Scintillation efficiency of liquid argon in low energy neutron-argon scattering

Experiments searching for weak interacting massive particles with noble gases such as liquid argon require very low detection thresholds for nuclear recoils. A determination of the scintillation efficiency is crucial to quantify the response of the detector at low energy. We report the results obtained with a small liquid argon cell using a monoenergetic neutron beam produced by a deuterium-deuterium fusion source. The light yield relative to electrons was measured for six argon recoil energies between 11 and 120 keV at zero electric drift field.

First $pi K$ atom lifetime and $pi K$ scattering length measurements

The results of a search for hydrogen-like atoms consisting of $pi^{mp}K^{pm}$ mesons are presented. Evidence for $pi K$ atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic $pi K$ pairs from their breakup in the same target ($178 pm 49$) and from Coulomb final state interaction ($653 pm 42$). Using these results the analysis yields a first value for the $pi K$ atom lifetime of $tau=(2.5_{-1.8}^{+3.0})$ fs and a first model-independent measurement of the S-wave isospin-odd $pi K$ scattering length $left|a_0^-right|=frac{1}{3}left|a_{1/2}-a_{3/2}right|= left(0.11_{-0.04}^{+0.09} right)M_{pi}^{-1}$ ($a_I$ for isospin $I$).

Study of nuclear recoils in liquid argon with monoenergetic neutrons

For the development of liquid argon dark matter detectors we assembled a setup in the laboratory to scatter neutrons on a small liquid argon target. The neutrons are produced mono-energetically (E_kin=2.45 MeV) by nuclear fusion in a deuterium plasma and are collimated onto a 3 liquid argon cell operating in single-phase mode (zero electric field). Organic liquid scintillators are used to tag scattered neutrons and to provide a time-of-flight measurement. The setup is designed to study light pulse shapes and scintillation yields from nuclear and electronic recoils as well as from {alpha}-particles at working points relevant to dark matter searches. Liquid argon offers the possibility to scrutinise scintillation yields in noble liquids with respect to the populations of the two fundamental excimer states. Here we present experimental methods and first results from recent data towards such studies.

Evidence for $pi K$-atoms with DIRAC

We present evidence for the first observation of electromagnetically bound $pi^pm K^mp$-pairs ($pi K$-atoms) with the DIRAC experiment at the CERN-PS. The $pi K$-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the $pi^pm$ and $K^mp$-mesons from the atom dissociation are analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173 $pm$ 54 $pi K$-atoms. The mean life of $pi K$-atoms is related to the s-wave $pi K$-scattering lengths, the measurement of which is the goal of the experiment. From these first data we derive a lower limit for the mean life of 0.8 fs at 90% confidence level.