Pentaquark $\Theta^+$ search at HERMES

TL;DR

This study re-examines the HERMES experiment data for narrow baryon states, finding no significant evidence for the pentaquark $ heta^+$ in the updated analysis, especially in hydrogen targets.

Contribution

The paper extends previous HERMES searches with improved techniques and larger data sets, providing a more sensitive analysis that challenges earlier potential signals of the $ heta^+$ pentaquark.

Findings

No significant $ heta^+$ signal in hydrogen data.

Reduced significance of earlier observed structure at 1522 MeV.

Increased event sample size and improved particle identification.

Abstract

The earlier search at HERMES for narrow baryon states excited in quasi-real photoproduction, decaying through the channel $pK_S^0\rightarrow p\pi^+\pi^-$, has been extended with improved decay-particle reconstruction, more advanced particle identification, and increased event samples. The structure observed earlier at an invariant mass of 1528 MeV shifts to 1522 MeV and the statistical significance drops to about 2$\sigma$ for data taken with a deuterium target. The number of events above background is $68_{-31}^{+98}\text{(stat)}\pm13\text{(sys)}$. No such structure is observed in the hydrogen data set.