arXiv subjects

David Milstead

Publications and source records attributed to David Milstead.

11 recordsLinked to original sources

arXiv2026

Vertex reconstruction for a search for neutron-antineutron conversions with HIBEAM

The HIBEAM/NNBAR programme (incorporated into the FINESSE/NNBAR programme) at the European Spallation Source is proposed to search for neutrons converting to antineutrons. An important observable is the reconstructed vertex arising from charged particles produced by an antineutron annihilating on a thin target foil and which pass through a time projection chamber. This paper studies track clustering and foil-plane vertex reconstruction for this topology. Both non-machine-learning methods and graph-neural-network methods are tested and compared with each other, including deterministic clustering, trackless projection and a hybrid clustering/graph-neural-network chain with track classification and vertex refinement. We conclude that, for the geometrically simple events in the HIBEAM TPC, classical reconstruction methods perform on-par with machine-learning based methods in terms of vertex coordinate reconstruction. Custom machine-learning based methods can, however, deliver event-shape information that may be important in downstream analyses.

hep-ex↗

arXiv2026

New Limits on $n \rightarrow n'$ Transformation from HFIR Cold Neutron Beam

Hypothetical neutron $n$ to sterile neutron $n'$ transformations would violate baryon number $\mathcal{B}$ and point to the nature of Dark Matter. We performed a new search for $n \rightarrow n'$ using an intense cold neutron beam from the High Flux Isotope Reactor at Oak Ridge National Laboratory. We used a theoretical model that describes the transformation $n \rightarrow n'$ with two parameters: a small mass difference $\Delta{m}$ between the interaction states $n$ and $n'$ and a mixing vacuum angle $\theta_0$. A thin absorbing cadmium wafer was used in the center of the superconducting 6.6 T magnet which provided a large gradient for the non-adiabatic $n \rightarrow n'$ transition. No signal was observed above background in the $^{3}\text{He}$ neutron detector 20 meters downstream of the magnet. This result gives an order of magnitude improvement in the lower limit for the probability $2\theta_0^2$ of the $n \rightarrow n'$ transformation in vacuum in the range of $\Delta{m}$ between $0.1$ neV and $1000$ neV.

hep-ex↗

arXiv2025

Avoiding Blindness in Baryon Number Violating Processes: Free-Beam and Intranuclear Paths to Neutron-Antineutron Transitions

Experimental searches for neutron--antineutron ($n \rightarrow \bar n$) transitions can be considered via two approaches: conversion in free-neutron beams and intranuclear transformation leading to matter instability in large-mass detectors. Plans for next-generation searches make it timely to highlight the complementarity, necessity, and limitations of each method. Converting the bound neutron limit into one for free neutrons traditionally utilizes nucleus-specific estimates of the in-medium suppression of $n \rightarrow \bar n$, obtained within mean-field theory under a single-operator assumption. This paper highlights how this suppression can be scenario-dependent, which can lead to deviations from the standard approach that can span several orders of magnitude. A further goal of the paper is to point out the need for a broader phenomenology program for $n\rightarrow \bar{n}$ that is akin to those developed for electric dipole moments and other systems for which short-distance new physics must be studied in-medium.

hep-ph↗

arXiv2025

Advancements in Computing and Simulation Techniques for the HIBEAM-NNBAR Experiment

The HIBEAM-NNBAR program is a proposed two-stage experiment at the European Spallation Source focusing on searches for baryon number violation processes as well as ultralight dark matter. This paper presents recent advancements in computing and simulation, including machine learning for event selection, fast parametric simulations for detector studies, and detailed modeling of the time projection chamber and readout electronics.

physics.ins-det↗

arXiv2024

Improved Limits on $n \rightarrow n'$ Transformation from the Spallation Neutron Source

Conversions between neutrons $n$ and Dark Matter candidate sterile neutrons $n'$ have been proposed as a mechanism for Baryon Number $\mathcal{B}$ violation. In the case that there is a small mass difference $\Delta{m}$ between the $n$ and the $n'$ states, oscillations can be induced by compensating for $\Delta{m}$ with a magnetic field. A search for such neutron oscillations was performed at the Spallation Neutron Source by looking for anomalous neutron transmission through a strongly absorbing cadmium wafer inside of a $6.6$~T magnet. The approach described here saw no regenerated neutrons above background, which provides an improved limit for neutron - sterile neutrons transformations for a range of $\Delta{m}$ between $0.1$~neV and $1000$~neV.

hep-ex↗

arXiv2022

Beamline Simulation for the NNBAR Experiment at the European Spallation Source

The HIBEAM and NNBAR experiments are a proposed fundamental science experiments at the European Spallation Source, performing high precision searches for neutron conversions in several baryon number violating (BNV) channels. For simulations of the NNBAR beamline, a new sampling method has been developed. The method is based on probability density evaluation and duct source biasing and enables the simulations of the entire NNBAR beamline with high statistics while also preserving correlations of the neutron tracks.

physics.data-an↗

arXiv2016

Baryon number violation in supersymmetry: Neutron-antineutron oscillations as a probe beyond the LHC

We summarize the current status of baryon number violation in supersymmetry and provide prospects for going beyond the present reach by means of a new search for neutron-antineutron oscillations. The main motivation is the recently proposed neutron-antineutron oscillation experiment at the European Spallation Source in Lund, Sweden, which is projected to be able to improve the current bound on the transition probability in the quasi-free regime by three orders of magnitude. We consider various processes involving superpartners that give rise to neutron-antineutron oscillations and extract the corresponding simplified models, including only the most relevant superpartners and couplings. In terms of these models we recast and determine the exclusion limits from LHC searches as well as from searches for flavor transitions, CP violation and di-nucleon decays. We find that, for certain regions of the parameter space, the proposed neutron-antineutron experiment has a reach that goes beyond all other experiments, as it can probe gluino and squark masses in the multi-TeV range.

hep-ph↗

arXiv2015

A new high sensitivity search for neutron-antineutron oscillations at the ESS

A sensitive search for neutron-antineutron oscillations can provide a unique probe of some of the central questions in particle physics and cosmology: the energy scale and mechanism for baryon number violation, the origin of the baryon-antibaryon asymmetry of the universe, and the mechanism for neutrino mass generation. A remarkable opportunity has emerged to search for such oscillations with the construction of the European Spallation Source (ESS). A collaboration has been formed which has proposed a search at the ESS, which would provide a sensitivity to the oscillation probability which is three orders of magnitude greater than that achieved at an ILL experiment at which the present best limit on free neutron-antineutron oscillations was obtained.

physics.ins-det↗

arXiv2014

Non-collider searches for stable massive particles

The theoretical motivation for exotic stable massive particles (SMPs) and the results of SMP searches at non-collider facilities are reviewed. SMPs are defined such that they would be sufficiently long-lived so as to still exist in the cosmos either as Big Bang relics or secondary collision products, and sufficiently massive to be beyond the reach of any conceivable accelerator-based experiment. The discovery of SMPs would address a number of important questions in modern physics, such as the origin and composition of dark matter in the Universe and the unification of the fundamental forces. This review outlines the scenarios predicting SMPs and the techniques used at non-collider experiments to look for SMPs, eg in cosmic rays and bound in matter. The limits so far obtained on the fluxes and matter densities of SMPs which possess various detection-relevant properties such as electric and magnetic charge are given.

hep-ph↗

arXiv2009

An Updated Description of Heavy-Hadron Interactions in Geant-4

Exotic stable massive particles (SMP) are proposed in a number of scenarios of physics beyond the Standard Model. It is important that LHC experiments are able both to detect and extract the quantum numbers of any SMP with masses around the TeV scale. To do this, an understanding of the interactions of SMPs in matter is required. In this paper a Regge-based model of R-hadron scattering is extended and implemented in Geant-4. In addition, the implications of $R$-hadron scattering for collider searches are discussed.

hep-ph↗

arXiv2002

Precision Tests of QCD

The electron-proton collider HERA allows the study of the partonic content of the proton in regions of Q-squared up to 50,000 GeV$^2$ and values of Bjorken-x down to 10^-5. This paper presents recent precision measurements of structure functions and hadronic final state observables which test QCD over this wide region of phase space.

hep-ex↗