search for a higgs boson decaying into two photons at the ... file(0.89 cm), small moliere radius...
TRANSCRIPT
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
06/06/2011 M. Grassi 3
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
06/06/2011 M. Grassi 4
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
06/06/2011 M. Grassi 5
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
06/06/2011 M. Grassi 6
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
06/06/2011 M. Grassi 7
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
06/06/2011 M. Grassi 8
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
06/06/2011 M. Grassi 10
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
06/06/2011 M. Grassi 11
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):
06/06/2011 M. Grassi 12
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
06/06/2011 M. Grassi 13
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
06/06/2011 M. Grassi 14
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)
06/06/2011 M. Grassi 15
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
06/06/2011 M. Grassi 16
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
06/06/2011 M. Grassi 17
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
06/06/2011 M. Grassi 18
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!)
06/06/2011 M. Grassi 19
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!