Physics

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The Heavy Photon Search test detector
(2015-01-01) Battaglieri, M. a; Celentano, A. a; Boyarinov, S. b; Griffioen, K. k
The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment's technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e(+)e(-) invariant mass spectrum above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW0(4) crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top bottom. This arrangement provides sensitivity to low mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e(+)e(-) pairs requires the first layer of silicon sensors be placed only 10 cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab. (C) 2014 The Authors. Published by Elsevier B.V.
Single and double spin asymmetries for deeply virtual Compton scattering measured with CLAS and a longitudinally polarized proton target
(2015-01-01) Pisano, S.; Mirazita, M.; Pereira, S. Anefalos; Bosted, P.; Griffioen, K. A.
Single-beam, single-target, and double spin asymmetries for hard exclusive electroproduction of a photon on the proton (e) over right arrow(p) over right arrow. e'p'gamma are presented. The data were taken at Jefferson Lab using the CEBAF large acceptance spectrometer and a longitudinally polarized (NH3)-N-14 target. The three asymmetries were measured in 165 four-dimensional kinematic bins, covering the widest kinematic range ever explored simultaneously for beam and target-polarization observables in the valence quark region. The kinematic dependences of the obtained asymmetries are discussed and compared to the predictions of models of generalized parton distributions. The measurement of three DVCS spin observables at the same kinematic points allows a quasi-model-independent extraction of the imaginary parts of the H and (H) over tilde Compton form factors, which give insight into the electric and axial charge distributions of valence quarks in the proton.
Directed search for gravitational waves from Scorpius X-1 with initial LIGO data
(2015-01-01) Aasi, J.; Abbott, B. P.; Abbott, R.; Mikhailov, E. E.; Romanov, G.; Zhang, M.
We present results of a search for continuously emitted gravitational radiation, directed at the brightest low-mass x-ray binary, Scorpius X-1. Our semicoherent analysis covers 10 days of LIGO S5 data ranging from 50-550 Hz, and performs an incoherent sum of coherent F-statistic power distributed amongst frequency-modulated orbital sidebands. All candidates not removed at the veto stage were found to be consistent with noise at a 1% false alarm rate. We present Bayesian 95% confidence upper limits on gravitational-wave strain amplitude using two different prior distributions: a standard one, with no a priori assumptions about the orientation of Scorpius X-1; and an angle-restricted one, using a prior derived from electromagnetic observations. Median strain upper limits of 1.3 x 10(-24) and 8 x 10(-25) are reported at 150 Hz for the standard and angle-restricted searches respectively. This proof-of-principle analysis was limited to a short observation time by unknown effects of accretion on the intrinsic spin frequency of the neutron star, but improves upon previous upper limits by factors of similar to 1.4 for the standard, and 2.3 for the angle-restricted search at the sensitive region of the detector.
Measurement of muon plus proton final states in nu(mu) interactions on hydrocarbon at < E-nu >=4.2 GeV
(2015-01-01) Walton, T.; Christy, M. E.; Datta, M.; Aliaga, L.; Devan, J.; Nelson, J. K.; Norrick, A.; Zhang, D.
A study of charged-current muon neutrino scattering on hydrocarbon in which the final state includes a muon, at least one proton, and no pions is presented. Although this signature has the topology of neutrino quasielastic scattering from neutrons, the event sample contains contributions from quasielastic and inelastic processes where pions are absorbed in the nucleus. The analysis accepts events with muon production angles up to 70 degrees and proton kinetic energies greater than 110 MeV. The cross section, when based completely on hadronic kinematics, is well described by a relativistic Fermi gas nuclear model including the neutrino event generator modeling for inelastic processes and particle transportation through the nucleus. This is in contrast to the quasielastic cross section based on muon kinematics, which is best described by an extended model that incorporates multinucleon correlations. This measurement guides the formulation of a complete description of neutrino-nucleus interactions that encompasses the hadronic as well as the leptonic aspects of this process.
Measurement of parity-violating asymmetry in electron-deuteron inelastic scattering
(2015-04-01) Wang, D.; Armstrong, D. S.
The parity-violating asymmetries between a longitudinally polarized electron beam and an unpolarized deuterium target have been measured recently. The measurement covered two kinematic points in the deep-inelastic scattering region and five in the nucleon resonance region. We provide here details of the experimental setup, data analysis, and results on all asymmetry measurements including parity-violating electron asymmetries and those of inclusive pion production and beam-normal asymmetries. The parity-violating deep-inelastic asymmetries were used to extract the electron-quark weak effective couplings, and the resonance asymmetries provided the first evidence for quark-hadron duality in electroweak observables. These electron asymmetries and their interpretation were published earlier, but are presented here in more detail.
Locally smeared operator product expansions in scalar field theory
(2015-04-01) Monahan, Christopher J.; Orginos, Kostas
We propose a new locally smeared operator product expansion to decompose nonlocal operators in terms of a basis of smeared operators. The smeared operator product expansion formally connects nonperturbative matrix elements determined numerically using lattice field theory to matrix elements of nonlocal operators in the continuum. These nonperturbative matrix elements do not suffer from power-divergent mixing on the lattice, which significantly complicates calculations of quantities such as the moments of parton distribution functions, provided the smearing scale is kept fixed in the continuum limit. The presence of this smearing scale complicates the connection to the Wilson coefficients of the standard operator product expansion and requires the construction of a suitable formalism. We demonstrate the feasibility of our approach with examples in real scalar field theory.
Measurements of ep -> e 'pi(+)n at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS
(2015-01-01) Arizona State Univ, Tempe
Differential cross sections of the exclusive process ep -> e 'pi(+)n were measured with good precision in the range of the photon virtuality Q(2) = 1.8-4.5 GeV2 and the invariant mass range of the pi(+)n final state W = 1.6-2.0 GeV using the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the n pi(+) center-of-mass system. More than 37 000 cross-section points were measured. The contributions of the isospin I = 1/2 resonances N(1675) 5/2(-), N(1680) 5/2(+), and N(1710) 1/2(+) were extracted at different values of Q(2) using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-t dispersion relations, were employed in the analysis. We observe significant strength of the N(1675)5/2(-) in the A(1/2) amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the N(1680)5/2(+) we observe a slow changeover from the dominance of the A(3/2) amplitude at the real photon point (Q(2) = 0) to a Q(2) where A(1/2) begins to dominate. The scalar amplitude S-1/2 drops rapidly with Q(2) consistent with quark model prediction. For the N(1710)1/2(+) resonance our analysis shows significant strength for the A1/2 amplitude at Q(2) < 2.5 GeV2.
The Q(weak) experimental apparatus
(2015-05-01) Allison, T.; Armstrong, David S.
The Jefferson Lab experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise (e) over right arrowp asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 mu A of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Muller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q(2)=0.025 GeV2 was determined using dedicated low-current (similar to 100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
Precision Measurement of the p(e,e ' p)pi(0) Reaction at Threshold
(2015-05-01) Chirapatpimol, K.; Armstrong, David S.
New results are reported from a measurement of pi(0) electroproduction near threshold using the p(e , e'p)pi(0) reaction. The experiment was designed to determine precisely the energy dependence of s- and p-wave electromagnetic multipoles as a stringent test of the predictions of chiral perturbation theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the. phi*(pi) and. theta*(pi) angles in the p pi(0) center of mass was obtained for invariant energies above threshold from 0.5 up to 15 MeV. The 4-momentum transfer Q(2) coverage ranges from 0.05 to 0.155 (GeV/c)(2) in fine steps. A simple phenomenological analysis of our data shows strong disagreement with p-wave predictions from ChPT for Q(2) > 0.07 (GeV/c)(2), while the s-wave predictions are in reasonable agreement.
Disorder-tuned selection of order in bilayer graphene
(2015-01-01) Zhang, Junhua; Rossi, E.; Nandkishore, Rahul; Nandkishore, Rahul
The nature of the interaction-driven spontaneously broken-symmetry state in charge-neutral bilayer graphene (BLG) has attracted a lot of interest. Theoretical studies predict various ordered states as the candidates for the ground state of BLG in the absence of external fields. Several experiments have been performed by different groups to identify the nature of the collective ground state in BLG. However, so far, there is no consensus: some experiments show evidence that suggests the establishment of a nematic gapless state, while others present results that are more consistent with the establishment of a fully gapped state. Moreover, even among the experiments that appear to see a bulk gap, some of the samples are found to be conducting (suggesting the existence of gapless edge states), while others are insulating. Here we explore the hypothesis that disorder might explain the discrepancy between experiments. We find that the pair-breaking effect due to nonmagnetic short-range disorder varies among the candidate ground states, giving rise to different amounts of suppression of their mean-field transition temperatures. Our results indicate that BLG can undergo a transition between different ordered states as a function of the disorder strength, providing a possible scenario to resolve the discrepancy between experimental observations.
Determination of the beam-spin asymmetry of deuteron photodisintegration in the energy region E-gamma=1.1-2.3 GeV
(2015-01-01) Zachariou, N.; Ilieva, Y.; Baltzell, N. A.; Griffioen, K. A.
The beam-spin asymmetry, Sigma, for the reaction gamma d -> pn has been measured using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility (JLab) for six photon-energy bins, between 1.1 and 2.3 GeV, and proton angles in the center-of-mass frame, theta(c.m.), between 25 degrees and 160 degrees. These are the first measurements of beam-spin asymmetries at theta(c.m.) = 90. for photon-beam energies above 1.6 GeV, and the first measurements for angles other than theta(c.m.) = 90 degrees. The angular and energy dependence of Sigma is expected to aid in the development of QCD-based models to understand the mechanisms of deuteron photodisintegration in the transition region between hadronic and partonic degrees of freedom, where both effective field theories and perturbative QCD cannot make reliable predictions.
Surface plasmon resonance modulation in nanopatterned Au gratings by the insulator-metal transition in vanadium dioxide films
(2015-01-01) Beebe, M.; Wang, L.; Madaras, S. E.; Klopf, J. M.; Li, Z.; Brantley, D.; Heimburger, M.; Lukaszew, R. A.; Wincheski, R. A.; Kittiwatanakul, S.; Lu, J.
Correlated experimental and simulation studies on the modulation of Surface Plasmon Polaritons (SPP) in Au/VO2 bilayers are presented. The modification of the SPP wave vector by the thermallyinduced insulator-to-metal phase transition (IMT) in VO2 was investigated by measuring the optical reflectivity of the sample. Reflectivity changes are observed for VO2 when transitioning between the insulating and metallic states, enabling modulation of the SPP in the Au layer by the thermally induced IMT in the VO2 layer. Since the IMT can also be optically induced using ultrafast laser pulses, we postulate the viability of SPP ultrafast modulation for sensing or control. (C)2015 Optical Society of America
Direct observation of quark-hadron duality in the free neutron F-2 structure function
(2015-01-01) Niculescu, I.; Niculescu, G.; Melnitchouk, W.; Griffioen, K. A.
Using the recently published data from the BONuS(Barely Off-shell Nucleon Structure) experiment at Jefferson Lab, which utilized a spectator tagging technique to extract the inclusive electron-free neutron scattering cross section, we obtain the first direct observation of quark-hadron duality in the neutron F-2 structure function. The data are used to reconstruct the lowest few (N = 2, 4, and 6) moments of F-2 in the three prominent nucleon resonance regions, as well as the moments integrated over the entire resonance region. Comparison with moments computed from global parametrizations of parton distribution functions suggest that quark-hadron duality holds locally for the neutron in the second and third resonance regions down to Q(2) approximate to 1 GeV2, with violations possibly up to 20% observed in the first resonance region.
Excess optical quantum noise in atomic sensors
(2015-01-01) Novikova, Irina; Mikhailov, Eugeniy E.; Xiao, Yanhong; Xiao, Yanhong
Enhanced nonlinear optical response of a coherent atomic medium is the basis for many atomic sensors, and their performance is ultimately limited by the quantum fluctuations of the optical readout. Here we demonstrate that the off-resonant interactions, with the aid of the near-resonant process, can significantly modify the quantum noise of a coherent light field, even when its effect on the mean signal is negligible. The altered quantum optical noise distribution results in excess noise in the measurement quantity. We illustrate this concept by using an atomic magnetometer based on the nonlinear Faraday effect. These results show the existence of previously unnoticed factors in fundamental limitations in atomic magnetometry and could have impact in a wide range of atom-light-based precision measurements.
Development of high-performance alkali-hybrid polarized He-3 targets for electron scattering
(2015-01-01) Singh, Jaideep T.; Dolph, P. A. M.; Tobias, W. A.; Averett, Todd; Kelleher, A.
Background: Polarized He-3 targets have been used as effective polarized neutron targets for electron scattering experiments for over twenty years. Over the last ten years, the effective luminosity of polarized He-3 targets based on spin-exchange optical pumping has increased by over an order of magnitude. This has come about because of improvements in commercially-available lasers and an improved understanding of the physics behind the polarization process. Purpose: We present the development of high-performance polarized He-3 targets for use in electron scattering experiments. Improvements in the performance of polarized He-3 targets, target properties, and operating parameters are documented. Methods: We utilize the technique of alkali-hybrid spin-exchange optical pumping to polarize the He-3 targets. Spectrally narrowed diode lasers used for the optical pumping greatly improved the performance. A simulation of the alkali-hybrid spin-exchange optical pumping process was developed to provide guidance in the design of the targets. Data was collected during the characterization of 24 separate glass target cells, each of which was constructed while preparing for one of four experiments at Jefferson Laboratory in Newport News, Virginia. Results: From the data obtained we made determinations of the so-called X-factors that quantify a temperaturedependent and as-yet poorly understood spin-relaxation mechanism that limits the maximum achievable He-3 polarization to well under 100%. The presence of the X-factor spin-relaxation mechanism was clearly evident in our data. Good agreement between the simulation and the actual target performance was obtained by including details such as off-resonant optical pumping. Included in our results is ameasurement of the K-He-3 spin-exchange rate coefficient k(se)(K) = (7.46 +/- 0.62) x 10(-20) cm(3)/s over the temperature range 503 K to 563 K. Conclusions: In order to achieve high performance under the operating conditions described in this paper, the K to Rb alkali vapor density ratio should be about 5 +/- 2 and the line width of the optical pumping lasers should be no more than 0.3 nm. Our measurements of the X-factors under these conditions seem to indicate the He-3 polarization is limited to approximate to 90%. The simulation results, now benchmarked against experimental data, are useful for the design of future targets. Further work is required to better understand the temperature dependence of the X-factor spin-relaxation mechanism and the limitations of our optical pumping simulation.
Quantum Monte Carlo Calculations in Solids with Downfolded Hamiltonians
(2015-01-01) Ma, Fengjie; Purwanto, Wirawan; Zhang, Shiwei; Krakauer, Henry
We present a combination of a downfolding many-body approach with auxiliary-field quantum Monte Carlo (AFQMC) calculations for extended systems. Many-body calculations operate on a simpler Hamiltonian which retains material-specific properties. The Hamiltonian is systematically improvable and allows one to dial, in principle, between the simplest model and the original Hamiltonian. As a by-product, pseudopotential errors are essentially eliminated using frozen orbitals constructed adaptively from the solid environment. The computational cost of the many-body calculation is dramatically reduced without sacrificing accuracy. Excellent accuracy is achieved for a range of solids, including semiconductors, ionic insulators, and metals. We apply the method to calculate the equation of state of cubic BN under ultrahigh pressure, and determine the spin gap in NiO, a challenging prototypical material with strong electron correlation effects.
Modification of electronic structure in compressively strained vanadium dioxide films
(2015-05-01) Huffman, T. J.; Xu, Peng; Hollingshad, A. J.; Qazilbash, M. M.
Vanadium dioxide (VO2) undergoes a phase transition between an insulating monoclinic phase and a conducting rutile phase. Like other correlated electron systems, the properties of VO2 can be extremely sensitive to small changes in external parameters such as strain. In this paper, we investigate a compressively strained VO2 film grown on (001) quartz substrate in which the phase transition temperature (T-c) has been depressed to 325 K from the bulk value of 340 K. Infrared and optical spectroscopy reveals that the lattice dynamics of this strained film are similar to unstrained VO2. However, some of the electronic interband transitions of the strained VO2 film are significantly shifted in energy from those in unstrained VO2. The lattice dynamics remain largely unchanged while the T-c and some of the electronic interband transitions differ substantially from the bulk values, which highlights the role of electronic correlations in driving this metal-insulator phase transition.
Observation of seasonal variation of atmospheric multiple-muon events in the MINOS Near and Far Detectors
(2015-01-01) Anghel, I.; Goodman, M. C.; Paley, J.; Devan, A. V.; Kordosky, M.; Nelson, J. K.; Radovic, A.; Vahle, P.
We report the first observation of seasonal modulations in the rates of cosmic ray multiple-muon events at two underground sites, the MINOS Near Detector with an overburden of 225 mwe, and the MINOS Far Detector site at 2100 mwe. At the deeper site, multiple-muon events with muons separated by more than 8 m exhibit a seasonal rate that peaks during the summer, similar to that of single-muon events. In contrast and unexpectedly, the rate of multiple-muon events with muons separated by less than 5-8 m, and the rate of multiple-muon events in the smaller, shallower Near Detector, exhibit a seasonal rate modulation that peaks in the winter.
FFLO order in ultra-cold atoms in three-dimensional optical lattices
(2015-01-01) Rosenberg, Peter; Chiesa, Simone; Zhang, Shiwei
We investigate different ground-state phases of attractive spin-imbalanced populations of fermions in three-dimensional optical lattices. Detailed numerical calculations are performed using Hartree-Fock-Bogoliubov theory to determine the ground-state properties systematically for different values of density, spin polarization and interaction strength. We first consider the high density and low polarization regime, in which the effect of the optical lattice is most evident. We then proceed to the low density and high polarization regime where the effects of the underlying lattice are less significant and the system begins to resemble a continuum Fermi gas. We explore the effects of density, polarization and interaction on the character of the phases in each regime and highlight the qualitative differences between the two regimes. In the high-density regime, the order is found to be of Larkin-Ovchinnikov type, linearly oriented with one characteristic wave vector but varying in its direction with the parameters. At lower densities the order parameter develops more structures involving multiple wave vectors.
Quarkonium-nucleus bound states from lattice QCD
(2015-01-01) Beane, S. R.; Chang, E.; Lin, H. -W.; Orginos, K.
Quarkonium-nucleus systems are composed of two interacting hadronic states without common valence quarks, which interact primarily through multigluon exchanges, realizing a color van der Waals force. We present lattice QCD calculations of the interactions of strange and charm quarkonia with light nuclei. Both the strangeonium-nucleus and charmonium-nucleus systems are found to be relatively deeply bound when the masses of the three light quarks are set equal to that of the physical strange quark. Extrapolation of these results to the physical light-quark masses suggests that the binding energy of charmonium to nuclear matter is B-phys(NM) less than or similar to 40 MeV.

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