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.
The direct detection of gravitational waves (GWs) of frequencies above MHz has recently received considerable attention. In this work, we present a precise study of the reach of a cubic cavity resonator to GWs in the microwave range, using for the first time tools allowing us to perform realistic simulations. Concretely, the boundary integral—resonant mode expansion (BI-RME) 3D method, which allows us to obtain not only the detected power but also the detected voltage (magnitude and phase), is used here. After analyzing three cubic cavities for different frequencies and working simultaneously with three different degenerate modes at each cavity, we conclude that the sensitivity of the experiment is strongly dependent on the polarization and incidence angle of the GW. The presented experiment can reach sensitivities to the strain of the gravitational waves up to 1×10-19 at 100 MHz, 2×10-20 at 1 GHz, and 6×10-19 at 10 GHz for optimal angles and polarizations, and where in all cases we assumed an integration time of Δt=1 ms.
These results provide a strong case for further developing the use of cavities to detect GWs. Moreover, the possibility of analyzing the detected voltage (magnitude and phase) opens a new interferometric detection scheme based on the combination of the detected signals from multiple cavities. All these ideas will be further developed in the recently awarded ERC-2024-SYG grant to our group.
If dark matter is composed of axions, then axion stars form in the cores of dark matter halos. These stars are unstable above a critical mass, decaying to radio photons that heat the intergalactic medium, offering a new channel for axion indirect detection. We recently provided the first accurate calculation of the axion decay rate due to axion star mergers in a companion work (Du et al 2024). In this work we show how existing data concerning the CMB optical depth leads to strong constraints on the axion photon coupling in the mass range 10-14 eV≲m≲10-8 eV. Axion star decays lead to efficient reionization of the intergalactic medium during the dark ages. By comparing this nonstandard reionization with the experiment Planck legacy measurements of the Thomson optical width, we show that couplings gaγγ of axions to light in the range 10-14 GeV-1≲gaγγ≲10-10 GeV-1 are excluded for our benchmark model of axion star abundance. Future measurements of the 21 cm emission of neutral hydrogen at high redshift could improve this limit by an order of magnitude or more, providing complementary indirect constraints on axion dark matter in parameter space also targeted by direct detection haloscopes. In summary, we have suggested a new method to detect dark matter in an unexplored region of parameter space that could lead to its detection in upcoming surveys.
Unstable domain wall (DW) networks in the early universe are cosmologically viable and can emit a large amount of gravitational waves (GW) before annihilating. As such, they provide an interpretation for the recent signal reported by Pulsar Timing Array (PTA) collaborations. A related important question is whether such a scenario also leads to significant production of Primordial Black Holes (PBH). We investigated both GW and PBH production using 3D numerical simulations in an expanding background, with box sizes up to 32403, including the annihilation phase. We found that: i) the network decays exponentially, i.e. the false vacuum volume decays like exp(−t3), with t the conformal time; ii) the GW spectrum is larger than traditional estimates by more than one order of magnitude, due to a delay between DW annihilation and the sourcing of GWs. We presented a novel semi-analytical method to estimate the PBH abundances: rare false vacuum pockets of super-Hubble size collapse to PBHs if their energy density becomes comparable to the background when they cross the Hubble scale. Smaller (but more abundant) pockets will instead collapse only if they are close to spherical. This introduces very large uncertainties in the final PBH abundance. The first phenomenological implication is that the DW interpretation of the PTA signal is compatible with observational constraints on PBHs, within the uncertainties. Second, in a different parameter region, the dark matter can be entirely in the form of asteroid-mass PBHs from the DW collapse. Remarkably, this would also lead to a GW background in the observable range of LIGO-Virgo-KAGRA and future interferometers, such as LISA and Einstein Telescope.
We have extended the Standard Model by including a non-minimal Higgs coupling to gravity, and explored the phenomenology of the Higgs inflation model. We point out that even configurations that would be metastable in the Standard Model, become viable for inflation if the non-minimal coupling is large enough to flatten the Higgs potential at field values below the barrier. Moreover for R**2-Higgs inflation we show that the observed baryon asymmetry of the Universe can be obtained when this model is supplemented by a dimension-six CP-violating Chern-Simons effective term (suppressed by the scale Lambda) in the hypercharge sector. The plot of the allowed region in terms of the reheat temperature after inflation and the cutoff Lambda is exhibited in the attached plot.
We have studied a solvable class of five-dimensional dilaton gravity models that continuously interpolate between anti-de Sitter, linear dilaton and positively curved space-times as a function of a continuous parameter ν. We find that the spectrum of metric fluctuations can be either continuous or discrete. It features a massless graviton mode confined between the brane and the curvature singularity, and a massive radion mode tied to brane-dilaton stability. We show that, in the presence of a bulk black hole, the holographic theory living on the brane features a perfect fluid, with an equation of state interpolating between radiation, pressureless matter (a candidate to dark matter) and vacuum energy (a candidate to dark energy) as a function of ν.
We studied how causality and unitarity affect graviton scattering amplitudes and set new limits on theories with U(1)-gravitational anomalies, such as axion models or strongly-coupled gauge theories. We find a universal scale at which states with spin J ≥ 4 must appear. For axion models, we find that this scale is roughly equal to the axion decay constant, f_a, times the Planck scale. In strongly-coupled gauge theories with a large number of colors (large-Nc), glueballs can avoid these limits, as long as the number of fermions (N_F) is much smaller than Nc, and the ’t Hooft coupling is not too large. However, in models with a holographic 5D dual (large ’t Hooft coupling), this causality scale corresponds to a new cutoff of the models.
We looked at conformal transitions that happen when an infrared (IR) and ultraviolet (UV) fixed points merge, which is expected in QCD with a large number of flavors. We examine how physical quantities change during this transition, which is mainly affected by the logarithmic breaking of conformal symmetry. Using holography, we study the transition both inside and outside the conformal window and find that the dynamics are the same in both cases. We show that the mass of spin-1 mesons, the pion decay constant, and the dilaton mass remain continuous across the transition. Our analysis suggests that the light scalar seen in QCD lattice simulations is a meson that becomes lighter as the QQ operator dimension reaches its minimum value.
We explored heavy dark matter production from bubble collisions during a first-order phase transition, especially when bubble walls reach high energies. In such cases, dark matter can have masses far exceeding the symmetry breaking scale. We also pointed out that current methods for calculating particle production from bubble dynamics are not gauge invariant and may give incorrect results. Our approach avoids unphysical contributions and offers reliable estimates. We highlight the importance of three-body decays of field excitations, which dominate at high energies. Our findings show that dark matter across a wide mass range can be studied in future gravitational wave experiments.
The article “Footprints of the QCD Crossover on Cosmological Gravitational Waves at Pulsar Timing Arrays ”, has been awarded the Buchalter Cosmology Prize (Third Prize) of $2500 for “providing an interesting and timely test for a potential early universe contribution to the stochastic background of gravitational waves, arising from features imprinted by the QCD phase transition that could be detected by pulsar timing arrays”, according to the competition’s jury. The prize will be awarded to the three authors of the paper, Fabrizio Rompineve, Gabriele Franciolini (CERN) and Davide Racco (ETH and U. Zürich).
The paper showed that the spectral shape of the low-frequency (causality) tail of GW signals sourced at temperatures around T≳1 GeV is distinctively affected by confinement of strong interactions (QCD), due to the corresponding sharp decrease in the number of relativistic species, and significantly deviates from ∼f3 commonly adopted in the literature. Bayesian analyses in the NANOGrav 15 years and the previous international PTA datasets reveal a significant improvement in the fit with respect to cubic power-law spectra, previously employed for the causality tail. While no conclusion on the nature of the signal can be drawn at the moment, the results of the work show that the inclusion of standard model effects on cosmological GWs can have a decisive impact on model selection.
The Buchalter Cosmology Prize is an annual award that stimulates ground-breaking theoretical, observational, or experimental work in cosmology that has the potential to produce a breakthrough advance in our understanding. It was created to support the development of new theories, observations, or methods that can help illuminate the puzzle of cosmic expansion from first principles.
Goodness of fit is the basic scientific problem of assessing the absolute level of compatibility with data of one prior hypothesis on their statistical distribution. It is fundamentally different from a test of hypothesis, which aims instead at assessing the relative compatibility of two or several distinct hypotheses, with the final goal of discriminating between them. The lack of a viable goodness of fit methodology is a widespread limitation in several areas of science and technology. For example, we cannot assess the compatibility with General Relativity of the waveform of gravitational waves, but only compare the relative compatibility of the General Relativity model prediction with the prediction of alternative models, often of modest theoretical credibility. Similarly, we cannot assess the compatibility of collider and cosmological data with, respectively, the SM and ΛCDM predictions. We can only compare with the predictions of selected alternative models, with strategies that would be effective in order to falsify the “standard” SM and ΛCDM models only if the selected alternative was the true underlying model. In Computer Science, goodness of fit methods are also regularly needed, for instance in order to assess the quality of generative models. For technology applications, it is often needed to monitor the working conditions of a complex apparatus like for instance a particle detector. Data Quality Monitoring is precisely a goodness of fit problem aimed at assessing if the collected data follow the normal distribution expected in good operating conditions, or if they don’t, signalling a malfunction.
Our paper investigates a method for goodness of fit in many dimensions proposed by my group in 2018, dubbed NPLM, that exploits contemporary AI (in particular, machine learning) techniques. The paper advances along two directions. First, we revisit the statistical foundations of the NPLM method showing that it is robustly based on the classical Neyman-Pearson theory hypothesis testing, with the key modern ML innovation providing a flexible set of alternative hypotheses that effectively turns the hypothesis test into goodness of fit. Motivated by this theoretical finding, we perform a comprehensive comparison of the NPLM performances with the performances of current methods for goodness of fit that are available in the statistics and computer science literature. The plots below are a snapshot of the superior NLPM performances in a variety of test cases, in comparison with a variety of goodness of fit approaches. NPLM has a higher probability to detect a large Z-score (exclusion level).