We perform a detailed analysis of flavour changing neutral current processes in the charm sector in the context of 331 models. As pointed out recently, in the case of $Z^prime$ contributions in these models there are no new free parameters beyond tho
se already present in the $B_{d,s}$ and $K$ meson systems analyzed in the past. As a result, definite ranges for new Physics (NP) effects in various charm observables could be obtained. While generally NP effects turn out to be small, in a number of observables they are much larger than the tiny effects predicted within the Standard Model. In particular we find that the branching ratio of the mode $D^0 to mu^+ mu^-$, despite remaining tiny, can be enhanced by 6 orders of magnitude with respect to the SM. We work out correlations between this mode and rare $B_{d,s}$ and $K$ decays. We also discuss neutral charm meson oscillations and CP violation in the charm system. In particular, we point out that 331 models provide new weak phases that are a necessary condition to have non-vanishing CP asymmetries. In the case of $Delta A_{CP}$, the difference between the CP asymmetries in $D^0 to K^+ K^-$ and $D^0 to pi^+ pi^-$, we find that agreement with experiment can be obtained provided that two conditions are verified: the phases in the ranges predicted in 331 models and large hadronic matrix elements.
The Glashow-Iliopoulos-Maiani mechanism is extremely efficient to suppress the flavour-changing neutral current decays of charmed hadrons induced by the $c to u$ transitions, making such processes particularly sensitive to phenomena beyond the Standa
rd Model. In particular, $c to u$ decays with a neutrino pair in the final state are theoretically appealing due to the small long-distance contributions. Moreover, in the framework of the Standard Model Effective Field Theory (SMEFT), the $SU(2)_L$ invariance allows to relate the Wilson coefficients in the effective Hamiltonian governing the $c to u u {bar u}$ decays to the coefficients in the $c to u ell^+ ell^-$ Hamiltonian. We analyze the $B_c to B^{(*)+} u {bar u}$ decays, for which branching fractions of at most ${cal O}(10^{-16})$ are predicted in the Standard Model including short- and long-distance contributions, so small that they can be considered as null tests. Using SMEFT and the relation to the $c to u ell^+ ell^-$ processes we study the largest enhancement achievable in generic new physics scenarios. Then we focus on a particular extension of the Standard Model, the 331 model. SMEFT relations and the connection with $c to u ell^+ ell^-$ imply that ${cal B}(B_c to B^{(*)+} u {bar u})$ could even reach ${cal O}(10^{-6})$, an extremely large enhancement. A less pronounced effect is found in the 331 model, with ${cal O}(10^{-11})$ predicted branching fractions. Within the 331 model correlations exist among the $B_c to B^{(*)+} u bar u$ and $Kto pi u bar u$, $Bto (X_s, K, K^*) u bar u$ channels.
Inclusive semileptonic decays of beauty baryons are studied using the heavy quark expansion to ${cal O}(1/m_b^3)$, at leading order in $alpha_s$. The case of a polarized decaying baryon is examined, with reference to $Lambda_b$. An extension of the S
tandard Model effective Hamiltonian inducing $b to U ell {bar u}_ ell$ transitions ($U=u,,c$ and $ell=e,,mu,,tau$) is considered, which comprises the full set of D=6 semileptonic operators with left-handed neutrinos. The effects of the new operators in several observables are described.
Besides being important to determine Standard Model parameters such as the CKM matrix elements $|V_{cb}|$ and $|V_{ub}|$, semileptonic $B$ decays seem also promising to reveal new physics (NP) phenomena, in particular in connection with the possibili
ty of uncovering lepton flavour universality (LFU) violating effects. In this view, it could be natural to connect the tensions in the inclusive versus exclusive determinations of $|V_{cb}|$ to the anomalies in the ratios $R(D^{(*)})$ of decay rates into $tau$ vs $mu, e$. However, the question has been raised about the role of the parametrization of the hadronic $B to D^{(*)}$ form factors in exclusive $B$ decay modes. We focus on the fully differential angular distributions of $bar B to D^* ell^-{bar u}_ell$ with $D^* to D pi$ or $D^* to D gamma$, the latter mode being important in the case of $B_s to D_s^*$ decays. We show that the angular coefficients in the distributions can be used to scrutinize the role of the form factor parametrization and to pin down deviations from SM. As an example of a NP scenario, we include a tensor operator in the $b to c$ semileptonic effective Hamiltonian, and discuss how the angular coefficients allow to construct observables sensitive to this structure, also defining ratios useful to test LFU.
We reconsider the possibility that the tension in the $|V_{cb}|$ determinations from inclusive and exclusive $B$ decay modes is due to a new physics effect. We modify the Standard Model effective Hamiltonian for semileptonic $b to c$ transitions incl
uding a tensor operator with a lepton flavour dependent coupling $epsilon_T^ell$, and investigate separately the muon and electron modes. The interference term between SM and NP, proportional to the lepton mass, has different impact in the inclusive and exclusive $B$ modes to muon. Moreover, even when the lepton mass is small as for the electron, the NP effect is different in inclusive and exclusive $B$ channels. For both $mu$ and $e$ we find a region of $epsilon_T^{mu,,e}$ where the constraints from $B^- to D^{(*)0} ell^- {bar u}_ell$ and $B to X_c , ell , bar u_ell$ are satisfied for the same $|V_{cb}|$.
We use a holographic method to investigate thermalization of a boost-invariant strongly interacting non-Abelian plasma. Boundary sourcing, a distorsion of the boundary metric, is employed to drive the system far from equilibrium. Thermalization is an
alyzed through nonlocal probes: the equal-time two-point correlation function of large conformal dimension operators in the boundary theory, and Wilson loops of different shapes. We study the dependence of the thermalization time on the size of the probes, and compare the results to the ones obtained using local observables: the onset of thermalization is first observed at short distances.
We study a set of exclusive decay modes of the Standard Model Higgs boson into a vector meson and a dilepton pair: $hto V ell^+ ell^-$, with $V=Upsilon, J/psi,phi$, and $ell=mu, tau$, determining the decay rates, the dilepton mass spectra and the $V$
longitudinal helicity fraction distributions. In the same framework, we analyze the exclusive modes into neutrino pairs $hto V u bar u$. We also discuss the implications of the recent CMS and ATLAS results for the lepton flavor-changing process $hto tau^+ mu^-$ on the $hto V tau^+ mu^-$ decay modes.
Mesons with quantum numbers $J^{PC}=1^{-+}$ cannot be represented as simple quark-antiquark pairs. We explore hybrid configurations in the light meson sector comprising a quark, an antiquark and an excited gluon, studying the properties of such state
s in a phenomenological model inspired by the gauge/gravity correspondence. The computed mass, compared to the experimental mass of the $1^{-+}$ candidates $pi_1(1400)$, $pi_1(1600)$ and $pi_1(2015)$, favous $pi_1(1400)$ as the lightest hybrid state. An interesting result concerns the stability of hybrid mesons at finite temperature: they disappear from the spectral function (i.e. they melt) at a lower temperature with respect to other states, light vector and scalar mesons, and scalar glueballs.
The lowest dimensional gluon condensate $G_2$ is analyzed at finite temperature and chemical potential using a holographic model of QCD with conformal invariance broken by a background dilaton. Starting from the free energy of the model, the thermody
namical quantities needed to determine the $T$ and $mu$ dependence of the gluon condensate are evaluated. At high temperature the gluon condensate is independent of chemical potential. Moreover, at $mu=0$ and in the string frame, the temporal and spatial Wilson loops at low temperature are computed; they are related to the (chromo) electric and magnetic components of $G_2$, respectively. The $T$-dependence of the two components is separately determined.
