BSM-Astro/Cosmo

Despite the astounding successes of the SM, there are several reasons why the Standard Model of strong and electroweak interactions cannot be the ultimate theory of particle interactions, both experimental and theoretical character. Among them, the nature of dark matter, the origin of neutrino masses, the inclusion of gravity and black holes, the origins of the matter-antimatter asymmetry and the flavor structure of the SM, the electroweak naturalness and the strong CP problems. 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.
The group consists of UAB Profs. Alex Pomarol, Eduard Massó, Clara Murgui and Fabrizio Rompineve, ICREA Research Professors Diego Blas and Andrea Wulzer, IFAE researchers Oriol Pujolas and former ICREA Research Professor Mariano Quirós, now IFAE Emeritus Professor as well as postdocs Ricardo Z. Ferreira, Mario Herrero-Valea, Evangelos Sfakianakis, Teng Ma, Juan Sebastian Valbuena Bermúdez and Ricardo Vicente, George Zahariade and Ziyu Dong. The group activities are mainly in Beyond the Standard Model, Astro-Particle and Cosmology.

Electromagnetic cavities as mechanical bars for gravitational waves

We revived the old idea of using a superconducting cavity as a Weber bar to search for Gravitational Waves. We performed updated calculations of the sensitivity of both the mechanical and electromagnetic signals in the cavity, accounting for various noise sources that had previously been neglected. We computed the sensitivity of a tunable experiment across a wide range of frequencies from kHz to GHz. We quantified the advantages of using a superconducting cavity as a Weber bar over traditional approaches, demonstrating that the cavity can have a better broadband sensitivity across a wide range of frequencies. Our efforts have led to a revival of the MAGO program by experimental groups at FNAL and DESY.

A. Berlin, D. Blas, R. Tito D’Agnolo, S. A. R. Ellis, R. Harnik, Y. Kahn, J. Schu ̈tte-Engel and M. Wentzel, “Electromagnetic cavities as mechanical bars for gravitational waves”, Phys. Rev. D 108 (2023) no.8, 084058 doi:10.1103/PhysRevD.108.084058 [arXiv:2303.01518 [hep-ph]].

Figure 1: Reach of a MAGO-like setup to monochromatic GWs. The mechanical (purple) and EM (blue) signals are separated for visual comparison, but they would both be present in a single experiment. The shaded purple and blue regions labeled “scanning” and “scanning (EM)” show the sensitivity to mechanical and EM signals, respectively, for a scanning setup in which the EM mode splitting is matched to the GW frequency, and assuming that vibrational noise as inferred by recent measurements of the DarkSRF experiment. The solid and dashed light-shaded contours labeled “scanning (thermal)” and “non-scanning (thermal)” show the sensitivity when vibrational noise is attenuated to its irreducible thermal value, for a scanning or broadband setup, respectively. In the latter case, the EM mode splitting is fixed to the lowest-lying mechanical resonance. Also shown in gray are existing limits from LIGO-Virgo, AURIGA, bulk acoustic wave (BAW) resonator, and the Holometer experiment. The possible new parameter space to be accessible with the ideas of the paper is clear, and quite dramatic. The green shaded region corresponds to signals generated from superradiant bosonic clouds around black holes of mass depending on the frequency as M~ M⊙ (10^5 Hz/ω_g), where M⊙ is the mass of the Sun, at a distance of 1 kpc.

Invisible Higgs boson decay from forward muons at a muon collider

Engineering muon collisions for the first time entails new and yet unexplored physics opportunities, and novel strategies for measurements that will first become available at a future muon collider. Specifically, the paper exploits the highly-penetrating nature of the muons, which can travel through the components of the collider and hence be detected even if emitted very close to the beam line, unlike other particles. This entails novel opportunities for the study of Z-boson fusion processes, depicted on the left panel of the figure. If the ``X’’ particle produced in Z fusion is the Higgs boson, this gives new opportunities to study its properties. The right panel of the figure shows that this strategy would allow to measure with high precision the Higgs boson branching ratio to invisible (like neutrinos) particles. Small new physics (BSM) corrections to the branching ratio predicted by the Standard Model could be observed, provided the forward muon detection is possible up to angles as small as 1/100 of a radiant (i.e., half a degreed).

Ruhdorfer M, Salvioni E & Wulzer A 2023, ‘Invisible Higgs boson decay from forward muons at a muon collider’, Physical Review D, 107, 9, 095038.

Pulsar timing array stochastic background from light Kaluza-Klein resonances

We propose to accommodate the Pulsar Timing Arrays NANOGrav data on stochastic gravitational waves background at nHz frequencies, by means of the holographic first order phase transition associated with the radion confinement on a brane at the (sub)-GeV scale in a warped extradimensional setup. A corner plot fitting the NANOGrav data is produced in the attached figure. Blue points fall in the NANOGrav 95 percent favorite region of the “bubble collision only” hypothesis, the region covered by blue + red points corresponds to the “bubble collision + SMBHB” hypothesis, and the gray points belong to none of the previous regions.

E. Megías, G. Nardini and M. Quirós, “Pulsar timing array stochastic background from light Kaluza-Klein resonances”, Phys. Rev. D 108 (2023) no.9, 095017 [arXiv:2306.17071 [hep-ph]].

Pulsar Timing Arrays have recently reported evidence for a gravitational wave (GW) background, and the pending question is whether the signal originates from mergers ofsupermassive black holes or the early Universe. This paper provides a distinctive signature of the GW background sourced by a wide class of primordial phenomena: the low-frequency tail of the spectrum is modified by the occurrence of QCD confinement. The inclusion of these Standard Model effects has an important impact on model comparison in latest PTA datasets. The paper is published on PRL and has received the 2024 Buchalter Cosmology Prize (Third Prize), with the motivation of “providing an interesting and timely test for a potential early universe contributionto the stochastic background of gravitational waves, arising from features imprinted by the QCD phase transition that could be detected by pulsar timing arrays.

Oscillon spectroscopy

Ocillons are bound states with a finite lifetime composed of bosons with attractive self-interactions. We unveil the rich spectrum of oscillons in an axion-like model. All but the two lowest states decay by fragmentation.

Figure caption: maximal amplitude and frequency of spherically symmetric oscillons. Coloured dots show different states, progressing in time towards the right.

Cornering large-Ncc QCD with positivity bounds

We provide precise predictions on low-energy observables using the analytic structure of meson scattering amplitudes in the large-N limit, combined with positivity of the spectral density.We also give an explanation for the success of Vector Meson Dominance and holographic QCD and emphasize the complementarity between our results and Lattice computations in the exploration of large-N QCD.