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MSSM neutral Higgs in μμ analysis

MSSM neutral Higgs in μμ analysis. Stefano Marcellini – Gianni Masetti. Introduction. 5 Higgs bosons in the MSSM: 3 neutral (H and h with CP even; A with CP odd). 2 charged (H ± ) At the tree level, the Higgs sector is defined by tan β and M A .

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MSSM neutral Higgs in μμ analysis

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  1. MSSM neutral Higgs in μμ analysis Stefano Marcellini – Gianni Masetti

  2. Introduction • 5 Higgs bosons in the MSSM: • 3 neutral (H and h with CP even; A with CP odd). • 2 charged (H±) • At the tree level, the Higgs sector is defined by tanβ and MA. • Other parameters should be considered if radiative corrections are taken into account. • For the analysis, the Mhmax benchmark scenario is considered: • MSUSY = 1 TeV;µ = 200 GeV; M2 = 200 GeV; Xt=√6 MSUSY

  3. Effects of MA variation • If the MA variation is taken into account, there are 3 interesting regimes: • Decoupling regime(MA >> Mhmax): MA MH and very heavy; Mh  Mhmax. • Low MA regime (MA < Mhmax): MA Mh; MHMhmax. • Intense coupling regime (MAMhmax): MA Mh  MH (see hep-ph/0307079).

  4. Effects of MA variation Low MA regime Decoupling regime Intense coupling regime

  5. For high tanβ, the associate production becomes dominant. The cross section is proportional to tan2β. Production and decay   = h,A,H • The Higgs bosons mainly decay into bb (90%) and into  (10%). • The BR in two muons is about 3∙10-4.

  6. The h/A/H2μchannel • Channel characteristics: • The Branching ratio in two muons is very small. • The final state is quite clean. • The Higgs masses and widths can be reconstructed very precisely. • In particular it is possible to exploit the measurement of the width to determine tanβ. • The h/A/H→µµ channelis interestingfor the discovery of the MSSM neutral Higgs boson and for the study of the MSSM parameters.

  7. Signal • This analysis is performed assuming the cases of collected luminosity = 10, 20, 30 fb-1. • Higgs masses and widths calculated with FeynHiggs (Heinemeyer); cross sections with HQQ (Spira); branching ratios with HDECAY (Spira). • 23 signal samples have been generated trying to cover all the MA/tanβplane.

  8. Background • The main background comes from: • Drell-Yan with muon pair production (Z/γ* → μ+μ-). • tt, with t → Wb →μνμb. • About 3% of the DY is Z/γ*bb → μ+μ-bb which is the only irreducible background of this analysis. The production mechanism is the same as the signal. • Negligible backgrounds come from: • bb → μ+μ-+ X • WW → μ+μ-+ X • ZZ → μ+μ-+ X • Wt → μ+μ-+ X • h/A/H bb→ bb→ + bb + X

  9. Background (Drell-Yan) • It’s the dominant background. • Cross-section known at the NLO: • Z/γ*→μ+μ : calculated by J.Campbell with MCFM. • Z/γ* bb→μ+μ bb : compHEP cross sections. • Generated samples: • Z/γ*→μ+μ (generated with PYTHIA 6.2): • 300k events with Mμμ> 115 GeV (σ = 27.8 pb). • 2M events with Mμμ> 80 GeV (σ = 1891 pb). • Z/γ* bb→μ+μ bb (generated with compHEP): • 100k events with Mμμ> 100 GeV (σ = 1.05 pb). • 300k events with 60 GeV < Mμμ< 100 GeV (σ = 26.2 pb).

  10. Background (tt) • tt production with decay chain t → Wb → μνμbleads to a final state very similar to the signal: two isolated, high pT muons and two b-quarks. • Cross-section known at the NLO (from F.Maltoni, inclusive tt cross section = 840 pb). • Generated sample (with PYTHIA 6.2) : • 600k events with Mμμ> 10 GeV, pTμ1 > 20 GeV, pTμ2 > 10 GeV.

  11. Selection • Level 1 Trigger • High Level Trigger • Muon identification: to select events with 2 muons in final state • tt pairs rejection: Missing ET and Jet Veto • B-tagging: to reject the Drell-Yan

  12. Trigger selection • Summary of the online selection efficiency:

  13. Muon identification • The event is accepted if there are at least 2 muons with opposite charge that satisfy: • Number of hits in the Muon detector > 8 • pT > 20 GeV • Muon isolation: less then 10 GeV in a ΔR=0.35 cone around the muon

  14. Missing ET • As there is a neutrino in the top decay, a cut on the Missing transverse energy is applied. • The event is rejected if MET > 40 GeV

  15. JET veto • Jets in the events are reconstructed using the Iterative Cone Algorithm with the calibration “JetPlusTracks” using JetSeedEt=1GeV and CutCone = 0.5 . • The event is rejected if a jet with transverse energy > 45 GeV is found.

  16. B-Tagging • The B-jets generated by the signal are with low transverse momentum and in the forward region.

  17. B-Tagging • Two possibilities: • Based on jets: CombinedBTagging with discriminant > 0.4 • Based on tracks: At least two tracks with Transverse Impact Parameter (IP) in the range 0.01 < IP < 0.1 cm (only one track if 0.02 < IP < 0.075 cm)

  18. B-Tagging • Summary of the selection efficiency for the b-tag:

  19. B-Tagging • The analysis is performed with both selections independently. • The CombinedBTagging has a lower efficiency for the signal, but is more efficient in rejecting the background. It gives the best significance in the case of low Higgs mass and high collected luminosity. • In the case of high Higgs mass, and low luminosity, with such selection it becomes difficult to perform the fit to the Higgs peak (further on in this talk). • For such cases the “soft” B-Tagging, which has higher efficiency but is also less efficient in rejecting the background, gives the best significance.

  20. The Higgs peak region is found exploiting the TSpectrum class in root. The background is fitted in the region out from the peak with the three parameter function: fb = k0×Breit-Wigner(Mµµ;MZ,ΓZ) + k1 + k2×Mµµ. Fitting procedure N.B The analysis does not require an a-priori knowledge of the position of the peak!

  21. The signal is fitted with a Voigt function (convolution of Breit-Wigner and Gaussian): ftot = fb + k3×V(Mµµ;MA,σμμ,ΓA) σμμ is the CMS resolution for Mμμ. It is determined from real data fitting the Z peak. Fitting procedure

  22. An additional mass window cut is applied: MAmeas± (σμμ + 0.8 x Γmeas) NB integral of the background function in the mass windows. NS NTOT - NB Fitting procedure

  23. Systematic uncertainty • NS and NB are entirely determined from data. • When real data will be available, there will be no use of the Monte Carlo simulation to calculate the significance. • Thus the analysis is unaffected by systematic uncertainties coming from the modelling of the performance of the detector. • Possible uncertainty comes from the fitting procedure (next slide).

  24. Systematic uncertainty • To estimate the uncertainty for the fit parameters, 10000 toy Monte Carlo experiments were developed. • The standard deviation of the distributions of the results is taken as the uncertainty. • The fitting procedure was repeated fixing one of the parameters to the measured value increased by its error. • A new NB is obtained, and the difference with the previous one is taken as the systematic uncertainty. • The uncertainty varies from 1% to 7% (the worst values are obtained for MA = 200 GeV, where the background fluctuation makes difficult to perform the fit). • From this uncertainty a new significance is obtained.

  25. Decoupling regime

  26. Decoupling regime (MA = 150 GeV)

  27. Decoupling regime (MA = 200 GeV)

  28. To compute the significance the probability from Poisson distribution with mean NB to observe ≥ (NS + NB) events is used. The ScP program is used. It takes into account the systematic uncertainty too. Decoupling regime

  29. Low MA regime

  30. Low MA regime (MA = 100 GeV) No hope to see the signal in the low luminosity phase.

  31. Intensive coupling regime

  32. Intensive coupling regime • The signal peak is quite clear, but nothing can be said about the mass separation. • Significance > 5 already for 20 fb-1.

  33. Results • Discovery contour plot for 30 fb-1. • The tanβ limit is 22 for MA = 150 GeV, and 47 for 200 GeV. • In the intense coupling regime, there are three Higgs that contribute to the cross section. • The dashed line refers to the analysis without systematics. • Below 170 GeV, the contour is the limit for the fitting procedure and the systematics don’t modify it.

  34. tanβ measurement • The 2μ channel allows a good Higgs boson width reconstruction. • The measurement of tanβ can be obtained by exploiting the proportionality betweenΓA and tan2β. • The not perfect degeneration of A and H must be taken into account (negligible effect for high tanβ).

  35. tanβmeasurement • Higgs boson width uncertainty estimated with toy experiments. • The Γ=Γ(tanβ) curve is chosen by fixing the measured MA.. • The theoretical uncertainty of 15% (Sven Heinemeyer) is taken into account.

  36. tanβ measurement • Uncertainty, as a function of MA, for the tanβ measurement exploiting the Higgs boson width. • The method can be applied only for 150 < MA < 200 GeV. • Better results are obtained for high tanβ and low MA.

  37. Conclusions • Study of the discovery potential for the MSSM neutral Higgs bosons in the two muon decay channel. • Decoupling regime: for MA = 150 GeV the discovery is possible with tanβ > 22, for MA = 200 GeV tanβ > 47 (30 fb-1). • Intense coupling regime: it’s not possible to distinguish the single Higgs peaks, but the discovery is reachable for tanβ > 25. • Low MA regime: no hope to see the signal in the low luminosity fase. • It is possible to determine tanβ exploiting the measurement of the Higgs boson width. • Thanks to the referees for the useful comments.

  38. Summary table • Summary table in pb. In brackets efficiency w.r.t. previous cut.

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