  The ALICE Collaboration reports a search for jet quenching effects in
high-multiplicity (HM) proton$-$proton collisions at $\sqrt{s}$ = 13 TeV, using
the semi-inclusive azimuthal-difference distribution $\Delta\varphi$ of
charged-particle jets recoiling from a high transverse momentum
(high-$p_{\mathrm{T,trig}}$) trigger hadron. Jet quenching may broaden the
$\Delta\varphi$ distribution measured in HM events compared to that in minimum
bias (MB) events. The measurement employs a $p_{\mathrm{T,trig}}$-differential
observable for data-driven suppression of the contribution of multiple partonic
interactions, which is the dominant background. While azimuthal broadening is
indeed observed in HM compared to MB events, similar broadening for HM events
is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which
does not incorporate jet quenching. Detailed analysis of these data and
simulations show that the azimuthal broadening is due to bias of the HM
selection towards events with multiple jets in the final state. The
identification of this bias has implications for all jet quenching searches
where selection is made on the event activity.
