficiencies for all the cutsare not shown. The table givesan idea of the efficiency of themain cuts for the analysis. Thetotal number of surviving eventsfor each sample was calculatedfor a luminosity of 100pb−1 af-ter considering the efficiency ofall cuts. The significance shownhere is calculated as Nsignal√

Nbackground.

The R(HmissT ) is larger than 1 when lower

pT jets significantly contribute towardsbalancing of of the jets. The cut onR(Hmiss

T ) significantly affects the QCDbackground but leaves the signal effi-ciency high.

The αT distribution is by con-struction extremely good at elim-inating QCD background. Thedistribution sharply falls at 0.5for well measured jets and a cutof 0.55 eliminates almost all QCDbackground.

The ∆φ(j1, j2) distribution is flatfor signal like events whereas hasa peak at π for QCD and otherbackground events. In the di-jet scenario, the QCD distribu-tion extends between 2.0 and πwhereas for the multijet scenarioan optimal cut on ∆φ(j1, j2) willbe necessary

Background for this study arises from W → lν + Jets and Z → ll + Jets,semileptonic tt̄ and γ+Jets channels. All of the above are reducible backgroundsexcept for detector acceptance and identification inefficiencies. Z → νν + Jetsconstitute an irreducible background and must be estimated from Z → µµusing Monte-Carlo. QCD background rates must be determined from data. For,this purpose one may use the matrix-element method where two uncorrelatedvariables are identified with signal rich and signal depleted regions. This studyposes a perfect situation, where, αT can be used as one of the variables withsinal rich regions as αT > 0.55 and signal depleted regions as αT < 0.55, inconjunction with any of the other kinematic variables used in this study.

This study explores the possibilities of searching for R-parity-conserving SUSY withthe ATLAS detector, at the LHC, in the n-jet, n=2,3,... channel without explicitlyusing missing transverse energy (MET). This study is motivated by the difficultyof MET reconstruction during the early phases of ATLAS. Detector imperfectionsand uncertainties in energy scale measurements are among the major constraints toan accurate MET measurement. A set of Kinematic Variables is constructed, thatmay distinguish SUSY type events from dominant backgrounds like QCD.

10 TeV MC08 official samples were used in this analysis. The primarybackground samples for this study was estimated to be QCD jets,W and Z with associated jets, di-boson productions and Top pairproduction. Except for low pT QCD samples, all background Monte-Carlo consisted of an equivalent of 1 fb−1 of events.

Preselection: Dijet Preselection : MultijetP J1

T > 150GeV P J1T > 50GeV

P J2T > 50GeV P J2

T > 50GeVN [j(PT > 50GeV ) = 2 N [j(PT > 50GeV ) > 2

ηj1 < 3 ηj1 < 3∑i pji

T > 300GeV∑

i pji

T > 300GeVFor all El in event El(pT ) < 10GeV Electron veto

For all Mu in event Mu(pT ) < 10GeV Muon Veto

Jets were reconstructed using the Cone4H1Tower algorithm. Electrons werechosen following the standard ‘medium’ definition and muons were reconstructedusing the ‘Staco’ algorithm. Overlaps between jets and electrons were removed.The following selections were applied.

Further studies are currently ongoing, where all cuts are optimized in these variablesto get best significance. Once optimal cuts are found, this can be extended toN1(Jets)+N2(Leptons)+missingET scenarios. Uncorrelated variables need to beidentified to make background estimates from data.

Even though this study has ben performed for a N-Jet scenario, the methodis still valid for N-Jet + Leptons cases. Thus, one can have a good handle onany generic processes with a high mass scale (new physics) and a presenceof missing transverse energy. It could also be a starting point for search forHidden Valley phenomenologies with low Etmiss and leptons along withmultiple jets. Any analysis based on explicit missing transverse energycuts can also benefit from this method by lowering the threshold of cuts.As only topological dimensionless variables are used in the analysis, thestudy does not depend on accurate energy scale measurements. Moreover,variables like R(Hmiss

T ) were identified that can filter contribution to αT

giving a very good discrimination over the most dominant background fordiscovering any new physics at ATLAS with early data.

Ref.[1] arXiv : 0806.1049v1 Dijet Searches for Supersymmetry at the LHC , Lisa Radall and DavidTucker-Smith.I would like to thank everyone who has helped me with this study, specially my advisor Dr KaushikDe and Dr Amir Farbin. I would also like to thank Dr Alden Stradling and Dr Nurcan Ozturk forhelping me run and book-keep all the Monte Carlo samles.

Monday, August 3, 2009