Physics

Huge CERN experiment detects two extremely rare particle decay events

For several years now, physicists at the European Organisation for Nuclear Research (CERN) have been running a landmark experiment, recording tens of billions of particles break apart in the hopes of catching a few oddballs. And they finally have some intriguing results to share.

This experiment, called NA62, has researchers building and destroying pairs of quarks called kaons, looking for examples of a one-in-10 billion event that could verify some of the predictions of the Standard Model of particle physics. Last year they found one. Now they’ve added another possible two.

The findings were presented at a recent CERN Seminar and were based on data collected in 2017; ten times the amount of data harvested the previous year.

It’s a solid start for NA62. But to be confident in their results, the team is going to need at least a few more examples of a positively charged kaon (K+) decaying into a positively charged pion and a neutrino-antineutrino pair.

Ok, so it’s not exactly edge-of-your seat science. But the goal of this particle lottery could make it all worthwhile. So let’s just skip forward into the future to the much anticipated conclusion of this massive accelerator-based experiment.

There are two potential outcomes. The first is that the insanely rare K+ decay happens just as often as the Standard Model of particle physics says it should. Model verified, all good.

The second possibility is potentially a more exciting one. After crunching the statistics on the positively charged pairs of quarks recombining into other particles, the researchers might find that something just doesn’t add up.

The Standard Model doesn’t currently explain things like dark matter, why matter and antimatter failed to obliterate each other in the early Universe, or why there are differences in the masses of certain fundamental particles.

So finding out there’s something it doesn’t predict very well after all, something we can test with great precision… that could well pave the way towards a Standard Model V2.0.

Using this rather strange quark marriage isn’t an arbitrary decision. Kaons played a key role in establishing the physics of the Standard Model in the first place. So, if we’ve gotten something wrong about the way these particle partnerships behave, it’s going to have some serious consequences.

“This kaon decay process is called the ‘golden channel’ because of the combination of being ultra-rare and excellently predicted in the Standard Model,” says University of Birmingham physicist and NA62 spokesperson, Cristina Lazzeroni.

“It is very difficult to capture, and holds real promise for scientists searching for new physics.”

To give you a small idea of how much effort the experiment requires, here’s their process: a powerful synchrotron is used to shoot protons at super-speeds into a target made out of the metal beryllium.

Amid the resulting carnage of around a billion particles, a handful become kaons – around 60,000,000 of them, in fact. These are channelled off to have their decay is analysed, to pick out signs of the rare one or two transforming into something a bit different.

Thanks to the higher risk of biases affecting such a high precision method, the experiment has a blind phase where the researchers analyse the entire field of particle decays before homing in on areas where they expect to find the all-important signal.

By putting limits on how often this rare process does happen, and comparing it with how often it should happen, physicists get to test their maths with an extreme degree of precision.

So far, the evidence suggests K+ will become a pion, neutrino and antineutrino at most 24.4 out of every 100 billion decays, which is still in line with the Standard Model’s prediction of about 8.4 times per 100 billion.

But the hunt is not over yet. With just three unusual K+ decay events in the box so far, we’re going to need to analyse many more particle collisions before there’s a verdict.

There’s data left over from last year to be analysed, but we’ll need to wait until 2021 before CERN stokes up their super proton collider again.

Even if the Standard Model refuses to budge, little goes to waste in experiments such as these.

“The new result has still limited statistics but has already enabled us to begin putting constraints on some new physics models,” says Lazzeroni.

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