BSM-Astro/Cosmo

Mariano Quirós

The group consists of Profs. Alex Pomarol and Eduard Masso, the postdoc Dr. Benedict von Harling and the IFAE researchers Dr. Oriol Pujolas and former ICREA Research Professor Mariano Quiros, now IFAE Emeritus Professor. The group activities are mainly in Beyond the Standard Model, astro-particles and Cosmology.

Introduction to the year's activities

M. Quiros, in collaboration with A. Delgado and A. Martin (University of Notre Dame, USA), has considered a minimal natural supersymmetric model based on an extra dimension with supersymmetry breaking provided by the Scherk-Schwarz mechanism and all Standard Model fields propagating in the bulk of the extra dimension. Motivated by the actual possibility of identification of the Dark Matter with an almost pure Higgsino state with a mass $\sim 1.1$ TeV they have built a realistic model where electroweak symmetry breaking is correctly implemented with a Higgs mass of 125 GeV. The model does not have any free parameter and predicts a compressed spectrum with a mass at around 2.4 TeV which can be easily explored in gluino searches at the LHC.

M. Quiros, in collaboration with M. Carena (Fermilab, Chicago, USA) and Y. Zhang (Fermilab and Northwestern University, Evanston, USA), has proposed a novel mechanism of electroweak baryogenesis where CP violation occurs in a dark sector, comprised of standard model gauge singlets, thereby evading the strong electric dipole moment constraints. In this framework, the background of time-like component of the gauge boson $Z^\prime_0$ of a lepton symmetry, generated at electroweak temperatures, drives the electroweak sphaleron processes to create the desired baryon asymmetry. The models under consideration have a rich phenomenology and can be experimentally probed in leptophilic $Z^\prime$ searches, dark matter searches, heavy Majorana neutrino searches, as well as through hunting for new Higgs portal scalars in multi-lepton channels at colliders. The allowed region in the space of $g’$ (the $Z’$ gauge coupling) and $M_{Z^\prime}$ (the $Z’$ mass) is shown in Fig. 1.

Figure 1: Scan over the model parameters to find points that allow for successful EWBG (blue points). We show various experimental constraints by the correspondingly labeled shaded regions, in the gauged $L_e+L_\mu+L_\tau$ (left) and $L_\mu+L_\tau$ (right) models.

In view of the elusiveness of LHC experimental data on the presence of new narrow resonances (in particular Kaluza-Klein modes in theories with an extra dimension) M. Quiros, in collaboration with E. Megias (Universidad de Granada), has considered the possibility of a continuum of Kaluza-Klein resonances in a warped five-dimensional model with an ultraviolet (UV) brane and, on top of the Standard Model isolated modes, continua of KK modes with different mass gaps (at the TeV scale) for all particles: gauge bosons, fermions, graviton, radion and Higgs boson. The model can be considered as a modelization in five dimensions of gapped unparticles. The bosonic continuum of KK modes with the smallest mass gap are those of gauge bosons, and so they are the most likely produced at the LHC. Mass gaps of the continuum of KK fermions do depend on their localization in the extra dimension. We have computed the spectral functions, and arbitrary Green’s functions, and shown how they can modify some Standard Model processes. We show, in Fig. 2 the spectral density function for a massless gauge boson where it is shown a mass gap where the spectral function vanishes.

Figure 2: Spectral density $\rho_A(z_0,z_0;p)$ for a continuum gauge boson.

Finally M. Quiros, in collaboration with S. Bansal, C. Kolda and A. Delgado (University of Notre Dame), has studied bounds from Drell-Yan processes at LHC to Parity violating coupling in supersymmetric theories, providing the most updated set of bounds on the parameters $\lambda^\prime_{ijk}$, with superpotential $W=\lambda^\prime_{ijk}LQD^c$, in the literature from mono-lepton and di-lepton data.

E Masso and O Pujolas in collaboration with F Ferrer, G Panico and F Rompineve described a new mechanism to generate Primordial Black Holes (PBHs) which is independent of inflation and occurs below the cosmological QCD phase transition. It relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number $>1$ and Peccei-Quinn symmetry broken after inflation. The resulting dark matter is mostly composed of axions in the meV mass range along with a small fraction, $\Omega_{PBH}\gtrsim10^{-6}\Omega_{CDM}$ of heavy PBHs, with masses in the range $10^4-10^7,M_\odot$. The latter could play a role in alleviating some of the shortcomings of the $\Lambda CDM$ model on sub-galactic scales. The scenario has distinct signatures in ongoing axion searches as well as gravitational wave observatories.

Figure 3: Constraints on the Peccei-Quinn scale F for a representative value of the final temperature $T_2 \simeq 7 MeV$ where the bias term appears. The figure of merit (dashed lines) indicates the probability of PBH formation as a function of the temperature $T_*$ at which a closed structure fits in the horizon scale. The masses of the resulting PBHs are also shown (red lines).

A. Pomarol in collaboration with B.~Von Harling, O.~Pujolàs and F.~Rompineve has studied the possibility that the LIGO/VIRGO observatories detect stochastic gravitational waves arising from the phase transition of a Peccei-Quinn breaking which could have happened in the early universe at temperatures around ???$10^8$ GeV. They showed that this can be possible if the phase transition occurs after some supercooling, which can arise either in Coleman-Weinberg-type symmetry breaking or in strongly-coupled models.

A. Pomarol in collaboration with T.~Gherghetta, V.~V.~Khoze, A.~Pomarol and Y.~Shirman have calculated a new contribution to the axion mass that arises from gluons propagating in a 5th dimension at high energies. By uplifting the 4D instanton solution to five dimensions, the positive frequency modes of the Kaluza-Klein states generate a power-law term in the effective action that inversely grows with the instanton size. This causes 5D small instantons to enhance the axion mass in a way that does not spoil the axion solution to the strong CP problem. This result suggests that the mass range of axions (or axion-like particles), which is important for ongoing experimental searches, can depend sensitively on the UV modification of QCD.

B.~von Harling, in collaboration with V.~Domcke, E.~Morgante and K.~Mukaida, studied baryogenesis at the electroweak scale from a helical background of hypercharge gauge fields. During the electroweak phase transition, these hypermagnetic fields are transformed into ordinary magnetic fields. The U(1) of hypercharge contributes to the anomaly of B+L, while the U(1) of electromagnetism does not. From the anomaly equation, the transformation of hypermagnetic fields with net helicity into ordinary magnetic fields then generates a compensating asymmetry in B+L, thereby allowing to generate the baryon asymmetry. The helical hypermagnetic fields in turn can be naturally generated if inflation is driven by an axion-like field that couples to the hypercharge gauge field. They revisited this scenario and took into account the backreaction from fermion production on the gauge field production during axion inflation as well as constraints that had not been properly included in the literature before. They found that the observed baryon asymmetry can be reproduced if the energy scale of inflation is around $10^10 – 10^12$ GeV, with only a moderate dependence on the other parameters of the model.

B.~von Harling, in collaboration with N.~Fonseca, L.~de Lima and C.~S.~Machado, presented a construction to generate axion-like scalars with super-Planckian decay constants which are useful for various phenomenological applications. In order to avoid large corrections to the axion potential which are expected due to the weak gravity conjecture, super-Planckian decay constants should arise in the low-energy effective description from initially sub-Planckian ones. To this end, two axion-like scalars with different decay constants $f_1,f_2$ were considered which are charged under an Abelian gauge symmetry. One linear combination of the axions is then eaten by the gauge field. The two axions are also coupled to a non-Abelian gauge field. For a certain choice of these couplings, the remaining, uneaten axion couples to the non-Abelian gauge field with a decay constant of order $f_1^2/f_2$. For $f_1 \sim M_{\rm Pl} \gg f_2$, this is hierarchically larger than the Planck scale. The required hierarchy $f_1 \gg f_2$ can be naturally obtained if the axions arise from two nearly-conformal sectors with hierarchically different confinement scales. Via the AdS/CFT duality, this setup can be described by a Randall-Sundrum model which was used to perform the calculations.