The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro-and photo-induced reactions at very high luminosity and good momentum and angular resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2 x 10(-4). A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of 10(38) CM-2 s(-1). The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium. (C) 2003 Elsevier B.V. All rights reserved

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Large experimental programmes in the fields of nuclear and particle physics search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson completed the set of particles predicted by the standard model, which currently provides the best description of fundamental particles and forces. However, this theory's limitations include a failure to predict fundamental parameters, such as the mass of the Higgs boson, and the inability to account for dark matter and energy, gravity, and the matter-antimatter asymmetry in the Universe, among other phenomena. These limitations have inspired searches for physics beyond the standard model in the post-Higgs era through the direct production of additional particles at high-energy accelerators, which have so far been unsuccessful. Examples include searches for supersymmetric particles, which connect bosons (integer-spin particles) with fermions (half-integer-spin particles), and for leptoquarks, which mix the fundamental quarks with leptons. Alternatively, indirect searches using precise measurements of well predicted standard-model observables allow highly targeted alternative tests for physics beyond the standard model because they can reach mass and energy scales beyond those directly accessible by today's high-energy accelerators. Such an indirect search aims to determine the weak charge of the proton, which defines the strength of the proton's interaction with other particles via the well known neutral electroweak force. Because parity symmetry (invariance under the spatial inversion (x, y, z) → (-x, -y, -z)) is violated only in the weak interaction, it provides a tool with which to isolate the weak interaction and thus to measure the proton's weak charge . Here we report the value 0.0719 ± 0.0045, where the uncertainty is one standard deviation, derived from our measured parity-violating asymmetry in the scattering of polarized electrons on protons, which is -226.5 ± 9.3 parts per billion (the uncertainty is one standard deviation). Our value for the proton's weak charge is in excellent agreement with the standard model and sets multi-teraelectronvolt-scale constraints on any semi-leptonic parity-violating physics not described within the standard model. Our results show that precision parity-violating measurements enable searches for physics beyond the standard model that can compete with direct searches at high-energy accelerators and, together with astronomical observations, can provide fertile approaches to probing higher mass scales.

A search for nuclear-bound states of the 77 meson has been carried out. Targets of lithium, carbon, oxygen, and aluminum were placed in a /r"^ beam at 800 MeV/c. A predicted T] bound state in ^^O* {Ex « 540 MeV) with a width of « 9 MeV was not observed. A bound state of a size j of the predicted cross section would have been seen in this experiment at a confidence level of 3CT {P> 0.9987).PACS numbers: 25.80.-e, 21.90.+f, 27.20.+n This Letter describes a search for a novel nuclear excitation involving the creation of a bound r] meson in the nuclear medium. The concept is similar in spirit to a number of ideas which have recently been vigorously pursued. Some familiar examples are A hypernuclear states, Z hypernuclear states, antiprotonic nuclear states, and various dibaryon resonances. In each case an attractive particle-nucleus potential is required together with some mechanism to inhibit the decay process, such as strangeness conservation in the case of the A.Several suggestions of the existence of bound states of the 7] meson in a wide range of nuclei have recently been published. ^"^ The suggestions of this novel nuclear excitation are based on bound-state formation through the attractive N-T] channel of the TV* (1535), where A^*(1535) is the {KN) resonance with (/,/'') = (T, y") and mass 1535 MeV/c^. This resonance dominates 77 production near threshold. Bhalerao and Liu'* have shown, by a coupled-channels analysis, that the lowenergy rfN interaction is attractive with a scattering length of 0.28 + 0.202/ fm. The attractive interaction is a consequence of the threshold being below the A^*(1535) resonance.Liu and his collaborators have examined the consequences of this attractive interaction in the formation of a bound-77 state as a function of mass number. Their study indicates that nuclear bound states could exist for mass numbers larger than A^\Q. At low mass numbers, only 5-state bound r/'s are predicted. At larger mass numbers, p and d states could become bound. Both binding energies and widths increase with A. The optimum case, in their analysis, is \^0, formed from the in^.p) reaction on *^0 at a momentum near 740 MeV/c. ^ At an angle near 15°, the momentum transfer is favorable for the transition involving the conversion of a p-shell neutron to an ^-shell ry. For higher mass numbers, the increase in predicted width would make this excitation more difficult to see over the continuum {K^,p) background which is present.An experiment to test these predictions was devised with the positive pion beam available at the low-energy separated beam I at the Brookhaven alternating-gradient synchrotron (AGS), and the Moby Dick spectrometer. The experimental arrangement is virtually identical with that used for the production and measurement of hypernuclei, and it has been described in detail in a number of publications (see Milner et al.^ and related references). The only differences involve the selection of pions, rather