Baryon Number Violation and String Topologies

TL;DR

This work develops a comprehensive framework for studying baryon-number-violating (BNV) decays in supersymmetry with broken $R$-parity, focusing on the nonperturbative hadronization stage where a colour junction carries baryon number. It implements BNV decays within Pythia and extends the Lund string model to include Y-shaped junction topologies, providing detailed prescriptions for junction motion and hadronization and contrasting results with the Herwig generator. The paper presents extensive model tests and semi-realistic experimental studies, including jet-topology analyses and dedicated strategies to search for the junction-associated baryon in collider data. It argues that the distinctive low-momentum junction baryon offers a viable observable to establish baryon-number violation experimentally, and discusses broader implications for QCD string topology and detector performance.

Abstract

In supersymmetric scenarios with broken R-parity, baryon number violating sparticle decays become possible. In order to search for such decays, a good understanding of expected event properties is essential. We here develop a complete framework that allows detailed studies. Special attention is given to the hadronization phase, wherein the baryon number violating vertex is associated with the appearance of a junction in the colour confinement field. This allows us to tell where to look for the extra (anti)baryon directly associated with the baryon number violating decay.

Figures (26)

• Figure 1: The breakup of an original $\mathrm{q} \overline{\mathrm{q}}$ system into a set of mesons, each represented by a yo-yo state. For simplicity quarks are here assumed massless, and so move along lightcones. The broken horizontal lines illustrate the string pieces at a few discrete times.
• Figure 2: The string motion in a $\mathrm{q} \overline{\mathrm{q}} \mathrm{g}$ system, neglecting hadronization. The $\mathrm{q}$, $\overline{\mathrm{q}}$ and $\mathrm{g}$ move out from the common origin, all with the speed of light and along straight lines, in the limit that quark masses are neglected. The connecting dashed lines illustrate the string pieces at two times. A possible colour assignment is indicated within brackets.
• Figure 3: The string motion in a junction system, neglecting hadronization. The $\mathrm{u}_i$, $\mathrm{d}_j$ and $\mathrm{d}_k$ move out from the common origin, all with the speed of light and along straight lines, in the limit that quark masses are neglected. The thin arrow indicates the resulting motion of the junction J. The connecting dashed lines illustrate the Y-shaped string topology at two discrete times. A possible colour assignment is indicated within brackets.
• Figure 4: Hadronization by $\mathrm{q}'\overline{\mathrm{q}}'$ production in a junction string topology. The figure is in an abstract "flavour space", to be related to a space--time topology like Fig. \ref{['fig:Ytopology']}, with flavours belonging to the same hadron connected by string pieces. The labelling of new quarks is arbitrary. The $\mathrm{q}_4\mathrm{q}_7\mathrm{q}_9$ hadron carries the original baryon number, while $\mathrm{q}_1\mathrm{q}\mathrm{q}_2$ and $\overline{\mathrm{q}}\overline{\mathrm{q}}_2\overline{\mathrm{q}}_3$ correspond to the possibility of baryon--antibaryon pair production in the hadronization process, and the rest represents normal meson production.
• Figure 5: String topology of a $\tilde{\chi}_1^0 \to \mathrm{u}_i\mathrm{d}_j\mathrm{d}_k$ decay, where the two $\mathrm{d}_{(j,k)}$ quarks each has radiated a gluon. The event is drawn in the rest frame of the junction at early times, where the $\mathrm{u}$ and the two gluons are separated by 120$^{\circ}$, and the string topology is shown by dashed lines. A possible colour assignment is indicated within brackets.