Accelerator Neutrinos

Abstract:

A measurement is presented of elliptic ( $v_2$ ) and triangular ( $v_3$ ) azimuthal anisotropy coefficients for charged particles produced in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{{}_{\text{NN}}}}=5.02 \text{TeV}$ using a dataset corresponding to an integrated luminosity of $0.44{\text{nb}}^{-1}$ collected with the ATLAS detector at the LHC in 2018. The values of $v_2$ and $v_3$ are measured for charged particles over a wide range of transverse momentum ( $p_{\text{T}}$ ), 1–400 $\text{GeV}$ , and $\mathrm{Pb}+\mathrm{Pb}$ collision centrality, 0–60%, using the scalar-product and multiparticle cumulant methods. These methods are sensitive to event-by-event fluctuations and nonflow effects in the measurements of azimuthal anisotropies. Positive values of $v_2$ are observed up to a $p_{\text{T}}$ of approximately 100 $\text{GeV}$ from both methods across all centrality intervals. Positive values of $v_3$ are observed up to approximately 25 $\text{GeV}$ using both methods, though the application of the three-subevent technique to the multiparticle cumulant method leads to significant changes at the highest $p_{\text{T}}$ . At high $p_{\text{T}}$ ( $p_{\text{T}}⪆10 \text{GeV}$ ), charged particles are dominantly from jet fragmentation. These jets, and hence the measurements presented here, are sensitive to the path-length dependence of parton energy loss in the quark-gluon plasma produced in $\mathrm{Pb}+\mathrm{Pb}$ collisions.

Abstract:

We report the first measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasielasticlike interactions at the gadolinium-loaded Super-Kamiokande detector using the T2K neutrino beam, which has a peak energy of about 0.6 GeV. A total of 30 neutral-current quasielasticlike event candidates were selected from T2K data corresponding to an exposure of $1.76\times {10}^{20}$ protons on target. The $\gamma$ ray signals resulting from neutron captures were identified using a neural network. The flux-averaged mean neutron capture multiplicity was measured to be ${1.37\pm 0.33(\mathrm{stat}.)}_{-0.27}^{+0.17}(\mathrm{syst}.)$ , which is compatible within 2.3 sigma than predictions obtained using our nominal simulation. We discuss potential sources of systematic uncertainty in the prediction and demonstrate that a significant portion of this discrepancy arises from the modeling of hadron-nucleus interactions in the detector medium.

Abstract:

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R=0.2$$R=0.2$$ are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W,  Z,  and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R=0.2$$R=0.2$$ and 0.6,  obtained using the ATLAS detector. The results are based on 37fb-1$$37~\text {fb}^{-1}$$ of proton–proton collision data from the Large Hadron Collider at a centre-of-mass energy of s=13$$\sqrt{s}=13$$ TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (|η|<1.2)$$(|\eta | < 1.2)$$ typically vary from 1% to about 5% as a function of |η|$$|\eta |$$ at very low transverse momentum, pT,$$p_{\text {T}},$$ of around 20 GeV for both R=0.2$$R=0.2$$ and 0.6 jets. The relative energy resolution ranges from (35±6)%$$(35 \pm 6)\%$$ at pT=20$$p_{\text {T}} = 20$$ GeV to (6±0.5)%$$(6 \pm 0.5)\%$$ at pT=300$$p_{\text {T}} = 300$$ GeV for central R=0.2$$R=0.2$$ jets, and is found to be slightly worse for R=0.6$$R=0.6$$ jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.