R. Subedi

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)

Probing Cold Dense Nuclear Matter

The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, in which a proton is knocked out of the nucleus with high-momentum transfer and high missing momentum, show that in carbon-12 the neutron-proton pairs are nearly 20 times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.

Parity-violating electron scattering from 4He and the strange electric form factor of the nucleon.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ^4He at an average scattering angle= 5.7 degrees and a four-momentum transfer Q^2 = 0.091 GeV^2. From these data, for the first time, the strange electric form factor of the nucleon G^s_E can be isolated. The measured asymmetry of A_PV = (6.72 +/- 0.84 (stat) +/- 0.21 (syst) parts per million yields a value of G^s_E = -0.038 +/- 0.042 (stat) +/- 0.010 (syst), consistent with zero.

Precision Measurements of the Nucleon Strange Form Factors at Q**2 ~ 0.1-GeV**2

We report new measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and 4He targets with~6.0 degrees. The 4He result is A_PV = (+6.40 +/- 0.23 (stat) +/- 0.12 (syst)) x10^-6. The hydrogen result is A_PV = (-1.58 +/- 0.12 (stat) +/- 0.04 (syst)) x10^-6. These results significantly improve constraints on the electric and magnetic strange form factors G_E^s and G_M^s. We extract G_E^s = 0.002 +/- 0.014 +/- 0.007 at= 0.077 GeV^2, and G_E^s + 0.09 G_M^s = 0.007 +/- 0.011 +/- 0.006 at= 0.109 GeV^2, providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.

First Determination of the Weak Charge of the Proton

The Q(weak) experiment has measured the parity-violating asymmetry in ep elastic scattering at Q(2)=0.025(GeV/c)(2), employing 145 μA of 89% longitudinally polarized electrons on a 34.4 cm long liquid hydrogen target at Jefferson Lab. The results of the experiments commissioning run, constituting approximately 4% of the data collected in the experiment, are reported here. From these initial results, the measured asymmetry is A(ep)=-279±35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in ep scattering. The small Q(2) of this experiment has made possible the first determination of the weak charge of the proton Q(W)(p) by incorporating earlier parity-violating electron scattering (PVES) data at higher Q(2) to constrain hadronic corrections. The value of Q(W)(p) obtained in this way is Q(W)(p)(PVES)=0.064±0.012, which is in good agreement with the standard model prediction of Q(W)(p)(SM)=0.0710±0.0007. When this result is further combined with the Cs atomic parity violation (APV) measurement, significant constraints on the weak charges of the up and down quarks can also be extracted. That PVES+APV analysis reveals the neutrons weak charge to be Q(W)(n)(PVES+APV)=-0.975±0.010.

Constraints on the nucleon strange form factors at Q2∼0.1 GeV2

We report the most precise measurement to date of a parity-violating asymmetry in elastic electron-proton scattering. The measurement was carried out with a beam energy of 3.03 GeV and a scattering angle=6 degrees, with the result A_PV = -1.14 +/- 0.24 (stat) +/- 0.06 (syst) parts per million. From this we extract, at Q^2 = 0.099 GeV^2, the strange form factor combination G_E^s + 0.080 G_M^s = 0.030 +/- 0.025 (stat) +/- 0.006 (syst) +/- 0.012 (FF) where the first two errors are experimental and the last error is due to the uncertainty in the neutron electromagnetic form factor. This result significantly improves current knowledge of G_E^s and G_M^s at Q^2 ~0.1 GeV^2. A consistent picture emerges when several measurements at about the same Q^2 value are combined: G_E^s is consistent with zero while G_M^s prefers positive values though G_E^s=G_M^s=0 is compatible with the data at 95% C.L.

Constraints on the nucleon strange form factors at Q(2)similar to 0.1 GeV2

We report the most precise measurement to date of a parity-violating asymmetry in elastic electron-proton scattering. The measurement was carried out with a beam energy of 3.03 GeV and a scattering angle=6 degrees, with the result A_PV = -1.14 +/- 0.24 (stat) +/- 0.06 (syst) parts per million. From this we extract, at Q^2 = 0.099 GeV^2, the strange form factor combination G_E^s + 0.080 G_M^s = 0.030 +/- 0.025 (stat) +/- 0.006 (syst) +/- 0.012 (FF) where the first two errors are experimental and the last error is due to the uncertainty in the neutron electromagnetic form factor. This result significantly improves current knowledge of G_E^s and G_M^s at Q^2 ~0.1 GeV^2. A consistent picture emerges when several measurements at about the same Q^2 value are combined: G_E^s is consistent with zero while G_M^s prefers positive values though G_E^s=G_M^s=0 is compatible with the data at 95% C.L.

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.

Low

We present an updated extraction of the proton electromagnetic form factor ratio, {mu}{sub p}G{sub E}/G{sub M}, at low Q{sup 2}. The form factors are sensitive to the spatial distribution of the proton, and precise measurements can be used to constrain models of the proton. An improved selection of the elastic events and reduced background contributions yielded a small systematic reduction in the ratio {mu}{sub p}G{sub E}/G{sub M} compared to the original analysis.

Q^2

We present final results on the photon electroproduction (