Prospect for WW Study with Early ATLAS Data

Transcription

1 Prospect for WW Study with Early ATLAS Data N. Vranješ, Lj. Simić, D.S.Popović of Physics Motivation and WW production at LHC Event reconstruction and selection WW production measurement with first ATLAS data

2 Motivation Test of the non Abelian gauge symmetry of the SM WW production is the most important background for a number of new physics signals

3 Test of the SM WWZ and WWγ couplings with W-fermion couplings describe WW production in the lowest order of the SM Deviations from the SM predictions would appear if new physics at scale Λ is present Need to measure total and differential cross sections Feynman diagrams for WW production in qq annihilation (LO) Complementary to direct search

4 WW as a background for new physics Higgs, SUSY, W ± LW L ± SM Higgs production with m H ~160 GeV

5 WW production at the LHC W boson pairs will dominantly be produced in qq anihilation process qq->ww NLO QCD graphs contribute: qg->qww, qq->wwg due to collinear enhancement factor log 2 [p T (W)/M W ] in the gq->wwq partonic cross section and large gq flux at LHC energies. gg->ww contribute at the O(α s2 ), 5% correction to NLO σ

6 Experimental signatures Fully hadronic: WW->qqqq ~4/9 of all events, resulting in 4 jets in the final state swamped by large QCD background expected at the LHC Semileptonic: WW->lνqq ~4/9 of all events, 2 jets, lepton and E T miss from neutrino large QCD and W+jet(s) background Fully leptonic: WW->lνlν ~1/9 of all events, clean signature 2 leptons and large E T miss WW->jjjj 46% No narrow mass peak, need good S/B ratio and good understanding of the background Target for this analysis! l=e,μ (τ not considered) WW->lnlnu 10% WW->lnujj 44%

7 WW->lνlν simulation Parton shower MC generators (PSMC): PYTHIA, HERWIG (LO) Numerical parton - level MC s: MCFM, DKS, BHO calculate di-boson production to NLO MC@NLO works like any other PSMC but incorporates NLO QCD matrix elements into parton shower. HERWIG used for fragmentation and showering GG2WW (gluon induced WW production). Here signal modeled with MC@NLO 3.1 σ MC@NLO (pp->ww)=111.3 pb σ(nlo)/σ(lo) 1.3

8 Background processes All sources of multilepton final states accompanied by missing transverse energy are potential background to the WW->lνlν signal. ttbar with t->wb and W->lnu Drell Yan Z/γ*->ll, l= l=e,μ,τ Diboson production WZ->lnull and ZZ->llnunu W+jet/γ, with jet/γ missidentified as a lepton WW->l τnunu ttbar and WZ modeled with MC@NLO, DY, ZZ and W+ γ with Pythia W+jet considered negligible!

9 Full detector simulation and event reconstruction Signal and background passed through full simulation of ATLAS detector Event reconstruction - electrons (ID tracks + EM) - muons (ID + MS tracks) - MET (Calo( + MS information) - jets (EM+HAD clusters)

10 Electron reconstruction Standard Reconstruction (egamma) Construct and calibrate EM cluster Use shower shapes to discriminate against jets Try to match a track Use Tracker information Use combined Tracker+Calo information ATLAS e/jet separation Rej(jet) > 10 5 achived (important for W+jet reduction) pt>25 GeV ε=81.4%, fake rate<0.1%

11 Muon reconstruction Traversing ATLAS a μ is detected in 2 high precision tracking systems: Inner Detector and Muon Spectrometer Muon energy loss in calorimeters is taken into account 2 parallel algorithms: MUID and STACO MUID performance: ε=91.9%, fake rate<0.001%

12 Lepton isolation Further background with high hadron activity Isolation criterial: summ of E T in ΔR=0.45 less than 8(5) GeV for electrons (muons) Electron isolatio looser to preserve flat efficiency vs lepton p T WW->ee WW->μμ ε_ele=75.4%, ε_μ=87.2%,

13 Jet reconstruction Jet veto: efficient for ttbar background suppression p T jet cannot be arbitrary low (fake jets, pileup) We have used R=0.7 jets in this analysis (better ttbar suppresion) Do not consider jets near the leptons

14 MET reconstruction ETmiss reconstructed from calo informatioan and SA muons (to avoid double counting) Worse resolution in the channels involving muons

15 Event selection 2 opposite sign leptons p T >25 GeV and pseudorapidity η <2.5 E T miss >50 GeV Jet veto: p T >20 GeV and η <3 M Z veto: m ll -MZ >15 GeV case A: φ(ll)<2 rad case B: Φ(pT(ll),ETmiss)>175 0

16 Basic distributions

17 gg->ww process contribution gg2ww generator interfaced to Pythia and ATlfast used σ(gg->ww->lnulnu)=482.5 fb (l=e,mu,tau e,mu,tau) after selection cuts, gg contribution is 12% (selection A), 8% (selection B)

18 Event selection case A φ(ll)<2 rad Case B Φ(pT(ll),ETmiss)>175 0 L=1fb -1 L=1fb -1 Higher statistics; reducing high p T spectra Lower statistic; more suitable for TGC

19 ee ATLAS potential for WW production measurement with first data μμ eμ ll WW ttbar WZ ZZ < Wγ WW->tau bckg S/B L=1fb -1 φ(ll)<2 rad High significance in the first fb -1! Systematics from limited MC statistics taken into account.

20 Summary We have investigated ATLAS potential for W boson pair production with first data using full simulation of ATLAS detector By establishing proper event selection cuts S/B 4 ratio achieved with high signal significance WW cross section measurement with 1fb -1 of data and less To be included (systematic uncertainties, background from data )

21 BACKUP

22 WW production at LHC

23 LO vs NLO

24 Test of the SM q q q q W W 0 Z Feynman diagrams for WW production in qq annihilation (LO) γ q q W W W W WWZ and WWγ couplings with W-fermion couplings describe WW production in the lowest order of the SM Devations from the SM predictions would appear if the SM particle spectrum should be inlarged with new particles such as Z, or if gauge bosons are composite bla bla bla If there are new physics at large energies bla bla bla

25 The ATLAS detector Inner Detector Measures the momentum of each charged particle. Muon Spectrometer Identifies and measures the momenta of muons. Calorimeters Measure the energies carried by the particles. Magnet System Bends charged particles. for momentum measurement. The solenoid magnet surrounds the inner detector. Arrows points to toroid magnets.

26 ATLAS detector Tracking ( η <2.5, B=2T) : -- Si pixels and strips -- Transition Radiation Detector Calorimetry ( η <5) : -- EM : Pb-LAr with Accordion shape -- HAD: Fe/scintillator (central), Cu/W-LAr(fwd) Muon Spectrometer ( η <2.7, B=0.5T) : air-core toroids with muon chambers Length : ~45 m Radius : ~12 m Weight : ~ 7000 tons Electronic channels : ~ 108 ~ 3000 km of cables

27 ee channel

28 μμ channel

29 eμ channel