Transcript Search of Higgs boson: The H gg channel at ATLAS

Comparisons between
Diphox/ResBos/Pythia
for gg bkg & signal at LHC/ATLAS
• Signal
• Background
• Reducible
• Irreducible
• significance
Eurogdr Supersymmetry workshop on SM Backgrounds
http://lyoinfo.in2p3.fr/gdrsusy05/
Marc Escalier, LPNHE Paris, 13-14 octobre 2005
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ATLAS and CMS at LHC
•
•
•
•
very high energy accelerator : pp √s=14 TeV
interaction rate: 40 MHz
inelastic cross section: σpp=70 mb
luminosity: (2x)1033 cm-2s-1
1034 cm-2s-1
– per year:
10 fb-1
100 fb-1
– 1 year of LHC at 1033 cm-2s-1 ~10 years at prev. machines
• big detectors
Detectors optimised for Higgs boson and SUSY searches
ATLAS: 25 m x 46 m
CMS: 15 m x 21.5 m
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first collisions in Summer 2007
Constraints on Higgs mass
High constraints
• Unitarity (WLWL diffusion)
• Triviality
Low constraints
• vacuum stability l>0
mH<700 GeV
mH<750 GeV
mH>139 GeV (mt=178,1 GeV) (L=1016 GeV)
mH>74 GeV (L=1 TeV)
Experimental constraints
• direct research at LEP:
mH>114.4 GeV (95% CL=2s)
• indirect research:
global analysis of electroweak measurements
(sensibles to mH)
mH=126+73-48 GeV, MH<280 GeV
The Hgg channel
uncertainty on mH
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Signal
To discuss what is background, we have to discuss what is signal
significance
S  NS / NB
if S>5
signal >5 x error on background
P(bck fluctuates > 5 s)=10-7
discovery
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Higgs production at LHC
Opening of Htt-
VBF
MH(GeV)
assoc. prod. ttH, bbH
W/Z H associated production
Dominant channel over the whole range of MH :ggH
VBF and assoc. prod. : distinct signatures↑S/B, ↓statistics
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Higgs decay at LHC
• MH<140 GeV:
dominant decay mode bb and tt
overwhelmed by bck (inclusive)
s(ggHbb)~20 pb; s (bb)~500 µb
Accessible channels :
ttH(Hbb), VBF (Htt)
rare decay mode Hgg
cleaner signature, inclusive production
LEP
• 140<MH<180 GeV
2mW
2mt
WW* and ZZ* channels have significant BR
HZZ* 4l good mass reconstruction possible, but low stat.
HWW*lnln either inclusive or VBF production (better S/B)
• MH>180 GeV
HZZ4l gold-plated channel « easy »
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Signal
Direct production
dominant
Associated production WH, ZH,
ttH
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ggH
Tools available for signal
s (mb) production from Pythia
LO
Pythia
NLO: K=sNLO/sLO=1.8
• HiGlu (cross section only)
• ResBos (MC events)
NNLO (Anastasiou et al.), prog FEHiP
hep-ph/0501130, www.phys.hawaii.edu/~kirill/FEHiP
Higgs production (eg: 120 GeV)
LO
VBF
KNLO=1,76
(HiGlu: 1,8)
• HiGlu K=1.04
+BR corrections HDecay (NLO)
NLO
NNLO
KNNLO=1.16
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Uncertainty signal
~40 %
Need data (bck)
To better understand
KSV=Kulesza, Sterman, Vogelsang, CTEQ6M
RSVN=Ravindran, Smith, Van Neerven, CTEQ6M
BCFG=Bozzi, Catani, De Florian, Grazzini,
MRST2002
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Study mass resolution of Hgg
Parameters for analysis
• photons cuts: PT(1)>40 GeV, PT(2)>25 GeV
|h|<2.4, exclude transition region at |h|=1.45
• f corrections
• clusters sizes (TDR): EMB:3x7:conv., 3x5 non-conv., EC :5x5
• calibration
• constant term 0.6 %
• vertex correction:
• low lumi: vertex z : inner detector
truth+smearing s=40 µm
• high lumi: vertex z : elmg clusters
Resolution : 2 cm
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Resolution obtained
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Reducible background
• g+jet
20 millions > signal
s jets quarks ~10>jets gluons
Partonshadrons (« jets »)
• jet+jet
58 % jet gluon
4.7 % jet quark
37.3 % mixture
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g/jet separation
Full simulation of detector
Some examples
Hadronic leakages
Width h (3x5)
h (3x7/7x7)
f
h
g
jets
f (3x3/3x7)
f
2ème max.
h
min-2nd max
+1st layer: (width 40 strips, 3 strips, % E outside hearth shower)
Goal: efficiency 80 %, R=5000
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Efficiency for photons
|h|
ET
14/33
Jets rejection
PT>25GeV
~3
• all jets:
R~6680
• quarks jets
R~2880
• gluons jets
R~20650
p0=70 % remaining events
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Irreducible background
• Bremsstrahlung
(correction to Born)
• Born(aQED2)
• Box gggg (as2aQED2)
NLO computations
~10 > signal
Pythia distributions
NLO: cross-sections, scale dependence,
final state PT
Diphox
*NLO Born+brem w/o resum.
*LO Box
Eur. Phys. J. C 16,311-330
ResBos
*NLO Born+brem w/ resum.
*NLO Box w/ resum. 16/33
Mgg (GeV) Phys. Rev. D 57, 6934-6947,1998 [hep-ph/9712471]
fragmentation
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Resummation
Factorisation theorem
Partonic cross-section
At NLO, divergent terms as log PT/Q
long distance physics (pdf)
PT<<Q collinear and infrared divergences
isolation cone for photon
(eg:R=0.4 ET<15 GeV)
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Background with ResBos
• P « Perturbative » contains all terms (divergent terms also)
• A « Asymptotic » contains divergent terms only
• W contains resummed « divergent terms »
• W+P-A, but resummation not valid at high Ptsmoothing fW+P(+F)-fA
PTgg Born construction
fW+P(+F)-fA
Smoothing of background
Frag=fragmentation
PT(gg)
PT(gg)
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ResBos not easy to use: need 3 distributions to have one physical distribution
ResBos included Feynman diagrams
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Born+brem
No resummation in Diphox
Phase space cut in PT
Regularization of divergent terms
better fragmentation in
Diphox
Mgg (GeV)
PTgg (GeV)
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Box
Bern, Dixon, Schmidt NLO
ResBos NLO
Diphox (LO)
Bern, Dixon, Schmidt LO
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Events at LHC for 120 GeV and 30
-1
fb
Diphox is scaled by 1.6 to take into accound NLO of box
Gives an uncertainty of 5 %
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Parton shower problem of Pythia
News from Pythia 6.3
« contains a completely new multiple interactions model, with new transversemomentum-ordered showers for initial- and final-state radiation »
To have backward compatibility: PYEVNT (old) and PYEVNW (new)
http://www.thep.lu.se/~torbjorn/Pythia.html
Hep-ph/0408302: transverse-momentum-ordered showers and interleaved multiple interactions
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Comparison data/simulations
Diphox suffers infrared divergence in the « qT » (PTgg) distribution
ResBos resums effects of soft and/or collinear gluon emissions to all orders,
predicts a smooth qT distribution.
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Invariant mass Mgg at CDF, run II
w/ gg contribution
wo/ gg contribution
207 pb-1
NLO Diphox
Pythia contrib x 2
R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu
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http://yliu.home.cern.ch/yliu/diphotonReBlessed_30Jan_2004/diphoton_reblessed.html
Transverse momentum of pair qT=PTgg
≠scales
may give 10% effect
• ResBos < Diphox
BUT
Old version of ResBos
(since NLO box
Pythia contrib x 2 included)
• What about high PT ?
need more statistics
R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu
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DFgg
Pythia contrib x 2
R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu
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Details of values
Mgg
qT=PTgg
DFgg
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http://yliu.home.cern.ch/yliu/diphotonReBlessed_30Jan_2004/April25_2004/numbers.ps
• background is not so well constrainted:
-resummation important to have a well description of high PT
-difficult to have same parameters for the comparisons
-need update comparisons background/data at Tevatron with
*new version of ResBos, *more statistics (used 200 pb-1 so far…)
*new version of Pythia (new parton shower)
*take same theoretical assumptions (scales, pdfs, order of the pert. devel.)
need manpower
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Significance
↑50 % (NLO)
100 fb-1
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Likelihood ratio method
qq- et qg have fermions
exchange in t channelg
angles lower than those of
Higgs boson (isotropic in q)
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Significance: 120 GeV, 30
fb-1:
6.24likelihood:9.24
Conclusion
• First years of LHC are near : summer 2007 ?
SM Higgs may be discovered in first years of operation with Hgg
Need understanding of systematics
• Use data for
• detector performance understanding
• validation of pdf for cross-sections
• validation of MC tools
More information
• M. Escalier, F. Derue, L. Fayard, M. Kado, B. Laforge, C. Reifen, G. Unal
Search for a Standard Model Higgs boson in the ATLAS experiment on the H  gamma
gamma channel, ATL-COM-PHYS-2005-054
• M. Escalier, F. Derue, L. Fayard, M. Kado, B. Laforge, C. Reifen, G. Unal, Photon/jet
separation with DC1 data, ATL-COM-PHYS-2005-048
• M. Escalier, B. Laforge (dir.), Recherche expérimentale de la brisure spontanée de symétrie
électrofaible dans le canal H \to \gamma \gamma et d'une solution au problème de hiérarchie
dans ATLAS. Participation à la préparation de l'électronique du calorimètre
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électromagnétique, 2005 Paris : Paris 11, CERN-THESIS-2005-023
Appendix
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Rumour on Diphox
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Correlations entre variables
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g/jet separation (II)
Inner detector
g
p0
• track veto
Non converted g
request : no tracks with PT>5 GeV in DR=0.2
• Isolation
request: SPT tracks in [0.1<DR<0.3] <4 GeV low lumi.
<10 GeV high lumi.
goal
• efficiency 80 %
• R=5000
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The Higgs mechanism
solution : new field fPredict existence of a scalar boson
V(|f|)
f=v+c
(µ2<0)
non sym.
v≠0
v
• bosons get a dynamical mass :
|f|
sym
+interaction H-(W; Z)
r=MW/MZcos qW=1
• f other mechanism for fermions
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Constraints on Higgs mass
High constraints
• Unitarity (WLWL diffusion)
• Triviality
Low constraints
• void stability
l>0
mH<700 GeV
mH<750 GeV
mH>139 GeV (mt=178.1 GeV) (L=1016 GeV)
mH>74 GeV (L=1 TeV)
Experimental constraints
• direct search at LEP:
mH>114.4 GeV (95% CL=2s)
• indirect search :
Global analysis of electroweak measurements
mH=126+73-48 GeV, MH<280 GeV
39/33
SM Discovery potential
Almost all allowed mass range explored in 1st year (10 fb-1) for ATLAS-CMS
With 30 fb-1, more than 7 s for the whole range (provided systematics on the
40/33
background are under control)
Higgs search in the Hgg channel
low masses
(80-150 GeV)
Very rare decay (BR~10-3), σ(ppH115)xBr(H→gg)=76 fb (NLO), S/B~0.05
bck:irreducible gg continuum, reducible: g-jet and jet-jet
Keys: excellent energy and angular resolutions
H
excellent g efficiency, jet rejection
p
p
high granularity and response uniformity
LAr (ATLAS), PbWO4 (CMS)
g
GH width negligible, resolution
dominated by detector
σ(mgg)/mgg≈1%
NLO tools available for signal and bck.
PT improve these results
g
CERN/LHCC 96-40 ATLAS TDR 1
CERN/LHCC 96-41 ATLAS TDR 2
CERN/LHCC 97-33 CMS TDR 4
bck from sidebands
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Reducible background after
rejection
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