Collider Physics Analysis Procedures

Transcription

1 Collider Physics Analysis Procedures Alex Tapper Slides available at:

2 Aim Overview of analysis techniques at CMS Contrast with Tevatron (see DØ lecture) New energy regime New detector Emphasis on robustness and data-driven techniques Nothing fancy (b-tagging, NN, BDT...) Graduate lectures, February Page 2

3 Outline Quick reminder of CMS detector Basic physics objects Analysis - SUSY search Summary Graduate lectures, February Page 3

4 The CMS detector Lead Tungstate Electromagnetic Calorimeter Plastic-Brass Hadronic Calorimeter Iron Yoke Muon Detectors Superconducting Solenoid (4 Tesla) Silicon Microstrip Tracker Silicon Pixel Detector Graduate lectures, February Page 4

5 The CMS detector Graduate lectures, February Page 5

6 Analysis objects Basic list of objects needed for analysis Leptons muons, electrons, taus Hadronic jets Energy sums MET, ET... Typically want the ET, η and ϕ of each reconstructed object Also often quality flags or isolation Graduate lectures, February Page 6

7 Analysis objects: muons Muons Easiest to reconstruct Use tracks in silicon and muon chambers Graduate lectures, February Page 7

8 Analysis objects: electrons & photons Electrons Search for isolated, compact EM energy Matched track Strikingly in 4T B-field all electrons have Bremsstrahlung photons Algorithms collect photons in electrons Graduate lectures, February Page 8

9 Analysis objects: hadronic jets Jets Attempt to cluster together energies from parton Traditionally jets made from clustering HCAL and ECAL energies according to some algorithm (anti-kt) Graduate lectures, February Page 9

10 Analysis objects: taus Taus Hadronic decays of tau leptons Either one or three pronged decays to pions Thin jets with few constituents Backgrounds from QCD jets No taus in CMS yet... Graduate lectures, February Page 10

11 Analysis objects: MET Et(miss) GeV MET Vector sum of all energy deposits in the event Very sensitive to mis-calibration, non-uniformity, beam backgrounds Tevatron experience scary Graduate lectures, February Page 11

12 Analysis objects: particle flow Particle flow - The Rolls Royce of reconstruction Graduate lectures, February Page 12

13 Analysis objects: particle flow Particle flow - MET comes naturally Charged hadrons Neutral hadrons P-flow: MET (1.9 GeV) Photons Graduate lectures, February Page 13

14 Analysis objects: particle flow Particle flow - jets too PFJet 1 pt 41.5 GeV/c PFJet 2 pt 37.5 GeV/c PFJet 3 pt 21.8 GeV/c P-flow: MET (1.9 GeV) Graduate lectures, February Page 14

15 Analysis objects: particle flow Graduate lectures, February Page 15

16 Analysis strategy: overview 1. Take data Detector, trigger, DAQ 2. Reconstruct physics objects Muons, electrons, taus, jets, MET Simulation Generate events, detector simulation 4. All the tools to make a measurement Graduate lectures, February Page 16

17 SUSY search strategy Be as model independent as possible But the MSSM has > 100 parameters Need more constrained models Choose a set of benchmark points that are representative of a range of topologies and areas of phase space Range of models MSUGRA (high and low masses) GMSB Split SUSY In this talk MSUGRA at low masses, just above the Tevatron (LM0 and LM1) m 1/2 (GeV) MSUGRA, tanβ = 10, A 0 = 0, µ > τ ~ 1 LSP 7 HM1 Br( χ ~ 2 0 l ~ l) > 0.15 HM2 LM6 LM5 LM2 LM4 LM1 LM LM8 m(e ~ L )<m(χ 2 0 ) HM3 8 m χ = 103 GeV m 0 (GeV) m(u ~ L ) > m(g~ ) Br( χ ~ 0 2 h 0 χ ~ 0 1) > 0.5 Br( χ ~ 2 0 Z 0 χ ~ 1 0 ) > 0.5 m h = 122 GeV m h = 120 GeV HM4 m(t ~ 1 ) < m(g~ ) m h = 114 GeV ~ l Teva tron NO EWSB J. Phys. G: Nucl. Part. Phys. 34 (2006) 9 LM LM10 LM7 200 Graduate lectures, February Page 17

18 SUSY search strategy Production Squark and gluino expected to dominate Strong production so high cross section Cross section depends only on masses Approx. independent of SUSY model Graduate lectures, February Page 18

19 SUSY search strategy Production Squark and gluino expected to dominate Strong production so high cross section Cross section depends only on masses Approx. independent of SUSY model Decay Details of decay chain depend on SUSY model (mass spectra, branching ratios, etc.) Assume RP conserved decay to lightest SUSY particle (LSP) Assume squarks and gluinos are heavy long decay chains Signatures MET from LSPs, high-et jets and leptons from long decay chain Focus on robust and simple signatures Common to wide variety of models Let Standard Model background and detector performance define searches not models Graduate lectures, February Page 19

20 Backgrounds Physics Standard Model processes that give the same signatures as SUSY Cannot rely on Monte Carlo predictions measure in data Detector effects Detector noise, mis-measurements etc. that generate MET or extra jets Commissioning and calibration Beam related Beam-halo muons (and cosmic-ray muons), beam-gas events Data and simulation already measure in situ too Graduate lectures, February Page 20

21 Backgrounds Data-driven background estimates are the key challenge in early SUSY searches General idea is find a control region where SM is dominant and use this to predict SM background in signal region Two approaches pursued: Matrix (ABCD) methods playing variables off against each other Replacement methods modify SM with same topology as signal to predict signal In both cases need to identify clean SM control region Difficult to avoid using Monte Carlo in some way Will discuss all hadronic (jets + MET) search giving examples of data-driven methods Graduate lectures, February Page 21

22 All-hadronic SUSY search SUSY particles produced strongly and decay through long cascade Search for excess of events with large MET (from LSP) and several hadronic jets Veto events with leptons Graduate lectures, February Page 22

23 All-hadronic SUSY search Simple (pre)selection At least two jets with ET>50 GeV and η <3.0 Veto events with an electron or muon PT>10 GeV Use energy sums based on jets More robust since you can put minimum ET cut HT scalar sum of jet ET MHT vector sum of jet ET Enhance SUSY-like processes ET of two highest ET jets > 100 GeV ηj1 <2.0 Look at simulation to see what processes form backgrounds to your signal Graduate lectures, February Page 23

24 All-hadronic SUSY search QCD is by far largest background Z-boson decays Top-pair production and W-boson decays Graduate lectures, February Page 24

25 Background from QCD QCD processes lead to di-jet events Gluon radiation gives >2 jets When perfectly measured no MET but... Not a perfect detector Semi-leptonic decays in jets (b and c quarks) Graduate lectures, February Page 25

26 Background from QCD Mis-measurement of a jet leads to MET along the jet axis Remove with ΔΦ(jeti,MET) > 0.3 rad Also several methods developed to predict MET tail from QCD events Graduate lectures, February Page 26

27 All-hadronic search PRL101: (2008) & CMS-PAS-SUS jet LSP LSP jet jet jet A novel approach combining angular and energy measurements α T = E T j 2 M T j1 j 2 = E T j 2 / E T j1 2(1 cosδϕ) Perfectly balanced events have αt=0.5 Mis-measurement of either jet leads to lower values Graduate lectures, February Page 27

28 All-hadronic search PRL101: (2008) & CMS-PAS-SUS Originally proposed for di-jet events generalised up to six jets Perfectly balanced events have αt=0.5 (cut at αt>0.55) Mis-measurement of either jet leads to lower values Graduate lectures, February Page 28

29 All-hadronic SUSY search After cut on αt>0.55 QCD under control Look at other backgrounds Graduate lectures, February Page 29

30 Z-boson background Data-driven background estimate Find a control region in phase space where SM background dominates ν MET ν Use measurements in this region to infer SM background in signal region Z Example Z νν + jets irreducible background Replacement technique µ µ µ ν Z W γ Z ll + jets Strength: very clean Weakness: low statistics W lν + jets Strength: larger statistics Weakness: background from SM and SUSY γ + jets Strength: large statistics and clean at high ET Weakness: background at low ET, theoretical errors Graduate lectures, February Page 30

31 Z-boson background Select γ + 3 jets with Eγ>150 GeV Clean sample S/B>20 Remove photon from the event Recalculate MET Normalise with σ(z+jets)/σ(γ+jets) from MC or measurements CMS-PAS-SUS Graduate lectures, February Page 31

32 Background estimation SUSY signal more central than W, Z and QCD Consequence of s-channel pair-production of heavy particles Try and use this property Graduate lectures, February Page 32

33 Background estimation Use high η as a signal free control region Use this to extrapolate background prediction into central signal region Check behaviour at low HT which is signal free Scan HT and see excess to discover SUSY! Graduate lectures, February Page 33

34 All-hadronic SUSY search Hundreds of SUSY events with Standard Model backgrounds of 10s of events Robust early search with possibility to discover SUSY in 2010 Data-driven methods to control and check background estimates Graduate lectures, February Page 34

35 Summary Overview of collider physics techniques Emphasis on early searches Incomplete! Much more to learn... Graduate lectures, February Page 35

36 Backup: Benchmark points Graduate lectures, February Page 36