E. Long

Search for a New Gauge Boson in Electron-Nucleus Fixed-Target Scattering by the APEX Experiment

S. Abrahamyan,1 Z. Ahmed,2 K. Allada,3 D. Anez,4 T. Averett,5 A. Barbieri,6 K. Bartlett,7 J. Beacham,8 J. Bono,9 J.R. Boyce,10 P. Brindza,10 A. Camsonne,10 K. Cranmer,8 M.M. Dalton,6 C.W. de Jager,10, 6 J. Donaghy,7 R. Essig,11, ∗ C. Field,11 E. Folts,10 A. Gasparian,12 N. Goeckner-Wald,13 J. Gomez,10 M. Graham,11 J.-O. Hansen,10 D.W. Higinbotham,10 T. Holmstrom,14 J. Huang,15 S. Iqbal,16 J. Jaros,11 E. Jensen,5 A. Kelleher,15 M. Khandaker,17, 10 J.J. LeRose,10 R. Lindgren,6 N. Liyanage,6 E. Long,18 J. Mammei,19 P. Markowitz,9 T. Maruyama,11 V. Maxwell,9 S. Mayilyan,1 J. McDonald,11 R. Michaels,10 K. Moffeit,11 V. Nelyubin,6 A. Odian,11 M. Oriunno,11 R. Partridge,11 M. Paolone,20 E. Piasetzky,21 I. Pomerantz,21 Y. Qiang,10 S. Riordan,19 Y. Roblin,10 B. Sawatzky,10 P. Schuster,11, 22, † J. Segal,10 L. Selvy,18 A. Shahinyan,1 R. Subedi,23 V. Sulkosky,15 S. Stepanyan,10 N. Toro,24, 22, ‡ D. Walz,11 B. Wojtsekhowski,10, § and J. Zhang10 Yerevan Physics Institute, Yerevan 375036, Armenia Syracuse University, Syracuse, New York 13244 University of Kentucky, Lexington, Kentucky 40506 Saint Mary’s University, Halifax, NS B3H 3C3, Canada College of William and Mary, Williamsburg, Virginia 23187 University of Virginia, Charlottesville, Virginia 22903 University of New Hampshire, Durham, New Hampshire 03824 New York University, New York, New York 10012 Florida International University, Miami, Florida 33199 Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606 SLAC National Accelerator Laboratory, Menlo Park, California 94025 North Carolina Agricultural and Technical State University, Greensboro, North Carolina 27411 Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Longwood University, Farmville, Virginia 23909 Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 California State University at Los Angeles, Los Angeles, California 90032 Norfolk State University, Norfolk, Virginia 23504 Kent State University, Kent, Ohio 44242 University of Massachusetts, Amherst, Massachusetts 01003 University of South Carolina, Columbia, South Carolina 29225 Tel Aviv University, Tel Aviv, 69978 Israel Perimeter Institute for Theoretical Physics, Waterloo, ON N2L 2Y5, Canada George Washington University, Washington DC 20052 Stanford University, Menlo Park, California 94025 (Dated: February 1, 2013)

High-precision measurement of the proton elastic form factor ratio mu_pG_E/G_M at low Q^2.

Abstract We report a new, high-precision measurement of the proton elastic form factor ratio μ p G E / G M for the four-momentum transfer squared Q 2 = 0.3 – 0.7 ( GeV / c ) 2 . The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio μ p G E / G M from unity observed in previous polarization measurements at high Q 2 continues down to the lowest Q 2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q 2 range. We obtain new extractions of the proton electric and magnetic radii, which are 〈 r E 2 〉 1 / 2 = 0.875 ± 0.010 fm and 〈 r M 2 〉 1 / 2 = 0.867 ± 0.020 fm . The charge radius is consistent with other recent extractions based on the electron–proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.

Measurements of parity-violating asymmetries in electron-deuteron scattering in the nucleon resonance region.

We report on parity-violating asymmetries in the nucleon resonance region measured using inclusive inelastic scattering of 5-6 GeV longitudinally polarized electrons off an unpolarized deuterium target. These results are the first parity-violating asymmetry data in the resonance region beyond the Δ(1232). They provide a verification of quark-hadron duality-the equivalence of the quark- and hadron-based pictures of the nucleon-at the (10-15)% level in this electroweak observable, which is dominated by contributions from the nucleon electroweak γZ interference structure functions. In addition, the results provide constraints on nucleon resonance models relevant for calculating background corrections to elastic parity-violating electron scattering measurements.

Precision Measurement of the Neutron Twist-3 Matrix Element d^n_2: Probing Color Forces

Double-spin asymmetries and absolute cross sections were measured at large Bjorken x  (0.25≤x≤0.90), in both the deep-inelastic and resonance regions, by scattering longitudinally polarized electrons at beam energies of 4.7 and 5.9 GeV from a transversely and longitudinally polarized (3)He target. In this dedicated experiment, the spin structure function g(2)((3)He) was determined with precision at large x, and the neutron twist-3 matrix element d(2)(n) was measured at ⟨Q(2)⟩ of 3.21 and 4.32  GeV(2)/c(2), with an absolute precision of about 10(-5). Our results are found to be in agreement with lattice QCD calculations and resolve the disagreement found with previous data at ⟨Q(2)⟩=5  GeV(2)/c(2). Combining d(2)(n) and a newly extracted twist-4 matrix element f(2)(n), the average neutron color electric and magnetic forces were extracted and found to be of opposite sign and about 30  MeV/fm in magnitude.

Rosenbluth Separation of the π0 Electroproduction Cross Section

We present deeply virtual π^{0} electroproduction cross-section measurements at x_{B}=0.36 and three different Q^{2} values ranging from 1.5 to 2  GeV^{2}, obtained from Jefferson Lab Hall A experiment E07-007. The Rosenbluth technique is used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component and, thus, is far from the asymptotic limit predicted by perturbative quantum chromodynamics. Nonetheless, an indication of a nonzero longitudinal contribution is provided by the measured interference term σ_{LT}. Results are compared with several models based on the leading-twist approach of generalized parton distributions (GPDs). In particular, a fair agreement is obtained with models in which the scattering amplitude includes convolution terms of chiral-odd (transversity) GPDs of the nucleon with the twist-3 pion distribution amplitude. This experiment, together with previous extensive unseparated measurements, provides strong support to the exciting idea that transversity GPDs can be accessed via neutral pion electroproduction in the high-Q^{2} regime.

Precision Measurements of

We have performed precision measurements of the double-spin virtual-photon asymmetry A_1 on the neutron in the deep inelastic scattering regime, using an open-geometry, large-acceptance spectrometer and a longitudinally and transversely polarized ^3He target. Our data cover a wide kinematic range 0.277 ≤ x ≤0.548 at an average Q^2 value of 3.078 (GeV/c)^2, doubling the available high-precision neutron data in this x range. We have combined our results with world data on proton targets to make a leading-order extraction of the ratio of polarized-to-unpolarized parton distribution functions for up quarks and for down quarks in the same kinematic range. Our data are consistent with a previous observation of an A_1^n zero crossing near x=0.5. We find no evidence of a transition to a positive slope in (Δd+Δd)/(d+d) up to x=0.548x=0.548.

A_1^n

Abstract We report a new, high-precision measurement of the proton elastic form factor ratio μ p G E / G M for the four-momentum transfer squared Q 2 = 0.3 – 0.7 ( GeV / c ) 2 . The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio μ p G E / G M from unity observed in previous polarization measurements at high Q 2 continues down to the lowest Q 2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q 2 range. We obtain new extractions of the proton electric and magnetic radii, which are 〈 r E 2 〉 1 / 2 = 0.875 ± 0.010 fm and 〈 r M 2 〉 1 / 2 = 0.867 ± 0.020 fm . The charge radius is consistent with other recent extractions based on the electron–proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.

in the Deep Inelastic Regime

We report on new p(e,e′p)π∘p(e,e′p)π∘ measurements at the Δ+(1232)Δ+(1232) resonance at the low momentum transfer region, where the mesonic cloud dynamics is predicted to be dominant and rapidly changing, offering a test bed for chiral effective field theory calculations. The new data explore the Q2Q2 dependence of the resonant quadrupole amplitudes and for the first time indicate that the Electric and the Coulomb quadrupole amplitudes converge as Q2→0Q2→0. The measurements of the Coulomb quadrupole amplitude have been extended to the lowest momentum transfer ever reached, and suggest that more than half of its magnitude is attributed to the mesonic cloud in this region. The new data disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations that include pion cloud effects, chiral effective field theory and lattice calculations. The measurements indicate that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.

High-precision measurement of the proton elastic form factor ratio μpGE/GM at low Q2

We report on the results of the E06-014 experiment performed at Jefferson Lab in Hall A, where a precision measurement of the twist-3 matrix element d_2 of the neutron (d^n_2) was conducted. The quantity dn_2 represents the average color Lorentz force a struck quark experiences in a deep inelastic electron scattering event off a neutron due to its interaction with the hadronizing remnants. This color force was determined from a linear combination of the third moments of the ^3He spin structure functions, g_1 and g_2, after nuclear corrections had been applied to these moments. The structure functions were obtained from a measurement of the unpolarized cross section and of double-spin asymmetries in the scattering of a longitudinally polarized electron beam from a transversely and a longitudinally polarized ^3He target. The measurement kinematics included two average Q^2 bins of 3.2  GeV^2 and 4.3  GeV^2, and Bjorken-x 0.25≤ x ≤0.90 covering the deep inelastic and resonance regions. We have found that d^n_2 is small and negative for ⟨Q^2⟩=3.2  GeV^2, and even smaller for ⟨Q^2⟩=4.3  GeV^2, consistent with the results of a lattice QCD calculation. The twist-4 matrix element f^n_2 was extracted by combining our measured d^n_2 with the world data on the first moment in x of g^n_1, Γ^n_1. We found f^n_2 to be roughly an order of magnitude larger than dn2. Utilizing the extracted d^n_2 and f^n_2 data, we separated the Lorentz color force into its electric and magnetic components, F^(y,n)_E and F^(y,n)_B, and found them to be equal and opposite in magnitude, in agreement with the predictions from an instanton model but not with those from QCD sum rules. Furthermore, using the measured double-spin asymmetries, we have extracted the virtual photon-nucleon asymmetry on the neutron A^n_1, the structure function ratio g^n_1/F^n_1, and the quark ratios (Δu+Δu)/(u+u) and (Δd+Δd)/(d+d). These results were found to be consistent with deep-inelastic scattering world data and with the prediction of the constituent quark model but at odds with the perturbative quantum chromodynamics predictions at large x.

Electroexcitation of the Δ+(1232) at low momentum transfer

M. Mihovilovič, ∗ G. Jin, E. Long, Y.-W. Zhang, K. Allada, B. Anderson, J. R. M. Annand, T. Averett, W. Boeglin, P. Bradshaw, A. Camsonne, M. Canan, G. D. Cates, C. Chen, J. P. Chen, E. Chudakov, R. De Leo, X. Deng, A. Deltuva, 13 A. Deur, C. Dutta, L. El Fassi, D. Flay, S. Frullani, F. Garibaldi, H. Gao, S. Gilad, R. Gilman, O. Glamazdin, J. Golak, S. Golge, J. Gomez, O. Hansen, D. W. Higinbotham, T. Holmstrom, J. Huang, H. Ibrahim, C. W. de Jager, E. Jensen, X. Jiang, M. Jones, H. Kang, J. Katich, H. P. Khanal, A. Kievsky, P. King, W. Korsch, J. LeRose, R. Lindgren, H.-J. Lu, W. Luo, L. E. Marcucci, P. Markowitz, M. Meziane, R. Michaels, B. Moffit, P. Monaghan, N. Muangma, S. Nanda, B. E. Norum, K. Pan, D. Parno, E. Piasetzky, M. Posik, V. Punjabi, A. J. R. Puckett, X. Qian, Y. Qiang, X. Qui, S. Riordan, A. Saha, † P. U. Sauer, B. Sawatzky, R. Schiavilla, 9 B. Schoenrock, M. Shabestari, A. Shahinyan, S. Širca, 1, ‡ R. Skibiński, J. St. John, R. Subedi, V. Sulkosky, W. A. Tobias, W. Tireman, G. M. Urciuoli, M. Viviani, D. Wang, K. Wang, Y. Wang, J. Watson, B. Wojtsekhowski, H. Wita la, Z. Ye, X. Zhan, Y. Zhang, X. Zheng, B. Zhao, and L. Zhu