MSSM neutral Higgs in

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
  tanß 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 Xtv6
  MSUSY

3
Effects of MA variation

• If the MA variation is taken into account, there
  are 3 interesting regimes
• Decoupling regime (MA gtgt Mhmax) MA ? MH and very
  heavy Mh ? Mhmax.
• Low MA regime (MA lt 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
Production and decay

• For high tanß, the associate production becomes
  dominant.
• The cross section is proportional to tan2ß.

??
? h,A,H

• The Higgs bosons mainly decay into bb (?90) and
  into ?? (?10).
• The BR in two muons is about 310-4.

6
The h/A/H?2µ channel

• 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 tanß.
• The h/A/H?µµ channel is interesting for 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/tanß plane.

8
Background

• The main background comes from
• Drell-Yan with muon pair production (Z/? ?
  µµ-).
• tt, with t ? Wb ?µ?µb.
• About 3 of the DY is Z/?bb ? µµ-bb which is
  the only irreducible background of this analysis.
  The production mechanism is the same as the
  signal.
• Negligible backgrounds come from
• bb ? µµ- X
• WW ? µµ- X
• ZZ ? µµ- X
• Wt ? µµ- X
• h/A/H bb??? bb??? bb X

9
Background (Drell-Yan)

• Its the dominant background.
• Cross-section known at the NLO
• Z/??µµ calculated by J.Campbell with MCFM.
• Z/? bb?µµ bb compHEP cross sections.
• Generated samples
• Z/??µµ (generated with PYTHIA 6.2)
• 300k events with Mµµ gt 115 GeV (s 27.8 pb).
• 2M events with Mµµ gt 80 GeV (s 1891 pb).
• Z/? bb?µµ bb (generated with compHEP)
• 100k events with Mµµ gt 100 GeV (s 1.05 pb).
• 300k events with 60 GeV lt Mµµ lt 100 GeV (s 26.2
  pb).

10
Background (tt)

• tt production with decay chain t ? Wb ? µ?µb
  leads 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 gt 10 GeV, pTµ1 gt 20 GeV,
  pTµ2 gt 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

tt? µµ Z? µµ Zbb?µµbb Signal
Level 1 94.7 91.3 92.5 92.2
HLT 86.1 99.3 98.9 98.7
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 gt 8
• pT gt 20 GeV
• Muon isolation
• less then 10 GeV in
• a ?R0.35 cone
• around the muon

tt? µµ Z? µµ Zbb?µµbb Signal
Muon id 30.7 87.9 79.9 81.8
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 gt 40 GeV

tt? µµ Z? µµ Zbb?µµbb Signal
Missing ET 23.0 91.7 88.4 89.2
15
JET veto

• Jets in the events are reconstructed using the
  Iterative Cone Algorithm with the calibration
  JetPlusTracks using JetSeedEt1GeV and CutCone
  0.5 .
• The event is rejected if a
• jet with transverse
• energy gt 45 GeV is found.

tt? µµ Z? µµ Zbb?µµbb Signal
Jet veto 26.4 88.1 83.1 84.5
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
  gt 0.4
• Based on tracks At least two tracks with
  Transverse Impact Parameter (IP) in the range
  0.01 lt IP lt 0.1 cm (only one track if 0.02 lt IP lt
  0.075 cm)

18
B-Tagging

• Summary of the selection efficiency for the b-tag

tt? µµ Z? µµ Zbb?µµbb Signal
CombinedBT (hard b-tag) 54.7 0.8 14.7 11.2
CombinedBT OR IP tracks (soft b-tag) 75.2 10.9 35.0 30.9
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
Fitting procedure

• 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 k0Breit-Wigner(MµµMZ,GZ) k1 k2Mµµ.

N.B The analysis does not require an a-priori
knowledge of the position of the peak!
21
Fitting procedure

• The signal is fitted with a Voigt function
  (convolution of Breit-Wigner and Gaussian)
• ftot fb k3V(MµµMA,sµµ,GA)
• sµµ is the CMS resolution for Mµµ. It is
  determined from real data fitting the Z peak.

22
Fitting procedure

• An additional mass window cut is applied
• MAmeas (sµµ 0.8 x Gmeas)
• NB ? integral of the background function in the
  mass windows.
• NS ? NTOT - NB

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
Decoupling regime

• 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.

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 gt 5 already for 20 fb-1.

L (fb-1) MA125 MA130 MA135
20 7.5 6.0 5.9
30 9.3 7.4 7.0
33
Results

• Discovery contour plot for 30 fb-1.
• The tanß 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 dont
  modify it.

34
tanß measurement

• The 2µ channel allows a good Higgs boson width
  reconstruction.
• The measurement of tanß can be obtained by
  exploiting the proportionality between GA and
  tan2ß.
• The not perfect degeneration of A and H must be
  taken into account (negligible effect for high
  tanß).

35
tanß measurement

• Higgs boson width uncertainty estimated with toy
  experiments.
• The GG(tanß) curve is chosen by fixing the
  measured MA..
• The theoretical uncertainty of 15 (Sven
  Heinemeyer) is taken into account.

36
tanß measurement

• Uncertainty, as a function of MA, for the tanß
  measurement exploiting the Higgs boson width.
• The method can be applied only for 150 lt MA lt 200
  GeV.
• Better results are obtained for high tanß 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 tanß gt 22, for MA 200 GeV tanß
  gt 47 (30 fb-1).
• Intense coupling regime its not possible to
  distinguish the single Higgs peaks, but the
  discovery is reachable for tanß gt 25.
• Low MA regime no hope to see the signal in the
  low luminosity fase.
• It is possible to determine tanß 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.