Search for a Higgs boson decaying intotwo photons at the CMS experiment

Marco Grassi

Exploiting neutral meson π0 to understand Higgs

06/06/2011 M. Grassi 2

The Higgs Boson

► Standard Model of Particle Physics: very successful in explaining high energy experimental data

► Open question: origin of W,Z masses (i.e. electroweak symmetry breaking mechanism)

► Possible answer: Higgs Mechanism → Higgs Boson experimentally observable

► LEP2 experiments excluded mH< 114 GeV/c2; Tevatron excluded 158 < m

H < 173 GeV/c2

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Outline

Why the Diphoton Channel

Experimental Setup

Neutral meson π0 to understand the detector

► Detector Calibration

► Probing radiation hardness

Higgs Hunting

► Reconstruction of the decay vertex

► Photon Selection

► Category based analysis

► Confidence Level for exclusion or discovery

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The Diphoton Decay

► Diphoton channel: relevant for low mass Higgs

► Small Branching ratio (~0.2%) but clean final state topology (2 high energy photons)

► Photon background well known: very promising channel

► Narrow resonance: energy resolution is the key parameter in this discovery channel

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The CMS Electromagnetic Calorimeter

Electromagnetic Calorimeter (ECAL)

► More than 75000 PbWO4 crystals

► High density (8.3g/cm3), short radiation length (0.89 cm), small Moliere radius (2.2cm)

► Fine Granularity and Compact Calorimeter

ECAL

pseudorapidity

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Understanding π0 to Understand Higgs

► 99% of neutral meson π0 decays into two photons

► Well known resonance available with high statistics

► Useful to study► Crystal inter-calibration and absolute energy scale► Crystal Radiation Hardness► Background Rejection

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Crystal Inter-Calibration Using π0

► Thanks to high statistics available, π0 peak position can be evaluated for each crystal

► Equalize crystal response is fundamental to improve constant term in resolution

► Above 100 GeV it starts to affect strongly the energy reconstruction

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Crystal Transparency Loss due to Radiation

►Proton-proton collisions generate highly radioactive environment

►ECAL crystals loose transparency when strongly irradiated

►Laser system provides continuous check of transparency

►Correction factors are applied offline to account for the effect

►π0 is a perfect tool to validate and improve corrections

Radiation Dose in Gy

Higgs Hunting

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Higgs Boson in Proton-Proton Collisions

► Higgs invariant mass

► Two main issues: photon reconstruction/selection and Vertex Identification

► Any unaccounted effected in the two quantities spoils the mass reconstruction

Techical Design Report Irreducible background

► Direct diphoton production

Reducible background

► pp → γ + jet

► pp → jet + jet

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Vertex Reconstruction of Neutral Objects

► High Luminosity: many hard interactions at each collision → many vertexes

► Neutral particles (photons): no signal in tracker → no tracks to assign vertex

► Use correlation between kinematic of vertex tracks and di-photon kinematics

► Vertexes ranked according to the following three variables (based on simulation):

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Test of Vertex Assignment Using Z Boson

► Z boson decaying into charged particles (e,µ) : capability to reconstruct vertex

► Remove tracks associated to e (µ): mimic di-photon Higgs decay

► Vertex identification (using ranking method) measured on Z sample in data

► Check that data and simulation agree in assigning right vertex

► Predict efficiency for Higgs vertex assignment

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Photons Might Convert in e+e-

► Average material in front of ECAL: ~1 radiation length (X0)

► Significant probability for photons to convert in e+ e- pairs

► Electrons and positions from conversions have to match several requirements

► Tracks belonging to opposite charge electrons

► Tracks have to be parallel at conversion vertex

► Converted vertex inside tracker fiducial volume

electron

Nu

mb

er

of

Co

nv

ers

ion

s

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Vertex Assignment With Converted Photons

► Converted photons exploited to measure longitudinal coordinate of Primary Vertex

► Track Fit: measure ZCONV

(A) and RCONV

(B)

► Compute angle α between conversion direction

and z axis: R cot(α) ↔(C)

►

z

R

photon

e+

e-(C)

(A)

(B)

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Photon Identification

► Photon energy reconstructed clusterizing measurements from several crystals

► Accounts for energy irradiated by bremsstrahlung from electrons in the shower

► Variables to separate signal (isolated photons coming from primary interaction) from background (low multiplicity jets with high electromagnetic content)

1) Topological variable R9: E

3X3/E

CLUSTER: energy sum of 3x3 crystal centered on the most

energetic divided by energy of the cluster

► Cluster becomes photon if R9 > 0.94

2) H/E: ratio between hadronic and electromagnetic energy of photon candidate

► Isolated photons H/E~0 → ECAL thickness is 25 radiation lengths

3) σηη

Transverse shape of the electromagnetic cluster

► Smaller for single isolated photons than for background of π0 → γγ

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Isolation in Pile-Up Condition

► Photon Isolation is key requirement to select Higgs photons

► Increasing luminosity → much many collisions in the same bunch crossing

►Much busier environment

► Usual cut: variable < threshold → New definition: variable < a + b*ρ

► ρ is a parameter related to number of vertexes in event

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Signal Modelling

► Signal shape for standard model Higgs in low mass range is driven by detector resolution

► Function: convolution of Crystal Ball and Gaussian

► Crystal Ball: good description of ECAL resolution and bremsstrahlung losses

► Gaussian: additional resolution effects (misreconstructed vertexes)

► Derive shape parameters from data

► Resolution term σ(CB) and scale offset ∆(m0) are extracted from Z→ee events

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Analysis Categories & Limit Extraction

Categories

► Improve search sensitivity: selected events split into categories based on

►Compactness of photon shower

►Pseudorapidity of photon candidates

► Motivation: use all the events accounting for better resolution and S/B where expected

Confidence Level Calculation

► All Categories combined in confidence level (CL) calculation as independent channels

► Two statistical approach

► Bayesian with flat prior for signal strength

► Frequentist using profile likelihood and CLs technique

Unfortunately no plot to show (yet!)

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Conclusions

► Before computing γγ invariant mass, several steps have to be accomplished

► Detector has to be perfectly calibrated (crystal intercalibration goal 0.5%)

► Simulated energy resolution has to agree with data

► Corrections accounting for crystal transparency loss have to be reliable within 0.1%

► Neutral meson π0 is the right tool to address all the detector issues

► The knowledge gained so far on π0 is the first step to look for Higgs boson

► Up to one week ago, LHC experiment collected more than 0.5 fb-1 of data

► Now Higgs hunting starts to be exciting!