Search Results (18 total)

We present experimental measurements of a granular slope under horizontal vibration. We use optical particle tracking to measure the motion of surface beads as the slope fails. We find that for all but the largest inclination angles, initial bead motion leads to strengthening rather than an avalanche. The initial motion of the beads is usually intermittent and evolves differently for different preparations, slope angles, and rates of increase in the vibration amplitude. When a specific criterion is chosen to define failure, the Coulomb friction model adequately describes the average acceleration required to produce failure, as long as slope preparation and experimental protocol are constant. However, the observed intermittent motion and rate dependence indicate that strengthening microrearrangements are important features that affect failure of slopes under external perturbations.

A detailed model of the Cornell Electron Storage Ring (CESR) guide field, including beam-beam interaction computed in the weak-strong regime, is the basis for a multiturn simulation of luminosity. The simulation reproduces the dependence of luminosity on bunch current that is measured in the storage ring, at both high-energy (5.3   GeV/beam) and in the wiggler-dominated low energy (CESR-c) configuration (1.9   GeV/beam). Dynamics are determined entirely by the physics of propagation through the individual guide field elements with no free parameters. Energy dependence of the compensation of the transverse coupling introduced by the experimental solenoid is found to significantly degrade specific luminosity. The simulation also indicates a strong dependence of limiting beam-beam tune shift parameter on the geometric mean of synchrotron tune and bunch length.

We report the first evidence for a nonzero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic kinematic region. Data for the reaction ep→e^′π⁺X have been obtained using a polarized electron beam of 4.3 GeV with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The amplitude of the sin φ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range z=0.5–0.8 the average amplitude is 0.038±0.005±0.003 for a missing mass M_X>1.1 GeV and 0.037±0.007±0.004 for M_X>1.4 GeV.

Measurement of the betatron phase in the vertical and horizontal planes as well as the transverse horizontal-vertical coupling is a standard procedure now used at the Cornell Electron/Positron Storage Ring. The measurement is made by shaking the beam and observing the phase of oscillation at detectors located around the ring. The measurements allow quadrupolar errors to be corrected.

A formalism for describing the coupled two-dimensional motion of high energy particle beams in a storage ring is developed and extended to circumstances where the coupling is very strong, such as for the Möbius twist accelerator.

The Cornell electron storage ring (CESR) phase-III upgrade plan includes very strong permanent magnet quadrupoles in front of the cryostat for the superconducting quadrupoles and physically as close as possible to the interaction point. Together with the superconducting quadrupoles, they provide tighter vertical focusing at the interaction point. The quadrupoles are built with neodymium iron boron (NdFeB) material and operate inside the 15 kG solenoid field. Requirements on the field quality and stability of these quadrupoles are discussed and test results are presented.

Using the silicon strip detector of the CLEO experiment operating at the Cornell Electron-Positron Storage Ring (CESR), we have observed that the horizontal size of the luminous region decreases in the presence of the beam-beam interaction from what is expected without the beam-beam interaction. The dependence on the bunch current agrees with the prediction of the dynamic beta effect. This is the first direct observation of the effect.

We report measurements of the temperature dependence of the surface resistivity of highquality YBa₂Cu₃O₇ crystals at 5.95 GHz. A narrow transition with 89-K onset is observed. Resistivity in the normal state is found to be R_norm=126×10^-3 Ω. We establish an upper limit for the resistivity at 77 K of 1.0×10^-3 Ω and at 2 K of 1.5×10^-5 Ω. The dissipation in four crystals with a total area of about 20 mm² is measured to be nearly independent of peak microwave magnetic field over the range of 0.5 to 93 G in sharp contrast to the strong field dependence observed in bulk sintered samples.

Data from the High Resolution Spectrometer at the SLAC storage ring PEP have been used to study the inclusive production of baryons and mesons. Time-of-flight measurements are used to identify the charged hadrons. Neutral hadrons are identified from effective-mass peaks associated with their decay into two charged particles. Cross sections and other inclusive production characteristics are presented for π^±, K^±, and K⁰ (K¯⁰) mesons, and for the baryons (antibaryons) p (p¯) and Λ (Λ¯). The ratio of the inclusive cross section to the point cross section for the K⁰ and K¯⁰ mesons is R(K⁰,K¯⁰)=6.15±0.13±0.25, and for Λ and Λ¯, R(Λ,Λ¯)=0.846±0.036±0.085. The neutral-hadron differential cross sections are compared with the predictions of the Lund string model.

We report new and precise measurements of the decay branching fractions of the τ lepton to one and three charged particles. The data, corresponding to an integrated luminosity of 176 pb^-1, were collected by the high resolution spectrometer HRS at the e⁺e^- storage ring PEP at SLAC operated at sqrt[s]=29 GeV. The fractions of τ decays into one and three charged particles are 0.869±0.002±0.003 and 0.130±0.002±0.003, respectively.

Data from the high-resolution spectrometer at PEP have been used to study the inclusive production of φ mesons and F^± mesons decaying into φπ^±. Fragmentation functions and cross sections are presented and compared to existing data. The total φ cross section at 29 GeV is 40±6 pb. The observed F signal in the region z>0.4, given the assumption that R(F) / [R(F)+R(D)]=0.15, corresponds to an F→φπ branching ratio of (3.3±1.1)%. The measured F mass is 1963±3±3 MeV/c².

We have measured the process e⁺e^-→μ⁺μ^- at √s =29 GeV using the High Resolution Spectrometer at SLAC PEP. The forward-backward charge asymmetry is A_μμ=-(4.9±1.5±0.5)% based on 5057 events. A subsample of 3488 μ⁺μ^- events in the angular range ‖cosθ‖<0.55 gives a cross-section ratio of R_μμ=0.990±0.017±0.030. The resulting couplings of the weak neutral current are g_a^eg_a^μ=0.208±0.064± 0.021 and g_v^eg_v^μ=0.027 ±0.051±0.089. The QED cutoff parameters are Λ₊>170 GeV and Λ_->146 GeV at 95% C.L.

The production of τ-lepton pairs in e⁺e^- collisions at 29 GeV has been used to study the inclusive τ decay into five charged particles. Ten events are observed in the high-resolution spectrometer giving a branching ratio of B(τ→5π^±+nγ+ν_τ)=(1.3±0.4)×10^-3. Five of the events contain photons which are consistent with coming from a single π⁰. With this interpretation we measure B(τ→5π^±+ν_τ), B(τ→5π^±+π⁰+ν_τ)=(6.7±3.0)×10^-4. The branching-ratio upper limit for the decay mode τ→K⁰+3π^±+nγ+ν_τ is found, at 90% confidence level, to be 2.7×10^-3.

In this paper, results are presented from a study of the hadronic final states in e⁺e^- annihilation at 29 GeV. The data were obtained with the High Resolution Spectrometer (HRS) at the SLAC PEP e⁺e^- colliding-beam facility. The results are based on 6342 selected events corresponding to an integrated luminosity of 19.6 pb^-1. The distributions of the events in sphericity (S), thrust (T), and aplanarity (A) are given and compared to other e⁺e^- data in the same energy range. We measure 〈S〉=0.130±0.003±0.010 and 〈1-T〉=0.100±0.002. The sphericity distribution is compared to sphericity measurements made for beam jets in hadronic collisions as well as jets studied in neutrino scattering. The data sample is further reduced to 4371 events with the two-jet selections, S≤0.25 and A≤0.1. The single-particle distributions in the longitudinal and transverse directions are given. For low values of the momentum fraction (z=2p/W), the invariant distribution shows a maximum at z∼0.06, consistent with a QCD expectation. The data at high Feynman x (x_F) show distribution consistent with being dominated by a (1-x_f)² variation for the leading quark-meson transition. The rapidity distribution shows a shallow central minimum with a height (1/N_evdN^h/dY‖_Y=0=2.3±0.02±0.07. The mean charged multiplicity is measured to be 〈n_ch〉=13.1±0.05±0.6. The mean transverse momentum relative to the thrust axis 〈p_T〉 rises as a function of z to a value of 0.70±0.02 GeV/c for z≳0.3. The distributions are compared to those measured in other reactions.

We have observed inclusive production of D⁰ and D⁺ mesons, and their charge conjugates, in e⁺e^- annihilation at 29 GeV on the basis of a data sample of 106 pb^-1. These signals correspond to R values of R (D⁰+D―⁰)=1.8±0.5 and R (D⁺+D^-)=1.2±0.4. Taking the D⁺ and D⁰ data together, we measure a charge asymmetry of A=-0.08±0.12 for charmed quarks. A comparison of R (D+D―) with R (D^*+D―^*) obtained via the process D^*+→D⁰π⁺ gives a D / D^* ratio of 1.0_-0.2^+0.3, indicating that direct D^* production dominates over direct D production.

During the initial data run with the High Resolution Spectrometer (HRS) at SLAC PEP, an integrated luminosity of 19.6 pb^-1 at a center-of-mass energy of 29 GeV was accumulated. The data on Bhabha scattering and muon pair production are compared with the predictions of QED and the standard model of electroweak interactions. The measured forward-backward charge asymmetry in the angular distribution of muon pairs is -8.4%±4.3%. A comparison between the data and theoretical predictions places limits on alternative descriptions of leptons and their interactions. The existence of heavy electronlike or photonlike objects that alter the structure of the QED vertices or modify the propagator are studied in terms of the QED cutoff parameters. The Bhabha-scattering results give a lower limit on a massive photon and upper limits on the effective size of the electron of Λ₊>121 GeV and Λ_->118 GeV at the 95% confidence level. Muon pair production yields Λ₊>172 GeV and Λ_->172 GeV. If electrons have substructure, the magnitude and character of the couplings of the leptonic constituents affects the Bhabha-scattering angular distributions to such an extent that limits on the order of a TeV can be extracted on the effective interaction length of the components. For models in which the constituents interact with vector couplings of strength g² / 4π∼1, the energy scale Λ_VV for the contact interaction is measured to be greater than 1419.0 GeV at the 95% confidence level. We set limits on the production of supersymmetric scalar electrons through s-channel single-photon annihilation and t-channel inelastic scattering. Using events with two noncollinear electrons and no other charged or observed neutral particles in the final state, we see one event which is consistent with a simple supersymmetric model but which is also consistent with QED. This allows us to exclude the scalar electron to 95% confidence level in the mass range 1.8 to 14.2 GeV/c².

Inclusive production of (D⁰, D̅ ⁰) and D^± mesons have been observed in e⁺e^- annihilation at 29 GeV. The signals correspond to R values of R(D⁰+D̅ ⁰)=3.25±1.2 and R(D⁺+D^-)=1.35±0.6. D^*± production is also observed via the process D^*+→D⁰π⁺ and its charge conjugate. The D and D^* production rates are compared.