T. Holmstrom

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)

Scaling tests of the cross-section for deeply virtual compton scattering

We present the first measurements of the \vec{e}p->epg cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q^2 dependence (from 1.5 to 2.3 GeV^2) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q^2. The helicity-independent cross section is also measured at Q^2=2.3 GeV^2. We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

Evidence for the decay sigma+ --> pmu+ mu-.

We report the first evidence for the decay Sigma(+)-->pmu(+)mu(-) from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(Sigma(+)-->pmu(+)mu(-))=[8.6(+6.6)(-5.4)(stat)+/-5.5(syst)]x10(-8). The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, Sigma(+)-->pP(0),P0-->mu(+)mu(-) with a P0 mass of 214.3+/-0.5 MeV/c(2) and branching ratio B(Sigma(+)-->pP(0),P0-->mu(+)mu(-))=[3.1(+2.4)(-1.9)(stat)+/-1.5(syst)]x10(-8).

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.

New Precision Limit on the Strange Vector Form Factors of the Proton

The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q2 = 0.624  GeV2 and beam energy E(b) = 3.48  GeV to be A(PV) = -23.80 ± 0.78(stat) ± 0.36(syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G(E)(s) + 0.517G(M)(s) = 0.003 ± 0.010(stat) ± 0.004(syst) ± 0.009(ff), where the third error is due to the limits of precision on the electromagnetic form factors and radiative corrections. With this measurement, the world data on strange contributions to nucleon form factors are seen to be consistent with zero and not more than a few percent of the proton form factors.

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.

Observation of the decay K- ---> pi- mu+ mu- and measurements of the branching ratios for K+- ---> pi+- mu+ mu-

Using data collected with the HyperCP (E871) spectrometer during the 1997 fixed-target run at Fermilab, we report the first observation of the decay K--->pi(-)mu(+)mu(-) and new measurements of the branching ratios for K+/--->pi(+/-)mu(+)mu(-). By combining the branching ratios for the decays K+-->pi(+)mu(+)mu(-) and K--->pi(-)mu(+)mu(-), we measure Gamma(K+/--->pi(+/-)mu(+)mu(-))/Gamma(K+/--->all) = (9.8+/-1.0+/-0.5)x10(-8). The CP asymmetry between the rates of the two decay modes is [Gamma(K+-->pi(+)mu(+)mu(-))-Gamma(K--->pi(-)mu(+)mu(-))]/[Gamma(K+-->pi(+)mu(+)mu(-))+Gamma(K--->pi(-)mu(+)mu(-))] = -0.02+/-0.11+/-0.04.

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 (