Higgs rush at CERN. Frank Filthaut Radboud University Nijmegen / Nikhef

Transcription

1 Higgs rush at CERN Frank Filthaut Radboud University Nijmegen / Nikhef

2 Contents: Particle physics: general picture The Higgs boson The recent results

3 Particle Physics: the General Picture

4 Going beyond the naked eye Antoni van Leeuwenhoek, : invention of the microscope discovery first bacteria ( kleine beestjes ), μm E. coli (size ~ 1 μm) Minimum discernible dimensions ~ λ limit when using visible light: 0.5 μm improvement to ~ 1Å posssible using STM, AFM 4

5 The atom cracked Idea: use particles to see smaller structures Geiger, Marsden,1909 (working for Rutherford): scattering of α-particles ( 4 He nuclei, Eα 3 MeV) off a gold foil Quantum mechanical translation: de Broglie wavelength λ ~ h/p Planck s constant projectile momentum diffuse charge distribution (Thomson) charge distribution with nucleus (Rutherford) Repeated in the 60 s with scattering of 180 MeV electrons on protons the proton (r ~ 1 fm) contains further sub-structure (quarks)! 5

6 State of the art Present scheme in CERN s Large Hadron Collider: accelerate proton beams to energies of 3.5 TeV per proton v/c (energy to be doubled in 2014) in both directions! make them collide in the centres of the detectors experiments analyze outcome of collisions and select interesting events stochastic process, no control over outcome of individual collision can only select after the fact 6

7 Quantum Electrodynamics Einstein (1905): photo-electric effect particle nature of light Paradigm change! EM interaction described as photon exchange p!%# i p!"# i q p!%# $ p!"# $ Graphical representation: Feynman diagrams (intuitive way to compute outcome of scattering processes in QM) 7

8 Techniques High energy allows for the creation of other, usually short-lived particles τ < s for interesting particles in collisions: convert kinetic energy into mass in decay processes: reconstruct mass of the decaying particle 8

9 Techniques High energy allows for the creation of other, usually short-lived particles τ < s for interesting particles in collisions: convert kinetic energy into mass in decay processes: reconstruct mass of the decaying particle Entries / 1 MeV Minimum Bias Stream, Data 2009 ( ATLAS Preliminary Invariant Mass Gauss (+poly) fit µ = ± 0.1 (stat) MeV = 3.2 ± 0.1 (stat) MeV PDG (2009) m = ± MeV s=900 GeV) Both tracks: p > 100 MeV, Si hits > 6 T cos( ) > 0.8, flight distance > 0.2 mm Data Simulation 200 mλ m p [MeV]

10 The Weak Interaction Exchange / production of heavy particles! W-boson production (and decay) ū d W l l W and Z bosons are heavy! MW = (25) GeV (~ Sr, Kr) MZ = (2) GeV (~ Ru) Discovered in p-p collisions, ECM = 630 GeV e ± Most collisions between protons involve the strong interaction look for leptons (only EM and weak interactions) 9

11 The Weak Interaction Exchange / production of heavy particles! e Z-boson production and decay q q Z, l l! W and Z bosons are heavy! MW = (25) GeV (~ Sr, Kr) MZ = (2) GeV (~ Ru) Discovered in p-p collisions, ECM = 630 GeV e ± Most collisions between protons involve the strong interaction look for leptons (only EM and weak interactions) 9

12 The Higgs Boson

13 Leptons?? particles not involved in the strong interaction (only weak, EM) also heavier counterparts of the electron and its neutral partner, νe Quarks: particles also susceptible to the strong interaction again, 3 generations involving heavier partners than the u, d that are constituents of the proton Force carriers: The Particle Family photon (EM), W/Z bosons (weak interaction), gluon(s) (strong interaction) The only missing particle: the Higgs boson 11

14 Rotational Symmetry The Universe at large is isotropic and homogeneous! 12

15 Rotational Symmetry The Universe at large is isotropic and homogeneous! CMWB: temperature fluctuations ~ 10-5 K WMAP 7-year results, full sky 12

16 Extended Symmetry Rotational symmetry: laws of physics do not depend on any direction Symmetries are important in many areas of physics e.g. conserved quantities like angular momentum in the case of rotational symmetry In particle physics, this idea is extended to internal symmetries that can turn particles into one another the origin of our description of all (EM, weak, strong) interactions but this symmetry must be broken! 13

17 The Higgs Mechanism Particles become effectively massive by means of their interaction with the Higgs field! More physical analogy: refractive index caused by different speed of light in medium caused by forward scattering of light by the medium Photons in the medium are effectively massive 14

18 The Higgs Mechanism Particles become effectively massive by means of their interaction with the Higgs field! More physical analogy: refractive index caused by different speed of light in medium caused by forward scattering of light by the medium Photons in the medium are effectively massive 14

19 Magnetic Analogues Spontaneous symmetry breaking equivalent ground states excitation 15

20 Magnetic Analogues Spontaneous symmetry breaking equivalent ground states Massive photons excitation Meißner effect: superconductor repels magnetic field lines massive photons but needs a medium (e - pair condensate)! In the particle physics case, the medium is the vacuum! 15

21 LEP Heritage & Electroweak Constraints MH unknown, but for given MH all Higgs boson properties fixed know exactly what to look for LEP: mh > GeV + a wealth of EW constraints... now dominated by Tevatron W W H W t b W W m W [GeV] August 2009 LEP2 and Tevatron (prel.) LEP1 and SLD 68% CL 80.3 α m H [GeV] m t [GeV]

22 LEP Heritage & Electroweak Constraints MH unknown, but for given MH all Higgs boson properties fixed know exactly what to look for LEP: mh > GeV + a wealth of EW constraints... now dominated by Tevatron W W H W t b W W Focus on a (relatively) light Higgs boson! m W [GeV] August 2009 LEP2 and Tevatron (prel.) LEP1 and SLD 68% CL 80.3 α m H [GeV] m t [GeV]

23 The Recent Results

24 The LHC: a Success Story! 18

25 The LHC: a Success Story! ] -1 Total Integrated Luminosity [pb ATLAS Online Luminosity LHC Delivered ATLAS Recorded -1 Total Delivered: 48.1 pb -1 Total Recorded: 45.0 pb s = 7 TeV 0 24/03 19/05 14/07 08/09 03/11 Day in

26 The LHC: a Success Story! ] -1 Total Integrated Luminosity [pb ATLAS Online Luminosity LHC Delivered ATLAS Recorded Total Delivered: 48.1 pb Total Recorded: 45.0 pb -1 ] -1-1 Total Integrated Luminosity [fb s = 7 TeV /03 19/05 14/07 08/09 03/11 2 Day in Expectations for 2011 exceeded by a factor 5 7 ATLAS Online Luminosity s = 7 TeV LHC Delivered ATLAS Recorded -1 Total Delivered: 5.61 fb -1 Total Recorded: 5.25 fb 0 28/02 30/04 30/06 30/08 31/10 Day in

27 Experimental conditions We need this performance! interactions at hadron colliders dominated by strong interaction when searching for Higgs boson production, need to suppress backgrounds by ~ Look for striking signatures setting the Higgs boson apart from more ubiquitous background processes 1 pb = cm 2 s ECM now 19

28 H W + W - Relatively large event rate, but leptonic W boson decays lead to unobserved neutrinos cannot reconstruct mass of a system decaying to WW After all cuts (selection for m H =130 GeV) Observed in data 94 events 10 ee, 42 eµ, 42 μµ Expected background 76 (±11) Expected signal m H =130 GeV 19 (±4) Transverse mass spectrum after all cuts (except M T ) m H =130 GeV m H =150 GeV 20

29 H W + W - Relatively large event rate, but leptonic W boson decays lead to unobserved neutrinos cannot reconstruct mass of a system decaying to WW 95% CL Limit on / SM Large mass range excluded already in Summer ATLAS Observed Expected (*) H WW l l -1 Ldt = 2.05 fb ± 1 s = 7 TeV ± 2 After all cuts (selection for m H =130 GeV) Observed in data 94 events 10 ee, 42 eµ, 42 μµ Expected background 76 (±11) Expected signal m H =130 GeV 19 (±4) Transverse mass spectrum after all cuts (except M T ) m H [GeV] 20 m H =130 GeV m H =150 GeV

30 Requires excellent discrimination between single high-energy photons from hadrons but offers good energy resolution H γγ Events / 1 GeV Inclusive diphoton sample Data 2011 Background model SM Higgs boson m H s = 7 TeV, = 120 GeV (MC) -1 Ldt = 4.9 fb ATLAS Preliminary Data - Bkg model m [GeV] 21 Looking for small excess on top of large (but smooth) background

31 Requires excellent discrimination between single high-energy photons from hadrons but offers good energy resolution H γγ Events / 1 GeV Inclusive diphoton sample Data 2011 Background model SM Higgs boson m H s = 7 TeV, = 120 GeV (MC) -1 Ldt = 4.9 fb ATLAS Preliminary Data - Bkg model m [GeV] 21 Looking for small excess on top of large (but smooth) background

32 H ZZ Very rare process, especially with both Z bosons decaying to leptons but very clean, and with good mass resolution Events/5 GeV DATA Background Signal (m =125 GeV) H Signal (m =150 GeV) H Signal (m =190 GeV) H H ZZ Ldt = 4.8 fb s = 7 TeV (*) 4l ATLAS -1 Preliminary Found 3 candidate events at low mass: 4 2 in e + e - μ + μ - final state (124.3 GeV, GeV) 2 1 in μ + μ - μ + μ - final state (124.6 GeV) [GeV] m 4l 22

33 The Competition: CMS entries data m H =130 WW W+jets Z+jets top WZ/ZZ CMS preliminary -1 L = 4.6 fb H W + W - 95% CL limit on!/! SM 10 5 median expected expected ± 1! expected ± 2! observed CMS preliminary H " WW (BDT based) -1 L = 4.6 fb BDT Output Higgs mass [GeV] ) 2 Events / ( 1 GeV/c CMS preliminary -1 s = 7 TeV L = 4.76 fb All Categories Combined Data Bkg Model!1"!2 " 5xSM m H =120 GeV H γγ /#(H"!!) SM 95%CL #(H"!!) Observed CLs Limit Median Expected CLs Limit " 1# Expected CLs " 2# Expected CLs CMS preliminary -1 s = 7 TeV L = 4.76 fb !# SM (GeV/c ) m!! m H (GeV/c )

34 The Competition: CMS (2) 2 Events/2 GeV/c 2 Events/2 GeV/c CMS Preliminary 2011 LEP excluded (99% C.L.) Events/10 GeV/c 2-1 s = 7 TeV L = 4.71 fb DATA Z+X ZZ 2 m H =140 GeV/c 2 m H =120 GeV/c H ZZ Found 2 candidate events near 126 GeV 1 in e + e - e + e - 1 in e + e - μ + μ M 4l Combination 2 [GeV/c ] m4l [GeV/c 2 ] of SM Higgs hypothesis CL S CMS Preliminary, s = 7 TeV Observed -1 Combined, L = fb int Expected ± 1! Expected ± 2! 90% 95% 99% Higgs boson mass (GeV/c 2 ) 24

35 ATLAS: excess in W + W - final states: broad but compatible with low-mass Higgs boson excess in ZZ final state (124 GeV) excess in γγ final state (126 GeV) Summary 25

36 Summary ATLAS: excess in W + W - final states: broad but compatible with low-mass Higgs boson excess in ZZ final state (124 GeV) excess in γγ final state (126 GeV) CMS: excess in W + W - final states: broad but compatible with low-mass Higgs boson excess in ZZ final state (126 GeV) excess in γγ final state (123 GeV) 25

37 Summary ATLAS: excess in W + W - final states: broad but compatible with low-mass Higgs boson excess in ZZ final state (124 GeV) excess in γγ final state (126 GeV) CMS: excess in W + W - final states: broad but compatible with low-mass Higgs boson excess in ZZ final state (126 GeV) excess in γγ final state (123 GeV) Caveat emptor! each individual excess not statistically significant masses are close but do not match within resolution questions: are the energy calibrations as well understood as we think? is this just a statistical fluctuation after all? Time (and additional investigation) will tell But one way or the other, we expect to make a much more definite statement within a year 25

38 Thank you! 26

39 Further Information CERN press release including pointers to further information: 27

40 Finally... Finding the Higgs boson does not mean particle physics is finished! The Standard Model cannot incorporate gravity in a consistent way The Higgs boson s mass is not stable against radiative corrections The Standard Model does not explain Dark Matter / Dark Energy 28

41 Outlook 29

42 Outlook 29

43 Symmetries and Conserved Quantities Noether Theorem: Every symmetry of a physical system comes with an associated conserved quantity (and vice versa) also indicates how to construct these conserved quantities, given the symmetry Examples: translational symmetry conservation of momentum rotational symmetry conservation of angular momentum Emmy Noether 30

44 The Electron s Magnetic Dipole Moment Well-known system: interaction of magnetic dipole moments with external magnetic field H = µ B, µ = γ q S g S Zeeman splitting of (atomic) energy levels 2m Spin precession around B-field axis, Larmor frequency ω=ϒb Unlike regular QM, QED provides a prediction for g! Applying the gauge principle to the Dirac equation (relativistic equation of motion for spin-1/2 particles): g=2 Computing quantum corrections: expansion in powers (up to fifth power) of fine structure constant α e2 4π contributions at lowest orders + 31

45 The Electron s Magnetic Dipole Moment Well-known system: interaction of magnetic dipole moments with external magnetic field H = µ B, µ = γ q S g S Zeeman splitting of (atomic) energy levels 2m Spin precession around B-field axis, Larmor frequency ω=ϒb Unlike regular QM, QED provides a prediction for g! Applying the gauge principle to the Dirac equation (relativistic equation of motion for spin-1/2 particles): g=2 Computing quantum corrections: expansion in powers (up to fifth power) of fine structure constant α e2 4π subset of contributions at 5 th order 31

46 The Electron s Magnetic Dipole Moment Well-known system: interaction of magnetic dipole moments with external magnetic field H = µ B, µ = γ q S g S Zeeman splitting of (atomic) energy levels 2m Spin precession around B-field axis, Larmor frequency ω=ϒb Unlike regular QM, QED provides a prediction for g! G. Gabrielse et al., 2008 Cylindrical Penning trap Observe a single electron for months 31

47 The Electron s Magnetic Dipole Moment Well-known system: interaction of magnetic dipole moments with external magnetic field H = µ B, µ = γ q S g S Zeeman splitting of (atomic) energy levels 2m Spin precession around B-field axis, Larmor frequency ω=ϒb Unlike regular QM, QED provides a prediction for g! The comparison: 31

48 The Electron s Magnetic Dipole Moment Well-known system: interaction of magnetic dipole moments with external magnetic field H = µ B, µ = γ q S g S Zeeman splitting of (atomic) energy levels 2m Spin precession around B-field axis, Larmor frequency ω=ϒb Unlike regular QM, QED provides a prediction for g! g/2 = The comparison: (28) (experiment) (76) (theory) A triumph for QED! A similar measurement for the muon (μ ± ) looks much more like a regular HEP setting... 31

49 A Colourful Interaction Three quarks forming baryons (and quark-antiquark pairs forming mesons): a new symmetry (and interaction), colour gauge principle interaction with gluons: q q r q g q b p p + g s T quarks change identity (colour) under exchange a G 2 of a a gluon! a α s = g2 s 4π 32 Quark confinement at low energy

50 A Colourful Interaction Three quarks forming baryons (and quark-antiquark pairs forming mesons): a new symmetry (and interaction), colour gauge principle interaction with gluons: q q r q g q b p p + g s T quarks change identity (colour) under exchange a G 2 of a a gluon! a α s = g2 s 4π 32 Quark confinement at low energy

51 The Weak Interaction Responsible for all nucleonic transmutations fusion radioactivity (β decay) 33

52 The Weak Interaction Responsible for all nucleonic transmutations and particle decays π + ν µ Decays of heavy particles e + π ν µ e µ + ν e ν µ µ ν e ν µ π + ν e π ν e e + e 33

53 The Weak Interaction Responsible for all nucleonic transmutations and particle decays π + ν µ Decays of heavy particles e + π ν µ e µ + ν e ν µ µ ν e ν µ π + ν e π ν e e + e Truly a weak interaction: solar ν flux on Earth: ~ m -2 s -1 during your lifetime, at most a few will interact with your body at all! 33

54 Standard Model Summary Three fermion generations doublet structure ordered by mass ψ e = νe e W-boson couples charged leptons to ν (and up- to down-type quarks) 34

55 QCD at High Energies At high energies, quarks and gluons do manifest themselves as free particles hadron jets e - p 1 p 3 p xp electron-proton scattering: 27.5 GeV GeV XY View jet 35

56 A Weighty Issue... QED, QCD: photon & gluons are strictly massless Weak interaction: massive W and Z bosons fermion masses: m = m ν (and similarly for quarks) 36

57 A Weighty Issue... QED, QCD: photon & gluons are strictly massless Weak interaction: massive W and Z bosons fermion masses: m = m ν (and similarly for quarks) And worse! W-boson deals with left-handed fermions (right-handed anti-fermions) only λ= -½ λ= +½ p left- and right-handed fermions should be different particles this requires them to be strictly massless S 36

58 The Higgs Mechanism to the Rescue Required: a mechanism to break the EW symmetry spontaneously Lagrangian maintains full EW symmetry but the ground state does not! Achieved through the introduction of the (complex scalar) Higgs field φ = 1 φ + V 2 φ 0 φ = µ(φ φ)+λ(φ φ) 2 With μ < 0: minimum at ϕ 0 SU(3) c SU(2) L U(1) Y Generation of fermion masses through Yukawa couplings: (φ = 0 v/ 2 L Y = g e er φ ψ L + ψ L φe R ) g ev (e R e L + e L e R ) = g ev ee

59 The Higgs Hunters ATLAS... and CMS 38

60 Particle Detection 39

61 Particle Detection In addition to individually observable particles: neutrinos (from apparent lack of momentum conservation) hadron jets (from calorimeter energy deposits/tracks) τ leptons (very narrow hadronic jet ) b-jets (from hadronisation of b-quarks: lifetime of B-hadrons, τb 1.5 ps) long 39

62 Higgs Boson Production and Decay SM Higgs boson production " (fb) qq! Wh qq! qqh bb! h gg,qq! tth TeV4LHC Higgs working group gg! h Tevatron qq! Zh M (GeV) H Total inelastic scattering cross section (strong interaction) ~ 60 mb: background suppression by orders of magnitude required use signatures not overwhelmed by the strong interaction 40

63 Higgs Boson Production and Decay SM Higgs boson production " (fb) qq! Wh qq! qqh bb! h gg,qq! tth TeV4LHC Higgs working group gg! h Tevatron qq! Zh M (GeV) H SM Higgs Branching Fraction bb + W ZZ -! +! gg - W HDECAY program M H (GeV) Strategy: use leptons! low MH ( 135 GeV): VH associated production, leptonic V decay (V=W,Z) high MH ( 135 GeV): H W + W -, both W bosons decaying leptonically A straightforward strategy, but leading to a large number of final states 40

64 LEP Higgs Boson Search Searches at LEP dominated by ZH associated production ( Higgsstrahlung ) e + Z " H e! Z Events / 3 GeV/c LEP!s = GeV Data Background Signal (115 GeV/c 2 ) all! 109 GeV/c 2 Data 18 4 Backgd Signal Tight 2 1 Example distribution (also other variables used) m H rec (GeV/c 2 ) 41

65 The Tevatron Collider pp collisions, s = 1.96 TeV mature collider and experiments running since 2001 CDF Tevatron DØ Main Injector 42

66 The Tevatron Collider pp collisions, s = 1.96 TeV mature collider and experiments running since 2001 CDF Tevatron DØ 8.0 fb -1 Main Injector 7.1 fb fb = cm 2 if σ =1 fb: need L=1 fb -1 to produce one event many interesting processes have σ ~ fb

67 How Credible is All This? cross section (pb) DØ s discovery track record kin. cuts leptonic FS jets W Z WW WZ Especially interesting: single top production: state as WH lνbb ZZ evidence only... single t WZ+ZZ same final similarly for WW: irreducible bg to high-mh search channel q q Yield [Events/0.04] W! t W! b b l! " l g q' (a) Final Discriminant D 2.3 fb 1 Data tb + tqb W+jets tt Multijets Ranked Combination Output W b t b q W + b l + " l 43

68 How Credible is All This? cross section (pb) DØ s discovery track record kin. cuts leptonic FS jets W Z WW WZ Especially interesting: single top production: state as WH lνbb ZZ evidence only... single t WZ+ZZ same final similarly for WW: irreducible bg to high-mh search channel q q Yield [Events/0.02] W! l! q' q W! t W + " l W t b b b g b b (b) Signal Region D 2.3 fb Ranked Combination Output l + " l 43

69 Combinations DØ only LLR 8 SM Higgs Combination LLR B ±2-1 LLR B ±1 6 DØ Preliminary, L= fb LLR B LLR S+B 4 LLR Obs 2 95% CL Limit / SM 10 SM Higgs Combination -1 DØ Preliminary, L= fb Observed Limit Expected Limit Expected ±1- Expected ± November 3, 2009 m H (GeV/c ) Standard Model = m H (GeV/c ) November 3,

70 Combinations DØ only LLR 8 SM Higgs Combination LLR B ±2-1 LLR B ±1 6 DØ Preliminary, L= fb LLR B LLR S+B 4 LLR Obs 2 95% CL Limit / SM 10 SM Higgs Combination -1 DØ Preliminary, L= fb Observed Limit Expected Limit Expected ±1- Expected ±2-0 LLR November 3, 2009 m H (GeV/c ) 14 LLR B ±2 12 LLR B ±1 10 LLR B LLR 8 S+B LLR Obs Tevatron Run II Preliminary -1 L= fb 95% CL Limit/SM November 6, 2009 m H (GeV/c ) Standard Model = m H (GeV/c ) November 3, 2009 Tevatron Run II Preliminary, L= fb -1 LEP Exclusion SM=1 Expected Observed ±1 Expected ±2 Expected Tevatron Tevatron Exclusion November 6, m H (GeV/c 2 )

71 Combinations DØ only LLR 8 SM Higgs Combination LLR B ±2-1 LLR B ±1 6 DØ Preliminary, L= fb LLR B LLR S+B 4 LLR Obs 2 95% CL Limit / SM 10 SM Higgs Combination -1 DØ Preliminary, L= fb Observed Limit Expected Limit Expected ±1- Expected ±2-0 LLR -2 Standard Model = 1.0 Note the consistent 1 (but not 170yet 180significant) signallike behaviour for low MH: Tevatron November 3, 2009 m H (GeV/c ) November 3, 2009 m H (GeV/c ) Tevatron Run II Preliminary, L= fb 14-1 LLR B ±2 Tevatron Run II Preliminary ±1 a first L= hint?! fb LEP Exclusion Tevatron 10 Exclusion LLR B LLR B LLR S+B LLR Obs 95% CL Limit/SM November 6, 2009 m H (GeV/c ) SM=1 Expected Observed ±1 Expected ±2 Expected November 6, m H (GeV/c 2 )

72 Limits No significant signal-like excess observed... set limits Procedure: Compare data compatibility with s+b / b-only hypotheses (each MH) Q = L(s + b m H) L(b) = i bins Calibrate outcome with toy experiments e (s i +b i ) (s i + b i ) n i n i! / e b i b n i i n i! Compare resulting distributions with observed Q CLb/s+b fraction of background-only/signal+bg experiments less signallike than data Reject s+b hypothesis if CLs+b < 0.05 probability density signal + bg bg only 1-CLb observed CLs+b ln(q) 45