7,298 research outputs found
Upper Energy Limit of Heavy Baryon Chiral Perturbation Theory in Neutral Pion Photoproduction
With the availability of the new neutral pion photoproduction from the proton
data from the A2 and CB-TAPS Collaborations at Mainz it is mandatory to revisit
Heavy Baryon Chiral Perturbation Theory (HBChPT) and address the extraction of
the partial waves as well as other issues such as the value of the low-energy
constants, the energy range where the calculation provides a good agreement
with the data and the impact of unitarity. We find that, within the current
experimental status, HBChPT with the fitted LECs gives a good agreement with
the existing neutral pion photoproduction data up to 170 MeV and that
imposing unitarity does not improve this picture. Above this energy the data
call for further improvement in the theory such as the explicit inclusion of
the \Delta (1232). We also find that data and multipoles can be well described
up to 185 MeV with Taylor expansions in the partial waves up to first
order in pion energy.Comment: 6 pages, 5 figures, version to be published in Physics Letters
Amplitude analysis and the nature of the Zc(3900)
The microscopic nature of the XYZ states remains an unsettled topic. We show
how a thorough amplitude analysis of the data can help constraining models of
these states. Specifically, we consider the case of the Zc(3900) peak and
discuss possible scenarios of a QCD state, virtual state, or a kinematical
enhancement. We conclude that current data are not precise enough to
distinguish between these hypotheses, however, the method we propose, when
applied to the forthcoming high-statistics measurements should shed light on
the nature of these exotic enhancements.Comment: 14 pages, 10 figures, 3 tables. Version accepted for publication on
Phys.Lett.
Exclusive tensor meson photoproduction
We study tensor meson photoproduction outside of the resonance region, at
beam energies of few GeVs. We build a model based on Regge theory that includes
the leading vector and axial exchanges. We consider two determinations of the
unknown helicity couplings, and fit to the recent a2 photoproduction data from
CLAS. Both choices give a similar description of the a2 cross section, but
result in different predictions for the parity asymmetries and the f2
photoproduction cross section. We conclude that new measurements of f2
photoproduction in the forward region are needed to pin down the correct
production mechanism. We also extend our predictions to the 8.5 GeV beam
energy, where current experiments are running.Comment: 13 pages, 7 figures, 3 tables. Version accepted for publication on
Phys.Rev.
Analysis of the quadrupole deformation of (1232) within an effective Lagrangian model for pion photoproduction from the nucleon
We present an extraction of the E2/M1 ratio of the (1232) from experimental data applying an effective Lagrangian model. We compare the result obtained with different nucleonic models and we reconcile the experimental results with the Lattice QCD calculations
Structure of Pion Photoproduction Amplitudes
We derive and apply the finite energy sum rules to pion photoproduction. We
evaluate the low energy part of the sum rules using several state-of-the-art
models. We show how the differences in the low energy side of the sum rules
might originate from different quantum number assignments of baryon resonances.
We interpret the observed features in the low energy side of the sum rules with
the expectation from Regge theory. Finally, we present a model, in terms of a
Regge-pole expansion, that matches the sum rules and the high-energy
observables.Comment: 19 pages, 15 figures and 4 table
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