Accelerator Neutrinos

Abstract:

This paper presents the first measurements of the azimuthal anisotropy coefficients $v_n$ , which quantify the $n\mathrm{th}$ -order Fourier modulation of charged-particle azimuthal distributions, for $n=2-4$ in $\sqrt{s_{NN}}=5.36\mathrm{TeV}{}^{16}\mathrm{O}+{}^{16}\mathrm{O}$ and ${}^{20}\mathrm{Ne}+{}^{20}\mathrm{Ne}$ 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 ( $p_{\text{T}}$ ), 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 $p_{\text{T}}$ , reaching a maximum around $2\mathrm{GeV}$ , consistent with trends observed in heavy-ion collisions. Detailed comparisons between the two collision systems reveal an enhanced $v_2$ in central ${}^{20}\mathrm{Ne}+{}^{20}\mathrm{Ne}$ 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.

Abstract:

The short-baseline near detector (SBND), the near detector in the short-baseline neutrino program at Fermi National Accelerator Laboratory, is located just 110 m from the booster neutrino beam target. Thanks to this close proximity, relative to its $4\mathrm{m}\times 4\mathrm{m}$ front face, neutrinos enter SBND over a range of angles from 0° to approximately 1.6°, enabling the detector to sample variations in the neutrino flux as a function of the angle—a technique known as precision reaction-independent spectrum measurement (PRISM), referred to here as SBND-PRISM. In this paper, we show how muon- and electron-neutrino fluxes vary as a function of the neutrino beam axis angle and how this can be exploited to expand the physics potential of SBND. We make use of a model that predicts an angle-dependent electron-neutrino excess signal to illustrate this effect, such as ${\nu }_{\mu }\to {\nu }_e$ oscillations. We present how SBND-PRISM provides a method to add robustness against uncertainties in cross-section modeling and, more generally, uncertainties that do not depend on the spatial position of neutrino interaction inside the detector. The fluxes, along with their associated covariance matrices, are made publicly available with this publication.

Abstract:

We present the strongest experimental limits to date on the mixing angle, $\theta$ , with which a new scalar particle, $S$ , mixes with the Higgs field in the mass range $110\mathrm{MeV}<m_S<155\mathrm{MeV}$ . This result uses the MicroBooNE liquid argon time projection chamber to search for decays of these Higgs-portal scalar particles through the $S\to e^{+}{e}^{-}$ channel with the decays of kaons in the NuMI neutrino beam acting as the source of the scalar particles. The analysis uses an exposure of $2.01\times {10}^{21}$ protons on target of NuMI beam data including periods when the beam focusing system was configured to focus positively charged hadrons and separate periods when negatively charged hadrons were focused. The analysis searches for scalar particles produced from kaons decaying in flight in the beam’s decay volume and at rest in the target and absorber. At $m_S=125\mathrm{MeV}$ ( $m_S=150\mathrm{MeV}$ ) we set a limit of $\theta <3.19\times {10}^{-4}$ ( $\theta <2.79\times {10}^{-4}$ ) at the 95% confidence level.

Abstract:

This Letter analyzes the sensitivity of event geometry estimators to the initial-state kinematics of hard scattering in proton–lead collisions. This analysis uses dijets as a proxy for the parton–parton scattering configuration, correlating it with event geometry estimators, namely the energy deposited in the Zero-Degree Calorimeter and the transverse energy recorded in the Forward Calorimeter in the Pb-going direction. The analysis uses data recorded by the ATLAS detector at the Large Hadron Collider with a nucleon–nucleon center-of-mass energy of 8.16 TeV, corresponding to an integrated luminosity of 56 nb − 1 . The jets are measured within the pseudorapidity interval −2.8  <  η  <  4.5, where positive η values correspond to the direction of the proton beam. Results are presented as a function of the Bjorken-x of the parton originating from the proton, xp . Both event geometry estimators are found to be dependent on xp , with the energy deposited in the Zero-Degree Calorimeter about six times less sensitive to xp compared with the transverse energy deposited in the Forward Calorimeter.

Abstract:

A search for the resonant production of a heavy scalar X decaying into a lighter scalar S and a Higgs boson, through the process X → S ( → b b ¯ ) H ( → γ γ ) , where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using integrated luminosities of 140 and 59 fb − 1 of proton–proton collision data at centre-of-mass energies of 13 and 13.6 TeV, respectively, recorded with the ATLAS detector at the LHC. The search is performed over the mass ranges of 170  ≤  mX  ≤  1000 GeV and 15  ≤  mS  ≤  500 GeV. No significant excess over the Standard Model background predictions is observed and limits at 95% confidence level are set on the product of cross section and branching fraction for the process X → S ( → b b ¯ ) H ( → γ γ ) at 13 TeV, ranging from 9 fb to 0.06 fb.