Search Results (53 total)

An experiment (E166) at the Stanford Linear Accelerator Center has demonstrated a scheme in which a multi-GeV electron beam passed through a helical undulator to generate multi-MeV, circularly polarized photons which were then converted in a thin target to produce positrons (and electrons) with longitudinal polarization above 80% at 6 MeV. The results are in agreement with Geant4 simulations that include the dominant polarization-dependent interactions of electrons, positrons, and photons in matter.

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.

We analyze the prospects of a feasible, Brookhaven National Laboratory based, very long baseline neutrino oscillation experiment consisting of a conventional horn produced low energy wideband beam and a detector of 500 kton fiducial mass with modest requirements on event recognition and resolution. Such an experiment is intended primarily to determine CP violating effects in the neutrino sector for 3-generation mixing. We analyze the sensitivity of such an experiment. We conclude that this experiment will allow determination of the CP phase δ_CP and the currently unknown mixing parameter θ₁₃, if sin²2θ₁₃>~0.01, a value ∼15 times lower than the present experimental upper limit. In addition to θ₁₃ and δ_CP, the experiment has great potential for precise measurements of most other parameters in the neutrino mixing matrix including Δm₃₂², sin²2θ₂₃, Δm₂₁²×sin 2θ₁₂, and the mass ordering of neutrinos through the observation of the matter effect in the ν_μ→ν_e appearance channel.

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.

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).

A Comment on the Letter by Bernhard Rau, T. Tajima, and H. Hojo, Phys. Rev. Lett. 78, 3310 (1997). The authors of the Letter offer a Reply.

A free electron can temporarily gain a very significant amount of energy if it is overrun by an intense electromagnetic wave. In principle, this process would permit large enhancements in the center-of-mass energy of electron-electron, electron-positron, and electron-photon interactions if these take place in the presence of an intense laser beam. Practical considerations severely limit the utility of this concept for contemporary lasers incident on relativistic electrons. A more accessible laboratory phenomenon is electron-positron production via an intense laser beam incident on a gas. Intense electromagnetic pulses of astrophysical origin can lead to very energetic photons via bremsstrahlung of temporarily accelerated electrons.

We report on measurements of quantum electrodynamic processes in an intense electromagnetic wave, where nonlinear effects (both multiphoton and vacuum polarization) are prominent. Nonlinear Compton scattering and electron-positron pair production have been observed in collisions of 46.6 GeV and 49.1 GeV electrons of the Final Focus Test Beam at SLAC with terawatt pulses of 1053 nm and 527 nm wavelengths from a Nd:glass laser. Peak laser intensities of ≈0.5×10¹⁸ W/cm² have been achieved, corresponding to a value of ≈0.4 for the parameter η=eE_rms/mω₀c and to a value of ≈0.25 for the parameter Υ_e=E_rms^⋆/E_crit=eE_rms^⋆ħ/m²c³, where E_rms^⋆ is the rms electric field strength of the laser in the electron rest frame. We present data on the scattered electron spectra arising from nonlinear Compton scattering with up to four photons absorbed from the field. A convolved spectrum of the forward high energy photons is also given. The observed positron production rate depends on the fifth power of the laser intensity, as expected for a process where five photons are absorbed from the field. The positrons are interpreted as arising from the collision of a high-energy Compton scattered photon with the laser beam. The results are found to be in agreement with theoretical predictions.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

A Comment on the Letter by G. Malka, E. Lefebvre, and J. L. Miquel, Phys. Rev. Lett. 78, 3314 (1997). The authors of the Letter offer a Reply.

A signal of 106±14 positrons above background has been observed in collisions of a low-emittance 46.6 GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the first laboratory evidence for inelastic light-by-light scattering involving only real photons.

Nonlinear Compton scattering has been observed in the collision of a low-emittance 46.6-GeV electron beam with terawatt pulses from a Nd:glass laser at 1054 and 527 nm wavelengths in an experiment at the Final Focus Test Beam at SLAC. Peak laser intensities of 10¹⁸  W/cm² have been achieved, corresponding to a value of 0.6 for the parameter η  =  eE_rms/mω₀c. Results are presented for multiphoton Compton scattering in which up to four laser photons interact with an electron, in agreement with theoretical calculations.