Quenching of polarized jets

TL;DR

This work develops a collinear-factorization framework to study Λ polarization in weak-boson–tagged jets, connecting longitudinal spin transfer in fragmentation $G_{1L}$ to hadronization of polarized partons in $pp$ collisions and exploring spin-dependent jet quenching in $AA$ collisions. It provides explicit LO partonic cross sections for $a+b\to V+d$ with $V=Z^0,W^{\pm}$, and demonstrates that gluon polarization can be sizable in $Z^0/W^{\pm}$–associated production, offering a clean channel to probe polarized fragmentation functions $G_{1L}$ via $\Lambda$ polarization. The paper also introduces two in-medium energy-loss schemes (single-hard-branching SHB and multiple-soft-branching MSB) to model spin-dependent jet quenching, showing that polarization can be enhanced at larger $z$ due to energy loss and that spin quenching of gluons is generally small, especially in the MSB limit. These results motivate measurements at Tevatron/LHC to constrain $G_{1L}$ and to illuminate spin-dependent medium effects in the QGP, while calling for comprehensive Monte-Carlo simulations of polarized jet interactions with the medium.

Abstract

Jets produced in association with a $Z^{0}$ or $W^{\pm}$ boson in hadronic collisions are automatically polarized due to the parity violation of weak interaction, making these processes ideal for understanding the spin transfer from polarized partons to polarized hadrons. Furthermore, leveraging this feature, we can also employ the weak-boson-tagged process to study the quenching phenomenon of polarized jets, an aspect that has rarely been discussed in the literature. In this work, we compute the polarization of $Λ$ hyperons in $pp$ collisions and investigate the nuclear modification due to the jet-medium interaction in $AA$ collisions. Our results highlight this process as a valuable probe of polarized parton hadronization and of spin-dependent medium effects in the QGP.

Figures (8)

• Figure 1: Feynman diagrams of the $q+g \to V +q$ channel with $V$ representing a $Z^0$ or $W^\pm$ boson.
• Figure 2: The gluon helicity $\lambda_g$ as a function of $\hat{t}/(\hat{s}-M_{Z/W}^2)$ in $Z^0/W^\pm$ associated production processes.
• Figure 3: Feynman diagrams of the $q+g \to V +q$ channel.
• Figure 4: Production ratio of different jet flavors in association with ${Z}^0$ boson in $p\bar{p}$ (left) and $pp$ (right) collisions.
• Figure 5: The gluon polarization as a function of the rapidity in $p\bar{p}$ (left) and $pp$ (right) collisions.