arXiv subjects

Joachim Kopp

Publications and source records attributed to Joachim Kopp.

At least 19 recordsLinked to original sources

arXiv2026

The QTF-Backbone: Proposal for a Nationwide Optical Fibre Backbone in Germany for Quantum Technology and Time and Frequency Metrology

The recent breakthroughs in the distribution of quantum information and high-precision time and frequency (T&F) signals over long-haul optical fibre networks have transformative potential for physically secure communications, resilience of timing infrastructure (such as that supporting Global Navigation Satellite Systems (GNSS)) and fundamental physics. To date, these capabilities remain confined to isolated testbeds, with quantum and T&F signals accessible, for example in Germany, to only a few institutions. In this white paper we propose the QTF Backbone: a dedicated national fibre-optic infrastructure in Germany for the networked distribution of Quantum and T&F signals using dark fibres and specialised hardware. The QTF Backbone is planned as a four-phase deployment over ten years to ensure scalable, sustainable access for research institutions and industry. The concept builds on successful demonstrations of time and frequency distribution at high Technology Readiness Levels (TRLs) across Europe, including PTB-MPQ links in Germany, REFIMEVE in France, and the Italian LIFT network. The QTF Backbone will enable transformative Research and Development (R&D), support a nationwide QTF ecosystem, and ensure the transition from innovation to deployment. As a national and European hub, it will position Germany and Europe at the forefront of quantum networking, as well as T&F transfer.

physics.ins-det↗

arXiv2026

Neutrino Fluxes at a Muon Collider

While muon colliders are primarily considered precision machines for new physics searches at the energy frontier, they are also intense sources of high-energy neutrinos. Large fluxes of electron and muon neutrinos are produced in the decay of beam muons, especially along the straight sections around the experiments. In this work, we re-evaluate these neutrino fluxes using the latest design of a \SI{10}{TeV} muon collider and find that fluxes around the interaction region are almost two orders of magnitude higher than earlier estimates. We also compute the fluxes of neutrinos produced in $μ^+μ^-$ collisions, electromagnetic showers induced by electrons from muon decay, and interactions of neutrinos close to the collider ring. We find that these additional neutrino sources are non-negligible and would lead to a sizable number of neutrino interactions of all flavors, including several tau neutrino events, in a ton-scale detector placed in the forward direction. We discuss the physics opportunities offered by muon collider neutrinos in the context of QCD and nuclear physics, electroweak precision measurements, and searches for new physics.

hep-ph↗

arXiv2026

High-Frequency Gravitational Wave Constraints from Precision Spectroscopy

Gravitational waves affect the propagation of electromagnetic waves in laser cavities, modulating the frequency of emitted photons. We use this effect to search for high-frequency gravitational waves between 100 kHz and 100 MHz using optical precision spectroscopy. Our limits constrain much of this frequency range for the first time. We discuss future improvements of the technique, which we expect to enhance the sensitivity by eight orders of magnitude, and to extend the frequency coverage up to at least 1 GHz.

gr-qc↗

arXiv2026

Visible Neutrino Decay As An Open Quantum System

Decays of heavier neutrino mass eigenstates into lighter ones, while very slow in the Standard Model, can be significantly enhanced in scenarios with more than three neutrino flavours, or in models with new ultra-light particles such as Majorons. A full theoretical description is challenging due to the intricate interplay between oscillations and decay, interference between different decay channels, and the possibility of multi-step decay cascades. In this paper, we develop a fully general description of arbitrarily complex systems of oscillating and decaying neutrinos using methods from the theory of open quantum systems. Notably, we demonstrate how such systems can be implemented using the Lindblad master equation, the Liouvillian superoperator, as well as Kraus operators. The last two methods eschew the need for solving a differential equation, thereby showing superior numerical performance.

hep-ph↗

arXiv2026

Quantum Simulation of Collective Neutrino Oscillations using Dicke States

In dense neutrino gases, which exist for instance in supernovae, the flavour states of different neutrinos may become entangled with one another. The theoretical description of such systems may therefore call for simulations on a quantum computer. Existing quantum simulations of simple toy systems are not optimal in the sense that they do not fully exploit the symmetries of the system. Here, we propose a new class of qubit-efficient algorithms based on Dicke states and the $su(2)$ spin algebra. We demonstrate the excellent performance of these algorithms both on classical and on quantum hardware.

quant-ph↗

arXiv2026

High-Frequency Gravitational Waves from Phase Transitions in Nascent Neutron Stars

Tentative evidence suggests that the cores of massive neutron stars consist of deconfined quark matter. We argue that the formation of such a quark matter core during a galactic supernova could be accompanied by the emission of gravitational waves in the MHz band. These signals constitute a new target for high-frequency gravitational wave detectors, demonstrating that such detectors may offer unique opportunities for testing quantum chromodynamics in an otherwise inaccessible regime.

hep-ph↗

arXiv2026

Gravitational Wave-Induced Freeze-In of Fermionic Dark Matter

The minimal coupling of massless fermions to gravity does not allow for their gravitational production solely based on the expansion of the Universe. We argue that this changes in presence of realistic and potentially detectable stochastic gravitational wave backgrounds. We compute the resulting energy density of Weyl fermions at 1-loop using in--in formalism. If the initially massless fermions eventually acquire mass, this mechanism can explain the dark matter abundance in the Universe. Remarkably, it may be more efficient than conventional gravitational production of superheavy fermions.

hep-ph↗

arXiv2026

Challenges and Opportunities of Gravitational Wave Searches above 10 kHz

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational-wave searches at frequencies above the LIGO/Virgo band. The scarcity of possible astrophysical sources in most of this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising of these sources. We review several detector concepts that have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of a series of workshops on the topic of high-frequency gravitational wave detection, held in 2019 (ICTP, Trieste, Italy), 2021 (online) and 2023 (CERN, Geneva, Switzerland).

gr-qc↗

arXiv2025

Physics Briefing Book: Input for the 2026 update of the European Strategy for Particle Physics

The European Strategy for Particle Physics (ESPP) reflects the vision and presents concrete plans of the European particle physics community for advancing human knowledge in fundamental physics. The ESPP is updated every five-to-six years through a community-driven process. It commences with the submission of specific proposals and other input from the community at large, outlining projects envisioned for the near-, mid-, and long-term future. All submitted contributions are evaluated by the Physics Preparatory Group (PPG), and a preliminary analysis is presented at a Symposium meant to foster a broad community discussion on the scientific value and feasibility of the various ideas proposed. The outcomes of the analysis and the deliberations at the Symposium are synthesized in the current Briefing Book, which provides an important input in the deliberations of the Strategy recommendations by the European Strategy Group (ESG).

hep-ex↗

arXiv2025

A LENS on DUNE-PRISM: Characterizing a Neutrino Beam with Off-Axis Measurements

Upcoming precision long-baseline neutrino oscillation experiments will be severely limited by the large systematic uncertainties associated with neutrino flux predictions and neutrino--nucleus cross sections. A promising remedy is the PRISM (Precision Reaction Independent Spectrum Measurement) technique, whereby the near detector measures the neutrino spectrum at different angles with respect to the beam axis. These measurements are then linearly combined into a prediction of the oscillated neutrino flux at the far detector. This prediction is data-driven, but still dependent on some theoretical knowledge about the neutrino flux. In this paper, we study to what extent off-axis measurements themselves can be used to directly constrain neutrino flux models. In particular, we use them to extract separately the fluxes and spectra of different meson species in the beam. We call this measurement LENS (Lateral Extraction of Neutrino Spectra). Second, we demonstrate how the thus improved flux model helps to further constrain the far detector flux prediction, thereby ultimately improving oscillation measurements.

hep-ph↗

arXiv2025

Effective Field Theory in Long-Baseline Neutrino Oscillation Experiments

We study the phenomenology of physics beyond the Standard Model in long-baseline neutrino oscillation experiments using the most general parametrisation of heavy new physics in the framework of Standard Model Effective Theory (SMEFT), as well as its counterpart below the electroweak scale, Weak Effective Field Theory (WEFT). We compute neutrino production, oscillation, and detection rates in these frameworks, consistently accounting for renormalisation group running as well as SMEFT/WEFT matching. We moreover use appropriately modified neutrino--nucleus cross sections, focusing specifically on the regime of quasi-elastic scattering. Compared to the traditional formalism of non-standard neutrino interactions (NSI), our approach is theoretically more consistent, and it allows for straightforward joint analyses of data taken at different energy scales and by different experiments including not only neutrino oscillation experiments, but also searches for charged lepton flavour violation, low-energy precision measurements, and the LHC. As a specific example, we carry out a sensitivity study for the DUNE experiment and compute projected limits on the WEFT and SMEFT Wilson coefficients. Together with this paper, we also release a public simulation package called ``GLoBES-EFT'' for consistently simulating long-baseline neutrino oscillation experiments in the presence of new physics parameterized either in WEFT or in SMEFT. GLoBES-EFT is available from \href{https://github.com/SalvaUrrea2/GLoBES-EFT}{GitHub}.

hep-ph↗

arXiv2025

On the sensitivity of nuclear clocks to new physics

The recent demonstration of laser excitation of the $\approx 8$ eV isomeric state of Thorium-229 is a significant step towards a nuclear clock. The low excitation energy likely results from a cancellation between electromagnetic and strong contributions, which new physics can disrupt. In this Letter, we quantify the enhancement of a nuclear clock's sensitivity to new physics using a geometric model and a novel $d$-wave halo model of the nucleus that reproduces measured differences between Thorium-229 states. We find likely enhancements of order $10^4$ while a worst case scenario with enhancement $\ll 1$ is unlikely.

hep-ph↗

arXiv2025

Primordial Black Holes from First-Order Cosmological Phase Transitions

We discuss the possibility of forming primordial black holes during a first-order phase transition in the early Universe. As is well known, such a phase transition proceeds through the formation of true-vacuum bubbles in a Universe that is still in a false vacuum. When there is a particle species whose mass increases significantly during the phase transition, transmission of the corresponding particles through the advancing bubble walls is suppressed. Consequently, an overdensity can build up in front of the walls and become sufficiently large to trigger primordial black hole formation. We track this process quantitatively by solving a Boltzmann equation, and we delineate the phase transition properties required for our mechanism to yield an appreciable abundance of primordial black holes.

astro-ph.CO↗

arXiv2025

Detailed Calculation of Primordial Black Hole Formation During First-Order Cosmological Phase Transitions

Primordial black holes could potentially form during a first-order cosmological phase transition due to a build-up of particles which are predominantly reflected from the advancing bubble walls. After discussing the general mechanism, we examine the criteria that need to be satisfied for a black hole to form. We then set out the Boltzmann equation that describes the evolution of the relevant phase space distribution function, carefully describing our treatment of the Liouville operator and the collision term. Assuming a spherical false vacuum pocket of sufficient size and a constant wall velocity, we find that black holes can form in a range of different scenarios.

astro-ph.CO↗

arXiv2025

Clarity through the Neutrino Fog: Constraining New Forces in Dark Matter Detectors

The PANDAX-4T and XENONnT experiments present indications of Coherent Elastic Neutrino Nucleus Scattering (CE$ν$NS) from ${}^{8}$B solar neutrinos at 2.6$σ$ and 2.7$σ$, respectively. This constitutes the first observation of the neutrino "floor" or "fog", an irreducible background that future dark matter searches in terrestrial detectors will have to contend with. Here, we first discuss the contributions from neutrino-electron scattering and from the Migdal effect in the region of interest of these experiments, and we argue that they are non-negligible. Second, we make use of the recent PANDAX-4T and XENONnT data to derive novel constraints on light scalar and vector mediators coupling to neutrinos and quarks. We demonstrate that these experiments already provide world-leading laboratory constraints on new light mediators in some regions of parameter space.

hep-ph↗

arXiv2025

Improving Neutrino Energy Reconstruction with Machine Learning

Faithful energy reconstruction is foundational for precision neutrino experiments like DUNE, but is hindered by uncertainties in our understanding of neutrino--nucleus interactions. Here, we demonstrate that dense neural networks are very effective in overcoming these uncertainties by estimating inaccessible kinematic variables based on the observable part of the final state. We find improvements in the energy resolution by up to a factor of two compared to conventional reconstruction algorithms, which translates into an improved physics performance equivalent to a 10-30% increase in the exposure.

hep-ph↗

arXiv2025

Detectable MeV Neutrino Signals from Neutron-Star Common-Envelope Systems

Common-envelope evolution - where a star is engulfed by a companion - is a critical but poorly understood step in, e.g., the formation pathways for gravitational-wave sources. However, it has been extremely challenging to identify observable signatures of such systems. We show that for systems involving a neutron star, the hypothesized super-Eddington accretion onto the neutron star produces MeV-range, months-long neutrino signals within reach of present and planned detectors. While there are substantial uncertainties on the rate of such events (0.01-1/century in the Milky Way) and the neutrino luminosity (which may be less than the accretion power), this signal can only be found if dedicated new analyses are developed. If detected, the neutrino signal would probe super-Eddington accretion, leading to significant new insights into the astrophysics of common-envelope evolution.

astro-ph.HE↗

arXiv2025

Reinterpretation and preservation of data and analyses in HEP

Data from particle physics experiments are unique and are often the result of a very large investment of resources. Given the potential scientific impact of these data, which goes far beyond the immediate priorities of the experimental collaborations that obtain them, it is imperative that the collaborations and the wider particle physics community publish and preserve sufficient information to ensure that this impact can be realised, now and into the future. The information to be published and preserved includes the algorithms, statistical information, simulations and the recorded data. This publication and preservation requires significant resources, and should be a strategic priority with commensurate planning and resource allocation from the earliest stages of future facilities and experiments.

hep-ph↗