Measurements at the LHC have shown that small collision systems (pp and p--Pb collisions) exhibit QGP-like features. However, the origin of these features remains uncertain. Event activity classifiers, such as the relative transverse activity classifier ($R_{\textrm {T}}$) and flattenicity, allow us to study the soft and hard physics regimes that drive particle production to better understand this behavior. This talk will present the first measurements of charged-particle $p_{\textrm {T}}$ spectra as a function of $R_{\textrm {T}}$ in pp and p–Pb collisions at $\sqrt{s_{\textrm {NN}}}=5.02$\,TeV. The selection biases observed in pp collisions at high $R_{\textrm {T}}$, present also in the standard analyses versus charged particle multiplicity, will be presented, discussing the need to reduce the sensitivity to local multiplicity fluctuations to mitigate them. Such a reduction can be achieved using flattenicity, which is measured using the forward V0 detectors of ALICE. The first results on the production of pions, kaons, protons, and unidentified charged particles at midrapidity as a function of flattenicity in pp collisions at $\sqrt{s} = 13$ TeV will be presented. The results will be discussed by comparing the data to PYTHIA 8 and EPOS LHC model predictions.

Measurements at the LHC have shown that small collision systems (pp and p--Pb collisions) exhibit QGP-like features. However, the origin of these features remains uncertain. Event activity classifiers, such as the relative transverse activity classifier ($R_{\textrm {T}}$) and flattenicity, allow us to study the soft and hard physics regimes that drive particle production to better understand this behavior. This talk will present the first measurements of charged-particle $p_{\textrm {T}}$ spectra as a function of $R_{\textrm {T}}$ in pp and p–Pb collisions at $\sqrt{s_{\textrm {NN}}}=5.02$\,TeV. The selection biases observed in pp collisions at high $R_{\textrm {T}}$, present also in the standard analyses versus charged particle multiplicity, will be presented, discussing the need to reduce the sensitivity to local multiplicity fluctuations to mitigate them. Such a reduction can be achieved using flattenicity, which is measured using the forward V0 detectors of ALICE. The first results on the production of pions, kaons, protons, and unidentified charged particles at midrapidity as a function of flattenicity in pp collisions at $\sqrt{s} = 13$ TeV will be presented. The results will be discussed by comparing the data to PYTHIA 8 and EPOS LHC model predictions.