Analyses with photons or electrons with early LHC data at the CMS experiment

Transcription

1 Analyses with photons or electrons with early LHC data at the CMS experiment Dottorando: Daniele Franci Relatori: Prof. Egidio Longo Dott. Daniele del Re Prof. Shahram Rahatlou Seminario progetto di tesi, 04/06/2009

2 Photons and electrons in LHC physics First beams circulating in LHC in early Fall 2009 Definitive test of Standard Model and new physics search Higgs sector Supersymmetry Extra Dimensions Crucial task: detection of high-energy photons and electrons Many physics processes with electromagnetic particles in final state (e.g. H γγ, Supersymmetric models) Also important for background knowledge (e.g. production) CMS detector: unique tool for γ/e detection Homogeneous crystal electromagnetic calorimeter (PbWO 4 ) Excellent resolution on both energy (0.5 %) and position 2

3 Why photons and electrons are so important? Higgs γ γ μ - Standard Model Higgs Golden channel for light SM Higgs Two photons in the final state γ p 0 ~χ 1 jet χ ~ χ ~+ 0 ~χ 1 p γ Supersymmetry: GMSB Valid candidate for new physics Two photons in the final state e - t - tbar production Leading bkg for many analyses Many leptons in the final state 3

4 Photon and electron detection Interactions γ (e - ) - matter Electromagnetic showers Moliere radius R M Describe lateral profile 95% of shower energy within an infinite cylinder of radius 2R M Electromagnetic calorimeter used for γ/e ± detection Response proportional to the energy of incoming particle Modular structure composed by independent crystals Energy escapes from central xtal Cluster of adiacent xtals Particle energy fully recovered 4

5 CMS electromagnetic calorimeter (ECAL) Cylindrical structure: - Central barrel ( η <1.479) -Twoendcaps (1.479< η <3) ~ PbWO 4 crystals θ 0.5% resolution for γ/e η = - ln( tan[ θ / 2 ] ) PbWO 4 ECAL crystal (barrel only): Lenght: 23 PbWO cm 4 Front face: cm 2 (R M R M ) Moliereradius[cm]: 2.2 5

6 The Compact Muon Solenoid detector ECAL 6

7 The Large Hadron Collider Proton - Proton collisions 27 Km long Nominal s: 14 TeV (10 TeV during low lumi phase) Four experiments: -ALICE -ATLAS -CMS - LHCb First collisions: Fall 2009 ~ 200 pb -1 expected by Fall 2010 for first physics analysis 7

8 My contribution on particle detection with ECAL I work on two issues related to γ/e detection After bkg rejection Background rejection: Algorithms to reject fake signals Crucial for channels overwhelmed by bkg M γγ [GeV] M γγ [GeV] After calibration Calibration: Response to particle energy Improve energy and position resolution M γγ [GeV] M γγ [GeV] 8

9 The role of π 0 Neutral meson π 0 plays an important role in both background rejection and calibration Mass: 135 MeV Lifetime: s Decay mode: π 0 γ γ Decay angle correlated with π 0 energy: 180 θ min sin 2 = M E π π 9

10 Background Rejection 10

11 Neutral pions as a source of background Neutral pions (copiusly produced at LHC) bkg for analyses with γ Photon and π 0 have the same signature! High-energy π 0 Low γ separation (δ) Both photons in the same crystal π 0 misinterpreted as a single photon π 0 δ 20 GeV π 0 in CMS ECAL δ 1.7cm < 2.2cm (Xtal size) 11

12 Pion cluster shape γ - π 0 identification relies on cluster shape studies Increasing π 0 energy φ index φ index δ~6cm, E~7GeV δ~2cm, E~20GeV δ~0.5cm, E~80GeV φ index η index η index η index Separated photons: compute invariant mass Elliptical shape due to overlapping photons (π 0 can be still identified) π 0 indistinguishable from a single photon 12

13 Energy covariance matrix and 2 moments Covariance matrix: describes geometrical properies of a cluster S = S S ηη ηφ COV ηφ with S ( )( ) μν = w i μi μ ν i ν φη Sφφ i= 1 Eigenvectors: principal axes of energy distribution directions of minimum/maximum energy spread Eigenvalues: 2 moments wrt the principal axes (S MAJ, S MIN ) amount of energy spread along those axes N γ π 0 S MIN S MAJ S MIN S MAJ Major axis Minor axis 13

14 Particle identification with 2 moments S MAJ discriminates between γ and π 0 γ π 0 PHOTON S MIN S MAJ S MAJ S MAJ NEUTRAL PION S MIN S MAJ S MIN discriminates between γ and hadrons HADRON S MIN γ, π 0 S MAJ S MIN S MIN 14

15 ECAL Calibration 15

16 The importance of calibration Energy resolution in a calorimeter a = stochastic term b = noise term c = constant term (calibration) σ/e % ( ) σ E a b = c E E E 100 GeV In photon range of interest (~100GeV), c dominates ECAL performance depends mainly on calibration quality Calibration purpose: find a constant for each crystal to bring ECAL response equal to particle energy E [GeV] 16

17 Calibration technique π 0 mass provides a kinematical constraint for calibration π M E 1 γγ ( θ ) = 2E E 1 cos = 135 MeV = CE E2 clu1 i i = CjE clu 2 j C i determined crystal by crystal (see CMS Detector Note /006 for further details) ADVANTAGES: High π 0 rate (~ 500 Hz), few days needed Tracker independent DISADVANTAGES: Limited energy range available (< 10 GeV) Background contamination 17

18 Biases in π 0 reconstruction Mass dependence on η Many η-rings calibrated at the same time Mass should be η-independent ~1.5% shift within barrel size M γγ [GeV] π 0 mass below 135 MeV ~97% of energy in 3 3 ~1.5% η(π 0 ) Bias produced by two effects: 1. Variation of lateral containment vs η Staggered crystals 2. Overlap between decay γ π 0 at low η mostly affected 18

19 Variation of lateral containment Staggering No staggering Xtals pointing to IP Staggering Energy lost into gaps Staggering increases with η γ Energy lost γ M γγ [GeV] Corrected mass Uncorrected mass M π Staggering can be corrected Mass closer to nominal value Residual mass dependence on η η(π 0 ) 19

20 Effect of overlapping photons Residual bias due to overlap between photons Less energetic γ mostly affected Effect of energy tails Excess in reco energy Dedicated study to address overlap effect Investigate 2D fit to energy deposits Determine energy and position of individual γ ONGOING 3D view of overlapping π 0 φ 20 η

21 Photons in Physics Analyses 21

22 Photons in physics analyses Standard Model analysis: H γγ Two high-energy photons Narrow resonance (detector resolution dominates) Calibration is a crucial task Long-term analysis O(year) Physics beyond Standard Model: GMSB Two high-energy photons Cluster shape is very important: - Neutral pion rejection - Determination of neutralino lifetime (see later) Analysis with early data O(month) Focus on search for new physics: Supersymmetry 22

23 Physics beyond the Standard Model Supersymmetry: possible scenario beyond Standard Model Basic idea: each SM particle has a SUSY partner (sparticle) Theory is invariant under particle sparticle transformation Topology determined by conservation of R-parity: Sparticles produced in pairs Decay chain composed by particles and sparticles Stable and neutral lightest SUSY particle (LSP) R = ( 1) 3( B L) + 2s 23

24 The GMSB model Gauge Mediated Supersymmetry Breaking (GMSB) model will be considered: Gravitino is the LSP Gravitino produced by neutralino decay: Process studied at the LHC: Other SM particles This channel satisfies 2 requirements: High energy photons in the final state Analysis with early LHC data High cross section (~3 pb) ~4K events/day 24

25 GMSB signature GMSB decay chain Missing transverse energy (MET) MET[GeV] P T spectrum of 2 γ Mean=93 GeV P T (γ) [GeV] Jets from quark fragmentation 25

26 Use of cluster shape to extract neutralino lifetime Some GMSB configurations predict a long-lived neutralino (cτ>0) Non-pointing photon to the interaction point, if cτ>o(10cm) IP Non-pointing γ elliptical shape Cluster shape can help Off-pointing α between major axis and φ Determine secondary vertex Non-pointing γ χ 1 lifetime can be extracted! 26

27 Conclusions & plans First collisions in LHC in Fall 2009, ~100pb -1 for analysis I work on two issues concerning particle detection with the CMS electromagnetic calorimeter: Photon/π 0 identification using cluster shape Calibration with neutral pions Photons and electrons crucial in many physics analyses Standard Model: t tbar production (bkg for many channels) Higgs sector: H γγ (golden channel, long term analysis) Physics beyond Standard Model: Supersymmetry I will focus on supersymmetric GMSB model High-energy photons in the final state Crucial role of cluster shape Discovery/exclusion possible with early LHC data 27

28 Backup Slides 28

29 Logarithmic weight Shower energy density decreases exponentially with lateral distance from shower core Cluster position: with w i = max 0; K + ln E E i TOT K=4.2 is used consider only xtals with E E i TOT 4.2 > = e 0.9% 29

30 Estimated separation δ δ Estimated separation between pion s decay photons δ is a function of Energy and η: Energy: related to decay angle θ sin 2 MIN = M E π π δ δ η: γ γ γ γ η = 0 η > 0 π π 30

31 S MAJ discriminating power Photon - π 0 discrimination based on S MAJ Particles energy = GeV (expected δ ~ 1.2 cm ~ ½xtalsize) 80% of γ efficiency 60% of π 0 rejection A similar tool is being developed to treat photon conversions 31

32 Identify conversions How photon conversions can be identified? Magnetic field can help e -(+) bent along φ direction Random major axis direction φ UNCONVERTED γ φ CONVERTED γ Major axis close to φ direction η η Define α as the angle between major and φ axis: α 32

33 Distributions of α Uncoverted photons Coverted photons α [rad] α [rad] α can be used to identify conversions 33

34 Calibration algorithm Iterative algorithm: After each step, crystal constants are obtained as N: iteration step C(n η,n φ ): calibration constant of xtal identified with n η,n φ n phot : number of photons containing this xtal in the 3x3 matrix w i : energy weight E i / E 3x3 M inv : invariant mass of γγ pair M π : nominal π 0 mass Few iterations needed to converge Target resolution 34