Higgs physics theory aspects experimental approaches Monika Jurcovicova Department of Nuclear...
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Transcript of Higgs physics theory aspects experimental approaches Monika Jurcovicova Department of Nuclear...
Higgs physics
theory aspects
experimental approaches
Monika Jurcovicova
Department of Nuclear Physics, Comenius University Bratislava
H f
~ mf
f
Reasons for Higgs
• the presence of mass terms for gauge fields
destroys the gauge invariance of Lagrangian
• no problem for gluons and photons
• serious problem for W, Z0
• problems with origin of fermion masses
Spontaneous Symmetry Breaking
• way to generate particle masses
• opposite of putting them by hand into Lagrangian
basic idea: -- there is a simple world consisting just of scalar particles described by -- where so not a usual mass term -- ground state (vacuum) is not there are 2 minima
)4/12/1()(2/1 4222 L
0,02 0
/, 2 vv
-v v
V
Spontaneous Symmetry Breaking
• perturbative calculations involve expansions around classical minimum or one of them has to be chosen ( )
• then the reflection symmetry of Lagrangian is broken
• the mass is revealed:
v vv
)()( xvx
constvvL 43222 4/1)(2/1
22 22 vm
The Higgs mechanism • spontaneous breaking of a local gauge symmetry
(simplest U(1) gauge symmetry)• procedure: add the Higgs potential to Lagrangian
translate the field to a true ground state• obtained particle spectrum: 1 Higgs field with mass
1 massive vector A - desired 1 massless Goldstone boson
- unwanted• with a special choice of gauge the unwanted Goldstone boson
becomes longitudinal polarization of the massive vector the Higgs mechanism has avoided massless particles
The EW Weinberg-Salam model• formulation of Higgs mechanism:
– W, Z0 - become massive
– photon remains massless
– SU(2) x U(1) gauge symmetry must be an isospin doublet
– special choice of vacuum
– U(1)em symmetry with generator remains unbroken => the photon remains massless
– W, Z0 masses:
0
1,2
1,
2
1with
0
2
1 3
YTT
v
23 YTQ
, 2
1gvMW
, 2
1 22 gggvM Z
WZ
W
M
M cos
Fermion masses
• the fermion mass term is excluded from the original Lagrangian by gauge invariance
• the same doublet which generates W, Z0 masses is sufficient to give masses to leptons and quarks
• however: the value of mass is not predicted - just parameters of the theory
• nevertheless: the Higgs coupling to fermions is proportional to their masses
this can be tested
Theory summary
• the existence of the Higgs field has 3 main consequences:– W, Z0 acquire masses in the ratio
– there are quanta of the Higgs field, called Higgs bosons
– fermions acquire masses
• deficiencies of the theory– fermion masses are not predicted
– the mass of the Higgs boson itself is not predicted either
WZW MM cos
What do we know today about
• mass not predicted by theory except that mH < 1000 GeV
• from direct searches at LEP mH > 114.4 GeV
• indirect limits from fit of SM to data from LEP, Tevatron (mW, mtop)
• Best fit (minimum χ2): mH=
81 +52-33 GeV
• mH < 193 GeV 95% C.L.
Higgs decays
• mH < 130 GeV: H dominates
• mH 130 GeV : H WW(*), ZZ(*) dominate
• important: H , H ZZ 4, HWW , etc.
H f
~ mf
f
bb
H
• select events with 2 photons with pT ~50
• measure energy and direction of each photon
• calculate invariant mass of photon pair: mγγ= ((E1+ E2 )2 -(p1+ p2 )2 )1/2
• plot the mγγ spectrum - Higgs should appear as a peak at mH
HW*
W*
W*
mH 150 GeV
Main backgrounds of H • γγ production:
irreducible (i.e. same final state as signal)
• γ jet + jet jet production where one/two jets fake photons : reducibleq
q
g
g
q
g
(s)
0q
)( Hjj
~ 108
) (
)(
H
60 m ~ 100 GeV
H ZZ(*) 4
• “gold-plated” channel for Higgs discovery at LHC
• select events with 4 high-pT leptons (excluded): e+e- e+e-, e+e-
• require at least one lepton pair consistent with Z mass
• plot 4 invariant mass distribution :
H Z(*)
Z
e,
e,
e, mZ
120 mH < 700 GeV
222 )( i
ii
i pEm
Higgs should appear as a peak at mH
Backgrounds of H ZZ(*) 4
• irreducible pp ZZ (*) 4
• reducible X 4l tt
t , t W
b
X 4l bZb g
g
b
b
Z
Both reducible rejected by asking:
-- m ~ mZ
-- leptons are isolated -- leptons come from interaction vertex ( B lifetime : ~ 1.5 ps leptons from B produced at 1 mm from vertex)
How can one claim a discovery
• Signal significancepeak width due to detectorresolution
m
B
S
N
N S
NS= number of signal eventsNB= number of background events
in peakregion
if S > 5 : signal is larger than 5x error of background probability that background fluctuates up by more than 5is 10-7
discovery
2 critical parameters to maximize S
• detector resolution S ~ 1 /m
detector with better resolution has larger probability to find signal (Note: only valid if H << m. If Higgs is broad, detector resolution is not relevant.)
• integrated luminosity S ~ L numbers of events increase with luminosity
Summary on Higgs at LHC
• LHC can discover Higgs over full mass range with S > 5 in < 2 years
• detector performance is crucial in most cases
• discovery faster for larger masses
• whole mass range can be excluded at 95% C.L. after 1 month of running
What about the Tevatron
• for mH ~ 115 GeV Tevatron needs:
• 2 fb-1 for 95% C.L. in 2003-2004 ?
• 5 fb-1 for 3σ observation in 2004-2005 ?
• 15 fb-1 for 5σ discovery end 2007-beg 2008 ? Discovery possible up to mH ~120 GeV
Conclusions
• Standard Model Higgs can be discovered:– at the Tevatron up to mH ~120 GeV
– at the LHC over the full mass region up to mH ~1 TeV final word about SM Higgs mechanism
• if SM Higgs is not found before/at LHC, then alternative methods for generation of masses will have to be found