  Measurements of dijet $p_{\mathrm{T}}$ correlations in Pb+Pb and $pp$
collisions at a nucleon-nucleon centre-of-mass energy of
$\sqrt{s_{\mathrm{NN}}}=2.76\textrm{ TeV}$ are presented. The measurements are
performed with the ATLAS detector at the Large Hadron Collider using Pb+Pb and
$pp$ data samples corresponding to integrated luminosities of 0.14 nb$^{-1}$
and 4.0 pb$^{-1}$, respectively. Jets are reconstructed using the anti-$k_t$
algorithm with radius parameter values $R=0.3$ and $R=0.4$. A background
subtraction procedure is applied to correct the jets for the large underlying
event present in Pb+Pb collisions. The leading and sub-leading jet transverse
momenta are denoted $p_{\mathrm{T_{\mathrm{1}}}}$ and
$p_{\mathrm{T_{\mathrm{2}}}}$. An unfolding procedure is applied to the
two-dimensional ($p_{\mathrm{T_{\mathrm{1}}}}$, $p_{\mathrm{T_{\mathrm{2}}}}$)
distributions to account for experimental effects in the measurement of both
jets. Distributions of $(1/N)\mbox{$\mathrm{d}$} N/\mbox{$\mathrm{d}$}
x_{\mathrm{J}}$, where $x_{\mathrm{J}}=p_{\mathrm{T}_{2}}/p_{\mathrm{T}_{1}}$,
are presented as a function of $p_{\mathrm{T_{\mathrm{1}}}}$ and collision
centrality. The distributions are found to be similar in peripheral Pb+Pb
collisions and $pp$ collisions, but highly modified in central Pb+Pb
collisions. Similar features are present in both the $R=0.3$ and $R=0.4$
results, indicating that the effects of the underlying event are properly
accounted for in the measurement. The results are qualitatively consistent with
expectations from partonic energy loss models.
