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ATLAS Collaboration(Aad, G. et al), Ahuja, S., Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., et al. (2026). Measurement of the azimuthal anisotropy of charged particles in √sNN=5.36 TeV 16O+16O and 20Ne+20Ne collisions with the ATLAS detector. Phys. Rev. C, 113(4), 045205–30pp.
Abstract: This paper presents the first measurements of the azimuthal anisotropy coefficients vn, which quantify the nth-order Fourier modulation of charged-particle azimuthal distributions, for n = 2-4 in root s(NN) = 5.36 TeV O-16+ O-16 and Ne-20+ Ne-20 collisions recorded with the ATLAS detector at the CERN Large Hadron Collider in 2025. The v(n) coefficients are measured as a function of transverse momentum (p(T)), collision centrality, and event multiplicity. They are extracted using two complementary methods: two-particle correlations with a template-fit subtraction of short-range nonflow contributions, and four-particle subevent cumulants, which intrinsically suppress nonflow effects and provide sensitivity to flow fluctuations. The results show a clear hierarchy v(2) > v(3) > v(4) and a nonmonotonic dependence on p(T), reaching a maximum around 2 GeV, consistent with trends observed in heavy-ion collisions. Detailed comparisons between the two collision systems reveal an enhanced v(2) in central Ne-20+ Ne-20 collisions, consistent with theory expectations based on the predicted prolate deformation of neon nuclei, in contrast to the slightly tetrahedral structure predicted for oxygen. The four-particle cumulant results highlight strong event-by-event fluctuations and provide the greatest sensitivity to nuclear shape effects. These measurements can place new constraints on the initial geometry and the hydrodynamic response in light-ion collisions, offering valuable input for models of nuclear structure.
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ATLAS Collaboration(Aad, G. et al), Ahuja, S., Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., et al. (2026). Measurement of the W-boson angular coefficients and transverse momentum in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 86(5), 483–41pp.
Abstract: The angular distributions of Drell-Yan lepton pairs provide sensitive probes of the underlying dynamics of quantum chromodynamics (QCD) effects in vector-boson production. This paper presents for the first time the measurement of the full set of angular coefficients together with the differential cross-section as a function of the transverse momentum of the W boson, in the full phase space of the decay leptons. The measurements are performed separately for the W- and W+ channels. The analysis uses proton-proton collision data recorded by the ATLAS experiment at the Large Hadron Collider in 2017 and 2018, during special low-luminosity runs with a reduced number of interactions per bunch crossings (pile-up). The data correspond to an integrated luminosity of 338 pb(-1) at a centre-of-mass energy of root s = 13 TeV. The low pile-up environment provides excellent experimental conditions for high-precision measurements of W-boson production. All results agree with theoretical predictions incorporating finite-order QCD corrections up to order alpha(2)(S).
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Barenboim, G., & Burns, A. K. (2026). Temperature-dependent CPT violation: Constraints from big bang nucleosynthesis. Phys. Rev. D, 113(9), 095031–15pp.
Abstract: In this study, we explore temperature-dependent CPT violation during big bang nucleosynthesis (BBN) through electron-positron mass asymmetries parametrized by b0(T) 1/4 alpha T2. The T2 scaling naturally evades stringent laboratory bounds at zero temperature while allowing for significant CPT violation at MeV scales in the early Universe [S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)]. Using a modified version of the BBN code PRyMordial with dynamically-solved chemical potentials and appropriate finite-mass corrections, we constrain electron-positron mass differences from observed abundances of helium-4, deuterium, and Neff. We find that alpha must be greater than or approximately equal to 10-6 GeV-1 for keV-scale mass differences at BBN. All three observables show no simultaneous 1 sigma overlap, though pairwise combinations allow for constrained regions of parameter space. We present two toy models demonstrating how b0(T) proportional to T2 arises from field-theoretic mechanisms, including temperature-driven phase transitions. These results provide the most stringent constraints on early-Universe CPT violation in this regime, probing parameter space inaccessible to laboratory experiments.
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del Rio, A., & Lopez-Oliver, P. (2026). Spontaneous particle creation by oscillating compact stars. Phys. Rev. D, 113(10), 105018–23pp.
Abstract: Quantum field theory predicts that dynamical curved spacetimes can spontaneously excite particle pairs from the quantum vacuum, a phenomenon extensively studied in expanding universes and in scenarios involving gravitational collapse. In this article, we explore particle creation driven by radial oscillations of 3 & thorn; 1-dimensional spherically symmetric compact objects, such as neutron stars, using a massless, minimally coupled scalar field as a reference model. We employ a toy model to describe the oscillatory dynamics and its coupling to the field modes, focusing on the resulting effects in the exterior spacetime of the star. The Bogoliubov coefficients relating the in and out vacua are computed nonperturbatively using high-precision numerical methods, without relying on weak-field, small-amplitude or small-velocity expansions. This allows us to determine the full particle spectrum and the total particle number in the strong-field and fully relativistic regime. Our analysis confirms the existence of particle creation in this setting and, crucially, reveals a distinct resonance structure in the spectrum.
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Esser, F., Salas-Bernardez, A., Sanz, V., & Ubiali, M. (2026). Global analysis of ALP-mediated multiboson production at the LHC. Phys. Rev. D, 113(9), 095033–23pp.
Abstract: Axionlike particles (ALPs) provide a well-motivated framework for physics beyond the Standard Model, coupling to gauge bosons through dimension-five operators protected by an approximate shift symmetry. At the LHC, such interactions lead to distinctive signatures in multiboson production, where the ALP appears as an off-shell mediator rather than a narrow resonance. In this work, we present the first global and vector-boson-fusion (VBF) final states. On the theory side, motivated from a UV perspective, we assume that the ALP couples only to the gauge sector of the SM and classify the ALP-multiboson vertices that directly modify collider observables. Using Run-2 LHC measurements, we extract bounds on the Wilson coefficients (cG, c W, cB) that parametrize gluonic and electroweak ALP interactions. Our results show that the dijet channel dominates the sensitivity to ALP couplings and determines the limits on cG, while diboson and VBF processes provide complementary constraints on the electroweak couplings. We further assess the validity of the effective field theory expansion at the multi-TeV scales probed by the data. This global study provides the most comprehensive picture to date of ALP-gauge interactions from multiboson production at the LHC and highlights the opportunities for significant improvements with future high-luminosity measurements.
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