Disentangling Sources of Momentum Fluctuations in Xe+Xe and Pb+Pb Collisions with the ATLAS Detector.

High-energy nuclear collisions create a quark-gluon plasma, whose initial condition and subsequent expansion vary from event to event, impacting the distribution of the eventwise average transverse momentum [P([p_{T}])]. Disentangling the contributions from fluctuations in the nuclear overlap size (geometrical component) and other sources at a fixed size (intrinsic component) remains a challenge. This problem is addressed by measuring the mean, variance, and skewness of P([p_{T}]) in ^{208}Pb+^{208}Pb and ^{129}Xe+^{129}Xe collisions at sqrt[s_{NN}]=5.

Search for the Exclusive W Boson Hadronic Decays W^{±}→π^{±}γ, W^{±}→K^{±}γ and W^{±}→ρ^{±}γ with the ATLAS Detector.

A search for the exclusive hadronic decays W^{±}→π^{±}γ, W^{±}→K^{±}γ, and W^{±}→ρ^{±}γ is performed using up to 140  fb^{-1} of proton-proton collisions recorded with the ATLAS detector at a center-of-mass energy of sqrt[s]=13  TeV. If observed, these rare processes would provide a unique test bench for the quantum chromodynamics factorization formalism used to calculate cross sections at colliders. Additionally, at future colliders, these decays could offer a new way to measure the W boson mass through fully reconstructed decay products.

Observation of the decay.

Using proton-proton collision data corresponding to an integrated luminosity of collected by the CMS experiment at , the decay is observed for the first time, with a statistical significance exceeding 5 standard deviations. The relative branching fraction, with respect to the decay, is measured to be , where the first uncertainty is statistical, the second is systematic, and the third is related to the uncertainties in and .

Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in pp Collisions at sqrt[s]=13 TeV.

A search for collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged particles using the CMS detector at the CERN LHC. The analysis uses data collected at a center-of-mass energy of sqrt[s]=13  TeV, corresponding to an integrated luminosity of 138  fb^{-1}. Jets are reconstructed with the anti-k_{T} algorithm with a distance parameter of 0.