HUNTING FOR THE HIGGS - PDF Free Download

Univ. of Sci. and Tech. of China Dec. 16th, 2011 HUNTING FOR THE HIGGS Tao Liu UC@ Santa Barbara

Why Higgs Mechanism? Two mysteries in the Electroweak (EW) theory : The cause of the EW symmetry breaking The origin of quark and lepton masses Simultaneously explained by Higgs mechanism The existence of a Higgs boson with a mass of the EW scale 2

Production and Decay Multiple options for the SM Higgs search: production X decay Guideline for Higgs search: more signal events, less background ones. 3

Main Search Strategies at the Tevatron and the LHC Tevatron LHC High mass region (> 140 GeV): WW (large branching ratio) Low mass region (< 140 GeV): bƃ (large branching ratio) High mass region: WW and ZZ (large branching ratio) Low mass region: γγ (clean background) 4

Current Status at the Tevatron [arxiv:1107.5518[hep-ex]] Decreased bb branching ratio WW pole region Decreased Higgs production xection 5

Progress at the LHC (CMS) γγ WW ZZ High mass region: excluded at 2 sigma C.L. up to ~ 600 GeV High mass region: completely excluded at 2 sigma C.L., if combined with precision EW measurements (mh < 185 GeV at 2 sigma C.L.) Low mass region: will be covered at 2 sigma soon Low mass region: ~ 2.5 sigma deviation excess has been observed for mh ~ 124 GeV 6

Questions If the observed di-photon excess is caused by Higgs decay, is new physics involved (hierarchy problem)? If no SM Higgs boson is observed in the di-photon searches at the LHC, how to understand the Higgs mechanism from a theoretical perspective? 7

Hierarchy Problem Higgs boson mass in the SM m 2 H = m 2 H tree + O Λ 2 Planck mh^2 is of the EW scale Loop correction is of the Planck scale => Large fine-tuning between the tree-level and the looplevel contributions (16 order cancelation: Lamda_Planck / Lamba_EW) 9

Supersymmetry Supersymmetry is one of the most popular candidate theories to solve the hierarchy problem Cancelation between contributions from the SM particles and their superpartners m 2 H = m 2 H tree + O log Λ2 Planck m 2 H tree 10

Can the LHC Results Be Explained in Supersymmetry? γγ decay rate is required to be mildly larger than the SM prediction Small alpha scenario in the MSSM: the coupling between SM-like Higgs boson and bƃ is suppressed => small bƃ decay width => large γγ branching ratio [Carena, Draper, Liu, Wagner, Phys. Rev. D 83 (2011)] 11

Solution I: Suppressed Gluon Fusion Process Recall: in the SM, Br(h -> γγ) ~ 10^{-3}, => sensitivity arises from the large xection of the gluon-fusion Higgs production Gluon-phobic scenario in the MSSM: gluon-fusion process can be suppressed due to loop cancelation between the contributions from top quark and its superpartners top quarks -> stop quarks 13

Solution II: Enhanced bƃ Decay Width SM-like Higgs boson can have nonstandard Yukawa couplings, with new physics involved. In the maximal and minimal mixing scenarios in the MSSM: for given tan, smaller ma h (SM-like) is more down-type like larger partial width of h bƃ x-axis: mass of the CP-odd Higgs suppressed Br (h γγ) y-axis: (σ ggφ Br(φ γγ)) MSSM (σ ggφ Br(φ γγ)) SM bƃ mode may play a complementary role to the γγ Higgs search [Carena, Draper, Liu, Wagner, Phys. Rev. D 83 (2011)] 14

Minimal Mixing Scenario Minimal mixing scenarios -- the radiative correction to m h is minimized (m h < 120 GeV) [Carena, Draper, Liu, Wagner, Phys. Rev. D 83 (2011)] 15

Minimal Mixing Scenario (c.d.) A combination of the Tevatron search and the 7 TeV LHC search: Almost the whole parameter region can be covered at 3 sigma C.L. because of the complementarity between the bb and γγ searches [Carena, Draper, Liu, Wagner, Phys. Rev. D 83 (2011)] 16

Run III Project at the Tevatron Run though 2014, collecting 16 fb^-1 for each detector Decision of Physics Advisory Committee at Fermi Lab (Aug. 2010). 17

Footcorners of the Run III Proposal 2011 Run III The sensitivity of bb mode is high MSSM analysis is based on the work by Draper, Liu, Wagner 18

Challenges for the bƃ Mode Search at the LHC Gluon-fusion can not work: huge di-jets background W/Z associated production: large backgrounds from W/Z + jets and ttbar Higgs cat and QCD raccoon 19

New Strategy I: jet Substructure [Butterworth et. al., Phys. Rev. Lett. 100 (2008)] BDRS algorithm Step I: consider boosted Higgs only and define jets to be fat Step II: split a fat jet into two subjets by undoing clustering + apply b- tagging for the two subjets Kills non-b and single-b backgrounds Step III: filter the neighborhood of the fat jet by reclustering the subjets with smaller cone size Eliminates the contamination of the underlying event 20

New Strategy II: Multiple b-jets New possibility: multiple (> or = 4) b-jets t 2 h b b For a heavy stop with m t 2 <m g, it mainly decays into the SM-like Higgs boson or Z boson and a light stop quark t 1 [Berenstein, Liu and Perkins, in progress] t χ 0 1 Pair production of the heavy stop quarks multiple b-jets via cascaded decays Large MET + multiple b-jets suppressed the SM and reducible supersymmetric backgrounds Main backgrounds: (1) events with both decay chains containing Z bosons; (2) events with the decay chains contain one or two hsm bosons but with a wrong reconstruction of the hsm candidate 21

Some Preliminary Restults [Berenstein, Liu and Perkins, in progress] 22

Solution III: Non-standard Higgs Physics Nonstandard decay: decay into particles absent in the SM It can suppress all standard decay modes Allows a light SM-like Higgs boson (<100 GeV) experimentally, a better fit to the precision EW measurements at the LEP and the Tevatron theoretically, improves the naturalness of the EW symmetry breaking. Two examples: R-symmetry and PQ-symmetry limits in the Next-to-MSSM [LEP+Tevatron+LHC, 2011] 23

Basic Elements on the NMSSM The NMSSM: W NMSSM = Y U QH u U c Y D QH d D c Y E LH d E c + λnh u H d + 1 3 κ V soft = m 2 H d H d 2 + m 2 H u H u 2 + m 2 N N 2 (λa λ H u H d N +h.c.)+ κn3 3 A κn 3 +h.c. There are two approximate global symmetries in the Higgs sector: R- symmetry and Peccei-Quinn (PQ) symmetry Three CP-even mass eigenstates (h₁, h₂, h₃) and two CP-odd ones (a₁, a₂) In the symmetry limits, a₁ plays a role of pseudo - Goldstone boson, with ma₁ << EW scale 24

Old Story: R-symmetry limit (Aλ 0, Aκ 0) [Dobrescu et al., Phys. Rev. D 63 (2001); Dermisek et al., Phys. Rev. Lett. 95 (2005)] bƃ mode is typically suppressed h₁ a₁a₁ becomes dominant e + Z 0, γ h 0 A0 A 0 c, g, τ c, g, τ + c, g, τ c, g, τ + h 2 h 1,a 1 µ, τ µ +, τ + µ e Z 0 ν, e +,µ + ν, e,µ h 1,a 1 µ + e Feynman diagram for the processes co LEP searches [Schael et al. [ALEPH Collaboration], JHEP 1005 (2010); Abbiendi et al. [The OPAL Collaboration], Eur. Phys. J. C 27, (2003)] Tevatron searches [Abazov et al. [D0 Collaboration], Phys. Rev. Lett. 103 (2009)] Current Exp bounds are pretty strong! 25

New Paradigm: PQ limit (κ/λ 0, Aκ 0) (GeV) m χ 2 140 120 100 h 2 χ χ 1 h 2 χ χ 1 h 2 χ χ 2 1 2 2 dominant dominant dominant h 2 bb dominant [Draper, Liu, Wagner, Wang and Zhang, Phys. Rev. Lett. 106 (2011)] 80 60 40 One interesting feature in the PQ symmetry limit: mh₁, ma₁, mχ₁ ~ 10 GeV or below 20 0 80 85 90 95 100 105 110 115 120 125 m h2 (GeV) [In progress, Liu, Wagner, Wang and Zhang] h₂ h₁h₁, a₁a₁ are generically suppressed (unlike the R-symmetry limit!) bƃ mode is dominant sometimes, but not generic. If kinematically allowed, h₂ tends to dominantly decay into χ₁ + χ₂ (new! See green points) Not very hard because χ₁ is light 26

Decay Topology of SM-like Higgs Boson χ 1 1.0 h 2 χ 1 0.8 ΜΜ On-shell resonance χ 2 h 1,a 1 f Br 0.6 0.4 0.2 ΤΤ ΠΠ KK ΗΗ gg f 0.0 0.5 1.0 2.0 5.0 10.0 m h1 GeV [Liu, Wagner, Wang and Zhang, in progress] Collider Signature A pair of soft, collimated leptons or quarks + MET + X 27

New Higgs Physics Requires New Search Strategies New non-standard Higgs decay new search strategies required. Multiple possibilities for the SM-like Higgs boson search: h₂ production + h₁, a₁ decay [Huang, Liu, Su, Wagner, Wang and Yu, in progress] μμ mode: W + h₂, with W-> l+nu, h₁ μμ; signature: tri-leptons + MET; main background: W+ γ*, with γ* μμ σ (fb) 10 1 Signal Background σ (fb) 14 12 10 Signal Background -1 10 8 6-2 10 4 2 0 20 40 60 80 100 120 140 160 180 200 MET (GeV) 0.5 0 1 1.5 2 2.5-3 m(µ + µ ) (GeV) Discovery is possible for 5 fb^-1 data at 7TeV LHC! ττ and bƃ modes: jet-substructure technique is required! 28

Conclusions No conclusions! 29

!ank y"!

LEP Bounds bb) 2 Br(h 1 0.9 0.8 0.7 h 2 1 h 2 1 h 2 2 1 2 2 dominant dominant dominant h 2 bb dominant h bb bound in NMSSMTools ) 1 1 2 Br(h 1 0.9 0.8 0.7 h 2 1 h 2 1 h 2 2 1 2 2 dominant dominant dominant h 2 bb dominant h inv. bound in NMSSMTools 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0 80 85 90 95 100 105 110 115 120 m h2 (GeV) 0 80 85 90 95 100 105 110 115 120 m h2 (GeV) 31

Search for Light Higgs Pair Decays of SM-like Higgs Motivated by the studies in the R- symmetry limit of the NMSSM e + e Z 0, γ A0 h 0 A 0 Z 0 c, g, τ c, g, τ + c, g, τ c, g, τ + ν, e +,µ + ν, e,µ e Feynman diagram for the processes co LEP searches: (1) (h -> aa)a -> 6b [Schael et al. [ALEPH, DELPHI, L3, and OPAL Collaborations], Eur. Phys. J. C 47(2006); S. Schael et al. [ALEPH Collaboration]], JHEP 1005 (2010)); (2) Z-associated Higgs production, with Z leptonically decayed [Schael et al. [ALEPH Collaboration], JHEP 1005 (2010); Abbiendi et al. [The OPAL Collaboration], Eur. Phys. J. C 27, (2003)]. Tevatron searches: h_sm -> a1a1, h1h1 -> 4 mu, 2 mu 2 tau [Abazov et al. [D0 Collaboration], Phys. Rev. Lett. 103 (2009)] h 2 h 1,a 1 h 1,a 1 µ, τ µ +, τ + µ µ + 32

Branching Ratios of h2 -> h1h1, a1a1 33

A Novel Supersymmetric Light DM Paradigm! [Xenon 100, Phys. Rev. Lett. 107 (2011)] MDM ~O(1-10) GeV Benchmark scenario? MDM ~O(100) GeV Benchmark scenario: MSSM LSP 34

A Novel Supersymmetric Light DM Paradigm! χ 1 χ 1 Ñ f a 1 h i Ñ f N N A t-channel process is dominant in spinindependent direct-detection => xection be strongly enhanced by a small mh1 σ SI ε 0.04 +0.46 λ 0.1 mh1 1GeV 2 v yh1 χ 1 χ 1 µ 0.003 2 4 10 40 cm 2 35

A Novel Supersymmetric Light DM Paradigm! χ 1 χ 1 Ñ f a 1 h i Ñ f N N A t-channel process is dominant in spinindependent direct-detection => xection be strongly enhanced by a small mh1 Breit - Wigner enhancement effect! -> Right relic density σ SI ε 0.04 +0.46 λ 0.1 mh1 1GeV 2 v yh1 χ 1 χ 1 µ 0.003 2 4 10 40 cm 2 35

Dark Matter Results [Draper, Liu, Wagner, Wang and Zhang, Phys. Rev. Lett. 106 (2011)] 0.09 Ωh 2 0.13 0.05 λ 0.15, 0.001 κ 0.005, ε 0.25, 30GeV A κ 15GeV, 5 tan β 50, 100GeV µ 250GeV All points have passed the current exp. bounds of flavor physics, meson decays, and collider exp. The blue points fall in a 3 sigma range of the observed relic density. Their Sigma_SI can be as large as above 10^{-40} cm^2 36

PQ Symmetry Limit DLH DLH Red Points: [Draper, Liu, Wagner, Wang and Zhang, Phys. Rev. Lett. 106 (2011)] Blue Points: ε = λµ m Z Aλ µ tan β 1 For λ < 0.3 (blue and red points), mh₁, mχ₁ ~ 10 GeV or below h₁, a₁ and χ₁ are singlet-like or singlino-like and h₂ is SM-like ``Dark Light-Higgs (DLH) Scenario Nearly PQ limit: κ/λ 0, Aκ 0 + Moderate or small λ: λ < or ~ 0.3 37