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H. -W. Hammer

Publications and source records attributed to H. -W. Hammer.

177 records · Page 10Linked to original sources
arXiv1998

Renormalization of the Three-Body System with Short-Range Interactions

We discuss renormalization of the non-relativistic three-body problem with short-range forces. The problem becomes non-perturbative at momenta of the order of the inverse of the two-body scattering length, and an infinite number of graphs must be summed. This summation leads to a cutoff dependence that does not appear in any order in perturbation theory. We argue that this cutoff dependence can be absorbed in a single three-body counterterm and compute the running of the three-body force with the cutoff. We comment on relevance of this result for the effective field theory program in nuclear and molecular physics.

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arXiv1998

Effective Theory for the non-relativistic three-body System

We discuss renormalization of the non-relativistic three-body problem with short-range forces. The problem becomes non-perturbative at momenta of the order of the inverse of the two-body scattering length, and an infinite number of graphs must be summed. This summation leads to a cutoff dependence that does not appear in any order in perturbation theory. We argue that this cutoff dependence can be absorbed in a single three-body counterterm and compute the running of the three-body force with the cutoff.

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arXiv1998

The Three-Boson System with Short-Range Interactions

We discuss renormalization of the non-relativistic three-body problem with short-range forces. The problem is non-perturbative at momenta of the order of the inverse of the two-body scattering length. An infinite number of graphs must be summed, which leads to a cutoff dependence that does not appear in any order in perturbation theory. We argue that this cutoff dependence can be absorbed in one local three-body force counterterm and compute the running of the three-body force with the cutoff. This allows a calculation of the scattering of a particle and the two-particle bound state if the corresponding scattering length is used as input. We also obtain a model-independent relation between binding energy of a shallow three-body bound state and this scattering length. We comment on the power counting that organizes higher-order corrections and on relevance of this result for the effective field theory program in nuclear and molecular physics.

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arXiv1998

Parity-Violating $Δ$(1232) Electroweak Production: Axial Structure and New Physics

We consider prospects for studying the parity-violating electroweak excitation of the $Δ(1232) $ resonance with polarized electron scattering (see nucl-th/9801025). We discuss the experimental feasibility and theoretical interpretability of such a measurement as well as the prospective implications for hadron structure theory. We also analyze the extent to which a PV $N\toΔ$ measurement could constrain various extensions of the Standard Model.

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arXiv1998

K^* Mesons and Nucleon Strangeness

We study contributions to the nucleon strange quark vector current form factors from intermediate states containing K^* mesons. We show how these contributions may be comparable in magnitude to those made by K mesons, using methods complementary to those employed in quark model studies. We also analyze the degree of theoretical uncertainty associated with K^* contributions.

hep-ph↗
arXiv1998

Parity-Violating Excitation of the Δ(1232): Hadron Structure and New Physics

We consider prospects for studying the parity-violating (PV) electroweak excitation of the Δ(1232) resonance with polarized electron scattering. Given present knowledge of Standard Model parameters, such PV experiments could allow a determination of the N -> Δelectroweak helicity amplitudes. We discuss the experimental feasibility and theoretical interpretability of such a determination as well as the prospective implications for hadron structure theory. We also analyze the extent to which a PV N -> Δmeasurement could constrain various extensions of the Standard Model.

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arXiv1998

KN Scattering and the Nucleon Strangeness Radius

The leading non-zero electric moment of the nucleon strange-quark vector current is the mean square strangeness radius, $ $. We evaluate the lightest OZI-allowed contribution to $ $, arising from the kaon cloud, using dispersion relations. Drawing upon unitarity constraints as well as $K^{+}N$ scattering and $e^+e^-\to K\bar{K}$ cross section data, we find the structure of this contribution differs significantly from that suggested by a variety of QCD-inspired model calculations. In particular, we find evidence for a strong $ϕ$-meson resonance which may enhance the scale of kaon cloud contribution to an observable level.

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arXiv1997

Nucleon Vector Strangeness Form Factors: Multi-pion Continuum and the OZI Rule

We estimate the 3 πcontinuum contribution to the nucleon strange quark vector current form factors, including the effect of a 3 π<----> ρπresonance. We find the magnitude of this OZI-violating contribution to be comparable to that of typical OZI-allowed contributions. We also study the isoscalar electromagnetic form factors, and find that the presence of a ρπresonance in the multi-pion continuum may generate an appreciable contribution.

hep-ph↗
arXiv1997

Contributions of strange quarks to the magnetic moment of the proton

Using the Gerasimov-Drell-Hearn sum rule and experimental total cross sections for photoproduction of $η$, $K$ and $ϕ$ mesons on the proton, we obtain upper bounds for the contribution of strange quarks to the anomalous magnetic moment of the proton, $|κ_{s}| \le 0.20 κ_{p}$. The proposed experiments to measure the spin-dependence of the absorption cross section are expected to lower these bounds considerably. The existing data on $η$ production and phenomenological models for $K$ production agree with a negative sign for $κ_{s}$.

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arXiv1996

Nucleon Strangeness and Unitarity

The strange-quark vector current form factors of the nucleon are analyzed within the framework of dispersion relations. Particular attention is paid to contributions made by $K\bar{K}$ intermediate states to the form factor spectral functions. It is shown that, when the $K\bar{K}\to N\bar{N}$ amplitude is evaluated in the Born approximation, the $K\bar{K}$ contributions are identical to those arising from a one-loop calculation and entail a serious violation of unitarity. The mean square strangeness radius and magnetic moment are evaluated by imposing unitarity bounds on the kaon-nucleon partial wave amplitudes. The impact of including the kaon's form factor in the dispersion integrals is also evaluated.

hep-ph↗
arXiv1996

Dispersion-theoretical analysis of the nucleon electromagnetic form factors: Inclusion of time-like data

We update a recent dispersion--theoretical fit to the nucleon electromagnetic form factors by including the existing data in the time--like region. We show that while the time--like data for the proton can be described consistently with the existing world space--like data, this is not the case for the neutron. Another measurement of the process $e^+ e^- \to \bar n n$ is called for. We furthermore sharpen the previous estimate of the separation between the perturbative and the non--perturbative regime, which is characterized by a scale parameter $Λ^2 \simeq 10\,$GeV$^2$.

hep-ph↗
arXiv1995

The Strangeness Radius and Magnetic Moment of the Nucleon Revisited

We update Jaffe's estimate of the strange isoscalar radius and magnetic moment of the nucleon. We make use of a recent dispersion--theoretical fit to the nucleon electromagnetic form factors and an improved description of symmetry breaking in the vector nonet. We find $μ_s = -0.24 \pm 0.03$~n.m. and $r_s^2 = 0.21 \pm 0.03$~fm$^2$. The strange formfactor $F_2^s (t)$ follows a dipole with a cut--off mass of 1.46~GeV, $F_2^s (t)= μ_s (1-t/2.14 \, {\rm GeV}^2 )^{-2}$. These numbers should be considered as upper limits on the strange vector current matrix--elements in the nucleon.

hep-ph↗
arXiv1995

Parity violating pion electroproduction off the nucleon

Parity violating (PV) contributions due to interference between $γ$ and $Z^0$ exchange are calculated for pion electroproduction off the nucleon. A phenomenological model with effective Lagrangians is used to determine the resulting asymmetry for the energy region between threshold and $Δ(1232)$ resonance. The $Δ$ resonance is treated as a Rarita-Schwinger field with phenomenological $N Δ$ transition currents. The background contributions are given by the usual Born terms using the pseudovector $πN$ Lagrangian. Numerical results for the asymmetry are presented.

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