Spring Institute: Challenging the Standard Model after the Higgs discovery

Friday 12 May 2017, 10:00 → 17:05 Europe/Rome

High Energy Building, A1 Room (Laboratori Nazionali di Frascati)

Gennaro Corcella (LNF)

Aim of the workshop:

The Spring Institute will gather theorists and experimentalists, working mostly in the Rome area in the field of collider physics. In an informal environment, we shall investigate a few selected topics on possible tests of the Standard Model and its extensions after the discovery of the Higgs boson. In particular, we will discuss of recent progresses in effective field theories, of the trilinear Higgs self-coupling, as well as of the hunting for heavy resonances at the LHC, from both experimental and theoretical viewpoints.

 

1 Standard Model Higgs Effective Field Theory and its extension to the Two-Higgs Doublet Model

In this talk l will review effective field theories (EFTs) for Higgs physics. In this framework one can study deviations from the Standard Model (SM) in a systematic way, under the assumption that New Physics is heavier than the electroweak scale and can be decoupled. I will show how the naive LO framework can be consistently extended to NLO in perturbation theory, and present some details of the renormalization procedure. The SM EFT can be extended to include new dynamical degrees of freedom, in case they are around the electroweak scale and an energy gap separates them from the other New Physics resonances. In particular, in the second part of the talk I will present the full set of dimension-6 operators for the 2-Higgs Doublet Model, as this model is one of the simplest extensions of the SM and well motivated by supersymmetry. Moreover, I will discuss some aspects of the breaking of the electroweak symmetry and of the diagonalization of the mass matrices of the Higgs bosons in presence of the effective operators.

Speaker: Margherita Ghezzi (PSI)

Slides LNF_spring_institute_ghezzi.pdf

2 Study of the production modes of the Higgs boson and EFT interpretations in the decay channel H ->ZZ*->4l at 13 TeV with the ATLAS detector

Studies have been performed in order to pose limits on BSM couplings related to additional EFT contributions to the SM Lagrangian. In this picture, the VBF and VH production mechanisms shows a good sensitivity to BSM contributions to the HZZ vertex. Measurements are therefore performed in reduced phase spaces enriched in each production mode, obtained by categorizing the events in fiducial volumes. In each category, multivariate discriminants are built to separate the different production modes contribution and to investigate possible deviations with respect to the SM predictions. The proposed approach allows also to minimize the model dependence. Results are shown in the H -> ZZ* -> 4l decay channel with the 13 TeV data recorded by ATLAS at LHC.

Speaker: Giada Mancini (LNF)

Slides gMancini_SpringInstitute.pdf

3 Bounding the trilinear Higgs self coupling by means of precision electroweak measurements

I will talk about the constraints on the trilinear Higgs self-coupling that arise from loop effects in the W boson mass and the Weinberg angle predictions. The contributions to these precision observables arise from two-loop diagrams featuring an anomalous trilinear Higgs self coupling, parametrized by means of a single parameter $\k_\lambda$, effectively rescaling the Standard Model coupling. The computation has been carried out in the Unitary Gauge, but I will explicitly show that these contributions are gauge independent. The bounds found on the trilinear Higgs self-coupling are competitive with those coming from Higgs-pair production.

Speaker: Marco Fedele (Università "La Sapienza" - Roma)

Slides SpringInstitute_Fedele.pdf

12:15 Lunch Break

4 Searching for high-mass resonances with ATLAS

A broad program of searches of new heavy resonances with masses from O(100 GeV) to O(1 TeV) is performed by the ATLAS experiment. In this talk, the most recent results on these searches using the Run2 data will be presented. Various event topologies will be covered, including both hadronic and leptonic final states.

Speaker: Marianna Testa (LNF)

Slides testa_SpringLNF2017.pdf

5 Search for high-mass resonances decaying in 2 muons at CMS

The dimuon final-state signature is a key search channel for various new phenomena expected in theories that go beyond the Standard Model (SM). One of the most clean signatures would be the observation of a narrow resonance in the invariant-mass spectrum of lepton pairs, predicted by many models at the TeV scale. Examples include models described with extended gauge groups, featuring additional U(1) symmetries, based on GUTs, and the Sequential Standard Model (SSM), that includes a heavy boson with SM-like couplings. This search channel benefits from high-signal selection efficiency and relatively small and well-understood backgrounds. The most recent result obtained by the CMS experiment with the 2016 proton-proton LHC collisions at 13 TeV, corresponding to 13/fb, will be presented. No significant deviations from the SM expectation are observed. Upper bounds are set on the masses of the hypothetical particles.

Speaker: Federica Primavera (LNF)

Slides Frascati_workshop.pdf

6 Supersymmetric signals in Z' decays at the LHC

I discuss possible supersymmetric decays of heavy bosons Z', predicted in GUT-inspired U(1) models and in the Sequential Standard Model. In particular, Z' decays into pairs of sleptons and gauginos, leading to final states with charged leptons and missing energy, are investigated. The impact of the inclusion of supersymmetric channels on the Z' mass exclusion limits at the LHC is also presented.

Speaker: Gennaro Corcella (LNF)

Slides zlnf1.pdf