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M. T. Peña

Publications and source records attributed to M. T. Peña.

48 records · Page 3Linked to original sources
arXiv2013

Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction

Studies of the structure of excited baryons are key to the N* program at Jefferson Lab. Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with QCD-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the non-perturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

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arXiv2001

Relativistic effects and quasipotential equations

We compare the scattering amplitude resulting from the several quasipotential equations for scalar particles. We consider the Blankenbecler-Sugar, Spectator, Thompson, Erkelenz-Holinde and Equal-Time equations, which were solved numerically without decomposition into partial waves. We analyze both negative-energy state components of the propagators and retardation effects. We found that the scattering solutions of the Spectator and the Equal-Time equations are very close to the nonrelativistic solution even at high energies. The overall relativistic effect increases with the energy. The width of the band for the relative uncertainty in the real part of the scattering $T$ matrix, due to different dynamical equations, is largest for backward-scattering angles where it can be as large as 40%.

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arXiv2000

Nonexistence of a $ηNN$ quasibound state

We have solved the Faddeev equations for $ηd$ elastic scattering using realistic separable interactions for the $NN$ and coupled $ηN$-$πN$ subsystems. We found that including explicitly the pion channel in the integral equations drastically reduces the attraction that is present in the system. As a consequence, the existence of a $ηNN$ quasibound state is excluded by the modern $ηN$ amplitude analysis.

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arXiv2000

Multiplicity distributions associated to subthreshold events in heavy-ion collisions

Subthreshold events (pion production, for instance) at energies $E< m_π$ are treated as rare events. The associated multiplicity distribution $P_c (n)$ and the unconstrained distribution P(n), when there is no rare event-trigger, are related by the model independent relation $P_c (n) = {n^2 \over \average{n^2}} P(n)$. This relation, and in particular its improved version inspired in clustering of nucleons, is in fair agreement with data. Moreover, it allows to extract from the multiplicity data information on the number of nucleons involved in meson production processes, occuring at energies below the production threshold in free nucleon collisions.

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arXiv2000

The Reaction $pp\to ppη$ and the eta-Nucleon and Nucleon-Nucleon Interactions

The $π$, $η$ and $ρ$ exchange contributions to the cross-section for the reaction $pp\to ppη$ near threshold are calculated with a phenomenological description of the intermediate S11 (N(1535)) resonance for all exchange mechanisms. The final state interaction in the $pp$ system is described by realistic nucleon-nucleon interaction models. The sensitivity of the results to the phenomenological models for the $η$-nucleon transition amplitude and the nucleon-nucleon interaction is explored. The $η$ exchange mechanism is found to play a dominant role. The off-shell behavior of the $ηN \to ηN$ amplitude within the $η-$exchange amplitude leads to a result significantly different from that obtained with the constant scattering length approximation. The two-nucleon amplitudes that are associated with the short range components of the nucleon-nucleon interaction contribute significantly to the cross section.

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arXiv1999

Nucleon Resonance Effects in $pp\to ppπ^0$ near Threshold

The role of the low lying nucleon resonances beyond the $Δ(1232)$ in the reaction $pp\to ppπ^0$ near threshold is shown to be numerically significant by a calculation, which takes into account the pion re-scattering contribution described by chiral perturbation theory and the short-range mechanisms that are implied by the nucleon-nucleon interaction model. The intermediate N(1440) (P$_{11}$) resonance is excited by the short-range exchange mechanisms, while the N(1535) ($S_{11}$) and N(1520) ($D_{13}$) resonances are excited by $η$ and $ρ$ meson exchange, respectively. The $P_{11}$ increases the calculated cross section, whereas the $S_{11}$ and $D_{13}$ resonances decrease it. The calculation takes full account of the initial and final state interactions.

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arXiv1998

Two-pion exchange and strong form-factors in covariant field theories

In this work improvements to the application of the Gross equation to nuclear systems are tested. In particular we evaluate the two pion exchange diagrams, including the crossed-box diagram, using models developed within the spectator-on-mass-shell covariant formalism. We found that the form factors used in these models induce spurious contributions that violate the unitary cut requirement. We tested then some alternative form-factors in order to preserve the unitarity condition. With this new choice, the difference between the exact and the spectator-on-mass-shell amplitudes is of the order of the one boson scalar exchange, supporting the idea that this difference may be parameterized by this type of terms.

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arXiv1997

Effects of Relativistic Dynamics in $pp \to pp π^0$ near Threshold

The cross-section for threshold $π^0$ production in proton-proton collisions is evaluated in the framework of the covariant spectator description. The negative energy intermediate states are included non-perturbatively and seen to yield a considerably smaller contribution, when compared to perturbative treatments. A family of OBE-models with different off-shell scalar coupling is considered.

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arXiv1996

Two-pion exchange potential and the $πN$ amplitude

We discuss the two-pion exchange potential which emerges from a box diagram with one nucleon (the spectator) restricted to its mass shell, and the other nucleon line replaced by a subtracted, covariant $πN$ scattering amplitude which includes $Δ$, Roper, and $D_{13}$ isobars, as well as contact terms and off-shell (non-pole) dressed nucleon terms. The $πN$ amplitude satisfies chiral symmetry constraints and fits $πN$ data below $\sim$ 700 MeV pion energy. We find that this TPE potential can be well approximated by the exchange of an effective sigma and delta meson, with parameters close to the ones used in one-boson-exchange models that fit $NN$ data below the pion production threshold.

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arXiv1995

Repulsive Short Range Three-Nucleon Interaction

The three nucleon interaction that arises from pion and the effective scalar and vector meson exchange components of the nucleon-nucleon interaction is constructed and shown to be repulsive. Using several wavefunction models we show that this interaction reduces the calculated binding energy of the trinucleons by about 200 keV. The contributions of intermediate $N$(1440) resonances to this three nucleon interaction and of the $D$-state component in the trinucleon model are estimated, and shown to be small.

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arXiv1993

Three-Nucleon Force and the $Δ$-Mechanism for Pion Production and Pion Absorption

The description of the three-nucleon system in terms of nucleon and $Δ$ degrees of freedom is extended to allow for explicit pion production (absorption) from single dynamic $Δ$ de-excitation (excitation) processes. This mechanism yields an energy dependent effective three-body hamiltonean. The Faddeev equations for the trinucleon bound state are solved with a force model that has already been tested in the two-nucleon system above pion-production threshold. The binding energy and other bound state properties are calculated. The contribution to the effective three-nucleon force arising from the pionic degrees of freedom is evaluated. The validity of previous coupled-channel calculations with explicit but stable $Δ$ isobar components in the wavefunction is studied.

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