The freeze-in mechanism describes the out-of-equilibrium production of dark matter (DM) particles via feeble couplings or non-renormalisable interactions with large suppression scales. In the latter case, predictions suffer from a strong sensitivity to the initial conditions of the universe, such as the details of reheating. In this work, we investigate how this sensitivity is altered in the presence of a cosmological first-order phase transition. We show that freeze-in via non-renormalisable interactions is not always dominated by the highest temperatures of the Standard Model (SM) thermal bath, but instead may be governed by the period immediately after the phase transition, during which the decaying scalar field transfers its energy density to the SM radiation. We refer to this alternative production regime as $\textit{DM phase-in}$. Using numerical and approximate analytical solutions of the relevant Boltzmann equations, we determine the conditions under which phase-in or conventional freeze-in production dominates the final DM abundance in terms of the type of interaction between the DM and SM particles, the amount of supercooling before and the evolution of the scalar field after the phase transition. In the phase-in regime, the DM abundance is correlated with the peak frequency of the gravitational wave signal associated with the phase transition, opening up new observational possibilities.

Germany

Cristina Benso

Felix Kahlhoefer

Henda Mansour

Dark matter phase-in: producing feebly-interacting particles after a first-order phase transition

<p>Superfluid <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math> is an ideal candidate for the direct detection of light dark matter via nuclear recoils thanks to its low nuclear mass and the possibility to reach a low-detection-energy threshold by exploiting the generated quasiparticles. The design of future detectors based on this target, such as the DELight experiment, requires a proper understanding of the formation and partitioning of the signal for different energy depositions from various sources. This work presents an overview of the physical processes involved in the energy deposition of recoiling electrons and ions, and it describes a Monte Carlo approach to the partitioning of the signal into different channels. Despite an overall good agreement with existing literature, differences in the region of interest for light dark matter searches below 200 eV are observed.</p>

Francesco Toschi

Axel Brunold

Lea Burmeister

Klaus Eitel

Christian Enss

Eleanor Fascione

Torben Ferber

Rahel Gabriel

Lena Hauswald

Sebastian Kempf

Markus Klute

Belina von Krosigk

Sebastian Lindemann

United Kingdom

Benedikt Maier

Marc Schumann

Melih Solmaz

Kathrin Valerius

Friedrich Carl Wagner

Signal partitioning in superfluid <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mn>4</mn></mmultiscripts></mrow></math>: A Monte Carlo approach

Models of inelastic (or pseudo-Dirac) dark matter commonly assume an accidental symmetry between the left-handed and right-handed mass terms in order to suppress diagonal couplings. We point out that this symmetry is unnecessary because for Majorana fermions the diagonal couplings are not strongly constrained. Removing the requirement of such an ad hoc symmetry instead relaxes the relic density constraint due to additional annihilation modes. We consider a simple UV-complete model realizing this setup and study constraints from (in)direct detection, beam dump experiments and colliders. We identify two viable mass regions for the dark matter mass, around a few hundred MeV and around a few GeV, respectively. The former region will be fully tested by near-future analyses of the NA64 and Belle II data, while the latter turns out to be challenging to explore even with future experiments.

Giovani Garcia

Netherlands

Maksym Ovchynnikov

Thomas Schwetz

Not-so-inelastic Dark Matter

Simulation-based inference (SBI) makes it possible to infer the parameters of a model from high-dimensional low-level features of the observed events. In this work we show how this method can be used to establish the presence of a weak signal on top of an unknown background, to discard background events and to determine the signal properties. The key idea is to use SBI methods to identify events that are similar to each other in the sense that they agree on the inferred model parameters. We illustrate this method for the case of axion-like particles decaying to photons at beam-dump experiments. For poor detector resolution the diphoton mass cannot be reliably reconstructed, so there is no simple high-level observable that can be used to perform a bump hunt. Since the SBI methods do not require explicit high-level observables, they offer a promising alternative to increase the sensitivity to new physics.

Kierthika Chathirathas

Alessandro Morandini

Finding excesses in model parameter space

Stable dark matter particles may arise as pseudo-Goldstone bosons from the confinement of dark quarks interacting via a non-Abelian gauge force. Their relic abundance is determined not by annihilations into visible particles but by dark pion number-changing processes within the dark sector, such as <math><mn>3</mn><msub><mi>π</mi><mi>D</mi></msub><mo>→</mo><mn>2</mn><msub><mi>π</mi><mi>D</mi></msub></math>. However, if the dark vector mesons <math><msub><mi>ρ</mi><mi>D</mi></msub></math> are light enough for <math><mn>3</mn><msub><mi>π</mi><mi>D</mi></msub><mo>→</mo><msub><mi>π</mi><mi>D</mi></msub><msub><mi>ρ</mi><mi>D</mi></msub></math> annihilations to be kinematically allowed, this process dominates and significantly delays freeze-out. As a result, the preferred dark matter mass scale increases and bounds from the Bullet Cluster can be evaded.

United States

Elias Bernreuther

Nicoline Hemme

Austria

Suchita Kulkarni

Dark matter relic density in strongly interacting dark sectors with light vector mesons

Axionlike particles (ALPs) emerge in many extensions of the Standard Model as pseudo-Goldstone bosons of a spontaneously broken global symmetry. Understanding their phenomenology in high-energy collisions is crucial for optimizing experimental searches and understanding the exploration potential of future experiments. In this paper, we revise the phenomenology of ALPs with universal couplings to fermions. In particular, we analyze the hierarchy and uncertainty of the various ALP production channels depending on the proton collision energy and the placement of the experiment, and provide improved calculations of the hadronic decay modes.

Giovani Dalla Valle Garcia

Ukraine

Andrii Zaporozhchenko

Phenomenology of axionlike particles with universal fermion couplings revisited

Axion-like particles (ALPs) that decay into photon pairs pose a challenge for experiments that rely on the construction of a decay vertex in order to search for long-lived particles. This is particularly true for beam-dump experiments, where the distance between the unknown decay position and the calorimeter can be very large. In this work we use machine learning to explore the possibility to reconstruct the ALP properties, in particular its mass and lifetime, from such inaccurate observations. We use a simulation-based inference approach based on conditional invertible neural networks to reconstruct the posterior probability of the ALP parameters for a given set of events. We find that for realistic angular and energy resolution, such a neural network significantly outperforms parameter reconstruction from conventional high-level variables while at the same time providing reliable uncertainty estimates. Moreover, the neural network can quickly be re-trained for different detector properties, making it an ideal framework for optimizing experimental design.

Reconstructing axion-like particles from beam dumps with simulation-based inference

We note the possibility to perform a parametrically improved search for gauged baryon (B) and baryon minus lepton (<math><mi>B</mi><mo>−</mo><mi>L</mi></math>) Dark Photon Dark Matter (DPDM) using auxiliary channel data from LISA Pathfinder. In particular we use the measurement of the differential movement between the test masses (TMs) and the space craft (SC) which is nearly as sensitive as the tracking between the two TMs. TMs and SC are made from different materials and therefore have different charge-to-mass ratios for both <math><mi>B</mi><mo>−</mo><mi>L</mi></math> and B. Thus, the surrounding DPDM field induces a relative acceleration of nearly constant frequency. For the case of <math><mi>B</mi><mo>−</mo><mi>L</mi></math>, we find that LISA Pathfinder can constrain previously unexplored parameter space, providing the world leading limits in the mass range <math><mn>4</mn><mo>⋅</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>19</mn></mrow></msup><mspace width="0.25em"></mspace><mtext>eV</mtext><mo>&lt;</mo><mi>m</mi><mo>&lt;</mo><mn>3</mn><mo>⋅</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>17</mn></mrow></msup><mspace width="0.25em"></mspace><mtext>eV</mtext></math>. This limit can easily be recast also for dark photons that arise from gauging other global symmetries of the SM.

Jonas Frerick

Joerg Jaeckel

Kai Schmidt-Hoberg

Riding the dark matter wave: Novel limits on general dark photons from LISA Pathfinder

Australia

Christopher Chang

Pat Scott

Tomás Gonzalo

Martin White

Publisher Erratum to: Global fits of simplified models for dark matter with GAMBIT

Global fits explore different parameter regions of a given model and apply constraints obtained at many energy scales. This makes it challenging to perform global fits of simplified models, which may not be valid at high energies. In this study, we derive a unitarity bound for a simplified vector dark matter model with an s-channel vector mediator and apply it to global fits of this model with GAMBIT in order to correctly interpret missing energy searches at the LHC. Two parameter space regions emerge as consistent with all experimental constraints, corresponding to different annihilation modes of the dark matter. We show that although these models are subject to strong validity constraints, they are currently most strongly constrained by measurements less sensitive to the high-energy behaviour of the theory. Understanding when these models cannot be consistently studied will become increasingly relevant as they are applied to LHC Run 3 data.

Global fits of simplified models for dark matter with GAMBIT