Rare particle decays support standard model

The standard model of particle physics describes the processes observed in the world of elementary particles extremely well. However, there are objects in the physical world that the standard model does not describe, such as the so-called dark matter, which is observed by astronomers in space and forms a major part of the universe, and whose nature is completely unknown. Consequently, researchers are looking for modified or extended versions of the standard model that also include this additional matter. In order to decide whether the classical standard model or a modified form describes nature correctly, researchers examine processes that can be observed, for which the various models make different predictions.

Rare decays as test of the standard model

An example of such a process is the decay of the B_s meson (see box). A B_s meson decays into other particles in a very short time (around 0.000,000,000,001,5 seconds). There are many different decay channels which result in different particles as end products, one resulting in only two muons – one positive, one negative. Two aspects make this decay particularly attractive as a test of the standard model. On the one hand, the probability the Standard Model predicts for this decay can be calculated with high precision. On the other hand, the decay is extremely improbable (according to predictions, only one in 300,000,000 B_s mesons decays in this manner), which means that even minor influences of a new physics beyond the standard model would be evident in a considerable deviation of the decay probability from the predicted value. In the new physics, for instance, additional particles would exist that the standard model does not describe. These could serve as an additional intermediate stage in the transformation of the B_s mesons into two muons, opening up a new path for this decay that alters its probability. Although a decay via this new channel would also be extremely improbable, its probability would be comparable to that predicted by the standard model, enabling us to detect the deviation clearly, explains Urs Langenegger, a physicist from the high energy physics group at PSI who is heading the project. For a decay that is much more probable such a small contribution would not be detectable.

PSI researchers detect rare decays for first time

Along with other PSI researchers, Urs Langenegger has been responsible for the search for this decay within the data of the CMS detector at CERN since 2005. Their crucial task is to find the muons that stem from the decay sought for reliably among the trillions of trillions of particles that have been observed in the CMS detector in the last four years. For the first time, their analysis has now revealed that the probability of these decays is in agreement with the predictions of the standard model. The results were confirmed simultaneously by colleagues in another CERN experiment, LHCb.

Challenge: filtering out the right muons

The challenge in these studies is that a large number of the particles observed appear to be similar to the muons sought for. These similar particles have to be filtered out reliably. The search starts with requiring that the trajectories of a pair of muons start from a common point, which means that they can stem from the decay of one particle. Then the two muons have to have a kinetic energy that corresponds to the mass of the decayed meson. Another key criterion is isolation – in other words, there are no other particles in the vicinity of the muons that come from the same point, explains Langenegger. Otherwise, you can end up studying two muons that come from two different decays that took place next to each other. Besides the muons, however, many other particles are created that signal to us that the muons do not come from the decay we are looking for. In addition there are other particles in the detector that look like muons at first, which also need to be identified reliably. The multitude of criteria for distinguishing between the right and wrong particles had to be tested and translated into corresponding computer programmes, which could then be used to sift through the CMS data.

Nevertheless, there will always be individual muon pairs that look as if they came from the right decay because their energies happen to correspond to the mass of the B_s meson and thus cannot be filtered out. As these pairs are produced randomly, there will be other pairs that look as if they came from the decay of a particle with another mass, producing a continuum in the mass spectrum. This means that evidence for that elusive decay has been found if a distinctly larger number of muon pairs is observed for the correct mass, above the continuum background.

Detector from PSI reveals exact particle trajectories

Researchers from the PSI were not only responsible for analysing the data for the decay of the B_s meson; they also constructed and developed a significant part of the CMS detector – the pixel detector – which constitutes the innermost component of the entire experiment. The pixel detector is able to determine the trajectories of the particles so accurately that it can be clearly distinguished if tracks emerge form the decay of the same particle or from a different one close by. All researchers analysing data from the CMS detector take advantage of information from this detector – including those searching for the Higgs particle.

Search for new physics continues

The fact that the decay probability of the B_s meson into two muons confirms the predictions of the standard model does not mean to say that there is no new physics. It merely rules out a series of models that indicate a different probability for this decay. There are other experiments that test predictions of the standard model, e.g. the MEG experiment, which is being performed at PSI and searches for an extremely rare decay of the muon. Next, the researchers collaborating with Urs Langenegger want to target the usual B mesons. Its decays into two muons are also well suited to test the standard model. However, the data taken by the CERN experiments so far are insufficient to study these decays, explains Langenegger. This will change when the large accelerator of the LHC at CERN is restarted in 2015. Perhaps the observation of the B decay will reveal physics beyond the standard model. And we’ll be right in the thick of the action again.

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.