XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Lesson #4 Higgs boson searches at LHC...

XXVIII Ph.D in PhysicsEzio TorassaPadova, May 13th 2013

Lesson #4

Higgs boson searches at LHC

Standard Model


Higgs serches at LHC

ECM = 7,8 TeV

L max = 7.7 1033 cm-2 sec-1 (Hz / nb )

CMS


Integrated Luminosity 2012 8 TeV

Delivered 23.3 fb-1 Recorded 21.8 fb-1

Integrated Luminosity 2011 7 TeV

Delivered 6.13 fb-1 Recorded 5.55 fb-1


Cosmic Rays

LHC 8~ 100 mb

(AKENO, FLY’S EYE)

SPS (SppS) (UA1, UA4 UA5)

TEVATRON (CDF, E710, E811)

( ISR )

LHC8 TeV

Total cross section at LHCCERN-PH-EP-2012-354 December 11, 2012

( 101.7 ± 2.9 ) mb

in agreement with the extrapolation from lower energies

XXVIII Ph.D in PhysicsEzio TorassaPadova, May 13th 2013Padova 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


The pileup is the multiple interactionin 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 5Average PU 2011 RUNB 8Average PU 2012 all 21



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


Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC

Received 31 July 2012Accepted 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


H (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)

Higgs boson production at LHC SM Higgs production cross section

NNLO/NLO QCD corrections


H → H→WW , H→ZZ

In the low mass region all the channels can help to increase the significancein 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→ is a fundamental decay channel for the discovery and mass measurement

H →

b

b_

H →bb

Higgs boson decays


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 coupling constant of the Higgs to the fermions and bosons are proportional to the mass of the particles:

ff

fhfm

v

mg W

VhVV m

v

mg

22 2GeV/c246sin

WWmv


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)

tt turn-on

WW/ZZ turn-on

bb


LEP results were described with the CLs plots. The mass limit is the mass corresponding to CLs=5%. (exclusion at 95%).

At LHC we decided to multiply the signal cross section with a factor (>1 or <1) needed to exclude the signal at 95%.The real exclusion is the mass range wereThis factor is equal or lower than 1 (where you do need a cross section larger than SM to obtain the exclusion.


Higgs search at LHC


HZZ (*) 4l

4 July HCP (Nov. 2012) Moriond (March 2013)

N1 = 9 B1=4 N2=21 B2=20 Significance 3.2

N2 = 47 B2=37 Significance 4.5

R1: 121.5 ÷ 130.5 GeV R2: 110 ÷ 160 GeV

N2 = 71 B2=46 Significance 6.7

https://twiki.cern.ch/twiki/pub/CMSPublic/Hig13002TWiki/HZZ4l_animated_slower.gif


Dominant ZZ background

FSR Z background

Signal


Considering mH < 2 mZ one of the two Z is off shell

All the 4l combinations (4e , 4 , 2e2)show the excess at M=126 GeV


NTOT = 23 + 32 + 16 = 71


The SM Higgs can be excluded everywhere with CL > 95% except in the region where we see the mass peak and for mH > 800 GeV

Exclusion plot Discovery plot

6.7 (right) corresponds to a probability to be a statistical fluctuation of 10-11 (left)


Higgs mass best fit


signal strength = best fit / SM


H

Large background contribution:-irriducible from QCD - mis identification -jet and di-jet

Small branching ration: BR HWW ~ 2 10-1 BR HZZ ~ 3 10-2 BR H→ ~ 2 10-3 but no additional BR (ZZ 4,4e,2e2 36 10-4)

Signal

Background


Several photon quality selection (class 0.1.2.3)and different tags dominated by different production diagrams.


The diphoton invariant mass distribution with each event category weighted by its S/(S+B). The lines represent the fitted background and signal

4 July Moriond (March 2013)

Significance 4.1(expected 2.8

Significance 3.2(expected 4.2





mH = 124.3 ± 0.6 ± 0.5 GeV mH = 125.8 ± 0.5 ± 0.2 GeVZZ

mH = 126.8 ± 0.2 ± 0.7 GeV mH = 125.4 ± 0.5 ± 0.6 GeV


(at mH=125.4 GeV) = 0.78 ± 0.28 (at mH=126.8 GeV) = 1.6 ± 0.4

ZZ (at mH=125.8 GeV) = 0.91 ± 0.30 (at mH=124.3 GeV) = 1.7 ± 0.5


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


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.


Distributions of the transverse mass in the 0-jet category compared with the backgrounds and the signal mH = 125 GeV expectedThe cut-based H → WW selection, except for the requirement on the transverse mass itself, is applied.

)cos1(2llE

misT

llTT miss

TEpm

The transverse mass is the invariant mass between W+W- with the missing energy used to estimate the neutrino momentum and all the z components set to zero.

)0,,( 2121 missy

ly

ly

missx

lx

lxinv EppEppm



Significance @ 125 GeV 4.0 (expected 5.1


ATLAS largest significance @ mH=140 GeV

CMS largest significance @ mH=135 GeV

Is interesting to compare the mass having largest significance with the mass fitted for the H→ZZ and H→ channels

(at mH=125 GeV) = 0.76 ± 0.21

(at mH=125 GeV) = 1.0 ± 0.3


The fact that the reconstruction of the pair decay kinematics is underconstrained by the measured observables is addressed by a maximum-likelihood fit method. The mass mis reconstructed by combining the measured observables Ex

miss and Ey

miss with a likelihood model

H



Significance @ 120 GeV 3.0 (expected 2.6


best-fit value of the signal strength =1.1±0.4


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 k2

VV

Combined results


ff

fhfm

v

mg

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.

(the minimum of the Higgs potential)is a common constant


(at mH=125.7 GeV) = 0.80 ± 0.14CMS mH =125.7 ± 0.3 ± 0.3 GeV

ATLAS mH =125.5 ± 0.2 ± 0.5 GeV (at mH=125.5 GeV) = 1.3 ± 0.2


It is crucial to determine the spin and quantum numbers of the new bosonWe can use the HZZ decay channel to distinguish between the scalar (SM Higgs)JP=0+ and the pseudoscalar JP =0- hypothesis.We start to remember the M.E. likelhood analysis used in the mass estimation

Considering the following angles:

},,,,{ 211*

: angle between Z direction (z’) and z axis:: angles between the leptons and the Z

: angle between the two leptons pair planes angle between z’z plane and two lept. plane

We can build a kinematic discriminant between signal and background1

421

421

)|,,(

)|,,(1

lZZsig

lZZbkg

bkgsig

sigD

mmmP

mmmP

PP

PK

(MELA)Matrix Element Likelihood Analysis

Higgs JP measurement


The mass of the boson is measured with a maximum-likelihood fit to 3D distributions combining for each event:- m4l, - m4l (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


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 > 0.5 has been applied

A similar kinematic discriminant (pseudo-MELA) ca be used to distinguish scalar JP=0+ from pseudoscaral JP=0-

1

421

4210

0)|,,(

)|,,(1

lZZSM

lZZJ

JSM

SMJ mmmP

mmmP

PP

PD P


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.

The data disfavor the pseudo scalar hypothesis with 3.3 or 0.16%

Excluded 0.16%



Excluded < 0.1% Excluded < 0.1%

Excluded < 0.1%Excluded 1.5%


HZZ 4μ candidate


HWW 2μ + MET candidate


Higgs searches at LHC:

CERN-PH-EP-2012-354 TOTEM Luminosity measurementCERN-PH-EP/2013-035 arXiv:1303.4571v1 (19 March 2013)Observation of a new boson with mass near 125 GeV in pp collisions at √s = 7 and 8 TeV

CMS-PAS-HIG-13-005Combination of standard model Higgs boson searches and measurements of the properties of the new boson with a mass near 125 GeV

CMS-PAS-HIG-13-002Properties of the Higgs-like boson in the decay H to ZZ to 4l in pp collisions at √s =7 and 8 TeV


Backup



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.)

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XXVIII Ph.D in Physics Ezio TorassaPadova, May 13th 2013 Lesson #4 Higgs boson searches at LHC...

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