The experiment

With the aSPECT experiment we aim to measure the energy spectrum of the protons that are emitted in the beta-decay of the free neutron. From this spectrum the angular correlation of the beta-particle and the neutrino from the decay is extracted. This correlation is represented by the angular correlation coefficient a in the decay probability


Beta-Neutrino angular correlation


 On the theoretical side a depends on the coupling constants of the electroweak interaction. Its measurement can be used to determine parameters of the standard model like the coupling constants  gV and gA (V-A interaction)  and to determine the first element of the quark mixing matrix Vud, which is used to test the unitarity of the CKM-matrix.  A measurement of a can also be used to test the symmetries of the standard model, e.g. whether parity really is maximally violated, and to search for exotic interactions like scalar or tensor interaction, which are not present in the standard model. aSPECT uses a cold neutron beam as source of the neutrons. Protons from neutron decays that take place in the decay volume of aSPECT are guided by a magnetic field into a spectrometer, which is an electrostatic filter of MAC-E type (link). Protons with sufficient kinetic energy to pass the retardation potential in the analyzing plane of the spectrometer are refocused onto a silicon drift detector where they are counted. By variation of the retardation potential a cumulative energy spectrum is obtained. At present aSPECT ist set-up at the Institut Laue-Langevin, ILL, Grenoble, France, which operates a high-flux research reactor for neutron production. This reactor has the most intense continuous neutron flux in the world. A spherical panorama of the guidehall with aSPECT inside can be found here.




The planned sensitivity of aSPECT is Δa/a=0.3%, compared with the best measurements in the world to date of Δa/a=5%. In a measurement in 2008 the major systematic errors have been identified. They have been fixed for a beamtime in summer and autumn 2011. In this beamtime we want to reach a precision of Δa/a≈1%. During this beamtime also more detailed studies of several second order systematic effects and of the background will be performed. Their understanding will be crucial to reach the sensitivity of Δa/a=0.3%  in the future.

The principle of aSPECT is described more thoroughly here. For more details on the experiment click here.

For open positions click here. In addition we have Hiwi positions for students.

Most of the aSPECT collaboration during the successful beam time in 2013:

aSPECT collaboration photo 2013




More Information

PhD theses:

F. Ayala Guardia
M. Borg
G. Konrad
R. Munoz Horta
M. Simson

aSPECT is funded by the PERC Schwerpunktprogramm of the DFG.

People involved

Prof. W. Heil, JGU Mainz (homepage, e-mail)

Dr. M. Beck, JGU Mainz (homepage, external homepage, e-mail)

Dr. M. Borg, JGU Mainz (homepage, e-mail)

F. A. Guardia, JGU Mainz (homepage, e-mail)

A. Wunderle, JGU Mainz (homepage, e-mail)

Ch. Schmidt, JGU Mainz



G. Konrad, JGU Mainz/TU Wien, Austria (homepage, e-mail)

Prof. O. Zimmer, ILL, France

Dr. T. Soldner, ILL, France


Dr. M. Simson, ILL, France

R. Maisonobe, ILL, France

Dr. F. Glück, KIT, Karlsruhe

Prof. S. Baessler, University of Virginia, USA (homepage, e-mail)









Prof. Dr. Werner Heil (office 02-627), Tel. +49 (0) 6131 39 22885, email

Dr. Marcus Beck (office 02-625), Tel. +49 (0) 6131 39 25974, email