Addressing the B-physics anomalies in a fundamental Composite Higgs Model

I present a model addressing coherently the naturalness problem of the electroweak scale and the observed pattern of deviations from the Standard Model in semi-leptonic decays of $B$ mesons. The Higgs and the two scalar leptoquarks responsible for the $B$-physics anomalies, $S_1 = ({\bf \bar 3}, {\bf 1}, 1/3)$ and $S_3 = ({\bf \bar 3}, {\bf 3}, 1/3)$, arise as pseudo Nambu-Goldstone bosons of a new strongly coupled sector at the multi-TeV scale. I focus on an explicit realization of such a dynamics in terms of a new strongly coupled gauge interaction and extra vectorlike fermions charged under it. The model presents a very rich phenomenology, ranging from flavour observables, Higgs and electroweak precision measurements, and direct searches of new states at the LHC.Comment: 46 pages, 4 figures, 1 table. v3: Minor modifications, matches the published versio

High-pTp_T dilepton tails and flavour physics

We investigate the impact of flavour-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in $p~p \to \ell^+ \ell^-$ processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb$^{-1}$ of data, we derive the best up-to-date limits on the full set of 36 relevant four-fermion operators, as well as estimate the sensitivity achievable at the HL-LHC. We discuss how these high-$p_T$ measurements can provide complementary information to the low-$p_T$ rare meson decays. In particular, we find that the recent hints on lepton flavour universality violation in $b \to s \mu^+ \mu^-$ transitions are already in mild tension with the dimuon spectrum at high-$p_T$ if the flavour structure follows minimal flavour violation. Even if the mass scale of New Physics is well beyond the kinematical reach for on-shell production, the signal in the high-$p_T$ dilepton tail might still be observed, a fact that has been often overlooked in the present literature. In scenarios where new physics couples predominantly to third generation quarks, instead, the HL-LHC phase is necessary in order to provide valuable information.Comment: 9 pages, 5 figures, 1 tabl

Composite Dark Matter and LHC Interplay

The actual realization of the electroweak symmetry breaking in the context of a natural extension of the Standard Model (SM) and the nature of Dark Matter (DM) are two of the most compelling questions in high-energy particle physics. Composite Higgs models may provide a unified picture in which both the Higgs boson and the DM particle arise as pseudo Nambu-Goldstone bosons of a spontaneously broken global symmetry at a scale $f\sim$ TeV. In this paper we analyze a general class of these models based on the coset SO(6)/SO(5). Assuming the existence of light and weakly coupled spin-1 and spin-1/2 resonances which mix linearly with the elementary SM particles, we are able to compute the effective potential of the theory by means of some generalized Weinberg sum rules. The properties of the Higgs boson, DM, top quark and the above resonances are thus calculable and tightly connected. We perform a wide phenomenological analysis, considering both collider physics at the LHC and astrophysical observables. We find that these models are tightly constrained by present experimental data, which are able to completely exclude the most natural setup with $f\simeq 800$ GeV. Upon increasing the value of $f$, an allowed region appears. In particular for $f\simeq 1.1$ TeV we find a concrete realization that predicts $m_{DM}\simeq 200$ GeV for the DM mass. This DM candidate lies close to the present sensitivity of direct detection experiments and will be ruled out - or discovered - in the near future.Comment: 36 pages + 2 appendices, 9 figure

On the breaking of Lepton Flavor Universality in B decays

In view of recent experimental indications of violations of Lepton Flavor Universality (LFU) in $B$ decays, we analyze constraints and implications of LFU interactions, both using an effective theory approach, and an explicit dynamical model. We show that a simple dynamical model based on a $SU(2)_L$ triplet of massive vector bosons, coupled predominantly to third generation fermions (both quarks and leptons), can significantly improve the description of present data. In particular, the model decreases the tension between data and SM predictions concerning: i) the breaking of $\tau$-$\mu$ universality in $B\to D^{(*)} \ell \nu$ decays; ii) the breaking of $\mu$-$e$ universality in $B \to K \ell^+\ell^-$ decays; iii) the difference between exclusive and inclusive determinations of $|V_{cb}|$ and $|V_{ub}|$. The minimal version of the model is in tension with ATLAS and CMS direct searches for the new massive vectors (decaying into $\tau^+\tau^-$ pairs), but this tension can be decreased with additional non-standard degrees of freedom. Further predictions of the model both at low- and high-energies, in view of future high-statistics data, are discussed.Comment: 20 pages, 7 figures. v2: discussion on flavor structure clarified; added discussion on the associated production of the heavy vectors and a few refs. Published versio

Knocking on New Physics' door with a Scalar Resonance

We speculate about the origin of the recent excess at ~750 GeV in diphoton resonance searches observed by the ATLAS and CMS experiments using the first 13 TeV data. Its interpretation as a new scalar resonance produced in gluon fusion and decaying to photons is consistent with all relevant exclusion bounds from the 8 TeV LHC run. We provide a simple phenomenological framework to parametrize the properties of the new resonance and show in a model-independent way that, if the scalar is produced in gluon fusion, additional new colored and charged particles are required. Finally, we discuss some interpretations in various concrete setups, such as a singlet (pseudo-) scalar, composite Higgs, and the MSSM.Comment: 20 pages, 4 figures, 1 table. Extraction of the ATLAS 13 TeV diphoton signal and 8 TeV ZZ bound corrected, no sizable change in the final combination. Presentation improved, references added, and conclusions unchanged. Version to appear in EPJ

Higgs and beyond in the LHC era

The last few years witnessed some major breakthroughs in the field of fundamental particle physics, which had a big impact in our understanding of Nature at a microscopic level. On March 30th, 2010, the first proton-proton collisions took place at the Large Hadron Collider (LHC), marking the beginning of a new era in particle physics. The excellent performance of the machine and the detectors, due to the fantastic work of all the researchers involved in the experiments, lead, in only two years, to the announcement of the discovery of the Higgs boson on July 4th, 2012. This event could be considered as the peak of success for the Standard Model (SM) of elementary particles, which predicted the existence of this particle \u2013 as well as all its properties \u2013 since more than forty years before. In the following two years the ATLAS and CMS experiments at the LHC measured the properties of the Higgs particle with a good accuracy, showing no significant deviation from the SM. In the meanwhile, also the numerous direct searches for other new particles turned out to give only negative results, against all expectations from the theory community, pushing the scale of new physics to higher and higher values. Also, while the cosmological evidence for Dark Matter (DM) is now stronger than ever, so far all direct and indirect searches provided negative results (albeit with some isolated exceptions which, however, are still much debated in the literature and seem to be incompatible with other negative results) and the bounds on weakly interacting massive particle DM are extremely strong. In neutrino physics an important event took place in June 2011, when the Tokay-to-Kamioka (T2K) collaboration reported an evidence for a non-zero, and sizable, value of the reactor neutrino mixing angle, \u3b813. This was confirmed in March 2012 by the Daya Bay collaboration, which measured this mixing angle with a very high precision, confirming that its value lies on the high-end of previous upper bounds. Since many popular and well motivated models of neutrino mixing predicted a zero, or very small, value of the reactor angle, this result was very important and offered a new insight in the quest for understanding the origin of flavor in the lepton sector. Also, since CP violation in the lepton sector effects vanish in the \u3b813 \u2192 0 limit, the fact that this angle is sizable opens up many interesting possibilities for measuring CP violation in the neutrino sector. The work presented in this thesis was largely stimulated by these two major breakthroughs in particle physics. On the one hand the Higgs discovery and the measurement of its properties, in particular its mass, lead us to study the consequences of these measurements for a specific class of models beyond the SM: composite Higgs models (and also in supersymmetric versions of these models). In particular, we found that a very definite (and testable) prediction for the spectrum of new physics can be obtained: fermionic top partners are expected to be near the 3c1TeV scale. Also, the measurements of the Higgs couplings and the fact that the bounds for the new physics scale are often much higher than the electroweak scale, open up the possibility of studying possible deformations from the SM in an effective field theory framework. In this context we studied the possibility of linking the properties of the Higgs with other electroweak observables, very well constrained by LEP, via renormalization group effects, finding that they already allow to derive constraining, and independent, bounds on some Higgs properties. In the future, when some deviation from the SM will be \u2013 hopefully \u2013 observed, these effects could provide a new window on the new physics sector. On the other hand, we studied how the measured value of \u3b813 can be accommodated in some motivated models of neutrino mixing by exploiting corrections due to the mixing among the charged leptons. Such corrections are expected, for example, in Grand Unified Theories, which allow to link the charged lepton sector with the quark sector, and therefore the neutrino mixing matrix with the quark mixing one. This analysis allowed us to obtain a precise prediction for the value of the Dirac CP violating phase in neutrino mixing, testable by future neutrino experiments

Toward a coherent solution of diphoton and flavor anomalies

We propose a coherent explanation for the 750 GeV diphoton anomaly and the hints of deviations from Lepton Flavor Universality in B decays in terms a new strongly interacting sector with vectorlike confinement. The diphoton excess arises from the decay of one of the pseudo-Nambu-Goldstone bosons of the new sector, while the flavor anomalies are a manifestation of the exchange of the corresponding vector resonances (with masses in the 1.5-2.5 TeV range). We provide explicit examples (with detailed particle content and group structure) of the new sector, discussing both the low-energy flavor-physics phenomenology and the signatures at high $p_T$. We show that specific models can provide an excellent fit to all available data. A key feature of all realizations is a sizable broad excess in the tails of $\tau^+ \tau^-$ invariant mass distribution in $p p \to \tau^+ \tau^-$, that should be accessible at the LHC in the near future.Comment: v2: 32 pages, 9 figures, 2 tables. Published version. Extended discussion about the flavor structure of the model and high-PT phenomenology, typos corrected. Added note about the relevance of the paper in light of the absence of the diphoton signal at the LH

B-physics anomalies: a guide to combined explanations

Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a $U(2)_q \times U(2)_\ell$ flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an $SU(2)_L$-singlet vector leptoquark emerges as a particularly simple and successful framework.Comment: 33 pages, 7 figures, 2 tables. Extended discussion and one plot added on single production of leptoquarks, typos corrected, references adde

Pseudo-observables in electroweak Higgs production

We discuss how the leading electroweak Higgs production processes at the LHC, namely vector-boson fusion and Higgs+W/Z associated production, can be characterized in generic extensions of the Standard Model by a proper set of pseudo-observables (PO). We analyze the symmetry properties of these PO and their relation with the PO set appearing in Higgs decays. We discuss in detail the kinematical studies necessary to extract the production PO from data, and present a first estimate of the LHC sensitivity on these observables in the high-luminosity phase. The impact of QCD corrections and the kinematical studies necessary to test the validity of the momentum expansion at the basis of the PO decomposition are also discussed.Comment: 34 pages, 12 figures, 1 tabl