The 2013 CERN-Latin-American School of High-Energy Physics Arequipa, Peru (6–19 March 2013) LHC Results Highlights (Lecture III: Results on Higgs and New Physics Searches) ´ Oscar Gonz´ alez (CIEMAT) 1 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
Outline of Lecture III (Results on Higgs and New Physics Searches) • The SM Higgs boson ⇒ The search of the boson: the last two years ⇒ The Higgs discovery ⇒ Measure as many channels as posssible ⇒ Measuring its properties: Is the 125-GeV boson the Higgs? ⇒ Other searches for Higgs-like particles • Searches of other SM-like Higgs bosons • Searches of New Physics (SUSY covered in Lecture II) ⇒ Mostly an overview. . . too much to cover, no obvious hint to follow ⇒ Inclusive searches: resonances, tails, . . . ⇒ Common models: extradimensions, leptoquarks, . . . ⇒ Searches motivated by “Natural Higgs” • Upgrades and plans for the “Run 2” (2015 and beyond) 2 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
The SM-Higgs Search 3 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
Where should the Higgs be? (before Dec 2011) The Higgs is the missing keystone of the Standard Model. Its existance is strongly motivated by the success of the model but there is nothing proving it. The EWK constraints from pre-LHC colliders indicated a mass around 100 GeV. There are also theoretical considerations that motivates a light Higgs. The idea is that SM-related parameters that are sensitive to the Higgs mass allows to make es- timations of preferred values. Even with the addition of new physics (e.g. Supersym- metry) the bounds were close to what the EWK fits sug- gested. On the other hand, most of these assume the Higgs sec- tor is as the SM indicated. Nature might not be as predictable as we think 4 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
Search for the SM Higgs (before Dec 2011) The search at the LHC experiments was performed assuming a SM-Higgs-like boson at any not-excluded (by Tevatron or LEP) mass. For low masses (115-135 GeV) ⇒ H → bb is the dominant decay channel Impossible to detect the direct production channel ( pp → H ) Associated production with a weak vector ⇒ others: γγ , ττ Branching ratios are small, but the LHC produces many Higgses For medium masses (135-200 GeV) ⇒ Main channel is H → W W so use direct production ⇒ Need of leptons prevents full decay reconstruction ⇒ Associated channels helped on this, but smaller yield. Masses higher than 200 were not reachable at Tevatron, but the LHC opened them: ⇒ Specifically the “Golden channel” H → ZZ → lll ′ l ′ This simple structure was “violated” since using off-shell bosons the different channels contributed beyond their optimal regions. 5 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
December 2011 On December 13 of 2011, ATLAS and CMS presented at CERN the status of the SM Higgs searches and for the first time hints of a particle with mass close to 125 GeV. Signal was not completely significant, but excesses appeared in several channels and seemed consistent with a reasonance in that area decaying to several final states. In addition, the analyses performed were able to ex- clude all the medium masses, allowing only the re- gion of the excesses to be reasonable compatible with the EWK fits. Due to its theoretical motivation within the SM, the Higgs boson becomes the first candidate to be the particle causing the excess. So all the focus was in searching for a possible SM- like Higgs boson with a mass around 125 GeV. And we enter in 2012, the year of the 8 TeV and the Higgs search. . . 6 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
The search of the 125-GeV Higgs: strategy After the “December 2011 event” the plan was to confirm the presence of a signal (and also reach the “discovery”, 5 σ level) using the new data collected from April. • LHC energy raised to 8 TeV to increase yield. • Efforts focusing on most sensitive channels: H → γγ H → ZZ ∗ → lll ′ l ′ These also provide the cleanest channels to measure the properties since we reconstruct the full decay. They are also complementary: one with reasonable yield but high background and the other with small yield but very low background. • Of course, secondary channels also very relevant H → ττ H → W W ∗ → lνl ′ ν H → bb (associated production) because they provide further sensitivity (but low-significant signal) and because they provide additional information (additional cou- plings to the boson) 7 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
Higgs Search: update for ICHEP-2012 For ICHEP-2012 the size of the available dataset was ∼ 5 . 5 fb − 1 of 8 TeV collisions Higher available energy but tougher conditions (pile-up, triggering) led to a comparable sensitivity (a bit better) with respect to the 7-TeV sample. Analysis focused to the observed excesses appear- ing in the region that is not excluded. ⇒ ATLAS presented the results for the most sensi- tive channels ( γγ and lll ′ l ′ ) leading the quest. -1 -1 CMS s = 7 TeV, L = 5.1 fb s = 8 TeV, L = 5.3 fb 1 Local p-value σ 1 σ 2 It provided a clean result for the discovery of a new boson. -2 10 σ 3 -4 10 σ 4 ⇒ CMS used the five channels that has reasonable -6 10 σ 5 sensitivity. -8 10 Combined obs. Combined obs. Exp. for SM H Exp. for SM H σ 6 → → γ γ γ γ H H → → H H ZZ ZZ -10 10 More prone to fluctuation in less sensitive channels, but it pro- → → H H WW WW → → τ τ τ τ H H → → σ H H bb bb 7 vided a more general picture about the boson. -12 10 110 115 120 125 130 135 140 145 m (GeV) H It is worth it to discuss the details of the current results involving this boson: sev- eral updates after ICHEP! 8 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
A new boson at 125 GeV: CMS Results (I) • The CMS H γγ ( CMS-PAS-HIG-12-016 ) is per- → fraction of events/0.04 MC Background CMS Preliminary 0.12 ggh 124GeV Simulation vbf 124GeV formed by using several categories of diphoton (for in- wzh 124GeV 0.10 tth 124GeV clusive production mode) and two categories for tagging 0.08 0.06 Vector-Boson Fusion processes. 0.04 0.02 • For Higgs, VBF process is very important since it is siz- 0.00 -1.0 -0.5 0.0 0.5 1.0 di-photon MVA output able (LO gg → H process is via loops) and involve very 4 SM Observed (Asymptotic) CMS Preliminary ) different couplings γ Median Expected (Asymptotic) -1 s = 7 TeV, L = 5.1 fb 3.5 γ ± σ 1 Expected → -1 s = 8 TeV, L = 5.3 fb ± σ 2 Expected (H 3 σ / 95%CL 2.5 ) γ 2 γ → 1.5 (H → Specially atractive for fermiophobic Higgss σ 1 × σ 1 SM 0.5 0 110 115 120 125 130 135 140 145 150 → Tagged with forward jets. m (GeV) H CMS preliminary Di-jet loose -1 s = 7 TeV, L = 5.1 fb -1 s = 8 TeV, L = 5.3 fb Di-jet tight 8TeV Event Class • Analysis with MVA cross-checked with cut-based analy- Combined Untagged 3 m = 125.0 GeV H σ σ ± / = 1.56 0.43 SM Untagged 2 sis: comparable result. Untagged 1 Untagged 0 Di-jet • Local significance: 4 . 1 σ ,a bit higher yield than expected. 7TeV Untagged 3 Untagged 2 Untagged 1 • Updated result expected during the Conferences at Untagged 0 -4 -2 0 2 4 6 8 10 Moriond. . . σ σ Best Fit / SM 9 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
A new boson at 125 GeV: CMS Results (II) • The H → 4 l analysis by CMS ( CMS-PAS-HIG-12-041 ) has been updated several times since July. • Also expected new results for Moriond, but mostly focused on properties since signal is well established. • Using a kinematic discriminant based on the masses of the reconstructed Z and angular correlations (which are based on the scalar nature of the boson). • The yield is a bit lower, but still in agreement with SM: µ = 0 . 80 +0 . 35 − 0 . 28 • This analysis is the central reference for properties of the boson (see later). 10 O. Gonz´ alez (CIEMAT) (March 2013) Lecture III on LHC Results Highlights (CLASHEP 2013)
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