Isotensor Component Research Articles

Analysis of Negative-Pion Photoproduction Near theP33Resonance: Test of theΔI<~1Rule and Time-Reversal Invariance

Angular distribution for the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\gamma}n$ are analyzed. Existing sets of multipoles do not agree with the data. An acceptable fit is obtained with the Sanda-Shaw model of an electromagnetic current with an isotensor component of $\ensuremath{-}0.1>x>\ensuremath{-}0.4$. The model of Christ and Lee is used to estimate the magnitude of a possible $T$-invariance violation implied by the difference between the data at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}=1245$ MeV and the inverse reaction deduced from $\ensuremath{\gamma}d$ work. A strong violation in the isovector or isotensor amplitude is excluded, but a maximum violation in the isoscalar amplitude is possible.

Possible tests for the presence of an isotensor current

The possibility that the electromagnetic current has an isotensor component has never been excluded. To be sure, if an I ~ 2 electromagnetic current existed, it would have rather peculiar properties provided one wants to maintain the usual structure ot the charge operator. (In particular, the elastic electric isotensor form factors must vanish at zero momentum transfer.) However unattract ive the existence of isotensor currents may be from the theoretical point of view, the question ultimatel~ has to be answered experimentally. In recent works, BLIN-STOYLE analysed possible isotensor electromagnetic effects in nuclear physics (1) with the conclusion that an isotensor admixture-i f present-is at most of the order of ~ 20~o; while SANDA and SHAW claim from their analysis of pion photoproduetion off nucleons (2) that there is a substantial isotensor admixture present (of the order of 20 ~o). While the results of ref. (1.2) are not inconsistent with each other, in our opinion, they are not quite conclusive. In nuclear physics on the one hand, various effects tend to suppress an isotensor contribution, mainly due to the fact that nucleons in a nucleus are almost on their mass shells. On the other hand, the quanti tat ive analysis of Sands and Shaw is based on some dynamical calculations which may not be entirely reliable. In view of the importance of the problem, in this note we want to suggest some further--essential ly quali tat ive--tests to decide whether the electromagnetic current has an isotensor component. Specifically, we propose that double pion photoproduction and electron-positron annihilation into 7:+re-T: ° can provide adequate tests. Assume that an isotensor current exists. Our tests depend crucially on its charge parity. If the isotensor current had C = ~1 (abnormal charge parity), its presence could be perhaps best tested in the processes ~o__> ~+7:-~0 or ~o~ r:+7:-y. Such tests have been performed by a number of groups and the (( Rosenfeld Tables ~> (3) give a small world average ((1--2)% with a large error) for the asymmetry parameter. (The a n a -

Low-Energy Nuclear Physics Test for a Possible Isotensor Component of the Electromagnetic Interaction

It is shown how a comparison of the $\ensuremath{\gamma}$ widths for corresponding $T=\frac{3}{2}\ensuremath{\rightarrow}T=\frac{1}{2}$ transitions in mirror nuclei can give information about a possible isotensor component of the electromagnetic interaction. From data available at present it is concluded that the isotensor $\ensuremath{\gamma}$ transition amplitude in the decays considered is less than about 10% of the isovector amplitude.

Determination of the Isotensor Electromagnetic Current from Pion Photoproduction

Some consequences of an isotensor component of the electromagnetic current are examined in simple pion photoproduction. An experiment to detect the isotensor contribution in pion photoproduction from deuterium (already suggested by several authors) is reformulated to improve the sensitivity of the test. To estimate how accurately the experiment should be performed, upper bounds on the strength of the isotensor contribution are obtained from our experimental and theoretical knowledge of pion photoproduction from the nucleon. This limit turns out to be much smaller than previously stated in the literature.