Experimental search  for the Higgs Boson in di‐photon channel

Kajari Mazumdar

for a summary of the salient features of the discovery please visit  http://www.tifr.res.in/~mazumdar

Higgs seminar, TIFR                                                                                      August 6, 2012

•Standard Model is very successful in explaining all the experimental observations.• Electroweak symmetry breaking (EWSB) is the reason behind massive mediators of the weak interaction (W and Z bosons) while that of the electromagnetic interaction, the photon remains massless.

• One of the corner stone of the theory, the Higgs mechanism, needed to be established to prove this idea of EWSB, ie, mass generation of elementary particles.

• The Higgs mechanism invokes at least one Higgs boson, a scalar particle in SM.• Without the confirmation about the Higgs particle, the SM could not be complete.

The experimental observation of the Higgs particle crucial for completeness of SM.

• LHC machine has been mainly builtto resolve the issue of Higgs mechanism.to probe the physics of TeV energy scale.

•It is argued theoretically that the mass of the Higgs boson should be below 1000 GeV.

•LHC is capable of producing the Higgs boson in the whole mass range anticipated.

Motivation

LHC dataExcellent performance of LHC machine, ATLAS and CMS experimentsas well as LHC computing grid made the discovery possible.

During April 5 to June 18, 2012, ~ 6 fb-1 p-p collision at 8 TeVMean pile-up: 19 eventsPeak luminosity: 6.8 X 1033 /cm2 /s integrated lumi: 1 fb-1/week

CMS : 5.1 fb-1 @ 7 TeV5.3 fb-1 @ 8 TeV

ATLAS : 4.8 fb-1 @ 7 TeV5.8 fb-1 @ 8 TeV

Data used

Data already collectedat 8 TeV /experiment~ 10 fb-1

Expect ~ 20 fb-1 by end 2012

Higgs search as of beginning of 2012

• Standard Model Higgs boson excluded in mass range 127.5 to 600 GeVat 95% CL by ATLAS and CMS experiments.

• Excess of events corresponding to Higgs mass value of ~125 GeV

• CMS with local significance 2.8 and global significance of

Similar situation with ATLAShep-ex: 1207.0319

What is new in 2012 analysis:• Reoptimization performed only using

simulation.• Analysis validated in data using control

regions.

Phys. Lett. B (2012) 403

Announcement of Higgs boson discovery on July 4.

5 evidence of resonance observed by both CMS and ATLAS .

•Papers based on combination of Higgs searches in , ZZ, WW, , bb channels using data collected in 2011 and 2012 at cm energy 7 and 8 TeV.

submitted by CMS and ATLAS collaborations on 31st July.To be published in Phys.Lett. B

CMS: Fitted mass: 125.3 ±0.4 (stat) ±0.5(sys.) GeVCombined significance: 5 corresponding to p-value: 2 X 10-7

CERN-PH-EP-2012-220 arXiv: hep-ex 1207.723575 names from India

ATLAS: Fitted mass: 126 ±0.4 (stat) ±0.4(sys.) GeVCombined significance: 5.9 corresponding to p-value: 1.9 X 10-9

CERN-PH-EP-2012-218 arXiv: hep-ex 1207.7214

Big News

Higgs  production at LHC

gluon-gluon fusion Vector boson fusion Associated productions with W, Z, top

• Higgs boson decays within ~ 10 -24 s.

• Since the mass is unknown all modesat any mass value must be considered

87.4%

@ 120 GeV6.7% 5.4% 0.5%

Experimental StrategyDivide the mass region according to the decay mode which is easy to identify and measure. avoid jets as much as  possible!

1) For Higgs mass below 140  GeV look for H H WW(*), H ZZ(*)

2) For  mass 140 ‐180 GeV, H WW,  H ZZ(*)

3) Above 180 GeV, H  ZZ

Note, Br(W l l = eBrZ  l+ l‐) = 3.3% ,  l = e,  ,

H WW* : 23H ZZ* : 2.9

H bb : 56H cc: 2.8H 6.2H : 0.021

Branching ratios (%) for mH= 125 GeVH gg : 8.5H : 0.23H Z : 0.16

X Br (H ) ~ 50 fb at 125 GeV

Candles for search of needle in million-million haystack

Early studies for LHC indicated thedaunting task for the experiments:

1. The rate of Higgs production is quite low, naturally.

2. High energy proton collisionsproduce huge amount of debris.

3. The detectors had to be excellent.

Today’s beautiful detectors withtheir fantastic capabilities of doing the jobs at hand are fruits of long and extensive R&D programmes .

TIFR joined some of these efforts in ‘90s: radiation hardness studies of crystals,

performance of Shashlik calorimeter,detector and physics simulations.

Higgs decaying to a pair of photons

•The signal is simple and gives a clean signature: final state with 2 energetic photons.

•Identified as the discovery channel for low mass Higgs at a hadron collider in late 80’s.

• Narrow peak to be identified on top of huge continuous background in the invariant mass distribution.

• Need powerful jet-rejection to suppress dominant background (O (10 4))

Advanced analysis required to enhance small S/B

H 2 photons

• Photon reconstruction from clusters in electromagnetic calorimeter cells

• Recovery of conversion in inner detector

• Identification of isolated photons

• Calibration of photon energy

• Thorough understanding of the background

m2γγ= 2 E1 E2 (1-cosα)

Crucial measurement s of individual energy and angle between 2 s.

The calorimeter material chosen to have low radiation length and Moliere radius. compact detector with good energy, position, and angular resolutions. excellent mass resolution ~1%

• 75,000 crystals: 24 X 2 X 2 cm3

• Compact inorganic, scintillators.transparent but 96% metal by mass,

supported by 0.4 mm thick glass/carbon fibre structure.

Electromagnetic calorimeter in CMS

Barrel : ||<1.479Endcap :1.653< ||< 3

Analysis strategy in CMSMain analysis based on multivariate technique:Improves expected limit by 15% wrt cut-based analysisBackground model derived from data fit mass distribution in different categories

Cross-checked with independent cut-based and sideband background models.All approaches give consistent results within experimental uncertainties.

Blind analysis in 2012Analysis repeated on 2011 data but treated as separate set.

Events classified in total 6 categories:According to mass resolution, event kinematics and primary vertex probability.• 4 diphoton MVA• 2 dijet tagged include Vector Boson Fusion production processes reasonably forward-backward jets well-separated in rapidity improvement in signal to background ratio

• Inter-crystal calibration : 0 , •Energy scale W  e Z  eestability 0.12% in barrel

0.45% in endcap• ECAL cluster energies corrected using multivariate technique based on monte carlo

3 sources of correction:•Containment of shower•Recovery of conversion•Robustness against pile up

Photon identification : discriminate prompt photon vs. 0 from jet : Boosted Decision Tree (BDT) method using•Isolation•Cluster shape•Preshower energy in endcaps•Pseudorapidity

Energy scale, mass resolution, trigger efficiency, systematic uncertainties etc. derived from Z e+e- events

ECAL with collision data

Association of photons with primary vertex

• Mass resolution crucially depends on angle between 2 s

• Identification of hard interaction vertex crucial•High pile up in data many vertices distributed

within 5-6 cm •Difficult to identify which one produced 2 photons.

Efficiency decreases with increasing PU.Use boosted decision tree methodInput from tracks and photon pair

Event selection

• Identify photons based on BDT output• Associate photons with primary vertex • Associate tracks from conversion

• PT m

• PT m

•Background model derived from data•Reasonable agreement between MC and data•MC scaled by k-factors deduced from study of QCD processes in data

• Now optimize selection• classify events on various counts based on multivariate analysis (MVA)•MVA input independent of mass

Event classification

High score event in MVA •Photons are well identified in BDT

•Signal like kinematics of the event high score for events with high

PT (presence of high recoil)

• Good di-photon mass resolution

Class boundaries optimized to give the best expected exclusion limit using MC . •Both photons are fakes

•One photon fake•Contribution of Drell-Yan + W

k-factor of gg H process ~ 2 More than 50% of events have highly boosted diphoton system

Polynomial shape (order ranging from 3 to 5) for background modellingNegligible biasStatistical error , 20%

Distribution of diphoton invariant mass

Cat 0 Cat 1

Cat 2 Cat 3

Dijet tagging

Include • jet systematics: 10%• Jet energy scale and resolution : 6%• Underlying event modeling : 8%• Parton Density Function: 1.5%

Category migration due to :• uncertainty in higher order gg H process ( H+ 2jets) • different tunes of underlying events

VBF Candidate

Mass resolution

CMS result

Probability for the background to fluctuate upwards to create the observed Excess is less than10-4, corresponding to better than 4 sigma (S/√B) effect.

• Based on data collected in 2011 and 2012 at energies of 7 and 8 TeV• Observed 4.1 significance excess at 125 GeV, expected significance 2.8 • Excluded regions at 95% CL: 112-123, 132 – 143 GeV

Sensitivity almost flat over mass.Position unchanged in 2 datasets.

•Based on data collected in 2011 and 2012 at energies of 7 and 8 TeV• 4.5 significance excess at 126.5 GeV •Expected significance 2.5 , signal strength: 1.8± 0.5•Excluded regions at 95% CL: 112-123, 132 – 143 GeV

Results from ATLASPublished result based on analysis of data colelcted in 2011:Excess in the region of 124-126 GeV, with significance of 2.9

Conclusion

• Both CMS and ATLAS experiments have observed significant excess of events above all possible backgrounds in di-photon final state.

• This supports the hypothesis of a resonance production of mass around125 GeV.

• Observations in other channels also corroborate the same hypothesis.

•The nature of the resonance remains to be studied in detail to establish if itis indeed the Higgs boson expected in Standard Model.

• LHC machine is performing fantastically.

• Proton-on-proton collisions at energy 8 TeV will conclude by end of this year.

• Stay tuned for updates!

Back up

• Cut‐based [Straightforward] [2 frameworks]– Cut‐based PhotonID and event classification (EB/EE and 

high/low R9).– Background model from fitting the data.

• Mass factorized [Main result] [2 frameworks]– PhotonID MVA based on PFIsolation and shower shapes, and 

split events in classes using mass resolution and kinematics DiPhoton MVA. 

– Background model from fitting the data.

• Mass sideband [Alternate BG treatment]– BDT combining mγγ and DiPhoton MVA.– Background from mγγ sidebands.

CMS analysis

1. Cut based, with background model derived from fitting diphoton mass distribution

2. Multi-variate analysis (MVA) photon identification and event classification

3. Same as above and background estimation from sidebands of mass spectrum

• The calorimeter typically measures the energies of charged (exclude ) and neutral particles (exclude ).

• Calorimeter design criteria driven by Physics goals: signal linearity as well as energy& position resolution (Higgs discovery)Experimental conditions: radiation hard, fast responsePractical aspects: cost and ease of construction

• Homogeneous calorimeter : inorganic crystal• Sampling calorimeter : dense absorbers and sensing (active) elements

• Calorimeter characterized by power of fast and compact destruction :1. Radiation length and Moliere radius for

electromagnetic shower2. Hadronic interaction length

Essentials on Calorimeter

CMS : PbW04 crystal calorimeterATLAS: accordian geometry of steel

filled with liquid Argon

CMS DetectorHOTIFR, U.Panjab

Silicon preshowerBARC, U.Delhi

Energy corrections

• Standard model (SM) of particle physics encompasses 3 of the 4 fundamental interactions: Electromagnetic, Weak and Strong (leaving out gravitation). •The interactions have spin-1 mediators.

• The matter particles, which are fermions, classified into leptons and quarks. • They corroborate to these interactions.

• The mathematical description dictating the behaviour of the particles was developed during1960s.

• During the following decades, in particular, the experiments towards the end of the last century established Standard Model firmly.

1. Weak Interactions among fundamental matter particles are mediated by heavy gauge bosons, W, Z

2. The mediator of strong interaction, photon, is massless infinite range

3. The predictions of the theory were beautifully confirmed.

Particles’ masses are inputs to the theory (like the electromagnetic charge, spin, etc.).

Recapitulations

However, all was not well!

Behind the Tracker and in front of EE2 Pre-shower Endcaps (SE)Coverage: 1.653<||< 2.61000 detectors, 3 Xo

Silicon PreshowerBARC+ Delhi University• fabrication at Bangalore with close 

supervision from BARC• Quality of detectors comparable to that of 

Hamamatsu  CMS requested for more  production.

Physics motivation: discriminate between and

Crucial for measurement of H .

Photon-id MVA

• This single piece has been absent, rather,elusive in experiments,making the pictureincomplete.

• Big question: What isthe mechanism behindthe generation of massesof the particles?

What happens in LHC experiment

Proton‐Proton             1600 bunch/beam

Protons/bunch             2. 1011

Beam energy                4 TeV

Luminosity                   7.1033 /cm 2/s

Crossing rate                 20 MHz

Collisions                       108 Hz

Summer, 2012

• The largest silicon based detector• Total area ~ 205 sq.m• 76 Million electronic channels• To be operated at -200 C• Innermost layer: 100 X 150 m pixels

• 74,000 crystals: 24 X 2 X 2 cm3

• Compact inorganic, scintillatorstransparent but 96% metal by mass,

supported by 0.4 mm thick glass/carbon fibre structure.

Some of the subsystems of CMS detector