Diapositiva 1

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Standard Model
Lesson 4 Higgs boson searches at LHC
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Higgs serches at LHC
ECM 7,8 TeV
CMS
L max 7.7 1033 cm-2 sec-1 (Hz / nb )
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Integrated Luminosity 2011 7 TeV Delivered
6.13 fb-1 Recorded 5.55 fb-1
Integrated Luminosity 2012 8 TeV Delivered
23.3 fb-1 Recorded 21.8 fb-1
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Total cross section at LHC

Cosmic Rays
(AKENO, FLYS EYE)
CERN-PH-EP-2012-354 December 11, 2012
TEVATRON (CDF, E710, E811)
LHC 8 100 mb
( 101.7 2.9 ) mb
SPS (SppS) (UA1, UA4 UA5)
( ISR )
in agreement with the extrapolation from lower
energies
LHC 8 TeV
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Interazione principale
ISR e FSR
Creazione dei Jet
Frammentazione e Adronizzazione
Interazioni Multi Partoniche
Beam Remnant
protone
protone
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Underlying Event, Minimum Bias, Pile-Up
protone
protone
The Underlying Event is the residual part of the
event excluding the high pt process ISR, FSR,
Multi partonic interactions, Beam remanent
Together with the p-p interaction producing the
high pt process, we can find additional p-p
interactions in the same beam-crossing ( 1011
protons/buch) ? Pile-Up
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The pileup is the multiple interaction in the
same bunch crossing. Increasing the luminosity
(i.e. with more protons / beam) will produce
also higher pileup environment for the triggered
events
Average PU 2011 RUNA 5 Average PU 2011
RUNB 8 Average PU 2012 all 21
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• Minimum Bias soft inelastic scattering
• Observable fro the detector (Pt min 100 MeV)
• None (or few) tracks produced at significant
  Pt ( 2 GeV)

Elastic scattering (25)
Not diffractive inelastic (55)
Double diffractive inelastic (8)
Single diffractive inelastic (8)
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From LEP to LHC
LHC
LHC Higgs factory inside a little bit hostile
environment
LEP
E.W. background
QCD background
?107
?103
H
H
? 1/hour
? 1/year
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Observation of a new boson at a mass of 125 GeV
with the CMS experiment at the LHC Received 31
July 2012 Accepted 11 August 2012 5.1 fb-1 7
TeV 5.3 fb-1 8 TeV M 125.3 0.4
(stat) 0.5 (sys) GeV
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Higgs boson production at LHC
SM Higgs production cross section NNLO/NLO QCD
corrections
sH (mH 126 GeV) 19.22 pb gg fusion
1.57 pb VBF
1.06 pb
ZH,WH 0.13
pb ttH (105 pb for W)
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Higgs boson decays
H ?gg
H?WW , H?ZZ
H ?tt
H ?bb
In the low mass region all the channels can help
to increase the significance in addition we are
interested to check all the theoretical branching
ratios The Higgs does not couple with gluons and
photons because they are massless particles
anyway the gg fusion is the dominant contribution
in the production and the H?gg is a fundamental
decay channel for the discovery and mass
measurement
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The coupling constant of the Higgs to the
fermions and bosons are proportional to the mass
of the particles
This rule can be broken when the two mass are
very close BR(WW) gt BR (ZZ) but mW lt mZ In
the Lagrangian the ZZ has a factor two of penalty
in comparison to WW because they are
indistinguishable. This factor 2 it becomes a
factor 4 in the BR, reduced to a factor 3
considering the different masses
BR(h?WW) / BR(h?ZZ) g2hWW / g2hZZ 4mW2 /
mZ2 3
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tt turn-on
WW/ZZ turn-on
The width changes from few MeV for low masses to
hundreds of GeV for high masses due to his
dependece on m3H (from H?VV coupling)
bb
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At LHC we decided to multiply the signal cross
section with a factor (gt1 or lt1) needed to
exclude the signal at 95. The real exclusion is
the mass range were This factor is equal or
lower than 1 (where you do need a cross section
larger than sSM to obtain the exclusion.
LEP results were described with the CLs plots.
The mass limit is the mass corresponding to
CLs5. (exclusion at 95).
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Higgs search at LHC
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H?ZZ () ?4l
https//twiki.cern.ch/twiki/pub/CMSPublic/Hig13002
TWiki/HZZ4l_animated_slower.gif
4 July
HCP (Nov. 2012)
Moriond (March 2013)
R1 121.5 130.5 GeV
R2 110 160 GeV
N1 9 B14 N221 B220 Significance 3.2s
N2 47 B237 Significance 4.5s
N2 71 B246 Significance 6.7s
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FSR Zg background
Dominant ZZ background
Signal
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Considering mH lt 2 mZ one of the two Z is off
shell
All the 4l combinations (4e , 4m , 2e2m) show the
excess at M126 GeV
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NTOT 23 32 16 71
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Discovery plot
Exclusion plot
The SM Higgs can be excluded everywhere with CL
gt 95 except in the region where we see the
mass peak and for mH gt 800 GeV
6.7 s (right) corresponds to a probability to be
a statistical fluctuation of 10-11 (left)
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Higgs mass best fit
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signal strength m best fit s / sSM
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H?gg
Signal
Small branching ration BR H?WW 2 10-1
BR H?ZZ 3 10-2 BR H?gg 2 10-3 but no
additional BR (ZZ ?4m,4e,2e2m 36 10-4)
Background

• Large background contribution
• irriducible gg from QCD
• mis identification g-jet and di-jet

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Several photon quality selection (class
0.1.2.3) and different tags dominated by
different production diagrams.
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The diphoton invariant mass distribution with
each event category weighted by its S/(SB). The
lines represent the fitted background and signal
4 July
Moriond (March 2013)
Significance 4.1s (expected 2.8s)
Significance 3.2s (expected 4.2s)
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Discovery plot
Exclusion plot
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mH 126.8 0.2 0.7 GeV
mH 125.4 0.5 0.6 GeV
gg
mH 125.8 0.5 0.2 GeV
mH 124.3 0.6 0.5 GeV
ZZ
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m (at mH125.4 GeV) 0.78 0.28
m (at mH126.8 GeV) 1.6 0.4
gg
m (at mH125.8 GeV) 0.91 0.30
m (at mH124.3 GeV) 1.7 0.5
ZZ
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H?WW () ?2l 2n
Signal

• The signal signature is
• - 2 high Pt leptons
• missing Et
• veto for high energy Jet
• angular correlation between W-W

Background
Direct production of WW
Wt
DY
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Three categories are considered WW 0 jets
, WW 1 jet , WW 2 jet
Distributions of the azimuthal angle difference
between two selected leptons in the 0-jet
category for data, for the main backgrounds, and
for a SM Higgs boson signal with mH 125 GeV.
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Distributions of the transverse mass in the 0-jet
category compared with the backgrounds and the
signal mH 125 GeV expected The cut-based H ? WW
selection, except for the requirement on the
transverse mass itself, is applied.
The transverse mass is the invariant mass
between WW- with the missing energy used to
estimate the neutrino momentum and all the z
components set to zero.
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Discovery plot
Exclusion plot
Significance _at_ 125 GeV 4.0 s (expected 5.1 s)
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Is interesting to compare the mass having largest
significance with the mass fitted for the H?ZZ
and H?gg channels
ATLAS largest significance _at_ mH140 GeV
m (at mH125 GeV) 1.0 0.3
CMS largest significance _at_ mH135 GeV
m (at mH125 GeV) 0.76 0.21
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H?tt
The fact that the reconstruction of the tt pair
decay kinematics is underconstrained by the
measured observables is addressed by a
maximum-likelihood fit method. The mass mtt is
reconstructed by combining the measured
observables Exmiss and Eymiss with a likelihood
model
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Exclusion plot
Discovery plot
Significance _at_ 120 GeV 3.0 s (expected 2.6 s)
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best-fit value of the signal strength m1.10.4
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Combined results
The signal strength is a measurement of the
coupling but the relation is not trivial due to
the different production channels. In the
following table ki are the coupling scale
factors. Only for VBF/VH production and H?VV
decay the signal strength factor is simply k2VV
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Remeber coupling with fermions (and bosons) are
proportional to the messes
Summary of the fits for deviations in the
coupling for the generic five-parameter model not
effective loop couplings, expressed as function
of the particle mass. n (the minimum of the
Higgs potential) is a common constant
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CMS mH 125.7 0.3 0.3 GeV
m (at mH125.7 GeV) 0.80 0.14
ATLAS mH 125.5 0.2 0.5 GeV
m (at mH125.5 GeV) 1.3 0.2
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Higgs JP measurement
It is crucial to determine the spin and quantum
numbers of the new boson We can use the H?ZZ
decay channel to distinguish between the scalar
(SM Higgs) JP0 and the pseudoscalar JP 0-
hypothesis. We start to remember the M.E.
likelhood analysis used in the mass estimation
Considering the following angles
q angle between Z direction (z) and z
axis q1,q2 angles between the leptons and the
Z F angle between the two leptons pair
planes F1 angle between zz plane and two
lept. plane
We can build a kinematic discriminant
between signal and background
(MELA) Matrix Element Likelihood Analysis
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• The mass of the boson is measured with a
• maximum-likelihood fit to 3D distributions
  combining for each event
• m4l,
• dm4l (from the individual
• lepton momentum errors)
• - KD

BKG
SGN
Expected density of points (max set to 1) for
background and signal as a function of m4l and KD
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A similar kinematic discriminant (pseudo-MELA)
ca be used to distinguish scalar JP0 from
pseudoscaral JP0-
The fit is performed using the DJP (parity) and
the the KD (sgn-bkg) discriminants. The
discriminating power of D0- becomes more clear
looking to the right plot where the condition
DBKG gt 0.5 has been applied
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The fit is performed using the DJP (parity) and
the the KD (sgn-bkg) discriminants. The relevant
distribution is the log-likelihood ratio -2 ln
(L0- / L0 ) from pseudoexperiments under the
assumptions of either a pure pseudoscalar or a
pure scalar model. The arrow indicates the
observed value.
Excluded 0.16
The data disfavor the pseudo scalar hypothesis
with 3.3 s or 0.16
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Excluded lt 0.1
Excluded lt 0.1
Excluded lt 0.1
Excluded 1.5
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H?ZZ ? 4µ candidate
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H?WW ? 2µ MET candidate
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Higgs searches at LHC CERN-PH-EP-2012-354
TOTEM Luminosity measurement CERN-PH-EP/2013-035
arXiv1303.4571v1 (19 March 2013) Observation
of a new boson with mass near 125 GeV in pp
collisions at vs 7 and 8 TeV CMS-PAS-HIG-13-005
Combination of standard model Higgs boson
searches and measurements of the properties of
the new boson with a mass near 125
GeV CMS-PAS-HIG-13-002 Properties of the
Higgs-like boson in the decay H to ZZ to 4l in pp
collisions at vs 7 and 8 TeV
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Backup
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Look elsewhere effect
The statistical significance that is associated
to the observation of new phenomena is usually
expressed using a p-value, that is, the
probability that a similar or more extreme effect
would be seen when the signal does not exist.
p-value p0
CLs (1 - p1) / (1 - p0)
arXiv1005.1891v3
Looking everywhere (elsewhere) i.e. the invariant
mass in a wide mass range, the probability to
observe somewhere a background fluctuation is
boosted. The effect can be quantified in terms of
a trial factor, which is the ratio between the
probability of observing the excess at some fixed
mass point, to the probability of observing it
anywhere in the range.
p-value (ATLAS Hgg) 2.8 s (1.5 s L.E.E.)