Four seas conference
17 April 2002 1
Vanina Ruhlmann-KleiderCEA/DAPNIA/SPP
Status of EW symmetry breaking
1) Introduction
2) The SM Higgs boson
3) Other scenarios
4) Conclusions
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1)The Standard Model of particle physics : the ingredients
12 elementary constituents
6 leptons 6 quarks
e-
-
-
e u
c
t
d
s
b
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3 interactions propagated by intermediate bosons of spin 1: (massless) electromagnetic interaction W et Z (massive) weak interaction 8 gluons (massless) strong interaction
one example :
The and Z fields are linear combinations of two vector fields which do not know the difference between the em and weak interactions : before mass generation that is, at high energy
ElectroWeak symmetry
Fundamental interactions
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is assumed to be spontaneous i.e. due to a non-zero v.e.v. is therefore responsible for the generation of the particle
masses :
M = 0 MZ ~ 91 GeV MW ~ 80 GeV
confirmed when discovering the W,Z at CERN in the 80’s
what is the exact mechanism of the
breaking ?
the SM minimal solution: Higgs mechanism with one doublet of scalar fields with a non-zero v.e.v.
EW symmetry breaking
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the SM minimal solution: Higgs mechanism with one doublet of scalar fields acquiring a non-zero v.e.v. one Higgs boson all properties predicted
its mass, which is poorly constrained by theory: 0 MH 1000 GeV
search ALL experimental clues to such a Higgs boson
Other more complicated scenarios exist, too …
EW symmetry breaking
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2) Status of the search for the SM Higgs boson
Over the past decade, the search strategy was twofold: Direct search for a Higgs boson actually produced in collisions:
LEP sensitivity to low masses
Indirect constraints from precise EW measurements sensitive to the quantum corrections due to loops with the Higgs boson:
LEP, SLC, TeVatron sensitivity to low and high masses
see W.Adam’s talk
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Direct search at LEP: the environment Initial S/B ratios:
at LEP 1 :
MH = 10 GeV: 10-3
MH = 60 GeV: 10-5
at LEP 2 :
MH = 60 GeV: 10-2
MH = 115 GeV: 10-3
LEP 1 result:
Before LEP:
MH > 100 MeV (95%CL)
MH > 60 GeV (95% CL)
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Direct search at LEP: experimental signature
LEP provided the ideal experimental environment to search for a light Higgs boson, i.e. with mass MH S – MZ :
the dominant production process:
main decay is H bb (80% at 100 GeV) clean signature
Z boson easy to tag: mass, decays:
Z hadrons 70% Z charged leptons 10% Z neutrinos 20%
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Tools for the direct search at LEP: b-tagging
Step 1: a silicon vertex detectorAfter full alignement, hit precisions are :
~10 m in R ~15 m in Rzin the central part of the detector.
Ex: DELPHI
~3 double-sided layers
~0.5 X0
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Tools for the direct search at LEP: b-tagging
Step 2: impact parameters B hadron lifetimes <>= 1.6 ps
flight distances ~3 mm
impact parameters ~c ~400 m
Experimental resolutions:
R: = 20 60/p sin3/2 m
Rz: = 39 71/p m ( ~90o)
IP/ (= significances)as basic inputs of b-tagging
PV
SV
IP
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Step 2: secondary vertices B hadron decays lead to tracks originating from secondary
vertices information from reconstructed SV add more discrimination between b quarks and other flavours
more powerful b-tagging:
e.g. SV masses
c quarks
b quarks
Tools for the direct search at LEP: b-tagging
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Tools for the direct search at LEP: b-tagging
Step 3: tuning and control of performance Simulated IP distributions and resolutions tuned on data.
data/simulation agree within 5%
R significances
before tuning after tuning
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Step 3: control of performance b-tagging performance checked on control samples:
Z data
Z data
b-tagging variable
data/simulation agree within 5%
Tools for the direct search at LEP: b-tagging
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To reach the best S/B discrimination: multidimensional analyses (NN, likelihood …)
L3
Hqq channel
likelihood
To improve on signal mass reconstructions: kinematic fits with E,p conservation and the Z mass constraint
H channel
Tools: multidimensional analyses and kinematic fits
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Tools… : statistical interpretation of the results
To make an unbiased and powerful statistical analysis of the search results:
stop selection of signal-like events at a loose level test compatibility of data with B-only and S+B hypotheses rates and 2d
pdf’s (H mass vs a second variable such as b-tagging, NN …)
e.g. Hqq channel, S = 206.5 GeV, DELPHI
Tools : likelihood ratio test-statistics (-2lnQ) and confidence levels (CLs,CLb)
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The HZ production cross-section rises fast once the kinematic threshold is crossed a few pb-1 are enough to test a given MH
hypothesis as soon as SMH+MZ
e.g. july 2000: hypothesisMH=110 GeV is excluded
LEP 2 result :
MH > 114.1 GeV (95% CL)
Direct search for the SM Higgs boson at LEP: results
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A possible signal ?
likel
ihoo
d ra
tio te
st-s
tatis
tics
expected behaviour from background only (mean, ±1
and ±2 bands)
expected behaviour from a a 115 GeV signal + bkg
data: consistent with a signal of mass:
compatibility with the hypothesis of a background fluctuation: 3.4%compatibility with the hypothesis of a 115.6 GeV signal: 44.%a handful of events makes most of the effect
mass hypothesis
MH = 115.6 ± 0.8 GeV
Direct search for the SM Higgs boson at LEP: results
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Final state: e+e- HZ qq bb
Comparing signal and background probabilities:
ln(1+s/b) = 1.73
4 jets of particles2 b-jets
Reconstructed mass: MH = 114.3 3 GeV
One event consistent with the SM Higgs boson production
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EW precise measurements:(LEP,SLC,TeVatron..)
(95% CL)
Direct searches (LEP):
(95% CL)
MH 196 GeV
MH 114.1 GeV
MH = 115.6 GeV ?
Summary about the SM Higgs boson:
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3) Other more complicated scenarios
One Higgs boson with non-standard properties: Same decays but different cross-section:
MH 105 GeV: BR /SM 20%
Non-b hadronic decays and different cross-section:
MH 105 GeV: BR /SM 30%
114.1 GeV
112.9 GeV
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Invisible decays and different cross-section:
MH 105 GeV: BR /SM
25%
Photonic decays and different cross-section:
MH 105 GeV: BR /SM
5%
114.4 GeV
115. GeV
One Higgs boson with non-standard properties:
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two representative scenarios: results in the (mh, tan) plane
Mh 91.5 GeV , MA 92.2 GeV (95% CL)0.7 tan 10.5 excluded (95% CL)
Mh 91.0 GeV , MA 91.9 GeV (95% CL)0.5 tan 2.4 excluded (95% CL)
ratio
of
the
tw
o H
igg
s d
ou
ble
t v.
e.v
.’s e+e- h A
e+e- h Z
More Higgs bosons: h, A, H, H+, H- from SUperSYmmetry
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the same two scenarios: results in the (mA, tan) plane
the mass limits in the representative scenarios have been checked to be valid in most scenarios Large mu scenario (h, A decoupled from b’s): completely excluded reinterpretation of the analyses in models with explicit CP violation : under progress
More Higgs bosons: h, A, H, H+, H- from supersymmetry
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charged Higgs bosons:
New decays open at high mass (H+ W A) : dedicated analyses under progress
assuming : Br(H± ) + Br(H± cs) = 1
MH 78.6 GeV (95% CL)
More Higgs bosons: h, A, H, H+, H- in general 2HD models
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neutral Higgs bosons: masses and couplings no longer constrained as in SUSY Models, more final states to be expected and hence analysed, e.g.
More general analyses of LEP data to cover less constrainedtopologies than in SM or SUSY-driven analyses
is forbidden in SUSY models(Mh ~ MA when cos(-) is large)
but allowed in 2HD models
is negligible in the SM and experimentally excluded in SUSY models
but possible in 2HD models
More Higgs bosons: h, A, H, H+, H- in type II 2HD models
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Other results: analyses of other final states (non-b hadrons, 4’s), reinterpretation of the existing analyses in models with two doublets and a singlet …
reduction factor 1
reduction factor 0.1
Z bb h/A bb
enhancement factor of the bb h/A couplings
Neutral Higgs bosons in type II 2HD models: examples of results
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EW symmetry breaking due to condensation in the vacuum of strongly-interacting fermions (technifermions):
most models are disfavoured by EW precision constraints some models fulfill them direct searches
MT 79.8 GeV (95% CL) MT 206.7GeV (95%CL)
No elementary Higgs bosons: technicolor models
J.Ellis et al., Phys. Lett. B343 (1995) 282.
SMdata
TC models
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Conclusions The past decade did open the era of the search for
the exact EW symmetry breaking mechanism with both the precise EW measurements and the direct searches (LEP, SLC, TeVatron)
SM Higgs boson:
Many other scenarios have also been investigated The main question : is the EW symmetry breaking
due to doublet(s) of scalar fields or not ?
TeVatron run II, LHC, LC
MH < 196 GeV MH > 114 GeV MH = 115.6 GeV ?
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