What if the odds of an event occurring were about one in ten billion? This is the case for the decay of a positively charged particle known as a kaon into another positively charged particle called a pion and a neutrino-antineutrino pair. Yet, such a rare event, which has never been observed with certainty, is something that particle physicists really want to get their hands on.

The reason? The Standard Model of Physics predicts such one-in-ten-billion odds with an uncertainty of less than ten percent. A deviation from this prediction, revealed by a precise measurement of the decay, could therefore be a clear indicator of physics beyond the Standard Model.

In a seminar taking place today (Tuesday 27th March, 2018) at CERN, the NA62 collaboration reports a candidate event of this ultra-rare kaon decay found using a new "in-flight decay" approach. The result was also presented earlier this month at the Rencontres de Moriond conference in Italy.

While this single event cannot be used to probe beyond-Standard-Model physics, it demonstrates that the approach works well and can be applied to catch more events in the next run of data-taking, which kicks off in mid-April.

Professor Cristina Lazzeroni, from the University of Birmingham's School of Physics and Astronomy, said: 'We are delighted to have played a leading role in the NA62 experiment. Today we have shown that we are indeed able to measure the ultra-rare decay K+ to pi+ nu nu. This comes after years of work on the detector and the data analysis. Scientists here at Birmingham have designed and built the detector and readout system that identifies the kaon particles in the beam which is mainly formed by pions with only 6% of kaons; this detector is therefore an essential element of the experiment, and has the best time resolution of all the components; we are very proud that it works brilliantly and has allowed this measurement to be made. With the additional data that we have and will keep collecting, we will be able to establish if this particular decay agrees or not with the very clear prediction from the Standard Model, and so we will be able to discover or constrain new physics scenarios.'

What is the NA62 experiment?

The NA62 experiment is a particle physics experiment at CERN using a 400 GeV proton beam from the SPS (Super Proton Synchrotron) accelerator. The experiment began taking data taking in 2016. The main aim of the NA62 experiment is to study rare kaon decays and to pin down possible effects of physics beyond the Standard Model that appear in short distance interactions involving quarks. Specifically, NA62 will measure the rate at which the charged kaon decays into a charged pion and a neutrino-antineutrino pair. This process is one of the rarest among meson decays, with a probability of happening of about 1 over 10,000,000,000, and it is extremely well predicted by the Standard Model.

Several models of new physics make a different prediction for this decay. If the measurement of the rate deviates from the Standard Model prediction, it would indicate a new phenomenon for physics, beyond the Standard Model. If it turns out to be consistent with the Standard Model, it is further evidence of its accuracy and can be used to put stringent constraints on models of new physics.

How do you do such a thing?

First, you have to make a beam that contains kaons. Colliding high-energy protons from the Super Proton Synchrotron (SPS) into a stationary beryllium target creates a beam of secondary particles which contains and propagates almost one billion particles per second, about 6% of which are kaons.

Before entering a large vacuum tank, each particle in the beam has its momentum measured by a silicon-pixel detector. A detector called KTAG (kaon tagger) determines the types of particle in the beam from their Cherenkov radiation, and identifies which ones are kaons.

Further detectors inside the tank look for decay particles: a magnetic spectrometer measures the momentum of charged tracks from kaon decays, a ring imaging Cherenkov (RICH) detector tells the team the nature of decay particles, and electromagnetic and hadronic calorimeters measure their energy. A large system of photon, muon and charged particle detectors reject unwanted decays.

When trying to measure very rare processes like this one, predicted to take place only once every 10,000-100,000 million events, the team has to be very careful not to apply selection criteria that might bias the result. For this reason, it is custom to perform a 'blind analysis', where physicists initially only look at the background to check that their understanding of the various sources is correct. Only once they are satisfied with that, they look at the region of the data where the signal is expected to be (the "opening of the blind box").

What was the UK involvement?

The University of Birmingham played a leading role in the detector construction for NA62, built the readout system for the KTAG (kaon tagger detector that identifies kaons in the beam line), designed and implemented the high-level (offline) trigger system of the experiment, and part of the (online) L0 trigger, designed and implemented the experiment control system (Run Control), and has given crucial contributions to the experiment commissioning and operation, to the calibration and data quality assurance systems.

The UK has also contributed to other aspects of the detector construction and to the MonteCarlo simulation.

The UK also has a leading role in data analysis and production of physics results, with Birmingham physicists driving all of the NA62 publications so far. UK physicists cover leadership positions in the data analysis, including the Physics Coordinator (Giuseppe Ruggiero, University of Lancaster) and the Lepton Flavour Working Group coordinator (Evgueni Goudzovski, University of Birmingham).

What's next for the NA62?

The team will continue to collect more data and refine the analysis technique using 2017 and 2018 data. This optimisation will allow the team to increase the number of signal events, and reach the sensitivity required to show to what extent this process agrees with the Standard Model prediction.

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