Particle production as a function of charged-particle flattenicity in $ pp$ collisions at $ \sqrt{s}=13\text{}\mathrm{TeV}$

This paper reports the first measurement of the transverse momentum ($ {p}_{\mathrm{T}}$) spectra of primary charged pions, kaons, (anti)protons, and unidentified particles as a function of the charged-particle flattenicity in pp collisions at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$. Flattenicity is a novel event shape observable that is measured in the pseudorapidity intervals covered by the V0 detector, $ 2.8<\eta <5.1$ and $ -3.7<\eta <-1.7$. According to QCD-inspired phenomenological models, it shows sensitivity to multiparton interactions and is less affected by biases toward larger $ {p}_{\mathrm{T}}$ due to local multiplicity fluctuations in the V0 acceptance than multiplicity. The analysis is performed in minimum-bias (MB) as well as in high-multiplicity events up to $ {p}_{\mathrm{T}}=20\text{}\text{}\mathrm{GeV}/c$. The event selection requires at least one charged particle produced in the pseudorapidity interval $ |\eta |<1$. The measured $ {p}_{\mathrm{T}}$ distributions, average $ {p}_{\mathrm{T}}$, kaon-to-pion and proton-to-pion particle ratios, presented in this paper, are compared to model calculations using pythia 8 based on color strings and EPOS LHC. The modification of the $ {p}_{\mathrm{T}}$-spectral shapes in low-flattenicity events that have large event activity with respect to those measured in MB events develops a pronounced peak at intermediate $ {p}_{\mathrm{T}}$ ($ 2<{p}_{\mathrm{T}}<8\text{}\text{}\mathrm{GeV}/c$), and approaches the vicinity of unity at higher $ {p}_{\mathrm{T}}$. The results are qualitatively described by pythia, and they show different behavior than those measured as a function of charged-particle multiplicity based on the V0M estimator.