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M. P. Locher

Publications and source records attributed to M. P. Locher.

16 recordsLinked to original sources
arXiv2002

Pion Correlations and Resonance Effects in ppbar Annihilation to 4pi0 at Rest

We study pi0pi0 correlations in the exclusive reaction ppbar -> 4pi0 at rest with complete reconstruction of the kinematics for each event. The inclusive distribution shows a dip at small invariant mass of the pion pair while a small enhancement in the double differential distribution is observed for small invariant masses of both pion pairs. Dynamical models with resonances in the final state are shown to be consistent with the data while the stochastic HBT mechanism is not supported by the present findings.

hep-ex↗
arXiv2000

Higher Order Two Step Mechanisms in Nucleon Antinucleon Annihilation and the OZI Rule

We evaluate three meson doorway mechanisms for nucleon-antinucleon annihilation at rest for the first time. Detailed results are presented for the final state phi pi0 originating from the 3S1 initial state and for the phi rho channel originating from 1S0. The results presented also include the improved contributions from two meson doorway states and from the tree diagrams. For all the channels considered a consistent explanation of large and small OZI violations emerges.

hep-ph↗
arXiv1999

Structure of the Light Scalar Mesons from a Coupled Channel Analysis of the s-wave pi pi - k kbar Scattering

The analysis of the poles of the $ππ$ scattering amplitude is shown to favour a dynamical origin of the sigma meson: it is created by a strongly attractive $ππ$ interaction in the $J^{PC}=0^{++}I=0$ channel. In the limit $N_c\to\infty$, the $σ$ meson disappears, contrary to the case of conventional $q\bar{q}$ states. Implications for the search of the scalar $q\bar{q}$ and glueball states are discussed.

hep-ph↗
arXiv1999

Dispersion relation analysis of the neutral kaon regeneration amplitude in carbon

We apply a forward dispersion relation to the regeneration amplitude for kaon scattering on 12C using all available data. The CPLEAR data at low energies allow the determination of the net contribution from the subthreshold region which turns out to be much smaller than earlier evaluations, solving a long standing puzzle.

hep-ex↗
arXiv1998

Structure of f0(980) from a Coupled Channel Analysis of S-wave pi pi Scattering

A coupled channel model is used to study the nature of the f0(980) resonance. It is shown that the existence of two poles close to KKbar threshold, as found in many data fits and confirmed here, can be reconciled with the KKbar molecular origin of the f0(980). Due to a strong coupling between channels the number of the S-matrix poles close to the physical region in the physical case exceeds the number of bare states introduced in the model.

hep-ph↗
arXiv1996

Nucleon Form Factors in a Relativistic Three-Quark Model

We report the calculation of electromagnetic form factors of nucleons within a relativistic three-quark model with Gaussian shape for the nucleon-quark vertex. The allowed region for two adjustable parameters, the range parameter $Λ_N$ in the Gaussian and the constituent quark mass $m_q$, is obtained from fitting the data for magnetic moments and electromagnetic radii of nucleons. It is found that their values calculated with $m_q$=420 MeV and $Λ_N$=1.25 GeV agree very well with the experimental data. It is turned out that the electric proton and magnetic nucleon form factors fall faster than the dipole fit of the experimental data for momentum transfers $0\le Q^2\le 1$ GeV$^2$.

hep-ph↗
arXiv1996

Pion Absorption Cross Section for ^2H and ^3He in the Delta-Isobar Region: A Phenomenological Connection

The absorption of π^+ on ^3He in the $Δ$-region is evaluated with exact inclusion of the final state interaction among the three emerging protons. The absorption is described by a $πN \to Δ$ vertex and a $NΔ- NN$ transition t-matrix which are calculated from a phenomenological model for NN and πd reactions. In a calculation where the initial pion scattering effects are neglected, the predicted peaks of the pion absorption cross sections for ^2H and ^3He lie too high in energy in relation to the data. The effect of the final state three-nucleon interaction turns out to be too small for changing the magnitude and shifting the peak position of the total absorption cross section for ^3He. We demonstrate that the adjustment of the peak position for the deuteron cross section by small modifications of the $Δ$-parameters, automatically leads to the correct peak position in ^3He.

nucl-th↗
arXiv1996

The $ππ$ Final State Interaction in $K\toππ$, $pp\to ppππ$ and Related Processes

Final state interactions in the $S$--wave $ππ$ system (I=0,2) are re-examined on the basis of the Omnès-Mus\-khe\-li\-shvili equation and the coupled channel formalism. The contributions to the pion scalar form factor from $ρ$ and $f_2(1270)$ exchange in the $t$--channel and from the $f_0(980)$ $s$--channel resonance are separately evaluated and the role of the nontrivial polynomial in the Omnès function in a coupled channel situation is elucidated. Applications are made to $K\to ππ$ and $pp\to ppππ$. It is found that the contribution from the $f_0$ resonance to the form-factor is strongly reduced by a nearby zero.

hep-ph↗
arXiv1996

Electromagnetic form factors of nucleons in a relativistic three-quark model

We report the calculation of electromagnetic form factors of nucleons within a relativistic three-quark model with Gaussian shape for the nucleon-quark vertex. The allowed region for two adjustable parameters, the range parameter $Λ_N$ in the Gaussian and the constituent quark mass $m_q$, is obtained from fitting the data for magnetic moments and electromagnetic radii of nucleons. The shapes of electric and magnetic nucleon form factors calculated in this approach with the central parameters $m_q$=420 MeV and $Λ_N$=1.25 GeV are close to the data for momentum transfers $0\le Q^2\le 1$ GeV$^2$

hep-ph↗
arXiv1995

On the relationship of the scaled phase space and Skyrme-coherent state treatments of proton antiproton annihilation at rest

We discuss pion multiplicities and single pion momentum spectra from proton antiproton annihilation at rest. Both the scaled phase space model and the Skyrme-coherent state approach describe these observables well. In the coherent state approach the puzzling size of the scale parameter relating the phase space integrals for different multiplicities is replaced by a well defined weight function. The strength of this function is determined by the intensity of the classical pion field and its spatial extent is of order 1 fm.

nucl-th↗
arXiv1994

Coherent state formulation of pion radiation from nucleon antinucleon annihilation

We assume that nucleon antinucleon annihilation is a fast process leading to a classical coherent pion pulse. We develop the quantum description of such pion waves based on the method of coherent states. We study the consequences of such a description for averages of charge types and moments of distributions of pion momenta with iso-spin and four-momentum conservation taken into account. We briefly discuss the applicability of our method to annihilation at rest, where we find agreement with experiment, and suggest other avenues for its use.

hep-ph↗
arXiv1993

Rates for the reactions antiproton-proton --> pi phi and gamma phi

We study antiproton-proton annihilation at rest into $πϕ$ and $γϕ$. Rescattering by $\overline{K^*}K+K^*\overline{K}$ and $ρ^{+}ρ^{-}$ for $\overline{p}p\rightarrowπϕ$ states is sizable, of order $(0.90\, {\rm to}\,2.6)\times 10^{-4}$ in the branching ratio, but smaller than experiment. For $\overline{p}p\rightarrowγϕ$ the rescattering contributions are negligible, but the $γϕ$ channel is well explained by a $ρϕ$ intermediate state combined with vector meson dominance.

nucl-th↗
arXiv1993

Coherent Pion Radiation From Nucleon Antinucleon Annihilation

A unified picture of nucleon antinucleon annihilation into pions emerges from a classical description of the pion wave produced in annihilation and the subsequent quantization of that wave as a coherent state. When the constraints of energy-momentum and iso-spin conservation are imposed on the coherent state, the pion number distribution and charge ratios are found to be in excellent agreement with experiment.

nucl-th↗