Transverse momentum
= Transverse momentum
{title2=$\mathbf p_T=(p_x,p_y)$}
= Transverse momenta
{synonym}
Choose the beam as the $z$ direction. A <particle>'s <transverse momentum> is its <momentum> projected onto the perpendicular plane; $p_T=\sqrt{p_x^2+p_y^2}$. A <Lorentz boost> along the beam changes <energy> and longitudinal <momentum>, while leaving <transverse momentum> unchanged. Large <transverse momentum> identifies motion substantially across the beam rather than merely along it.