Speaker
Description
Recent studies from various experiments at the LHC and RHIC suggest that small collision systems exhibit quark-gluon plasma (QGP)-like signatures similar to those previously observed in large systems. While QGP-like signatures have been observed in the soft sector, strong constraints on possible partonic energy loss were established in the hard sector. At the same time, there are multiple pieces of evidence pointing to correlations between hard and soft particle production in proton-proton collisions. The search for energy loss in small systems can be expanded to very low transverse momentum by studying charged hadrons correlated with $Z$ bosons, thus obtaining a proxy for low-energy jets. The initial condition of the hard probe is controlled via the kinematics of the leptonically decaying $Z$ boson, which is unaffected by the QGP due to the absence of color charges. Results from ATLAS on the hadron production in events with $Z$ bosons in $p$+Pb and $pp$ collisions at $\sqrt{s_{\mathrm{NN}}} 8$~TeV are presented. For the first time, these results are separated between the underlying event and hard scattering components, and a relation between the two is discussed.
The correlations between hard and soft particle production in $pp$ collisions are studied in events with $\Upsilon$ mesons, which provide a valuable hard probe to study such effects. The multiplicity and distributions of kinematic variables of charged particles produced in association with an $\Upsilon$ meson are measured using 139 fb$^{-1}$ of data at $\sqrt{s}\ = 13$~TeV collected by the ATLAS experiment at the LHC. The measurement is performed separately for the first three $\Upsilon$(nS) states in intervals of their transverse momentum.
At low transverse momentum, the associated charged-particle multiplicity is observed to be systematically lower in collisions where $\Upsilon$(2S) or $\Upsilon$(3S) is produced compared to $\Upsilon$(1S), with the difference decreasing as the $\Upsilon$ transverse momentum increases. The study also includes measurements of $\eta-\phi$ correlations between $b\bar{b}$ states and charged particles.
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