294 research outputs found
Halla A JLaba nakon postizanja 12 GeV
An overview is presented of the planned physics program for JLab\u27s Hall A following the 12 GeV upgrade with emphasis on the equipment needed to achieve the desired experimental goals. Results of simulations of sample experiments with anticipated uncertainties are presented.Predstavlja se plan fizičkih istraživanja u Halli A JLaba nakon podizanja energije na 12 GeV, s naglaskom na potrebnu opremu za postizanje željenih eksperimentalnih ciljeva. Izlažu se ishodi simulacija nekih mjerenja s očekivanim pogreškama
Performance of the Two Aerogel Cherenkov Detectors of the JLab Hall A Hadron Spectrometer
We report on the design and commissioning of two silica aerogel Cherenkov
detectors with different refractive indices. In particular, extraordinary
performance in terms of the number of detected photoelectrons was achieved
through an appropriate choice of PMT type and reflector, along with some design
considerations. After four years of operation, the number of detected
photoelectrons was found to be noticeably reduced in both detectors as a result
of contamination, yellowing, of the aerogel material. Along with the details of
the set-up, we illustrate the characteristics of the detectors during different
time periods and the probable causes of the contamination. In particular we
show that the replacement of the contaminated aerogel and parts of the
reflecting material has almost restored the initial performance of the
detectors.Comment: 18 pages, 9 Figures, 4 Tables, 44 Reference
A Search for Sigma^0_5, N^0_5 and Theta^++ Pentaquark States
A high-resolution (sigma_instr. = 1.5 MeV) search for narrow states (Gamma <
10 MeV) with masses of M_x approx 1500-1850 MeV in ep -> e'K^+ X, e'K^- X and
e' pi^+ X electroproduction at small angles and low Q^2 was performed. These
states would be candidate partner states of the reported Theta^+(1540)
pentaquark. No statistically significant signal was observed in any of the
channels at 90% C.L. Upper limits on forward production were determined to be
between 0.7% and 4.2% of the Lambda(1520) production cross section, depending
on the channel and the assumed mass and width of the state.Comment: 5 pages, 5 figures, to appear in Phys. Rev. C, update with responses
to referee suggestion
Search for a new gauge boson in the Experiment (APEX)
We present a search at Jefferson Laboratory for new forces mediated by
sub-GeV vector bosons with weak coupling to electrons. Such a
particle can be produced in electron-nucleus fixed-target scattering and
then decay to an pair, producing a narrow resonance in the QED trident
spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV,
found no evidence for an reaction, and set an upper limit of
. Our findings demonstrate that fixed-target
searches can explore a new, wide, and important range of masses and couplings
for sub-GeV forces.Comment: 5 pages, 5 figures, references adde
Polarization transfer in the d(epol,e' ppol)n reaction up to Q^2=1.61 (GeV/c)^2
The recoil proton polarization was measured in the d(epol,e' ppol)n reaction
in Hall A of the Thomas Jefferson National Accelerator Facility (JLab). The
electron kinematics were centered on the quasielastic peak (x_{Bj}~1) and
included three values of the squared four-momentum transfer, Q^2=0.43, 1.00 and
1.61 (GeV/c)^2. For Q^2=0.43 and 1.61 (GeV/c)^2, the missing momentum, p_m, was
centered at zero while for Q^2=1.00 (GeV/c)^2 two values of p_m were chosen: 0
and 174 MeV/c. At low p_m, the Q^2 dependence of the longitudinal polarization,
P_z', is not well described by a state-of-the-art calculation. Further, at
higher p_m, a 3.5 sigma discrepancy was observed in the transverse
polarization, P_x'. Understanding the origin of these discrepancies is
important in order to confidently extract the neutron electric form factor from
the analogous d(epol,e' npol)p experiment.Comment: 6 pages, 4 figures; updated text, figures and table
Cross Section Measurement of Charged Pion Photoproduction from Hydrogen and Deuterium
We have measured the differential cross section for the gamma n --> pi- p and
gamma p --> pi+ n reactions at center of mass angle of 90 degree in the photon
energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at photon
energies greater than 3.3 GeV exhibit a global scaling behavior for both pi-
and pi+ photoproduction, consistent with the constituent counting rule and the
existing pi+ photoproduction data. Possible oscillations around the scaling
value are suggested by these new data The data show enhancement in the scaled
cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio
of exclusive pi- to pi+ photoproduction at high energy is consistent with the
prediction based on one-hard-gluon-exchange diagrams
Quark-Hadron Duality in Neutron (3He) Spin Structure
We present experimental results of the first high-precision test of
quark-hadron duality in the spin-structure function g_1 of the neutron and
He using a polarized 3He target in the four-momentum-transfer-squared range
from 0.7 to 4.0 (GeV/c)^2. Global duality is observed for the spin-structure
function g_1 down to at least Q^2 = 1.8 (GeV/c)^2 in both targets. We have also
formed the photon-nucleon asymmetry A_1 in the resonance region for 3He and
found no strong Q^2-dependence above 2.2 (GeV/c)^2.Comment: 13 pages, 3 figure
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