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[show abstract]
[hide abstract]
ABSTRACT: We apply a sum rule for the forward light-by-light scattering process within
the context of the $\phi^4$ quantum field theory. As a consequence of the sum
rule a stringent causality criterion is presented and the resulting constraints
are studied within a particular resummation of graphs. Such resummation is
demonstrated to be consistent with the sum rule to all orders of perturbation
theory. We furthermore show the appearance of particular non-perturbative
solutions within such approximation to be a necessary requirement of the sum
rule. For a range of values of the coupling constant, these solutions manifest
themselves as a physical bound state and a $K$-matrix pole. For another domain
however, they appear as tachyon solutions, showing the inconsistency of the
approximation in this region.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We calculate the two-photon exchange (TPE) corrections in the region where
the kinematical variables describing the elastic $ep$ scattering are moderately
large momentum scales relative to the soft hadronic scale. For such kinematics
we use the QCD factorization approach formulated in the framework of the
soft-collinear effective theory (SCET). Such technique allows us to develop a
description for the soft-spectator scattering contribution which is found to be
important in the region of moderately large scales.
Together with the hard-spectator contribution we present the complete
factorization formulas for the TPE amplitudes at the leading power and leading
logarithmic accuracy. The momentum region where both photons are hard is
described by only one new nonperturbative SCET form factor. It turns out that
the same form factor also arises for wide-angle Compton scattering which is
also described in the framework of the SCET approach. This allows us to
estimate the soft-spectator contribution associated with the hard photons in a
model independent way.
The main unknown in our description of the TPE contribution is related with
the configuration where one photon is soft. The nonperturbative dynamics in
this case is described by two unknown SCET amplitudes. We use a simple model in
order to estimate their contribution.
The formalism is then applied to a phenomenological analysis of existing data
for the reduced cross section as well as for the transverse and longitudinal
polarization observables.
12/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We study the annihilation process $p\bar{p} \to \pi^0 e^+ e^-$ within a Regge
framework, as a means to provide constraints on timelike nucleon form factors.
We present results for the $e^+e^-$ angular distributions and the differential
cross sections in kinematics which will be accessible by PANDA@FAIR. To check
the consistency of the model, we first test the approach on the process of real
photon production, $\bar{p} p \to \pi^0 \gamma$, where data in the energy range
of 2.911 GeV$\leq\sqrt{s} \leq 3.686$ GeV exist. We find that a Regge pole
model is able to well reproduce the available data. The analysis is then
extended to a timelike virtual photon in the final state.
10/2012;
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H. Fonvieille,
G. Laveissière,
N. Degrande,
S. Jaminion,
C. Jutier,
L. Todor,
R. Di Salvo,
L. Van Hoorebeke,
L. C. Alexa,
B. D. Anderson, [......],
E. Voutier,
J. W. Watson,
L. B. Weinstein,
K. Wijesooriya,
R. Wilson,
B. B. Wojtsekhowski,
D. G. Zainea,
W-M. Zhang,
J. Zhao,
Z.-L. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Virtual Compton scattering (VCS) on the proton has been studied at the Jefferson Laboratory using the exclusive photon electroproduction reaction ep→epγ. This paper gives a detailed account of the analysis which has led to the determination of the structure functions PLL−PTT/ε and PLT and the electric and magnetic generalized polarizabilities (GPs) αE(Q2) and βM(Q2) at values of the four-momentum transfer squared Q2=0.92 and 1.76 GeV2. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured Q2 range and point to their nontrivial behavior.
Phys. Rev. C. 07/2012; 86(1).
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H. Fonvieille,
G. Laveissiere,
N. Degrande,
S. Jaminion,
C. Jutier,
L. Todor,
R. Di Salvo,
L. Van Hoorebeke,
L. C. Alexa,
B. D. Anderson, [......],
E. Voutier,
J. W. Watson,
L. B. Weinstein,
K. Wijesooriya,
R. Wilson,
B. B. Wojtsekhowski,
D. G. Zainea,
W. -M. Zhang,
J. Zhao,
Z. -L. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson
Lab using the exclusive photon electroproduction reaction (e p --> e p gamma).
This paper gives a detailed account of the analysis which has led to the
determination of the structure functions P_LL-P_TT/epsilon and P_LT, and the
electric and magnetic generalized polarizabilities (GPs) alpha_E(Q^2) and
beta_M(Q^2) at values of the four-momentum transfer squared Q^2= 0.92 and 1.76
GeV^2. These data, together with the results of VCS experiments at lower
momenta, help building a coherent picture of the electric and magnetic GPs of
the proton over the full measured Q^2-range, and point to their non-trivial
behavior.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Using the available cross-section and polarization data for elastic electron-proton scattering, we provide an extraction of
the two-photon exchange amplitudes at a common value of four-momentum transfer, around Q
2 = 2.5 GeV^2. This analysis also predicts the e
+
p/e
-
p elastic-scattering cross-section ratio, which will be measured by forthcoming experiments.
European Physical Journal A 05/2012; 47(6):1-5. · 2.19 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We review recent developments in the theoretical investigation of the nucleon polarizabilities. We first report on the static
polarizabilities as measured in real Compton scattering, comparing and interpreting the results from various theoretical approaches.
In a second step, we extend the discussion to the generalized polarizabilities which can be accessed in virtual Compton scattering,
showing how the information encoded in these quantities can provide a spatial interpretation of the induced polarization densities
in the nucleon.
The European Physical Journal Special Topics 04/2012; 198(1):269-285. · 1.56 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present a systematic study of generalized transverse-momentum dependent parton distributions (GTMDs). By taking specific
limits or projections, these GTMDs yield various transverse-momentum dependent and generalized parton distributions, thus
providing a unified framework to simultaneously model different observables. We present such simultaneous modeling by considering
a light-cone wave function overlap representation of the GTMDs. We construct the different quark-quark correlation functions
from the 3-quark Fock components within both the light-front constituent quark model and the chiral quark-soliton model. We
provide a comparison with available data and make predictions for different observables.
KeywordsDeep Inelastic Scattering–Phenomenological Models–QCD
Journal of High Energy Physics 04/2012; 2011(5):1-40. · 5.83 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: This colloquium addresses the issue of the shape of hadrons and in particular
that of the proton. The concept of shape in the microcosm is critically
examined. Special attention is devoted to properly define the meaning of shape
for bound-state systems of near massless quarks. The ideas that lead to the
expectation of non-sphericity in the shape of hadrons, the calculations that
predict it, and the experimental information obtained from recent
high-precision measurements are examined. Particular emphasis is given to the
study of the electromagnetic transition between the nucleon and its first
excited state, the $\Delta$(1232)-resonance. The experimental evidence is is
critically examined and compared with lattice calculations, as well as with
effective-field theories and phenomenological models.
01/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Recent progress on the extraction of electromagnetic properties of nucleon
resonance excitation through pion photo- and electroproduction is reviewed.
Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared
to the analysis of other groups. On this basis, we derive longitudinal and
transverse transition form factors for most of the four-star nucleon
resonances. Furthermore, we discuss how the transition form factors can be used
to obtain empirical transverse charge densities. Contour plots of the thus
derived densities are shown for the Delta, Roper, S11, and D13 nucleon
resonances.
09/2011;
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[show abstract]
[hide abstract]
ABSTRACT: An expansion of the electromagnetic form factors of the nucleon and
Delta(1232) in small momentum transfer and pion mass is performed in a
manifestly-covariant EFT framework consistent with chiral symmetry and
analyticity. We present the expressions for the nucleon and Delta(1232)
electromagnetic form factors, charge radii, and electromagnetic moments in the
framework of SU(2) baryon chiral perturbation theory, with nucleon and
Delta(1232)-isobar degrees of freedom, to next-to-leading order. Motivated by
the results for the proton electric radius obtained from the muonic-hydrogen
atom and electron-scattering process, we extract values for the second
derivative of the electric form factor which is a genuine prediction of the p^3
BChPT. The chiral behavior of radii and moments is studied and compared to that
obtained in the heavy-baryon framework and lattice QCD. The chiral behavior of
Delta(1232)-isobar properties exhibits cusps and singularities at the threshold
of Delta->pi N decay, and their physical significance is discussed.
08/2011;
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D Androić,
D S Armstrong,
J Arvieux,
S L Bailey,
D H Beck,
E J Beise,
J Benesch,
F Benmokhtar,
L Bimbot,
J Birchall, [......],
V Tadevosyan,
W T H van Oers,
M Versteegen,
E Voutier,
W Vulcan,
S P Wells,
S E Williamson,
S A Wood,
B Pasquini, M Vanderhaeghen
[show abstract]
[hide abstract]
ABSTRACT: We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasielastic scattering on the deuteron, at backward angles (lab scattering angle of 108°) for Q² = 0.22 GeV²/c² and 0.63 GeV²/c² at beam energies of 362 and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single-photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (πN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasistatic deuterium approximation, and is also in agreement with theory.
Physical Review Letters 07/2011; 107(2):022501. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We discuss the recent interpretation of quark-distribution functions in the plane transverse to the light-cone direction. Such a mapping is model independent and allows one to build up multidimensional pictures of the hadron and to develop a semi-classical interpretation of the quark dynamics. We comment briefly the results obtained from the form factors of the nucleon. We show that a generalization to a target with arbitrary spin leads to a set of preferred values for the electromagnetic coupling characterizing structureless particles. Finally, we present the Wigner distribution for an unpolarized quark in an unpolarized proton and we propose an interpretation of the observed distortion as due to the orbital angular momentum of the quark.
Journal of Physics Conference Series 05/2011; 295(1):012050.
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[show abstract]
[hide abstract]
ABSTRACT: We review recent developments in the theoretical investigation of the nucleon
polarizabilities. We first report on the static polarizabilities as measured in
real Compton scattering, comparing and interpreting the results from various
theoretical approaches. In a second step, we extend the discussion to the
generalized polarizabilities which can be accessed in virtual Compton
scattering, showing how the information encoded in these quantities can provide
a spatial interpretation of the induced polarization densities in the nucleon.
05/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We discuss the chiral behavior of nucleon and Delta(1232) electromagnetic
properties within the framework of a SU(2) covariant baryon chiral perturbation
theory. Our one-loop calculation is complete to the order p^3 and p^4/Delta
with Delta as the Delta(1232)-nucleon energy gap. We show that the magnetic
moment of a resonance can be defined through the linear energy shift only when
an additional relation between the involved masses and the applied magnetic
field strength is fulfilled. Singularities and cusps in the pion mass
dependence of the Delta(1232) electromagnetic moments reflect a
non-fulfillment. We show results for the pion mass dependence of the nucleon
iso-vector electromagnetic quantities and present results for finite volume
effects on the iso-vector anomalous magnetic moment.
05/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We address electromagnetic properties of quasi-stable baryons in the context
of chiral extrapolations of lattice QCD results. For particles near their decay
threshold we show that the application of a small external magnetic field
changes the particle's energy in a non-analytic way. Conventional
electromagnetic moments are only well-defined when the background field B
satisfies |eB|/(2M_*|M_*-M-m|) where M_* is the mass of the resonance and M, m
the masses of the decay products. An application of this situation is the
chiral extrapolation of Delta(1232)-isobar electromagnetic properties. We
discuss such an extrapolation of the Delta(1232)-isobar magnetic dipole,
electric quadrupole and magnetic octupole moments by a covariant chiral
effective field theory.
04/2011;
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M Meziane,
E J Brash,
R Gilman,
M K Jones,
W Luo,
L Pentchev,
C F Perdrisat,
A J R Puckett,
V Punjabi,
F R Wesselmann, [......], M Vanderhaeghen,
M Veilleux,
B B Wojtsekhowski,
S Wood,
Z Ye,
Y Zanevsky,
X Zhang,
Y Zhang,
X Zheng,
L Zhu
[show abstract]
[hide abstract]
ABSTRACT: Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, G(E)/G(M), obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H(e[over →],e(')p[over →]) reaction for three different beam energies at a Q(2)=2.5 GeV(2), spanning a wide range of the kinematic parameter ε. The ratio R, which equals μ(p)G(E)/G(M) in the Born approximation, is found to be independent of ε at the 1.5% level. The ε dependence of the longitudinal polarization transfer component P(ℓ) shows an enhancement of (2.3±0.6)% relative to the Born approximation at large ε.
Physical Review Letters 04/2011; 106(13):132501. · 7.37 Impact Factor
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The Collaboration,
D. Androić,
D. S. Armstrong,
J. Arvieux,
S L Bailey,
D. H. Beck,
E. J. Beise,
J. Benesch,
F. Benmokhtar,
L. Bimbot, [......],
V. Tadevosyan,
W T H Van Oers,
M. Versteegen,
E. Voutier,
W. Vulcan,
S.P. Wells,
S. E. Williamson,
S. A. Wood,
B. Pasquini, M. Vanderhaeghen
[show abstract]
[hide abstract]
ABSTRACT: We have measured the beam-normal single-spin asymmetries in elastic
scattering of transversely polarized electrons from the proton, and performed
the first measurement in quasi-elastic scattering on the deuteron, at backward
angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63
GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry
arises due to the imaginary part of the interference of the two-photon exchange
amplitude with that of single photon exchange. Results for the proton are
consistent with a model calculation which includes inelastic intermediate
hadronic (piN) states. An estimate of the beam-normal single-spin asymmetry for
the scattering from the neutron is made using a quasi-static deuterium
approximation, and is also in agreement with theory.
03/2011;
-
M. Meziane,
E. J. Brash,
R Gilman,
M K Jones,
W Luo,
L. Pentchev,
C. F. Perdrisat,
A. J. R. Puckett,
V. Punjabi,
F. R. Wesselmann, [......], M. Vanderhaeghen,
M. Veilleux,
B. B. Wojtsekhowski,
S. Wood,
Z Ye,
Y. Zanevsky,
X Zhang,
Y Zhang,
X Zheng,
L Zhu
[show abstract]
[hide abstract]
ABSTRACT: Intensive theoretical and experimental efforts over the past decade have
aimed at explaining the discrepancy between data for the proton electric to
magnetic form factor ratio, $G_{E}/G_{M}$, obtained separately from cross
section and polarization transfer measurements. One possible explanation for
this difference is a two-photon-exchange (TPEX) contribution. In an effort to
search for effects beyond the one-photon-exchange or Born approximation, we
report measurements of polarization transfer observables in the elastic
$H(\vec{e},e'\vec{p})$ reaction for three different beam energies at a fixed
squared momentum transfer $Q^2 = 2.5$ GeV$^2$, spanning a wide range of the
virtual photon polarization parameter, $\epsilon$. From these measured
polarization observables, we have obtained separately the ratio $R$, which
equals $\mu_p G_{E}/G_{M}$ in the Born approximation, and the longitudinal
polarization transfer component $P_\ell$, with statistical and systematic
uncertainties of $\Delta R \approx \pm 0.01 \mbox{(stat)} \pm 0.013
\mbox{(syst)}$ and $\Delta P_\ell/P^{Born}_{\ell} \approx \pm 0.006
\mbox{(stat)}\pm 0.01 \mbox{(syst)}$. The ratio $R$ is found to be independent
of $\epsilon$ at the 1.5% level, while the $\epsilon$ dependence of $P_\ell$
shows an enhancement of $(2.3 \pm 0.6) %$ relative to the Born approximation at
large $\epsilon$.
12/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We discuss the recent interpretation of quark distribution functions in the plane transverse to the light-cone direction. Such a mapping is model independent and allows one to build multidimensional pictures of the hadron and to develop a semi-classical intuition of the quark dynamics. We comment briefly the results obtained from the Form Factors of the nucleon. A generalization to a target with arbitrary spin led to a set of preferred values for the electromagnetic coupling characterizing structureless particles. Generalized polarizabilities can also be interpreted in that frame as the distortion of the charge densities due to an external electromagnetic field. Finally, we present preliminary results for the Generalized Transverse-Momentum dependent Distributions which encode in principle the most complete information about quark distributions. Comment: 8 pages, 2 figures, 1 table; contribution to the proceedings of "The 4th Workshop on Exclusive Reactions at High Momentum Transfer", Jefferson Lab., May 18-21, 2010
08/2010;
