  Being able to distinguish light-quark jets from gluon jets on an
event-by-event basis could significantly enhance the reach for many new physics
searches at the Large Hadron Collider. Through an exhaustive search of existing
and novel jet substructure observables, we find that a multivariate approach
can filter out over 95% of the gluon jets while keeping more than half of the
light-quark jets. Moreover, a combination of two simple variables, the charge
track multiplicity and the $p_T$-weighted linear radial moment (girth), can
achieve similar results. While this pair appears very promising, our study is
only Monte Carlo based, and other discriminants may work better with real data
in a realistic experimental environment. To that end, we explore many other
observables constructed using different jet sizes and parameters, and highlight
those that deserve further theoretical and experimental scrutiny. Additional
information, including distributions of around 10,000 variables, can be found
on this website http://jets.physics.harvard.edu/qvg .
