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H -> 4 m in the low mass region E.Meoni, L.Larotonda, M.Antonelli, F.Cerutti

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H -> 4 m in the low mass regionE.Meoni, L.Larotonda, M.Antonelli, F.Cerutti

- Introduction
- ATLFAST++ and MOORE/MuID performance
- Irreducible background rejection
- Reducible bkg. Rejection
- Status and prospects

Introduction

- Study started more then 1 year ago with twofold goals:
- Validate ATLFAST++ and ATHENA Moore/MuID
- Improve analysis w.r.t. TDR by using multivariate techniques against irreducible (ZZ->4m) and reducible (tt and Zbb) backgrounds
- Started with ATHENA release 6.0.3
- Mass region studied: MH[130-180] GeV (low mass is the most challenging because of the off-shell Z and higher bkg)
- Here results showed for MH=130 GeV

Introduction

- Samples produced with PYTHIA 6.2 with the exception of Zbb (ACERMC ME)
- Filter: 4 m with Pt>4 GeV and |h|<2.7

Introduction: analyses chain

COMMON PRESELECTION

4 muons, null total charge :

2 with pT > 20 GeV and | | < 2.5

2 with pT > 7 GeV and | | < 2.5

Analysis with

Multivariate methods

TDR analysis

Couples µ+ µ- with invariant mass :

M12= Mz ± 15 GeV M34> 20 GeV

( mH= 130GeV )

M12= Mz ± 10 GeV M34>30 GeV

( mH= 150GeV )

M12=Mz ± 6 GeV M34>60 GeV

( mH= 180GeV )

Angular cut

likelihood/NN (with angular variables and M12 & M34) cuts

Lepton isolation cut

likelihood/NN (with isolation variables) cut

Mass window cut (mH 2 )

to compute significance

Lepton isolation cuts

(single variable cuts)

Mass window cut (mH 2 )

to compute significance

Introduction: software codes

- ATLFAST++ (object oriented version of ALTAS fast simulation implemented in ATHENA framework)
- ATHENA: MOORE/MuID with muon spectrometer in standalone and combined
- started with version 6.0.3 many bugs found
- latest results with 7.0.2
- First step check of general performance
- Efficiency
- Pt resolution
- MH resolution

Selection efficiency

- Acceptance after kinematic cuts (4m and M12 and M34 cuts):
- ATLFAST++: 33.0%
- TDR: 33.5%
- MOORE/MuID combined 6.0.3: 9%
- Inefficiency concentrated in low Pt region

Muid Combined

Athena6.0.3

Selection efficiency

- Improved with version 7.0.2
- MOORE/MuID combined 7.0.2: 23%
- Inefficiency concentrated eta~2 region

Muid Combined

Athena7.0.2

Mass resolution

- Performance muon spectrometer:
- TDR: 2.7 GeV
- MOORE 7.0.2: 3.0 GeV
- Combined (including Z mass constraint):
- TDR: 1.4 GeV
- ATLFAST++: 1.5 GeV
- MOORE/MUID comb 7.0.2: 1.7 GeV

Irreducible bkg.: ZZ->4m

- Multivariate analyses: in addition to MH, M12 and M34 there are other 9 independent kinematic variables (12 in total)
- Try to select variable sensitive to the spin and parity of the signal
- Combine all variables with multivariate techniques: likelihood and NN

- Likelihood function (and neural network)
- with 11 variables:
- Angle of the decay planes of the two Z in Higgs rest frame
- (see ATL-COM-PHYS-2003-001,Buszello et al.)
- Angle between m- in Z rest frame and Z boost in Higgs rest
- frame (both for on-shell Z and off-shell Z)
- (see ATL-COM-PHYS-2003-001,Buszello et al.)
- Angle between Z (both on-shell and off-shell) direction in
- Higgs rest frame and the Higgs boost
- Angle between the two m+ in Higgs rest frame
- Angle between the two m- in Higgs rest frame
- Angle between the two m of Z (both on-shell and off-shell)
- Invariant masses of the two m+ m- couples (M12 and M34)

rest frame and Z boost in Higgs rest frame

Angle between the decay

planes of the two Z

in Higgs rest frame

Angle between on-shell Z direction in Higgs rest frame and Higgs boost

H4

H4

H4

ATLFAST

ATLFAST

ATLFAST

ZZ4

ZZ4

ZZ4

ATLFAST

ATLFAST

ATLFAST

H4

H4

H4

FULL REC.

FULL REC.

FULL REC.

ZZ4

ZZ4

ZZ4

FULL REC.

FULL REC.

FULL REC.

Results with Fast simulation

Improvement: mainly coming from M12 and M34 optimization

angles relevant only at higher MH

Reducible background

- 2 out of 4 muons not isolated in tt and Zbb background

- Likelihood (and neural network) with 6 variables:
- the 2 largest normalized impact parameters(IP) in trasverse plane of the 4 IP
- the 2 largest pT reconstructed inside a cone of R=0.2 around the 4 µ tracks
- the 2 largest total transverse energy depositions in calorimeters (EM+HC) in a cone of R=0.2 around the 4µ tracks

We have added in CBNT ntuple block of Moore/Muid the energy deposition in cones of different radii around the “muon track”

“muon track” defined in 4 ways: moore trk, muid statandalone trk, muid combined trk, iPat trk

Best results with: energy of radius R=0.2 around “iPat” trk

Around iPat track

Signal

ttbar

Zbb

Signal

ttbar

Zbb

Largest IP

Signal

ttbar

Zbb

Largest pT

After pT & cuts

and m12 & m34 cuts

Mass plots

- Preselection (as in TDR) : 4 m , total charge =0, pT & cuts
- Angular cut: likelihood- 11 variables
- Isolation cut : likelihood– 6 variables

After preselection (4m,Qtot=0,pT& h cuts)

Signal

ZZ 4m

ZZ 2m2t

ttbar

Zbb

All channels

Signal

ZZ 4m

ZZ 2m2t

ttbar

Zbb

All channels

After overall analysis (preselection+ ang. lik cut+isol lik cut)

Conclusions and Prospects

- ATLFAST++ and MOORE/MuID (7.0.2) comb. performance studied on H->4m (low mass): worse performance then TDR, still low efficiency at |h|~2 to be understood

Prospects

- Add Noise and pileup, relevant for lepton isolation
- Control samples to study lepton isolation variables on data: tt-> WWbb: W->l W->jj select b jet with Mbjj=Mtop (b forced to leptonic decay)
- Wait for bug fixes ?
- Produce documentation: ATLAS note and SN
- Participation to DC2 validation very important:
- New digitization
- New simulation GEANT4
- New output data format
- New reconstruction release

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