<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>

Germany

Francesco Toschi

Axel Brunold

Lea Burmeister

Klaus Eitel

Christian Enss

Eleanor Fascione

Torben Ferber

Rahel Gabriel

Lena Hauswald

Felix Kahlhoefer

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

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

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

Ultrasensitive cryogenic calorimeters have become a favored technology with widespread application where eV-scale energy resolutions are needed. In this article, we characterize the performance of an x-ray magnetic microcalorimeter (MMC) using a <math><mrow><mmultiscripts><mrow><mi>Fe</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>55</mn></mrow></mmultiscripts></mrow></math> source. Employing an optimum filter-based amplitude estimation and energy reconstruction, we demonstrate that a full-width half-maximum (FWHM) resolution of <math><mrow><mi>Δ</mi><msub><mrow><mi>E</mi></mrow><mrow><mi>FWHM</mi></mrow></msub><mo>=</mo><mo>(</mo><mn>1.25</mn><mo>±</mo><mn>0.17</mn><mo>(</mo><mi>stat</mi><msubsup><mrow><mo>)</mo></mrow><mrow><mo>−</mo><mn>0.07</mn></mrow><mrow><mo>+</mo><mn>0.05</mn></mrow></msubsup><mo>(</mo><mi>syst</mi><mo>)</mo><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math> can be achieved, leading to an unprecedented energy resolving power <math><mi>E</mi><mo>/</mo><mi>Δ</mi><msub><mi>E</mi><mrow><mi>FWHM</mi></mrow></msub><mo>∼</mo><mn>4700</mn></math> among existing energy-dispersive detectors for soft and tender x-rays. We also derive the best possible resolution and discuss limiting factors affecting the measurement. The analysis pipeline for the MMC data developed in this paper is furthermore an important step for the realization of the proposed superfluid helium-based experiment DELight, which will search for direct interaction of dark matter particles with masses below <math><mrow><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi><mo>/</mo><msup><mrow><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>.

Greta Heine

Optimum filter-based analysis for the characterization of a high-resolution magnetic microcalorimeter

Dark showers from strongly interacting dark sectors that confine at the GeV scale can give rise to novel signatures at e + e  −  colliders. In this work, we study the sensitivity of B factory experiments to dark showers produced through an effective interaction arising from a heavy off-shell mediator. We show that a prospective search for displaced vertices from GeV-scale long-lived particles at Belle II can improve the sensitivity to dark showers substantially compared to an existing search at BaBar. We compare the sensitivity of searches for displaced signals to searches for promptly produced resonances at BaBar and KLOE and calculate sensitivity projections for a single-photon search at Belle II to invisible dark showers produced through an effective interaction. The underlying structure of the effective interaction can be resolved at higher-energy experiments, where the mediator can be produced on-shell. To study the resulting constraints, we update electroweak precision bounds on kinetically mixed Z′ bosons and reinterpret a search for low-mass di-muon resonances at LHCb in terms of dark showers. We find that LHCb and Belle II are most sensitive to different particle decay lengths, underscoring the complementarity of LHC and intensity frontier experiments.

United States

Elias Bernreuther

Kai Böse

Canada

Christopher Hearty

Kai Schmidt-Hoberg

Forecasting dark showers at Belle II

In this letter we point out that the BelleII experiment has a unique sensitivity to visibly decaying long-lived dark photons. Concentrating on the signatures with a single high-energy photon in association with a displaced pair of charged particles, we find that BelleII will be able to probe large regions of parameter space that cannot be covered by any other running or proposed experimental facility. While the signature with charged muons or pions in the final state is expected to be background-free after all selections are applied, the case of final state electrons is more involved and requires an in-depth study. We discuss possible ways to further suppress backgrounds and the corresponding experimental prospects.

Camilo Garcia-Cely

Belle II sensitivity to long–lived dark photons

Inelastic dark matter is an interesting scenario for light thermal dark matter which is fully consistent with all cosmological probes as well as direct and indirect dark matter detection. The required mass splitting between dark matter χ 1 and its heavier twin χ 2 is naturally induced by a dark Higgs field which also provides a simple mechanism to give mass to the dark photon A′ present in the setup. The corresponding dark Higgs boson h′ is naturally the lightest dark sector state and therefore decays into Standard Model particles via Higgs mixing. In this work we study signatures with displaced vertices and missing momentum at Belle II, arising from dark Higgs particles produced in association with dark matter. We find that Belle II can be very sensitive to this scenario, in particular if a displaced vertex trigger is available in the near future.

Michael Duerr

Long-lived dark Higgs and inelastic dark matter at Belle II

A mistake has been found in the numerical code used to reproduce the bounds from proton beam dump experiments from ref. [1] in figures 2 and 7 of ref. [2]. Correcting this mistake leads to slightly stronger bounds as shown below. We note that this correction does not include recent improvements in the analysis of proton beam dump experiments [3]. Additional recent bounds on GeV-scale ALPs can be found in refs. [4–8].

Australia

Matthew Dolan

Erratum to: Revised constraints and Belle II sensitivity for visible and invisible axion-like particles

Many dark matter models generically predict invisible and displaced signatures at Belle II, but even striking events may be missed by the currently implemented search programme because of inefficient trigger algorithms. Of particular interest are final states with a single photon accompanied by missing energy and a displaced pair of electrons, muons, or hadrons. We argue that a displaced vertex trigger will be essential to achieve optimal sensitivity at Belle II. To illustrate this point, we study a simple but well-motivated model of thermal inelastic dark matter in which this signature naturally occurs and show that otherwise inaccessible regions of parameter space can be tested with such a search. We also evaluate the sensitivity of single-photon searches at BaBar and Belle II to this model and provide detailed calculations of the relic density target.

Patrick Tunney

Invisible and displaced dark matter signatures at Belle II

Light pseudoscalars interacting pre-dominantly with Standard Model gauge bosons (so-called axion-like particles or ALPs) occur frequently in extensions of the Standard Model. In this work we review and update existing constraints on ALPs in the keV to GeV mass region from colliders, beam dump experiments and astrophysics. We furthermore provide a detailed calculation of the expected sensitivity of Belle II, which can search for visibly and invisibly decaying ALPs, as well as long-lived ALPs. The Belle II sensitivity is found to be substantially better than previously estimated, covering wide ranges of relevant parameter space. In particular, Belle II can explore an interesting class of dark matter models, in which ALPs mediate the interactions between the Standard Model and dark matter. In these models, the relic abundance can be set via resonant freeze-out, leading to a highly predictive scenario consistent with all existing constraints but testable with single-photon searches at Belle II in the near future.

Revised constraints and Belle II sensitivity for visible and invisible axion-like particles