Physics at the LHC. Academic Training, Part 3: Experiment and Standard Model

Transcription

1 Physics at the LHC Academic Training, Part 3: Experiment and Standard Model Dr. Sven Moch, DESY Zeuthen (Theory) Dr. (Experiment) June 2007 DESY Zeuthen, based on a lecture in WS 06/07 at Humboldt University Berlin 1

2 Plan LHC: Experimental Overview and Standard Model Physics Monday, June 11, 2007, LHC: Higgs Searches and Physics beyon the Standard Model Thursday, June 14, 2007,

3 Contents Discoveries at the energy frontier Overview: LHC and its experiments Hadron-Hadron interactions Jets: fragmentation, signature and algorithms Standard model channels at LHC top-physics and heavy flavor reconstruction Higgs: event topologies and search strategies SUSY: searches in the MSSM 3

4 Hadron-Hadron-Colliders over the world 4

5 Physics at the Energy Frontier 5

6 Cross Sections at Hadron Colliders with LHC a new energy region is accessible Hadron-Colliders are discovery machines 6

7 Evetns at Hadron Colliders background (frequent): strong interaction quarks, gluons ( LHC: ~ 20 p p collisions per bunch-crossing ) intresting events (rare): electroweak processes, decay of heavy particles high energetic (high-pt) leptons 7

8 Comparision Proton/Antiproton-Proton Scattering low energy: valence-quarks dominating the hard scattering: Proton / Antiproton > Proton Proton high energy: sea-quarks and gluons dominating the hard scattering: Proton / Antiproton = Proton Proton 8

9 Interaction Rates at LHC s 25 n dn/dt = L x σ (pp) 109 Hz Extremly good detectors and event selection necessary 9

10 Cross Section and Luminosity N =σ L event rate: Background strong IA p total inelastic cross section p 2 σ 10 fm 10 Signal point-like cross section σ 34 2 L=10 /cm /s 100 particles / collision 25 electroweak IA 2 α 36 s 10 cm huge background! particles /s cm

11 Hadron - Hadron collisions 11

12 Energy in the Center of Mass System enough energy to produce new particles with masses up to 1 TeV! gluon p p quark rough estimation: relevant Center of Mass energy needed: s 1 1 s 2 3 s ' ' of the colliding partons (q, g) s TeV 14 TeV 12

13 Kinematic Variables boost of the c.m.s. along the beam axis is unkown Lorentz Invariant variables needed transverse momentum pt (LI) energy E longitudinal momentum pl momentum p polar angle θ azimuthal angle φ (LI) Lorentz Invariant polar angle distribution rapidity y: if (as usual..) m << E, pt pseudo rapidity η 1 E pl y= ln 2 E pl η= ln tan 2 Usually, the invariant variables Δη = η1- η2 and Δφ = φ1- φ2 are used 13

14 η/φ Distribution in the Detector direction of particles η/φ Rotation: Δφ = const. Boost: Δη = const. distance measurement: 2 2 R= Δ Δη (η/φ) particle/jet grid in the detector 14

15 Rapidity and Pseudorapidity The rapidity and the azimuthal angular distributions are correlated. 15

16 Missing Transverse Momentum/Energy masses small: Energy = momentum transverse momentum measurable for all visible particles invisible particles: small angles: escaping into beam pipe, but pt small neutral particles: neutrinos, neutralinos, gravitinos,. balanced momentum: calculate missing transverse momentum/energy mis pt = pt E T = pt invis vis invis Example: W μ ν 16

17 Experiments at LHC CERN and LHC Experiment Atlas Experiment CMS Event topologies Signatures of new physics at LHC Measurement of particles 17

18 Centre Europeen de la Recherche Nucleaire 18

19 The ATLAS experiment and the LHC: Physics at the TeV - scale 19

20 The Large Hadron Collider 20

21 LHC superconducting Magnets 21

22 new physics at LHC? supersymmetric particles: chargino : (partner of W) neutralino : (partner of Z, g) smuon : (partner of muons) 22

23 new physics at LHC? Higgs boson: 23

24 The ATLAS - Experiment ATLAS is bigger than the office - building total diameter barrel toroid length total length total weight 25 m 26 m 46 m 7000 t 24

25 ATALS: schematic Overview 25

26 Particle Measurement at ATLAS Tracking : - high resolution HLT - TRT (e/π separation) Energy measurement: - EM : Pb-LAr - HAD: Fe/Szint. (cent), Cu/W-LAr (fwd) Muon spectrometer: Toroid with streamer tubes 26

27 Event Rates and Multiplicities cross section of p-p collisions σtot(14 TeV) 100 mb σinel(14 TeV) 70 mb R= LHC cm energy (GeV) With every bunch crossing 23 Minimum Bias events with ~1725 particles produced R = event rate σinel N Δt = luminosity = 1034 cm-2 s-1 = inel. Cross section = 70 mb = interactions / bunch crossing = bunch crossing interval = 25 ns x σinel = 1034 cm-2 s-1 x 70mb = Hz N = R / t = s-1 x s = 17.5 = 17.5 x 3564 / 2808 (not all bunches filled) = 23 interactions / bunch crossing (pileup) nch = charged particles / interaction Nch = charged particles / BC Ntot = all particles / BC nch 50 Nch= nch x 23 = ~ 1150 Nto= Nch x 1.5 = ~

28 ATLAS Trigger: Overview hardware 3-Level Trigger System: 1) LVL1 decision based on data 2.5 µs from calorimeters and muon trigger chambers; synchronous at 40 MHz; bunch crossing identification software 2) LVL2 uses Regions of ~ 10 ms ~ sec. Interest (identified by LVL1) data (ca. 2%) with full granularity from all detectors 3) Event Filter has access to full event and can perform more refined event reconstruction 28

29 Interface to HLT: RoI Mechanism LVL1 triggers on (high) pt objects L1Calo and L1Muon send Regions of Interest (RoI) to LVL2 for e/γ/τ-jet-µ candidates above thresholds LVL2 uses Regions of Interest as seed for reconstruction (full granularity) only data in RoI are used advantage: total amount of transfered data is small ~2% of the total event data can be dealt with at 75 khz EF runs after event building, full access to event 29

30 HLT Selection Strategy fundamental principles: Example: Dielectron Trigger 1) step-wise processing and decision inexpensive (data, time) algorithms first, complicated algorithms last. 2) seeded reconstruction algorithms use results from previous steps initial seeds for LVL2 are LVL1 RoIs LVL2 confirms & refines LVL1 EF confirms & refines LVL2 note: EF tags accepted events according to physics selection ( streams, offline analysis!) ATLAS trigger terminology: Trigger chain Trigger signature (called item in LVL1) Trigger element 30

31 The CMS-Experiment at LHC 31

32 CMS Detector 32

33 Particle reconstruction at CMS 33

34 Supersymmetric Particles and dark Matter The neutralino is a good candidate for dark matter in the universe. LHC discovery potential Time of data taking 1 month 1 year 3 yeras Upper limit Massen region ~ ~ ~ ~ 1.3 TeV 1.8 TeV 2.5 TeV 3 TeV neutralino masses measurable at LHC: discovery of SUSY and measurement of the neutralino masses at LHC could solve the problem of cold dark matter in the universe. 34

35 Example: a possible Higgs-Event in ATLAS H ZZ 4 g t g H Optimal channel for the discovery of the Higgs-boson at LHC e, µ e, µ Z(*) e, µ Z e, µ mz expected Higgs-signal after 1 year of data taking simulation of a H µµ ee event in ATLAS 35

36 The same Higgs Event in CMS 36

37 The Higgs Event in the CMS tracker (r,φ) 37

38 The Higgs Event in the CMS Tracker (r,z) 38

39 Realistic Event Topology of the Higgs Event More than 20 overlayed pp-interactions (pile up) 39

40 Realistic Higgs Event in (rz)-projection 40

41 Higgs Event in CMS: Total overview 41

42 Hadronic Cross Sectios partonic cross section (calculation in perturbative QCD (lecture of SM)) Parton-Density-Functions (measurements of HERA) models of fragmentations (cluster-, string-, and independent - fragmentation) formation of Jets 42

43 Hadron-Hadron Collisions perturbative calculation of point like partonic cross section renormalization scale μr = Q non-perturbative, but PDF are universal, take from other measurements (as HERA) factorization scale μf = Q Final state, formation of jets by hadronisation 43

44 Cross Section Measurement in pp wanted: differential cross section for a certain variable V at a certain Q2 calculable: partonic cross section for two interacting partons i,j 2 dσ FS s,q dv ij 2 dσ FS x i, x j,q dv 2 PartonDensityFunction known form other experiments: 2 f i x i,q ij 2 dσ FS x i, x j,q 2 dσ FS s,q = dx i dx j f i x i,q 2 f j x j,q dv i,j dv Factorisation! 44

45 From Parton-Density to Fragmentation 45

46 Hadronisation is Fragmentaion formation of jets non-perturbative process, model-dependent string-model 46

47 Formation of a Jet from parton interaction to energy measurement in the calorimeter 47

48 Jets jet definition jet reconstruction di- and multi-jet events standard model processes with jets at LHC searches for new particles with jets 48

49 Jet-Definitions detector - level parton - level 49

50 Jets: General Request hadrons: experiment LHC: about 100 particles per jet, 2 5 jets per event Jets are sensitive to the fundamental hadronic process (direction/energy) Good agreement of theory and experiment needed: jet-algorithms parton (quark/gluon) : theory Inelastic scattering of hadrons: beam of colliding partons c.m.s. energy boost unknown: lorentz invariant jet variables needed! rapidity, transverse momentum, azimuthal angle 50

51 2-Jet event at CDF (TEVATRON) 51

52 Cone - Algorithm Sum over all calorimeter activities within a certain cone of radius R around a high energetic cluster cut criteria for jet-cone: R= Δ 2 Δη 2 y cut y(cut) typically problem: overlaying jets 52

53 Jet-rates as Function of y(cut) LEP-data 53

54 Jets in the Calorimeter of CDF 2 jet event 2 jet event cone algorithm, with R < jet event 5 jet event 54

55 Jets in the Tracking system of CDF 2 jet events same events as before Clear separation partially difficult! 3 jet event 5 jet event 55

56 Multi-Jet Events CDF-data, R(cone) < 0.7 good test of QCD measurement of αs by measuring of the relative fraction of events with 2, 3,.. Jets 56

57 KT - Algorithm Make a list of all hadrons (= calo clusters) 2 d i = pt,i Calculate for each cluster: Check for each cluster pair d ij =min where is pt,i, pt, j R i, j R i, j = Δη i, j Δφi, j d i, j d i? Yes: add clusters i and j to the same jet No: put cluster i on the list of new jets repeat up to the list is finished 57

58 Cone-Algorithmus vs. KT - Algorithmus 58

59 Jet Algorithms for LHC, Examples from ATLAS distance between both partons in the example of W jj in the (η,φ)-plain ET of jets in comparison to ET of partons (ratio) in the example of W jj, comparison of different algorithms 59

60 W-Mass Reconstruction of Di-Jet Events (LHC) W j j H W W l υl j j W j j 60

61 Reconstruktion of Higgs and Z0 pp hh bb bb pp WH bb l υ pp ττ j υτ l υl υτ 61

62 Some glances on Standard Model Physics.. Heavy Flavours reconstruction methods and b-flavoured Jets W Production Top Physics: decay channels, Mass and Cross Section 62

63 Impact-Parameter in b-flavourd Jets 63

64 Reconstruction of Decay Vertices Experiment D0 (Tevatron): Z bb 64

65 Reconstruction of the Decay Length decay length in the lab-system: L=ctßγ 65

66 Discovery of the Top-Quark 66

67 Top-Decay Channels Top production at TEVATRON dominated by quark antiquark processes, At LHC by gluon-gluon interactions, decay channels are the same. 67

68 Observation in the Muon-Channel Lepton + Jet channel: 1 L + 2 J + 2 b-j + 1 MET 68

69 Observation in the Electron-Channel 2 Lepton channel: 2 L + 2 b-j + 2 MET 69

70 Observation based on Jets Jet channel: 4 J + 2 b-j 70

71 Mean Value of the Top-Mass (TEVATRON) 71

72 Cross Section Measurement 72

73 Mean Value of Top-Production Cross Section 73

74 Connection of W- and Top-Masse 74