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Experimental Particle PhysicsPHYS6011 Performing an analysis Lecture 5

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1 Experimental Particle PhysicsPHYS6011 Performing an analysis Lecture 5
7th May 2009 Fergus Wilson, RAL

2 Data Analysis Basic concepts Monte Carlo methods Signal Backgrounds
Extract physics from data Measure a quantity Search for new particles Basic concepts Monte Carlo methods Signal Backgrounds Errors Statistics 7th May 2009 Fergus Wilson, RAL

3 What do we measure? But in reality: In principle: Quark content Spin
Mass Lifetime Form factor Decay Modes Branching Fraction 7th May 2009 Fergus Wilson, RAL

4 Particle Properties Properties Mass Mass width → Lifetime
Measure momentum and energy: E2 = p2 + m2 Mass width → Lifetime Measure momentum and energy or: How many particles exist after t seconds Branching Fraction Reconstruct the decays and see how many there are. Charge Direction in a magnetic field Spin Angular distribution of decays Structure e.g. Proton/Neutron/Nucleus Scatter particles off the proton and look at distribution 7th May 2009 Fergus Wilson, RAL

5 High Signal:Background Low Signal:Background
Data Flow High Signal:Background Low Signal:Background Trigger and DAQ Event selection Collision Detector Analysis Storage e+ e- ? 7th May 2009 Fergus Wilson, RAL

6 Elements of Analysis Not only Data but... Detector response to signal
Background estimates Errors statistical systematic How to solve? Try and evaluate from data Sometimes need more... Monte Carlo 7th May 2009 Fergus Wilson, RAL

7 Monte Carlo Generate artificial data Simulate detector response
Analyse simulated data as if it were real Response to known input can be calculated Also used in detector design Computer intensive Must be carefully tuned and checked 7th May 2009 Fergus Wilson, RAL

8 Data and Monte Carlo Data MC Reconstruction Trigger and DAQ Collision
Detector Storage Event selection MC Event Simulator Trigger and DAQ Simulator Detector Simulator Analysis MC 7th May 2009 Fergus Wilson, RAL

9 Search for the Higgs Boson
Missing piece of Standard Model Standard Model Higgs theory well understood: Mass is only free parameter Clear predictions to test Most “New Physics” models have something equivalent to a Higgs boson (“MSSM Higgs”, “little Higgs”, etc…). Could be more than one type of Higgs boson Current limit MH>115 GeV (LEP) Particle masses are generated by interactions with the scalar (Higgs) field. Couplings are fixed by the masses. Once MH is known everything is predicted. So by measuring the coupling of the Higgs to particles of known mass we can test theory. 7th May 2009 Fergus Wilson, RAL

10 Search for the Higgs Boson
Direct searches MH > GeV @ 95% C.L. If no new physics up to Planck scale (~1019GeV quantum gravity significant) small mass range for Higgs: 130 < MH< 190 GeV 7th May 2009 Fergus Wilson, RAL

11 How is the Higgs produced? How often is it produced?
Higgs Production First: understand signal How is the Higgs produced? How often is it produced? WW/ZZ fusion gg fusion σ (pb) Associated WH, ZH Associated ttH Gluon fusion most promising 7th May 2009 Fergus Wilson, RAL

12 Higgs Decay Which decay to look at? Depends on Higgs Mass
MH <150 GeV gg → H→γγ H→ZZ* →4l gg → HW,Htt: H→bb H→WW* →2l 2ν qq→qqH : H →γγ, WW* ,τ+τ- MH <500 GeV H→ZZ →4l MH >500 GeV H→ZZ,WW → jets Branching Fraction: If produce 108 Higgs and measure only 20 decays H→gg with an efficiency of % then Branching Fraction: 7th May 2009 Fergus Wilson, RAL

13 Compare to list on previous slide
Best Modes to look at Compare to list on previous slide 7th May 2009 Fergus Wilson, RAL

14 Backgrounds - Tevatron to the LHC
Huge stats for Standard Model signals. 1033cm-2 s-1 ~109 events/10 fb-1 W (200 Hz) ~108 events/10 fb-1 Z (50 Hz) ~107 events/10 fb-1 tt (1 Hz) (10 fb-1 = 1 year of LHC running at low luminosity 1033 cm-2 s-1, hence by ~end 2010) Background is anything with signature similar to signal W+X (X can be W, Z or just 2 QCD jets) ZZ qql+l- (one lepton not identified) τ+τ- b-tags can be real, charm or fakes 7th May 2009 Fergus Wilson, RAL

15 Current Results - Tevatron
At 120 GeV Hbb dominates Signature ggHbb: 2 jets One or two b-tags Swamped by dijet production bb ~ μb qq ~ mb (fake b-tag rate small but not zero) Have to use W/Z+H channel (Associated Production) 7th May 2009 Fergus Wilson, RAL

16 Tevatron/CDF - Associated Production
Wqq 70% final state qqbb Four jet backgrounds still too large W eνe 10% W μνμ 10% Final state lνbb One electron or muon Missing transverse momentum Two jets One or two b-tags Easy to select in trigger and offline σ×Br ≈ 0.02 pb qqWH with Hbb W* H W q b 7th May 2009 Fergus Wilson, RAL

17 Efficiency at the Tevatron/CDF
Nature provides 20 fb of WHlνbb events – a handful per year How many pass CDF trigger and analysis selection? Cleanly identified electron or muon in acceptance Two jets At least one b-tag Large missing momentum None overlapping Run thousands of MC events Efficiency Observe 2 per fb-1 per year 7th May 2009 Fergus Wilson, RAL

18 How do we report this result?
Statistical Mostly counting events (data or MC) Poisson distribution: σ = √μ ≈ √N NB fractional error ~ 1/√N Efficiency follows binomial distribution: Systematic Anything not completely understood may affect result Detector performance, background rates, Monte Carlo modeling… Estimate range of parameter Vary in Monte Carlo 7th May 2009 Fergus Wilson, RAL

19 Significance In a given amount of data we expect: NB background events
Statistical error on background ≈ √NB Systematic error on background = σsys Add errors in quadrature to get σTOT Observe N(>NB) events in data. Could be: random fluctuation in NB ± σTOT background events NB background events & NS signal events Significance S = NS/σTOT S = 3: probability of fluctuation ~10-3 – interesting... S = 5: probability of fluctuation ~10-5 – discovery!! 7th May 2009 Fergus Wilson, RAL

20 Latest CDF/D0 Higgs Results (March 2009)
Look for H→WW (dominant above 135 GeV) and W→ lυ (W→bb not used as too much background). 7th May 2009 Fergus Wilson, RAL

21 Predicted Sensitivity – Tevatron v LHC
CDF expects a maximum of 6.5 fb-1 by 2009 15-20 signal events 2000 background 8 years of running — 5s Discovery % C.L. exclusion 1 month of running  Ldt (fb-1) needed per experiment L (fb-1) 6 fb-1 ATLAS + CMS preliminary mH (GeV) Compare to actual result on previous page mH (GeV) 7th May 2009 Fergus Wilson, RAL

22 Ruptured bus-bar interconnection
7th May 2009 Fergus Wilson, RAL 04/12/2018 LHC status and commissioning 22

23 Damage: magnet displacements
QQBI.27R3 7th May 2009 Fergus Wilson, RAL 04/12/2018 LHC status and commissioning 23

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