  Event-by-event fluctuations in the initial stages of ultrarelativistic
nucleus-nucleus collisions depend little on rapidity. The hydrodynamic
expansion which occurs in later stages then gives rise to correlations among
outgoing particles which depend weakly on their relative rapidity. Azimuthal
correlations, through which anisotropic flow ($v_n(p_T)$) is defined, have been
the most studied. Here we study a new observable introduced in 2020 by Schenke,
Shen and Teaney and dubbed $v_0(p_T)$, which quantifies the relative change in
the $p_T$ spectrum induced by a fluctuation. We describe how it can be
measured. Using hydrodynamic simulations, we make quantitative predictions for
$v_0(p_T)$ of charged and identified hadrons. We then discuss how $v_0(p_T)$
relates to two phenomena which have been measured: The increase of the mean
transverse momentum in ultracentral collisions, and the event-by-event
fluctuations of the transverse momentum per particle $[ p_T]$. We show that
$v_0(p_T)$ determines the dependence of these quantities on the $p_T$ cuts
implemented in the analysis. We quantitatively explain the rise of
$\sigma_{p_T}$ observed by ATLAS as the upper $p_T$-cut is increased from $2$
to $5$~GeV/$c$.
