The GERDA experiment at Gran Sasso: Search for neutrinoless double beta decay in germanium 76

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) designed to search for the rare neutrinoless double beta decay (0νββ) of 76Ge. In the first phase of the experiment, high purity germanium diodes inherited from the former Heidelberg-Moscow and IGEX experiments are operated “bare” and immersed in liquid argon, with an overall background environment of 2 · 10−2 cts/(keVkg yr), a factor of ten better than its predecessors. Preliminary measurements on two neutrinos double beta decay (2νββ) giving T2ν 1/2 = (1.88 ± 0.10) · 1021 yr and a recently published background model are discussed in the paper. Results on 0νββ are expected in summer 2013 and, in the absence of a signal, the expected sensitivity is T0ν 1/2 > 1.9 × 1025 yr. Phase II of the experiment is scheduled to start at the end of 2013, after an upgrade shutdown, with an additional set of new detectors. Thanks to the new design of the diodes and to the introduction of liquid argon instrumentation techniques, the experiment aims to reduce further the expected background to about 1 · 10−3 cts/(keVkg yr), and improve the 0νββ sensitivity to T0ν 1/2 > 1.35 · 1026 yr

Design and Tests of the Silicon Sensors for the ZEUS Micro Vertex Detector

To fully exploit the HERA-II upgrade,the ZEUS experiment has installed a Micro Vertex Detector (MVD) using n-type, single-sided, silicon micro-strip sensors with capacitive charge division. The sensors have a readout pitch of 120 micrometers, with five intermediate strips (20 micrometer strip pitch). The designs of the silicon sensors and of the test structures used to verify the technological parameters, are presented. Results on the electrical measurements are discussed. A total of 1123 sensors with three different geometries have been produced by Hamamatsu Photonics K.K. Irradiation tests with reactor neutrons and Co-60 photons have been performed for a small sample of sensors. The results on neutron irradiation (with a fluence of 1 x 10^{13} 1 MeV equivalent neutrons / cm^2) are well described by empirical formulae for bulk damage. The Co-60 photons (with doses up to 2.9 kGy) show the presence of generation currents in the SiO_2-Si interface, a large shift of the flatband voltage and a decrease of the hole mobility.Comment: 33 pages, 25 figures, 3 tables, accepted for publication in NIM

Beam Test of Silicon Strip Sensors for the ZEUS Micro Vertex Detector

For the HERA upgrade, the ZEUS experiment has designed and installed a high precision Micro Vertex Detector (MVD) using single sided micro-strip sensors with capacitive charge division. The sensors have a readout pitch of 120 microns, with five intermediate strips (20 micron strip pitch). An extensive test program has been carried out at the DESY-II testbeam facility. In this paper we describe the setup developed to test the ZEUS MVD sensors and the results obtained on both irradiated and non-irradiated single sided micro-strip detectors with rectangular and trapezoidal geometries. The performances of the sensors coupled to the readout electronics (HELIX chip, version 2.2) have been studied in detail, achieving a good description by a Monte Carlo simulation. Measurements of the position resolution as a function of the angle of incidence are presented, focusing in particular on the comparison between standard and newly developed reconstruction algorithms.Comment: 41 pages, 21 figures, 2 tables, accepted for publication in NIM

The OPERA magnetic spectrometer

The OPERA neutrino oscillation experiment foresees the construction of two magnetized iron spectrometers located after the lead-nuclear emulsion targets. The magnet is made up of two vertical walls of rectangular cross section connected by return yokes. The particle trajectories are measured by high precision drift tubes located before and after the arms of the magnet. Moreover, the magnet steel is instrumented with Resistive Plate Chambers that ease pattern recognition and allow a calorimetric measurement of the hadronic showers. In this paper we review the construction of the spectrometers. In particular, we describe the results obtained from the magnet and RPC prototypes and the installation of the final apparatus at the Gran Sasso laboratories. We discuss the mechanical and magnetic properties of the steel and the techniques employed to calibrate the field in the bulk of the magnet. Moreover, results of the tests and issues concerning the mass production of the Resistive Plate Chambers are reported. Finally, the expected physics performance of the detector is described; estimates rely on numerical simulations and the outcome of the tests described above.Comment: 6 pages, 10 figures, presented at the 2003 IEEE-NSS conference, Portland, OR, USA, October 20-24, 200

Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76

The GERDA experiment searches for the neutrinoless double beta decay of Ge-76 using high-purity germanium detectors enriched in Ge-76. The analysis of the signal time structure provides a powerful tool to identify neutrinoless double beta decay events and to discriminate them from gamma-ray induced backgrounds. Enhanced pulse shape discrimination capabilities of "Broad Energy Germanium" detectors with a small read-out electrode have been recently reported. This paper describes the full simulation of the response of such a detector, including the Monte Carlo modeling of radiation interaction and subsequent signal shape calculation. A pulse shape discrimination method based on the ratio between the maximum current signal amplitude and the event energy applied to the simulated data shows quantitative agreement with the experimental data acquired with calibration sources. The simulation has been used to study the survival probabilities of the decays which occur inside the detector volume and are difficult to assess experimentally. Such internal decay events are produced by the cosmogenic radio-isotopes Ge-68 and Co-60 and the neutrinoless double beta decay of Ge-76. Fixing the experimental acceptance of the double escape peak of the 2.614 MeV photon to 90%, the estimated survival probabilities at Qbb = 2.039 MeV are (86+-3)% for Ge-76 neutrinoless double beta decays, (4.5+-0.3)% for the Ge-68 daughter Ga-68, and (0.9+0.4-0.2)% for Co-60 decays.Comment: 27 pages, 17 figures. v2: fixed typos and references. Submitted to JINS

On the muon neutrino mass

During the runs of the PS 179 experiment at LEAR of CERN, we photographed an event of antiproton-Ne absorption, with a complete pi+ -> mu+ ->e+ chain. From the vertex of the reaction a very slow energy pi+ was emitted. The pi+ decays into a mu+ and subsequently the mu+ decays into a positron. At the first decay vertex a muon neutrino was emitted and at the second decay vertex an electron neutrino and a muon antineutrino. Measuring the pion and muon tracks and applying the momentum and energy conservation and using a classical statistical interval estimator, we obtained an experimental upper limit for the muon neutrino mass: m_nu < 2.2 MeV at a 90% confidence level. A statistical analysis has been performed of the factors contributing to the square value of the neutrino mass limit.Comment: 18 pages, 5 eps figure

DISTO data on Kpp

The data from the DISTO Collaboration on the exclusive pp -> p K+ Lambda production acquired at T_p = 2.85 GeV have been re-analysed in order to search for a deeply bound K- pp (= X) state, to be formed in the binary process pp -> K+ X. The preliminary spectra of the DeltaM_{K+} missing-mass and of the M_{p Lambda} invariant-mass show, for large transverse-momenta of protons and kaons, a distinct broad peak with a mass M_X = 2265 +- 2 MeV/c^2 and a width Gamma_X = 118 +- 8 MeV/c^2.Comment: 8 pages, 4 figures. Talk presented at the "10th International Conference on Hypernuclear and Strange Particle Physics" (HYP-X), Tokai, Ibaraki, Japan, September 14th-18th, 2009. To appear in the proceeding

Prospect for Charge Current Neutrino Interactions Measurements at the CERN-PS

Tensions in several phenomenological models grew with experimental results on neutrino/antineutrino oscillations at Short-Baseline (SBL) and with the recent, carefully recomputed, antineutrino fluxes from nuclear reactors. At a refurbished SBL CERN-PS facility an experiment aimed to address the open issues has been proposed [1], based on the technology of imaging in ultra-pure cryogenic Liquid Argon (LAr). Motivated by this scenario a detailed study of the physics case was performed. We tackled specific physics models and we optimized the neutrino beam through a full simulation. Experimental aspects not fully covered by the LAr detection, i.e. the measurements of the lepton charge on event-by-event basis and their energy over a wide range, were also investigated. Indeed the muon leptons from Charged Current (CC) (anti-)neutrino interactions play an important role in disentangling different phenomenological scenarios provided their charge state is determined. Also, the study of muon appearance/disappearance can benefit of the large statistics of CC muon events from the primary neutrino beam. Results of our study are reported in detail in this proposal. We aim to design, construct and install two Spectrometers at "NEAR" and "FAR" sites of the SBL CERN-PS, compatible with the already proposed LAr detectors. Profiting of the large mass of the two Spectrometers their stand-alone performances have also been exploited.Comment: 70 pages, 38 figures. Proposal submitted to SPS-C, CER

Indication of a deeply bound compact K-pp state formed in the pp -> p Lambda K+ reaction at 2.85 GeV

We have analyzed data of the DISTO experiment on the exclusive pp -> p Lambda K+ reaction at 2.85 GeV to search for a strongly bound compact K-pp (= X) state to be formed in the pp -> K+ + X reaction. The observed spectra of the K+ missing-mass and the p Lambda invariant-mass with high transverse momenta of p and K+ revealed a broad distinct peak with a mass M_X = 2265 +- 2 (stat) +- 5 (syst) MeV/c2 and a width Gamma_X = 118 +- 8 (stat) +- 10 (syst) MeV.Comment: 4 pages, 4 figure

K−^- Meson Production in the Proton-Proton Reaction at 3.67 GeV/c

The total cross section of the reaction $pp\to ppK^+K^-$ has been determined for proton--proton reactions with $p_{beam}=3.67 GeV/c$. This represents the first cross section measurement of the $pp \to ppK^-K^+$ channel near threshold, and is equivalent to the inclusive $pp\to ppK^-X$ cross section at this beam momentum. The cross section determined at this beam momentum is about a factor 20 lower than that for inclusive $pp\to ppK^+X$ meson production at the same CM energy above the corresponding threshold. This large difference in the $K^+$ and $K^-$ meson inclusive production cross sections in proton-proton reactions is in strong contrast to cross sections measured in sub-threshold heavy ion collisions, which are similar in magnitude at the same energy per nucleon below the respective thresholds.Comment: 12 pages, 3 figures Phys. Lett. B in prin