We study the evolution of growing vacuum bubbles. The bubble walls interact with the surrounding fluid and may, consequently, reach a terminal velocity. If the mean free path of the particles in the fluid is much shorter than the bubble wall thickness, the fluid is locally in thermal equilibrium and the wall’s terminal velocity can be determined by entropy conservation. On the other hand, if local thermal equilibrium inside the wall cannot be maintained, the wall velocity can be estimated from the pressure impacted by ballistic particle dynamics at the wall. We find that the latter case leads to slightly slower bubble walls. Expectedly, we find the largest differences in the terminal velocity when the fluid is entirely ballistic. This observation indicates that the non-equilibrium effects inside walls are relevant. To study bubble evolution, we perform hydrodynamic lattice simulations in the case of local thermal equilibrium and -body simulations in the ballistic case to investigate the dynamical effects during expansion. Both simulations show that even if a stationary solution exists in theory it may not be reached depending on the dynamics of the accelerating bubble walls.

Poland

Tomasz Krajewski

Marek Lewicki

Estonia

Martin Vasar

Italy

Ville Vaskonen

Hardi Veermäe

Mateusz Zych

Thermalization effects on the dynamics of growing vacuum bubbles

We study the impact of the ambient fluid on the evolution of collapsing false vacuum bubbles by simulating the dynamics of a coupled bubble-particle system. A significant increase in the mass of the particles across the bubble wall leads to a buildup of those particles inside the false vacuum bubble. We show that the backreaction of the particles on the bubble slows or even reverses the collapse. Consequently, if the particles in the true vacuum become heavier than in the false vacuum, the particle-wall interactions always decrease the compactness that the false vacuum bubbles can reach, making their collapse to black holes less likely.

Kristjan Müürsepp

Joosep Pata

Dynamics of false vacuum bubbles with trapped particles

We investigate the stability of the electroweak vacuum in metric-affine gravity in which the Standard Model Higgs boson can be non-minimally coupled to both the Ricci scalar and the Holst invariant. We find that vacuum stability is improved in this framework across a wide range of model parameters.

Ioannis D. Gialamas

Electroweak vacuum decay in metric-affine gravity

We show that the physical conditions which induce the Thakurta metric, recently studied by Bœhm et al. in the context of time-dependent black hole masses, correspond to a single accreting compact object in the entire Universe filled with isotropic non-interacting dust. In such a case, accretion physics is not local but tied to the properties of the whole Universe. We show that radiation, primordial black holes or particle dark matter cannot produce the specific energy flux required for supporting the mass growth of the compact objects described by the Thakurta metric. In particular, this solution does not apply to black hole binaries. We conclude that compact dark matter candidates and their mass growth cannot be described by the Thakurta metric, and thus existing constraints on the primordial black hole abundance from the LIGO-Virgo and the CMB measurements remain valid.

Gert Hütsi

Tomi Koivisto

Martti Raidal

Spain

Cosmological black holes are not described by the Thakurta metric: LIGO-Virgo bounds on PBHs remain unchanged

The new measurement of the muon's anomalous magnetic moment released by the Muon g-2 experiment at Fermilab sets strong constraints on the properties of many new particles. Using an effective field theory approach to the interactions of higher-spin fields, we evaluate the contribution of an electrically neutral and colour singlet spin-3/2 fermion to <math><msub><mrow><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><mo>)</mo></mrow><mrow><mi>μ</mi></mrow></msub></math> and derive the corresponding constraints on its mass and couplings. These constraints are then compared with the ones on spin-1/2 fermions, such as the vector-like leptons that are predicted by various extensions of the Standard Model, the excited leptons which appear in composite models, as well as the charginos and neutralinos of supersymmetric theories. Unlike these new spin-1/2 fermions, the spin-3/2 particles generate only small contributions to the muon anomalous magnetic moment unless the effective new physics scale is very close to the electroweak scale.

United Kingdom

Juan C. Criado

Abdelhak Djouadi

Niko Koivunen

Confronting spin-3/2 and other new fermions with the muon g-2 measurement

Using an effective field theory approach for higher-spin fields, we derive the interactions of colour singlet and electrically neutral particles with a spin higher than unity, concentrating on the spin-3/2, spin-2, spin-5/2 and spin-3 cases. We compute the decay rates and production cross sections in the main channels for spin-3/2 and spin-2 states at both electron-positron and hadron colliders, and identify the most promising novel experimental signatures for discovering such particles at the LHC. The discussion is qualitatively extended to the spin-5/2 and spin-3 cases. Higher-spin particles exhibit a rich phenomenology and have signatures that often resemble the ones of supersymmetric and extra-dimensional theories. To enable further studies of higher-spin particles at collider and beyond, we collect the relevant Feynman rules and other technical details.

Juan Criado

France

Higher-spin particles at high-energy colliders

We develop an effective field theory of a generic massive particle of any spin and, as an example, apply this to study higher-spin dark matter (DM). Our formalism does not introduce unphysical degrees of freedom, thus avoiding the potential inconsistencies that may appear in other field-theoretical descriptions of higher spin. Being a useful reformulation of the Weinberg’s original idea, the proposed effective field theory allows for consistent computations of physical observables for general-spin particles, although it does not admit a Lagrangian description. As a specific realization, we explore the phenomenology of a general-spin singlet with <math><mrow><msub><mrow><mi>Z</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></math>-symmetric Higgs portal couplings, a setup which automatically arises for high spin, and show that higher spin particles with masses above <math><mrow><mi>O</mi><mo>(</mo><mn>10</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> can be viable thermally produced DM candidates. Most importantly, if the general-spin DM has purely parity-odd couplings, it naturally avoids all DM direct detection bounds, in which case, its mass can lie below the electroweak scale. Our formalism reproduces the existing results for low-spin DM and allows one to develop consistent higher-spin particle physics phenomenology for high- and low-energy experiments and cosmology.

Dark matter of any spin: An effective field theory and applications

We show that the electron recoil excess around 2 keV claimed by the Xenon Collaboration can be fitted by dark matter (DM) or DM-like particles having a fast component with velocity of order <math><mrow><mo>∼</mo><mn>0.1</mn></mrow></math>. Those particles cannot be part of the cold DM halo of our Galaxy, so we speculate about their possible nature and origin, such as fast-moving DM subhalos, semiannihilations of DM and relativistic axions produced by a nearby axion star. Feasible new physics scenarios must accommodate exotic DM dynamics and unusual DM properties.

Kristjan Kannike

Alessandro Strumia

Daniele Teresi

Dark matter and the XENON1T electron recoil excess

4-Derivative gravity provides a renormalizable theory of quantum gravity at the price of introducing a physical ghost, which could admit a sensible positive-energy quantization. To understand its physics, we compute ghost-mediated scatterings among matter particles at tree-level, finding a new power-like infra-red enhancement typical of 4-derivative theories, that we dub ‘ghostrahlung’. Super-Planckian scatterings get downgraded to Planckian by radiating hard gravitons and ghosts, which are weakly coupled and carry away the energy.

CERN

Alberto Salvio

New infra-red enhancements in 4-derivative gravity

The recently claimed anomaly in the measurement of the 21 cm hydrogen absorption signal by EDGES at <math><mi>z</mi><mo>∼</mo><mn>17</mn></math>, if cosmological, requires the existence of new physics. The possible attempts to resolve the anomaly rely on either (i) cooling the hydrogen gas via new dark matter-hydrogen interactions or (ii) modifying the soft photon background beyond the standard CMB one, as possibly suggested also by the ARCADE 2 excess. We argue that solutions belonging to the first class are generally in tension with cosmological dark matter probes once simple dark sector models are considered. Therefore, we propose soft photon emission by light dark matter as a natural solution to the 21 cm anomaly, studying a few realizations of this scenario. We find that the signal singles out a photophilic dark matter candidate characterised by an enhanced collective decay mechanism, such as axion mini-clusters.