Thu·Oct 6, 2016Thursday·October 6, 2016

1. 01*

   Initial Results from the MAJORANA DEMONSTRATOR

   S.R. Elliott🇺🇸 · N. Abgrall🇺🇸 · I.J. Arnquist🇺🇸 · F.T. Avignone III🇺🇸 · A.S. Barabash🇷🇺 · F.E. Bertrand🇺🇸 · A.W. Bradley🇺🇸 · V. Brudanin🇷🇺 · M. Busch🇺🇸 · M. Buuck🇺🇸 · T.S. Caldwell🇺🇸 · Y-D. Chan🇺🇸 and 55 other authors

   Neutrinoless double-beta decay searches seek to determine the nature of neutrinos, the existence of a lepton violating process, and the effective Majorana neutrino mass. The {\sc Majorana} Collaboration is assembling an array of high purity Ge detectors to search for neutrinoless double-beta decay in ${}^{76}$Ge. The {\sc Majorana Demonstrator} is composed of 44.8~kg (29.7 kg enriched in ${}^{76}$Ge) of Ge detectors in total, split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. The initial goals of the {\sc Demonstrator} are to establish the required background and scalability of a Ge-based, next-generation, tonne-scale experiment. Following a commissioning run that began in 2015, the first detector module started physics data production in early 2016. We will discuss initial results of the Module 1 commissioning and first physics run, as well as the status and potential physics reach of the full {\sc Majorana Demonstrator} experiment. The collaboration plans to complete the assembly of the second detector module by mid-2016 to begin full data production with the entire array.

   nucl-exhep-exphysics.ins-detJ.Phys.Conf.Ser.(2017)·34 citations
2. 02*

   First observation of a reactor-status effect on the beta+ decay rate of 22Na

   Robert de Meijer (1 and 2) · Albert Zondervan (3) · Jan Stegenga (4) · Steph Steyn (5) · Robbie Lindsay (2) · Milton van Rooy (6) · Marco Tijs (7) · Han Limburg (7) ((1) Stichting EARTH, Netherlands, (2) University of the Western Cape, South Africa, (3) GNS Science, New Zealand, (4) INCAS^3, Netherlands, (5) ESKOM, South Africa, (6) NMISA, South Africa, (7) MEDUSA Sensing, Netherlands)

   In the search for an electron antineutrino detection method with sensitivity below the 1.8 MeV threshold for the inverse beta decay reaction, beta decay counting experiments with ca. 3 kBq 22Na and 60Co sources were conducted at unit #1 (2.775 GW_th) of the Koeberg Nuclear Power Station in South Africa. The setup consisted of one NaI crystal to measure de-excitation and annihilation photons associated with beta decay. Its volume and well shape were chosen to use coincidence summing in order to differentiate between electron capture and beta+ emission in 22Na. The setup was shielded from the reactor core by 8 m of uninterrupted concrete. Background radiation, responsible for ca. 1% of the total countrate with either source, increased by merely 3% when the reactor status changed from OFF to ON. Normalized countrates of three energy regions-of-interest (TOT, MED, HI) were parameterized to jointly describe the time dependence of two instrumental effects and a reactor-status step function in a least-squares regression analysis. With the 22Na source, the fractional countrate changes in the step from reactor OFF to ON were: (delA/A)_TOT = [-3.02 +- 0.14(stat) +- 0.07(syst)] x 10^-4, (delA/A)_MED = [+1.44 +- 0.42(stat) +- 0.07(syst)] x 10^-4, and (delA/A)_HI = [-2.70 +- 0.26(stat) +- 0.04(syst)] x 10^-4. The uncertainty budget is incomplete because it does not contain the possible influence from environmental factors and the finite stability of the MCA clock-oscillator. No reactor-status dependence was observed with the 60Co source. The corresponding cross sections are [1.55 +- 0.07(stat)] x 10^-25 cm^2 for EC + beta+ decay in 22Na and [0.5 +- 1.5(stat)] x 10^-26 cm^2 for beta- decay of 60Co. The negative sign for TOT and HI activity changes in 22Na points to an antineutrino related interference effect on the beta+ decay of 22Na and rules out reactor neutron induced reactions.

   nucl-exphysics.ins-det2 citations
3. 03*

   Pion-nucleon scattering in the Roper channel from lattice QCD

   C. B. Lang🇦🇹 · L. Leskovec🇺🇸 · M. Padmanath🇩🇪 · S. Prelovsek🇸🇮

   We present a lattice QCD study of $N\pi$ scattering in the positive-parity nucleon channel, where the puzzling Roper resonance $N^{\ast }(1440)$ resides in experiment. The study is based on the PACS-CS ensemble of gauge configurations with $N_f=2+1$ Wilson-clover dynamical fermions, $m_{\pi }\simeq 156$MeV and $L\simeq 2.9$fm. In addition to a number of $qqq$ interpolating fields, we implement operators for $N\pi$ in $p$-wave and $N\sigma$ in $s$-wave. In the center-of-momentum frame we find three eigenstates below 1.65 GeV. They are dominated by $N(0)$, $N(0)\pi (0)\pi (0)$ (mixed with $N(0)\sigma (0)$) and $N(p)\pi (-p)$ with $p\simeq 2\pi /L$, where momenta are given in parentheses. This is the first simulation where the expected multi-hadron states are found in this channel. The experimental $N\pi$ phase-shift would -- in the approximation of purely elastic $N\pi$ scattering -- imply an additional eigenstate near the Roper mass $m_R\simeq 1.43$GeV for our lattice size. We do not observe any such additional eigenstate, which indicates that $N\pi$ elastic scattering alone does not render a low-lying Roper. Coupling with other channels, most notably with $N\pi \pi$, seems to be important for generating the Roper resonance, reinforcing the notion that this state could be a dynamically generated resonance. Our results are in line with most of previous lattice studies based just on $qqq$ interpolators, that did not find a Roper eigenstate below $1.65$GeV. The study of the coupled-channel scattering including a three-particle decay $N\pi \pi$ remains a challenge.

   ↳ hep-lathep-phnucl-exnucl-thPRD(2017)·128 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.