Search Results (11 total)

A measurement of the ratio R_χc=(χ_c→J/ψ+γ)/J/ψ in pC, pTi, and pW interactions at 920  GeV/c (sqrt[s]=41.6  GeV) in the Feynman-x range -0.35<x_F^J/ψ<0.15 is presented. Both μ⁺μ^- and e⁺e^- J/ψ decay channels are observed with an overall statistics of about 15 000 χ_c events, which is by far the largest available sample in pA collisions. The result is R_χc=0.188±0.013_st+0.024 / -0.022_sys averaged over the different materials, when no J/ψ and χ_c polarizations are considered. The χ_c1 to χ_c2 production ratio R₁₂=R_χc1/R_χc2 is measured to be 1.02±0.40, leading to a cross section ratio σ(χ_c1) / σ(χ_c2)=0.57±0.23. The dependence of R_χc on the Feynman-x of the J/ψ, x_F^J/ψ, and its transverse momentum, p_T^J/ψ, is studied, as well as its dependence on the atomic number, A, of the target. For the first time, an extensive study of possible biases on R_χc and R₁₂ due to the dependence of acceptance on the polarization states of J/ψ and χ_c is performed. By varying the polarization parameter, λ_obs, of all produced J/ψ’s by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible χ_c1 and χ_c2 polarizations, it is shown that R_χc could change by a factor between 1.02 and 1.21 and R₁₂ by a factor between 0.89 and 1.16.

Detection of the field induced by a beam outside of the beam pipe can be used as a beam diagnostic. Wires placed in longitudinal slots in the outside wall of the beam pipe can be used as a beam pickup. This has a very small beam-coupling impedance and avoids complications of having a feedthrough. The signal can be reasonably high at low frequencies. We present a field waveform at the outer side of a beam pipe, obtained as a result of calculations and measurements. We calculate the beam-coupling impedance due to a long longitudinal slot in the resistive wall and the signal induced in a wire placed in such a slot and shielded by a thin screen from the beam. These results should be relevant for impedance calculations of the slot in an antechamber and for slots in the PEP-II distributed ion pump screens. The design of the low-frequency beam position monitor is very simple. It can be used in storage rings, synchrotron light sources, and free electron lasers, like LINAC coherent light source.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H^- beam for the Spallation Neutron Source (SNS) ring. First, H^- atoms are converted to H⁰ by a magnetic field, then H⁰ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H^- beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

A new measurement of the bb̅ production cross section in 920 GeV proton-nucleus collisions is presented by the HERA-B Collaboration. The bb̅ production is tagged via inclusive bottom quark decays into J/ψ mesons by exploiting the longitudinal separation of J/ψ→l⁺l^- decay vertices from the primary proton-nucleus interaction point. Both e⁺e^- and μ⁺μ^- channels are reconstructed for a total of 83±12 inclusive b→J/ψX events found. The combined analysis yields a bb̅ to prompt J/ψ cross section ratio of Δσ(bb̅ ) / Δσ_J/ψ=0.032±0.005_stat±0.004_sys measured in the x_F acceptance (-0.35<x_F<0.15), extrapolated to σ(bb̅ )=14.9±2.2_stat±2.4_sys  nb/nucleon in the total phase space.

We have searched for Θ⁺(1540) and Ξ^--(1862) pentaquark candidates in proton-inducedreactions on C, Ti, and W targets at midrapidity and sqrt[s]=41.6  GeV. In 2×10⁸ inelastic eventswe find no evidence for narrow (σ≈5  MeV) signals in the Θ⁺→pK_S⁰ and Ξ^--→Ξ^-π^- channels; our 95% C.L. upper limits (UL) forthe inclusive production cross section times branching fraction B dσ/dy|_y≈0 are (4–16)μb/N for a Θ⁺ mass between 1521 and 1555 MeV,and 2.5μb/N for the Ξ^--. The UL of the yield ratio of Θ⁺/Λ(1520)<(3–12)% is significantly lower than model predictions.Our UL of B Ξ^--/Ξ(1530)⁰<4% is at variance with the results that have provided the first evidencefor the Ξ^--.

Electron-impact ionization of low-temperature water ice leads to   H⁺, H₂⁺, and H⁺(H₂O)_n=1–8 desorption. The 22 eV H⁺ desorption threshold is correlated with localized 2-hole 1-electron and 2-hole final states which Coulomb explode, while the 22 eV H₂⁺ threshold is due to H₂O⁺ unimolecular dissociation. The 70 eV primary cluster ion threshold is consistent with holes in the 2a₁ level and secondary ionization channels. All cation yields are sensitive to local structural changes and probe surface acidity. The cluster size distribution indicates hole-hole screening distances of 1–2 nm.

This paper presents a scheme for three-step laser-based stripping of an H^- beam for charge exchange injection into a high-intensity proton ring. First, H^- atoms are converted to H⁰ by Lorentz stripping in a strong magnetic field, then neutral hydrogen atoms are excited from the ground state to upper levels by a laser, and the remaining electron, now more weakly bound, is stripped in a strong magnetic field. The energy spread of the beam particles gives rise to a Doppler broadened absorption linewidth, which makes for an inefficient population of the upper state by a narrow-band laser. We propose to overcome this limitation with a “frequency sweeping” arrangement, which populates the upper state with almost 100% efficiency. We present estimates of peak laser power and describe a method to reduce the power by tailoring the dispersion function at the laser-particle beam interaction point. We present a scheme for reducing the average power requirements by using an optical ring resonator. Finally, we discuss an experimental setup to demonstrate this approach in a proof-of-principle experiment.

A halo generation mechanism in the nonperiodic lattices such as the SNS (Spallation Neutron Source) linac MEBT [medium-energy beam transport between radio-frequency quadrupole and DTL (drift tube linac)] is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity ∼2:1 in the ∼1.6 m-long MEBT chopper section is responsible for halo formation. As a result, the beam distribution, based on the front end emittance measurements and multiparticle simulation studies, develops halo that leads to beam loss and radioactivation of the SNS linac. Designing lattices with transverse beam eccentricity close to 1:1 suppresses this kind of halo generation. Modifying the MEBT optics and introducing adjustable collimators in the MEBT significantly reduced beam losses in the coupled cavity linac, which is a preferred scheme for mitigating halo. It turns out that the DTL collimation does not effectively remove halo and presents a risk of overheating drift tubes.

A new method for the measurement of loss factor for an rf cavity is presented. The method consists of measuring the above quantity by means of the detection of both the rf voltage induced by an electron bunch in the device under test and the bunch charge. The device to be investigated is a copper reentrant T-shaped cavity. The experimental results and their comparison with analytical and numerical results are presented.

An experimental investigation was carried out to measure the response time of GaAs in negative electron affinity conditions as a photoemitter. During the experiment, the photocathode was excited by a short-pulse (38 ps rms) frequency-doubled Nd:YLF laser. Results show that the rms response time of GaAs is shorter than 40 ps.

In order to produce electron beams with the minimum possible energy spread we have investigated the possibility of replacing the usual thermocathodes with a photoemissive source such as GaAs. A comparison carried out between these two sources indicated the latter as the best device to obtain a very-low-energy spread. Beam relaxation after emission also leads to an increase in the energy spread. Therefore an experimental study on the nature of relaxations occurring in electron beams, yielded both by a thermocathode and by a photocathode, has been performed. More specifically, we investigated the possibility of reducing the transverse-longitudinal and the longitudinal-longitudinal relaxations. With this aim, the features of adiabatic acceleration, which damp the pure longitudinal relaxation, have been examined. The experience gained during the measurement cycle demonstrated that an adiabatic structure, accelerating electrons emitted by a GaAs photocathode, leads to the best performances.