1. Search for supersymmetric neutral Higgs bosons in the decay channel A/H->m+m- in the ATLAS detector
George Dedes , Sandra Horvat
Max-Planck-Institut für Physik München
Deutsche Physikalische Gesellschaft 2005

2. Outline The ATLAS detector MSSM Higgs decay to m+ m-
Fast – Full Simulation comparison
Analysis
Improvements
Summary

3. Electromagnetic Calorimeter
The ATLAS detector
Hadronic Calorimeter
S. C. Air core Toroids
S. C. Solenoid
Inner Detector
Electromagnetic Calorimeter
Muon Spectrometer
Initial luminosity : 20 fb-1 per year
Nominal luminosity : 100 fb-1 per year
Operating for 1.5 years at initial luminosity : 30 fb-1

4. A/H->mm decay channel
Enhanced production rates w.r.t. Standard Model
For mA>150 GeV both H,A degenerate in mass
Clean signature from 2 high pT muons
Two production mechanisms :
For large tanb (>10),
associated production
is dominant

5. Background Large background contribution :
(sxBR)B up to x(sxBR)S Strong rejection needed in order to suppress background
b – tagging essential for Z/g*->mm rejection

6. ATLFAST – Full Simulation comparison
PYTHIA generator program
ATLFAST – A fast simulation program with parametrized description of the detector response
FULL SIMULATION – A detailed description of the detector response, in each measurement unit of the detector
b-jet efficiency vs pT
b-jet efficiency vs eta
b-jet efficiency vs phi
Full simulated b-jets / muons
efficiency
efficiency
efficiency
Fast simulated b-jets / muons
Full simulated fake b-jets / muons
Fast simulated fake b-jets / muons
pT (GeV)
eta
phi (rads)
muon efficiency vs pT
muon efficiency vs eta
muon efficiency vs phi
efficiency
efficiency
efficiency
pT (GeV)
eta
phi (rads)

7. ATLFAST Analysis strategy
Analysis of fast simulated data (ATLFAST,ATHENA version 8.7.0)
First goal: The reproduction and comparison with the most recent study
D.Cavalli , P.Bosatelli : ATL-COM-PHYS
Data samples produced with ATLFAST
Data
N events
A→m+m-
20.000
bbA→m+m-
Z/g*→m+m-
Z(→m+m-)+jet
tt→(mnb) (mnb)
2 times higher statistics for the signal
10 times higher statistics for background

8. ATLFAST Analysis strategy
ATLFAST parametrization (for 1.5 years at initial luminosity):
average muon pT resolution of 3%
muon detection efficiency 93%
average b-tagging efficiency 60%
N(b-jets)=0
mmm=mA±dm
Analysis 1
Standard set of Cuts
Event Loop
N(b-jets)>0
mmm=mA±dm
Analysis 2
Standard set of cuts: 1. pT > 20 GeV
2. pTmiss < 20 GeV
3. pT2m < 100 GeV
mA/H = 150 , 300 , 450 GeV
dm = ± 5 , ± 10 , ± 15 GeV

9. Discriminating variablesZ
A/H , 150 GeV
pT > 20 GeV , trigger selection tt
A/H , 300 GeV
pTmiss < 20 GeV
A/H , 450 GeV Z
A/H Z tt
A/H
pT2m < 100 GeV tt

10. Signal significance at L = 30 fb-1,tanb = 30
Analysis 1: Lower signal significance, large contribution of the Z-background
Analysis 2: Larger signal significance, Z-background suppressed
Need more statistics for better precision
Analysis 1
mA = 150 GeV
mA = 300 GeV
mA = 450 GeV NS
1196 59
3.8 NB
29828
2137
337
NS / √NB
6.924
1.28
0.21
error
±0.006
±0.02
Nevents
Analysis 2
mA = 150 GeV
mA = 300 GeV
mA = 450 GeV NS
479 37
2.9 NB
624 47
8.8
NS / √NB
19.2
5.4
0.97
error
±0.8
±2.9
±2.80
A/H mass (MeV)

11. b-jets properties
b – tagging efficiency decreases for low pT and high |h|
b – jets from bbA/H populate a more forward region
b – jets from tt are more energetic Z
bb(A/H) tt
bb(A/H) tt
possible improvement!

12. Additional cuts
Introducing new selection criteria according to b-jets information :
Require exactly one b-jet (instead of more than 0) in Analysis 2
Rejecting events with PTbjet > 60 GeV
Analysis 2
mA = 150 GeV
mA = 300 GeV
mA = 450 GeV NS
380 27
2.0 NB
310 11
1.9
(NS / √NB) before
19.2
5.4
0.97
(NS / √NB) after
21.6
8.3
1.49

13. Signal significance after additional cuts at L = 30 fb-1, tanb = 30
Invariant mass of A/H - before
Nevents
5s discovery curves at L=30 fb-1
before – after additional cuts
tanb
previous analysis
After additional cuts
A/H mass (MeV)
Invariant mass of A/H - after
Nevents
mA(GeV)
A/H mass (MeV)

14. Summary
Study of the A/H discovery potential in the A/H -> m+ m- decay channel , using ATLFAST data
Good agreement between ATLFAST and Full Simulation
Obtained results agree with those from the previous analysis
Larger statistics in both signal and background needed for a better precision on the signal significance
Higher rejection of the tt background achieved by using b-jet properties
Next step:
analysis refinements , using further information from b-jets …