Predrag Milenović , Institute for Particle Physics, ETH Zürich on behalf of the CMS collaboration

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Processes with prompt same-sign dileptons: qq → WZ, qq → ZZ, qq → qqW±W±, 2 × (qq → W±), qq → ttW, qq → WWW Evaluated using simulation, 10 - 40% contribution, assigned 50% systematic uncertainty

Processes with opposite-sign dileptons and charge mis-reconstruction: Z + jets → l±l± + jets, top-pair → l±l± + jets Evaluation completely based on data, < 10% contribution, 20% systematic uncertainty

Processes with one or two fake leptons: top-pair → l± + jets, W + jets → l± + jets, single-top → l± + jets, QCD jets Evaluation completely based on data, dominant, below 50% systematic uncertainty fake leptons: leptons originating from heavy-flavor decays and misidentified light flavor jets

Predicted Measured (in)efficiencies

Reconstructed

Tight leptons: leptons passing the final selection criteria; Loose leptons: leptons passing loosened criteria, but not the final selection criteria;

       predrag.milenovic@cern.ch

Search for new physics in events with same-sign isolated dileptons at CMS

We identify four search regions and one baseline selection region: ①①  HT > 400 GeV, miss. ET > 120 GeV [cMSSM, low m0 region] ②②  HT > 200 GeV, miss. ET > 120 GeV [simplified model of squark-gluino production] ③③  HT > 400 GeV, miss. ET > 50 GeV [cMSSM, high m0 region] ④④  HT > 80 GeV, miss. ET > 100 GeV [pMSSM with sneutrino as the LSP]

HT > 80 GeV, miss. ET > 30 GeV [to asses the performance of the methods] The four search regions are consistent with our search motivations and are chosen to maximize the sensitivity of the analysis to several new physics scenarios. We analyze events for three dilepton selections / categories:

Strategy 3

Experiment

Compact Muon Solenoid experiment at the Large Hadron Collider

1

Events with same-sign isolated lepton pairs from hadron collisions are rare in the Standard Model but appear naturally in SUSY [1-3] and other new physics scenarios [4-7]. Two guiding principles:   Astrophysical evidence for the dark matter [8] suggests us to search for the final states

with significant missing ET.   Observable new physics signals are likely to be produced by strong interactions and we

thus expect significant hadronic activity in association with the two same-sign leptons. Search is signature-based, as independent from a particular model as possible

Motivation 2

Baseline  

Backgrounds 4

Search region

Control regions

Intrinsic to the physics process

2 prompt(true dileptons)

1 prompt, 1 fake(ttbar, W + jets)

2 fake(QCD jets)

2 tight 1 tight, 1 loose 2 loose

Data-driven background prediction methods:   Two sets of complimentary methods are used to measure the backgrounds for the low-pT and

high-pT dilepton categories (two predictions are shown next to each other in plots below) [9,10]   A similar method is used to measure the backgrounds for the hadronic τh dilepton category [9,10]

Predictions in the baseline selection region:   Baseline selection region is expected to be largely dominated by the background events. It is

used for each of the three dilepton categories in order to assess the performance of methods with a statistically meaningful sample.

A very good agreement between the predicted and observed number of events and between complementary background prediction methods.

.

Predictions 5

Low-pT High-pT Hadronic τh

Yields and predictions for each search region and the dilepton category:

  We observe no evidence of an excess over the background predictions and we set the 95% CLS upper limits on the number of new physics events (using the modified frequentist method [11]):

Limits on the new physics events in 0.98 fb-1 of data

In case of lepton selections with complementary prediction methods, limits are set using the background estimate that provides the more conservative limit.

  The limits include uncertainties on the signal efficiency of 14%, 17%, and 20% for the low-pT, high-pT, and hadronic τh di-lepton categories respectively.

Results 6

Low-pT and Hadronic τh High-pT

Dilepton category Low-pT Had. τh High-pT HT [GeV], MET [GeV] 400, 120 400, 50 200, 120 400, 120 400, 120 400, 50 200, 120 80, 100

predicted 2.3 ± 1.2 5.3 ± 2.4 6.6 ± 2.9 2.9 ± 1.7 1.4 ± 0.7 4.0 ± 1.7 4.5 ± 1.9 10 ± 4 observed 1 7 6 3 0 5 3 7

upper limit CLS 95% 3.7 8.9 7.3 5.8 3.0 7.5 5.2 6.0

[1] R. Barnett, J. Gunion, and H. Haber, “Discovering supersymmetry with like sign dileptons”, Phys. Lett. B 315 (1993) 349. doi:10.1016/0370-2693(93)91623-U. [2] M. Guchait and D. P. Roy, “Like sign dilepton signature for gluino production at the CERN LHC including top quark and Higgs boson effects”, Phys. Rev. D 52 (1995) 133. doi:10.1103/PhysRevD.52.133. [3] H. Baer et al., “Signals for minimal supergravity at the CERN Large Hadron Collider II: Multilepton channels”, Phys. Rev. D 53 (1996) 6241. doi:10.1103/PhysRevD.53.6241. [4] H. Cheng, K. Matchev, and M. Schmaltz, “Bosonic supersymmetry? Getting fooled at the CERN LHC”, Phys. Rev. D 66 (2002) 056006. doi:10.1103/PhysRevD.66.056006. [5] R. Contino and G. Servant, “Discovering the top partners at the LHC using same-sign dilepton final states”, JHEP 06 (2008) 026. doi:10.1088/1126-6708/2008/06/026.

[6] F. Almeida et al., “Same-sign dileptons as a signature for heavy Majorana neutrinos in hadron-hadron collisions”, Phys. Lett. B 400 (1997) 331. doi:10.1016/S0370-2693(97)00143-3. [7] Y. Bai and Z. Han, “Top-antitop and Top-top Resonances in the Dilepton Channel at the CERN LHC”, JHEP 04 (2009) 056. doi:10.1088/1126-6708/2009/04/056. [8] G. Bertone, D. Hooper, and J. Silk, “Particle dark matter: Evidence, candidates and constraints”, Phys. Rept. 405 (2005) 279. doi:10.1016/j.physrep.2004.08.031. [9] CMS Collaboration, “Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy at the LHC”, JHEP 1106 (2011) 077. doi:10.1007/JHEP06(2011)077 [10] CMS Collaboration, “Search for new physics with same-sign isolated dilepton events with jets and missing energy”, CMS-PAS-SUS-11-010, CERN, 2011. [11] Particle Data Group Collaboration, “Review of particle physics”, J. Phys. G 37 (2010) 075021. doi:10.1088/0954-3899/37/7A/075021.

R e f e r e n c e s 9

ü  Background estimates derived from data for all major background sources ü  No evidence for new physics observed ü  Set 95% CLS upper limits on the number of new physics events in 0.98 fb−1 of data

Conclusions 8

Interpretation 7

Low-pT [ee, µµ, eµ]

High-pT [ee, µµ, eµ]

Hadronic τh [τe, τµ, ττ]

Constrained MSSM model: Exclusion Limits extended

Model independent information:   We provide approximate efficiency models for

the lepton, HT and missing ET selections in a parametric form that can be used to test a broad range of specific physics models.

Parameterized efficiencies for HT and missing ET:

HT miss.ET

Eidgenössische Technische Hochschule ZürichSwiss Federal Institute of Technology Zurich

collected in proton–proton collisions at c.m. energy of 7 TeV

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Search for new physics in events with same-sign isolated dileptons at CMS

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