Search Results (93 total)

We used a torsion pendulum containing ≈10²³ polarized electrons to search new interactions that couple to electron spin. We limit CP-violating interactions between the pendulum’s electrons and unpolarized matter in the Earth or the Sun, test for rotation and boost-dependent preferred-frame effects using the Earth’s rotation and velocity with respect to the entire cosmos, and search for exotic velocity-dependent potentials between polarized electrons and unpolarized matter in the Sun and Moon. We find CP-violating parameters |g_P^eg_S^N|/(ℏc)<9.4×10^-37 and |g_A^eg_V^N|/(ℏc)<1.2×10^-56 for λ>1  AU. We test for preferred-frame interactions of the form V=-σ^e·A, V=-Bσ^e·v/c, or V=-∑σ_i^eC_ijv_j/c, where v is the velocity of the Earth with respect to the cosmic microwave background restframe and i, j represent the equatorial inertial coordinates X, Y, and Z. We constrain all 3 components of A, obtaining 1σ upper limits |A_X,Y|≤1.5×10^-22  eV and |A_Z|≤4.4×10^-21  eV that may be compared to the benchmark value m_e²/M_Planck=2×10^-17  eV. Interpreting our constraint on A in terms of noncommutative geometry, we obtain an upper bound of (355l_GUT)² on the minimum observable area, where l_GUT=ℏc/(10¹⁶  GeV) is the grand unification length. We find that |B|≤1.2×10^-19  eV. All 9 components of C are constrained at the 10^-17 to 10^-18  eV level. We determine 9 linear combinations of parameters of the standard model extension; rotational-noninvariant and boost-noninvariant terms are limited at roughly the 10^-31  GeV and 10^-27  GeV levels, respectively. Finally, we find that the gravitational mass of an electron spinning toward the galactic center differs by less than about 1 part in 10²¹ from an electron spinning in the opposite direction. As a byproduct of this work, the density of polarized electrons in Sm  Co₅ was measured to be (4.19±0.19)×10²²  cm^-3 at a field of 9.6 kG.

We determined the absolute branch of the T=2 superallowed decay of ³²Ar by detecting the β⁺-delayed protons and γ decays of the daughter state. We obtain b_SA^β=(22.71±0.16)%, which represents the first determination of a proton branch to better than 1%. Using this branch along with the previously determined ³²Ar half-life and energy release, we determined ft=(1552±12) s for the superallowed decay. This ft value, together with the corrected Ft value extracted from previously known T=1 superallowed decays, yields a measurement of the isospin symmetry breaking correction in ³²Ar decay δ_C^exp=(2.1±0.8)%. This can be compared to a theoretical calculation δ_C=(2.0±0.4)%. As by-products of this work, we determined the γ and proton branches for the decay of the lowest T=2 state of ³²Cl, made a precise determination of the total proton branch and relative intensities of proton groups that leave ³¹S in its first excited state and deduced an improved value for the ³²Cl mass.

We used a continuously rotating torsion balance instrument to measure the acceleration difference of beryllium and titanium test bodies towards sources at a variety of distances. Our result Δa_N,Be-Ti=(0.6±3.1)×10^-15  m/s² improves limits on equivalence-principle violations with ranges from 1 m to ∞ by an order of magnitude. The Eötvös parameter is η_Earth,Be-Ti=(0.3±1.8)×10^-13. By analyzing our data for accelerations towards the center of the Milky Way we find equal attractions of Be and Ti towards galactic dark matter, yielding η_DM,Be-Ti=(-4±7)×10^-5. Space-fixed differential accelerations in any direction are limited to less than 8.8×10^-15  m/s² with 95% confidence.

We use data from our recent search for violations of the gravitational inverse-square law to constrain dilaton, radion, and chameleon exchange forces as well as arbitrary vector or scalar Yukawa interactions. We test the interpretation of the PVLAS Collaboration effect and a conjectured “fat-graviton” scenario and constrain the γ₅ couplings of pseuodscalar bosons and arbitrary power-law interactions.

We conducted three torsion-balance experiments to test the gravitational inverse-square law at separations between 9.53 mm and 55  μm, probing distances less than the dark-energy length scale λ_d=⁴\ ℏc/ρ_d≈85  μm. We find with 95% confidence that the inverse-square law holds (|α|≤1) down to a length scale λ=56  μm and that an extra dimension must have a size R≤44  μm.

We used a torsion pendulum containing ∼9×10²² polarized electrons to search for CP-violating interactions between the pendulum’s electrons and unpolarized matter in the laboratory’s surroundings or the Sun, and to test for preferred-frame effects that would precess the electrons about a direction fixed in inertial space. We find |g_P^eg_S^N|/(ℏc)<1.7×10^-36 and |g_A^eg_V^N|/(ℏc)<4.8×10^-56 for λ>1  AU. Our preferred-frame constraints, interpreted in the Kostelecký framework, set an upper limit on the parameter |b˜^e|≤5.0×10^-21  eV that should be compared to the benchmark value m_e²/M_Planck=2×10^-17  eV.

We produced the lowest T=2 state in ³²S with the ³¹P(p,γ) reaction and measured the energies of the deexcitation γ rays, obtaining an excitation energy of 12047.96±0.28 keV that disagrees with a previous value of 12045.0±0.4 keV. Our result, together with a recent measurement of the ³²Ar mass, makes the A=32 multiplet the most precisely measured T=2 quintet and provides easily the most stringent test of the isobaric multiplet mass equation. A significant violation of the isobaric multiplet mass equation is observed that could be explained by mixing with a nearby T=0 level.

We studied β-delayed singles-α spectra from ⁸Li and ⁸B decays with special emphasis on a careful calibration of the energy scale. ⁸Li and ⁸B activities were produced by ⁷Li(d,p) and ⁶Li(³He,n) reactions, respectively, and deposited in thin C foils. Delayed α's were counted in thin, cooled Si detectors with small solid angles to reduce summing with β's. The energy scale and detector response were calibrated with spectroscopic grade radioactive sources. We extracted the ⁸Li and ⁸B final-state continuum shapes from our spectra by using R-matrix analyses that included effects of lepton-recoil broadening and detector response. Our results are in excellent agreement with a recent measurement using ⁸B's implanted in a Si counter and in good agreement with our reanalysis of the older Wilkinson-Alburger ⁸Li and ⁸B data, but disagree with a recent ⁸B experiment using a coincidence technique.

Motivated by a variety of theories that predict new effects, we tested the gravitational 1/r² law at separations between 10.77 mm and 137 μm using two different 10-fold azimuthally symmetric torsion pendulums and rotating 10-fold symmetric attractors. Our work improves upon other experiments by up to a factor of about 100. We found no deviation from Newtonian physics at the 95% confidence level and interpret these results as constraints on extensions of the standard model that predict Yukawa or power-law forces. We set a constraint on the largest single extra dimension (assuming toroidal compactification and that one extra dimension is significantly larger than all the others) of R_*<~160 μm, and on two equal-sized large extra dimensions of R_*<~130 μm. Yukawa interactions with |α|>~1 are ruled out at 95% confidence for λ>~197 μm. Extra-dimensions scenarios stabilized by radions are restricted to unification masses M_*>~3.0 TeV/c², regardless of the number of large extra dimensions. We also provide new constraints on power-law potentials V(r)∝r^-k with k between 2 and 5 and on the γ₅ couplings of pseudoscalars with m<~10 meV/c².

We present new measurements of the ⁷Be(p,γ)⁸B cross section from E̅ _c.m.=116 to 2460  keV (where c.m. means center-of-mass), which incorporate several improvements over our previously published experiment, also discussed here. Our new measurements lead to S₁₇(0)=22.1±0.6(expt)±0.6(theor)  eV  b based on data from E̅ _c.m.=116 to 362  keV, where the central value is based on the theory of Descouvemont and Baye. The theoretical error estimate is based on the fit of 12 different theories to our low-energy data. We compare our results to other S₁₇(0) values extracted from both direct [⁷Be(p,γ)⁸B] and indirect (Coulomb-dissociation and heavy-ion reaction) measurements, and show that the results of these three types of experiments are not mutually compatible. We recommend a “best” value, S₁₇(0)=21.4±0.5(expt)±0.6(theor)  eV  b, based on the mean of all modern direct measurements below the 1⁺ resonance. We also present S factors at 20  keV which is near the center of the Gamow window: the result of our measurements is S₁₇(20)=21.4±0.6(expt)±0.6(theor)  eV  b, and the recommended value is S₁₇(20)=20.6±0.5(expt)±0.6(theor)  eV  b.

The simultaneous exchange of two pseudoscalars between fermions leads to a spin-independent force between macroscopic objects. Previous work has demonstrated that one can combine this interaction with tests of the weak equivalence principle, gravitational inverse square law, and studies of laser beam propagation in magnetic fields, to set significant new constraints on the Yukawa couplings of massless pseudoscalars to nucleons. Here we extend these results to massive pseudoscalars, and derive new constraints which relate the strengths of these couplings to the pseudoscalar mass.

We reanalyzed the Wilkinson-Alburger delayed-α spectra from ⁸Li and ⁸B decay, accounting for lepton-recoil broadening that had been neglected in previous analyses. This substantially improved the quality of the fits. The ⁸Li and ⁸B delayed-α spectra are now consistently described by the same ⁸Be final-state continuum. In our analysis, which did not invoke a low-lying intruder state, the discrepancy between the final-state continua inferred from the delayed-α spectra and from the L=2 α+α phase shifts is much less than found by previous authors. This largely resolves discrepancies noted by Barker and Warburton; the remaining differences may be artifacts of the assumption that the β-decay matrix elements are independent of excitation energy. Our analysis of the Wilkinson-Alburger data is consistent with results from the recent coincidence study of Ortiz et al., which gives additional confidence in using delayed-α spectra to infer the spectrum of ⁸B neutrinos that dominate the counting rate of many solar neutrino detectors.

We measured the ⁷Be(p,γ)⁸B cross section from E̅ _c.m.  =  186 to 1200 keV, with a statistical-plus-systematic precision per point of better than ±5%. All important systematic errors were measured including ⁸B backscattering losses. We obtain S₁₇(0)  =  22.3±0.7(expt)±0.5(theor) eV b from our data at E̅ _c.m.≤300 keV and the theory of Descouvemont and Baye.

Motivated by higher-dimensional theories that predict new effects, we tested the gravitational 1/r² law at separations ranging down to 218 μm using a 10-fold symmetric torsion pendulum and a rotating 10-fold symmetric attractor. We improved previous short-range constraints by up to a factor of 1000 and find no deviations from Newtonian physics.

We tested the equivalence principle at short length scales by rotating a 3 ton ²³⁸U attractor around a compact torsion balance containing Cu and Pb test bodies. The observed differential acceleration of the test bodies toward the attractor, a_Cu-a_Pb=(1.0±2.8)×10^-13 cm/s², should be compared to the corresponding gravitational acceleration of 9.2×10^-5 cm/s². Our results set new constraints on equivalence-principle violating interactions with Yukawa ranges down to 1 cm, and improve by substantial factors existing limits for ranges between 10 km and 1000 km. Our data also set strong constraints on certain power-law potentials that can arise from two-boson exchange processes.

The lunar-ranging test of the equivalence principle for gravitational self-energy is ambiguous. Although the Earth has more gravitational self-energy than the Moon, its sizable Fe/Ni core also gives it a different composition than the Moon. We removed this ambiguity by comparing, in effect, the accelerations of “miniature” earths and moons toward the Sun. Our composition-dependent Earth-Moon acceleration, Δa_CD/a_s  =  (+0.1±2.7±1.7)×10^-13, and lunar-ranging data provide an unambiguous test at the 1.3×10^-3 level.

The positron-neutrino correlation in the 0⁺→0⁺ β decay of ³²Ar was measured at ISOLDE by analyzing the effect of lepton recoil on the shape of the narrow proton group following the superallowed decay. Our result is consistent with the standard model prediction. For vanishing Fierz interference we find a  =  0.9989±0.0052±0.0039, which yields improved constraints on scalar weak interactions.

We studied β-delayed proton and γ emission from ⁴⁰Ti decay. We found t_1/2=52.7±1.5 ms and observed 28 proton groups that we organized into a ⁴⁰Ti decay scheme with 21 branches. The reduced transition strengths of these decay branches were then used to compute the neutrino detection efficiency of the ICARUS liquid argon time-projection chamber. We found ⁴⁰Ar(ν,e) cross sections (for an electron energy threshold W=5 MeV) of (14.0±0.3)×10^-43 cm², (75.1±1.8)×10^-43 cm², and (3.2±0.1)×10^-41 cm² for ⁸B neutrinos, hep neutrinos, and supernova neutrinos characterized by a temperature of 4.5 MeV.

We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

We outline an analytic method for calculating inner and outer multipoles of a 1/r potential about an arbitrary point in terms of known multipoles about a given point.

We rotated a 3 ton ²³⁸U attractor around a compact torsion balance and compared the accelerations of Cu and Pb toward U. We found that a_Cu-a_Pb  =  (-0.7±5.7)×10^-13 cm/s², compared to the 9.8×10^-5 cm/s² gravitational acceleration toward the attractor. Our results set new constraints on equivalence-principle violating interactions with Yukawa ranges down to 1 cm and rule out an earlier suggestion of a Yukawa interaction coupled predominantly to N-Z.

We measured ³⁶Ar(p→,p₀) cross sections and analyzing powers for E_p between 3.22 and 5.98 MeV, and used these data to deduce spin, parity, and widths of 17 levels in ³⁷K. Our results, combined with existing information on ³⁷Ca β⁺ decay, yield information on the distribution of Fermi and Gamow-Teller strength in this decay. A T=1/2 level lying 31 keV below the J^π=3/2⁺, T=3/2 analog of the ³⁷Ca ground state was found to have J^π=3/2⁺. The relative proton widths of the two levels imply an isospin-mixing matrix-element of 4.84±0.43 keV. This agrees with the value 5.93_-0.65^+0.23 keV extracted from the β⁺-decay intensities to the two levels, plus the Gamow-Teller to Fermi strength ratio for the 5018 keV level. We decompose the Gamow-Teller strength in ³⁷Ca decay into components feeding daughter levels with J^π=1/2⁺, 3/2⁺, or 5/2⁺, and compare the results to theory. None of the available shell-model calculations gives a satisfactory account of the data.

We used the ¹⁶O(³He,n)¹⁸Ne, ¹²C(¹²C,⁶He)¹⁸Ne, and ²⁰Ne(p,t)¹⁸Ne reactions to study ¹⁸Ne states up to an excitation energy of 10 MeV, with emphasis on levels corresponding to ¹⁴O(α,p)¹⁷F and ¹⁷F(p,γ)¹⁸Ne resonances that could strongly affect these reaction rates in hot stellar environments. Excitation energies, widths, absolute cross sections, and angular distributions were measured. We found previously unidentified states at E_x=6.15±0.01 MeV, 7.12±0.02 MeV, 7.35±0.02 MeV, 7.62±0.02 MeV, 8.30±0.02 MeV, (8.45±0.03 MeV), 8.55±0.03 MeV, 8.94±0.02 MeV, and 9.58±0.02 MeV. We combined level width, cross section, and angular distribution data to infer J^π values for a number of the new levels as well as for the previously known 5.1-MeV doublet. Using information from our experiments, we recalculated the ¹⁴O(α,p)¹⁷F reaction rate, which constitutes a possible path out of the hot CNO cycle into the rp process and could play an important role in transforming nuclei involved in the hot CNO cycle into heavier nuclei with Z≥10. © 1996 The American Physical Society.