19 research outputs found
The point spread function of electrons in a magnetic field, and the decay of the free neutron
Experiments in nuclear and particle physics often use magnetic fields to
guide charged reaction products to a detector. Due to their gyration in the
guide field, the particles hit the detector within an area that can be
considerably larger than the diameter of the source where the particles are
produced. This blurring of the image of the particle source on the detector
surface is described by a suitable point spread function (PSF), which is
defined as the image of a point source. We derive simple analytical expressions
for such magnetic PSFs, valid for any angular distribution of the emitted
particles that can be developed in Legendre polynomials. We investigate this
rather general problem in the context of neutron beta decay spectrometers and
study the effect of limited detector size on measured neutron decay correlation
parameters. To our surprise, insufficient detector size does not affect much
the accuracy of such measurements, even for rather large radii of gyration.
This finding can considerably simplify the layout of the respective
spectrometers.Comment: 24 pages, 12 figure
Determination of the Weak Axial Vector Coupling from a Measurement of the Beta-Asymmetry Parameter A in Neutron Beta Decay
We report on a new measurement of the neutron beta-asymmetry parameter
with the instrument \perkeo. Main advancements are the high neutron
polarization of from a novel arrangement of super mirror
polarizers and reduced background from improvements in beam line and shielding.
Leading corrections were thus reduced by a factor of 4, pushing them below the
level of statistical error and resulting in a significant reduction of
systematic uncertainty compared to our previous experiments. From the result
, we derive the ratio of the axial-vector to the vector
coupling constant Comment: 5 pages, 4 figure
Measurement of the Neutrino Asymmetry Parameter B in Neutron Decay
A new measurement of the neutrino asymmetry parameter B in neutron decay, the
angular correlation between neutron spin and anti-neutrino momentum, is
presented. The result, B=0.9802(50), agrees with the Standard Model expectation
and earlier measurements, and permits improved tests on ``new physics'' in
neutron decay.Comment: 4 pages, 2 figures; v2: revised PRL versio
Characterization of a ballistic supermirror neutron guide
We describe the beam characteristics of the first ballistic supermirror
neutron guide H113 that feeds the neutron user facility for particle physics
PF1B of the Institute Laue-Langevin, Grenoble (ILL). At present, the neutron
capture flux density of H113 at its 20x6cm2 exit window is 1.35x10^10/cm^2/s,
and will soon be raised to above 2x10^10/cm^2/s. Beam divergence is no larger
than beam divergence from a conventional Ni coated guide. A model is developed
that permits rapid calculation of beam profiles and absolute event rates from
such a beam. We propose a procedure that permits inter-comparability of the
main features of beams emitted from ballistic or conventional neutron guides.Comment: 15 pages, 11 figures, to be submitted to Nuclear Instruments and
Methods
A clean, bright, and versatile source of neutron decay products
We present a case study on a new type of cold neutron beam station for the
investigation of angular correlations in the beta-decay of free neutrons. With
this beam station, called PERC, the 'active decay volume' lies inside the
neutron guide, and the charged neutron decay products are magnetically guided
towards the end of the neutron guide. Hence, the guide delivers at its exit a
beam of decay electrons and protons, under well-defined and precisely variable
conditions, which can be well separated from the cold neutron beam. In this way
a general-purpose source of neutron decay products is obtained which can be
used for various different experiments in neutron decay correlation
spectroscopy. A gain in phase space density of several orders of magnitude can
be achieved with PERC, as compared to existing neutron decay spectrometers.
Neutron beam related background is separately measurable in PERC, and magnetic
mirror effects on the charged neutron decay products and edge effects in the
active neutron beam volume are both strongly suppressed. Therefore the spectra
and angular distributions of the emerging decay particles will be
distortion-free on the level of 10^-4, more than 10 times better than achieved
today.Comment: 20 pages, 6 figure
The neutron and its role in cosmology and particle physics
Experiments with cold and ultracold neutrons have reached a level of
precision such that problems far beyond the scale of the present Standard Model
of particle physics become accessible to experimental investigation. Due to the
close links between particle physics and cosmology, these studies also permit a
deep look into the very first instances of our universe. First addressed in
this article, both in theory and experiment, is the problem of baryogenesis ...
The question how baryogenesis could have happened is open to experimental
tests, and it turns out that this problem can be curbed by the very stringent
limits on an electric dipole moment of the neutron, a quantity that also has
deep implications for particle physics. Then we discuss the recent spectacular
observation of neutron quantization in the earth's gravitational field and of
resonance transitions between such gravitational energy states. These
measurements, together with new evaluations of neutron scattering data, set new
constraints on deviations from Newton's gravitational law at the picometer
scale. Such deviations are predicted in modern theories with extra-dimensions
that propose unification of the Planck scale with the scale of the Standard
Model ... Another main topic is the weak-interaction parameters in various
fields of physics and astrophysics that must all be derived from measured
neutron decay data. Up to now, about 10 different neutron decay observables
have been measured, much more than needed in the electroweak Standard Model.
This allows various precise tests for new physics beyond the Standard Model,
competing with or surpassing similar tests at high-energy. The review ends with
a discussion of neutron and nuclear data required in the synthesis of the
elements during the "first three minutes" and later on in stellar
nucleosynthesis.Comment: 91 pages, 30 figures, accepted by Reviews of Modern Physic
Constraints on the Dark Matter Interpretation of the Neutron Decay Anomaly with the PERKEO II experiment
International audienceDiscrepancies from in-beam- and in-bottle-type experiments measuring the neutron lifetime are on the 4σ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being n→χ+e+e-. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95% of the allowed mass range for the dark matter particle