Beyond the Standard Model

Mariano Quirós

There are a number of reasons, both theoretical (hierarchy problem, strong CP problem, flavor problem, the origin of matter-antimatter asymmetry,…) and experimental (Dark Matter…) why we believe that the Standard Model of strong and electroweak interactions cannot be the ultimate theory of particle interactions. This has motivated the development of theories beyond the Standard Model (BSM), which is the main task of the BSM subgroup of the IFAE Theory Group, and the experimental search of BSM physics, which in particular is being undertaken at the LHC.

Introduction

The group consists of Profs. Alex Pomarol and Eduard Masso, the ICREA Research Professor Jose Ramon Espinosa, the SO postdoc Dr. Giuliano Panico and the IFAE researchers Dr. Oriol Pujolas and former ICREA Research Professor Mariano Quiros. The group activities are mainly in Beyond the Standard Model and Cosmology.

Instability of the Higgs potential in the Standard Model

J.R. Espinosa studied some of the physics associated to the instability of the Higgs potential in the Standard Model. The scale of this instability, determined as the Higgs field value at which the potential drops. Together with Davide Racco and Antonio Riotto (U. Geneva), he showed that a cosmological signature of such instability could be primordial gravitational waves, produced at second order and seeded by large Higgs fluctuations during inflation if the Higgs enters the unstable range of the potential during that epoch. The predicted spectrum of primordial gravitational waves via this mechanism is shown in figure 1.

Figure 1: Power spectra of gravitational waves for the scalar power spectra generated by Higgs fluctuations compared with the estimated sensitivities for LISA, the Einstein Telescope, and the design sensitivity of Advanced LIGO + Virgo. The Higgs and top mass values are $M_h = 125.09 \pm 0.24$ GeV and $M_t = 172.47 \pm 0.5$ GeV.

J.R. Espinosa also developed a new approach to the calculation of tunneling actions, that control the exponential suppression of the decay of metastable phases (like the unstable electroweak vacuum). The new approach reformulates the calculation as an elementary variational problem in field space. This alternative approach circumvents the use of bounces in Euclidean space by introducing an auxiliary function, a tunneling potential Vt that connects smoothly the metastable and stable phases of the field potential V. The tunneling action is obtained as the integral in field space of an action density that is a simple function of Vt and V and can be considered as a generalization of the thin-wall action to arbitrary potentials. This formalism provides new handles for the theoretical understanding of different features of vacuum decay, can be easily extended to include gravitational effects in an elegant way and has a number of useful applications, like the study of multi-field potentials.

RD(∗) in custodial warped space

M. Quiros in collaboration with M. Carena (Fermilab), C. Wagner (Argonne and University of Chicago) and E. Megias (universidad de Granada) suggested that the apparent anomaly in $R_{D^{(\ast)}} = \mathcal B(B \to D^{(\ast)} \tau \nu)/ \mathcal B(B \to D^{(\ast)} \ell \nu )$ may be explained by new gauge bosons coupled to right-handed currents of quarks and leptons, involving light right-handed neutrinos. They proposed a well-motivated ultraviolet complete realization of this idea, embedding the SM in a warped space with an $SU(2)_L\otimes SU(2)_R \otimes U(1)_{B-L}$ bulk gauge symmetry. Besides providing a solution to the hierarchy problem, they showed that this model, which has an explicit built-in custodial symmetry, can explain the $R_{D^{(\ast)}}$ anomaly and at the same time allow for a solution to the $R_{K^{(\ast)}}$ anomalies, related to the decay of $B$-mesons into $K$-mesons and leptons, $R_{K^{(*)}} = \mathcal B(B\to K^{(*)} \mu \mu)/ \mathcal B(B \to K^{(*)} e e)$. In addition, a model prediction is an anomalous value of the forward-backward asymmetry $A^b_{FB}$, driven by the $Z\bar b_R b_R$ coupling, in agreement with LEP data. M. Quiros, in collaboration with G. Nardini (University of Stavanger) and E. Megias (Universidad de Granada) have studied the electroweak phase transition within a 5D warped model including a scalar potential with an exponential behavior, and strong back-reaction over the metric, in the infrared. By means of a novel treatment of the superpotential formalism, they explore parameter regions that were previously inaccessible. They find that for large enough values of the t’Hooft parameter (e.g.~$N\simeq 25$) the holographic phase transition occurs, and it can force the Higgs to undergo a first order electroweak phase transition, suitable for electroweak baryogenesis. The model exhibits gravitational waves and colliders signatures. It typically predicts a stochastic gravitational wave background observable both at the Laser Interferometer Space Antenna and at the Einstein Telescope as shown in figure 2. Moreover the radion tends to be heavy enough such that it evades current constraints, but may show up in future LHC runs.

Figure 2: The $T_R$–$\beta/H_\star$ parameter space that exhibits SNR $>10$ at SKA, LISA, aLIGO, and ET for $v_w=v_2$. The etched areas are in tension with present aLIGO O1, EPTA, PPTA and NANOGRAV constraints. The BBN bound excludes the blue area. The considered benchmark scenarios B$_i$ (blue points), C$_i$ (orange points), D$_1$ (green point) and E$_1$ (red point) are detectable at both LISA and ET. The stepwise behavior shown in the inserted figure, a zoom of the main one, is a consequence of the continuous change of regime from $O(4)$ to $O(3)$ bubbles.

Collider phenomenology, BSM and cosmology

G. Panico has been working on different topics connected to collider phenomenology, beyond the Standard Model physics and cosmology. He followed three main research directions. One of them, in collaboration with L. Di luzio and R. GrÂber, was focussed on the exploitation of precision measurements to set bounds on the presence of new heavy multiplets with electroweak quantum numbers. In particular he assess the sensitivity to these states via the modification of neutral and charged Drell-Yan processes at the high-luminosity phase of the LHC and at future lepton and hadron colliders.

A second research line, in collaboration with A. Pomarol and M. Riembau, was devoted to the study of new-physics corrections to the electron electric dipole moment (EDM). He performed an analysis of the leading contributions up to two-loop order within an effective field theory framework. The relevance of this work is related to the new, recently released ACME bounds that improve the constraint on the electron EDM by nearly one order of magnitude. The new bounds allow to test a significant fraction of the parameter space of theories in which the electron EDM is generated only at two-loop level, which were instead not testable with the previous experimental constraints.

The third research line, done in collaboration with F. Ferrer, E. Masso, O. Pujolas and F. Rompineve, was focussed on the study of a new formation mechanism for primordial black holes. The new mechanism relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number larger than one and Peccei-Quinn symmetry broken after inflation. A nice feature of this mechanism is its independence from cosmological inflation.