Abstract
ATLAS measurements of two-particle correlations are presented for $\sqrt{s} = 5.02$ and $13$ TeV $pp$ collisions and for $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV $p$+Pb collisions at the LHC. The correlation functions are measured as a function of relative azimuthal angle $\Delta \phi$, and pseudorapidity separation $\Delta \eta$, using charged particles detected within the pseudorapidity interval $|\eta|{<}2.5$. Azimuthal modulation in the long-range component of the correlation function, with $|\Delta\eta|{>}2$, is studied using a template fitting procedure to remove a "back-to-back" contribution to the correlation function that primarily arises from hard-scattering processes. In addition to the elliptic, $\cos{(2\Delta\phi)}$, modulation observed in a previous measurement, the $pp$ correlation functions exhibit significant $\cos{(3\Delta\phi)}$ and $\cos{(4\Delta\phi)}$ modulation. The Fourier coefficients $v_{n,n}$ associated with the $\cos{(n\Delta\phi)}$ modulation of the correlation functions for $n =$ $2$-$4$ are measured as a function of charged-particle multiplicity and charged-particle transverse momentum. The Fourier coefficients are observed to be compatible with $\cos{(n\phi)}$ modulation of per-event single-particle azimuthal angle distributions. The single-particle Fourier coefficients $v_n$ are measured as a function of charged-particle multiplicity, and charged-particle transverse momentum for $n {=} $ $2$-$4$. The integrated luminosities used in this analysis are, $64$ $\mathrm{nb^{-1}}$ for the $\sqrt{s}=13$ TeV $pp$ data, $170$ $\mathrm{nb^{-1}}$ for the $\sqrt{s}=5.02$ TeV $pp$ data and $28$ $\mathrm{nb^{-1}}$ for the $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV $p$+Pb data.
Highlights
Observations of azimuthal anisotropies in the angular distributions of particles produced in proton-lead (p + Pb) collisions at the LHC [1,2,3,4,5] and in deuteron-gold (d + Au) [6,7,8] and 3He + Au [9] collisions at RHIC have garnered much interest due to the remarkable similarities between the phenomena observed in those colliding systems and the effects of collective expansion seen in the Pb + Pb and Au + Au collisions [3,10,11,12,13]
The amplitude of the modulation relative to the average differential particle yield dN/dφ, was observed to be constant, within uncertainties, as a function of the charged particle multiplicity of the pp events and to be consistent between the two energies, suggesting that the modulation is an intrinsic feature of high-energy pp collisions. These results provide further urgency to address the question of whether strong coupling and collective dynamics play a significant role in small systems, including the smallest systems accessible at collider energies—pp collisions
The measurements presented in this paper were performed using the ATLAS [54] inner detector (ID), minimum-bias trigger scintillators (MBTS), calorimeter, zero-degree calorimeters (ZDC), and the trigger and data acquisition systems
Summary
Observations of azimuthal anisotropies in the angular distributions of particles produced in proton-lead (p + Pb) collisions at the LHC [1,2,3,4,5] and in deuteron-gold (d + Au) [6,7,8] and 3He + Au [9] collisions at RHIC have garnered much interest due to the remarkable similarities between the phenomena observed in those colliding systems and the effects of collective expansion seen in the Pb + Pb and Au + Au collisions [3,10,11,12,13].1 The most intriguing feature of the azimuthal anisotropies is the “ridge”: an enhancement in the production of particles with small azimuthal angle (φ) separation which extends over a large range of pseudorapidity (η) separation [1,2,14,15]. A recent study by the ATLAS Collaboration of two-particle angular correlations i√n proton–proton (pp) collisions at centerof-mass energies of s = 13 and 2.76 TeV obtained results that are consistent with the presence of an elliptic or cos(2φ) modulation of the per-event single particle azimuthal angle distributions [√41]. Application of the template fitting method to the pp data provided an excellent description of the measured correlation functions It indicated substantial bias resulting from the application of the ZYAM-subtraction procedure to the peripheral reference correlation function due to the nonzero v2,2 in low-multiplicity events.
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