arXiv subjects

R. Kaiser

Publications and source records attributed to R. Kaiser.

At least 55 records · Page 3Linked to original sources

arXiv2016

Temporal intensity correlation of light scattered by a hot atomic vapor

We present temporal intensity correlation measurements of light scattered by a hot atomic vapor. Clear evidence of photon bunching is shown at very short time-scales (nanoseconds) imposed by the Doppler broadening of the hot vapor. Moreover, we demonstrate that relevant information about the scattering process, such as the ratio of single to multiple scattering, can be deduced from the measured intensity correlation function. These measurements confirm the interest of temporal intensity correlation to access non-trivial spectral features, with potential applications in astrophysics.

physics.atom-ph↗

arXiv2016

Spin density matrix elements in exclusive $ω$ electroproduction on $^1$H and $^2$H targets at 27.5 GeV beam energy

Exclusive electroproduction of $ω$ mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of Q$^2$>1.0 GeV$^2$, 3.0 GeV < W < 6.3 GeV, and -t'< 0.2 GeV$^2$. Results on the angular distribution of the $ω$ meson, including its decay products, are presented. The data were accumulated with the HERMES forward spectrometer during the 1996-2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of Q$^2$ or -t'. Violation of s-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse $ω$ production by longitudinal and transverse virtual photons, $γ^{*}_{L} \to ω_{T}$ and $γ^{*}_{T} \to ω_{T}$, is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the $γ^*_T \to ω_T$ transition dominates over the two natural-parity-exchange amplitudes describing the $γ^*_L \to ω_L$ and $γ^*_T \to ω_T$ transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.

hep-ex↗

arXiv2015

Transverse-target-spin asymmetry in exclusive $ω$-meson electroproduction

Hard exclusive electroproduction of $ω$ mesons is studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. The amplitudes of five azimuthal modulations of the single-spin asymmetry of the cross section with respect to the transverse proton polarization are measured. They are determined in the entire kinematic region as well as for two bins in photon virtuality and momentum transfer to the nucleon. Also, a separation of asymmetry amplitudes into longitudinal and transverse components is done. These results are compared to a phenomenological model that includes the pion pole contribution. Within this model, the data favor a positive $πω$ transition form factor.

hep-ex↗

arXiv2015

Fluctuation properties of laser light after interaction with an atomic system: comparison between two-level and multilevel atomic transitions

The complex internal atomic structure involved in radiative transitions has an effect on the spectrum of fluctuations (noise) of the transmitted light. A degenerate transition has different properties in this respect than a pure two-level transition. We investigate these variations by studying a certain transition between two degenerate atomic levels for different choices of the polarization state of the driving laser. For circular polarization, corresponding to the textbook two-level atom case, the optical spectrum shows the characteristic Mollow triplet for strong laser drive, while the corresponding noise spectrum exhibits squeezing in some frequency ranges. For a linearly polarized drive, corresponding to the case of a multilevel system, additional features appear in both optical and noise spectra. These differences are more pronounced in the regime of a weakly driven transition: whereas the two-level case essentially exhibits elastic scattering, the multilevel case has extra noise terms related to spontaneous Raman transitions. We also discuss the possibility to experimentally observe these predicted differences for the commonly encountered case where the laser drive has excess noise in its phase quadrature.

quant-ph↗

arXiv2015

Bose-Einstein correlations in hadron-pairs from lepto-production on nuclei ranging from hydrogen to xenon

Bose-Einstein correlations of like-sign charged hadrons produced in deep-inelastic electron and positron scattering are studied in the HERMES experiment using nuclear targets of $^1$H, $^2$H, $^3$He, $^4$He, N, Ne, Kr, and Xe. A Gaussian approach is used to parametrize a two-particle correlation function determined from events with at least two charged hadrons of the same sign charge. This correlation function is compared to two different empirical distributions that do not include the Bose-Einstein correlations. One distribution is derived from unlike-sign hadron pairs, and the second is derived from mixing like-sign pairs from different events. The extraction procedure used simulations incorporating the experimental setup in order to correct the results for spectrometer acceptance effects, and was tested using the distribution of unlike-sign hadron pairs. Clear signals of Bose-Einstein correlations for all target nuclei without a significant variation with the nuclear target mass are found. Also, no evidence for a dependence on the invariant mass W of the photon-nucleon system is found when the results are compared to those of previous experiments.

hep-ex↗

arXiv2015

Optical pattern formation with a 2-level nonlinearity

We present an experimental and theoretical investigation of spontaneous pattern formation in the transverse section of a single retro-reflected laser beam passing through a cloud of cold Rubidium atoms. In contrast to previously investigated systems, the nonlinearity at work here is that of a 2-level atom, which realizes the paradigmatic situation considered in many theoretical studies of optical pattern formation. In particular, we are able to observe the disappearance of the patterns at high intensity due to the intrinsic saturable character of 2-level atomic transitions.

physics.atom-ph↗

arXiv2015

Pentaquark $Θ^+$ search at HERMES

The earlier search at HERMES for narrow baryon states excited in quasi-real photoproduction, decaying through the channel $pK_S^0\rightarrow pπ^+π^-$, has been extended with improved decay-particle reconstruction, more advanced particle identification, and increased event samples. The structure observed earlier at an invariant mass of 1528 MeV shifts to 1522 MeV and the statistical significance drops to about 2$σ$ for data taken with a deuterium target. The number of events above background is $68_{-31}^{+98}\text{(stat)}\pm13\text{(sys)}$. No such structure is observed in the hydrogen data set.

hep-ex↗

arXiv2014

Cooperative effects and disorder: A scaling analysis of the spectrum of the effective atomic Hamiltonian

We study numerically the spectrum of the non-Hermitian effective Hamiltonian that describes the dipolar interaction of a gas of $N\gg 1$ atoms with the radiation field. We analyze the interplay between cooperative effects and disorder for both scalar and vectorial radiation fields. We show that for dense gases, the resonance width distribution follows, both in the scalar and vectorial cases, a power law $P(Γ) \sim Γ^{-4/3}$ that originates from cooperative effects between more than two atoms. This power law is different from the $ P(Γ) \sim Γ^{-1}$ behavior, which has been considered as a signature of Anderson localization of light in random systems. We show that in dilute clouds, the center of the energy distribution is described by Wigner's semicircle law in the scalar and vectorial cases. For dense gases, this law is replaced in the vectorial case by the Laplace distribution. Finally, we show that in the scalar case the degree of resonance overlap increases as a power law of the system size for dilute gases, but decays exponentially with the system size for dense clouds.

cond-mat.mes-hall↗

arXiv2014

Assessing the Feasibility of Interrogating Nuclear Waste Storage Silos using Cosmic-ray Muons

Muon radiography is a fast growing field in applied scientific research. In recent years, many detector technologies and imaging techniques using the Coulomb scattering and absorption properties of cosmic-ray muons have been developed for the non-destructive assay of various structures across a wide range of applications. This work presents the first results that assess the feasibility of using muons to interrogate waste silos within the UK Nuclear Industry. Two such approaches, using different techniques that exploit each of these properties, have previously been published, and show promising results from both simulation and experimental data for the detection of shielded high-Z materials and density variations from volcanic assay. Both detector systems are based on scintillator and photomultiplier technologies. Results from dedicated simulation studies using both these technologies and image reconstruction techniques are presented for an intermediate-sized nuclear waste storage facility filled with concrete and an array of uranium samples. Both results highlight the potential to identify uranium objects of varying thicknesses greater than 5cm within real-time durations of several weeks. Increased contributions from Coulomb scattering within the concrete of the structure hinder the ability of both approaches to resolve objects of 2cm dimensions even with increased statistics. These results are all dependent on both the position of the objects within the facility and the locations of the detectors. Results for differing thicknesses of concrete, which reflect the unknown composition of the structures under interrogation, are also presented alongside studies performed for a series of data collection durations. It is anticipated that with further research, muon radiography in one, or both of these forms, will play a key role in future industrial applications within the UK Nuclear Industry.

physics.ins-det↗

arXiv2014

Transverse polarization of $Λ$ hyperons from quasireal photoproduction on nuclei

The transverse polarization of $Λ$ hyperons was measured in inclusive quasireal photoproduction for various target nuclei ranging from hydrogen to xenon. The data were obtained by the HERMES experiment at HERA using the 27.6 GeV lepton beam and nuclear gas targets internal to the lepton storage ring. The polarization observed is positive for light target nuclei and is compatible with zero for krypton and xenon.

hep-ex↗

arXiv2014

Signatures of Lévy flights with annealed disorder

We present theoretical and experimental results of Lévy flights of light originating from a random walk of photons in a hot atomic vapor. In contrast to systems with quenched disorder, this system does not present any correlations between the position and the step length of the random walk. In an analytical model based on microscopic first principles including Doppler broadening we find anomalous Lévy-type superdiffusion corresponding to a single-step size distribution P(x) proportional to x^(-1-alpha), with alpha=1. We show that this step size distribution leads to a violation of Ohm's law [T_(diff) proportional to L^(-alpha/2) different from 1/L], as expected for a Lévy walk of independent steps. Furthermore the spatial profile of the transmitted light develops power law tails [I(r) proportional to r^(-3-alpha)]. In an experiment using a slab geometry with hot rubidium vapor, we measured the total diffuse transmission T_(diff) and the spatial profile of the transmitted light T_{diff}(r). We obtained the microscopic Lévy parameter alpha under macroscopic multiple scattering conditions paving the way to investigation of Lévy flights in different atomic physics and astrophysics systems.

physics.optics↗

arXiv2014

Raman process under condition of radiation trapping in a disordered atomic medium

We consider the Raman process developing in a disordered medium of alkali-metal atoms when the scattered modes are trapped on a closed transition. Our theoretical analysis, based on numerical simulations of the Bethe-Salpeter equation for the light correlation function, which includes all Zeeman states and light polarization, lets us track the stimulated amplification as well as the losses associated with the inverse anti-Stokes scattering channel. We discuss possible conditions when this process could approach the instability point and enter the regime of random lasing.

physics.atom-ph↗

arXiv2014

Reevaluation of the parton distribution of strange quarks in the nucleon

An earlier extraction from the HERMES experiment of the polarization-averaged parton distribution of strange quarks in the nucleon has been reevaluated using final data on the multiplicities of charged kaons in semi-inclusive deep-inelastic scattering obtained with a kinematically more comprehensive method of correcting for experimental effects. General features of the distribution are confirmed, but the rise at low x is less pronounced than previously reported.

hep-ex↗

arXiv2014

The Atomic Lighthouse Effect

We investigate the deflection of light by a cold atomic cloud when the light-matter interaction is locally tuned via the Zeeman effect using magnetic field gradients. This "lighthouse" effect is strongest in the single-scattering regime, where deviation of the incident field is largest. For optically dense samples, the deviation is reduced by collective effects, as the increase in linewidth leads to a decrease of the magnetic field efficiency.

physics.atom-ph↗

arXiv2014

Beam-helicity asymmetry in associated electroproduction of real photons $ep \to eγπN$ in the $Δ$-resonance region

The beam-helicity asymmetry in associated electroproduction of real photons, $ep\to eγπN$, in the $Δ$(1232)-resonance region is measured using the longitudinally polarized HERA positron beam and an unpolarized hydrogen target. Azimuthal Fourier amplitudes of this asymmetry are extracted separately for two channels, $ep\to eγπ^0 p$ and $ep\to eγπ^+ n$, from a data set collected with a recoil detector. All asymmetry amplitudes are found to be consistent with zero.

hep-ex↗

arXiv2013

The OLYMPUS Experiment

The OLYMPUS experiment was designed to measure the ratio between the positron-proton and electron-proton elastic scattering cross sections, with the goal of determining the contribution of two-photon exchange to the elastic cross section. Two-photon exchange might resolve the discrepancy between measurements of the proton form factor ratio, $μ_p G^p_E/G^p_M$, made using polarization techniques and those made in unpolarized experiments. OLYMPUS operated on the DORIS storage ring at DESY, alternating between 2.01~GeV electron and positron beams incident on an internal hydrogen gas target. The experiment used a toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight detectors to measure rates for elastic scattering over the polar angular range of approximately $25^\circ$--$75^\circ$. Symmetric Møller/Bhabha calorimeters at $1.29^\circ$ and telescopes of GEM and MWPC detectors at $12^\circ$ served as luminosity monitors. A total luminosity of approximately 4.5~fb$^{-1}$ was collected over two running periods in 2012. This paper provides details on the accelerator, target, detectors, and operation of the experiment.

physics.ins-det↗

arXiv2013

Transverse target single-spin asymmetry in inclusive electroproduction of charged pions and kaons

Single-spin asymmetries were investigated in inclusive electroproduction of charged pions and kaons from transversely polarized protons at the HERMES experiment. The asymmetries were studied as a function of the azimuthal angle $ψ$ about the beam direction between the target-spin direction and the hadron production plane, the transverse hadron momentum relative to the direction of the incident beam, and the Feynman variable $x_F$. The $\sin(ψ)$ amplitudes are positive for positive pions and kaons, slightly negative for negative pions and consistent with zero for negative kaons, with particular transverse-momentum but weak $x_F$ dependences. Especially large asymmetries are observed for two small subsamples of events, where also the scattered electron was recorded by the spectrometer.

hep-ex↗

arXiv2013

Subradiant hybrid states in the open 3D Anderson-Dicke model

Anderson localization is a paradigmatic coherence effect in disordered systems, often analyzed in the absence of dissipation. Here we consider the case of coherent dissipation, occurring for open system with coupling to a common decay channel. This dissipation induces cooperative Dicke super- and subradiance and an effective long range hopping, expected to destroy Anderson localization. We are thus in presence of two competing effects, i.e localization driven by disorder and delocalization driven by dissipative opening. Here we demonstrate the existence of a {\it subradiant hybrid regime}, emerging from the interplay of opening and disorder, in which subradiant states are {hybrid with both features of localized and extended states}, while superradiant states are extended. We also provide analytical predictions for this regime, confirmed by numerical simulations.

cond-mat.mes-hall↗