127 research outputs found
Recent Results from Jefferson Lab
Recent results on studies of the structure of nucleons and nuclei in the
regime of strong interaction QCD are discussed. Use of high current polarized
electron beams, polarized targets, and recoil polarimeters, in conjunction with
modern spectrometers and detector instrumentation allow much more detailed
studies of nucleon and nuclear structure than has been possible in the past.
The CEBAF accelerator at Jefferson Lab was build to study the internal
structure of hadrons in a regime where confinement is important and strong
interaction QCD is the relevant theory. I discuss how the first experiments
already make significant contributions towards an improved understanding of
hadronic structure.Comment: Lecture presented at the International School of Nuclear Physics,
Erice, Sicily, Italy, September 17 - 25, 199
Status of the N* Program at Jefferson Lab
Recent results from JLab on the electromagnetic excitation of nucleon
resonances are presented, and confronted with theoretical predictions.
Preliminary data in the search for undiscovered states are discussed as well.Comment: 7 pages, 11 figures, talk presented at Electron-Nucleus Scattering
VII, Elba, June 24-28,2002, added reference in section 2 and section 4.1,
corrected misleading typographical error in section 4.
The Jlab Upgrade - Studies of the Nucleon with CLAS12
An overview is presented on the program to study the nucleon structure at the
12 GeV JLab upgrade using the CLAS12 detector. The focus is on deeply virtual
exclusive processes to access the generalized parton distributions,
semni-inclusive processes to study transverse momentum dependent distribution
functions, and inclusive spin structure functions and resonance transition form
factors at high Q^2 and with high precision.Comment: 7 pages, 12 figures, NSTAR 2007 conference, Bonn, September 5-8, 200
Gravitational Approach to Tachyon Matter
We found a gravity solution of p+1 dimensional extended object with
SO(p)xSO(9-p) symmetry which has zero pressure and zero dilaton charge. We
expect that this object is a residual tachyon dust after tachyon condensation
of brane and anti-brane system discussed by Sen, recently. We also discuss the
Hawking temperature and some properties of this object.Comment: 14 pages, LaTeX, reference added and typos correcte
Can black holes have Euclidean cores?
The search for regular black hole solutions in classical gravity leads us to
consider a core of Euclidean signature in the interior of a black hole.
Solutions of Lorentzian and Euclidean general relativity match in such a way
that energy densities and pressures of an isotropic perfect fluid form are
everywhere finite and continuous. Although the weak energy condition cannot be
satisfied for these solutions in general relativity, it can be when higher
derivative terms are added. A numerical study shows how the transition becomes
smoother in theories with more derivatives. As an alternative to the Euclidean
core, we also discuss a closely related time dependent orbifold construction
with a smooth space-like boundary inside the horizon.Comment: 14 pages with figures, version to appear in PR
Statistique mensuelle de la viande. 1968 N° 4 APRIL-AVRIL = Monthly statistiques of meat. 1968 No. 4 April
In high energy experiments such as active beam dump searches for rare decays and missing energy events, the beam purity is a crucial parameter. In this paper we present a technique to reject heavy charged particle contamination in the 100 GeV electron beam of the H4 beam line at CERN SPS. The method is based on the detection with BGO scintillators of the synchrotron radiation emitted by the electrons passing through a bending dipole magnet. A 100 GeV pi- beam is used to test the method in the NA64 experiment resulting in a suppression factor of 10−5 while the efficiency for electron detection is 95%. The spectra and the rejection factors are in very good agreement with the Monte Carlo simulation. The reported suppression factors are significantly better than previously achieved.ISSN:0168-9002ISSN:1872-957
Electroexcitation of nucleon resonances
We review recent progress in the investigation of the electroexcitation of
nucleon resonances, both in experiment and in theory. The most accurate results
have been obtained for the electroexcitation amplitudes of the four lowest
excited states, which have been measured in a range of Q2 up to 8 and 4.5 GeV2
for the Delta(1232)P33, N(1535)S11 and N(1440)P11, N(1520)D13}, respectively.
These results have been confronted with calculations based on lattice QCD,
large-Nc relations, perturbative QCD (pQCD), and QCD-inspired models. The
amplitudes for the Delta(1232) indicate large pion-cloud contributions at low
Q2 and don't show any sign of approaching the pQCD regime for Q2<7 GeV2.
Measured for the first time, the electroexcitation amplitudes of the Roper
resonance, N(1440)P11, provide strong evidence for this state as a
predominantly radial excitation of a three-quark (3q) ground state, with
additional non-3-quark contributions needed to describe the low Q2 behavior of
the amplitudes. The longitudinal transition amplitude for the N(1535)S11 was
determined and has become a challenge for quark models. Explanations may
require large meson-cloud contributions or alternative representations of this
state. The N(1520)D13 clearly shows the rapid changeover from helicity-3/2
dominance at the real photon point to helicity-1/2 dominance at Q2 > 0.5 GeV2,
confirming a long-standing prediction of the constituent quark model. The
interpretation of the moments of resonance transition form factors in terms of
transition transverse charge distributions in infinite momentum frame is
presented.Comment: 70 pages, 46 figures, will appear in Progress in Particle and Nuclear
Physics, v.67, p.1, 201
Spin asymmetry A_1^d and the spin-dependent structure function g_1^d of the deuteron at low values of x and Q^2
We present a precise measurement of the deuteron longitudinal spin asymmetry
A_1^d and of the deuteron spin-dependent structure function g_1^d at Q^2 < 1
GeV^2 and 4*10^-5 < x < 2.5*10^-2 based on the data collected by the COMPASS
experiment at CERN during the years 2002 and 2003. The statistical precision is
tenfold better than that of the previous measurement in this region. The
measured A_1^d and g_1^d are found to be consistent with zero in the whole
range of x.Comment: 17 pages, 10 figure
Interferometry of Direct Photons in Central 280Pb+208Pb Collisions at 158A GeV
Two-particle correlations of direct photons were measured in central
208Pb+208Pb collisions at 158 AGeV. The invariant interferometric radii were
extracted for 100<K_T<300 MeV/c and compared to radii extracted from charged
pion correlations. The yield of soft direct photons, K_T<300 MeV/c, was
extracted from the correlation strength and compared to theoretical
calculations.Comment: 5 pages, 4 figure
Gluon polarization in the nucleon from quasi-real photoproduction of high-pT hadron pairs
We present a determination of the gluon polarization Delta G/G in the
nucleon, based on the helicity asymmetry of quasi-real photoproduction events,
Q^2<1(GeV/c)^2, with a pair of large transverse-momentum hadrons in the final
state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV
polarized muon beam scattered on a polarized 6-LiD target. The helicity
asymmetry for the selected events is = 0.002 +- 0.019(stat.) +-
0.003(syst.). From this value, we obtain in a leading-order QCD analysis Delta
G/G=0.024 +- 0.089(stat.) +- 0.057(syst.) at x_g = 0.095 and mu^2 =~ 3
(GeV}/c)^2.Comment: 10 pages, 3 figure
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