71 research outputs found
Circular Dichroism in Atomic Resonance-Enhanced Few-Photon Ionization
We investigate few-photon ionization of lithium atoms prepared in the
polarized 2() state when subjected to femtosecond light pulses
with left- or right-handed circular polarization at wavelengths between 665 nm
and 920 nm. We consider whether ionization proceeds more favorably for the
electric field co- or counter-rotating with the initial electronic current
density. Strong asymmetries are found and quantitatively analyzed in terms of
"circular dichroism" (). While the intensity dependence of the measured
values is rather weak throughout the investigated regime, a very strong
sensitivity on the center wavelength of the incoming radiation is observed.
While the co-rotating situation overall prevails, the counter-rotating geometry
is strongly favored around 800 nm due to the 2-3 resonant transition,
which can only be driven by counter-rotating fields. The observed features
provide insights into the helicity dependence of light-atom interactions, and
on the possible control of electron emission in atomic few-photon ionization by
polarization-selective resonance enhancement
Using circular dichroism to control energy transfer in multi-photon ionization
Chirality causes symmetry breaks in a large variety of natural phenomena
ranging from particle physics to biochemistry. We investigate one of the
simplest conceivable chiral systems, a laser-excited, oriented, effective
one-electron Li target. Prepared in a polarized p state with |m|=1 in an
optical trap, the atoms are exposed to co- and counter-rotating circularly
polarized femtosecond laser pulses. For a field frequency near the excitation
energy of the oriented initial state, a strong circular dichroism is observed
and the photoelectron energies are significantly affected by the
helicity-dependent Autler-Townes splitting. Besides its fundamental relevance,
this system is suited to create spin-polarized electron pulses with a
reversible switch on a femtosecond timescale at an energy resolution of a few
meV
Nuclear recoil response of liquid xenon and its impact on solar 8B neutrino and dark matter searches
Knowledge of the ionization and scintillation responses of liquid xenon (LXe)
to nuclear recoils is crucial for LXe-based dark matter experiments. Current
calibrations carry large uncertainties in the low-energy region below
keV where signals from dark matter particles of 10 GeV/c masses are
expected. The coherent elastic neutrino-nucleus scattering (CENS) by solar
B neutrinos also results in a continuum of nuclear recoil events below 3.0
keV (99\% of events), which further complicates low-mass dark matter
searches in LXe experiments. In this paper, we describe a method to quantify
the uncertainties of low-energy LXe responses using published calibration data,
followed by case studies to evaluate the impact of yield uncertainties on
B searches and low-mass dark matter sensitivity in a typical ton-scale
LXe experiment. We conclude that naively omitting yield uncertainties leads to
overly optimistic limits by factor for a 6 GeV/c WIMP mass. Future
nuclear recoil light yield calibrations could allow experiments to recover this
sensitivity and also improve the accuracy of solar B flux measurements
Applying Superfluid Helium to Light Dark Matter Searches: Demonstration of the HeRALD Detector Concept
The SPICE/HeRALD collaboration is performing R&D to enable studies of sub-GeV
dark matter models using a variety of target materials. Here we report our
recent progress on instrumenting a superfluid He target mass with a
transition-edge sensor based calorimeter to detect both atomic signals (e.g.
scintillation) and He quasiparticle (phonon and roton) excitations. The
sensitivity of HeRALD to the critical "quantum evaporation" signal from He
quasiparticles requires us to block the superfluid film flow to the
calorimeter. We have developed a heat-free film-blocking method employing an
unoxidized Cs film, which we implemented in a prototype "HeRALD v0.1" detector
of 10~g target mass. This article reports initial studies of the atomic
and quasiparticle signal channels. A key result of this work is the measurement
of the quantum evaporation channel's gain of , which will
enable He-based dark matter experiments in the near term. With this gain
the HeRALD detector reported here has an energy threshold of 145~eV at 5 sigma,
which would be sensitive to dark matter masses down to 220~MeV/c.Comment: 14 pages, 9 figure
Fast and Flexible Analysis of Direct Dark Matter Search Data with Machine Learning
We present the results from combining machine learning with the profile
likelihood fit procedure, using data from the Large Underground Xenon (LUX)
dark matter experiment. This approach demonstrates reduction in computation
time by a factor of 30 when compared with the previous approach, without loss
of performance on real data. We establish its flexibility to capture non-linear
correlations between variables (such as smearing in light and charge signals
due to position variation) by achieving equal performance using pulse areas
with and without position-corrections applied. Its efficiency and scalability
furthermore enables searching for dark matter using additional variables
without significant computational burden. We demonstrate this by including a
light signal pulse shape variable alongside more traditional inputs such as
light and charge signal strengths. This technique can be exploited by future
dark matter experiments to make use of additional information, reduce
computational resources needed for signal searches and simulations, and make
inclusion of physical nuisance parameters in fits tractable
Extending light WIMP searches to single scintillation photons in LUX
We present a novel analysis technique for liquid xenon time projection chambers that allows for a lower threshold by relying on events with a prompt scintillation signal consisting of single detected photons. The energy threshold of the LUX dark matter experiment is primarily determined by the smallest scintillation response detectable, which previously required a twofold coincidence signal in its photomultiplier arrays, enforced in data analysis. The technique presented here exploits the double photoelectron emission effect observed in some photomultiplier models at vacuum ultraviolet wavelengths. We demonstrate this analysis using an electron recoil calibration dataset and place new constraints on the spin-independent scattering cross section of weakly interacting massive particles (WIMPs) down to 2.5 GeV/c2 WIMP mass using the 2013 LUX dataset. This new technique is promising to enhance light WIMP and astrophysical neutrino searches in next-generation liquid xenon experiments
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