4,026 research outputs found
Aksijalni i elektromagnetski nukleonske strukturne funkcije u nefiziÄkom podruÄju
We study the annihilation channel in antiproton-nucleon collisions with production of a single charged or neutral pion and a lepton-antilepton pair. These processes offer a unique possibility to study the nucleon electromagnetic form factors in the unphysical region. The differential cross section in an experimental set-up, where the pion is fully detected, is given with explicit dependence on the relevant nucleon form factors.ProuÄavamo kanal poniĆĄtavanja u sudarima antiprotonânukleon s tvorbom jednog nabijenog ili neutralnog piona i para lepton-antilepton. Ti procesi pruĆŸaju jedinstvenu moguÄnost za prouÄavanje nukleonskih elektromagnetskih strukturnih funkcija u nefiziÄkom podruÄju. Dajemo diferencijalni udarni presjek u mjernoj postavi u kojoj je pion cjelovito detektiran uz izriÄitu ovisnost o dotiÄnim nukleonskim strukturnim funkcijama
Aksijalni i elektromagnetski nukleonske strukturne funkcije u nefiziÄkom podruÄju
We study the annihilation channel in antiproton-nucleon collisions with production of a single charged or neutral pion and a lepton-antilepton pair. These processes offer a unique possibility to study the nucleon electromagnetic form factors in the unphysical region. The differential cross section in an experimental set-up, where the pion is fully detected, is given with explicit dependence on the relevant nucleon form factors.ProuÄavamo kanal poniĆĄtavanja u sudarima antiprotonânukleon s tvorbom jednog nabijenog ili neutralnog piona i para lepton-antilepton. Ti procesi pruĆŸaju jedinstvenu moguÄnost za prouÄavanje nukleonskih elektromagnetskih strukturnih funkcija u nefiziÄkom podruÄju. Dajemo diferencijalni udarni presjek u mjernoj postavi u kojoj je pion cjelovito detektiran uz izriÄitu ovisnost o dotiÄnim nukleonskim strukturnim funkcijama
Study of resonant processes for multi-pion production in annihilation
In frame of a phenomenological approach based on Compton-like Feynman
amplitudes, we study multi-pion production in antiproton nucleon collisions.
The main interest of this reaction is related to the possibility to study the
properties of the presumable atom and of its resonances. For the
case of formation of a scalar or pseudoscalar resonant state, with
numerical results are obtained. The
differential cross section in an experimental set-up where the pions invariant
mass is measured, is given with explicit dependence on the lepton pair and
pions invariant mass.Comment: 9 pages, 4 figure
The metabolic response of pteropods to acidification reflects natural CO2-exposure in oxygen minimum zones
© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 9 (2012): 747-757, doi:10.5194/bg-9-747-2012.Shelled pteropods (Thecosomata) are a group of holoplanktonic mollusks that are believed to be especially sensitive to ocean acidification because their aragonitic shells are highly soluble. Despite this concern, there is very little known about the physiological response of these animals to conditions of elevated carbon dioxide. This study examines the oxygen consumption and ammonia excretion of five pteropod species, collected from tropical regions of the Pacific Ocean, to elevated levels of carbon dioxide (0.10%, 1000 ppm). Our results show that pteropods that naturally migrate into oxygen minimum zones, such as Hyalocylis striata, Clio pyramidata, Cavolinia longirostris and Creseis virgula, were not affected by carbon dioxide at the levels and duration tested. Diacria quadridentata, which does not migrate, responds to high carbon dioxide conditions with reduced oxygen consumption and ammonia excretion. This indicates that the natural chemical environment of individual species may influence their resilience to ocean acidification.Funding of the National Science Foundation (grant
OCE-0526502 to Wishner and Seibel, OCE â 0526545 to Daly, and
OCE â 0851043 to Seibel), the University of Rhode Island, and
the Rhode Island Experimental Program to Stimulate Competitive
Research Fellowship Program
Measurement of Strange Quark Contributions to the Nucleon's Form Factors at Q^2=0.230 (GeV/c)^2
We report on a measurement of the parity-violating asymmetry in the
scattering of longitudinally polarized electrons on unpolarized protons at a
of 0.230 (GeV/c)^2 and a scattering angle of \theta_e = 30^o - 40^o.
Using a large acceptance fast PbF_2 calorimeter with a solid angle of
\Delta\Omega = 0.62 sr the A4 experiment is the first parity violation
experiment to count individual scattering events. The measured asymmetry is
A_{phys} =(-5.44 +- 0.54_{stat} +- 0.27_{\rm sys}) 10^{-6}. The Standard Model
expectation assuming no strangeness contributions to the vector form factors is
. The difference is a direct measurement of the
strangeness contribution to the vector form factors of the proton. The
extracted value is G^s_E + 0.225 G^s_M = 0.039 +- 0.034 or F^s_1 + 0.130 F^s_2
= 0.032 +- 0.028.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Letters on Dec 11, 200
Measurement of the Transverse Beam Spin Asymmetry in Elastic Electron Proton Scattering and the Inelastic Contribution to the Imaginary Part of the Two-Photon Exchange Amplitude
We report on a measurement of the asymmetry in the scattering of transversely
polarized electrons off unpolarized protons, A, at two Q values of
\qsquaredaveragedlow (GeV/c) and \qsquaredaveragedhighII (GeV/c) and a
scattering angle of . The measured transverse
asymmetries are A(Q = \qsquaredaveragedlow (GeV/c)) =
(\experimentalasymmetry alulowcorr \statisticalerrorlow
\combinedsyspolerrorlowalucor) 10 and
A(Q = \qsquaredaveragedhighII (GeV/c)) = (\experimentalasymme
tryaluhighcorr \statisticalerrorhigh
\combinedsyspolerrorhighalucor) 10. The first
errors denotes the statistical error and the second the systematic
uncertainties. A arises from the imaginary part of the two-photon
exchange amplitude and is zero in the one-photon exchange approximation. From
comparison with theoretical estimates of A we conclude that
N-intermediate states give a substantial contribution to the imaginary
part of the two-photon amplitude. The contribution from the ground state proton
to the imaginary part of the two-photon exchange can be neglected. There is no
obvious reason why this should be different for the real part of the two-photon
amplitude, which enters into the radiative corrections for the Rosenbluth
separation measurements of the electric form factor of the proton.Comment: 4 figures, submitted to PRL on Oct.
Evidence for Strange Quark Contributions to the Nucleon's Form Factors at = 0.108 (GeV/c)
We report on a measurement of the parity violating asymmetry in the elastic
scattering of polarized electrons off unpolarized protons with the A4 apparatus
at MAMI in Mainz at a four momentum transfer value of = \Qsquare
(GeV/c) and at a forward electron scattering angle of 30. The measured asymmetry is = (\Aphys
\Deltastat \Deltasyst) 10. The
expectation from the Standard Model assuming no strangeness contribution to the
vector current is A = (\Azero \DeltaAzero) 10. We
have improved the statistical accuracy by a factor of 3 as compared to our
previous measurements at a higher . We have extracted the strangeness
contribution to the electromagnetic form factors from our data to be +
\FakGMs = \GEsGMs \DeltaGEsGMs at = \Qsquare (GeV/c).
As in our previous measurement at higher momentum transfer for + 0.230
, we again find the value for + \FakGMs to be positive,
this time at an improved significance level of 2 .Comment: 4 pages, 3 figure
Weak Hyperon Production in ep Scattering
We study the kinematics and cross section of the scattering process . The cross section is expressed in terms of complex
form factors characterizing the hadron vertices. We estimate the cross section
for small momentum transfer using known experimental information. To first
order in the momentum transfer, we obtain a model independent result for the
photon-exchange part of the cross section, which is completely determined by
the decay width . For the kinematics of
the parity violation experiment at MAMI, this first order result gives rise to
a ratio of
. The
-exchange and interference parts give much smaller contributions due to
the suppression of the flavor changing weak neutral current in the standard
model. Feasibility of the experimental measurement is briefly discussed.Comment: ReVTeX, 9 pages, one figure embedde
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