From Precision Electro-weak Physics to Beyond-the-SM Physics Searches

Transcription

1 From Precision Electro-weak Physics to Beyond-the-SM Physics Searches PING TAN Oct. 22, 2012 Outline W charge asymmetry analysis Search for BSM physics with 2leptons +2jets

2 Precision Electro-weak Physics at the LHC The standard model (SM) of elementary particles: survived most stringent experimental tests, observation of a new particle/narrow resonance ~ 125 GeV at LHC experiments (2012)... Gauge sector is among the most motivated/ tested pieces: UA1/UA2, LEP, SLD,CDF/D0, and LHC experiments LHC is a W/Z factory: ~ 8 Ws/fb standard candle for detector commissioning dominant backgrounds for Higgs/BSM searches precision measurement of W/Z/multi-boson production to test SM predictions, search for atgcs,... 7 TeV J. Campbell, et al. 2

3 Precision Electro-weak Physics at the LHC (cont.) Remarkable development in phenomenology before LHC start. most of SM processes available at NLO (or NNLO), re-summation (NNLL) available for some processes, PDF errors: ~4% scale variation <1% PDF4LHC Collaboration C. Anastasiou et. al., Phys. Rev. D69, (2004) NNLO W/Z predictions yield <1% scale variations, QED correction and PDF uncertainties are more sizable Precision W/Z measurement can be used to improve knowledge of PDFs. 3

4 W/Z - Production Cross Sections 20 campaign: 0.2 pb-1 (ICHEP ) 2.9 pb-1 (first CMS publication) 36 pb-1 (detailed studies) Remarkable agreement with SM predictions: experimental uncertainty is much less than theoretical uncertainty and luminosity errors (4%) Ratios cancel luminosity errors/reduce theoretical uncertainties EWK-PAS JHEP, 132 (2011) 4

5 W Lepton Charge Asymmetry Direct accessible observable, lepton charge asymmetry Different PDF fits have significant different predictions at the LHC kinematic region. Cancellation of experimental uncertainties. ~ 3 Entries POWHEG MC + W rapidity - W rapidity + l pseudorapidity - l pseudorapidity W rapidity/lepton pseudorapidity Lepton Charge Asymmetry pl T > 25 GeV/c MCFM + MSTW2008NLO + CTW theory bands: 90% C.I. Lepton Pseudorapidity 5

6 W µν Analysis Overview Isolated high-pt muon Large missing transverse energy (due to neutrino) >90% trigger efficiency, (single muon trigger threshold cm -2 s) Muon identification: hits in silicon tracker and muon detector, track fitting chi-squared,... Background: QCD, Drell-Yan, W(τν), ttbar, cosmic background Expect M (W µν)/fb -1 ~4.5M after trigger/ reconstruction/selection known issue with QCD background simulation (LO only) W candidate events are divided into different pseudorapidity bins, [0,0.4, 0.8, 1.2, 1.5, 1.8, 2.1] 6

7 Signal Estimation Isolation variable: scalar sum of isolation and transverse muon ionization energy deposit. poor QCD modeling, minimizing PDF uncertainties,... Simultaneous fit to extract charge asymmetry and total signal yield. Signal region: 0 < ξ < GeV Within signal region, QCD: 13.9%, Electro-weak (EWK): 6.9%, ttbar: 0.5% 7

8 Results Electron channel was performed using a fit to the MET variable. Different methods for signal estimation: consistent check against each other. Providing inputs to the PDF global fits Both measurement and prediction not very sensitive to higher order QCD, EWK-corrections, etc. JHEP 04, 050(2011) 8

9 Latest Update Constraints from triggers, Pile-up,... Contributed 2011 Lepton-Photon conference (*): assuming no trigger constraint EWK-PAS Uncertainty (%) = 0.2 [0.0, 0.2] (sys.) [0.0, 0.2] (stat.) [1.0, 1.2] (sys.) [1.0, 1.2] (stat.) [1.8, 2.0] (sys.) [1.8, 2.0] (stat.) 1 CTW PDF (90% C.L.) Luminosity (fb -1 ) 2 9

10 Current Status Finishing up analyzing the TeV CMS data journal publication (major change in analysis due to change of single muon triggers) 7 institutions: Cornell, Rochester, Fermilab, Purdue-Calumet, University of Nebraska, TIFR (India), 2 students (thesis topic) + 2 postdocs (including myself) + senior physicists. Documentation: AN , SMP Primary analyzer/analysis contact person since Public results: 1 journal paper + 2 PAS (1 based MC) Two more papers coming

11 Beyond the-sm BSM physics is the final goal. So far, no hint of new physics at CMS. Most convincing experimental evidence for Beyond-the-SM: dark matter. Model-independent search for new resonances with 2l+2j: WR, Lepto-quarks,... 11

12 Validation Plots - Work-in-progress Analysis strategy: focus on understanding of SM background contributions Dominant background: Drell-Yan, ttbar, diboson,... Excellent modeling of Drell-Yan+jets data, ttbar MC simulation agrees well with data (more data would help) data/

13 Status and Plan Plan to analyze both 2011 and 2012 and publish final search results. Involving institutions: Fermilab, University of Colorado-Boulder, Leading analyzer and contact person Analysis is ready and closely examining coming 2012 LHC data three CMS internal analysis notes have been prepared AN , IN-12-0, AN

14 Backup Slides 14

15 W/Z - Parton Distribution Functions (Cont.) Q 2 -dependence can be described by Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution equation(splitting function available at NNLO). x-dependence is parametrized function, determined with experimental inputs. MSTW 2008 NLO PDFs (68% C.L.) 2 ) ) xf(x,q Q = GeV xf(x,q Q = GeV g/ g/ u 0.6 b,b u 0.4 d 0.4 c,c d 0.2 c,c s,s u d 0.2 s,s d u x x 15

16 W/Z - Proton Distribution Functions (Cont. II) Directly probing PDFs in unprecedented regions with W/Zs: rapidity distributions, lepton charge asymmetry 16

17 W Charge Tevatron There is tension to include Tevatron lepton charge asymmetry results into PDF global fits. LHC data can shed new lights on the PDFs. Phys.Rev.Lett 2, (2009) FERMILAB- CONF--299-E 17

18 Background Overview QCD large uncertainty, poor modeling. Drell-Yan background (mainly Z/γ* µµ): reduced by rejecting events with second muon (Drell- Yan veto), remaining 2-%, depending on pseudorapidity, W τν, ~3-4%. (normalized to W µν yield in data with ratios determined with MC) ttbar background, ~0.5 % (from MC) Cosmic muon contamination, ~ -5 Drell-Yan background after DY veto (MC) dxy: transverse distance of closest approach dxy<0.2 cm 18

19 Overview of Systematics Asymmetries: with statistical errors, Signal estimation (Drell-Yan background dominates), Charge mis-identification: < -4 (from cosmic data) FSR correction, Efficiency ratio, 1.0 within from Z(µµ) data Muon momentum scale correction ( ) charge-dependent muon scale bias CMS silicon tracker mis-alignment bias determined from Z(µµ) QCD control sample 19

20 Results (cont.) Prompt feedback from PDF global fit community. 45% improvement in sea-quark uncertainty! (included in the latest NNPDF 2.3) Complementarity among LHC experiments M. Ubiali, J. Rojo et al., NNPDF collaboration 20

21 Latest Update - Muon Channel Ongoing-analysis to include most of 2011 CMS data (~ 5 fb-1) in muon channel: maintained lower pt threshold Much work remained to reduce systematics: ~ 20 M W events to deal with EWK-PAS

22 PDFs with W/Zs An extensive program to utilize the LHC data as inputs for PDFs: Z rapidity distributions, inclusive jet production, W charge asymmetry, Z/W ratios, observables with W+1 jet, extensive interactions with theorists... arxiv: hep-ex/ rs = 0.5(s+sbar)/dbar 22

23 PDFs with W+c First CMS measurement in W+c production single muon trigger, isolated muon pt>25, η <2.1, mt>50 GeV, anti-kt jet pt>20 GeV, η <2.1 (R=0.5), Δ(µ, j)>0.3, DY-veto, jet veto,... W + c/w - c: Wc/w+j: EWK-PAS (36 pb -1 ) 23

24 A rich physics program with W/Zs: σ (inclusive/differential), PDFs, τ-related decays, angular analysis, W polarization, mw, ΓW,... Beyond W/Zs Established processes across many orders of magnitude: W/Z+jets, di-bosons. Probing processes at sub pb-levels. [pb] σ tot Production Cross Section, j W 2j 3j 4j 1j 2j jet E T > 30 GeV jet η < 2.4 Z 3j 4j Wγ γ E T Zγ > GeV ΔR(γ,l) > 0.7 CMS 95%CL limit CMS measurement (stat syst) theory prediction WW WZ ZZ H(127) ZZ CMS pb pb fb fb JHEP(2011)132 CMS-PAS-EWK--012 PLB701(2011)535 CMS-PAS-EWK-11-0 CMS-PAS-HIG