Potential Discoveries at the Large Hadron Collider Chris Quigg Fermilab quigg@fnal.gov XXIII Taiwan Spring School Tainan 31 March - 3 April 2010
Electroweak theory successes Theoretical Physics Department, Fermi National Accelerator Laboratory, PO Box 500, Chris Batavia, Quigg IL(Fermilab) 60510, USA Potential Discoveries @ LHC 23rd Spring School Tainan 68 / 105 search for agent of EWSB IOP PUBLISHING Rep. Prog. Phys. 70 (2007) 1019 1053 REPORTS ON PROGRESS IN PHYSICS doi:10.1088/0034-4885/70/7/r01 Spontaneous symmetry breaking as a basis of particle mass Chris Quigg
Higgs (then) Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 69 / 105
Kibble, Guralnik, Hagen, Englert, Brout (now) Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 70 / 105
What the LHC is not really for... Find the Higgs boson, the Holy Grail of particle physics, the source of all mass in the Universe. Celebrate. Then particle physics will be over. We are not ticking off items on a shopping list... We are exploring a vast new terrain... and reaching the Fermi scale Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 71 / 105
Electroweak Questions for the LHC What hides electroweak symmetry: a Higgs boson, or new strong dynamics? If a Higgs boson: one or several? Elementary or composite? Is the Higgs boson indeed light, as anticipated by the global fits to EW precision measurements? Does H only give masses to W ± and Z 0, or also to fermions? (Infer t th from production) Are the branching fractions for f f decays in accord with the standard model? If all this: what sets the fermion masses and mixings? Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 72 / 105
Search for the Standard-Model Higgs Boson Γ(H f f ) = G F m 2 f M H 4π 2 M H in the limit of large Higgs mass; N c ( ) 3/2 1 4m2 f MH 2 β 3 for scalar Γ(H W + W ) = G F M 3 H 32π 2 (1 x)1/2 (4 4x + 3x 2 ) x 4M 2 W /M 2 H Γ(H Z 0 Z 0 ) = G F M 3 H 64π 2 (1 x ) 1/2 (4 4x + 3x 2 ) x 4M 2 Z/M 2 H asymptotically M 3 H and 1 2 M 3 H, respectively 2x 2 and 2x 2 terms decays into transverse gauge bosons Dominant decays for large M H : pairs of longitudinal weak bosons Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 73 / 105
SM Higgs Boson Branching Fractions 10 0 bb WW ZZ Branching fraction 10 1 10 2 ττ gg cc tt 10 3 γγ ss μμ Zγ 10 4 100 130 160 200 300 500 700 1000 M H (GeV) Djouadi, hep-ph/0503172 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 74 / 105
Dominant decays at high mass 1000 Partial Width [GeV] 100 10 W + W! _ t t Z 0 Z 0 1 200 400 600 800 1000 M Higgs [GeV/c 2 ] For M H 1 TeV, Higgs boson is ephemeral: Γ H M H. Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 75 / 105
Total width of the standard-model Higgs boson 10 3 10 2 10 1 Γ(H) (GeV) 10 0 10 1 10 2 10 100 130 160 200 300 500 700 1000 M H (GeV) Below W + W threshold, Γ H < 1 GeV Far above W + W threshold, Γ H MH 3 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 76 / 105
A few words on Higgs production... e + e H: hopelessly small µ + µ H: scaled by (m µ /m e ) 2 40 000 e + e HZ: prime channel Hadron colliders: gg H b b: background?! gg H ττ, γγ: rate?! gg H W + W : best Tevatron sensitivity now pp H(W, Z): prime Tevatron channel for light Higgs At the LHC: Many channels accessible, search sensitive up to 1 TeV Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 77 / 105
Higgs search in e + e collisions σ(e + e H all) is minute, me 2 Even narrowness of low-mass H is not enough to make it visible... Sets aside a traditional strength of e + e machines pole physics Most promising: associated production e + e HZ (has no small couplings) Z Z H e e+ σ = πα2 24 K(K 2 + 3MZ 2)[1 + (1 4x W ) 2 ] s (s MZ 2)2 xw 2 (1 x W ) 2 K: c.m. momentum of H x W sin 2 θ W Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 78 / 105
l + l X... σ(e + e H) = (m e /m µ ) 2 σ(µ + µ H) σ(µ + µ H)/40 000 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 79 / 105
H couples to gluons through quark loops H Q i Q i Q i g g Only heavy quarks matter: heavy 4th generation?? 0.4 0.3 "(!) 2 0.2 0.1 0.0 0 1 2 3! = 4m Q 2 /M H 2 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 80 / 105
Higgs-boson production at the Tevatron Djouadi Update 1 Update 2 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 81 / 105
Current Tevatron Sensitivity Tevatron Run II Preliminary, L=2.0-5.4 fb -1 95% CL Limit/SM 10 LEP Exclusion Expected Observed ±1 Expected ±2 Expected Tevatron Exclusion 1 SM=1 November 6, 2009 100 110 120 130 140 150 160 170 180 190 200 m H (GeV/c 2 ) combining experiments, channels: Fall 2009 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 82 / 105
Electroweak theory projection Global fit + exclusions 2 12 10 8 6 4 2 0 LEP exclusion at 95% CL Tevatron exclusion at 95% CL Theory uncertainty G fitter SM Fit including theory errors Fit excluding theory errors 100 150 200 250 300 M H Dec 09 [GeV] 3 2 1 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 83 / 105
Higgs Reach Konigsberg, La Thuile 2010 Expected sensitivity Analyzed Lum/Exp (fb-1) Tevatron prospects... 2 x CDF Projections MH (GeV) With projected improvements achieved Chris Quigg (Fermilab) Potential Discoveries @ LHC 1 23rd Spring School Tainan 84 / 105
Standard Tevatron prospects Model Higgs... Projections Denisov, La Thuile 2010 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 85 / 105
LHC cross sections... Djouadi Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 86 / 105
SM (electroweak theory) shortcomings No explanation of Higgs potential No prediction for M H Doesn t predict fermion masses & mixings M H unstable to quantum corrections No explanation of charge quantization Doesn t account for three generations Vacuum energy problem Beyond scope: dark matter, matter asymmetry, etc. imagine more complete, predictive extensions Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 87 / 105
Fermion Mass Generation 10 0 Charged leptons Up quarks Down quarks 10 1 t Mass/weak scale 10 2 10 3 μ c s τ b 10 4 10 5 u d 10 6 e Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 88 / 105
Fermion mass is accommodated, not explained All fermion masses physics beyond the standard model! ζ t 1 ζ e 3 10 6 ζ ν 10 11?? What accounts for the range and values of the Yukawa couplings? There may be other sources of neutrino mass Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 89 / 105
The Problem of Identity Quark and Lepton a Mixing b t! b' 100 0 100 0 90 10 90 10 80 20 80 20 70 30 70 30 60 40 60 40 b s τ μ 50 50 50 υ 50 3 40 60 40 υ 2 60 30 70 30 70 20 80 20 80 10 90 10 90 u! d' c! s' υ 1 0 100 0 100 100 90 80 70 60 50 40 30 20 0 100 90 80 70 60 50 40 30 20 10 0 d e What makes a top quark a top quark,...? Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 90 / 105
The Hierarchy Problem Evolution of the Higgs-boson mass M 2 H(p 2 ) = M 2 H(Λ 2 ) + + + quantum corrections from particles with J = 0, 1 2, 1 Potential divergences: M 2 H(p 2 ) = M 2 H(Λ 2 ) + Cg 2 Λ 2 p 2 dk 2 +, Λ: naturally large, M Planck or U 10 15 16 GeV How to control quantum corrections? Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 91 / 105
A Delicate Balance... even for Λ = 5 TeV δm 2 H = G FΛ 2 4π 2 2 (6M2 W + 3M2 Z + M2 H 12m2 t ) 2 ΔM H 1.5 1.0 0.5 0 Desired Scalar output loops 0.04 0.209 Top quark loops Gauge boson loops 0.333 Tuned input 1.34 0.5 1.0 1.5 2.0 1.84 Light Higgs + no new physics: LEP Paradox Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 92 / 105
The Hierarchy Problem Possible paths Fine tuning A new symmetry (supersymmetry) fermion, boson loops contribute with opposite sign Composite Higgs boson (technicolor... ) form factor damps integrand Little Higgs models, etc. Low-scale gravity (shortens range of integration) All but first require new physics near the TeV scale Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 93 / 105
Why is empty space so nearly massless? Natural to neglect gravity in particle physics... Gravitational ep interaction 10 41 EM ( c G Newton small M Planck = q G Newton ) 1 2 1.22 10 19 GeV large G E M Planck q Estimate B(K πg) ( MK M Planck ) 2 10 38 Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 94 / 105
But gravity is not always negligible... The vacuum energy problem At the minimum, Higgs potential V (ϕ ϕ) = µ 2 (ϕ ϕ) + λ (ϕ ϕ) 2 V ( ϕ ϕ 0 ) = µ2 v 2 4 = λ v 4 4 Identify M 2 H = 2µ2 < 0. V 0 contributes position-independent vacuum energy density ϱ H M 2 H v 2 8 10 8 GeV 4 10 24 g cm 3 Adding vacuum energy density ϱ vac adding cosmological constant Λ to Einstein s equation R µν 1Rg 2 µν = 8πG N T c 4 µν + Λg µν Λ = 8πG N c 4 ϱ vac Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 95 / 105
CMB ϱ vac < 10 46 GeV 4 ϱ crit = 3H 2 0 /8πG N Supernova Cosmology Project Kowalski, et al., Ap.J. (2008) 1.5 Union 08 SN Ia compilation 1.0 SNe Ω Λ 0.5 BAO Flat 0.0 0.0 0.5 1.0 Ω m ϱ H > 10 8 GeV 4 : mismatch by 10 54 A dull headache for thirty years... H constraints Chris Quigg (Fermilab) Potential Discoveries @ LHC 23rd Spring School Tainan 96 / 105