Lepton (non-) unversality in (flavor changing) neutral current B decays
The following proceedings contain a theory perspective on the flavor changing neutral current anomalies reported by LHCb on the ratios $B\to K^{(*)} μμ/B\to K^{(*)} ee$
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arXiv subjects
Rodrigo Alonso
Publications and source records attributed to Rodrigo Alonso.
On amplitudes, resonances and the ultraviolet completion of gravity
This letter constructs, making use of the on-shell spinor-helicity formalism, a possible ultraviolet completion of gravity following a "bottom-up" approach. The assumptions of locality, unitarity and causality i) require an infinite tower of resonances with increasing spin and quantized mass, ii) introduce a duality relation among crossed scattering channels, and iii) dress all gravitational amplitudes in the Standard Model with a form factor that closely resembles either the Veneziano or the Virasoro-Shapiro amplitude in string theory. As a consequence of unitarity, the theory predicts leading order deviations from General Relativity in the coupling of gravity to fermions that could be explained if space-time has torsion in addition to curvature.
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Tau Polarimetry in B Meson Decays
This article summarizes recent developments in $B\to D^{(\ast)}τν$ decays. We explain how to extract the tau lepton's production properties from the kinematics of its decay products. The focus is on hadronic tau decays, which are most sensitive to the tau polarizations. We present new results for effects of new physics in tau polarization observables and quantify the observation prospects at BELLE II.
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Scattering of light dark matter in atomic clocks
We present a detailed analysis of the effect of light Dark Matter (DM) on atomic clocks, for the case where DM mass and density are such that occupation numbers are low and DM must be considered as particles scattering off the atoms, rather than a classical field. We show that the resulting atomic clock frequency shifts are first order in the scattering amplitudes, and particularly suited to constrain DM models in the regime where the DM mass $m_χ\ll$ GeV. We provide some rough order of magnitude estimates of sensitivity that can be confronted to any DM model that allows for non zero differential scattering amplitudes of the two atomic states involved in the clock.
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A clockwork solution to the flavor puzzle
We introduce a set of clockwork models of flavor that can naturally explain the large hierarchies of the Standard Model quark masses and mixing angles. Since the clockwork only contains chains of new vector-like fermions without any other dynamical fields, the flavor constraints allow for relatively light new physics scale. For two benchmarks with gear masses just above 1 TeV, allowed by flavor constraints, we discuss the collider searches and the possible ways of reconstructing gear spectra at the LHC. We also examine the similarities and differences with the other common solutions to the SM flavor puzzle, i.e., with the Froggatt-Nielsen models, where we identify a new {\it clockworked } version, and with the Randall-Sundrum models.
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Flavoured $B-L$ Local Symmetry and Anomalous Rare $B$ Decays
We consider a flavoured $B-L$ gauge symmetry under which only the third generation fermions are charged. Such a symmetry can survive at low energies (~TeV) while still allowing for two superheavy right-handed neutrinos, consistent with neutrino masses via see-saw and leptogenesis. We describe a mechanism for generating Yukawa couplings in this model and also discuss the low-energy phenomenology. Interestingly, the new gauge boson could explain the recent hints of lepton universality violation at LHCb, with a gauge coupling that remains perturbative up to the Planck scale. Finally, we discuss more general $U(1)$ symmetries and show that there exist only two classes of vectorial $U(1)$ that are both consistent with leptogenesis and remain phenomenologically viable at low-energies.
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Anomaly-free local horizontal symmetry and anomaly-full rare B-decays
The largest global symmetry that can be made local in the Standard Model + 3$ν_R$ while being compatible with Pati-Salam unification is $SU(3)_H\times U(1)_{B-L}$. The gauge bosons of this theory would induce flavour effects involving both quarks and leptons, and are a potential candidate to explain the recent reports of lepton universality violation in rare B meson decays. In this letter we characterise this type of models and show how they can accommodate the data and naturally be within reach of direct searches.
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Wormholes and masses for Goldstone bosons
There exist non-trivial stationary points of the Euclidean action for an axion particle minimally coupled to Einstein gravity, dubbed wormholes. They explicitly break the continuos global shift symmetry of the axion in a non-perturbative way, and generate an effective potential that may compete with QCD depending on the value of the axion decay constant. In this paper, we explore both theoretical and phenomenological aspects of this issue. On the theory side, we address the problem of stability of the wormhole solutions, and we show that the spectrum of the quadratic action features only positive eigenvalues. On the phenomenological side, we discuss, beside the obvious application to the QCD axion, relevant consequences for models with ultralight dark matter, black hole superradiance, and the relaxation of the electroweak scale. We conclude discussing wormhole solutions for a generic coset and the potential they generate.
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Tau properties in $B\to Dτν$ from visible final-state kinematics
In semi-leptonic $B$ decays with a tau lepton, features of the production process are imprinted on the tau helicity states. Since the tau momentum cannot be fully reconstructed experimentally, the available information on the tau properties is encoded in its visible decay products. Focusing on the process $B\to Dτν$, we find explicit relations between the tau properties and the kinematics of the charged particles in the decays $τ\toπν$, $τ\toρν$, and $τ\to\ellν\barν$. In particular, we show that the perpendicular polarization, $P_\perp$, and the forward-backward asymmetry, $A_τ$, of the tau lepton can simultaneously be extracted from an angular asymmetry of the charged particle against the $D$ meson. For the most sensitive decay channel, $τ\toπν$, we expect a relative statistical precision of about $10\%$ for $P_\perp$ and $A_τ$ in a measurement based on $50\,$ab$^{-1}$ of data at BELLE II.
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The lifetime of the $B_c^-$ meson and the anomalies in $B\to D^{(*)}τν$
We investigate a new constraint on new-physics interpretations of the anomalies observed in $B\to D^{(*)}τν$ decays making use of the lifetime of the $B_c^-$ meson. A constraint is obtained by demanding that the rate for $B_c^-\toτ^-\barν$ does not exceed the fraction of the total width that is allowed by the calculation of the lifetime in the standard model. This leads to a very strong bound on new-physics scenarios involving scalar operators since they lift the slight, but not negligible, chiral suppression of the $B_c^-\toτ^-\barν$ amplitude in the standard model. The new constraint renders a scalar interpretation of the enhancement measured in $R_{D^*}$ implausible, including explanations implementing extra Higgs doublets or certain classes of leptoquarks. We also discuss the complementarity of $R_{D^{(*)}}$ and a measurement of the longitudinal polarization of the $τ$ in the $B\to D^*τν$ decay in light of our findings.
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New physics in the kinematic distributions of $\bar B\to D^{(*)}τ^-(\to\ell^-\barν_\ellν_τ)\barν_τ$
We investigate the experimentally-accessible kinematic distributions of the $\bar B\to D^{(*)}τ^-(\to\ell^-\barν_\ellν_τ)\barν_τ$ decays. Specifically, we study the decay rates as functions of the $B\to D^{(*)}$ transferred squared momentum, the energy of the final charged lepton and the angle of its 3-momentum relative to the 3-momentum of the recoiling $D^{(*)}$. The angular distribution allows to introduce new observables, like a forward-backward asymmetry, which are complementary to the total rates. We present analytic formulas for the observable 3-fold 5-body differential decay rates, study the predictions in the Standard Model and investigate the effects in different new-physics scenarios that we characterize using an effective field theory framework.
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Geometry of the Scalar Sector
The $S$-matrix of a quantum field theory is unchanged by field redefinitions, and so only depends on geometric quantities such as the curvature of field space. Whether the Higgs multiplet transforms linearly or non-linearly under electroweak symmetry is a subtle question since one can make a coordinate change to convert a field that transforms linearly into one that transforms non-linearly. Renormalizability of the Standard Model (SM) does not depend on the choice of scalar fields or whether the scalar fields transform linearly or non-linearly under the gauge group, but only on the geometric requirement that the scalar field manifold ${\mathcal M}$ is flat. We explicitly compute the one-loop correction to scalar scattering in the SM written in non-linear Callan-Coleman-Wess-Zumino (CCWZ) form, where it has an infinite series of higher dimensional operators, and show that the $S$-matrix is finite. Standard Model Effective Field Theory (SMEFT) and Higgs Effective Field Theory (HEFT) have curved ${\mathcal M}$, since they parametrize deviations from the flat SM case. We show that the HEFT Lagrangian can be written in SMEFT form if and only if ${\cal M}$ has a $SU(2)_L \times U(1)_Y$ invariant fixed point. Experimental observables in HEFT depend on local geometric invariants of ${\mathcal M}$ such as sectional curvatures, which are of order $1/Λ^2$, where $Λ$ is the EFT scale. We give explicit expressions for these quantities in terms of the structure constants for a general $\mathcal G \to \mathcal H$ symmetry breaking pattern. (Full abstract in pdf)
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A Geometric Formulation of Higgs Effective Field Theory: Measuring the Curvature of Scalar Field Space
A geometric formulation of Higgs Effective Field Theory (HEFT) is presented. Experimental observables are given in terms of geometric invariants of the scalar sigma model sector such as the curvature of the scalar field manifold $\mathcal M$. We show how the curvature can be measured experimentally via Higgs cross-sections, $W_L$ scattering, and the $S$ parameter. The one-loop action of HEFT is given in terms of geometric invariants of $\mathcal M$. The distinction between the Standard Model (SM) and HEFT is whether $\mathcal M$ is flat or curved, not whether the scalars transform linearly or non-linearly under the electroweak group.
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Sigma Models with Negative Curvature
We construct Higgs Effective Field Theory (HEFT) based on the scalar manifold H^n, which is a hyperbolic space of constant negative curvature. The Lagrangian has a non-compact O(n,1) global symmetry group, but it gives a unitary theory as long as only a compact subgroup of the global symmetry is gauged. Whether the HEFT manifold has positive or negative curvature can be tested by measuring the S-parameter, and the cross sections for longitudinal gauge boson and Higgs boson scattering, since the curvature (including its sign) determines deviations from Standard Model values.
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Holomorphy without Supersymmetry in the Standard Model Effective Field Theory
The anomalous dimensions of dimension-six operators in the Standard Model Effective Field Theory (SMEFT) respect holomorphy to a large extent. The holomorphy conditions are reminiscent of supersymmetry, even though the SMEFT is not a supersymmetric theory.
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Lepton universality violation and lepton flavor conservation in $B$-meson decays
Anomalies in (semi)leptonic $B$-meson decays present interesting patterns that might be revealing the shape of the new physics to come. In order to understand the experimental data, we explore symmetry arguments that lead to the hypothesis of minimal flavor violation. In particular, under the assumption of negligible neutrino mass effects in charged lepton processes, the presence of lepton universality violation without lepton flavor violation naturally arises. This can account for a deficit of $B^+\to K^+μμ$ over $B^+\to K^+ee$ decays with new physics coupled predominantly to muons and a new physics scale of a few TeV. A prediction of this scenario is the modification of processes involving the third generation. In particular, accounting for the above ratio implies a large enhancement, by a factor $\sim10^3$ with respect to the standard model, of all the $b\to sττ$ decay rates. Although these are still below current experimental limits, they should be easily at reach in future experiments at $B$-factories. Another important consequence is the prediction of sizable effects in charge-current $B$ tauonic decays which could also explain the enhancements that have been observed in the $B\to D^{(*)}τ\bar ν$ and $B^-\toτ^-\bar ν$ decays. For the most part, the study is carried out in an effective field theory framework with an underlying $SU(2)_L\times U(1)_Y$ symmetry that emphasizes the model-independent correlations arising between low- and high-energy observables. For example, a connection between $B$-decays and top physics is pointed out. To complement the discussion, all possible (spin 0 and 1) leptoquark models are matched to the low-energy field theory so that the effective analysis can be used to survey these candidates for new physics. These models also serve as concrete examples where the hypotheses of this work can be implemented.
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Sigma Decomposition
In composite Higgs models the Higgs is a pseudo-Goldstone boson of a high-energy strong dynamics. We have constructed the effective chiral Lagrangian for a generic symmetric coset, restricting to CP-even bosonic operators up to four momenta which turn out to depend on seven parameters, aside from kinetic terms. Once the same sources of custodial symmetry breaking as in the Standard Model are considered, the total number of operators in the basis increases up to ten, again aside from kinetic terms. Under these assumptions, we have then particularised the discussion to three distinct frameworks: the original $SU(5)/SO(5)$ Georgi-Kaplan model, the minimal custodial-preserving $SO(5)/SO(4)$ model and the minimal $SU(3)/(SU(2)\times U(1))$ model, which intrinsically breaks custodial symmetry. The projection of the high-energy electroweak effective theory into the bosonic sector of the Standard Model is shown to match the low-energy chiral effective Lagrangian for a dynamical Higgs, and it uncovers strong relations between the operator coefficients. Finally, the relation with the bosonic basis of operators describing linear realisations of electroweak symmetry breaking is clarified.
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$SU(2)\times U(1)$ gauge invariance and the shape of new physics in rare $B$ decays
New physics effects in $B$ decays are routinely modeled through operators invariant under the strong and electromagnetic gauge symmetries. Assuming the scale for new physics is well above the electro-weak scale, we further require invariance under the full Standard-Model gauge symmetry group. Retaining up to dimension-6 operators, we unveil new constraints between different new-physics operators that are assumed to be independent in the standard phenomenological analyses. We illustrate this approach by analyzing the constraints on new physics from rare $B_{q}$ (semi-)leptonic decays.
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