Higgs rush at CERN Frank Filthaut Radboud Universiteit

Higgs rush at CERN Frank Filthaut Radboud Universiteit Nijmegen / Nikhef

Contents: Particle physics: general picture The Higgs particle The recent results

Particle Physics: the General Picture

Going beyond the naked eye Antoni van Leeuwenhoek, 16321723: invention of the microscope discovery first bacteria (“kleine beestjes”), 0. 5 - 500 μm E. coli (size ~ 1 μm) Minimum discernible dimensions ~ λ limit when using visible light: 0. 5 μm improvement to ~ 1Å possible using STM, AFM 4

The atom “cracked” Idea: use particles to “see” smaller structures Rutherford: scattering of α-particles (4 He nuclei, Eα≈3 Me. V) 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 Me. V electrons on protons 5

State of the art Present scheme in CERN’s Large Hadron Collider: accelerate proton beams to energies of 3. 5 Te. V per proton v/c ≈ 0. 99999996 (energy to be doubled in 2014: 6→ 8) 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

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

Techniques High energy allows for the creation of other, usually short-lived particles τ < 10 -22 s for “interesting” particles in collisions: convert kinetic energy into mass in decay processes: reconstruct mass of the decaying particle (if all decay products are measured) (not the Higgs particle. . . ) mΛ 8

The Weak Interaction e∓ Exchange / production of heavy particles! W-boson Z-boson production (and decay) and decay e± W and Z particles are heavy! e± Most collisions between MW = 80. 398(25) Ge. V (~ Sr, Kr) protons involve the strong interaction ➟ MZ = 92. 188(2) Ge. V (~ Ru) look for leptons (only EM Discovered in p-p collisions, ECM = 630 Ge. V and weak interactions) 9

The Higgs Particle

The Particle Family “Leptons”? ? particles not involved in the strong interaction (only weak, EM) also heavier counterparts of the electron and its neutral partner, νe 5⋅10 -3 1. 5 173 5⋅10 -3 0. 3 4. 8 Quarks: particles also susceptible to the strong interaction • 5⋅10 -4 0. 1 91 0 80 0 1. 8 < 10 -6 again, 3 “generations” involving heavier partners than the u, d that are constituents of the proton Force carriers: • photon (EM), W/Z (weak interaction), gluon(s) (strong interaction) The only missing family member: the Higgs particle 11

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

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! This is what the Higgs field does: interactions obey symmetry • ground state does not The Standard Model is invalid w/o Higgs! 13

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

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

Constraints & Previous Searches MH unknown, but for given MH all Higgs boson properties fixed ➟ know “exactly” what to look for direct searches at previous colliders indirect evidence from precision measurements 16

The Recent Results

The LHC: a Success Story! Expectations for 2011 exceeded by a factor 5 18

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 ~ 1010 Look for striking signatures setting the Higgs boson apart from more ubiquitous “background” processes 1 pb = 10 -36 cm 2 √s ≡ ECM 19 now

+ WW 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 Large mass range excluded already in Summer 20

H → γγ Requires excellent discrimination between single high-energy photons from hadrons but offers good energy resolution Looking for small excess on top of large (but smooth) background 21

H → ZZ Very rare process, especially with both Z particles decaying to leptons but very clean, and with good mass resolution Found 3 candidate events at low mass: 2 in e+e-μ+μ- final state (124. 3 Ge. V, 123. 6 Ge. V) 1 in μ+μ- final state (124. 6 Ge. V) 22

Combining It All 23

The Competition: CMS H→W+W- H→γγ 24

The Competition: CMS (2) H→ZZ Found 2 candidate events near 126 Ge. V 1 in e+e-e+e 1 in e+e-μ+μ- Combination 25

Summary CMS: ATLAS: excess in W+W- final states: broad but compatible with lowmass Higgs boson excess in ZZ final state (124 Ge. V) excess in ZZ final state (126 Ge. V) excess in γγ final state (123 excess in γγ final state (126 Caveat emptor! Ge. V) each individual excess not statistically significant masses in γγ, ZZ are close but do not match ➠ 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 onewe way the. Higgs other, particle we expect to rule make Either findorthe or we outa much morethe definite statement within a year Standard Model! 26

Thank you! 27

Further Information CERN press release including pointers to further information: http: //press. web. cern. ch/press/Press. Releases/Releases 2011/PR 25. 11 E. ht ml 28

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 29

Outlook 30

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 31 Emmy Noether

The Electron’s Magnetic Dipole Moment Well-known system: interaction of magnetic dipole moments with external magnetic field Zeeman splitting of (atomic) energy levels Spin precession around B-field axis, Larmor frequency ω=ϒB Unlike regular QM, QED provides a prediction for g! th order Applying the gauge principle to the G. Gabrielse et al. , 2008 subset contributions of contributions at lowest at orders 5 Observe a single electron for months The comparison: Dirac equation. Penning (relativistic Cylindrical trapequation of motion for spin-1/2 particles): g=2 A triumph for QED! Computing quantum corrections: expansion in powers (up to fifth ±) looks much more like a A similar measurement for the muon (μ power) of fine structure constant regular HEP setting. . . 32

A Colourful Interaction Three quarks forming baryons (and quark-antiquark pairs forming mesons): a new symmetry (and interaction), colour “gauge principle” interaction with gluons: quarks change identity (colour) under exchange of a gluon! Quark confinement at low energy 33

The Weak Interaction Responsible for all nucleonic transmutations and particle decays fusion. Decays of heavy particles radioactivity (β decay) Truly a weak interaction: solar ν flux on Earth: ~ 6⋅1014 m-2 s-1 during your lifetime, at most a few will interact with your body at all! 34

Standard Model Summary Three fermion “generations” doublet structure ordered by mass W-boson couples charged leptons to ν (and up- to down-type quarks) 35

QCD at High Energies At high energies, quarks and gluons do manifest themselves as “free” particles → hadron jets e- electron-proton scattering: 27. 5 Ge. V + 920 Ge. V jet 36

A Weighty Issue. . . QED, QCD: photon & gluons are strictly massless Weak interaction: massive W and Z bosons fermion masses: (and similarly for quarks) And worse! W-boson deals with left-handed fermions (right-handed anti-fermions) only λ= -½ p λ= +½ S • left- and right-handed fermions should be different particles this requires them to be strictly massless 37

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 With μ < 0: minimum at ϕ≠ 0 Generation of fermion masses through “Yukawa” couplings: 38

The Higgs Hunters ATLAS. . . and CMS 39

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) 40 “long”

Higgs Boson Production and Decay Strategy: use leptons! Total inelastic scattering cross section (strong interaction) ~ 60 low MH (≲ 135 Ge. V): VH associated production, leptonic V decay mb: (V=W, Z) background suppression by 10 -11 orders of magnitude required + high MH (≳ 135 Ge. V): H→ W W , both W bosons decaying use signatures not overwhelmed by the strong interaction leptonically A straightforward strategy, but leading to a large number of final states 41

LEP Higgs Boson Searches at LEP dominated by ZH associated production (“Higgsstrahlung”) Example distribution (also other variables used) 42

The Tevatron Collider pp collisions, 1. 96 Te. V √s = CDF mature collider and experiments DØ Tevatron running since 2001 Main Injector 8. 0 fb-1 7. 1 fb-1 43 1 fb = 10 -43 cm 2 if σ =1 fb: need L=1 fb-1 to produce one event many interesting processes have σ ~ 100 -1000 fb

How Credible is All This? DØ’s discovery track record. . . kin. cuts leptonic FS jets evidenc e only. . . Especially interesting: single top production: final state as WH→lνbb same similarly for WW: irreducible bg to high-MH search channel 44

Combinations Note the consistent (but not yet significant) signal -like behaviour for low MH : a first hint? ! 45 DØ only Tevatron

Limits No significant signal-like excess observed. . . ➟ set limits Procedure: Compare data compatibility with s+b / b-only hypotheses (each MH) Calibrate outcome with toy experiments Compare resulting distributions with observed Q observed CLb/s+b ≡ fraction of backgroundonly/signal+bg experiments less signallike than data Reject s+b hypothesis if CLs+b < 0. 05 1 -CLb 46 CLs+b