Speaker
Brian Cole
(Columbia University (US))
Description
(speaker known later)
ATLAS measurements of correlations between particle pairs in relative
azimuthal angle ($\Delta\phi$) and pseudorapidity separation
($\Delta\eta$), in $pp$ collisions at $\sqrt{s}$=2.76, 5.02 and
13~TeV, and in $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=5.02~TeV
are presented. Prior measurements have shown that in $pp$ collisions
with a large multiplicity of produced particles, a long-range
structure,commonly called the ``ridge'', develops along $\Delta\eta$
at $\Delta\phi\sim$0. However, due to the presence of the large
away-side jet, the full $\Delta\phi$ dependence of the long-range
correlation could not be studied previously. In this analysis, a
template fitting procedure is implemented to determine the
contributions from dijets to the correlations, using low-multiplicity
events, and extract the genuine long-range correlation. The Fourier
harmonics of the genuine long-range correlation for orders $n$=2--4
are extracted, and their dependence on $p_{\mathrm{T}}$,
event-multiplicity and collision energy are studied. It is shown that
the second Fourier coefficient, $v_{2,2}$, dominates the long range
correlation, with small but significant contributions from $v_{3,3}$
and $v_{4,4}$. The $v_{2,2}$ is shown to factorize into the product of
single-particle anisotropies $v_2$. A large $v_2$ is shown to be
present even in events with a small multiplicity of produced
particles, implying that the long-range correlations are not unique to
rare high multiplicity events, but are present even in low
multiplicity $pp$ collisions. Comparisons to the $v_{n,n}$ and $v_{n}$
obtained when applying the template fitting procedure to $p$+Pb
collisions are also presented. These measurements can help determine
if the long range correlations in $pp$ and $p$+Pb collisions arise
from similar mechanisms or not.
| Collaboration | ATLAS |
|---|
