Foundations of Particle Physics: CERN, LHC, ATLAS

Transcription

1 Foundations of Particle Physics: CERN, LHC, ATLAS Richard Teuscher Institute of Particle Physics University of Toronto (1) Physics Motivation (2) CERN (3) LHC (4) ATLAS (5) Movie 1

2 Recap: The Standard Model (SM) 2

3 Questions Do the fundamental forces of nature unify? What is the origin of mass? What is dark matter made of? Einstein / World Year of Physics 3

4 Steps to Unification Famous example (1864) J.C. Maxwell: Theoretical unification of electricity and magnetism (electromagnetism) Glashow, Salam, and Weinberg: Theoretical unification of electromagnetic and weak nuclear interactions (electroweak). (Nobel Prize in Physics 1979) Rubbia (spokesperson UA1 experiment) + van der Meer (accelerator physicist): Discovery of the W and Z bosons at CERN (Nobel Prize in Physics 1984). 4

5 Nobel Prize 2004 Wilczek, Politzer, Gross: Theoretical discovery of asymptotic freedom in the theory of the strong interaction strong nuclear force decreases at higher energies. Strong coupling constant α 3 (Q) α 3 is the gauge coupling in in QCD (Quantum Chromo Dynamics = theory of strong force) Also known as α s = alpha strong 5

6 Q1: Do these Forces Unify? 1 / α 1 1/ α 1 1/ α 2 1/ α 3 Standard Model World Average (GeV) Physics at electroweak scale 10 2 GeV well-understood. 1 / α 2 α 1 = Electromagnetic coupling α 2 = Weak coupling 1/ α 3 α 3 = Strong coupling SUSY World Average Can we extrapolate the gauge couplings of the Standard Model to the Planck scale GeV? No unification. If Supersymmetry is added, the forces unify at about GeV. A hint that we re on the right track? GeV) (more on gravity later) 6

7 Q2: What Gives Particles Mass? The Higgs Mechanism To understand the Higgs mechanism, imagine that a room full of physicists chattering quietly is like space filled with the Higgs field a well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field... 7

8 Q2 : What Gives Particles Mass? The missing piece of the SM is the Higgs (H) Spin-0 particle forming a scalar field everywhere in the universe Example of a scalar field (temperature) Coupling of the Higgs field to a particle generates its mass n.b. p/n mass from gluon fields, quark motion Breaks electroweak symmetry: W,Z massive Higgs properties predicted except its own exact mass We know it must lie in the range 114 GeV < M H < 1.2 TeV Previous direct searches at CERN Precision electroweak data (CERN, Fermilab) Higgs 8

9 Q3: What is Dark Matter? Planets, stars, H/He, Matter-Energy Budget of Universe make up ~5% of the universe. Heavy Elements 0.03% Evidence for dark matter: galaxy rotation Neutrinos 0.3% Observed Dark Energy 75% Stars 0.5% H He 4% Dark Matter 25% Cold Dark Matter Candidate: SUSY Neutralino χ Another hint for SUSY? Expected 9

10 2. CERN Founded in 1954, CERN (European Council for Nuclear Research) in Geneva, Switzerland is now the largest particle physics laboratory in the world. 10

11 WWW was invented at CERN The World Wide Web was invented in 1989 by researchers at CERN. It began as a way of sharing Information amongst physicists The first Web site. Inventor Tim Berners- Lee meets UN Secretary General in Geneva, October, 2003 CERN also mentioned in Dan Brown s book Angels and Demons. To see what s false / true: 11

12 CERN Users

13 Age Distribution of CERN Users Age Distribution of CERN Users All Countries Age May

14 1984 Nobel Prize at CERN Discovery of W boson p p _ W + debris W e +ν Neutrino signal is missing energy pp Z Z + debris e + + e - 14

15 2. The LHC 15

16 The Large Hadron Collider (LHC) Questions about unification, dark matter, and the Higgs should be answered by probing physics at the TeV scale To probe physics at ~ 1 TeV in p-p collisions, need: E(quark) > 1 TeV E(proton) > 6 TeV LHC p-p collider: 7 TeV x 7 TeV High luminosity: Design luminosity of L=10 34 cm -2 s -1 Physics Rate = L x nb. σ ~ 1/s -> 2*s requires factor 10 in L Design 100 fb -1 / year per experiment Luminosity cm -2 s -1 Luminosity [cm -2.s -1 ] 1.E+35 1.E+34 1.E+33 1.E+32 1.E+31 1.E+30 ISR LEP2 LEP1 HERA SppS 7 x TeVatron LHC 100 x Center-of-mass energy [GeV] s Energy (GeV) 16

17 LHC First collisions 2007 pp collider 14 TeV 100 fb -1 / y / experiment 25 ns bunch spacing ~ billion collisions / s 9T Supercond. magnets 26.7 km ring Energy (2 LHC beams) = 11 GJ = 7% of energy stored in an aircraft carrier at 30 knots 17

18 Alice Heavy-ion programme Atlas General-purpose detector 4LHC Experiments CMS General-purpose detector LHCb Dedicated b-physics 18

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20 LHC Status: First superconducting dipole magnet lowered March 7, 2005 ~1000 of 1232 dipoles delivered to CERN 20

21 LHC Schedule LHCC Review March 05 Jan-Mar. 2007: cooldown. All 8 LHC Sectors July 07: LHC commissioning finished Ready for beam. Initial L = 6 x 1031 cm-2 s-1 to ~ 1032 cm-2 s-1 ~1% of Lmax for the LHC, as in SppS and TeVatron early runs Then 3 month shutdown? Then ~7 months L = 2 x 1033 cm-2 s-1 End 2008: 1 to 10 fb-1 per experiment

22 3. LHC Physics 22

23 Production Rates for LHC at L = cm -2 s -1 (10% design Luminosity) Process Events/s Events / year (10 fb 1 ) W eν Z ee SM Higgs SUSY Exotics tt bb QCD jets (p T > 200 GeV) H (m =130 GeV) g ~ g ~ (m = 1 TeV) Black holes m > 3 TeV (M D =3 TeV, n=4) Wealth of physics: LHC is a b-factory, top factory, W/Z factory, Higgs factory, SUSY factory, 23

24 The Price: Pileup Total event rate: R = L x σ inelastic (pp) cm 2 s 1 x 70 mb 700 MHz ~ 25 inelastic minimum bias low-p T events produced on average in each bunch crossing of 25 ns pile-up e.g. Golden Higgs channel: H ZZ 4µ Reconstructed tracks p T >25 GeV 24

25 SM Higgs Production at LHC 4 processes: gg fusion tt fusion W,Z Bremsstrahlung WW,ZZ Vector-Boson Fusion 25

26 Search Channel Depends on M H bb W W ZZ 26

27 Intermediate Mass Higgs 130 < M H < 500 GeV + H ZZ l l l l ( l = e, µ ) 4 high p T leptons golden channel Narrow mass peak, small background + Entries/ 0.25GeV Corrections: BREM+eISO+Presampler mzz Entries χ / ndf / 14 Prob Constant ± Mean 130 ± Sigma ± H->ZZ->4e Entries/0.5 GeV 600 H->ZZ->µµµµ 400 σ = 1.4 GeV σ=1.4 GeV ZZ Invariant Mass (GeV) m ZZ (GeV) m ZZ (GeV) m µµµµ (GeV) 27

28 Strategy for Higgs Discovery Signal significance Signal Significance 5σ ATLAS 30 fb -1 LEP excluded L dt = 30 fb -1 (no K-factors) ATLAS H γ γ tth (H bb) H ZZ (*) 4 l H WW (*) lνlν qqh qq WW (*) qqh qq ττ Total significance LHC can probe full range of allowed Higgs masses. 5σ observation possible in first year at L = 2x m H (GeV) m H (GeV) Eur.Phys.J. C32S2 (2004)

29 Extensions to SM: Supersymmetry Recall: symmetry between fermions and bosons Q boson> = fermion> Q fermion> = boson> All SM particles have SUSY partners differing by spin ½ e.g. quark <> squark Deeper than just doubling # of particles SUSY is maximal extension of Poincaré group: 4D spacetime translations 3D rotations Lorentz group Local SUSY implies gravity (msugra) Spin 1 2 Spin 0 q q~ Spin1/ Spin 0 : g, Spin : g~, 1 2 l, ν ~ l, ~ ν W ~ χ ± ± 1,2 Two Higgs Doublets : h : :,, 0 H ±, H 0 γ, Z, H ~ 0 χ, A 0, H ± 0 1, H 1,2,3,4 0 2 Corrections from SUSY particles cancel divergences to Higgs mass 29

30 Strategy for SUSY Discovery at LHC SUSY search channels: Large E T miss ; Large jet multiplicity; Large E T sum. Background estimation crucial Estimations from data. pp gg ~ ~ cascade Missing E T 5σ ATLAS decay jets (R-Parity conserving) 30

31 Signal of a SUSY Discovery Powerful background rejection using effective mass : M eff = E miss T + 4 i= 1 p T (jet i ) (GeV) dσ/dm eff (mb) M SUSY (GeV) msugra : 5 parameters M eff (TeV) Peak position correlated to M SUSY Area ~ SUSY cross-section M eff (GeV) M eff (GeV) Effective mass tracks SUSY scale Constrained MSSM with 15 parameters 31

32 SUSY at LHC ATLAS 5σ discovery curves Large cross-sections for squark and gluino production at LHC With L= 10 33, mass reach: M ~ 1.3 TeV in 1 week M ~ 1.8 TeV in 1 month M 1 / 2 (GeV) M ~ TeV (ultimate 300 fb -1 ) Main limitation not statistics but understanding the detectors SUSY may be the first discovery beyond the SM at the LHC M 0 (GeV) 32 M 0, M 1/2 mass of SUSY particles at GUT scale.

33 Many other physics searches Excited quarks: q* qγγ, up to ~ 6 TeV Leptoquarks: X l + q, up to ~ 1.5 TeV Compositeness: dijets up to 40 TeV Lepton flavour violation: τ µγ up to 10-6 to 10-7 Monopoles, 4 th generation fermions, extra dimensions 33

34 One Possibility: Extra Dimensions Graviton Bulk Graviton The weakness of gravity compared to the SM forces could be explained by extra dimensions beyond 4D, inspired by string theory. Gravity leaks into the extra dimensions and appears to be weaker. A gravity (string) scale of M S ~ 1 TeV is then possible. A solution to the hierarchy problem! M Planck2 ~ M S n+2 R n n, R = number and size of extra dimensions 4 D Standard Model n x-dim If Ms ~ 1 TeV: n = 1 -> R = m (excluded by macroscopic gravity) n = 2 -> R = 0.7 mm (limit of small-scale gravity expts) Extra dimensions compact over R < mm R 34

35 Black Hole Production at LHC If R < mm M s ~ TeV Spectacular signature 35

36 4. The ATLAS Experiment 36

37 Basic Detector Components Tracking chamber Electromagnetic calorimeter Hadronic calorimeter Muon chambers photons e± muons π±, p neutrons Interior Exterior Hadros = Greek for strong 37

38 ATLAS (A Toroidal LHC ApparatuS) 2 T Solenoid Hadronic Tile calorimeter Muon spectrometer Toroid magnets p 7 TeV p 7 TeV 22 m Inner Detectors Electromagnetic barrel calorimeter 46 m Total mass ~ 7000 tonnes Forward calorimeter Endcap calorimeter 38

39 Scale of ATLAS 92 m ATLAS assembled 92 m below ground at CERN 39

40 ATLAS Calorimetry LAr EM Barrel LAr EM Endcap (EMEC) Extended Barrel Barrel Extended Barrel LAr Hadronic Endcap (HEC) LAr forward Calorimeter (FCAL) Hadronic Tile Calorimeter (TileCal): - Physics: Jets + Missing E T - Scintillating tiles orthogonal to particle trajectories - Iron plates (82%) + Scintillating Tiles (18%) - 3 Longitudinal samples: λ TileCal Front-End Electronics 40

41 Operation of the TileCal: Measure light produced by charged particles in plastic scintillator. PMT WLS fiber Front End Electronics 7λ Φ Particles Plastic scintillator inside steel absorber structure 2 fibres / scintillator. Bundle fibres to form cells of 0.1 x 0.1 (η,φ) 2 PMT sper cell PMT s 41

42 Scintillating tiles production Granulated polysterene Pressing of tiles Mixing components tiles ; 80 tons Done by injection molding (2min/tile) 3 mm thick tiles in 11 sizes Polystyrene + PTP (1.5 %) + POPOP (0.4 %). Peak of light emission at 420 nm. Tiles finished 42

43 TileCal Mechanics and Optics Mechanics Laminated steel Epoxy glue + press submodule Assembled Module Optics 43 Insert tiles in module Insert profiles with fibres Fibre routing

44 One TileCal Barrel Module ~ 20 Tonnes 44

45 TileCal PMT Amplifier Card plugged into anode of each PMT cards on detector Analogue + digital components Performs shaping of PMT signal Signal then digitized by 10-bit ADC s on detector Bigain output high gain (0-10 GeV) low gain ( GeV) Effective dynamic range of 16 bits Also performs calibration functions Example of calibration pulse ADC Counts Digitized Samples (25 ns bins) 45

46 5. ATLAS Commissioning and Physics 46

47 TileCal Commissioning in ATLAS Pit Record calibration data from detector, measure noise levels, debug electronics Calibration Pulses OK Photo from Sept OK: uninstrumented channels Noise Test OK RMS (ADC counts) Channel 47

48 Fall 2004: Lowering EM Barrel A-Frames = final LAr supports Alignment Tooling (during insertion) Completion of Tile Cal 48

49 December 2004 TileCal + Liquid Argon EM barrel assembly in ATLAS pit completed. 49

50 Next Step: Commissioning ATLAS with Cosmic-Ray Muons ~ 5 million cosmic muons enter the ATLAS cavern in 15 minutes 50

51 First Cosmic Ray Data Not a simulation! June 21: first cosmic ray data from ATLAS pit Barrel hadronic Tile Calorimeter (TileCal) custom trigger on back-toback cosmic rays no external scintillators Test full chain from frontend electronics to offline software. See Nature Article 14 July

52 The Road to ATLAS Physics 1: Testbeam (past years 2004) 2: Installation, Cosmic Ray Commissioning (Now) 3: Single beam 4: First LHC collisions 5: First Physics

53 Video about CERN 53

54 Constructing ATLAS Episode I 54

55 Links CERN Particle Data Book 55