arXiv subjects

S. Penttila

Publications and source records attributed to S. Penttila.

15 recordsLinked to original sources

arXiv2025

Fundamental Nuclear and Particle Physics At Neutron Sources

Fundamental neutron and neutrino physics at neutron sources, combining precision measurements and theory, can probe new physics at energy scales well beyond the highest energies probed by the LHC and possible future high energy collider facilities. The European Spallation Source (ESS) will in the not too far future be a most powerful pulsed neutron source and simultaneously the world's brightest pulsed neutrino source. The ESS, and neutron sources in general, can provide unprecedented and unique opportunities to contribute to the search for the missing elements in the Standard Model of particle physics. Currently there are no strong indications where hints of the origin of the new physics will emerge. A multi-pronged approach will provide the fastest path to fill the gaps in our knowledge and neutron sources have a pivotal role to play. To survey the ongoing and proposed physics experiments at neutron sources and assess their potential impact, a workshop was held at Lund University in January, 2025. This report is a summary of that workshop and has been prepared as input to the European Strategy Update.

nucl-ex↗

arXiv2021

First Measurement of Coherent Elastic Neutrino-Nucleus Scattering on Argon

We report the first measurement of coherent elastic neutrino-nucleus scattering (\cevns) on argon using a liquid argon detector at the Oak Ridge National Laboratory Spallation Neutron Source. Two independent analyses prefer \cevns over the background-only null hypothesis with greater than $3σ$ significance. The measured cross section, averaged over the incident neutrino flux, is (2.2 $\pm$ 0.7) $\times$10$^{-39}$ cm$^2$ -- consistent with the standard model prediction. The neutron-number dependence of this result, together with that from our previous measurement on CsI, confirms the existence of the \cevns process and provides improved constraints on non-standard neutrino interactions.

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arXiv2020

COHERENT Collaboration data release from the first detection of coherent elastic neutrino-nucleus scattering on argon

Release of COHERENT collaboration data from the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) on argon. This release corresponds with the results of "Analysis A" published in Akimov et al., arXiv:2003.10630 [nucl-ex]. Data is shared in a binned, text-based format representing both "signal" and "backgrounds" along with associated uncertainties such that the included data can be used to perform independent analyses. This document describes the contents of the data release as well as guidance on the use of the data. Included example code in C++ (ROOT) and Python show one possible use of the included data.

nucl-ex↗

arXiv2020

First Precision Measurement of the Parity Violating Asymmetry in Cold Neutron Capture on $^3$He

We report the first precision measurement of the parity-violating asymmetry in the direction of proton emission with respect to the neutron spin, in the reaction $^{3}\mathrm{He}(\mathrm{n},\mathrm{p})^{3}\mathrm{H}$, using the capture of polarized cold neutrons in an unpolarized active $^3\rm{He}$ target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the most poorly understood aspects of electro-weak theory. The measurement provides an important benchmark for modern effective field theory (EFT) calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is $A_{PV} = \left( 1.58 \pm 0.97 ~\mathrm{(stat)} \pm 0.24~\mathrm{(sys)}\right)\times10^{-8}$, which has the smallest uncertainty of any parity-violating asymmetry measurement so far.

nucl-ex↗

arXiv2020

The n$^3$He Experiment: Parity Violation in Polarized Neutron Capture on $^{3}$He

Significant progress has been made to experimentally determine a complete set of the parity-violating (PV) weak-interaction amplitudes between nucleons. In this paper we describe the design, construction and operation of the n$^3$He experiment that was used to measure the PV asymmetry $A_{\mathrm{PV}}$ in the direction of proton emission in the reaction $\vec{\mathrm{n}} + {^3}\mathrm{He} \rightarrow {^3}\mathrm{H} + \mathrm{p}$, using the capture of polarized cold neutrons in an unpolarized gaseous $^3\mathrm{He}$ target. This asymmetry has was recently calculated \cite{Viviani,Viviani2}, both in the traditional style meson exchange picture, and in effective field theory (EFT), including two-pion exchange. The high precision result (published separately) obtained with the experiment described herein forms an important benchmark for hadronic PV (HPV) theory in few-body systems, where precise calculations are possible. To this day, HPV is still one of the most poorly understood aspects of the electro-weak theory. The calculations estimate the size of the asymmetry to be in the range of $(-9.4 \rightarrow 3.5)\times 10^{-8}$, depending on the framework or model. The small size of the asymmetry and the small overall goal uncertainty of the experiment of $δA_{\mathrm{PV}} \simeq 1\times10^{-8}$ places strict requirements on the experiment, especially on the design of the target-detector chamber. In this paper we describe the experimental setup and the measurement methodology as well as the detailed design of the chamber, including results of Garfield++ and Geant4 simulations that form the basis of the chamber design and analysis. We also show data from commissioning and production and define the systematic errors that the chamber contributes to the measured $A_{\mathrm{PV}}$. We give the final uncertainty on the measurement.

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arXiv2019

Sensitivity of the COHERENT Experiment to Accelerator-Produced Dark Matter

The COHERENT experiment is well poised to test sub-GeV dark matter models using low-energy recoil detectors sensitive to coherent elastic neutrino-nucleus scattering (CEvNS) in the $π$-DAR neutrino beam produced by the Spallation Neutron Source. We show how a planned 750-kg liquid argon scintillation detector would place leading limits on scalar light dark matter models, over two orders of magnitude of dark matter mass, for dark matter particles produced through vector and leptophobic portals in the absence of other effects beyond the standard model. The characteristic timing structure of a $π$-DAR beam allows a unique opportunity for constraining systematic uncertainties on the standard model background in a time window where signal is not expected, enhancing expected sensitivity. Additionally, we discuss future prospects, further increasing the discovery potential of CEvNS detectors. Such methods would test the calculated thermal dark matter abundance for all couplings $α'\leq1$ within the vector portal model over an order of magnitude of dark matter masses.

hep-ex↗

arXiv2019

The neutron electric dipole moment experiment at the Spallation Neutron Source

Novel experimental techniques are required to make the next big leap in neutron electric dipole moment experimental sensitivity, both in terms of statistics and systematic error control. The nEDM experiment at the Spallation Neutron Source (nEDM@SNS) will implement the scheme of Golub & Lamoreaux [Phys. Rep., 237, 1 (1994)]. The unique properties of combining polarized ultracold neutrons, polarized $^3$He, and superfluid $^4$He will be exploited to provide a sensitivity to $\sim 10^{-28}\,e{\rm \,\cdot\, cm}$. Our cryogenic apparatus will deploy two small ($3\,{\rm L}$) measurement cells with a high density of ultracold neutrons produced and spin analyzed in situ. The electric field strength, precession time, magnetic shielding, and detected UCN number will all be enhanced compared to previous room temperature Ramsey measurements. Our $^3$He co-magnetometer offers unique control of systematic effects, in particular the Bloch-Siegert induced false EDM. Furthermore, there will be two distinct measurement modes: free precession and dressed spin. This will provide an important self-check of our results. Following five years of "critical component demonstration," our collaboration transitioned to a "large scale integration" phase in 2018. An overview of our measurement techniques, experimental design, and brief updates are described in these proceedings.

nucl-ex↗

arXiv2019

First Constraint on Coherent Elastic Neutrino-Nucleus Scattering in Argon

Coherent elastic neutrino-nucleus scattering (CEvNS) is the dominant neutrino scattering channel for neutrinos of energy $E_ν< 100$ MeV. We report a limit for this process using data collected in an engineering run of the 29 kg CENNS-10 liquid argon detector located 27.5 m from the Oak Ridge National Laboratory Spallation Neutron Source (SNS) Hg target with $4.2\times 10^{22}$ protons on target. The dataset yielded $< 7.4$ observed CEvNS events implying a cross section for the process, averaged over the SNS pion decay-at-rest flux, of $<3.4 \times 10^{-39}$ cm$^{2}$, a limit within twice the Standard Model prediction. This is the first limit on CEvNS from an argon nucleus and confirms the earlier CsI non-standard neutrino interaction constraints from the collaboration. This run demonstrated the feasibility of the ongoing experimental effort to detect CEvNS with liquid argon.

hep-ex↗

arXiv2018

COHERENT Collaboration data release from the first observation of coherent elastic neutrino-nucleus scattering

This release includes data and information necessary to perform independent analyses of the COHERENT result presented in Akimov et al., arXiv:1708.01294 [nucl-ex]. Data is shared in a binned, text-based format, including both "signal" and "background" regions, so that counts and associated uncertainties can be quantitatively calculated for the purpose of separate analyses. This document describes the included information and its format, offering some guidance on use of the data. Accompanying code examples show basic interaction with the data using Python.

nucl-ex↗

arXiv2018

COHERENT 2018 at the Spallation Neutron Source

The primary goal of the COHERENT collaboration is to measure and study coherent elastic neutrino-nucleus scattering (CEvNS) using the high-power, few-tens-of-MeV, pulsed source of neutrinos provided by the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). The COHERENT collaboration reported the first detection of CEvNS [Akimov:2017ade] using a CsI[Na] detector. At present the collaboration is deploying four detector technologies: a CsI[Na] scintillating crystal, p-type point-contact germanium detectors, single-phase liquid argon, and NaI[Tl] crystals. All detectors are located in the neutron-quiet basement of the SNS target building at distances 20-30 m from the SNS neutrino source. The simultaneous measurement in all four COHERENT detector subsystems will test the $N^2$ dependence of the cross section and search for new physics. In addition, COHERENT is measuring neutrino-induced neutrons from charged- and neutral-current neutrino interactions on nuclei in shielding materials, which represent a non-negligible background for CEvNS as well as being of intrinsic interest. The Collaboration is planning as well to look for charged-current interactions of relevance to supernova and weak-interaction physics. This document describes concisely the COHERENT physics motivations, sensitivity, and next plans for measurements at the SNS to be accomplished on a few-year timescale.

physics.ins-det↗

arXiv2017

Observation of Coherent Elastic Neutrino-Nucleus Scattering

The coherent elastic scattering of neutrinos off nuclei has eluded detection for four decades, even though its predicted cross-section is the largest by far of all low-energy neutrino couplings. This mode of interaction provides new opportunities to study neutrino properties, and leads to a miniaturization of detector size, with potential technological applications. We observe this process at a 6.7-sigma confidence level, using a low-background, 14.6-kg CsI[Na] scintillator exposed to the neutrino emissions from the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. Characteristic signatures in energy and time, predicted by the Standard Model for this process, are observed in high signal-to-background conditions. Improved constraints on non-standard neutrino interactions with quarks are derived from this initial dataset.

nucl-ex↗

arXiv2016

The COHERENT Experiment at the Spallation Neutron Source

The COHERENT collaboration's primary objective is to measure coherent elastic neutrino-nucleus scattering (CEvNS) using the unique, high-quality source of tens-of-MeV neutrinos provided by the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). In spite of its large cross section, the CEvNS process has never been observed, due to tiny energies of the resulting nuclear recoils which are out of reach for standard neutrino detectors. The measurement of CEvNS has now become feasible, thanks to the development of ultra-sensitive technology for rare decay and weakly-interacting massive particle (dark matter) searches. The CEvNS cross section is cleanly predicted in the standard model; hence its measurement provides a standard model test. It is relevant for supernova physics and supernova-neutrino detection, and enables validation of dark-matter detector background and detector-response models. In the long term, precision measurement of CEvNS will address questions of nuclear structure. COHERENT will deploy multiple detector technologies in a phased approach: a 14-kg CsI[Na] scintillating crystal, 15 kg of p-type point-contact germanium detectors, and 100 kg of liquid xenon in a two-phase time projection chamber. Following an extensive background measurement campaign, a location in the SNS basement has proven to be neutron-quiet and suitable for deployment of the COHERENT detector suite. The simultaneous deployment of the three COHERENT detector subsystems will test the $N^2$ dependence of the cross section and ensure an unambiguous discovery of CEvNS. This document describes concisely the COHERENT physics motivations, sensitivity and plans for measurements at the SNS to be accomplished on a four-year timescale.

physics.ins-det↗

arXiv2014

Determination of the Free Neutron Lifetime

We present the status of current US experimental efforts to measure the lifetime of the free neutron by the "beam" and "bottle" methods. BBN nucleosynthesis models require accurate measurements with 1 second uncertainties, which are currently feasible. For tests of physics beyond the standard model, future efforts will need to achieve uncertainties well below 1 second. We outline paths achieve both.

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arXiv2008

Neutron Beam Effects on Spin Exchange Polarized He-3

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal-spin-exchange polarized He-3 cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable He-3 polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at LANSCE and ILL. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as the square root of the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the recombination-limited ion concentration, but is much larger than expected from earlier work.

nucl-ex↗

arXiv2001

Measurement of the 3He mass diffusion coefficient in superfluid 4He over the 0.45-0.95 K temperature range

We have measured the mass diffusion coefficient D of 3He in superfluid 4He at temperatures lower than were previously possible. The experimental technique utilizes scintillation light produced when neutron react with 3He nuclei, and allows measurement of the 3He density integrated along the trajectory of a well-defined neutron beam. By measuring the change in 3He density near a heater as a function of applied heat current, we are able to infer values of D with 20% accuracy. At temperatures below 0.7 K and for concentrations of order 10^{-4} we find D=(2.0+2.4-1.2)T^-(6.5 -/+ 1.2) cm^2/s, in agreement with a theoretical approximation.

cond-mat↗