1. LPNHE Paris CNRS/IN2P3-Universités Paris 6&7
Hgg analysis at ATLAS
M. Escalier, B. Laforge
LPNHE Paris CNRS/IN2P3-Universités Paris 6&7
Status of TDR analysis
Signal & background generation
Kinematic variables
Likelihood analysis
Conclusion and outlooks
Special thanks to Arthur for let me use his laptop for some update
Thanks L. Fayard, G. Unal (discussion, feedback)

2. Status of TDR Leading Order: CTEQ 2L no K factor Pythia 5.700 ISR, FSR
Atlfast 2.0
from full reconstruction

3. Signal/B
S/√B=6.5, signal & background computed at LO

4. Signal Hgg VBF direct production gg fusion associated production
WH,ZH,ttH

5. Signal pdf=CTEQ6L1 (comment of M. Spira) at Les Houches
Using correction of HDecay for decay Hgg

6. Background Irreductible background
NLO calculation using Diphox 1.2, pdf=CTEQ6M
reductible background (LO Pythia), pdf=CTEQ6L1
g jet, 1 jet misidentified
jet jet, 2 jets misidentified

7. Higgs mass resolution With Pythia/ATLFAST s=1.34 GeV
Next slides: use of s of full simu TDR for
window 1.4 s
Mgg (GeV)

8. Diphox definitions
Thanks J. Ph. Guillet for help with Diphox

9. Analysis Requirements
CTEQ 6L1 pdf (LO fit), CTEQ 6M pdf for diphox (NLO)
Pythia 6.210, Atlfast 2.53, Diphox 1.2
Cuts:
photon candidates: pT1>40 GeV, pT2>25 GeV
|h|<2.4
transition crack of |h|=1.45, Dh=0.15 considered
Photon efficiency: 80% (Cf. M. Wielers)
Jet rejection factor: 2900 (Cf. M. Wielers)
photon Isolation cone R= ET<15 GeV
In discussion (F. Derue: ) cf. Athens 4th Atlas Worshop

10. Branching ratio: depends on b mass and on QCD corrections
8.5%
56%
Branching ratio:
Pythia: running aQED for pythia (I mean by default)
HDecay: QCD corrections more relevant
No corrections aQED (better because real photons q2=0)
gg fusion :compatible with papers
R.Harlander and W.Kilgore   Phys Rev Lett 88,201801,2002
C.Anastasiou and K.Melnikov Nucl Phys B646,220,2002
K factor depending on higgs mass
Ravindran, Smith, Van Neerven
KNLO=1.76
VBF:compatible with talk of C. Oleari
Total production cross-section
What is the NLO Prediction ?
HiGlu package (M. Spira et. al)
gg fusion K=1.8 (using CTEQ6)
VBF K=1.04 (using CTEQ6)

11. Ptgg>30 GeV likelihood analysis
Problem of resummation: C. Balazs has done resummation, use/compare his soft. later
Ptgg
contributions
TDR
analysis
direct
16517
onef
8510
twof
883
gg background
29000
25910
g jet
5800
6737
jet jet
4600
5916
Higgs
1283
1097
S/√B
6.5
5.59
Ptgg<30 GeV
Ptgg>30 GeV likelihood analysis

12. Ptgg distribution
fsignal
fbackground

13. Cos q*
fsignal
fbackground

14. Likelihood ratio R
Search for optimal cut

15. Results
8.8
6 %
8.25

16. Conclusion Background: Irreductible background:
Use of NLO irreductible background
Need resummation
(but box is LO in Diphox)
use Dixon calculations (L. Fayard, G. Unal)
Does NNLO computation is possible ?
reductible background:
Know better the rejection factor (g jet)
Then look for NLO computation of g jet, jet jet

17. Signal:
gg fusion and VBF comp. at NLO (but K=f(Pt)need MC at NLO)
gg fusion: Ravindran, Smith, Van Neerven
KNLO is not smal (1.76), KNNLO=1.16 (use of KNNLO in the future)
NNLO of VBF ?
likelihood analysis:gain not so much important
use neural network ?
Understand background of gg jet jet with 2 tagged jets
jet veto in central region