Speaker
Description
The observation of collective phenomena, a hallmark of the quark-gluon plasma (QGP) formed in heavy-ion collisions, in small systems like proton-proton (pp) collisions, remains a topic of intense debate. This study presents the first investigation of radial flow in high-multiplicity (HM) pp collisions using the novel transverse momentum correlation observable, $v_{0}[p_{\rm T}]$. Analogous to anisotropic flow coefficients, the $v_{0}[p_{\rm T}]$ probes event-by-event fluctuations of the mean transverse momentum and is sensitive to the isotropic, collective expansion of the system [1]. Our analysis is performed using PYTHIA8 (v8.315) simulated events at Run 3 LHC energy ($\sqrt{s}$ = 13.6 TeV). We measure $v_{0}[p_{\rm T}]$ for inclusive charged and identified particles ($p/\overline p, \pi^{\pm}, K^{\pm}$). To suppress non-flow correlations from jets and resonance decays, we employ a pseudorapidity gap ($\eta_{\rm gap}$) technique and extend it to include additional azimuthal gaps. The robustness of the signal is verified using two independent centrality estimators: charged particle multiplicity at mid-rapidity and the FT0 amplitude. The results reveal two key signatures of hydrodynamic collectivity: (i) A distinct mass ordering at low-$p_{\rm T}$ ($p_{\rm T} \leq 1.5 $ GeV/c), where $v_{0}[p_{\rm T}]$ is largest for pions and smallest for protons. A characteristic baryon-meson splitting at intermediate $p_{\rm T}$, with protons exhibiting a larger $v_{0}[p_{\rm T}]$ than mesons. Furthermore, the magnitude of $v_{0}[p_{\rm T}]$ shows a clear multiplicity dependence, with the largest magnitude observed for low-multiplicity (LM) (60-70\%) class, gradually following a decreasing trend for HM classes (30-40\% and 10-20\%). The persistence of these signals after applying stringent $\eta_{\rm gap}$ and azimuthal gaps strongly suggests a collective origin. These findings, extracted for the first time using the $v_{0}[p_{\rm T}]$ observable, provide compelling evidence that radial flow can be generated in HM pp collisions, challenging the traditional paradigm that collective effects are exclusive to larger collision systems.
References:
[1] B. Schenke, C. Shen, and D. Teaney, “Transverse momentum fluctuations and their correlation with elliptic flow in nuclear collisions,” Physical Review. C, vol. 102, no. 3, Sep. 2020, doi: https://doi.org/10.1103/physrevc.102.034905.
