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Single-spin asymmetries for pions and charged kaons are measured in semi-inclusive deep-inelastic scattering of positrons and electrons off a transversely nuclear-polarized hydrogen target. The dependence of the cross section on the azimuthal angles of the target polarization ([phi]S) and the produced hadron ([phi]) is found to have a substantial sin([phi]+[phi]S) modulation for the production of [pi]+, [pi]- and K+. This Fourier component can be interpreted in terms of non-zero transversity distribution functions and non-zero favored and disfavored Collins fragmentation functions with opposite sign. For [pi]0 and K- production the amplitude of this Fourier component is consistent with zero

Multiplicities in semi-inclusive deep-inelastic scattering are presented for each charge state of pi(+/-) and K-+/- mesons. The data were collected by the HERMES experiment at the HERA storage ring using 27.6 GeV electron and positron beams incident on a hydrogen or deuterium gas target. The results are presented as a function of the kinematic quantities x(B), Q(2), z, and P-h perpendicular to. They represent a unique data set for identified hadrons that will significantly enhance our understanding of the fragmentation of quarks into final-state hadrons in deep-inelastic scattering. DOI: 10.1103/PhysRevD.87.07402

Azimuthal single-spin asymmetries of leptoproduced pions and charged kaons were measured on a transversely polarized hydrogen target. Evidence for a naive-T-odd, transverse-momentum-dependent parton distribution function is deduced from nonvanishing Sivers effects for pi(+), pi(0), and K(+/-), as well as in the difference of the pi(+) and pi(-) cross sections.

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Current models predict the hose instability to crucially limit the applicability of plasma-wakefield accelerators. By developing an analytical model which incorporates the evolution of the hose instability over long propagation distances, this work demonstrates that the inherent drive-beam energy loss, along with an initial beam-energy spread, detunes the betatron oscillations of beam electrons and thereby mitigates the instability. It is also shown that tapered plasma profiles can strongly reduce initial hosing seeds. Hence, we demonstrate that the propagation of a drive beam can be stabilized over long propagation distances, paving the way for the acceleration of high-quality electron beams in plasma-wakefield accelerators. We find excellent agreement between our models and particle-in-cell simulations. DOI: 10.1103/PhysRevLett.118.174801 Plasma-based accelerators can provide accelerating fields in excess of 10 GV=m [1,2]. As a result, these devices can potentially contribute to a future generation of more compact particle accelerators and radiation sources. Plasma-wakefield accelerators (PWFAs) [3,4] employ charged particle beams as drivers of large-amplitude plasma waves. Significant experimental results [2,5] were obtained in the blowout regime, in which a particle beam with a charge density greater than the ambient plasma density expels all plasma electrons within its vicinity, thereby generating a copropagating ion channel with linear electron focusing and extreme accelerating fields [6].Identified by Whittum et al. in the early 1990s [7], the hose instability (HI) remains a long-standing challenge for PWFAs. Hosing is seeded by initial transverse asymmetries of the beam or plasma phase-space distributions. According to current models, the beam-centroid displacement is amplified exponentially during propagation in the plasma [7][8][9][10][11], ultimately leading to a beam breakup. The most recent description for the coupled evolution of the ionchannel centroid X c ðξ; tÞ and the beam centroid X b ðξ; tÞ in the blowout regime is given by [11]with the time t and the comoving coordinate ξ ¼ ct − z, where z is the longitudinal coordinate and c is the speed of light. The plasma wave number is denoted by k p ¼ ω p =c, and the betatron frequency by ω β ¼ ω p = ffiffiffiffiffi 2γ p , with the Lorentz factor γ, where ω p ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4πn 0 e 2 =m p is the plasma frequency with the ambient plasma density n 0 , the elementary charge e, and the electron rest mass m. The coefficients c ψ ðξÞ and c r ðξÞ account for the relativistic motion of electrons in the blowout sheath and for a ξ dependence of the blowout radius and the beam current [11]. According to Eq. (1), a beam-centroid displacement X b leads to a displacement of the ion-channel centroid X c along the beam. The displacement X c couples back to the temporal evolution of X b according to Eq. (2). The case where c ψ ¼ c r ¼ 1 recovers the seminal hosing model [7]. This limit, which accounts for a linear resp...

We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness bunches. This injection principle is explained by example of three-dimensional particle-in-cell calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blowout regime inside a fully ionized hydrogen plasma of density 5×10(17)cm-3. Within an embedded 100  μm long plasma column contaminated with neutral helium gas, the wakefields trigger ionization, trapping of a defined fraction of the released electrons, and subsequent acceleration. The hereby generated electron beam features a 1.5 kA peak current, 1.5  μm transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.