Electroweak Corrections to Higgs to γγ\gamma\gamma and W+W−W^+W^- in the SMEFT

Higgs decays to gauge boson pairs are a crucial ingredient in the study of Higgs properties, with the decay $H\rightarrow\gamma\gamma$ being particularly sensitive to new physics effects. Assuming all potential new physics occurs at energies much above the weak scale, deviations from Standard Model predictions can be parameterized in terms of the coefficients of an effective field theory (SMEFT). When experimental limits on the SMEFT coefficients reach an accuracy of a few percent, predictions must be done beyond the lowest order in the SMEFT in order to match theory and experimental accuracy. This paper completes a program of computing the one-loop electroweak SMEFT corrections to $H\rightarrow VV^\prime$, $V=W^\pm,Z,\gamma$. The calculation of the real contribution to $H\rightarrow W^+W^-\gamma$ is performed by mapping two-loop amplitudes to the $3-$ body phase space.Comment: 21 pages, 3 figures; V2: Version accepted for publicatio

Aspects of LHC phenomenology

In the Standard Model the electroweak symmetry is broken due to the existence of an elementary scalar particle: the Higgs boson. The Higgs boson was for a long time the missing piece of the Standard Model puzzle. Furthermore in the absence of direct signal of new physics, the Higgs boson couplings might indirectly indicate a portal to Beyond Standard Model theories. In this context, the discovery made at LHC of a new particle with a mass of 125.66 ± 0.34 GeV and the characteristics of the Higgs boson is of great importance: it could be the conclusive achievement of the Standard Model, or it could give a renewed impulse to the search for new physics. The discovery of this new particle was announced by the ATLAS and CMS collaborations during 2012. After that, all LHC and TeVatron collaborations presented at the Moriond 2013 conference the results based on the full collected data. In the first part of this thesis we study the properties of this particle in order to be sure that the new resonance is, indeed, the Higgs boson. In particular we study its couplings with the other particles: the SM has definite predictions for the gauge boson and fermion couplings with the Higgs boson. Those affect both the Higgs boson production mechanism at the LHC as well as its dominant decay modes. We find that these couplings are compatible with the SM ones. Thus, although the discovery of the Higgs boson was expected to be the herald of new physics soon to be found at the TeV scale, so far no signal of new physics nor any clear deviation from the SM Higgs properties have been detected at the LHC. Moreover, the Higgs mass has not provided unambiguous indications for new physics. Thus, in the second part of the thesis, we try to extrapolate the SM to high energy in search for clues. In order to do that we needed a precise computation of the corrections to the SM parameters. The intriguing result is that, assuming the validity of the SM up to very high energy scales, the measured value of the Higgs mass is near-critical, in the sense that it places the EW vacuum right at the border between absolute stability and metastability

Exo-Higgs at 750 GeV and Genesis of Baryons

We propose that the diphoton excess at 750 GeV reported by ATLAS and CMS is due to the decay of an ${\it exo-Higgs}$ scalar $\eta$ associated with the breaking of a new $SU(2)_e$ symmetry, dubbed ${\it exo-spin}$. New fermions, ${\it exo-quarks}$ and ${\it exo-leptons}$, get TeV-scale masses through Yukawa couplings with $\eta$ and generate its couplings to gluons and photons at 1-loop. The matter content of our model yields a $B-L$ anomaly under $SU(2)_e$, whose breaking we assume entails a first order phase transition. A non-trivial $B-L$ asymmetry may therefore be generated in the early universe, potentially providing a baryogenesis mechanism through the Standard Model (SM) sphaleron processes. The spontaneous breaking of $SU(2)_e$ can in principle directly lead to electroweak symmetry breaking, thereby accounting for the proximity of the mass scales of the SM Higgs and the exo-Higgs. Our model can be distinguished from those comprising a singlet scalar and vector fermions by the discovery of TeV scale exo-vector bosons, corresponding to the broken $SU(2)_e$ generators, at the LHC.Comment: 9 pages, 3 figure

Constraints on the trilinear Higgs self coupling from precision observables

We present the constraints on the trilinear Higgs self coupling that arise from loop effects in the $W$ boson mass and the effective sine predictions. We compute the contributions to these precision observables of two-loop diagrams featuring an anomalous trilinear Higgs self coupling. We explicitly show that the same anomalous contributions are found if the analysis of $m_{ \scriptscriptstyle W}$ and \mbox{\sin^2 \theta^{{\rm lep}}_{{\rm eff}}} is performed in a theory in which the scalar potential in the Standard Model Lagrangian is modified by an (in)finite tower of $(\Phi^\dagger \Phi)^n$ terms with $\Phi$ the Higgs doublet. We find that the bounds on the trilinear Higgs self coupling from precision observables are competitive with those coming from Higgs pair production.Comment: 23 pages, 4 figures; V2: References added, version published on JHE

Higgs Decays to ZZZZ and ZγZ\gamma in the SMEFT: an NLO analysis

We calculate the complete one-loop electroweak corrections to the inclusive $H\rightarrow ZZ$ and $H\rightarrow Z\gamma$ decays in the dimension-$6$ extension of the Standard Model Effective Field Theory (SMEFT). The corrections to $H\rightarrow ZZ$ are computed for on-shell $Z$ bosons and are a precursor to the physical $H\rightarrow Z f {\overline{f}}$ calculation. We present compact numerical formulas for our results and demonstrate that the logarithmic contributions that result from the renormalization group evolution of the SMEFT coefficients are larger than the finite NLO contributions to the decay widths. As a by-product of our calculation, we obtain the first complete result for the finite corrections to $G_\mu$ in the SMEFT.Comment: Published Version. The results for $\Delta r$ in the SMEFT are now given in $R_\xi$ gauge; V3: corrected a typo in eq 38 and 39, updated eq. 4

Probing the Higgs self coupling via single Higgs production at the LHC

We propose a method to determine the trilinear Higgs self coupling that is alternative to the direct measurement of Higgs pair production total cross sections and differential distributions. The method relies on the effects that electroweak loops featuring an anomalous trilinear coupling would imprint on single Higgs production at the LHC. We first calculate these contributions to all the phenomenologically relevant Higgs production ($gg{\rm F}$, VBF, $WH$, $ZH$, $t\bar tH$) and decay ($\gamma \gamma$, $WW^{*}/ZZ^{*}\to 4f$, $b\bar b$, $\tau \tau$) modes at the LHC and then estimate the sensitivity to the trilinear coupling via a one-parameter fit to the single Higgs measurements at the LHC 8 TeV. We find that the bounds on the self coupling are already competitive with those from Higgs pair production and will be further improved in the current and next LHC runs.Comment: 34 pages, 13 figures, 5 tables; V2: New appendix A added on the comparison with the Effective Field Theory approach; V3: Journal versio

LHC bounds on large extra dimensions

We derive new dominant bounds on the coefficient of the effective operator generated by tree-level graviton exchange in large extra dimensions from pp \rightarrow jj data at LHC: M_T > 2.1TeV (ATLAS after 3.1/pb of integrated luminosity), M_T > 3.4 TeV (CMS after 36/pb), MT > 3.2 TeV (ATLAS after 36/pb). We clarify the role of on-shell graviton exchange and compare the full graviton amplitude to ATLAS data, setting bounds on the fundamental quantum-gravity scale.Comment: 16 pages, 6 figures. v2: updated with CMS data. v3: updated with ATLAS data at 36/pb; final published versio

Unified Scenario for Composite Right-Handed Neutrinos and Dark Matter

We entertain the possibility that neutrino masses and dark matter (DM) originate from a common composite dark sector. A minimal effective theory can be constructed based on a dark $SU(3)_D$ interaction with three flavors of massless dark quarks; electroweak symmetry breaking gives masses to the dark quarks. By assigning a $\mathbb Z_2$ charge to one flavor, a stable "dark kaon" can provide a good thermal relic DM candidate. We find that "dark neutrons" may be identified as right handed Dirac neutrinos. Some level of "neutron-anti-neutron" oscillation in the dark sector can then result in non-zero Majorana masses for light Standard Model neutrinos. A simple ultraviolet completion is presented, involving additional heavy $SU(3)_D$-charged particles with electroweak and lepton Yukawa couplings. At our benchmark point, there are "dark pions" that are much lighter than the Higgs and we expect spectacular collider signals arising from the UV framework. This includes the decay of the Higgs boson to $\tau \tau \ell \ell^{\prime}$, where $\ell$($\ell'$) can be any lepton, with displaced vertices. We discuss the observational signatures of this UV framework in dark matter searches and primordial gravitational wave experiments; the latter signature is potentially correlated with the $H \to \tau \tau \ell \ell^{\prime}$ decay.Comment: 8 pages, 4 figures, 1 table. Version published on PR

Higgs boson properties from hadron colliders experiments

We perform a state-of-the-art global fit to all Higgs data. We synthesise them into a 'universal' form, which allows to easily test any desired model. We apply the proposed methodology to extract from data the Higgs branching ratios, production cross sections, couplings and to analyse composite Higgs models, extra particles in the loops, invisible Higgs decay into Dark Matter. Best fit regions lie around the Standard Model predictions and are well approximated by our fit