XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Lesson #3 Higgs boson searches at LEP1,...

XXVII Ph.D in PhysicsEzio TorassaPadova, March 16th 2012

Lesson #3

Higgs boson searches at LEP1 , LEP2 and LHC

Standard Model


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


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”


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


(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. )


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


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


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%


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


• 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”


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.


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


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-

qq

e+e- → e+e-qq


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


mH=100 GeV

Invariant mass distribution

for MC and real data.

mH=115 GeV

Final LEP selections

for 115 GeV search

(Loose and Tight)


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


!

))(()|(

))((

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

)())(|(


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

))(( )(

!

))((


green: 1 from the background yellow: 2 from the background

background(higher 2 for b+s)

signal+background(higher 2 for b)


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


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



Higgs serches at LHC

ECM = 7 TeV

L max = 3.54 1033 cm-2 sec-1

CMS


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


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


* 1.5m 1mgcrosbunch

p fN

L sin_2

2

4

/2*

!/)( NeNP NPU

gcrosbunchxPU fLN sin_sec /

Number of interactions / bunch crossing


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)


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


SM Higgs production cross section including NNLO/NLO QCD corrections

Higgs boson production at LHC

mH (GeV)


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)


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


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)


mH (GeV)


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


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


Data describes the predicted background well


< 90° (1.55 rad)

< 1.8 rad

Prima del taglio mll <50 GeV

Dopo il taglio mll < 45 GeV


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


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.


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


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


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


H

CMS arXiv:1202.1487v1 7 Feb 2012ATLAS arXiv:1202.1414v1 7 Feb 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


arXiv:1203.4254v1


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


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


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.)


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

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XXVII Ph.D in Physics Ezio TorassaPadova, March 16 th 2012 Lesson #3 Higgs boson searches at LEP1,...

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