Differential cross sections and spin density matrix elements for gamma p -> phi p from CLAS

TL;DR

This paper reports on differential cross sections and spin density matrix elements for the gamma p to phi p reaction, revealing a localized bump in cross sections and deviations from helicity conservation, based on CLAS data.

Contribution

First measurement of SDME and differential cross sections for gamma p to phi p across a wide energy range using CLAS, highlighting deviations from theoretical expectations.

Findings

Observation of a localized bump in cross sections between 2 and 2.2 GeV.

Confirmation of deviations from t-channel helicity conservation.

Evidence of s-channel helicity conservation breaking.

Abstract

Preliminary differential cross-sections and the $\rho^0_{MM'}$ spin density matrix elements (SDME) for the reaction $\gamma p \to \phi p$ for both charged- ($\phi \to K^+ K^-$) and neutral-mode ($\phi \to K^0_L K^0_S$) topologies obtained from CLAS are presented. Our kinematic coverage is from near production threshold ($\sqrt{s} \sim 1.97$ GeV) to $\sqrt{s} = 2.84$ GeV, with a wide coverage in the production angle. As seen in previous LEPS results, the differential cross-sections show a localized "bump" between $\sqrt{s} \sim 2$ and 2.2 GeV that is not expected from a simple Pomeron exchange picture. Comparisons between the charged- and neutral-mode results and possible effects from the $K^+ \Lambda(1520)$ channel are discussed. Our SDME results confirm the well-known deviations from t-channel helicity conservation (TCHC) for Pomeron exchange, but s-channel helicity conservation (SCHC) is also seen to be broken.