Search for high mass resonances decaying to τ-lepton pair in pp collisions at s=7 TeV

Transcription

1 Search for high mass resonances decaying to τ-lepton pair in pp collisions at s=7 TeV Nitish Dhingra (Exotica Tau Panjab University, Chandigarh 1

2 Outline Physics Motivation Analysis Strategy Tau Identification Overview Selection Criteria Background Estimation Results Summary/Conclusions 2

3 Motivation New heavy gauge bosons Z, W occur quite frequently in various extensions of the Standard Model like Sequential Standard model, Left-Right Symmetric model, Superstring-inspired E 6 model, Little Higgs model. Models exists where a Z decays preferentially to a pair of high p T tau leptons. Such models with enhanced couplings to third generation fermions motivate the possibility of exploring a Z in its τ + τ - decay mode. There is no guarantee for any Z state produced at the LHC to have universal couplings to fermions. If a Z state exists and discovered through its e + e - or μ+μ - decay mode, the BR(Z τ + τ - )/BR(Z e + e - OR μ+μ - ) must be measured. Previous searches reported by the CDF and CMS collaborations excluded a Z gauge boson with Standard Model couplings at 95% C.L. with mass less than 399 GeV [1] and 468 GeV [2] respectively. [1] CDF Collaboration, Search for New Physics Using High-Mass Tau Pairs from 1.96 TeV ppbar Collisions, Phys. Rev. Lett. 95 (2005) [2] CMS Collaboration, Search for New Ditau Resonances in proton-proton collisions at s = 7 TeV, Physics Analysis Summary, EXO (2011) [Previous iteration with Run2010 data of this analysis]. 3

4 Signal & Backgrounds Backgrounds 4

5 Analysis Strategy Search involves four distinct and dominant final states: 1) Z τ τ τ e τ h 2) Z τ τ τ μ τ h 3) Z τ τ τ e τ μ 4) Z τ τ τ h τ h Z τ τ ee/μμ final states are not considered due to their small BRs and large background contamination from Drell-Yan production. Search performed using full 4.94 fb -1 of pp collision TeV collected during Run2011. Estimation of backgrounds from collision data wherever possible and where MC may not model the data correctly (dominant backgrounds like: W+jets, ttbar, QCD). Obtain highly pure control regions (CRs) with minimal modifications to the signal selections. Extrapolation to the signal region is done by measuring efficiencies of cuts in these CRs. Use Data-to-MC scale factors where MC is expected to model data correctly and where a complete data-driven estimation is not possible (Z μμ, Z ee, Z τ τ). Cut and count method. In the absence of excess, extract the exclusion limits. 5

6 Tau Identification 1. PFJets are formed using anti-kt algorithm with a cone size of Tracks are searched within a cone of 0.5 around the PFJet axis. 3. The highest pt track (leading track) is searched for within a cone of 0.1 about the PFJet axis. 4. An isolation cone is defined about the leading track. 5. Reconstruct decay mode. 6. Populate tau-identification algorithm discriminants. 7. Hadron-Plus-Strip (HPS) algorithm has been used for this analysis. 6

7 PZeta Dongwook Jang s Thesis (Rutgers, 2006) Jan DAE-BRNS HEP Symposium, 7

8 Selection Criteria 8

9 Background Estimation W+jets estimation in τ μ τ h Control Region 1: Obtain high purity W+jets control sample to measure cosδφ(μ,τ h ) & P ζ cut efficiency. Remove cosδφ(μ,τ h ) and P ζ cut to enhance W+jets. Require 50 < M T (μ,met) < 100 GeV to reduce contamination from other non-w+jets backgrounds. The sample thus obtained is highly enriched with W+jets background (~90%) and is used to measure the efficiency of cosδφ(μ,τ h ) & P ζ cuts. Control Region 2: To measure the M T (μ,met) cut efficiency. Invert cosδφ(μ,τ h ) and P ζ cuts. A sample of ~90 % purity thus obtained is used to measure M T (μ,met) cut efficiency. 9

10 QCD estimation in τ h τ h Start with the signal selections but invert the opposite-sign (OS) requirement to select the like-sign (LS) tau-pair candidates. Subtract LS component of non-qcd backgrounds based on MC expectations. Measure R OS/LS by fitting (M T (τ 1,MET)) OS/LS one-dimensionally. R OS/LS = QCD OS MET>15 /QCD LS MET>15 Total prediction on opposite-sign QCD is measured as: QCD OS = R OS/LS x QCD LS = 467 ± 67 10

11 Z ττ estimation in τ e τ μ MC approach: We can t estimate Z τ τ using fully data-driven methods since the tails extend well into the high-mass region where a Z resonance is expected. Moreover this background is expected to be well modeled by MC. Z τ τ is identical to signal but dominated in the low mass region. Calculate a scale factor b/w Data and MC to extrapolate to the signal region (SR). Two separate measurements of the scale factor: Control Region 1: SR + M(e,μ,MET)< 150 GeV. Purity = 87 % Scale Factor = Control Region 2: SR + M(e,μ)< 120 GeV MET Cut. Purity = 89 % Scale factor consistent with one (cross-check). Scale factor from CR1 applied to MC expectations in the SR gives: events. 11

12 Final Event Count (Results) eμ eτ h μτ h τ h τ h 12

13 Exclusion limits(individual) 13

14 Combined Limit Combined limit of μτ h + τ h τ h + eτ h + eμ Z SSM excluded with M < 1.4 TeV. Z ψ excluded with M < 1.1 TeV. 14

15 Summary & Conclusions Presented the results of search for new heavy resonances Z τ τ, performed with full pp collision data at 7 TeV collected by CMS detector during LHC Run 2011 (with SSM and E 6 models as benchmarks). Explored four dominant decay channels of Z τ τ decay: τ e τ h, τ μ τ h, τ e τ μ, τ h τ h Backgrounds estimated with data-driven techniques whenever possible. No excess of events observed with respect to SM background processes. Extracted combined limit for σ Z τ τ x BR(τ τ) taking into account the correlation of systematics within and across the four channels. World s best limit on this subject till date! Public Result M Z SSM M Z ψ > 1.4 TeV > 1.1 TeV Phys. Lett. B, Volume 716, Issue 1, pp (2012). 15

16 Exotica Tau Team B. Calpas 7, J. Cumalat 1, N. Dhingra 6, A. Florez 10, S. Gennai 3, A. Gurrola 10, M. Hildreth 5, W. Johns 10, T. Kamon 9, E. Luiggi 1, K. Mazumdar 8, A. Perieanu 7, M. Pioppi 2, A. Safonov 9, A. Saha 4, P. Sheldon 10, J. Singh 6, I. Suarez 9, and N. Valls 5 1 University of Colorado 2 Imperial College London 3 INFN Sezione di Milano 4 INFN Perugia 5 University of Notre Dame 6 Panjab University 7 I. Physikalisches Institut B, RWTH Achen 8 Tata Institute of Fundamental Research 9 Texas A & M University 10 Vanderbilt University 16

17 Back Up 17

18 Systematics 18