xxvii ph.d in physics ezio torassapadova, march 16 th 2012 lesson #3 higgs boson searches at lep1,...
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XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Lesson #3
Higgs boson searches at LEP1 , LEP2 and LHC
Standard Model
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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”
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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:
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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. )
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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%
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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”
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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.
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
e+ f’
e-f
Z
W+, Z, e+
,e
e- W-, Z,
e+H
e- Z
Ze+ -
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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
)())(|(
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
))(( )(
!
))((
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
green: 1 from the background yellow: 2 from the background
background(higher 2 for b+s)
signal+background(higher 2 for b)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Higgs serches at LHC
ECM = 7 TeV
L max = 3.54 1033 cm-2 sec-1
CMS
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
( 98.3 ± 0.2 stat ± 2.8 sys ) mb
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012Padova 29 Giugno 2009 Ezio Torassa
protone protone
Interazione principale
ISR e FSR
Creazione dei Jet
Frammentazione e Adronizzazione
Interazioni Multi Partoniche
Beam Remnant
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Underlying Event, Minimum Bias, Pile-Up
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
protone protone
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
* 1.5m 1mgcrosbunch
p fN
L sin_2
2
4
/2*
!/)( NeNP NPU
gcrosbunchxPU fLN sin_sec /
Number of interactions / bunch crossing
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
SM Higgs production cross section including NNLO/NLO QCD corrections
Higgs boson production at LHC
mH (GeV)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
WWm
v
When mH is high enough to open a new decay channel this one becomes the dominant mH (GeV)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Higgs search at LHC 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
CMS arXiv:1202.1488v1Feb 7, 2012
ATLAS-CONF-2012-019March 7, 2012
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Data describes the predicted background well
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
< 90° (1.55 rad)
< 1.8 rad
Prima del taglio mll <50 GeV
Dopo il taglio mll < 45 GeV
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
ATLAS Exclusion window:Expected: 127 < MH < 234 GeV Observed: 130 < MH < 260 GeV
ATLAS-CONF-2012-012 7 Mar 2012
CMS Exclusion window:Expected: 129 < MH < 236 GeV Observed: 132 < MH < 238 GeV
CMS arXiv:1202.1489v1 7 Feb 2012
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Anziché mostrare il CLs in funzione della massa, si è scelto di moltiplicare la sezione d’urto del segnale per un fattore opportuno (maggiore o minore di 1) in modo da ottenere sempre l’esclusione al 95% per tutte le masse. Ovviamente solo dove non serve una sezione d’urto superiore a SM si ha una vera esclusione per l’Higgs SM.
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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 (*) 4lATLAS-CONF-2011-162 arXiv:1202.1415v3 1 Mar 2012
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
In the region mH < 160 GeV 13 events are observed: The excess is distributed in a wider mass range w.r.t. ATLAS
CMS arXiv:1202.1997v1 9 Feb 2012
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
134 < mH < 156 GeV
182 < mH < 233 GeV
256 < mH < 265 GeV
268 < mH < 415 GeV
134 < mH < 158 GeV
180 < mH < 305 GeV
340 < mH < 465 GeV
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
H
CMS arXiv:1202.1487v1 7 Feb 2012ATLAS arXiv:1202.1414v1 7 Feb 2012
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
ATLAS-CONF-2012-019 5 Mar 2012
Excluded from 129 GeV to 539 GeV
SM Higgs combinatio
n
CMS arXiv:1202.1488v1 7 Feb 2012
Excluded from 127 GeV to 600 GeV
118.5 GeV to 122.5 GeV100.0 GeV to 117.5 GeV
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 201249
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
~ 130
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 201250
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
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012
Look elsewhere effect
arXiv:1005.1891v3
The statistical significance that is associated tothe 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)
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 (1.5 L.E.E.)
XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 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