Baryon anti-Baryon Photoproduction Cross Sections off the Proton

TL;DR

This work presents the first wide-range photoproduction study of baryon–anti-baryon pairs off a proton target, using GlueX data up to $E_\gamma=11.6$ GeV across $p\bar{p}$, $\Lambda\bar{\Lambda}$, and $p\bar{\Lambda}$ channels. A phenomenological model with forward Regge-like single-$t$-channel exchange plus a double-$t$-channel component reproduces most kinematic distributions, including forward baryons and broad anti-baryon emissions, and yields total and differential cross sections that reveal a near-threshold mass-clustering effect with $c_m \approx 0.21$ GeV. The results show similar total cross sections for strange and non-strange channels and a strangeness suppression factor around $0.23$, consistent with quark-pair-production expectations in flux-tube models. The observed anti-baryon angular broadness, not mirrored by baryons, motivates interpretations involving double-exchange dynamics or quark-knockout mechanisms, and the study provides benchmarks for future theory and higher-energy measurements in hadronization dynamics.

Abstract

The GlueX experiment at Jefferson Lab has observed $p\bar{p}$ and, for the first time, $Λ\barΛ$ and $p\barΛ$ photoproduction from a proton target at photon energies up to 11.6 GeV. The angular distributions are forward peaked for all produced pairs, consistent with Regge-like $t$-channel exchange. Asymmetric wide-angle anti-baryon distributions show the presence of additional processes. In a phenomenological model, we find consistency with a double $t$-channel exchange process where anti-baryons are created only at the middle vertex. The model matches all observed distributions with a small number of free parameters. In the hyperon channels, we observe a clear distinction between photoproduction of the $Λ\barΛ$ and $p\barΛ$ systems but general similarity to the $p\bar{p}$ system. We report both total cross sections and cross sections differential with respect to momentum transfer and the invariant masses of the created particle pairs. No narrow resonant structures were found in these reaction channels. The suppression of $s\bar{s}$ quark pairs relative to $d\bar{d}$ quark pairs is similar to what has been seen in other reactions.

Figures (30)

• Figure 1: Possible production mechanisms for $\gamma p \rightarrow \{ \overline{p}p \} p$ in (a, b); $\gamma p \rightarrow \{ \overline{\Lambda}\Lambda \} p$ in (c, d); $\gamma p \rightarrow \{p \overline{\Lambda} \} \Lambda$ in (e, f). These involve either single Regge-like exchange (Panels a, c, e) or double Regge-like exchange (Panels b, d, f). Note the different exchange particles available in the non-strange (a, c) and strange (e) single-exchange cases.
• Figure 2: Missing-mass squared for kinematic fits to the $\gamma p \rightarrow p\bar{p} p$ reaction. The initial skimmed data (blue) are compared with signal Monte Carlo (red) with (a) minimal cuts applied, and (b) final cuts applied, except for the selection on the missing-mass squared itself (marked by black solid lines).
• Figure 3: The correlation between the hyperon decay path-length significance ($PLS$) and the invariant mass distributions for (a) $\Lambda$ and (b) $\bar{\Lambda}$ particles. The masses were unconstrained in the $\gamma p \rightarrow \hbox{$\Lambda\hbox{$\overline{\Lambda}$}$} p$ kinematic fits.
• Figure 4: Top row: data (blue) is compared to signal Monte Carlo simulation (red). (a) Invariant mass of the reconstructed $\Lambda$, (b) Invariant mass of the reconstructed $\bar{\Lambda}$, (c) the correlation of two hyperon masses from data, (d) the correlation of two hyperon masses from MC simulation. The distributions are shown after all selections except for the selection on the invariant mass itself (denoted by the black solid lines).
• Figure 5: Diagrams of the (a) single $t$-channel exchange and (b) double $t$-channel exchange model formulated for the reaction $\gamma + p \rightarrow 1+2 +3$. Particle 2 is the anti-baryon in each case.