High Energy Frontier Recent Results from the LHC. Lecture 2

Transcription

1 High Energy Frontier Recent Results from the LHC University of Heidelberg WS 2012/13 Lecture 2 LHC-Searches I 1

2 LHC: The Energy Frontiere ATLAS = An ToroidaL ApparatuS CMS = Compact Muon Solenoid ~ 2200 authors ~ 2900 authors about 350 publication in last two years! most are Searches for New Phyisics 2

3 ATLAS Detector 3

4 ATLAS Inner Detector 4

5 ATLAS Detector Lar calorimeter 5

6 ATLAS Muon Detector B-field 6

7 CMS Detector Bmagn~ 4 Tesla (inner), 2 Tesla (outer) 7

8 CMS Silicon Detector Tracker Inner Barrel (TIB) 8 Pixel Detector (PD)

9 CMS ECAL (PWO) low light yield and not very radiation hard but very good resolution σe E 0.03 E/GeV PbWO3 9

10 First Collisions: November 23rd

11 First Collisions: November 23rd

12 Motivation for New Physics Searches Hints from astroparticle physics Hints from experimental particle physics Theoretical arguments 12

13 The SM and Astro- Particle Physics Matter-Antimatter Asymmetry in Universe (problem of CP violation) N B 9 10 N Dark Matter Problem (25% of total energy): (rotation curves, formation of galaxies and structures in the universe) Dark energy (70% of energy) (from microwave back-ground anisotropy) Big-Bang (inflation) Nucleosynthesis constraints requires new physics (at high energies?) 13

14 Experimental Hints for New Physics Weinberg Angle Discrepancy NuTev: sin2 W = ± (2003) SM prediction (LEP): sin2 W = ± (2004) about 3 sigma discrepancy Muon g-2 μ= g μ B J μb= a= g 2 / 2 eh 2m a(μ) = (53) 10 3 a(μ)theor = (63) sigma discrepancy DAMA oscillations next page 14

15 Search for Dark Matter Several Search Experiments for Dark Matter WIMP = weakly interacting massive particle DAMA Collaboration reported 8.2 sigma evidence for dark matter candidate based on an annual modulation signature 15

16 Experimental Hints for New Physics Weinberg Angle Discrepancy NuTev: sin2 W = ± (2003) SM fprediction (LEP): sin2 W = ± (2004) about 3 sigma discrepancy Muon g-2 μ= g μ B J μb= a= g 2 / 2 eh 2m a(μ) = (53) 10 3 a(μ)theor = (63) sigma discrepancy DAMA oscillations next page No strong sign for new physics ye!t But Situation would be COMPLETELY different if no Higgs candidate found at LHC, recently! 16

17 Theory Arguments for New Physics (Too) many parameters (25)! Why three generations? Why so different masses (Yukawa couplings) Grand Unification (GUT) couplings? Fine Tuning and Naturalness Problem of the Higgs Mass (MH < Mplanck) Ultraviolet catastrophe at high energies Unification with Gravitation? electromagnetic (γ) α1~1/137 Mechanism of CP violation? weak IA (W, Z) α2~1/29 strong IA (gluon) α3~1/10 17

18 Vacuum Polarisation classical picture e=bare charge 18

19 How can New Physics be tested? Concepts: Look for new or hidden symmetries (model driven) Look for deviations from the Standard model (curiosity of experimentalists) 19

20 Examples for Symmetries QCD: color SU(3) symmetry gluons Number of generations (quarks + leptons) Quark Flavor: isospin SU(3) symmetry of light quarks hadrons Weak Interaction: SU(2) symmetry W,Z boson Higgs spontaneous symmetry breaking Periodic system of elements p,n 20

21 Searches for New Symmetries at LHC Fourth generation quarks (extension of the three generations) Heavy new vector bosons (W, Z ) Left-Right Symmetric Models Search for large extra dimensions (extension of 3D+1 space-time) Search for supersymmetry: fermion-boson symmetry and many more models (symmetries) and many variants... 21

22 Search for Heavy Quarks Heavy Top Quark partners t' or Bottom Quark partners b' Experimental Observables: higher rate of b-quarks Production: g t' t', b' b' W t' b, b' t + Decays: t' b' W+, t Z b' t W, b Z t' t H b' b H - (too high SM background) (strong coupling) (weak coupling) (flavor changing CC and NC decays) Higgs decay channels higher rate of top-quarks (inclusive measurement) specific final state topologies: bbww, ttww, ttzz, bbzz, ttw, bbw, tbz, Backgrounds: Top-pair production, QCD multijets, W-production + jets, Z-production+jets Important tools are b-tagging and top-tagging! 22

23 Top Pair Production Cross Section First Inclusive Measurement Counting all Tops! (limited sensitivity) 23

24 CERN-PH-EP

25 Search for Heavy Top Decays ATLAS results: mass distribution t' mass limit 25

26 Statistics and Limit Setting chi2 fit: yi measurement μi model prediction (nuisance parameter) σi uncertainty (statistical and systematical) 2 2 χ = i ( yi μi ) σi2 chi2 fit with correlated errors: χ 2 = i j ( y i μ i )cov 1 ij ( y j μ j ) covij covariance (error) matrix Parameter Fit: xk model parameter μi = μi (x 1, x 2,..., x n ) χ2 χ min+4 2 χ2min+1 χ2min 2σ 95% confidence level 1σ 68% confidence level xk,0 xk 26 good fit if: χ2min / degrees of freedom ~ 1

27 Example Fit Measurement of some mass (1-Parameter fit) from 4 experiments: M +2σ (95%) ±1σ (68%) confidence level bands -2σ (95%) (could also be a cross section) χ2 χ min+4 2 χ2min+1 χ2min 2σ 95% confidence level 1σ 68% confidence level xk,0 xk 27 good fit if: χ2min / degrees of freedom ~ 1

28 Probability Densities for the above example a Gaussian probability density was used Gaussian (normal )distribution: 2 ( x μ ) 2 2σ 1 f ( x) = e 2 π σ b P(a<x<b) = a f (x) dx used for systematic uncertainties (symmetric) Poisson distribution: e μ μ N P(N ) = N! used for statistical uncertainties Note, Poisson distribution approaches Gaussian distribution for large μ 28

29 Limit Setting Philosophies Bayesian Method: based on the experiment posterior, exclusion limits are calculated low probability models are excluded probabilities are assigned to models using a prior natural method but choose of prior is arbitrary Frequentist Method: based on Monte Carlo toy experiments probabilities are assigned to all possible experimental outcomes exclusion limit is set by that model which excludes this experimental outcome with certain confidence interval computationally expensive and might give unphysical results (e.g. negative cross sections) 29

30 Bayesian Method Model: N = N BG + N Sig (background + signal) σ = σ BG + σ Sig = N / L choose cross section as prior χ2 CL Probability: 1-CL not allowed excluded additional constraint: χ2min σ σ BG σbg σfit σ CL σ>σ CL = σ>σ P e ( χ (σ) χ fit ) 2 σlimit P(σ) d σ BG because σ Sig 0 σ CL = confidence level P(σ) d σ BG 30

31 Choice of Prior Cross sections depend on couplings Alternatively, choose coupling as prior σ Sig = σ 0 α 2 d σ Sig dσ = = 2 σ0 α dα dα αcl αcl σ CL α>0 P(α) d α α>0 P(α)/α d σ σ >0 P (σ) d σ CL = = α>0 P(α) d α α>0 P(α)/α d σ σ >0 P (σ) d σ Sig Sig Results depends on choice of prior! 31

32 Frequentist Method S.Schmitt set limit with 95% confidence level for mu=4.6 experiment has a 5% probability to happen 32

33 Frequentist Method In case of many observables xk a combined discriminator variable is often defined: D = D( x 1, x 2, x 3,... X k ) large discriminator means high probability small discriminator means low probability Often, the output from artificial neural nets or other multivariate methods is used as discriminator variable Background Background +Signal calculate smallest signal with: α < 1 CL α measurement 33 D

34 Problem with Frequentist Method Problem in case of a very small measurement value with P(BG)< (1-CL) would require a negative signal cross section: Background + Signal Background α measurement D unphysical solution! 34

35 CLS Method S.Schmitt CLS > CLSB by definition! 35

36 Another Example CLS Method Definition: CL SB CL S = CL B small Background Background + Signal = 1.0 α=0.1 measurement Background + Signal Background 0.2 = D α=0.1 measurement 0.5 = D Background + Signal large Background α= measurement D

37 Another Example CLS Method Definition: CL SB CL S = CL B small Background = 1.0 Background + Signal α=0.01 measurement Background 0.02 = D Background + Signal α=0.01 measurement 0.05 = large Background Background + Signal α=0.01 excluded at 95% CL D 37 measurement D

38 Search for Heavy Top Decays to be compared to indirect limit from e.g. mixing: Λ >500 TeV λ ij (B-mixing) Frequentists CLS method used 38

39 B-mixing W+ b SM: B0 Fourth Generation: t d B0 t d W- b b W+ d t' B0 d t' W- B0 b unknown couplings! Change of oscillation amplitude 39

40 Reminder Electroweak Symmetry Breaking )( ) ( ) ( cos θw sin θw W 3 Z = A sin θw cos θw B 1 Y Lew = g j W 3 + g ' j B 2 3 L symmetry breaking 1 Y Lelm = g j sin θw A + g ' j cos θw A Y L NC = g j L cos θw Z g ' j sin θw Z 2 3 L The Lagrangian can be extended by right handed VBs Couplings to SM fermions a priori unknown! 40

41 Extending the Electroweak Sector 1 Y '' 3 ' Lew = g j W 3 + g ' j B + g j R W L symmetry breaking 1 Y Lelm = g j sin θw A + g ' j cos θw A Y '' 3 ' L NC = g j L cos θw Z g ' j sin θw Z + g j R Z 2 3 L The Lagrangian can be extended by right handed VBs Couplings to SM fermions a priori unknown! 41

42 Search for Z' Partner of the well known Z-boson Experimental Observables: Production: qq Z' (weak coupling) Decays: Z' jet jet (qq) (light quarks) Backgrounds: QCD two-jets B-pair production Top-pair production Lepton-Pair production (Drell-Yan) Z' bb Z' t t Z' l+ lz' ν ν higher cross section for two-fermion processes (mz' > s1/2) resonance peak in two fermion (two jets) invariant mass (mz' <s1/2) not visible Important tools are b-tagging and top-tagging! 42

43 ATLAS Seach for Z' Lepton-Pair di-muon channel di-electron channel No sign for new physics up to ~ 1 TeV! 43

44 CMS Dijet Search Gaxi q q QE6 q q W' q q Z' q q limit (Z') ~ 1.5 TeV 44

45 Search for ttbar resonance with 1 lepton at CMS t Wb lνb t Wb jjb 45

46 CMS ttbar Resonance Limit Unknown branching ratio included in limit Assumption on resonance width 1.2% use boosted top finder at high mass 46

47 Boosted Top Quark For new heavy states decaying into top quarks, the top quark is highly Lorentz boosted The Top quark is identified by a jet finder as one jet from D.Sosa 47

48 HEPTopTagger Developed by Plehn et al. [arxiv ] and used by ATLAS group in Heidelberg Allows reconstruction of hadronic decaying tops t W+ b j j b no lepton tag! t W- b j j b filtering + substructure from D.Sosa 48

49 ttbar Resonance Search with HEPTopTagger fat jet mass Z' decaying only into quarks 49

50 Search for Large Extra Dimensions There might be more than 3+1 dimensions Higher dimensions compactified (might explain weakness of gravity) Compactification radius R Introduce new mass scale Ms replacing the Planck Scale Higher modes can be excited in compactified extra dimensions (Kaluzza, Klein) Tower of excited Kaluza-Klein states mediate new interactions R looks like Z' Kaluza-Klein Gluons (strong) 50

51 Kaluza-Klein Gluons even stronger limits 51

52 52