Single Top Strategies and Potentials at CMS

Transcription

1 Single Top Strategies and Potentials at CMS Institut für Experimentelle Kernphysik, KIT on behalf of the CMS Collaboration Workshop on single top physics and fourth generation quarks DESY,

2 Overview Introduction t-channel modeling Early single top analysis - Event selection - QCD estimation - Robust sensitive variable - Prospects 2

3 Overview Introduction t-channel modeling Early single top analysis - Event selection - QCD estimation - Robust sensitive variable - Prospects 3

4 CMS Detector 4

5 Single LHC s-channel t-channel associated production Tevatron [pb] s = 1.96 TeV LHC [pb] s = 14 TeV LHC [pb] s = 10 TeV s-channel ( 12) 5 ( 6) t-channel ( 125) ( 65) associated production ( 600) ( 310) T.Tait Phys. Rev. D61, (2001); N. Kidonakis et al. Phsy. Rev. D75, (2007); B.W. Harris et. al. Phys. Rev. D66, (2002); MCFM calculations by Maxim Perfilov Rise of t-channel x-section ~13 times larger than rise of W+jets x-section t-channel is most interesting channel for the first LHC data 5

6 Single Top CMS Louvain, Belgium: Andrea Giammanco Tehran, Iran: Nadjieh Jafari Mojtaba Mohammadi Najafabadi Aachen, Germany: Martin Erdmann Dennis Klingebiel Jan Steggemann (joined recently) Protvino, Russia: Dmitri Konstantinov Karlsruhe, Germany: Julia Bauer (née Weinelt) Thomas Müller Moskow, Russia: Eduard Boos Lev Dudko Anastasia Markina 6

7 Single Top CMS try Charge asymme Louvain, Belgium: Andrea Giammanco Tehran, Iran: Nadjieh Jafari Mojtaba Mohammadi Najafabadi Aachen, Germany: Martin Erdmann Dennis Klingebiel Jan Steggemann (joined recently) Protvino, Russia: Dmitri Konstantinov Karlsruhe, Germany: Julia Bauer (née Weinelt) Thomas Müller Moskow, Russia: Eduard Boos Lev Dudko Anastasia Markina 7

8 Single Top CMS try Charge asymme Louvain, Belgium: Andrea Giammanco Early analysis with 200/pb at s=10tev (PAS TOP ) Tehran, Iran: Nadjieh Jafari Mojtaba Mohammadi Najafabadi (joined recently) Protvino, Russia: Dmitri Konstantinov Karlsruhe, Germany: Julia Bauer (née Weinelt) Thomas Müller Aachen, Germany: Martin Erdmann Dennis Klingebiel Jan Steggemann Moskow, Russia: Eduard Boos Lev Dudko Anastasia Markina 8

9 Single Top CMS try Charge asymme Louvain, Belgium: Andrea Giammanco Early analysis with 200/pb at s=10tev (PAS TOP ) Tehran, Iran: Nadjieh Jafari Mojtaba Mohammadi Najafabadi Aachen, Germany: Martin Erdmann Dennis Klingebiel Jan Steggemann (joined recently) Protvino, Russia: Dmitri Konstantinov Neural network analysis Karlsruhe, Germany: Julia Bauer (née Weinelt) Thomas Müller Moskow, Russia: Eduard Boos Lev Dudko Anastasia Markina 9

10 Single Top CMS try Charge asymme Louvain, Belgium: Andrea Giammanco Early analysis with 200/pb at s=10tev Tehran, Iran: Nadjieh Jafari Mojtaba Mohammadi Najafabadi Aachen, Germany: Martin Erdmann Dennis Klingebiel Jan Steggemann (joined recently) Protvino, Russia: Dmitri Konstantinov (PAS TOP ) Neural network analysis Karlsruhe, Germany: Julia Bauer (née Weinelt) Thomas Müller MC generators: MadEvent SINGLETOP Moskow, Russia: Eduard Boos Lev Dudko Anastasia Markina 10

11 Single Top CMS try Charge asymme Louvain, Belgium: Andrea Giammanco Early analysis with 200/pb at s=10tev Tehran, Iran: Nadjieh Jafari Mojtaba Mohammadi Najafabadi Aachen, Germany: Martin Erdmann Dennis Klingebiel Jan Steggemann (joined recently) Protvino, Russia: Dmitri Konstantinov (PAS TOP ) Neural network analysis Karlsruhe, Germany: Julia Bauer (née Weinelt) Thomas Müller MC generators: MadEvent SINGLETOP Moskow, Russia: Eduard Boos Lev Dudko Anastasia Markina 11

12 Overview Introduction t-channel modeling Early single top analysis - Event selection - QCD estimation - Robust sensitive variable - Prospects 12

13 t-channel Modeling NLO contribution (2 3) LO process (2 2) W-g fusion W-b fusion 2nd b nd parton shower 2 b Modeling: MadEvent + PYTHIA for showering W-b and W-g fusion processes generated separately and matched in pt of 2nd b to match ZTOP NLO calc. 2 2 K 2 3 ZTOP NLO prediction (total x-section and rate of events with a hard 2nd b) ZTOP: PRD66, (2002); MadEvent: JHEP 0709:028 (2007) matching point 13

14 Comparison: MadEvent - ZTOP s=14tev top 2nd b top 2nd b Matched MadEvent sample reproduces kinematics of NLO ZTOP calculation well 14

15 Generator Comparison Top Quark MadEvent: Matched 2 2 and 2 3 process (Default, mb 0) MC@NLO: NLO MC, based on Herwig (mb=0 in ME, mb 0 in showering) SINGLETOP: Matched 2 2 and 2 3 process (mb 0) s=14tev Good agreement between all three generators 15

16 Generator Comparison Top Quark MadEvent: Matched 2 2 and 2 3 process (Default, mb 0) MC@NLO: NLO MC, based on Herwig (mb=0 in ME, mb 0 in showering) SINGLETOP: Matched 2 2 and 2 3 process (mb 0) s=14tev 2nd b: pt>20 GeV 2nd b: η < 4 Largest differences visible in variables of 2nd b 16

17 Overview Introduction t-channel modeling Early single top analysis - Event selection - QCD estimation - Robust sensitive variable - Prospects 17

18 Considered Processes Signal: Single top t-channel events, where the W decays leptonically into a muon and a neutrino s=10tev Important backgrounds: Diboson 18

19 Event Selection Muon, Jets Trigger: muon trigger (pt> 15 GeV) Single isolated muon, di-lepton veto 1μ with: pt>20 GeV, η <2.1 mainly QCD 0e with: pt>20 GeV, η <2.4 TkIso (caloiso): Scalar sum of pt (ET) of tracks (cal. deposits) in a cone of ΔR=0.3 around μ Strong reduction of QCD Single top t-channel Exactly 2 jets, far from the muon Iterative Cone algorithm (R=0.5), ptcalib>30 GeV, η <5, ΔR(μ, jets)>0.3 Calib: calibrated jets; scaled with factor that describes detector response depending on jet ET and η 19

20 Cut to Reduce Wlight One b jet Track counting High Purity Algorithm: DhighPur: impact par. significance (IPsig) of track in jet with third highest IPsig 1 jet with DhighPur>5.4 mainly Wlight High Purity Algorithm Di-jet events PAS: BTV

21 Cut to Reduce Top Pairs 2nd b veto Track counting High Efficiency Algorithm: DhighEff: impact par. significance (IPsig) of track in jet with second highest IPsig 2nd jet with DhighEff<1.5 High efficiency algorithm mainly top pairs Di-jet events PAS: BTV

22 Cut to Further Reduce QCD Transverse mass of W boson (t Wb) mainly QCD 22

23 Event Yield s=10tev L=200pb-1 Expect only a small contribution of QCD events, but we prefer not to rely on predictions from MC Expected background uncertainties at the level of (30-50)% Simple counting experiment not possible Stated uncertainties reflect stat. uncertainty of MC 23

24 QCD Background Estimation QCD rate: Determine number of QCD events in signal region by performing a fit to the MT distribution (data-driven method) Signal-like (S): Use either Z+jets sample (+ MW/MZ- rescaling, take one μ as ν), MC signal-like prediction or Signal region QCD in signal region W-enriched sample Parametrize samples with Crystal Ball functions QCD background (B): Use sample without b-tag requirement and anti-isolation cut Parametrize sample with a polynominal of rank 4 Uncertainty (syst.+stat.): ± 45% 24

25 Reconstruction of Single Top Events W boson reconstruction: Muon W mass constraint 2nd order equation in pz,ν - Complex solutions (36%) Varying px,ν, py,ν so that MT=MW Img(pz,ν)=0 MET non-b-jet W boson Top quark - Two real solutions (64%) Pick the one with smallest pz,ν Assigning the b quark from the top quark decay: b-jet Rec. top quark mass Take the b-tagged jet Correct in 92.2%, only in 4% the 2nd b is chosen 2nd b 25

26 Polarization of the Top Quark Single top s- and t-channel events: Polarization of the top quark Generator level s=14tev (due to V-A nature of Wtb coupling) passed to its decay particles Sensitive variable: cosθ*lj top rest frame Θ*lj Non-b jet Reconstruction level lepton Signal: linear dependence on cosθ*lj (gen. level) Backgrounds: flat in cosθ*lj 26

27 Single Top Prospects Binned likelihood fit to cosθlj*: Fit range: [-1,¾] Take single top template from MC, assume flat template for sum of backgrounds No assumption about background size Dip due to cut on pt,µ and MT Ensemble tests: Determine uncertainty on cross section and expected sensitivity (hypothesis test) cosθlj* is very robust against sources of uncertainty (extreme bkg shapes: 2.7σ 2.6σ) 27

28 Luminosity Projection Expected sensitivity as a function of integrated luminosity: s=10tev Method would need ~700/pb to manifest an observation There is a good chance to obtain an evidence with 200/pb stat. uncertainties only 28

29 Summary t-channel modeling: Top kinematics of different generators (SINGLETOP,MadEvent, agree well Some discrepancies visible in 2nd b variables (presumably mb effects) Early single top analysis - fit cosθlj*: (muon-jet angle in rec. top quark rest frame) Robust against systematics and size of backgrounds Scenario: TeV: Exp. Uncertainty on x-section: LHC???? ± 35% (stat.) ± 14% (syst.) ± 10% (lumi.) Can realistically achieve ~3σ 29