Potential Discoveries at the Large Hadron Collider. Chris Quigg

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1 Potential Discoveries at the Large Hadron Collider Chris Quigg Fermilab XXIII Taiwan Spring School Tainan 31 March - 3 April 2010

2 Electroweak theory successes Theoretical Physics Department, Fermi National Accelerator Laboratory, PO Box 500, Chris Batavia, Quigg IL(Fermilab) 60510, USA Potential LHC 23rd Spring School Tainan 68 / 105 search for agent of EWSB IOP PUBLISHING Rep. Prog. Phys. 70 (2007) REPORTS ON PROGRESS IN PHYSICS doi: / /70/7/r01 Spontaneous symmetry breaking as a basis of particle mass Chris Quigg

3 Higgs (then) Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 69 / 105

4 Kibble, Guralnik, Hagen, Englert, Brout (now) Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 70 / 105

5 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 LHC 23rd Spring School Tainan 71 / 105

6 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 LHC 23rd Spring School Tainan 72 / 105

7 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 LHC 23rd Spring School Tainan 73 / 105

8 SM Higgs Boson Branching Fractions 10 0 bb WW ZZ Branching fraction ττ gg cc tt 10 3 γγ ss μμ Zγ M H (GeV) Djouadi, hep-ph/ Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 74 / 105

9 Dominant decays at high mass 1000 Partial Width [GeV] W + W! _ t t Z 0 Z M Higgs [GeV/c 2 ] For M H 1 TeV, Higgs boson is ephemeral: Γ H M H. Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 75 / 105

10 Total width of the standard-model Higgs boson Γ(H) (GeV) M H (GeV) Below W + W threshold, Γ H < 1 GeV Far above W + W threshold, Γ H MH 3 Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 76 / 105

11 A few words on Higgs production... e + e H: hopelessly small µ + µ H: scaled by (m µ /m e ) 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 LHC 23rd Spring School Tainan 77 / 105

12 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 LHC 23rd Spring School Tainan 78 / 105

13 l + l X... σ(e + e H) = (m e /m µ ) 2 σ(µ + µ H) σ(µ + µ H)/ Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 79 / 105

14 H couples to gluons through quark loops H Q i Q i Q i g g Only heavy quarks matter: heavy 4th generation?? "(!) ! = 4m Q 2 /M H 2 Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 80 / 105

15 Higgs-boson production at the Tevatron Djouadi Update 1 Update 2 Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 81 / 105

16 Current Tevatron Sensitivity Tevatron Run II Preliminary, L= fb -1 95% CL Limit/SM 10 LEP Exclusion Expected Observed ±1 Expected ±2 Expected Tevatron Exclusion 1 SM=1 November 6, m H (GeV/c 2 ) combining experiments, channels: Fall 2009 Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 82 / 105

17 Electroweak theory projection Global fit + exclusions LEP exclusion at 95% CL Tevatron exclusion at 95% CL Theory uncertainty G fitter SM Fit including theory errors Fit excluding theory errors M H Dec 09 [GeV] Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 83 / 105

18 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 LHC 1 23rd Spring School Tainan 84 / 105

19 Standard Tevatron prospects Model Higgs... Projections Denisov, La Thuile 2010 Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 85 / 105

20 LHC cross sections... Djouadi Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 86 / 105

21 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 LHC 23rd Spring School Tainan 87 / 105

22 Fermion Mass Generation 10 0 Charged leptons Up quarks Down quarks 10 1 t Mass/weak scale μ c s τ b u d 10 6 e Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 88 / 105

23 Fermion mass is accommodated, not explained All fermion masses physics beyond the standard model! ζ t 1 ζ e 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 LHC 23rd Spring School Tainan 89 / 105

24 The Problem of Identity Quark and Lepton a Mixing b t! b' b s τ μ υ υ u! d' c! s' υ d e What makes a top quark a top quark,...? Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 90 / 105

25 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 GeV How to control quantum corrections? Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 91 / 105

26 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 Desired Scalar output loops Top quark loops Gauge boson loops Tuned input Light Higgs + no new physics: LEP Paradox Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 92 / 105

27 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 LHC 23rd Spring School Tainan 93 / 105

28 Why is empty space so nearly massless? Natural to neglect gravity in particle physics... Gravitational ep interaction EM ( c G Newton small M Planck = q G Newton ) GeV large G E M Planck q Estimate B(K πg) ( MK M Planck ) Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 94 / 105

29 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 GeV 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 LHC 23rd Spring School Tainan 95 / 105

30 CMB ϱ vac < 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 Ω m ϱ H > 10 8 GeV 4 : mismatch by A dull headache for thirty years... H constraints Chris Quigg (Fermilab) Potential LHC 23rd Spring School Tainan 96 / 105