Search Results (118 total)

We report on the acceleration of ion beams from ultrathin diamondlike carbon foils of thickness 50, 30, and 10 nm irradiated by ultrahigh contrast laser pulses at intensities of ∼7×10¹⁹  W/cm². An unprecedented maximum energy of 185 MeV (15  MeV/u) for fully ionized carbon atoms is observed at the optimum thickness of 30 nm. The enhanced acceleration is attributed to self-induced transparency, leading to strong volumetric heating of the classically overdense electron population in the bulk of the target. Our experimental results are supported by both particle-in-cell (PIC) simulations and an analytical model.

Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding 2  m² sr at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as E^-3.0 and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.

We derive expressions for shock formation based on the local curvature of the flow characteristics during dynamic compression. Given a specific ramp adiabat, calculated for instance from the equation of state for a substance, the ideal nonlinear shape for an applied ramp loading history can be determined. We discuss the region affected by lateral release, which can be presented in compact form for the ideal loading history. Example calculations are given for representative metals and plastic ablators. Continuum dynamics (hydrocode) simulations were in good agreement with the algebraic forms. Example applications are presented for several classes of laser-loading experiment, identifying conditions where shocks are desired but not formed, and where long-duration ramps are desired.

The TRIDENT laser was used to induce quasi-isentropic compression waves to ∼15  GPa in samples of Si, by ablative loading using a laser pulse whose intensity increased smoothly over 2.5 ns. The intensity history of the pulse and the velocity history at the opposite surface of the sample were recorded. Experiments were performed using samples of two different thicknesses simultaneously, in which the evolution of the compression wave was clearly visible. Isentropic stress states deduced were consistent with the previously investigated response of Si to uniaxial loading. The ablative loading was simulated using radiation hydrodynamics, with different equations of state in the plasma and condensed regions and including elasticity in the solid Si. These calculations reproduced the measured velocity histories quite well, demonstrating that quasi-isentropic compression was induced with no preheat from the laser drive. Normal continuum behavior was demonstrated to hold below nanosecond time scales for isentropic compression waves, with no evidence for nonequilibrium effects in the crystal lattice. Details of the velocity history over about 10 GPa were reproduced less well, suggesting a deficiency in the model used for compressed Si, which may be consistent with recent theoretical predictions of uniaxial compression at high strain rates.

Thomson scattering is used to measure Langmuir waves (LW) driven by stimulated Raman scattering (SRS) in a diffraction limited laser focal spot. For SRS at wave numbers kλ_D≲0.29, where k is the LW number and λ_D is the Debye length, multiple waves are detected and are attributed to the Langmuir decay instability (LDI) driven by the primary LW. At kλ_D≳0.29, a single wave, frequency-broadened spectrum is observed. The transition from the fluid to the kinetic regime is qualitatively consistent with particle-in-cell simulations and crossing of the LDI amplitude threshold above that for LW self-focusing.

The fast reduction of the six-dimensional phase space of muon beams is an essential requirement for muon colliders and also of great importance for neutrino factories based on accelerated muon beams. Ionization cooling, where all momentum components are degraded by an energy absorbing material and only the longitudinal momentum is restored by rf cavities, provides a means to quickly reduce transverse beam sizes. However, the beam energy spread cannot be reduced by this method unless the longitudinal emittance can be transformed or exchanged into the transverse emittance. Emittance exchange plans until now have been accomplished by using magnets to disperse the beam along the face of a wedge-shaped absorber such that higher momentum particles pass through thicker parts of the absorber and thus suffer larger ionization energy loss. In the scheme advocated in this paper, a special magnetic channel designed such that higher momentum corresponds to a longer path length, and therefore larger ionization energy loss, provides the desired emittance exchange in a homogeneous absorber without special edge shaping. Normal-conducting rf cavities imbedded in the magnetic field regenerate the energy lost in the absorber. One very attractive example of a cooling channel based on this principle uses a series of high-gradient rf cavities filled with dense hydrogen gas, where the cavities are in a magnetic channel composed of a solenoidal field with superimposed helical transverse dipole and quadrupole fields. In this scheme, the energy loss, the rf energy regeneration, the emittance exchange, and the transverse cooling happen simultaneously. The theory of this helical channel is described in some detail to support the analytical prediction of almost a factor of 10⁶ reduction in six-dimensional phase space volume in a channel about 56 m long. Equations describing the particle beam dynamics are derived and beam stability conditions are explored. Equations describing six-dimensional cooling in this channel are also derived, including explicit expressions for cooling decrements and equilibrium emittances.

We present the first direct experimental observation of the parametric two-ion decay instability of ion-acoustic waves driven by a high intensity (5×10¹⁵   W cm^-2) laser beam in a laser produced high-Z plasma. Using two separate Thomson scattering diagnostics simultaneously, we directly measure the scattering from thermal ion-acoustic fluctuations, the primary ion waves that are driven to large amplitudes by the high intensity beam, and the two-ion decay products. The decay products are shown to be present only where the interaction takes place and their k spectrum is broad.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

New measurements of D_s⁺ and D_s^*+ meson production rates from B decays and from qq̅ continuum events near the Υ(4S) resonance are presented. Using 20.8 fb^-1 of data on the Υ(4S) resonance and 2.6 fb^-1 off-resonance, we find the inclusive branching fractions B(B→D_s⁺X)=(10.93±0.19±0.58±2.73)% and B(B→D_s^*+X)=(7.9±0.8±0.7±2.0)%, where the first error is statistical, the second is systematic, and the third is due to the D_s⁺→φπ⁺ branching fraction uncertainty. The production cross sections σ(e⁺e^-→D_s⁺X)×B(D_s⁺→φπ⁺)=7.55±0.20±0.34 pb and σ(e⁺e^-→D_s^*±X)×B(D_s⁺→φπ⁺)=5.8±0.7±0.5 pb are measured at center-of-mass energies about 40 MeV below the Υ(4S) mass. The branching fractions ΣB(B→D_s^(*)+D^(*))=(5.07±0.14±0.30±1.27)% and ΣB(B→D_s^*+D^(*))=(4.1±0.2±0.4±1.0)% are determined from the D_s^(*)+ momentum spectra. The mass difference m(D_s⁺)-m(D⁺)=98.4±0.1±0.3 MeV/c² is also measured.

The branching fractions of the exclusive decays B⁰→K^*0γ and B⁺→K^*+γ are measured from a sample of (22.74±0.36)×10⁶ BB̅ decays collected with the BABAR detector at the PEP-II asymmetric e⁺e^- collider. We find B(B⁰→K^*0γ)  =  [4.23±0.40(stat)±0.22(syst)]×10^-5, B(B⁺→K^*+γ)  =  [3.83±0.62(stat)±0.22(syst)]×10^-5 and constrain the CP-violating charge asymmetry to be -0.170<A_CP(B→K^*γ)<0.082 at 90% C.L.

We search for direct CP violation in charmless hadronic B decays observed in a sample of about 22.7 million BB̅ pairs collected with the BABAR detector at the SLAC PEP-II asymmetric-energy e⁺e^- collider. We measure the following charge asymmetries: A_CP(B^±→η^′K^±)=-0.11±0.11±0.02, A_CP(B^±→ωπ^±)=-0.01_- 0.31^+ 0.29±0.03, A_CP(B^±→φK^±)=-0.05±0.20±0.03, A_CP(B^±→φK^*±)=-0.43_- 0.30^+ 0.36±0.06, and A_CP(B⁰→φK^*0)=0.00±0.27±0.03.

We present a measurement of the time-dependent CP-violating asymmetries in neutral B decays to the π⁺π^-CP eigenstate, and an updated measurement of the charge asymmetry in B⁰→K⁺π^- decays. In a sample of 33 million Υ(4S)→BB̅ decays collected with the BABAR detector at the SLAC PEP-II asymmetric B factory, we find 65_-11⁺¹² π⁺π^- and 217±18 K⁺π^- candidates and measure the asymmetry parameters S_ππ=0.03_-0.56^+0.53±0.11, C_ππ=-0.25_-0.47^+0.45±0.14, and A_Kπ=-0.07±0.08±0.02, where the first error is statistical and the second is systematic.

We report branching fraction measurements for exclusive decays of charged and neutral B mesons into two-body final states containing a charmonium meson. We use a sample of 22.72±0.36 million BB̅ events collected between October 1999 and October 2000 with the BABAR detector at the PEP-II storage rings at the Stanford Linear Accelerator Center. The charmonium mesons considered here are J/ψ, ψ(2S), and χ_c1, and the light meson in the decay is either a K, K^*, or π⁰.

We measure the branching fractions of the ψ(2S) meson to the leptonic final states e⁺e^- and μ⁺μ^- relative to that for ψ(2S)→J/ψπ⁺π^-. The method uses ψ(2S) mesons produced in the decay of B mesons at the Υ(4S) resonance in a data sample collected with the BABAR detector at the Stanford Linear Accelerator Center. Using previous measurements for the ψ(2S)→J/ψπ⁺π^- branching fraction, we determine the e⁺e^- and μ⁺μ^- branching fractions to be 0.0078±0.0009±0.0008 and 0.0067±0.0008±0.0007, respectively.

We present a limit on the branching fraction for the decay B⁰→γγ using data collected at the ϒ(4S) resonance with the BABAR detector at the PEP-II asymmetric energy e⁺e^- collider. Based on the observation of one event in the signal region, out of a sample of 21.3×10⁶ e⁺e^-→ϒ(4S)→BB̅ decays, we establish an upper limit on the branching fraction of B(B⁰→γγ)<1.7×10^-6 at the 90% confidence level. This result substantially improves upon existing limits.

We present a measurement of the decay amplitudes in B→J/ψK^*(892) channels using 20.7 fb ^-1 of data collected at the ϒ(4S) resonance with the BABAR detector at PEP-II. We measure a P-wave fraction R_⊥  =  (16.0±3.2±1.4)% and a longitudinal polarization fraction (59.7±2.8±2.4)%. The measurement of a relative phase that is neither 0 nor π, φ_∥  =  2.50±0.20±0.08 radians, favors a departure from the factorization hypothesis. Although the decay B→J/ψKπ proceeds mainly via K^*(892), there is also evidence for K₂^*(1430) and Kπ S-wave contributions.

We present the results of searches for B decays to charmless two-body final states containing η^′ or ω mesons, based on 20.7 fb^-1 of data collected with the BABAR detector. We find the branching fractions B(B⁺→η^′K⁺)  =  (70±8±5)×10^-6, B(B⁰→η^′K⁰)  =  (42_-11⁺¹³±4)×10^-6, and B(B⁺→ωπ⁺)  =  (6.6_-1.8^+2.1±0.7)×10^-6, where the first error quoted is statistical and the second is systematic. We give measurements of four additional modes for which the 90% confidence level upper limits are B(B⁺→η^′π⁺)<12×10^-6, B(B⁺→ωK⁺)<4×10^-6, B(B⁰→ωK⁰)<13×10^-6, and B(B⁰→ωπ⁰)<3×10^-6.

The B⁰ and B⁺ meson lifetimes have been measured in e⁺e^- annihilation data collected in 1999 and 2000 with the BABAR detector at center-of-mass energies near the ϒ(4S) resonance. Events are selected in which one B meson is fully reconstructed in a hadronic final state while the second B meson is reconstructed inclusively. A combined fit to the B⁰ and the B⁺ decay time difference distributions yields τ_B⁰  =  1.546±0.032(stat)±0.022(syst)  ps, τ_B⁺  =  1.673±0.032(stat)±0.023(syst)  ps, and τ_B⁺/τ_B⁰  =  1.082±0.026(stat)±0.012(syst).

The production of J/ψ mesons in continuum e⁺e^- annihilations has been studied with the BABAR detector at energies near the ϒ(4S) resonance. The mesons are distinguished from J/ψ production in B decays through their center-of-mass momentum and energy. We measure the cross section e⁺e^-→J/ψX to be 2.52±0.21±0.21 pb. We set a 90% C.L. upper limit on the branching fraction for direct ϒ(4S)→J/ψX decays at 4.7×10^-4.

A diffraction-limited laser interacts with a plasma whose conditions are uniform on the scale of the focused laser spot. Two distinct, narrow waves are observed in the backscattered spectrum with phase velocities of v_φ/v_e  =  1.4±0.08 and 4.2±0.1, where v_e is the electron thermal speed. The high-velocity wave is ordinary stimulated Raman scattering (SRS) from a Langmuir wave. The low-velocity wave corresponds to stimulated scattering from an electron-acoustic wave (SEAS), and implies strong electron trapping. Previous SRS data from low-density plasmas are reinterpreted in terms of SEAS.

We present measurements, based on a sample of approximately 23×10⁶ BB̅ pairs, of the branching fractions and a search for CP-violating charge asymmetries in charmless hadronic decays of B mesons into two-body final states of kaons and pions. We find the branching fractions B(B⁰→π⁺π^-)  =  (4.1±1.0±0.7)×10^-6, B(B⁰→K⁺π^-)  =  (16.7±1.6±1.3)×10^-6, B(B⁺→K⁺π⁰)  =  (10.8_-1.9^+2.1±1.0)×10^-6, B(B⁺→K⁰π⁺)  =  (18.2_-3.0^+3.3±2.0)×10^-6, B(B⁰→K⁰π⁰)  =  (8.2_-2.7^+3.1±1.2)×10^-6. We also report 90% confidence level upper limits for B meson decays to the π⁺π⁰, K⁺K^-, and K̅ ⁰K⁺ final states. In addition, charge asymmetries have been found to be consistent with zero, where the statistical precision is in the range of ±0.10 to ±0.18, depending on the decay mode.

We have observed the decays B→φK and φK^* in a sample of over 45 million B mesons collected with the BABAR detector at the PEP-II collider. The measured branching fractions are B(B⁺→φK⁺)  =  (7.7_-1.4^+1.6±0.8)×10^-6, B(B⁰→φK⁰)  =  (8.1_-2.5^+3.1±0.8)×10^-6, B(B⁺→φK^*+)  =  (9.7_-3.4^+4.2±1.7)×10^-6, and B(B⁰→φK^*0)  =  (8.7_-2.1^+2.5±1.1)×10^-6. We also report the upper limit B(B⁺→φπ⁺)<1.4×10^-6 ( 90% C.L.).

We present an updated measurement of time-dependent CP-violating asymmetries in neutral B decays with the BABAR detector at the PEP-II asymmetric B Factory at SLAC. This result uses an additional sample of ϒ(4S) decays collected in 2001, bringing the data available to 32×10⁶ BB̅ pairs. We select events in which one neutral B meson is fully reconstructed in a final state containing charmonium and the flavor of the other neutral B meson is determined from its decay products. The amplitude of the CP-violating asymmetry, which in the standard model is proportional to sin2β, is derived from the decay time distributions in such events. The result sin2β  =  0.59±0.14(stat)±0.05(syst) establishes CP violation in the B⁰ meson system. We also determine |λ|  =  0.93±0.09(stat)±0.03(syst), consistent with no direct CP violation.

We present measurements of time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurement uses a data sample of 23×10⁶ ϒ(4S)→BB̅ decays collected by the BABAR detector at the PEP-II asymmetric B Factory at SLAC. In this sample, we find events in which one neutral B meson is fully reconstructed in a CP eigenstate containing charmonium and the flavor of the other neutral B meson is determined from its decay products. The amplitude of the CP-violating asymmetry, which in the standard model is proportional to sin2β, is derived from the decay time distributions in such events. The result is sin2β  =  0.34±0.20 (stat)±0.05 (syst).

The transmitted angular distribution of a 527 nm nearly diffraction-limited laser is measured after it propagates through a plasma with supersonic transverse flow. The laser beam is deflected by as much as 10° and exhibits bowlike features in the flow direction, which is attributed to flow-induced beam steering. The finite interaction volume allows for direct comparison with a 3D hydrodynamic simulation, which is in good agreement with details of the experiment.