K. Gabathuler

Limits on cold dark matter from the Gotthard Ge experiment

Abstract Data from a high purity Ge detector operated in the Gotthard underground laboratory, corresponding to 51.6 kg day, were used to set limits on cold dark matter. In particular, Dirac neutrinos with masses between 10 and 2400 GeV are ruled out. A new lower limit on the half-life for the decay of the electron into weakly interacting particles of 1.9 × 1023 yr at 68% CL was also derived.

Search for ββ decay in 136Xe: new results from the Gotthard experiment

Abstract The Gotthard Xe experiment has taken an additional 6013 hours of data with an upgraded readout system. Lower limits for the half-life of the double beta decay in 136 Xe are T 1/2 0 ν >4.4×10 23 yr (including 6830 hours taken in an earlier phase), T 1/2 0 νχ 0 >0.72×10 22 yr and T 1/2 2 ν >3.6×10 20 yr at 90% confidence level. Alpha spectroscopy, as well as βα and αα coincidence techniques lead to a determination of the activity of the gas of 10 −12 g/g effective of 232 Th and 238 U. Evaluations of background strongly suggest that an appreciable fraction of the ββ -candidates comes from the allowed 2 ν decay.

A search for double beta decay in 76Ge

This thesis describes a search for the double beta (ββ) decay of ⁷⁶Ge using an narray of eight high purity, high resolution germanium detectors containing a total nof 3.9 x 10²⁴ ⁷⁶Ge nuclei. There are three modes of ββ decay: neutrinoless (0ѵ)ββ ndecay, ββ decay with Majoron emission (x⁰ββ decay) and two neutrino (2ѵ)ββ ndecay. The first two modes violate lepton number conservation, while the third is nallowed by the Standard Model. 0ѵββ decay may take place to the ground state of nthe daughter nucleus (0⁺ → 0⁺ transition) or to an excited state of the daughter nnucleus (0⁺ → 2⁺ transition). The detector was operated for a total time of 2033 h n(0.23 y) which translates to 1.30 kg-y. The background at the ββ decay transition nenergy was 0.53 counts keV⁻¹ y⁻¹ (10²³ ⁷⁶Ge nuclei)⁻¹. No evidence for double beta ndecay of any sort was found and the half life limits are T0ѵ½(0⁺ → 0⁺) > 1.2 x 10²³ y, nT0ѵ½(0⁺ → 2⁺) > 2 x 10²² y, Tχo½ > 8 x 10²⁰ y, and T2ѵ½ > 2 x 10²⁰ y, all at 90% c.l. nThe limit for the neutrinoless mode translates to an upper limit of between 16 and n1.6 eV on the Majorana mass of the neutrino, depending on the nuclear matrix nelement used.

Limit on double beta decay with majoron emission

Abstract A search for majoron emission accompanying neutrinoles double beta decay in 76 Ge was conducted with a low background germanium detector in the St. Gotthard underground laboratory. No evidence for neutrinoless double beta decay with or without majoron emission was found, with a lower limit for the half life of the majoron branch of t 1 2 > 1.2 × 10 21 yr at 90% confidence level.

Limits on neutrinoless double beta decay in 136Xe with a time projection chamber

Abstract A xenon Time Projection Chamber with an active volume of 207 liters has been built to study 0ν and 2ν double beta decay in 136Xe. Data were taken in the Gotthard Underground Laboratory, with 5 atm of xenon enriched to 62.5% in 136Xe. From 4861 hours of data, no evidence has been found for the 0+ → 0+ transition. Half-life limts of T 1 2 0ν (0 + → 0 + ) > 2.7(5.2) × 10 23 years in the mass mechanism mode, and T 1 2 0ν (0 + → 0 + ) > 1.9(3.5) × 10 23 years in the right-handed currents mode, at the 90(68)% C.L., were derived. Upper limits for the Majorana neutrino mass parameter were deduced.