Saturday, October 23, 2021

Physicists may have discovered the 'new force of nature' in the LHC's experiment

 The Large Hadron Collider (LHC) generated worldwide excitement in March, when particle physicists reported tantalizing evidence of a new physics — potentially a new force of nature.

 Now, our new, yet-to-be-peer-reviewed result of CERN's gigantic particle collider appears to be adding more support to the idea.

 Our best current theory of particles and forces is known as the Standard Model, which describes everything we know about the physical things that make up the world around us with unerring accuracy.

 The Standard Model is arguably the most successful scientific theory ever written, and at the same time, we know it must be incomplete.

 Famously, he describes only three of the four fundamental forces – the electromagnetic force and the strong and weak forces, leaving out gravity.  There is no explanation for the dark matter that astronomy tells us dominates the Universe and it does not explain how matter survived during the Big Bang.

 LHCb experiment.  Credit: CERN.

 Most physicists are therefore confident that there must be more cosmic ingredients to discover, and studying a variety of fundamental particles known as beauty quarks is a particularly promising way of getting clues to what else might be out there.

 Beauty quarks, sometimes called bottom quarks, are fundamental particles that, in turn, form larger particles.  There are six flavors of quarks that are called up, down, strange, charm, beauty/bottom and truth/top.  Up and down quarks, for example, constitute the protons and neutrons in the atomic nucleus.

 Beauty quarks are unstable, living on average for only about 1.5 trillionth of a second before decomposing into other particles.  How beauty quarks decay can be strongly influenced by the existence of other particles or fundamental forces.

 When a quark beauty decays, it transforms into a set of lighter particles, such as electrons, through the influence of the weak force.  One of the ways in which a new force of nature can reveal itself to us is by subtly altering the frequency with which beauty quarks decay into different types of particles.

 The study was based on data from the LHCb experiment, one of four giant particle detectors that record the result of ultra-high energy collisions produced by the LHC.  (The “b” in LHCb means “beauty”.)

 He found that beauty quarks were decaying into electrons and their heavier cousins ​​called muons at different rates.  This was really surprising because, according to the standard model, the muon is basically a carbon copy of the electron – identical in every way except that it's about 200 times heavier.

 This means that all forces must attract electrons and muons with equal strength – when a quark beauty decays into electrons or muons via the weak force, it must do so with the same frequency.

 Instead, my colleagues found that muon decay was only happening about 85% as often as electron decay.  Assuming the result is correct, the only way to explain such an effect would be if some new force of nature exerted on electrons and muons in a different way is interfering with how the beauty quarks decay.

 The result caused great excitement among particle physicists.  We've been looking for signs of something beyond the standard model for decades, and despite ten years of work at the LHC, nothing conclusive has been found so far.

 So discovering a new force of nature would be a big deal and could finally open the door to answering some of the deeper mysteries facing modern science.

 new results

 While the result was tempting, it was not conclusive.  All measurements come with a certain degree of uncertainty or “error”.  In this case, there was only one chance in 1,000 that the result would be reduced to a random statistical oscillation – or “three sigma”, as we say in particle physics jargon.

 One in 1,000 might not sound like much, but we do a large number of measurements in particle physics, so you can expect a small handful to come up with outliers just by chance.

 To be sure the effect is real, we would need to come up with five sigma – corresponding to less than a one-in-a-million chance that the effect will be reduced to a cruel statistical fluke.

 To get there, we need to reduce the size of the error, and for that we need more data.  One way to achieve this is to simply run the experiment longer and record more decays.

 The LHCb experiment is being updated to be able to record collisions at a much higher rate in the future, which will allow us to make much more accurate measurements.  But we can also get useful information from the data we've already recorded, looking for similar types of decays that are harder to detect.

 This is what my colleagues and I did.  Strictly speaking, we never study beauty quark decays directly, since all quarks are always linked to other quarks to form larger particles.

 The March study looked at beauty quarks combined with “up” quarks.  Our result studied two decays: one where beauty quarks were paired with “down” quarks and another where they were also paired with up quarks.

 The fact that the pairing is different shouldn't matter, though – the decay that's going on in the background is the same, and so we expect to see the same effect, if there really is a new force out there.

 And that's exactly what we saw.  This time, the muon decays were only happening about 70 percent as often as the electron decays, but with a larger error, meaning the result is about “two sigma” of the standard model (about two in one hundred chances of being an anomalous statistic).

 This means that while the result is not accurate enough on its own to claim firm evidence of a new strength, it aligns very much with the previous result and adds further support to the idea that we may be on the brink of a breakthrough.

 Of course, we must be cautious.  There is still a way to go before we can say with any degree of certainty that we are actually seeing the influence of a fifth force of nature.

 My colleagues are working hard to extract as much information as possible from the existing data, while actively preparing for the first run of the updated LHCb experiment.

 Meanwhile, other experiments at the LHC, as well as the Belle 2 experiment in Japan, are approaching the same measurements.  It is exciting to think that in the coming months or years a new window could be opened on the most fundamental ingredients of our Universe


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