arXiv subjects

Susanne Westhoff

Publications and source records attributed to Susanne Westhoff.

At least 19 recordsLinked to original sources

arXiv2025

ALP-mediated Dark Matter-Nucleon Scattering

We perform a comprehensive analysis of dark matter-nucleon scattering via the exchange of axion-like particles (ALPs). At first sight, this might appear of little practical use, as non-relativistic scattering through pseudo-scalar interactions is momentum-suppressed and spin-dependent, resulting in scattering rates below any experimental sensitivity. We show that the scattering rate can be drastically enhanced in two ways. First, light ALPs with masses below the typical momentum transfer at direct detection experiments lift the momentum suppression by acting as essentially massless mediators. Second, ALP exchange through loops induces coherent spin-independent scattering. If the ALP has flavor-changing couplings to up-type quarks, loop-induced scattering receives an extra strong enhancement by the top-quark mass. We deduce that, contrary to common lore, XENONnT and PandaX-4T are already sensitive to ALP-mediated dark matter-nucleon scattering. The next generation of direct detection experiments will probe far into the parameter space of the ALP effective theory, potentially exceeding the sensitivity of collider searches.

hep-ph↗

arXiv2025

Scattering meets absorption in dark matter detection

Direct detection experiments have started to explore dark matter scattering off electrons and nucleons through light mediators. Mediators with sub-keV masses are efficiently produced in the Sun and can be absorbed in the same detectors that probe dark matter scattering. We investigate the interplay of dark matter scattering and mediator absorption for two models with a dark photon as mediator. For Dirac dark matter, we find that scattering and absorption can be simultaneously observed at direct detection experiments in the near future. For atomic dark matter, we predict additional signals due to scattering of both dark atoms and constituents from ionized dark atoms. In both models, we determine the parameter space that respects bounds from cosmology and astrophysics, where the strongest constraints come from dark matter self-interactions. In this way, we identify viable targets for dark matter with light mediators at upcoming direct detection experiments. Distinguishing between the various signals, for instance by measuring energy distributions, will be crucial to reveal the underlying model in case of a discovery.

hep-ph↗

arXiv2024

Darkonia at Colliders

Dark matter may form bound states in a dark sector with an attractive force between two dark matter particles. Searches for dark matter at colliders can differ dramatically from routine searches if bound states, dubbed darkonia, are produced and decay into visible Standard-Model particles. In this work, we use three representative models with scalar, pseudo-scalar, and vector force carriers to map out the darkonium signatures at both high-energy and low-energy colliders. Some of the bound states can be stable due to generalized parity and charge-conjugation symmetries, while others decay into light dark-force carriers, which subsequently can decay at a displaced vertex. New signatures with a mix of missing energy and multiple di-lepton or di-jet vertices reconstructing intermediate darkonium resonances are within reach at the LHC and Belle II.

hep-ph↗

arXiv2024

Precise tests of the axion coupling to tops

We present an in-depth analysis of axions and axion-like particles in top-pair production at the LHC. Our main goal is to probe the axion coupling to top quarks at high energies. To this end, we calculate the top-antitop cross section and differential distributions including ALP effects up to one-loop level. By comparing these predictions with LHC precision measurements, we constrain the top coupling of axion-like particles with masses below the top-antitop threshold. Our results apply to all UV completions of the ALP effective theory with dominant couplings to top quarks, in particular to DFSZ-like axion models.

hep-ph↗

arXiv2023

FIMP Dark Matter at the LHC

This brief summary targets feebly interacting massive particles, FIMPs, which are interesting candidates for dark matter. The cosmic history of FIMP dark matter often leads to predictions of long-lived mediator particles at laboratory experiments. I point out the role of the LHC in searching for such particles and sketch its complementarity with low-energy experiments.

hep-ph↗

arXiv2023

Top Secrets: Long-Lived ALPs in Top Production

We investigate the discovery potential for long-lived particles produced in association with a top-antitop quark pair at the (High-Luminosity) LHC. Compared to inclusive searches for a displaced vertex, top-associated signals offer new trigger options and an extra handle to suppress background. We design a search strategy for a displaced di-muon vertex in the tracking detectors, in association with a reconstructed top-antitop pair. For axion-like particles with masses above the di-muon threshold, we find that the (High-Luminosity) LHC can probe effective top-quark couplings as small as $|c_{tt}|/f_a = 0.03(0.002)/$TeV and proper decay lengths as long as $20(300)$ m, assuming a cross section of $1$ fb, with data corresponding to an integrated luminosity of 150 fb$^{-1}$ (3 ab$^{-1}$). Our predictions suggest that searches for top-associated displaced di-muons will explore new terrain in the current sensitivity gap between searches for prompt di-muons and missing energy.

hep-ph↗

arXiv2023

Global Analysis of the ALP Effective Theory

We perform a global fit of the effective Lagrangian for axion-like particles (ALPs) to data. By combining LHC observables from top physics, dijet and di-boson production with electroweak precision observables, we resolve the full parameter space of ALPs with flavor-universal couplings. Using the renormalization group to evolve the effective ALP couplings to low energies allows us to investigate the impact of flavor observables on the global analysis. We show that resonance searches in $B\to K$ meson decays significantly enhance the sensitivity to ALPs with sub-GeV masses. The lifetime of the ALP plays a crucial role in resolving the multi-dimensional parameter space with searches for prompt, displaced and invisible ALP decays. Our analysis points out the differences in probing an effective theory with new light particles, compared to scenarios with only non-resonant effects of heavy particles at low energies, as in the Standard Model Effective Field Theory.

hep-ph↗

arXiv2023

Displaced or invisible? ALPs from $B$ decays at Belle II

At colliders, neutral long-lived particles can be detected through displaced decay products or as missing energy. Which search strategy is better depends on the particle's decay length just as on the detector properties. We investigate the complementarity of displaced and invisible signatures for the Belle II experiment. Focusing on axion-like particles $a$ produced from meson decays, we present a new search strategy for two-body decays $B^+ \to K^+ a, a\to E\!\!\!/$ with missing energy $E\!\!\!/$. With $50\,$ab$^{-1}$ of data, Belle II can probe light invisible resonances with branching ratio $\mathcal{B}(B^+\to K^+ a) \gtrsim 10^{-7}$ and decay length $cτ_a \gtrsim 1\,$m. For axion-like particles, we expect the sensitivity of $B^+ \to K^+ E\!\!\!/$ to small couplings to improve by up to two orders of magnitude compared to previous searches at collider and fixed-target experiments. For sub-GeV particles, $B^+ \to K^+ E\!\!\!/$ at Belle II and searches at beam-dump experiments are most sensitive; for heavier particles, searches for displaced vertices at Belle II, long-lived particle experiments at the LHC, and future fixed-target experiments can probe the smallest couplings.

hep-ph↗

arXiv2023

Near or Far Detectors? Optimizing Long-Lived Particle Searches at Electron-Positron Colliders

We explore the discovery potential for long-lived particles at the 250-GeV ILC. The goal is to investigate possible gains of a dedicated far detector over the main detector ILD. For concreteness, we perform our study for sub-GeV axion-like particles $a$ produced via $e^+e^- \to a γ$ or $e^+e^- \to Z γ\to (aγ)γ$ and decaying into pairs of charged leptons. In the ideal case of zero background and perfect detection efficiency, we find that far detectors placed in the planned underground cavities or a large cuboid on the ground can enhance the sensitivity to long-lived pseudo-scalars at best moderately. On the other hand, the ILD itself is a perfect environment to search for long-lived particles, due to its excellent angular coverage and radial thickness. For long-lived particles produced with cross sections of a few picobarns, the ILD could probe lifetimes up to 300$\,$ns, or proper decay lengths up to 100$\,$m, in 250$\,$fb$^{-1}$ of data. For axion-like particles produced through weak interactions, the ILC can reach an even higher sensitivity than searches for displaced vertices in meson decays at Belle II. Our findings apply similarly to other proposed electron-positron experiments with a high angular coverage, such as the FCC-ee and CEPC.

hep-ph↗

arXiv2022

Resolving the Flavor Structure in the MFV-SMEFT

We constrain the flavor structure of Wilson coefficients in the Standard Model Effective Field Theory (SMEFT) from data. In the SMEFT, new physics effects in couplings of up-type and down-type quarks are related through the Cabibbo-Kobayashi-Maskawa mixing matrix. We exploit this relation to pin down potential new sources of flavor symmetry breaking in a global analysis of high- and low-energy data from the LHC, LEP, and $b$ factory experiments. We demonstrate the power of such an analysis by performing a combined fit of effective four-quark and two-quark couplings contributing to a large set of flavor, top-quark, electroweak, and dijet observables. All four sectors are needed to fully resolve the flavor structure of the four-quark couplings without leaving blind directions in the parameter space. Although we work in the framework of minimal flavor violation, our strategy applies as well to other flavor patterns, like $U(2)$ flavor symmetry or leptoquark scenarios.

hep-ph↗

arXiv2022

Inelastic Dirac Dark Matter

Feebly interacting thermal relics are promising dark matter candidates. Among them, scenarios of inelastic Dark Matter evade direct detection by suppressed elastic scattering off atomic nuclei. We introduce inelastic Dirac Dark Matter, a new model with two Dirac fermions in the MeV-GeV mass range. At feeble couplings, dark matter can depart from chemical as well as kinetic equilibrium with the Standard Model before freeze-out. In this case, the freeze-out is driven by conversion processes like coscattering, rather than coannihilation. We show that inelastic Dirac relics are consistent with cosmological observations, in particular with nucleosynthesis and the cosmic microwave background. Searches for dark sectors at colliders and fixed-target experiments, in turn, are very sensitive probes. Compared to the strongly constrained pseudo-Dirac scenario, inelastic Dirac Dark Matter offers a new search target for existing and upcoming experiments like Belle II, ICARUS, LDMX and SeaQuest.

hep-ph↗

arXiv2022

Physics reach of a long-lived particle detector at Belle II

We have studied three realistic benchmark geometries for a new far detector GAZELLE to search for long-lived particles at the \superkekb accelerator in Tsukuba, Japan. The new detector would be housed in the same building as Belle II and observe the same $e^+e^-$ collisions. To assess the discovery reach of GAZELLE, we have investigated three new physics models that predict long-lived particles: heavy neutral leptons produced in tau lepton decays, axion-like particles produced in $B$ meson decays, and new scalars produced in association with a dark photon, as motivated by inelastic dark matter. We do not find significant gains in the new physics discovery reach of GAZELLE compared to the Belle II projections for the same final states. The main reasons are the practical limitations on the angular acceptance and size of GAZELLE, effectively making it at most comparable to Belle II, even though backgrounds in the far detector could be reduced to low rates. A far detector for long-lived particles would be well motivated in the case of a discovery by Belle II, since decays inside GAZELLE would facilitate studies of the decay products. Depending on the placement of GAZELLE, searches for light long-lived particles produced in the forward direction or signals of a confining hidden force could also benefit from such a far detector. Our general findings could help guide the design of far detectors at future electron-positron colliders such as the ILC, FCC-ee or CEPC.

hep-ph↗

arXiv2022

Unleashing the full power of LHCb to probe Stealth New Physics

In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment.

hep-ph↗

arXiv2021

Unveiling Hidden Physics at the LHC

The field of particle physics is at the crossroads. The discovery of a Higgs-like boson completed the Standard Model (SM), but the lacking observation of convincing resonances Beyond the SM (BSM) offers no guidance for the future of particle physics. On the other hand, the motivation for New Physics has not diminished and is, in fact, reinforced by several striking anomalous results in many experiments. Here we summarise the status of the most significant anomalies, including the most recent results for the flavour anomalies, the multi-lepton anomalies at the LHC, the Higgs-like excess at around 96 GeV, and anomalies in neutrino physics, astrophysics, cosmology, and cosmic rays. While the LHC promises up to 4/ab of integrated luminosity and far-reaching physics programmes to unveil BSM physics, we consider the possibility that the latter could be tested with present data, but that systemic shortcomings of the experiments and their search strategies may preclude their discovery for several reasons, including: final states consisting in soft particles only, associated production processes, QCD-like final states, close-by SM resonances, and SUSY scenarios where no missing energy is produced. New search strategies could help to unveil the hidden BSM signatures, devised by making use of the CERN open data as a new testing ground. We discuss the CERN open data with its policies, challenges, and potential usefulness for the community. We showcase the example of the CMS collaboration, which is the only collaboration regularly releasing some of its data. We find it important to stress that individuals using public data for their own research does not imply competition with experimental efforts, but rather provides unique opportunities to give guidance for further BSM searches by the collaborations. Wide access to open data is paramount to fully exploit the LHCs potential.

hep-ph↗

arXiv2021

Global Searches for New Physics with Top Quarks

This is a brief summary of the latest searches for virtual effects of new physics in the top sector. In the framework of the Standard Model Effective Field Theory (SMEFT), I show how to resolve the structure of effective couplings by combining observables at the LHC and at flavor experiments in a global fit. With this approach we start exploring the features of a UV theory at energies beyond current colliders.

hep-ph↗

arXiv2021

The Flavor of UV Physics

New physics not far above the TeV scale should leave a pattern of virtual effects in observables at lower energies. What do these effects tell us about the flavor structure of a UV theory? Within the framework of Standard Model Effective Field Theory (SMEFT), we resolve the flavor structure of the Wilson coefficients in a combined analysis of top-quark and $B$-physics observables. Our fit to LHC and $b$-factory measurements shows that combining top and bottom observables is crucial to pin down possible sources of flavor symmetry breaking from UV physics. Our analysis includes the full analytic expansion of SMEFT coefficients in Minimal Flavor Violation and a detailed study of SMEFT effects in $b\to s$ flavor transitions.

hep-ph↗

arXiv2020

Soft displaced leptons at the LHC

Soft displaced leptons are representative collider signatures of compressed dark sectors with feeble couplings to the standard model. Prime targets are dark matter scenarios where co-scattering or co-annihilation sets the relic abundance upon freeze-out. At the LHC, searches for soft displaced leptons are challenged by a large background from hadron or tau lepton decays. In this article, we present an analysis tailored for displaced leptons with a low transverse momentum threshold at 20 GeV. Using a neural network, we perform a comprehensive analysis of the event kinematics, including a study of the expected detection efficiencies and backgrounds at small momenta. Our results show that weak-scale particles decaying into soft leptons with decay lengths between 1mm and 1m can be probed with LHC Run 2 data. This motivates the need for dedicated triggers that maximize the sensitivity to displaced soft leptons.

hep-ph↗

arXiv2020

A Complete Framework for Tau Polarimetry in $B\to D^{(*)}τν$ Decays

The meson decays $B\to Dτν$ and $B\to D^* τν$ are sensitive probes of the $b\to cτν$ transition. In this work we present a complete framework to obtain the maximum information on the physics of $B\to D^{(*)}τν$ with polarized $τ$ leptons and unpolarized $D^{(*)}$ mesons. Focusing on the hadronic decays $τ\to πν$ and $τ\toρν$, we show how to extract seven $τ$ asymmetries from a fully differential analysis of the final-state kinematics. At Belle II with $50~\text{ab}^{-1}$ of data, these asymmetries could potentially be measured with percent level statistical uncertainty. This would open a new window into possible new physics contributions in $b\to cτν$ and would allow us to decipher its Lorentz and gauge structure.

hep-ph↗