120 research outputs found
Measurement of the ortho-positronium confinement energy in mesoporous thin films
In this paper, we present measurements of the ortho-positronium emission
energy in vacuum from mesoporous films using the time of flight technique. We
show evidence of quantum mechanical confinement in the mesopores that defines
the minimal energy of the emitted Ps. Two samples with different effective pore
sizes, measured with positron annihilation lifetime spectroscopy, are compared
for the data collected in the temperature range 50-400 K. The sample with
smaller pore size exhibits a higher minimal energy (5 meV), compared to
the sample with bigger pores (5 meV), due to the stronger confinement.
The dependence of the emission energy with the temperature of the target is
modeled as ortho-positronium being confined in rectangular boxes in
thermodynamic equilibrium with the sample. We also measured that the yield of
positronium emitted in vacuum is not affected by the temperature of the target.Comment: 8 pages, 9 figures *Added references. * Corrected typos and Fig. 3
label. * Revised argument in section IV B abd C, results unchanged
Muonium Lamb shift: theory update and experimental prospects
We review the theory of the Lamb shift for muonium, provide an updated
numerical value and present the prospects of the Mu-MASS collaboration at PSI
to improve upon their recent measurement. Due to its smaller nuclear mass, the
contributions of the higher-order recoil corrections (160kHz level) and nucleus
self-energy 40kHz level) are enhanced for muonium compared to hydrogen where
those are below the level of the latest measurement performed by Hessels et al.
and thus could not be tested yet. The ongoing upgrades to the Mu-MASS setup
will open up the possibility to probe these contributions and improve the
sensitivity of this measurement to searches for new physics in the muonic
sector.Comment: 8 page
Prospects for studies of the free fall and gravitational quantum states of antimatter
Different experiments are ongoing to measure the effect of gravity on cold
neutral antimatter atoms such as positronium, muonium and antihydrogen. Among
those, the project GBAR in CERN aims to measure precisely the gravitational
fall of ultracold antihydrogen atoms. In the ultracold regime, the interaction
of antihydrogen atoms with a surface is governed by the phenomenon of quantum
reflection which results in bouncing of antihydrogen atoms on matter surfaces.
This allows the application of a filtering scheme to increase the precision of
the free fall measurement. In the ultimate limit of smallest vertical
velocities, antihydrogen atoms are settled in gravitational quantum states in
close analogy to ultracold neutrons (UCNs). Positronium is another neutral
system involving antimatter for which free fall under gravity is currently
being investigated at UCL. Building on the experimental techniques under
development for the free fall measurement, gravitational quantum states could
also be observed in positronium. In this contribution, we review the status of
the ongoing experiments and discuss the prospects of observing gravitational
quantum states of antimatter and their implications.Comment: This work reviews contributions made at the GRANIT 2014 workshop on
prospects for the observation of the free fall and gravitational quantum
states of antimatte
Pulsed CW laser for long-term spectroscopic measurements at high power in deep-UV
We present a novel technique for in-vacuum cavity-enhanced UV spectroscopy
that allows nearly continuous measurements over several days, minimizing mirror
degradation caused by high-power UV radiation. Our method relies on pulsing of
the cavity's internal power, which increases the UV intensity to maximum only
for short periods when the studied atom is within the cavity mode volume while
keeping the average power low to prevent mirror degradation. Additionally, this
method significantly decreases laser-induced background on charged particle
detectors. The described 244 nm laser system is designed for 1S-2S two-photon
CW spectroscopy of muonium in the Mu-MASS project. It was tested to provide
intracavity powers above 20 W, requiring maintenance only a few times a day.
The pulsing technique demonstrates minimal impact on the radiation frequency,
with no observed shifts exceeding 15 kHz. Our approach represents a promising
new technique for high-precision spectroscopy of atoms in harsh UV environments
and demonstrates the feasibility of CW spectroscopy of muonium.Comment: 10 pages, 5 figure
Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN
We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e+e− pairs observed in the excited 8Be∗ nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e−Z→e−ZX by a 100 GeV e−beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a e+e− pair. With 5.4×1010 electrons on target, no evidence for such decays was found, allowing us to set first limits on the X−e− coupling in the range 1.3×10−4≲εe≲4.2×10−4 excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A′) from nonobservation of the decay A′→e+e−of the bremsstrahlung A′ with a mass ≲23 MeV
Search for vector mediator of dark matter production in invisible decay mode
A search is performed for a new sub-GeV vector boson (A′) mediated production of dark matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A′, called dark photon, can be generated in the reaction e−Z→e−ZA′ of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A′→χ¯χ. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to 4.3×1010 electrons on target no evidence of such a process has been found. New stringent constraints on the A′ mixing strength with photons, 10−5≲ε≲10−2, for the A′ mass range mA′≲1 GeV are derived. For models considering scalar and fermionic thermal dark matter interacting with the visible sector through the vector portal the 90% C.L. limits 10−11≲y≲10−6on the dark-matter parameter y=ε2αD(mχmA′)4 are obtained for the dark coupling constant αD=0.5and dark-matter masses 0.001≲mχ≲0.5 GeV. The lower limits αD≳10−3 for pseudo-Dirac dark matter in the mass region mχ≲0.05 GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using exact tree level calculations of the A′production cross sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region mA′≳0.1 Ge
Mesoporous silica films with varying porous volume fraction: direct correlation between ortho-positronium annihilation decay and escape yield into vacuum
accepted for publication in Applied Physics LettersThe behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low–energy positrons has been studied as a function of the film porous volume fraction. A lifetime spectrometer allowed determination of o-Ps annihilation decay both inside and outside of the film. A kinetic model is introduced that permits the determination of the yield and rate of escape of o-Ps into vacuum as well as the annihilation decay rate of the trapped o-Ps in the film. It is shown that these undergo a sudden change at a threshold porous volume fraction, above which the o-Ps escape rate to vacuum varies linearly with volume fraction
Positronium reemission yield from mesostructured silica films
The reemission yield of ortho-positronium (o-Ps) into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40% are found. The intensity of the 142 ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2gamma and 3gamma energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for Ps annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in CTACl-TEOS films
Dark sectors 2016 Workshop: community report
This report, based on the Dark Sectors workshop at SLAC in April 2016,
summarizes the scientific importance of searches for dark sector dark matter
and forces at masses beneath the weak-scale, the status of this broad
international field, the important milestones motivating future exploration,
and promising experimental opportunities to reach these milestones over the
next 5-10 years
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