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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2014). Measurements of spin correlation in top-antitop quark events from proton-proton collisions at root s=7 TeV using the ATLAS detector. Phys. Rev. D, 90(11), 112016–32pp.
Abstract: Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb(-1). Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2014). Measurement of Higgs boson production in the diphoton decay channel in pp collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector. Phys. Rev. D, 90(11), 112015–44pp.
Abstract: A measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using 4.5 fb(-1) of proton-proton collisions data at root s = 7 TeV and 20.3 fb(-1) at root s = 8 TeV collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be μ= 1.17 +/- 0.27 at the value of the Higgs boson mass measured by ATLAS, m(H) = 125.4 GeV. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of m(H). They are found to be mu(ggF) = 1.32 +/- 0.38, mu(VBF) = 0.8 +/- 0.7, mu(WH) = 1.0 +/- 1.6, mu(ZH) = 0.1(-0.1)(+3.7), and μt (t) over barH = 1.6(-1.8)(+2.7), for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a W or Z boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.
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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., Oyanguren, A., & Villanueva-Perez, P. (2014). Search for new pi(0)-like particles produced in association with a tau-lepton pair. Phys. Rev. D, 90(11), 112011–9pp.
Abstract: We report on a search in e(+)e(-) annihilations for new pi(0)-like particles produced in association with a tau-lepton pair. These objects, with a similar mass and similar decay modes to pi(0) mesons, could provide an explanation for the non-asymptotic behavior of the pion-photon transition form factor observed by the BABAR Collaboration. No significant signal is observed, and limits on the production cross section at the level of 73 fb or 370 fb, depending on the model parameters, are determined at 90% confidence level. These upper limits lie below the cross section values needed to explain the BABAR form factor data.
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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., Oyanguren, A., & Villanueva-Perez, P. (2014). Bottomonium spectroscopy and radiative transitions involving the chi(bJ)(1P, 2P) states at BABAR. Phys. Rev. D, 90(11), 112010–20pp.
Abstract: We use (121 +/- 1) million Upsilon(3S) and (98 +/- 1) million Upsilon(2S) mesons recorded by the BABAR detector at the PEP-II e(+)e(-) collider at SLAC to perform a study of radiative transitions involving the chi(bJ)(1P, 2P) states in exclusive decays with mu(+)mu(-)gamma gamma final states. We reconstruct twelve channels in four cascades using two complementary methods. In the first we identify both signal photon candidates in the electromagnetic calorimeter (EMC), employ a calorimeter timing-based technique to reduce backgrounds, and determine branching-ratio products and fine mass splittings. These results include the best observational significance yet for the chi(b0)(2P) -> gamma Upsilon(2S) and chi(b0)(1P) -> gamma Upsilon(1S) transitions. In the second method, we identify one photon candidate in the EMC and one which has converted into an e(+)e(-) pair due to interaction with detector material, and we measure absolute product branching fractions. This method is particularly useful for measuring Upsilon(3S) -> gamma chi(b1,2)(1P) decays. Additionally, we provide the most up-to-date derived branching fractions, matrix elements and mass splittings for chi(b) transitions in the bottomonium system. Using a new technique, we also measure the two lowest-order spin-dependent coefficients in the nonrelativistic QCD Hamiltonian.
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Borexino Collaboration(Bellini, G. et al), & Pena-Garay, C. (2014). Final results of Borexino Phase-I on low-energy solar neutrino spectroscopy. Phys. Rev. D, 89(11), 112007–68pp.
Abstract: Borexino has been running since May 2007 at the Laboratori Nazionali del Gran Sasso laboratory in Italy with the primary goal of detecting solar neutrinos. The detector a large unsegmented liquid scintillator calorimeter characterized by unprecedented low levels of intrinsic radioactivity is optimized for the study of the lower energy part of the spectrum. During Phase-I (2007-2010) Borexino first detected and then precisely measured the flux of the Be-7 solar neutrinos ruled out any significant day-night asymmetry of their interaction rate made the first direct observation of the pep neutrinos and set the tightest upper limit on the flux of solar neutrinos produced in the CNO cycle (carbon nitrogen oxigen) where carbon nitrogen and oxygen serve as catalysts in the fusion process. In this paper we discuss the signal signature and provide a comprehensive description of the backgrounds quantify their event rates describe the methods for their identification selection or subtraction and describe data analysis. Key features are an extensive in situ calibration program using radioactive sources the detailed modeling of the detector response the ability to define an innermost fiducial volume with extremely low background via software cuts and the excellent pulse-shape discrimination capability of the scintillator that allows particle identification. We report a measurement of the annual modulation of the Be-7 neutrino interaction rate. The period the amplitude and the phase of the observed modulation are consistent with the solar origin of these events and the absence of their annual modulation is rejected with higher than 99% C.L. The physics implications of Phase-I results in the context of the neutrino oscillation physics and solar models are presented.
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