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Charged Higgs search with top : example of single-top events in ATLAS

Charged Higgs search with top : example of single-top events in ATLAS. Ketevi Assamagan (BNL) Arnaud Lucotte (LPSC Grenoble). Outline Motivation Single-top production @ LHC Single-top cross-section measurements Events selection Results and performance

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Charged Higgs search with top : example of single-top events in ATLAS

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  1. Charged Higgs search with top : example of single-top events in ATLAS Ketevi Assamagan (BNL) Arnaud Lucotte (LPSC Grenoble) • Outline • Motivation • Single-top production @ LHC • Single-top cross-section measurements • Events selection • Results and performance • Charged Higgs contribution measurements • Cross-sections in (mH±,tan ) • Efficiency and event yields in (mH±,tan ) • Conclusion

  2. Single top Production @ LHC • Production at the LHC • 3 contributing mechanisms in SM • Cross-section theoretical uncertainties are significant : • – NLO/NLL available for s-, t- and (new) W+t channels • – Main uncertainty due to the choice for the (b,g) PDF • – Choice of factorization/renormalization scale • – Top mass uncertainty : ∆mtop - • σNLO(qb(g)tb(g)) = 150±6 pb • σNLO(qb(g)tb(g)) = 88.5±4 pb • dominant at the LHC • used for Vtb, t, FCNC … - t-channel • σLO(gbtW) = 60±15 pb • high level of bcgkd: tt • sensitive to extra W, H±… - W+t channel - σNLO(qq’tb) = 6.1±0.3 pb σNLO(qq’tb) = 3.8±0.2 pb - high level of bckgd: tt, W+jets - s-channel - ∆σtheo/σ= ± 4-8%

  3. Single top Production @ LHC • Production at the LHC • 3 contributing mechanisms in SM + Higgs in 2HDM • Our strategy in charged Higgs searches : • – Measure precisely single-top cross-section •  estimate stat. + systematic uncertainties • – Measure deviation from single-top cross-section •  interpretation in (mH±,tan) plane - • σNLO(qb(g)tb(g)) = 150±6 pb • σNLO(qb(g)tb(g)) = 88.5±4 pb • dominant at the LHC • used for Vtb, t, FCNC … - t-channel • σLO(gbtH±) in (mH±,tan) • Up to a few pb • High level of tt + t backgds W+t channel H± - σLO(H±tb) in (mH±,tan) - Up to a few pb… - High level of t + tt + W+jets H± s-channel -

  4. W* single top event selection • Analysis Strategy • Select and tag event • – 1 high-pT lepton (25 GeV/c) • – High missing ET (25 GeV/c) • – at least 2 high-pT central jets • with at least 1 b-tagged jet • Discriminate from non-W background • – Reconstruct the (leptonic) mWT • Discriminate from top-pair background • – Veto of a 3rd high-pT jet • – Keep low total energy events (HT or MTOT) • Discriminate from W+jets background • – Require a 2nd high-pT b-tagged jet • – Reconstruct a Top mass Mlvb • – Use event shape & high e,µ b b v q “l+jets” final state

  5. W* single top selection : jet multiplicity • Jet selection • Jet mulitplicity • – At least two high-pT central jets •  useful against W+jets… • – Veto of 3rd high-pT jet (difficult...but crucial) •  needed against top pair events • b-tagged jet • – Exactly two high-pT central b-jets (35-45 GeV/c) •  effb = 60% for rejection uds=100 effc=10% •  needed against QCD, W+jets, etc…

  6. W* single top selection : b-tagged jets • Jet selection • Jet mulitplicity • – At least two high-pT central jets •  useful against W+jets… • – Veto of 3rd high-pT jet (difficult...but crucial) •  needed against top pair events • b-tagged jet • – Exactly two high-pT central b-jets (35 GeV/c) •  effb = 60% for rejection uds=100 effc=10% •  needed against QCD, W+jets, etc…

  7. W* single top selection :leptonic top mass • Top mass reconstruction • 1) Determination of longitudinal pZ() • Interpret of mET as pT() • Compute pz() using W-mass constraint •  2-fold ambiguities for pz() : choose best M(lvb) •  If no solution (MWT > MW) take real part • 2) Recontruction of M(lb) • Choose b-jet resulting in the highest pTtop • - Optimization of the window on M(lb) •  lower bound : W+jets , WZ, etc..

  8. W* single top selection :total energy • Total transverse energy HT • HT Definition • HT = pT(jet) + pT(lepton) + mET • Note : can also use Mtot, Ptot… • Discriminating power • Optimization of the window on HT: •  lower bound : W+jets , WZ, etc.. •  upper bound : top pair • Depend upon : • – jet energy scale determination • – Lepton and mET determination

  9. W* single top selection :total energy • Total transverse energy HT • HT Definition • HT = pT(jet) + pT(lepton) + mET • Note : can also use Mtot, Ptot… • Discriminating power • Optimization of the window on HT: •  lower bound : W+jets , WZ, etc.. •  upper bound : top pair • Depend upon : • – jet energy scale determination • – Lepton and mET determination

  10. W* single top selection : event yields • Performance • Efficiency and rejection : • – Efficiency ε ≈ 1.7% and N(30fb-1) ~ 600 events • – Main backgrounds in 30 fb-1: • top pair: ~2,500 evts [lbjjb(60%) llbb(30%) bb(10%)] • WQQ, W+jets: ~1,400 evts • Wg channel : ~1,100 evts • Statistical sensitivity : • Statistical precision : from 12% to 7% Separating tb from tb final state helps: against top pair events (charge symmetric production) S/B ~ 0.14 √(S+B)/S ~ 12% _ _

  11. W* single top selection : statistical precision • Performance • Efficiency and rejection : • – Efficiency ε ≈ 1.7% and N(30fb-1) ~ 600 events • – Main backgrounds in 30 fb-1: • top pair: ~2,500 evts [lbjjb(60%) llbb(30%) bb(10%)] • WQQ, W+jets: ~1,400 evts • Wg channel : ~1,100 evts • Statistical sensitivity : • Statistical precision : from 12% to 7% Separating tb from tb final state helps: against top pair events (charge symmetric production) S/B ~ 0.14 √(S+B)/S ~ 12% _ _

  12. W* single top selection : main systematics • Systematic uncertainties • Main experimental biases : • – Knowledge of b-tagging efficiency (&rejection rate) •  important since double-tag evts •  consider 3% uncertainty in b • – Gluon Radiation Modeling : •  ISR’s affect the jet multiplicity •  FSR’s affect the jet energy & multiplicity • – Determination of Jet Energy Scale •  affects the reconstructed masses, HT… • Theoretical biases: • – Use of MC to estimate backgrounds … •  we will have to use the data ! σ/σ =6.6% σ/σ =8.7% σ/σ = 5% σ/σ = 8%

  13. W* single top with 30 fb-1 : where we talk about H± … • Charged Higgs & single-top • Production mode in 2 HDM : • 5 higgs: 3 neutral (A,h,H) + 2 charged (H±) • Mass spectrum predicted in MSSM •  (H+tb) couplings depends on mH± and tanβ • Cross-section x BR(H±tb): • May be as large as 1/3 of W* in (mH± ,tan) plane • – Increase with tan • – Decrease with mH± Cross-section in pb mH+ (GeV/c2)

  14. Charged Higgs sensitivity : cross-section and efficiency • Performance in (mH±,tan) plane • Strategy : • Use the very same analysis developed for the s-channel • **No specific analysis performed for a H± search** • – no use of spin-0 property of H± • – no use of other H± decays (tau..) •  Limited by the precision on σ(single-top), σ(top pair).. • Selection efficiency: • Increases with mH± up to 250 GeV/c2 •  higher pT of H±decay products with mH± • Then, decrease with higher HT values (out of window) Efficiency in % Cross-section in pb

  15. Charged Higgs sensitivity: event yields…(1) • Performance in (mH±,tan) plane • Selection efficiency: • – Cross-section increases with tan • – Cross-section decreases with mH± • – Efficiency increases with mH± • Event yields : mH±=220 GeV/c2 ,tan=50 • – efficiency  ~ 0.3% • – Event yield : ~140±10 evts (after Mlvb and HT window cuts) mH±= 220, tanβ=50 S/B ~ 1.3

  16. Charged Higgs sensitivity: event yields…(2) • Performance in (mH±,tan) plane • Selection efficiency: • – Cross-section increases with tan • – Cross-section decreases with mH± • – Efficiency increases with mH± • Event yields : mH±=250 GeV/c2 ,tan=50 • – efficiency  ~ 1.9% • – Event yield : ~285±15 evts (after Mlvb and HT window cuts) mH±= 250, tanβ=50 S/B ~ 5.7

  17. Charged Higgs sensitivity: event yields…(3) • Performance in (mH±,tan) plane • Selection efficiency: • – Cross-section increases with tan • – Cross-section decreases with mH± • – Efficiency increases with mH± • Event yields : mH±=300 GeV/c2 ,tan=50 • – efficiency  ~ 2.02% • – Event yield : 190±10 evts (after Mlvb and HT window cuts) mH±= 300, tanβ=50 S/B ~ 3.9

  18. Charged Higgs sensitivity : significance • Performance in (mH±,tan) plane • Significance : • A 5σ-discovery can be reached in a (mH±,tan) region • – provided σ(single-top), σ(top pair) are well determined • – provided we can control experimental systematics • Perspectives : • Many improvements expected: • – specific analysis for H± still to be done • – more sophisticated tools to be used • – extension to other H± decays • – complementarity with W+t measts &direct searches Significance in Nσ

  19. Charged Higgs sensitivity : significance • Performance in (mH±,tan) plane • Significance : • A 5σ-discovery can be reached in a (mH±,tan) region • – provided σ(single-top), σ(top pair) are well determined • – provided we can control experimental systematics • Perspectives : • Many improvements expected: • – specific analysis for H± still to be done • – more sophisticated tools to be used • – extension to other H± decays • – complementarity with W+t measts & direct searches ATL-PHYS-2001-017 tanβ=30, mH+=300 GeV Significance in Nσ direct gb H± tb search

  20. Conclusion & perspectives • Single-top cross-section measurements • Event Selection • S/B ~ 12-15% and about 1,000 evts / 10 fb-1 • Significant level of background contamination: •  top pair, W+jets events, QCD…other single-top • Performance • Statistical precision of 7-12% for W* channel • Measurements dominated by systematics : ~10% •  experimental : b-tag, JES, ISR/FSR •  theoretical : use of MC for backgrounds • Charged Higgs with top events • Indirect/direct measurements: • Low Higgs mass : top-pair production • – disappearance of semi-leptonic e/  events • – appearance of extra tau contributions • Low and High Higgs mass : single top production • – deviation in σ(Wt) measurement • – deviation from σ(W*) in (e,) final state •  5σ discovery possible with 30 fb-1 All these measuremetns are Complementary to other direct searches

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