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Second IDPASC schoolEzio TorassaUdine, February 1st 2012
LHC Physics
Lesson #2
Higgs boson searches at LEP1 , LEP2 and LHC
IDPASC school
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
From the experimental observables:
line shape (s) FB asymmetries AFB(s) polarization P(cos)
pseudo-osservables can be extrapolated:
MZ Z h Al
FB etc..
Using a fit program (ZFITTER) with 2 loop QEWD and 3 loop QED the best fit can be obtained for the parameters of the model and for the masses having some uncertainty (mt, ,mH ). The current version of ZFITTER (in C++) is Gfitter.
Global fits are performed in two versions: the standard fit uses all the available informations except results from direct Higgs searches, the complete fit includes everything
Global Electroweak Fit
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
20 pseudo-osservables
5 fitted parameters
With the fitted parameters we can obtain
also the fitted pseudo-osservables
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
usage of latest experimental input:
Z-pole observables: LEP/SLD results [ADLO+SLD, Phys. Rept. 427, 257 (2006)]
MW and W: latest LEP+Tevatron averages (03/2010)[arXiv:0908.1374][arXiv:1003.2826]
mtop: latest Tevatron average (07/2010) [arXiv:1007.3178]
mc and mb: world averages [PDG, J. Phys. G33,1 (2006)]
had(5)(MZ
2): latest value (10/2010) [Davier et al., arXiv:1010.4180]
direct Higgs searches at LEP and Tevatron (07/2010)[ADLO: Phys. Lett. B565, 61 (2003)], [CDF+D0: arXiv:1007.4587]
Updated Status of the Global Electroweak Fit and Constraints on New Physics July 2011 arXiv:1107.0975v1
2min /DOF = 16.6 / 14
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
mH=81+52-33 GeV (2002)
mHiggs< 193 GeV 95% C.L.
mH=91+58-37 GeV (2003)
mHiggs< 211 GeV 95% C.L.
mH=96+60-38 GeV (2004)
mHiggs< 219 GeV 95% C.L.
MW
Ab FB
, Ac FB
, Rb , R
c
mH=96+31-24 GeV (2011)
mHiggs< 171 GeV 95% C.L.
GeV 96 3124
HM
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs searches at LEP
Z Z*
H
H
Z* Z
ECM=206 GeV
The coupling of the Higgs field to the vectorial bosons and fermions it’s fully defined in the Standard Model
The cross section of the Higgs production and the decay modes as a function ofit’s mass are predicted by the theory
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs-strahlung WW fusion
Dominant modem(H) s-m(Z)
+interference
MH(GeV/c2)
ECM=206 GeV
The dominating Higgs production mechanism at LEP1 and LEP2 is the “Higgs-strahlung”
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs decay channels
For mH 120 GeV, the most important decay chanel is H bb
“b-tagging” is relevant !
4 jets 2 jets &
missing energy
19%60%
Or a instead of the b
2 jet &
2 lepton
6%
Hbb 85%
H 8%
Reaserch topology:
Second IDPASC schoolEzio TorassaUdine, February 1st 2012Padova 12 Aprile 2011 Ezio Torassa
Neutrino decay channel
2 jets &
missing energy
The signature is one unbalanced hadronic event.
The background is due to Z decay into b quarks
Background reduction:
• invariant mass of the two jets MZ
• jets not in collinear directions
• b-tagging
Leptons transverse momentum
bc
uds
Tracks impact parameters
udsc b
Higgs searches at LEP1
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
(1) Preselection:
Acollinearity > 8 0
20 GeV < Minvariant < 70 GeV
Zqq Z H (55GeV)X
Eff. ( Z HX) = 81.2%
Eff. (Zqq) = 1.5 %
(2) Neural network:
Neural network with 15 input variables. The output is a single quality variables: Q takes values between 0 and 1
Data analysis example (1991-1992)
Q ( )
Z HXZqq
Eff. ( Z HX) = 65.8%
Eff. (Zqq) = 0.23 %
Q > 0.95
( to be multiplied with the previous Eff. )
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Results
MH (GeV) 50 55 60 65
Eventi (simulati HZ) 7.90.4 3.60.2 1.40.1
0.410.05
# expected signal events
# observed events: 0 # expected background events : 0
Sum of the tree decay channels: Z Zee Z
For MH = 55.7 GeV we have 3 expected signal events events.
The probability to observe 0 events from a Poisson distribution with mean value 3 is 5%.
Higgs mass limit: MH > 55.7 GeV al 95 % di C.L.
LEP1 : 1989-19954 detectors , all channels
m(Higgs) > 65 GeV /c2 at 95%CL
DELPHI 1991-1992:
1 M hadronic events
~380 k events ee
LEP1 1989-1995
17 M hadronic events
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Large number of events Gauss distribution approximation Small number of events Poisson distribution n = number of observed events m = mean number of events
n=0 m 3 @ 95% CL n=2 m 6.3 @ 95% CL
For the Higgs search m is related to the Higgs mass m xx MH ≥ yy
Contributions to the mean value m: background (b) and signal (s) :
n is the measurement;
• Exclusion (at least at 95% CL): the probability to observe n events 5%
• Discovery (5 significance): signal 5 times larger than the error
;;;!
)|( mmnnme
mn n
nm
;;;!
)()|(
)(
sbsbnn
sbesbn n
nsb
Exclusion and discovery
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
EXCLUSIONThe observed small number of events could be due to
a statistical fluctuation with prob. 5×10-2
DISCOVERY
The observed large number of events could be due to a statistical fluctuation with prob. 5.7×10-5
Lexclusion
Increasing the Integrated luminosity the background uncertainty decreases. When the difference between background and background+signal is 2 the Luminosity for the exclusion is reached.
Ldiscovery
Similar definition for the discovery
Really observe n events and expect to observe n events at a given luminosity is not the same.At the exclusion (or discovery) Luminositythe probability to reach the goal is 50%
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Signaficance
;;;!
)()|(
)(
sbsbnn
sbesbn n
nsb
sb
sScP
When the background b
can be precisely estimated
The inclusion of the background error b with a Gaussian distribution needs a specific calculation, with the Gaussian approximation for the number of events n the significance can be expressed with the following relation:
2bb
sScl
b
sScP With high statistics, for few units of significance,
the denominator is only √b
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
• With a large number of observed events (n>>n), the statistical fluctuations do not have a big impact in the final result; for small numbers is the opposite:
small changes in the selection can produce big differences (i.e. 0 evts 2 evts)
• None is “neutral” , good arguments can be found to modify a little bit the cuts to obtain a sensible change of the final result;
• The selection criteria must be defined a priori with the MC to optimize the signal significance, only at the end we can open the box and look the impact on the real data. This method is called “blind analysis”.
The “blind analysis”
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs searches at LEP II
MH
ECM=206 GeV
The “Higgs-strahlung” is dominant production also at LEP II. At higher s
- the diboson fusion increas the relative relevance;
- higher Higgs masses can be produced.
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs decay channels at LEP II
The most relevant decay channel is H bb like at LEP IOver 115 GeV (LHC region) other decay channels (WW e ZZ) becames relevant or dominant
4 jets 2 jets &
missing energy
19%60%
Or a instead of the b
2 jet &
2 lepton
6%
Hbb 85%
H 8%
Research topology:
LEP I
LEP II
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
e+ f’
e-f
Z
W+, Z, e+
,e
e- W-, Z,
e+H
e- Z
Z
e+ -
e-
W+
W-
H
In addition to Zff we have also the WW , ZZ and production and decays.
e+
e-
e+
e-
e+e- → e+e-qq
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
ALEPH
HZ4jet, s=192 GeV
mH=90 GeV, L = 500 pb-1
OPAL
HZ2jet 2, s=192 GeV,
mH=80 GeV, L = 1000 pb-1.
Invariant mass distribution for the signal and the backgrounds (MC)
After the selection dibosons are the main source of background
mH=80 GeV mH=90 GeV
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
mH=100 GeV
Invariant mass distribution
for MC and real data.
mH=115 GeV
Final LEP selections
for 115 GeV search
(Loose and Tight)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Statistic approach for the global combination
We need to combine the results from different channels (Hqq, H, Hll) and different energies Ecm. They are grouped in the same two-dimensional space (mH rec , G)
mH rec reconstruced invariant mass
G discrimanant variable (QNN, b-tag)
For every k channel we obtain:
- bk estimanted background
- sk estimated signal (related to mH)
- nk number of Higgs candidate from the real data
We build the Likelihood for two hypothesis:
- candidates coming from signal + background Ls+b
- candidates coming from background Lb
mHrec
G
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
!
))(()|(
))((
n
msbesbn
nH
msb H
P
We want to discriminate the number of observed events (n)
w.r.t. the mean number of expected signal plus background (b+s) or only background (b)
The following is the probability for b+s , s is a function related to mH :
The Likelihood is the product of the probability density (k channel density)
kn
i kk
ikkHikk
kHkkk bs
BbmSsmsbnPL
1
)())(|(
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
The comparison between the two hypothesis is provided by the Likelihood ratio.
)(
)()(
|
|
Hbn
HsbnH mL
mLmQ
2))(ln(2 HmQ
We choose to describe the results with the log of the ratio because it provides the 2 difference :
We look to the function -2ln(Q(mH))
(i) For the real data
(ii) For the MC with n=b
(iii) For the MC with n=b+s
kkHkk n
i kk
ikkHikk
k k
nHkk
msb
bs
BbmSs
n
msbeL
1
))(( )(
!
))((
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
green: 1 from the background yellow: 2 from the background
background(higher 2 for b+s)
signal+background(higher 2 for b)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
mH > 114.4 GeV/c2 at 95% CLs
Finally we can estimate the exclusion at 95% of confidence level
(CLs = CLs+b / CLb)
Over 114 GeV/c2 the real data line (red) is closer the the s+b line (brown)
anyway the real data line is always (every mH ) within 2from the background line
LEP I mH > 65 GeV/c2 LEP II mH > 114.4 GeV/c2
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
The “window” for MHiggs
114.4 GeV
171 GeV
This exclusion window is at 95% of C.L. , masses outside this window are not forbidden, they have a smaller probability
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs serches at LHC
ECM = 7 TeV
L max = 3.54 1033 cm-2 sec-1
CMS
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Cosmic Rays
LHC~ 100 mb
(AKENO, FLY’S EYE)
SPS (SppS) (UA1, UA4 UA5)
TEVATRON (CDF, E710, E811)
( ISR )
LHC7 TeV
Total cross section at LHCEPL Volume 96, Number 2, October 2011 First measurement of the total proton-proton cross-section at the LHC energy of √s =7TeV
Second IDPASC schoolEzio TorassaUdine, February 1st 2012Padova 19 Aprile 2011 Ezio Torassa
protone protone
Main interaction
ISR e FSR
Jets from high pt particles
Fragmentation and hadronization
Multi partonic interacions
Beam Remnant
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Underlying Event and Minimum Bias
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 PileUp
protone protone
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Δ Ei = 0
Elastic scattering (25%)
Double diffractive inelastic (8%)
Not diffractive inelastic (55%)
Single diffractive inelastic (8%)
Minimum Bias: soft inelastic scattering
- Observable fro the detector (Pt min ~100 MeV)
- None (or few) tracks produced at significant Pt (~ 2 GeV)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
E.W. backgroundLEP
103
107
QCD background
HH
1/year
LHCLHC: Higgs factory inside a little bit hostile environment
1/hour
From LEP to LHC
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
SM Higgs production cross section including NNLO/NLO QCD corrections
Higgs boson production at LHC
mH (GeV)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs branching ratios
ff
fhfm
v
mg W
VhVV m
v
mg
22
Higgs boson decays
For Higgs masses over 135 GeV the main decay channels are WW(*) and ZZ(*)
under 135 GeV they are bb , +- and
The coupling constant of the Higgs to the fermions and bosons are proportional to the mass of the particles:
2GeV/c246sin
WWmv
When mH is high enough to open a new decay channel this one becomes the dominant mH (GeV)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
BR(hWW) / BR(hZZ) = g2hWW / g2
hZZ = 4mW2 / mZ
2 ~ 3
This rule can be broken when the two mass are very close:BR(WW) > BR (ZZ) but mW < 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
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
The Higgs boson width
The width changes from few MeV for low masses to hundreds of GeV for high masses due to his dependece on m3
H (from H→VV coupling)
mH (GeV)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
mH (GeV)
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs search at LHC
Higss search status report CERN seminar December 13th, 2011
In high mass region the discovery can be obtained using the WW and ZZ channelsIn the low mass region the contribution from several channels can be useful
ATLAS
CMS
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Direct production of WW
Wt
The signal signature is:
- 2 high Pt leptons - missing Et- veto for high energy Jet - angular correlation between W-W
DYtt
HWW (*) 2l 2Signal
Background
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Data describes the predicted background wellExclusion window:Expected: 129 < MH < 236 GeV Observed: 132 < MH < 238 GeV
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
In the region mH < 140 GeV 3 events are observed: two 2e2μ events (m=123.6 GeV, m=124.3 GeV) and one 4μ event (m=124.6 GeV)
HZZ (*) 4l
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
In the region mH < 160 GeV 13 events are observed: The excess is distributed in a wider mass range w.r.t. ATLAS
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
135 < mH < 156 GeV181 < mH < 234 GeV
255 < mH < 415 GeV
134 < mH < 158 GeV180 < mH < 305 GeV
340 < mH < 460 GeV
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
H
CMS PAPER HIG-11-033
Second IDPASC schoolEzio TorassaUdine, February 1st 201246
November 2011CMS PAS HIG-11-023, ATLAS-CONF-201-157
LEP (95%CL)
mH > 114.4 GeV
Tevatron exclusion (95%CL):
100 < mH < 109 GeV156 < mH < 177 GeV
ATLAS+CMS combination: based on data recorded until end August 2011 (~2.3 fb-1 / exp.)
Excluded 95% CL : 141-476 GeV Excluded 99% CL : 146-443 GeV (except ~222, 238-248, ~295 GeV)
Higgs exclusion window
114 - 141
Second IDPASC schoolEzio TorassaUdine, February 1st 201247
HZZ 4μ candidate with m4μ= 124.6 GeV
pT (μ-, μ+, μ+, μ-)= 61.2, 33.1, 17.8, 11.6 GeVm12= 89.7 GeV, m34= 24.6 GeV
Second IDPASC schoolEzio TorassaUdine, February 1st 2012
Higgs searches at LEP I :
Z Physics at LEP I CERN 89-08 Vol 2 – Higgs search (pag. 58)
Search for the standard model Higgs boson in Z decays – Nucl Physics B 421 (1994) 3-37
Higgs searches at LEP II :
Search for the Standard Model Higgs Boson at LEP – CERN-EP/2003- 011
Higgs searches at LHC:
CMS PAS HIG-011-32 SM Higgs Combination