arXiv subjects

R. Beck

Publications and source records attributed to R. Beck.

At least 199 records · Page 11Linked to original sources

arXiv2014

Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised proton

Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise.

hep-ex↗

arXiv2014

Measurement of the transverse target and beam-target asymmetries in $η$ meson photoproduction at MAMI

We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the $\vec γ\vec p\toηp$ reaction up to a center-of-mass energy of W=1.9 GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of $η$ meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.

nucl-ex↗

arXiv2014

Quasifree photoproduction of $η$ mesons off protons and neutrons

Differential and total cross sections for the quasifree reactions $γp\rightarrowηp$ and $γn\rightarrowηn$ have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $η\rightarrow 2γ$ and $η\rightarrow 3π^0 \rightarrow 6γ$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $η$-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W = (1670\pm 5)$ MeV with an intrinsic width of $Γ\approx 30$ MeV.

nucl-ex↗

arXiv2014

Search for exclusive photoproduction of Z$_c^{\pm}$(3900) at COMPASS

A search for the exclusive production of the $Z_c^{\pm}(3900)$ hadron by virtual photons has been performed in the channel $Z_c^{\pm}(3900)\rightarrow J/ψπ^{\pm}$. The data cover the range from 7 GeV to 19 GeV in the centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio $BR(Z_c^{\pm}(3900)\rightarrow J/ψπ^{\pm} )\times σ_{ γ~N \rightarrow Z_c^{\pm}(3900)~ N} /σ_{γ~N \rightarrow J/ψ~ N}$ of $3.7\times10^{-3}$ has been established at the confidence level of 90%.

hep-ex↗

arXiv2014

The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR

The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz. We produced an image of total emission of M51 at the mean frequency of 151 MHz with 20 arcsec resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm--interarm contrast, and the break scales of the radio--far-infrared correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by HII regions. The non-detection of polarisation from M51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.

astro-ph.GA↗

arXiv2014

Magnetic fields near the peripheries of galactic discs

Magnetic fields are observed beyond the peripheries of optically detected galactic discs, while numerical models of their origin and the typical magnitudes are still absent. Previously, studies of galactic dynamo have avoided considering the peripheries of galactic discs because of the very limited (though gradually growing) knowledge about the local properties of the interstellar medium. Here we investigate the possibility that magnetic fields can be generated in the outskirts of discs, taking the Milky Way as an example. We consider a simple evolving galactic dynamo model in the "no-z" formulation, applicable to peripheral regions of galaxies, for various assumptions about the radial and vertical profiles of the ionized gas disc. The magnetic field may grow as galaxies evolve, even in the more remote parts of the galactic disc, out to radii of 15 to 30 kpc, becoming substantial after times of about 10 Gyr. This result depends weakly on the adopted distributions of the half thickness and surface density of the ionized gas component. The model is robust to changes in the amplitude of the initial field and the position of its maximum strength. The magnetic field in the remote parts of the galactic disc could be generated in situ from a seed field by local dynamo action. Another possibility is field production in the central regions of a galaxy, followed by transport to the disc's periphery by the joint action of the dynamo and turbulent diffusivity. Our results demonstrate the possibilities for the appearance and strengthening of magnetic fields at the peripheries of disc galaxies and emphasize the need for observational tests with new and anticipated radio telescopes (LOFAR, MWA, and SKA).

astro-ph.GA↗

arXiv2014

Magnetic fields and star formation in low-mass Magellanic-type and peculiar galaxies

We investigate how magnetic properties of Magellanic-type and perturbed objects are related to star-forming activity, galactic type, and mass. We present radio and magnetic properties of 5 Magellanic-type and 2 peculiar low-mass galaxies observed at 4.85 and/or 8.35 GHz with the Effelsberg 100-m telescope. The sample is extended to 17 objects by including 5 Magellanic-type galaxies and 5 dwarf ones. The radio emission of low-mass galaxies at 4.85/8.35 GHz is closely connected with their optical discs. The strengths of total magnetic field are within 5-9 μG, while the ordered fields reach 1-2 μG. The magnetic field strengths are well correlated with the surface density of SFR and manifest a power-law relation with an exponent of 0.25 extending a similar relation found for dwarf galaxies. The production of magnetic energy per supernova event is similar for all the various galaxies. It constitutes about 3% of the individual SN energy release. We show that the total magnetic field energy in galaxies is almost linearly related to the galactic gas mass, which indicates equipartition of the magnetic energy and the turbulent kinetic energy of the ISM. The Magellanic-type galaxies fit very well with the radio-infrared relation constructed for surface brightness of galaxies of various types, including bright spirals and interacting objects. We found that the typical far-infrared relation based on luminosity of galaxies is tighter and steeper but more likely to inherit a partial correlation from a tendency that larger objects are also more luminous. The thermal fractions, radio spectral indices, and magnetic field strengths of the Magellanic-type galaxies are between the values determined for spirals and dwarf galaxies. The confirmed magnetic field-star formation and radio-infrared relations for low-mass galaxies point to similar physical processes that must be at work in all galaxies.

astro-ph.GA↗

arXiv2014

Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field

This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for the extraction of the cosmological 21-cm signal from the Low-Frequency Array - Epoch of Reionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. The brightness temperature of the detected Galactic emission is on average 4 K in polarized intensity and covers the range from -10 to +13rad m^-2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology, at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies. The wide frequency range, good angular resolution and good sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of 1-2 rad m^-2 in Faraday depth. The different polarised patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarised foregrounds is a serious complication for Epoch of Reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.

astro-ph.GA↗

arXiv2014

The N(1520) 3/2- helicity amplitudes from an energy-independent multipole analysis based on new polarization data on photoproduction of neutral pions

New data on the polarization observables T, P, and H for the reaction $γp \to pπ^0$ are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

nucl-ex↗

arXiv2014

$K^+Λ$ and $K^+Σ^0$ photoproduction with fine center-of-mass energy resolution

Measurements of $γp \rightarrow K^{+} Λ$ and $γp \rightarrow K^{+} Σ^0$ cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel $K^+$ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy ($W$) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark-diquark dynamics in the nucleon.

nucl-ex↗

arXiv2014

Experimental constraints on the $ω$-nucleus real potential

In a search for $ω$ mesic states, the production of $ω$-mesons in coincidence with forward going protons has been studied in photon induced reactions on $^{12}$C for incident photon energies of 1250 - 3100 MeV. The $π^0 γ$ pairs from decays of bound or quasi-free $ω$-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the $π^0 γ$ pairs, which would indicate the population and decay of bound $ω~^{11}$B states, are not observed. The $π^0 γ$ cross section of 0.3 nb/MeV/sr observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the $ω$-meson. A comparison of the measured total energy distribution with calculations suggests the real part $V_0$ of the $ω~^{11}$B potential to be small and only weakly attractive with $V_0(ρ=ρ_0) = -15\pm$ 35(stat) $\pm$20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100 - 150 MeV.

nucl-ex↗

arXiv2014

Initial Performance of the NEOWISE Reactivation Mission

NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft has been brought out of hibernation and has resumed surveying the sky at 3.4 and 4.6 um. The scientific objectives of the NEOWISE reactivation mission are to detect, track, and characterize near-Earth asteroids and comets. The search for minor planets resumed on December 23, 2013, and the first new near-Earth object (NEO) was discovered six days later. As an infrared survey, NEOWISE detects asteroids based on their thermal emission and is equally sensitive to high and low albedo objects; consequently, NEOWISE-discovered NEOs tend to be large and dark. Over the course of its three-year mission, NEOWISE will determine radiometrically-derived diameters and albedos for approximately 2000 NEOs and tens of thousands of Main Belt asteroids. The 32 months of hibernation have had no significant effect on the mission's performance. Image quality, sensitivity, photometric and astrometric accuracy, completeness, and the rate of minor planet detections are all essentially unchanged from the prime mission's post-cryogenic phase.

astro-ph.EP↗

arXiv2014

Neutron skin of $^{208}$Pb from Coherent Pion Photoproduction

Information on the size and shape of the neutron skin on $^{208}$Pb has been extracted from coherent pion photoproduction cross sections measured using the Crystal Ball together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections the half-height radius and diffuseness of the neutron distribution are found to be 6.70$\pm 0.03(stat)$ fm and 0.55$\pm 0.01(stat)$$^{+0.02}_{-0.03}(sys)$ fm respectively, corresponding to a neutron skin thickness $Δr_{np}$=0.15$\pm 0.03(stat)$$^{+0.01}_{-0.03}(sys)$ fm. The results give the first successful extraction of a neutron skin with an electromagnetic probe and indicate the skin of $^{208}$Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.

nucl-ex↗

arXiv2014

Spin alignment and violation of the OZI rule in exclusive $ω$ and $ϕ$ production in pp collisions

Exclusive production of the isoscalar vector mesons $ω$ and $ϕ$ is measured with a 190 GeV$/c$ proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable $x_{F}$ of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on $x_{F}$ and on the invariant mass $M_{p\mathrm{V}}$ of the system formed by fast proton $p_\mathrm{fast}$ and vector meson $V$ is discussed in terms of diffractive production of $p_\mathrm{fast}V$ resonances in competition with central production. The measurement of the spin density matrix element $ρ_{00}$ of the vector mesons in different selected reference frames provides another handle to distinguish the contributions of these two major reaction types. Again, dependences of the alignment on $x_{F}$ and on $M_{p\mathrm{V}}$ are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to $ω$ production which are absent in the case of the $ϕ$ meson. Removing the low-mass $M_{p\mathrm{V}}$ resonant region, the OZI rule is found to be violated by a factor of eight, independently of $x_\mathrm{F}$.

hep-ex↗

arXiv2014

Measurement of polarisation observables in $K^0_sΣ^+$ photoproduction off the proton

The reaction $γ\, p \rightarrow K^0_S\,Σ^+$ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to $E_γ= 2250$ MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay $Σ^+ \rightarrow p π^0$ up to $E_γ= 1650$ MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed cusp-like structure near the $K^*$ thresholds is discussed.

nucl-ex↗

arXiv2014

A new measurement of the rare decay eta -> pi^0 gamma gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron

A new measurement of the rare, doubly radiative decay eta->pi^0 gamma gamma was conducted with the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. New data on the dependence of the partial decay width, Gamma(eta->pi^0 gamma gamma), on the two-photon invariant mass squared, m^2(gamma gamma), as well as a new, more precise value for the decay width, Gamma(eta->pi^0 gamma gamma) = (0.33+/-0.03_tot) eV, are based on analysis of 1.2 x 10^3 eta->pi^0 gamma gamma decays from a total of 6 x 10^7 eta mesons produced in the gamma p -> eta p reaction. The present results for dGamma(eta->pi^0 gamma gamma)/dm^2(gamma gamma) are in good agreement with previous measurements and recent theoretical calculations for this dependence.

hep-ex↗

arXiv2014

Measurement of the g p -> K^0 Sigma^+ reaction with the Crystal Ball/TAPS detectors at the Mainz Microtron

The g p -> K^0 Sigma^+ reaction has been measured from threshold to Eg=1.45 GeV (W_cm=1.9 GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the 3pi^0 p final state, by assuming K^0_S -> pi^0 pi^0 and Sigma^+ -> pi^0 p. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve empirical knowledge about the g p -> K^0 Sigma^+ reaction in the measured energy range. The results are compared to previous measurements and model predictions. It is demonstrated that adding the present g p -> K^0 Sigma^+ results to existing data allowed a better description of this reaction with various models.

hep-ex↗

arXiv2014

A new determination of the eta transition form factor in the Dalitz decay eta -> e^+ e^- gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron

The Dalitz decay eta -> e^+ e^- gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon tagging facility at the Mainz Microtron MAMI. The experimental statistic used in this work is one order of magnitude greater than in any previous measurement of eta -> e^+ e^- gamma. The value obtained for the slope parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/- 0.15_stat +/- 0.10_syst) [1/GeV^2], is in good agreement with recent measurements conducted in eta -> e^+ e^- gamma and eta -> mu^+ mu^- gamma decays, as well as with recent form-factor calculations. The uncertainty obtained in the value of 1/Lambda^2 is lower compared to results from previous measurements of the eta -> e^+ e^- gamma decay.

hep-ex↗