About the Higgs boson

Transcript

1 About the Higgs boson (On June 2012) Majorana Lectures Naples, June 14, 2012 Riccardo Barbieri SNS and INFN, Pisa

2 Il modello di Higgs del Modello Standard φ T =(φ 1 (x), φ 2 (x), φ 3 (x), φ 4 (x)) E = 1 2 Σ i[( dφ i dt )2 +( φ i ) 2 ]+V (φ) V (φ) = µ 2 φ 2 + λ 2 ( φ 2 ) 2 φ 2 = φ φ φ φ 2 4 Soluzioni di minima energia: φ R (4) φ x =(x,t) W ±,Z < φ < φ > 2 = Σ i (φ 0 i ) 2 = v 2 = µ2 i >= φ 0 i λ 1 - Esiste una simmetria residua R (3) intatta: la simmetria dell elettromagnetismo e intatta la carica elettrica e conservata 2 - La degenerazione del vuoto e descritta da 3 angoli θ i (6-3) 3 - Delle 4 particelle che corrispondono alle vibrazioni quantistiche del campo φ, tre (i θ i ) spariscono dallo spettro, mentre la quarta, il bosone di Higgs ha massa m h = λv

3 Rottura spontanea della simmetria ElettroDebole Interpretazione fisica: Una condensazione di BE di particelle scalari, che riempie tutto lo spazio in una configurazione costante, lo stato di minima energia del sistema Conseguenze fisiche: L (EWSB) = D µ φ 2 V (φ) SU(2) U(1) U(1) em Viaggiando in questo mezzo, i bosoni intermedi e i fermioni acquistano massa L elettromagnetismo e le interazioni deboli sono distinti Nel Modello minimo, con 4 campi scalari per partire, esiste una particella scalare, il bosone di Higgs, con accoppiamenti ai bosoni di gauge e ai fermioni determinati dalle masse rispettive

4 Particle Physics in one page Una (una pagina pagina di di qualche tempo fafa!) L SM = 1 4 Fa µνf aµν + i ψ Dψ +ψ i λ ij ψ j h + h.c. + D µ h 2 V (h) +N i M ij N j The gauge sector (1) The flavor sector (2) The EWSB sector (3) The ν-mass sector (4) (if Majorana)

5 Higgs boson searches after 2011 Let s be patient Yes, but... σ obs σ bk σ SM (compiled by Gigi Rilandi) As case study, accept 125 GeV discovery

6 Standard/Non-standard Higgs couplings (at 125 GeV) Tree Level Not an elementary particle (other parameters for LHC14 only) L = a( 2m2 W v mixing with other scalars E.g.: g hb b ( with SU(2) singlets, doublets or even triplets) New missed final states E.g.: Loop Level W + µ W µ h χ DM χ DM + m2 Z v Z2 µ)h + c f m f v ffh a SM = c f SM 1 with new virtual particles: t, W ; t, T, ρ ±,0 µ, etc L eff = c γ α πv F µνh 2 + c g α S 12πv G2 µνh c γ SM =2/9, cg SM 1 Can have large effects, δa/a, δc/c = O(1), if new physics nearby

7 Higgs widths Relative to the SM, the Higgs widths scales get approximately modified as a 2 where, taking into account the W loop ĉ γ c γ c V ĉ γ,sm 0.8

8 Higgs rates For m h = 125 GeV the total width scales as and, as long as h b b dominates Higgs widths

9 Examples of preparatory gymnastics (at 125 GeV) full 2011 data end of 2012 projection c loop effects Carmi, Falkowski, Kuflik, Volansky tree level a Azatov, Contino et al

10 Is it the SM up to very high energies? ATLAS example for SUSY searches 7 T ev 1.2 T ev 6 (See tomorrow) M(T ev ) Very surprising if this were the case! Nevertheless...

11 The SM unchanged up to very high energies largest couplings Higgs self-coupling most accurate calculation to date Degrassi et al

12 A special meaning for λ 0 at MPl? M t Assume SM unchanged up to M Pl Degrassi et al M h Absolute stability at M Pl (λ(m Pl ) 0) not quite achieved for current best values of M t and M h Speculations about possible meaning of all this not lacking

13 The Fine Tuning problem of the Fermi scale δm 2 SM H = + New in isolation grows like Λ 2 where Λ the cutoff i.e. the highest scale at which the theory is valid: a signal of the sensitivity of m H knows about Λ NP i.e. any higher physical scale to which the Higgs boson is possibly coupled M Pl,M GUT...you name it δm 2 H λ2 16π 2 Λ2 NP The ElectroWeak scale determined by the Higgs mass Why G 1/2 F << Λ NP? Why G 1/2 F << G 1/2 N? led

14 Another look at the FT problem There are infinitely many theories at short distances, that give the same physics of the Standard Model, as long as their low energy spectrum coincides with the one of the SM including the Higgs boson We only know of approximate symmetries that can explain this h }SM In all explicit examples, without unwarranted cancellations, new phenomena required at a scale not far from Λ NP What are and where are they? m h

15 The (many) reactions to the FT problem 0. Ignore it and view the SM in isolation (if no other short distance scale, what about gravity?) 1. Cure it by symmetries: SUSY, Higgs boson as PGB 2. A new strong interaction nearby 3. A new strong interaction not so nearby: quasi-cft 4. Saturate the UV nearby: extra-dimensions around the corner 5. Warp space-time: RS 6. Accept it: anthropic selection, the multiverse, the vacua of string theory Anything else?