Direct Measurement of the Top Quark Charge at Hadron Colliders

TL;DR

This work evaluates whether photon radiation in ttbar events at the Tevatron and LHC can directly determine the top quark's electric charge, testing the SM value of +2/3 against exotic -4/3 possibilities. It combines ttbarγ production and radiative top decays at tree level, using b-tagged final states and phase-space cuts to suppress unwanted backgrounds, and analyzes photon kinematics and angular distributions to extract charge-sensitive signatures. Through χ^2 analyses with realistic normalization uncertainties, the study finds that Tevatron Run II could rule out a -4/3 top at about 95% CL, while the LHC could achieve roughly 10% precision on q_top with 10 fb^-1. The approach offers a complementary pathway to test the top's EM coupling and charge beyond traditional decay-based methods, and may be enhanced by incorporating b-jet charge measurements.

Abstract

We consider photon radiation in tbar-t events at the upgraded Fermilab Tevatron and the CERN Large Hadron Collider (LHC) as a tool to measure the electric charge of the top quark. We analyze the contributions of tbar-t-gamma production and radiative top quark decays to p-p, pbar-p -> gamma l^+/- nu bbar-b jj, assuming that both b-quarks are tagged. With 20~fb^{-1} at the Tevatron, the possibility that the ``top quark'' discovered in Run I is actually an exotic charge -4/3 quark can be ruled out at the 95% confidence level. At the LHC, it will be possible to determine the charge of the top quark with an accuracy of about 10%.

Figures (9)

• Figure 1: The photon transverse momentum distribution for $p\bar{p}\to \gamma\ell^\pm\nu b\bar{b}jj$ at the Tevatron, imposing $\bar{t}t\gamma$ selection cuts (see Eq. (\ref{['eq:top51']})), for a) $q_{top}=2/3$ and b) $q_{top}=-4/3$. Shown are the $\bar{t}t\gamma$ (solid line), $\bar{t}t$, $t\to Wb\gamma\to\ell\nu b\gamma$ (dashed line), and $\bar{t}t$, $t\to Wb\gamma\to jjb\gamma$ (dotted line) contributions. The additional cuts imposed are described in Sec. II.
• Figure 2: The photon transverse momentum distribution for $pp\to \gamma\ell^\pm\nu b\bar{b}jj$ at the LHC, imposing $\bar{t}t\gamma$ selection cuts (see Eq. (\ref{['eq:top51']})), for a) $q_{top}=2/3$ and b) $q_{top}=-4/3$. Shown are the $\bar{t}t\gamma$ (solid line), $\bar{t}t$, $t\to Wb\gamma\to\ell\nu b\gamma$ (dashed line), and $\bar{t}t$, $t\to Wb\gamma\to jjb\gamma$ (dotted line) contributions. The additional cuts imposed are described in Sec. II.
• Figure 3: The photon transverse momentum distribution for $p\bar{p}\to \gamma\ell^\pm\nu b\bar{b}jj$ at the Tevatron, imposing $t\to Wb\gamma\to\ell\nu b\gamma$ selection cuts (see text), for a) $q_{top}=2/3$ and b) $q_{top}=-4/3$. Shown are the $\bar{t}t$, $t\to Wb\gamma\to\ell\nu b\gamma$ (solid line), $\bar{t}t$, $t\to Wb\gamma\to jjb\gamma$ (dashed line) and $\bar{t}t\gamma$ (dotted line) contributions. The additional cuts imposed are described in Sec. II.
• Figure 4: The photon transverse momentum distribution for $pp\to \gamma\ell^\pm\nu b\bar{b}jj$ at the LHC, imposing $t\to Wb\gamma \to\ell\nu b\gamma$ selection cuts (see text), for a) $q_{top}=2/3$ and b) $q_{top}=-4/3$. Shown are the $\bar{t}t$, $t\to Wb\gamma\to\ell\nu b\gamma$ (solid line), $\bar{t}t$, $t\to Wb\gamma\to jjb\gamma$ (dashed line) and $\bar{t}t\gamma$ (dotted line) contributions. The additional cuts imposed are described in Sec. II.
• Figure 5: The photon transverse momentum distribution for $p\bar{p}\to \gamma\ell^\pm\nu b\bar{b}jj$ at the Tevatron. Part a) displays the individual contributions for $q_{top}=2/3$, imposing $\bar{t}t\gamma$ (solid line), $t\to Wb\gamma\to\ell\nu b\gamma$ (dashed line) and $t\to Wb\gamma\to jjb\gamma$ selection cuts (dotted line). In part b), the photon $p_T$ distributions for the phase space region defined by the $\bar{t}t\gamma$ selection cuts and the two radiative top decay regions combined are shown for $q_{top}=2/3$ and $q_{top}=-4/3$. The cuts imposed are described in the text.