Search Results (52 total)

Synchrotron radiation is used to excite Ar to the intermediate states 3p_1∕2⁵ (3d^′[3∕2]₁, 5d^′[3∕2]₁, 5s^′[1∕2]₁, 7s^′[1∕2]₁) and 3p_3∕2⁵ (6d[1∕2]₁, 6d[3∕2]₁, 8s[3∕2]₁), then excited by lasers to the autoionizing Rydberg series 3p_1∕2⁵ np^′ ([1∕2]_0,1, [3∕2]_1,2) and nf^′[5∕2]₂. For the intermediate states of 5s^′, 7s^′, and 8s, the np^′ ([1∕2]₀ and [3∕2]₂) series are observed with high intensity but not the np^′ ([1∕2]₁ and [3∕2]₁) series when the polarization vectors of two light beams are in parallel; but when they are in orthogonal, the np^′[1∕2]₀ series disappears, the np^′[3∕2]₂ intensity remains, and the np^′ ([1∕2]₁ and [3∕2]₁) series show up strongly. The intensity distribution of the np^′ series strongly depends on the intermediate state. The spectra of the np^′ series are assigned according to their intensity variation with the polarization vectors. The quantum defects determined for the series np^′[1∕2]₀(n=11–57), np^′[3∕2]₂(n=11–65), np^′[3∕2]₁(n=11–31), and nf^′[5∕2]₂(n=9–74) are 1.611±0.011, 1.683±0.013, 1.688±0.010, and 0.016±0.005, respectively. Our values are in excellent agreement with theoretical prediction. The spectral line shapes of autoionizing Rydberg states are analyzed with a Beutler-Fano profile. Reduced autoionization linewidths for the np^′[1∕2]₀(n=11–16) series vary in the range 2549–4145  cm⁻¹, and the nf^′[5∕2]₂(n=9–11) series in 186–247  cm⁻¹ in reasonable agreement with theoretical prediction.

The spin precession frequency of muons stored in the (g-2) storage ring has been analyzed for evidence of Lorentz and CPT violation. Two Lorentz and CPT violation signatures were searched for a nonzero Δω_a(=ω_a^μ+-ω_a^μ-) and a sidereal variation of ω_a^μ±. No significant effect is found, and the following limits on the standard-model extension parameters are obtained: b_Z=-(1.0±1.1)×  10^-23  GeV; (m_μd_Z0+H_XY)=(1.8±6.0)×10^-23  GeV; and the 95% confidence level limits bˇ_⊥^μ+<  1.4×10^-24  GeV and bˇ_⊥^μ-<2.6×10^-24  GeV.

We present the final report from a series of precision measurements of the muon anomalous magnetic moment, a_μ=(g-2)/2. The details of the experimental method, apparatus, data taking, and analysis are summarized. Data obtained at Brookhaven National Laboratory, using nearly equal samples of positive and negative muons, were used to deduce a_μ(Expt)=11659208.0(5.4)(3.3)×10^-10, where the statistical and systematic uncertainties are given, respectively. The combined uncertainty of 0.54 ppm represents a 14-fold improvement compared to previous measurements at CERN. The standard model value for a_μ includes contributions from virtual QED, weak, and hadronic processes. While the QED processes account for most of the anomaly, the largest theoretical uncertainty, ≈0.55  ppm, is associated with first-order hadronic vacuum polarization. Present standard model evaluations, based on e⁺e^- hadronic cross sections, lie 2.2–2.7 standard deviations below the experimental result.

One of the main sources of electrons in the Spallation Neutron Source’s Accumulator Ring is the stripped electrons in the injection region. A magnetic field guides the stripped electrons to the bottom of the beam pipe, where an electron catcher with overhanging surface traps them. This paper describes the stripped electrons’ motion, the optimization of the catcher, and the build up of an electron cloud in this region.

State-selective fragmentation dynamics for excited fragments and ionic fragments of gaseous and condensed Si(CH₃)₂Cl₂ following Cl 2p and Si 2p core-level excitations have been characterized. The Cl 2p→15a₁^* excitation of Si(CH₃)₂Cl₂ induces significant enhancement of the Cl⁺ desorption yield in the condensed phase and the Si(CH₃)₂⁺ and SiCH₃⁺ yields in the gaseous phase. The core-to-Rydberg excitations at both Si 2p and Cl 2p edges lead to enhanced production of the excited fragments. These complementary results provide deeper insight into the origin of state-selective fragmentation of molecules via core-level excitation.

The anomalous magnetic moment of the negative muon has been measured to a precision of 0.7 ppm (ppm) at the Brookhaven Alternating Gradient Synchrotron. This result is based on data collected in 2001, and is over an order of magnitude more precise than the previous measurement for the negative muon. The result a_μ^-=11 659 214(8)(3)×10^-10 (0.7 ppm), where the first uncertainty is statistical and the second is systematic, is consistent with previous measurements of the anomaly for the positive and the negative muon. The average of the measurements of the muon anomaly is a_μ(exp)=11 659 208(6)×10^-10 (0.5 ppm).

A higher precision measurement of the anomalous g value, a_μ=(g-2)/2, for the positive muon has been made at the Brookhaven Alternating Gradient Synchrotron, based on data collected in the year 2000. The result a_μ⁺=11 659 204(7)(5)×10^-10 (0.7 ppm) is in good agreement with previous measurements and has an error about one-half that of the combined previous data. The present world average experimental value is a_μ(expt)=11 659 203(8)×10^-10 (0.7 ppm).

Localized fluctuations in the multiplicity of charged particles and photons produced in central 158A GeV/c Pb+Pb collisions are studied. The charged versus neutral correlations in common η-φ phase space regions of varying azimuthal size are analyzed by two different methods. The analysis provides a model-independent demonstration of nonstatistical fluctuations in both charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charge-neutral fluctuations are observed, contrary to expectations for the production of a disoriented chiral condensate. The result is not explained by the widely used VENUS model.

A precise measurement of the anomalous g value, a_μ  =  (g-2)/2, for the positive muon has been made at the Brookhaven Alternating Gradient Synchrotron. The result a_μ⁺  =  11 659 202(14) (6)×10^-10 (1.3 ppm) is in good agreement with previous measurements and has an error one third that of the combined previous data. The current theoretical value from the standard model is a_μ(SM)  =  11 659 159.6(6.7)×10^-10 (0.57 ppm) and a_μ(exp)-a_μ(SM)  =  43(16)×10^-10 in which a_μ(exp) is the world average experimental value.

The collimation system for the Spallation Neutron Source accumulator ring is designed for a capture efficiency close to 95% of the proton beam halo, dissipating about 2 kW of beam power. The collimation system consists of a two-stage collimation system (one scraper and two absorbers) cleaning the transverse halo and a beam-in-gap kicker system cleaning the gap residual and longitudinal halo. Preliminary studies indicate that a maximum level of uncontrolled loss of 0.01% of the total beam is achievable. On the other hand, the energy lost in the primary scraper may kick protons outside the rf bucket concentrating uncontrolled losses in areas of maximum dispersion. We use Monte Carlo simulations to clarify some beam dynamic issues that may compromise the high efficiency required. The material interacting with the beam and the shape of the scraper and absorbers have been carefully chosen to maximize the collimation efficiency and to minimize radioactivation. Furthermore, a realistic distribution of losses around the machine is used to identify potential hot areas. Finally, we determine the sensitivity of the collimation efficiency to misalignments and closed orbit errors. This paper describes the latest design of the collimation system and summarizes the results of these numerical studies.

A measurement of direct photon production in ²⁰⁸Pb+²⁰⁸Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5<p_T<4 GeV/c. A significant direct photon signal, compared to statistical and systematical errors, is seen at p_T>1.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.

Three-particle correlations have been measured for identified π^- from central 158A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found as expected for a mainly chaotic and symmetric source.

A new measurement of the positive muon’s anomalous magnetic moment has been made at the Brookhaven Alternating Gradient Synchrotron using the direct injection of polarized muons into the superferric storage ring. The angular frequency difference ω_a between the angular spin precession frequency ω_s and the angular orbital frequency ω_c is measured as well as the free proton NMR frequency ω_p. These determine R=ω_a/ω_p=3.707 201(19)×10^-3. With μ_μ/μ_p=3.183 345 39(10) this gives a_μ⁺=11 659 191(59)×10^-10 (±5 ppm), in good agreement with the previous CERN and BNL measurements for μ⁺ and μ^-, and with the standard model prediction.

This paper summarizes the low-loss design for the Spallation Neutron Source accumulator ring [“Spallation Neutron Source Design Manual” (unpublished)]. A hybrid lattice consisting of FODO arcs and doublet straights provides optimum matching and flexibility for injection and collimation. For this lattice, optimization focuses on six design goals: a space-charge tune shift low enough (below 0.15) to avoid strong resonances, adequate transverse and momentum acceptance for efficient beam collimation, injection optimized for desired target beam shape and minimal halo development, compensation of magnet field errors, control of impedance and instability, and prevention against accidental system malfunction. With an expected collimation efficiency of more than 90%, the uncontrolled fractional beam loss is expected to be at the 10^-4 level.

Neutral pion production in central 158A GeV ²⁰⁸Pb+ ²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The π⁰ transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out.

The muon anomalous magnetic moment has been measured in a new experiment at Brookhaven. Polarized muons were stored in a superferric ring, and the angular frequency difference, ω_a, between the spin precession and orbital frequencies was determined by measuring the time distribution of high-energy decay positrons. The ratio R of ω_a to the Larmor precession frequency of free protons, ω_p, in the storage-ring magnetic field was measured. We find R  =  3.707 220(48)×10^-3. With μ_μ/μ_p  =  3.183 345 47(47) this gives a_μ⁺  =  1 165 925(15)×10^-9 ( ±13 ppm), in good agreement with the previous CERN measurements for μ⁺ and μ^- and of approximately the same precision.

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons, are larger than those of pions. The difference in the inverse slopes of pions, kaons, and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam, suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density dN/dy of both K⁺ and K^- increases more rapidly with system size than for π⁺ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K^-/K⁺ ratio falls with increasing system size but more slowly than the p̅ /p ratio. The π^-/π⁺ ratio is close to unity for all systems. From pBe to SPb the K⁺/p ratio decreases while K^-/p̅ increases and sqrt[(K⁺⋅K^-)/(p⋅p̅ )] stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.

The production of neutral pions in 158A GeV ²⁰⁸Pb+²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron (SPS). Transverse momentum spectra are studied for the range 0.3≤m_T-m₀≤4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons. This is most naturally explained by assuming an equilibrated system.

Experiment NA44 has measured proton and antiproton distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at midrapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to midrapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 A GeV S+S and 158 A GeV Pb+Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.

First results of the m_T dependence of π⁺π⁺ and K⁺K⁺ correlations from S+Pb collisions at 200 GeV /c per nucleon measured by the focusing spectrometer of the NA44 experiment at CERN are presented. Multidimensional fits characterize the pion and kaon emission volume. The pion radius parameter decreases with increasing p_T. Furthermore, the pion and kaon radii show a common 1/sqrt[m_T] dependence. This behavior can be interpreted as a result of a strong momentum-position correlation arising from collective flow.

Accelerating polarized protons to 22 GeV/c at the Brookhaven Alternating Gradient Synchro- tron required both extensive hardware modifications and a difficult commissioning process. We had to overcome 45 strong depolarizing resonances to maintain polarization up to 22 GeV/c in this strong-focusing synchrotron. At 18.5 GeV/c we measured the analyzing power A and the spin-spin correlation parameter A_nn in large- P_⊥² proton-proton elastic scattering, using the polarized proton beam and a polarized proton target. We also obtained a high-precision measurement of A at P_⊥²=0.3 (GeV/c)² at 13.3 GeV/c. At 18.5 GeV/c we found that A_nn=(-2±16)% at P_⊥²=4.7 (GeV/c)², where it was about 60% near 12 GeV at the Argonne Zero Gradient Synchrotron. This sharp change suggests that spin-spin forces may have a strong and unexpected energy dependence at high P_⊥².

The analyzing power A in 28-GeV/c proton-proton elastic scattering was measured with a polarized proton target and a high-intensity unpolarized proton beam at the Brook-haven National Laboratory alternating-gradient synchrotron. The P_⊥² range of 2.85 to 5.95 (GeV/c)² was covered with good precision. A small dip of about -3.5% was found near P_⊥²=3.5 (GeV/c)² where a 24-GeV/c CERN experiment had reported a deep dip of about -16% with large errors. In the previously unexplored large-P_⊥² region near 6 (GeV/c)² these new large-error points suggest that A may be rising.