151 research outputs found
Dispersive contributions to cross section ratio in forward regime
Two-photon exchange (TPE) contributions to elastic electron-proton scattering
in the forward regime are considered. The imaginary part of TPE amplitude in
these kinematics is related to the DIS nucleon structure functions. The real
part of the TPE amplitude is obtained from the imaginary part by means of
dispersion relations. We demonstrate that the dispersion integrals for the
relevant elastic -scattering amplitude converge and do not need
subtraction. This allows us to make clean prediction for the real part of the
TPE amplitude at forward angles. We furthermore compare and cross
sections which depends on the real part of TPE amplitude, and predict the
positron cross section to exceed the electron one by a few per cent, with the
difference ranging from 1.4% to 2.8% for electron energies in the range
from 3 to 45 GeV. We furthermore predict that the absolute value of this
asymmetry grows with energy, which makes it promising for experimental tests.Comment: 10 pages, 6 figure
Forward Compton Scattering with weak neutral current: constraints from sum rules
We generalize forward real Compton amplitude to the case of the interference
of the electromagnetic and weak neutral current, formulate a low-energy
theorem, relate the new amplitudes to the interference structure functions and
obtain a new set of sum rules. We address a possible new sum rule that relates
the product of the axial charge and magnetic moment of the nucleon to the 0th
moment of the structure function . We apply the GDH and the finite
energy sum rule for constraining the dispersive -box correction to
the proton's weak charge.Comment: 6 pages, 2 figure
Beam normal spin asymmetry in the quasi-RCS approximation
The two-photon exchange contribution to the single spin asymmetries with the
spin orientation normal to the reaction plane is discussed for elastic
electron-proton scattering in the equivalent photon approximation. In this
case, hadronic part of the two-photon exchange amplitude describes real Compton
scattering (RCS). We show that in the case of the beam normal spin asymmetry,
this approximation selects only the photon helicity flip amplitudes of RCS. At
low energies, we make use of unitarity and estimate the contribution of the
multipoles to the photon helicity flip amplitudes. In the Regge regime,
QRCS approximation allows for a contribution from two pion exchange, and we
provide an estimate of such contributions. We furthermore discuss the
possibility of the quark and gluon GPD's contributions in the QRCS kinematics.Comment: 10 pages, 5 figures, revtex, submitted to Phys. Rev. C; new version:
references adde
Probing Dark Matter with AGN Jets
We study the possibility of detecting a signature of particle dark matter in
the spectrum of gamma-ray photons from active galactic nuclei (AGNs) resulting
from the scattering of high-energy particles in the AGN jet off of dark matter
particles. We consider particle dark matter models in the context of both
supersymmetry and universal extra-dimensions (UED), and we present the complete
lowest-order calculation for processes where a photon is emitted in dark
matter-electron and/or dark matter-proton scattering, where electrons and
protons belong to the AGN jet. We find that the process is dominated by a
resonance whose energy is dictated by the particle spectrum in the dark matter
sector (neutralino and selectron for the case of supersymmetry, Kaluza-Klein
photon and electron for UED). The resulting gamma-ray spectrum exhibits a very
characteristic spectral feature, consisting of a sharp break to a hard
power-law behavior. Although the normalization of the gamma-ray flux depends
strongly on assumptions on both the AGN jet geometry, composition and particle
spectrum as well as on the particle dark matter model and density distribution,
we show that for realistic parameters choices, and for two prominent nearby
AGNs (Centaurus A and M87), the detection of this effect is in principle
possible. Finally, we compare our predictions and results with recent gamma-ray
observations from the Fermi, H.E.S.S. and VERITAS telescopes.Comment: 23 pages, 13 figures, submitte
How strange is pion electroproduction?
We consider pion production in parity-violating electron scattering (PVES) in
the presence of nucleon strangeness in the framework of partial wave analysis
with unitarity. Using the experimental bounds on the strange form factors
obtained in elastic PVES, we study the sensitivity of the parity-violating
asymmetry to strange nucleon form factors. For forward kinematics and electron
energies above 1 GeV, we observe that this sensitivity may reach about 20\% in
the threshold region. With parity-violating asymmetries being as large as tens
p.p.m., this study suggests that threshold pion production in PVES can be used
as a promising way to better constrain strangeness contributions. Using this
model for the neutral current pion production, we update the estimate for the
dispersive -box correction to the weak charge of the proton. In the
kinematics of the Qweak experiment, our new prediction reads Re, an improvement over the
previous uncertainty estimate of . Our new prediction in
the kinematics of the upcoming MESA/P2 experiment reads Re.Comment: 18 pages, 10 figure
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