Observation of long-range elliptic anisotropies in $\sqrt{s}=$13 and 2.76 TeV $pp$ collisions with the ATLAS detector

Abstract

ATLAS has measured two-particle correlations as a function of relative azimuthal-angle, $Δφ$, and pseudorapidity, $Δη$, in $\sqrt{s}$=13 and 2.76 TeV $pp$ collisions at the LHC using charged particles measured in the pseudorapidity interval $|η|$<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at $Δφ\sim 0$ that extends over a wide range of $Δη$, which has been referred to as the "ridge". Per-trigger-particle yields, $Y(Δφ)$, are measured over 2<$|Δη|$<5. For both collision energies, the $Y(Δφ)$ distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by $\cos{(2Δφ)}$. The fitted Fourier coefficient, $v_{2,2}$, exhibits factorization, suggesting that the ridge results from per-event $\cos{(2φ)}$ modulation of the single-particle distribution with Fourier coefficients $v_2$. The $v_2$ values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a $p_{\mathrm{T}}$ dependence similar to that measured in $p$+Pb and Pb+Pb collisions. The $v_2$ values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in $pp$ collisions arises from the same or similar underlying physics as observed in $p$+Pb collisions, and that the dynamics responsible for the ridge has no strong $\sqrt{s}$ dependence.

Figures (4)

• Figure 1: Distributions of the multiplicity, $N_{\mathrm{ch}}^{\mathrm{rec}}$, of reconstructed charged particles having $\pT$>0.4 for the 2.76 (left) and 13 (right) data used in this analysis.
• Figure 2: Two-particle correlation functions, $C(\Delta\eta, \Delta\phi)$, in 13 $pp$ collisions in $N_{\mathrm{ch}}^{\mathrm{rec}}$ intervals 0--20 (left) and $\geq 120$ (right) for charged particles having 0.5<$p_{\mathrm{T}}^{\mathrm{a,b}}\xspace$<5 . The distributions have been truncated to suppress the peak at $\Delta \eta\xspace$=$\Delta \phi\xspace$=0 and are shown over $|\eta|$<4.6 to avoid statistical fluctuations at larger $|\Delta \eta\xspace|$.
• Figure 3: Per-trigger-particle yields, $Y(\Delta\phi)$, for 0.5<$p_{\mathrm{T}}^{\mathrm{a,b}}\xspace$<5 in different $N_{\mathrm{ch}}^{\mathrm{rec}}$ intervals in 2.76 and 13 data. Panel (a): 0$\leq$$N_{\mathrm{ch}}^{\mathrm{rec}}\xspace$<20 for both data sets. Panels (c) and (e): 50--60 and 70--80 $N_{\mathrm{ch}}^{\mathrm{rec}}$ intervals for 2.76 data. Panels (b), (d) and (f): 40--50, 60--70, and $\geq$90 $N_{\mathrm{ch}}^{\mathrm{rec}}$ intervals for 13 data. In panels (b)--(f), the open points and curves show different components of the template (see legend) that are shifted, where necessary, for presentation.
• Figure 4: Measured $v_{2,2}$ (top) and $v_2$ (middle) values versus $N_{\mathrm{ch}}^{\mathrm{rec}}$ for different $p_{\mathrm{T}}^{\mathrm{a,b}}\xspace$ intervals for 2.76 (left) and 13 (right) data. Results are averaged over $N_{\mathrm{ch}}^{\mathrm{rec}}$ bins of width 10 spanning the range 20$\leq$$N_{\mathrm{ch}}^{\mathrm{rec}}\xspace$<100 and 20$\leq$$N_{\mathrm{ch}}^{\mathrm{rec}}\xspace$<130 for 2.76 and 13 data, respectively, except for the 2<$p_{\mathrm{T}}^{\mathrm{b}}\xspace$<3 results for the 2.76 data which are averaged over bins of width 20. Measured $v_2$ values versus $p_{\mathrm{T}}^{\mathrm{a}}\xspace$ (bottom) spanning the range 0.3<$p_{\mathrm{T}}^{\mathrm{a}}\xspace$<5.0 for 13 and 2.76 data for the 50$\leq$$N_{\mathrm{ch}}^{\mathrm{rec}}\xspace$<60 interval (left) and for three $N_{\mathrm{ch}}^{\mathrm{rec}}$ intervals in the 13 data (right). Results are averaged over the $p_{\mathrm{T}}^{\mathrm{a}}\xspace$ intervals indicated by horizontal error bars. On all points, the vertical error bars indicate statistical uncertainties. The shaded bands indicate systematic uncertainties. For clarity, they are only shown for the 0.5<$p_{\mathrm{T}}^{\mathrm{b}}\xspace$<5 GeV case in the middle, for 2.76 TeV data in the lower left, and for the 40$\leq$$N_{\mathrm{ch}}^{\mathrm{rec}}\xspace$<50 case in the lower right panels.