Search Results (45 total)

A novel, single shot, nondestructive scheme to measure the bunch length of submillimeter relativistic electron bunches using the electro-optical method is described. In this scheme, the birefringence induced by the electric field of the electrons converts the temporal characteristics of the bunch to a spatial intensity distribution of an optical pulse. Electric field characteristics, induced birefringence, and retardation are calculated for a few typical electron beam parameters and criteria limiting the resolution are established.

Nuclear disassembly into light and intermediate mass ejectiles is searched for in ⁴⁰Ar + Cu, Ag, Au reactions of 17-115 A MeV. Distributions in fragment mass versus velocity show that such nuclear disassembly does occur for 65-115 A MeV. However, it is very rare, with much less abundance than reactions leading to a massive residual nucleus.

The ensemble of charged isotropically emitted ejectiles is studied for central collisions of (17-115)A MeV ⁴⁰Ar + Cu, Ag, Au. Measurements of average multiplicities, spectral slopes, and masses of the heaviest fragments are compared to statistical models for multifragmentation or sequential evaporation. The multifragmentation models predict much more complete nuclear disassembly than is observed. The evaporation model reproduces the data much more closely except for the spectra of Z=1 ejectiles. The kinetic energies of Z=1 and 2 ejectiles are much less than found for 1A GeV ¹⁹⁷Au+¹²C for similar energy depositions. Entrance channel dynamics seem to affect the isotropic emission ensembles, often taken to define an equilibrated emission source.

For central collisions of (17–115)A MeV ⁴⁰Ar+Cu, Ag, Au, an overall balance is determined for the average mass, energy, and longitudinal momentum. Light charged particles and fragments are separated into forward-focused and isotropic components in the frame of the heaviest fragment. Energy removal by the isotropic component reaches 1–2 GeV. For such high deposition energies, statistical multifragmentation models predict much more extensive nuclear disassembly than is observed.

Spallation neutron production in proton induced reactions on Pb targets at 0.8, 1.2, and 1.6 GeV has been measured at the SATURNE accelerator. Double-differential cross sections were obtained over a broad angular range from which averaged neutron multiplicities per reaction were inferred for energies above 2 MeV. The results are compared with calculations performed with a high energy transport code including two different intranuclear cascade (INC) models: it is shown that the Cugnon INC model gives a better agreement with the data than the Bertini one, mainly because of improved nucleon-nucleon cross sections and Pauli blocking treatment.

Measurements are reported for fragment masses and velocities from the reactions (17–115)A MeV ⁴⁰Ar+Cu, Ag, and Au. Charged particle multiplicities were used to select the most violent reactions, and systematics are reported for the momentum and energy deposition. These reactions are fusionlike for ≲44A MeV ⁴⁰Ar with a large fraction of momentum and energy deposition in the heavy nuclear system. However, for ≳44A MeV ⁴⁰Ar, a majority of the projectile energy and momentum is carried away by a multibody spray of light ejectiles and only a minority is deposited in the heavy nucleus. Nevertheless heavy composite systems are formed with up to 9–12 MeV per source nucleon for 115A MeV ⁴⁰Ar.

Heavy residual nuclei are shown to result from the most violent (i.e., central) collisions for ⁴⁰Ar + Ag reactions of up to 1.36 GeV; their average velocities are ≳80% of the c.m. velocity. Angular and energy distributions for ^1,2,3H, 3,4He, and Li are measured in coincidence with these heavy nuclei. The dominant light particle components are nearly isotropic in a frame of reference having the velocity of the heavy residues. In addition there are forward-peaked high-energy components of the H, He, and Li emission attributable to prethermalization emission. Fractional abundances of these prethermalization components increase markedly with increasing incident energy. Mass and momentum balance preclude the presence of a projectilelike fragment and thus indicate fusionlike reactions with large but incomplete linear momentum transfer. The remainder of the momentum is carried away by the spray of forward-peaked ejectiles. For 1.36 GeV ⁴⁰Ar∼½ of the 900 MeV available is completely thermalized, and ∼½ goes into prethermalization emission after strong collisional mixing.

A previously unexploited experimental observable is used to explore emission times for intermediate mass fragments relative to directly emitted ²H and ³H particles. Small-angle correlations are reported in central collisions for 34A  MeV ⁴⁰Ar+^natAg. High-velocity ³H and ²H particles follow a direct emission scenario with mean lifetime τ∼30-60  fm/c. Fragmentation to Li is characterized by τ∼120 fm/c. Current model calculations suggest a delay time of ∼100-200  fm/c for expansion of the central collision zone prior to the onset of “freeze-out” into fragments. But the observed velocity difference spectra limit the delay time to ≲50 fm/c for expansion between direct emission and fragmentation.

We use trajectory calculations to analyze small-angle particle-particle correlations for three typical situations: ⁴⁰Ar+¹⁹⁷Au (E/A=60 MeV)→²H-²H pairs and ¹-¹H pairs, ²⁰Ne+⁵⁹Cu (E/A=30 MeV)→n-n pairs. For the ²-²H pairs our analysis of the gentle featureless anticorrelations suggests that the major driving force is Coulomb repulsion after a range of average time delays from ≊5×10^-21 s for the ²H pairs of lower energy to ≊10^-22 s for the ²H pairs of higher energy. Simulations are used to illustrate the separate dominance of source size and lifetime in the space-time extent of the emitter. For lifetimes ≤10^-22 s the emitter size dominates; for longer lifetimes the time delays become predominant. The peaks at ≊20 MeV/c in the correlation functions for ¹-¹H pairs can be accounted for by diproton ejection which decays into protons with a Q value of ≊0.35 MeV and a decay width of ≊1 MeV (or a meanlife of 6×10^-22 s). The positive correlations between neutron pairs can be accounted for by dineutron ejection which decays into neutrons with a near zero Q value and a decay width of ≊0.25 MeV (or a meanlife of ≊2×10^-21 s). If these diproton and dineutron clusters do indeed have a metastable existence, then one should reexamine the notion that their associated small-angle correlations reflect the space-time extent of the emission source.

In-plane and out-of-plane angular correlations have been measured between fragments of Z>3, Li fragments, ^3,4He, and ^1,2,3H. The changing patterns for ⁴⁰Ar induced reactions of 7A, 17A, 27A, and 34A MeV give an overview of the decreasing importance of mass-symmetric fissionlike reactions at the expense of a broad range of more mass-asymmetric breakups. Evidence is given that these fragments come from a central collision group of reactions that have similar violence and from which many combinations of fragments and particles are ejected. Very similar azimuthal angular correlations are observed for particles with a Li fragment and for particles with a pair of heavier fragments (Z>3). This similarity suggests comparable strengths of association with the reaction plane for single Li fragments and for fragment pairs of Z>3. Azimuthal angular correlations for Li-Li pairs exhibit distinct asymmetries; their interpretation via trajectory-model calculations indicates mean delay times of ≊5×10^-22 s.

Using an impact-parameter tag to select an enriched sample of Z⁰→bb̅ events, we have measured the difference between the average charged multiplicity of bb̅ and all hadronic Z⁰ decays to be 2.1±1.8(stat)±0.6(syst) tracks per event. The resulting total (nonleading) charged multiplicity for Z⁰→bb̅ events is 23.1(12.0)±1.8±0.6 tracks. A comparison of this nonleading multiplicity to hadronic multiplicity data in the range of 10 to 60 GeV supports the hypothesis of flavor-independent hadronic fragmentation, and yields a measurement of the average energy fraction of bottom hadrons in Z⁰ decays of 〈x_E〉_b=0.62±0.10±0.04.

We have measured the fraction of bb¯ events in hadronic Z⁰ decays, R_bb¯, using the vertex detector system of the Mark II detector at the SLAC Linear Collider. We tag bb¯ events by requiring the coincidence of three or more tracks with significant impact parameters. This tag is 50% efficient and results in a sample of 85% purity. We find R_bb¯=0.251±0.049±0.030, in good agreement with other measurements and the standard model prediction.

A 4π charged-particle multidetector has been used to study the reaction ⁴⁰Ar+^natAg from 280–1356 MeV. Charged-particle multiplicity distributions show a low-multiplicity group associated with peripheral collisions and a high-multiplicity group associated with central collisions. Average multiplicities for central collisions increase with increasing projectile energy, indicating ever-increasing collision violence. Angular distributions of emitted protons are essentially isotropic for θ≥80° in a reference frame characterized by the empirical systematics of linear momentum transfer (i.e., ≊100% to ≊70% from 7–34 MeV/nucleon). Spectra of these protons at side angles are evaporationlike in shape and indicate relative effective temperatures of 3, 6, 8, and 12 MeV for beam energies of 7, 17, 27, and 34A MeV, respectively. Azimuthal angular correlations between various particle pairs are consistent with spin-driven emission from emitter sources of reasonable spin values. In short, these results support a classical picture of extensively thermalized emitter nuclei even for initial excitation energies of ≊5 MeV per system nucleon and spins of ≥100ħ.

We have searched for supersymmetric particles in 528 Z decays with the Mark II detector at the SLAC Linear Collider. We place 95%-confidence-level lower mass limits on degenerate squarks, nondegenerate up-type squarks, nondegenerate down-type squarks, charginos, pair-produced unstable neutralinos, and neutralinos from associated production.

A search for pair production of stable charged particles from Z decay has been performed with the Mark II detector at the SLAC Linear Collider. Particle masses are determined from momentum, ionization energy loss, and time-of-flight measurements. A limit excluding pair production of stable fourth-generation charged leptons and stable mirror fermions with masses between the muon mass and 36.3 GeV/c² is set at the 95% confidence level. Pair production of stable supersymmetric scalar leptons with masses between the muon mass and 32.6 GeV/c² is also excluded.

Using the Mark II detector at the SLAC Linear Collider, we search for decays of the Z boson to a pair of nonminimal Higgs bosons (Z→H_s⁰H_p⁰), where one of them is relatively light (≲10 GeV). We find no evidence for these decays and we obtain limits on the ZH_s⁰H_p⁰ coupling as a function of the Higgs-boson masses.

We describe a search for the decay of the Z boson into pairs of doubly charged Higgs bosons with the Mark II detector operating at the SLAC Linear Collider. Each Higgs boson is assumed to decay into a same-sign leptonic pair. No event candidates are found in a sample of 528 Z decays. At the 95% confidence level, this result excludes the region of leptonic coupling g_ll>3×10^-7 and Higgs-boson mass 6.5<M_H<36.5 GeV/c² for isotriplet (left-handed) Higgs bosons. Isosinglet (right-handed) Higgs bosons are excluded in the same g_ll interval and in the mass interval 7.3<M_H<34.3 GeV/c².

Using the sample of neutral vector bosons (Z's) produced at the SLAC Linear Collider and detected with the Mark II detector, we search for the decay of the Z to a pair of particles, one of which decays to two charged particles. The observed number of Z decays with this signature excludes, at a confidence level greater than 95%, the decay of the Z to a pair of fourth-generation Dirac neutrinos with mass between 2.5 and 22 GeV, a decay length less than about 1 cm, and coupling to any of the first three generations of charged leptons. This is the first time the existence of such a lepton coupling to the τ^± has been excluded by a direct search.

We have measured inclusive distributions for charged particles in hadronic decays of the Z boson. The variables chosen for study were charged-particle multiplicity, scaled momentum, and momenta transverse to the sphericity axes. The distributions have been corrected for detector effects and are compared with data from e⁺e^- annihilation at lower energies and with the predictions of several QCD-based models. The data are in reasonable agreement with expectations.

Using isolated leptons reconstructed in the Mark II detector to tag bb¯ events, we measure the fraction of bb¯ events in hadronic Z⁰ decays to be 0.23_-0.09^+0.11, in good agreement with the standard-model prediction of 0.22. We find Γ(Z→bb¯)=0.43_-0.17^+0.21 GeV.

We search for events in the Mark II detector at SLAC Linear Collider with the topology of a Z boson decaying into a pair of long-lived massive particles. No events that are consistent with the search hypothesis are found. Interpreting the long-lived particle as a sequential Dirac neutrino ν₄ of the fourth generation, we exclude at the 95% confidence level a significant range of mixing-matrix elements of ν₄ to other-generation neutrinos for a ν₄ mass from 10 to 43 GeV/c².

We measured the differential jet-multiplicity distribution in e⁺e^- annihilation with the Mark II detector. This distribution is compared with the second-order QCD prediction and α_s is determined to be 0.123±0.009±0.005 at √s≊M_Z (at the SLAC Linear Collider) and 0.149±0.002±0.007 at √s=29 GeV (at the SLAC storage ring PEP). The running of α_s between these two center-of-mass energies is consistent with the QCD prediction.

We present measurements by the Mark II experiment of the ratios of the leptonic partial widths of the Z boson to the hadronic partial width. The results are Γ_ee/Γ_had=0.037_-0.012^+0.016, &=0.053_-0.015^0.020, and Γ_ττ/Γ_had=0.066_-0.017^+0.021, in good agreement with the standard-model prediction of 0.048. From the average leptonic width result, Γ_ll/Γ_had=0.053_-0.009^+0.010, we derive Γ_had=1.56_-0.24^+0.28 GeV. We find for the vector coupling constants of the tau and muon v_τ²=0.31±0.31_-0.30^+0.43 and v_μ²=0.05±0.30_-0.23^+0.34.

We have searched for events with new-particle topologies in 390 hadronic Z decays with the Mark II detector at the SLAC Linear Collider. We place 95%-confidence-level lower limits of 40.7 GeV/c² for the top-quark mass, 42.0 GeV/c² for the mass of a fourth-generation charge -(1/3 quark, and 41.3 GeV/c² for the mass of an unstable Dirac neutral lepton.

We have measured the mass of the Z boson to be 91.14±0.12 GeV/c², and its width to be 2.42_-0.35^+0.45 GeV. If we constrain the visible width to its standard-model value, we find the partial width to invisible decay modes to be 0.46±0.10 GeV, corresponding to 2.8±0.6 neutrino species, with a 95%-confidence-level upper limit of 3.9.