That Infamous Boson: from Obscure Curiosity to Holy Grail Higgs Hunting 2011 John Ellis CERN & King’s College London

Outline The context The proposal The baptism A phenomenological profile The quest for the Holy Grail When is a Higgs not a Higgs? The LHC has shown there is new physics beyond Higgs

Life before Higgs Guage theories irrelevant? Early steps towards quantization (Feynman 1963, deWitt, Faddeev & Popov 1967) But require massless gauge bosons? Spontaneous breaking of global symmetries (Nambu 1960, Goldstone 1961) Later shown renormalizable (Lee, Symanzik 1969) But require massless Goldstone boson? Could two wrongs make a right?

The Curses of the Massless Bosons Counter-example in theory of superconductivity (Anderson 1963) Speculated that the Goldstone and Yang-Mills zero-mass problems could cancel (also Klein & Lee 1964) But this was thought impossible in relativistic theory because no time-like vector (Gilbert 1964)

The Seminal Papers

The Englert-Brout-Higgs Mechanism Vacuum expectation value of scalar field Englert & Brout: June 26th 1964 First Higgs paper: July 27th 1964 Pointed out loophole in argument of Gilbert if gauge theory described in Coulomb gauge Accepted by Physics Letters Second Higgs paper with explicit example sent on July 31st 1964 to Physics Letters, rejected! Revised version (Aug. 31st 1964) accepted by PRL Guralnik, Hagen & Kibble (Oct. 12th 1964)

The Englert-Brout-Higgs Mechanism Guralnik, Hagen & Kibble

The Higgs boson Higgs pointed out a massive scalar boson “… an essential feature of [this] type of theory … is the prediction of incomplete multiplets of vector and scalar bosons” Englert, Brout, Guralnik, Hagen & Kibble did not comment on its existence

Spontaneous breaking of symmetry Nambu, EB, GHK and Higgs Spontaneous breaking of symmetry

Consecration & Baptism Incorporation in electroweak theory (Weinberg 1967, Salam 1968) Renormalizability breakthrough (‘t Hooft 1970, 1971) Comprehensive & influential review by Ben Lee (Batavia Conference, 1972) Refers repeatedly to ‘Higgs’ fields, scalars Ben Lee the Baptist

Ben Lee the Baptist

Early Phenomenological Bounds Emission from stars: MH > 0.7 me (Sato & Sato, 1975) Neutron-electron scattering: MH > 0.7 MeV (Rafelski, Muller, Soff & Greiner; Watson & Sundaresan; Adler, Dashen & Treiman; 1974) Neutron-nucleus scattering: MH > 13 MeV (Barbieri & Ericson, 1975) Nuclear 0+ – 0+ transitions: MH > 18 MeV (Kohler, Watson & Becker, 1974)

A Phenomenological Profile of the Higgs Boson Neutral currents (1973) Charm (1974) Heavy lepton τ (1975) Much attention to search for W±, Z0 For us, the Big Issue: is there a Higgs boson? Previously ~ 10 papers on Higgs bosons MH > 18 MeV First attempt at systematic survey

A Phenomenological Profile of the Higgs Boson Higgs decay modes and searches in 1975: 14

Higgs Boson on the Experimental Agenda Searches at LEP: (EG, Yellow report 76-18) e+e-  Z + H (EGN 76, Ioffe & Khoze 78, Lee, Quigg & Thacker 77) Z  H + μ+μ- (EG 76, Bjorken 1978) MH > 114.4 GeV

Higgs Boson on the LEP Agenda e+e-  Z + H Z  H + μ+μ- JE & Gaillard, 1976

Higgs Boson on the LEP Agenda

Higgs Boson on the pp Agenda _ Higgs Boson on the pp Agenda gg  H W±/Z0 + H Georgi, Glashow, Machacek & Nanopoulos, 1978 Glashow, Nanopoulos & Yildiz, 1978

Higgs Boson on the SSC Agenda

Higgs Boson on the LHC Agenda

Higgs Hunting goes Mainstream

Estimating the Mass of the Higgs Electroweak radiative corrections are sensitive to massive particles: Sensitivity to the top quark mass >> sensitivity to Higgs mass: But LEP measurements gave an indication for a light Higgs even before the top discovery JE, Fogli & Lisi, 1990/1

Estimating the Mass of the Higgs First attempts in 1990, 1991: Very difficult before the discovery of the top JE, Fogli & Lisi, 1990/1

Estimating the Mass of the Higgs After the discovery of the top: Solid indications of a light Higgs JE, Fogli & Lisi, 1994/5

Precision Tests of the Standard Model Lepton couplings Pulls in global fit

The State of the Higgs: Pre-EPS High-energy search: Limit from LEP: mH > 114.4 GeV High-precision electroweak data: Sensitive to Higgs mass: mH = 96+30–24 GeV Combined upper limit: mH < 157 GeV, or 186 GeV including direct limit Exclusion from high-energy search at Tevatron: mH < 158 GeV or > 173 GeV

Higgs Search @ Tevatron Tevatron excludes Higgs between 158 & 173 GeV 27

First Higgs Searches @ LHC No exclusion yet, but significant contribution to global fit 28

Information from Direct Higgs Searches Gfitter collaboration

Combining the Information from Direct Searches and Indirect Data mH = 120+ 12-5 GeV Gfitter collaboration

The Stakes in the Higgs Search How is particle symmetry broken? Is there an elementary scalar field? What is the fate of the Standard Model? Did mass appear when the Universe was a picosecond old? Did Higgs help create the matter in the Universe? Did a related inflaton make the Universe so big and old? Why is there so little dark energy?

Theoretical Constraints on Higgs Mass Large → large self-coupling → blow up at low energy scale Λ due to renormalization Small: renormalization due to t quark drives quartic coupling < 0 at some scale Λ → vacuum unstable Bounds on Higgs mass depend on Λ

What is the probable fate of the SM? Confidence Levels (CL) without/with Tevatron exclusion Confidence Levels (CL) for different fates Blow-up excluded at 96% CL CL as function of instability scale  Espinosa, JE, Giudice, Hoecker, Riotto, arXiv0906.0954 33

The LHC will Tell the Fate of the SM Examples with LHC measurement of mH = 120 or 115 GeV Espinosa, JE, Giudice, Hoecker, Riotto

How to Stabilize a Light Higgs Boson? Top quark destabilizes potential: introduce introduce stop-like scalar: Can delay collapse of potential: But new coupling must be fine-tuned to avoid blow-up: Stabilize with new fermions: just like Higgsinos Very like Supersymmetry!

Supersymmetry Search at LHC 95% CL region 68% CL region MasterCode collaboration: http://mastercode.web.cern.ch/mastercode/ χ2 = 30.2, fit probability = 11.6% 36

CMSSM HiggsMass Full LHC 2010 data CMSSM O.Buchmueller, JE et al: in preparation

Latest Higgs Searches @ Tevatron Exclude (100,109); (156,177) GeV 38

Latest Higgs Searches @ LHC Exclude (149,206); (300,440) Exclude (155,190); (295,450) Excess of events @ (120, 140) GeV? 39

Blogger’s LHC Higgs Combination “Nonsense” (Bill Murray) Is the much-discussed excess @ (130, 140) large enough to be likely to be a Standard Model Higgs? Watch out for an excess @ ~ 120 GeV!

Plots we would like to see? p0 for bkgrd only CLs for signal What would Higgs signal look like? p0 expected if also signal Best fit + bands for σ/σSM 41

What if the Higgs is not a Higgs? Tree-level Higgs couplings ~ masses Coefficient ~ 1/v Couplings ~ dilaton of scale invariance Broken by Higgs mass term –μ2, anomalies Cannot remove μ2 (Coleman-Weinberg) Anomalies give couplings to γγ, gg Generalize to pseudo-dilaton of new (nearly) conformal strongly-interacting sector Couplings ~ m/V (V > v?), additions to anomalies

A Phenomenological Profile of a Pseudo-Dilaton New strongly-interacting sector at scale ~ V Pseudo-dilaton only particle with mass << V Universal suppression of couplings to Standard Model particles ~ v/V Possible enhancement of coupling to gg Possible suppression of coupling to γγ Modified self-couplings: effective potential ~ χ4 [ln(χ/V) – ¼] + anomalous dimensions Pseudo-baryons as dark matter?

There must be New Physics beyond the Higgs Boson potential collapses Higgs coupling blows up Higgs coupling less than in Standard Model

Conversation with Mrs Thatcher: 1982 What do you do? Wouldn’t it be better if they found what you predicted? Think of things for the experiments to look for, and hope they find something different Then we would not learn anything! 45

Supersymmetry? Would unify matter particles and force particles Related particles spinning at different rates 0 - ½ - 1 - 3/2 - 2 Higgs - Electron - Photon - Gravitino – Graviton Many phenomenological motivations Would help fix particle masses Would help unify forces Predicts light Higgs boson Could fix discrepancy in gμ - 2 Could provide dark matter for the astrophysicists and cosmologists

Imagine a Room … What lies Beyond? … Open The Door

What lies Beyond?