The weak side of the proton

“The weak interaction is one of the four fundamental forces of nature. Even though it is not part of our everyday experience, it is nonetheless involved in many important processes, like energy production by the sun and particle decay”, explained Klaus Kirch, head of the Particle Physics Laboratory at PSI. It is also an essential part of the Standard model, which is currently the best description of the world of elementary particles. Now, an international team of scientists from the USA, Russia, Belgium and Switzerland have examined the way protons participate in the weak interaction. In particular, they have determined the “pseudo scalar coupling”, one of the coupling constants that determine how strong the weak interaction is for the proton. The proton is a fundamental building block of the matter that surrounds us. It is itself composed of further sub-particles – quarks and gluons. As a result, protons exhibit highly complex behaviour, which cannot yet be calculated accurately using current computers. However, a low-energy expansion of the fundamental theory has been developed, which provides precise predictions in agreement with the new experimental results.

Muon tests weak force for protons

In their experiment, the scientists investigated the probability with which a proton captures a muon – a reaction induced by the weak force. The muon is very similar to the electron, but about 200-times heavier, and unstable – it decays into other particles in about 2 millionths of a second. Just like an electron in a normal hydrogen atom, a muon can bind to a proton. But since it is much heavier, it is much closer to the proton, and so can easily lead to a capture reaction. The proton is then transformed into a neutron, and the muon is converted into a neutrino.

Development of new technologies

Several new technologies had to be developed and combined to make a precise experiment possible. “A key element of the experiment was a ‘time projection chamber’ which was immersed in a tank of hydrogen gas. With this chamber, we could record the track of each muon in three dimensions to verify that it stopped in the target – a critical condition for the success of the experiment. Following the pioneering studies by the collaborating Petersburg Nuclear Physics Institute (PNPI), the novel high purity detector was designed and built at PSI”, said Malte Hildebrandt, a scientist at PSI and head of the Detector Group. The required ultrahigh purity of the target hydrogen was achieved and maintained by a continuous hydrogen purification system built at PNPI.

How can you tell when a proton has captured a muon?

“In the time projection chamber, a single muon is introduced“, said Bernhard Lauss, an experimental physicist at the PSI. “It displaces the electron from the hydrogen atoms and takes its place around the proton.” Then the muon disintegrates, emitting an observable electron. This muon can also be captured by the proton and disappear by an alternative route. Because of this additional possibility, a muon near a proton “lives” shorter on average than a free muon. This lifetime can be determined by observing the decay of the resulting electrons in a surrounding detector. By comparing this to the lifetime of free muons, which is very accurately known from measurements made at the PSI, it is possible to compute the corresponding coupling constants.

Experiment only possible in a lifetime at PSI

“At the present time, an experiment like this can only be conducted at PSI“, stressed Peter Kammel, a scientist at the University of Washington in Seattle (USA), and one of the two spokespersons of the experiment. “Only the PSI accelerator produces enough muons to collect this massive data set in a realistic timeframe. Still, it took several years of data taking and plenty of hard work, in particular by our students, to analyse the experiment at US supercomputer facilities”.

High intensity versus high energy

This experiment is an example of particle physics research, where the large number of particles available -– in this case muons – is critical to measure an important physics quantity to high precision. These studies are complementary to experiments at the highest-energy accelerators, where one can look deep inside other particles, or can generate new particles with high masses. Switzerland, with PSI and CERN, has the best facilities in the world for both types of experiments.

Sources of “psychological” error ruled out

Long before the experiment was conducted, theoretical physicists had calculated the value of the coupling constant. So there was the risk that experimenters might be unconsciously influenced by these predictions when they analysed their measurements. To prevent this bias, the collaboration modified their results by a secret factor. In this way, they were unable to assess how close their result was to the theory. “It was only at a very exciting unblinding meeting held after our analysis had been completed, that the secret value of this factor was revealed, and the actual result could be determined”, said Claude Petitjean, the second experiment spokesperson.

Participants in the project included:

• USA: Universities of Washington-Seattle, Kentucky-Lexington, Illinois-Urbana-Champaign, California-Berkeley, Regis-Denver, and Boston
• Russia: Petersburg Nuclear Physics Institute
• Belgium: University of Louvain
• Switzerland: Paul Scherrer Institute PSI

Text: Paul Piwnicki

About PSI

The Paul Scherrer Institute develops, builds and operates large, complex research facilities, and makes them available to the national and international research community. The Institute's own key research priorities are in the investigation of matter and material, energy and the environment; and human health. PSI is Switzerland's largest research institution, with 1500 members of staff and an annual budget of approximately 300 million CHF.