Now We’re Looking for Lepton Flavour Violation
Wouldn’t we all like to think that the world that we’re living in is more or less stable? Isn’t there a certain pleasure to be sure that our feet will be pulled to the ground as firmly tomorrow as they are today? Isn’t it reassuring to know that the cup of tea we’ve just put on our desk won’t disappear instantly and reappear on the bottom of the sea on the other side of the planet having traveled its diameter on a straight line? In classical physics, Newton’s laws give us this reassurance. These laws bestow predictability on objects or events as they exist or happen in our reality - on a macroscopic level. On a microscopic level - in particle physics - Fermi’s interaction theory, for instance, postulates that the laws of physics remain the same even after a particle undergoes substantial transformation.
In 1964, however, it became apparent that this isn’t always the case. James Cronin and Val Fitch showed, by examining the decay of subatomic particles called kaons, that a reaction run in reverse does not necessarily retrace the path of the original reaction. This discovery opened a pathway to the theory of electroweak interaction, which in turn gave rise to the theory we all now know as the Standard Model of particle physics.
Although the Standard Model is currently the most convenient paradigm to live with, it doesn’t explain a number of problems, including gravity or dark matter. Other theories compete very actively for the leading role in describing the laws of nature in the most accurate and comprehensive way. To succeed, they have to provide evidence of something that happens outside the limitations of the Standard Model. A promising area to look for this kind of evidence is the decay of a charged lepton (tau lepton) into three lighter leptons (muons), which happen to have a certain characteristic - flavour - that is different from the same characteristic of their ‘mother’ particle. According to the Standard Model, the probability of this decay is vanishingly low, but it can be much higher in other theories.
One experiment at CERN, LHCb, aims at finding this τ → 3μ decay. How are they going to find it? By searching for statistically significant anomalies in an unthinkably large amount of data. How can they find statistically significant anomalies in an unthinkably large amount of data? By using algorithms. These can be trained to separate signal (lepton decays) from background (anything else, really) better than humans. The problem here, however, is not only to find these lepton decays, but also find them in statistically significant numbers. If the Standard Model is correct, the τ → 3μ decays are so rare that their observations are below experimental sensitivity.
To come up with a more sensitive and scale-appropriate solution that would help physicists find evidence of the tau lepton decay into three muons at a statistically significant level, Yandex and CERN’s LHCb experiment have launched a contest for a perfect algorithm. The contest, called ‘Flavours of Physics’, starts on July 20th with the deadline for code submissions on October 12th. It is co-organised with an associated member of the LHCb collaboration, the Yandex School of Data Analysis, and Yandex Data Factory - a big data analytics division of Yandex - and is hosted on a website for predictive modeling and analytics competitions, Kaggle. The winning team or participant will claim a cash prize of $7,000, with $5,000 and $3,000 awarded to the first and the second runners-up. An additional prize in the form of an opportunity to participate in an LHCb workshop at the University of Zurich and $2,000 provided by Intel will be given to the creator of an algorithm that will prove to be the most useful to the LHCb experiment. The data used in this contest will consist both of simulated and real data, acquired in 2011 and 2012, that was used for the τ → 3μ decay analysis in the LHCb experiment.
Contest participants can build on the algorithm provided by the Yandex School of Data Analysis and Yandex Data Factory to make an algorithm of their own.
The metric for evaluation of the algorithms submitted for this contest is very similar to the one used by physicists to evaluate significance of their results, but is much more simple and robust thanks to the collective effort of the Yandex School of Data Analysis and LHCb specialists who have adapted procedures routinely used in the LHCb experiment specifically for this contest. Our expectation is that this metric will help scientists choose the algorithms that they could use on data that will be collected in the LHCb experiment in 2015, and in a wide range of other experiments.
Finding the tau lepton decay might take us out of the comfort zone of the Standard Model, but it just as well may open the door to extra dimensions, shed light on dark matter, and finally explain how gravity works on a quantum level.
Collisions as seen within the LHCb experiment's detector (Image: LHCb/CERN)