PERKEO IIB

With the project PERKEO IIB we measured another neutron decay correlation coefficient: The neutrino- asymmetry B. This is the correlation between neutron spin and neutrino momentum.

Largely, we used the setup of the 2003/2004 A-measurement again. We only had to narrow the beam width to minimize the magnetic mirror effect, and we had to use a different detector system (see below) to measure B. The experiment was performed from june to august 2004 at the ILL (Grenoble).

The results of the measurement are the neutrino asymmetry parameter B = 0.9802(50) [Phys. Rev. Lett. 99, 191803 (2007); arXiv:0706.3788], and the proton asymmetry parameter C = -0.2377(26) [Phys. Rev. Lett. 100, 151801 (2008), arXiv:0712.2442].

Physical Motivation

In the Standard Model of Particle Physics parity is maximally violated, but there is no explanation for this symmetry breaking within the model. There are theories beyond the standard model that propose full parity conservation in the early universe (left-right-symmetric models). If these models are correct it should be possible to observe weak right-handed currents. The neutrino-asymmetry B is very sensitive to these currents.

The Spectrometer

The electron spectrometer PERKEO II consists out of two superconducting coils in almost split pair configuration (see figure). The coils produce a 1 Tesla magnetic field perpendicular to the neutron beam. Since the charged decay products (electrons, protons) propagate along the field lines, the spectrometer is divided into two hemispheres. A detector is placed at each side.

The Detector

Since neutrinos cannot be detected directly, we observe electron and proton from the beta decay and determine the neutrino momentum using momentum conservation. Due to the magnetic field there are two possibilities: Electron and proton can be detected in the same or in opposite hemispheres. Hence it must be possible to detect electrons and protons with the same detector. We developed a special detector setup for this problem: The protons are accelerated towards a thin carbon-foil on high voltage (-18 kV), where they are converted into electrons. These are detected by the same plastic scintillators that detect the primary electrons out of the decay.

The PERKEO IIB - Collaboration