When beauty becomes strange | Big nove …

The smallest bricks that make up the material universe are elementary particles. Its rules of the game are dictated by the standard model, designed in the sixties. Despite its modest name, it is the most precise scientific theory ever constructed. We can verify your predictions in several ways; the most common and the most robust: by collisions in particle accelerators

The most energetic particle accelerator is the Large Hadron Collider (LHC), an underground ring more than 27 kilometers in circumference built by CERN on the Franco-Swiss border, at a depth of around 100 meters. There, protons traveling in opposite directions are accelerated until they are caused to collide at 14 TeV (for reference, their temperature equivalent is nearly a trillion degrees) at four points in l ‘ring where huge detectors called Alice, Atlas, CMS and, today’s protagonist, LHCb.

The last letter of the LHCb experiment is the initial of the word beauty. And it is that physicists have given free rein to the imagination when it comes to baptizing the six quarks, and we call two of them strange and beauty (syb, for his initials in English). Quarks, remember, are the particles that make up the proton and neutron, which in turn populate the nucleus of atoms. But s and b quarks are not inside nuclei but in unstable particles that can only be seen in detectors by the traces they leave. Let’s look at an example.

There is a particle that contains an (anti) b quark called B +, heavier than five protons, whose fleeting existence barely exceeds a billionth of a second. What can happen after this period is a range of over five hundred possibilities. We will focus on two of them, which take place almost once in a million decays: B + becomes K + (what really happens is that the (anti) quark b becomes the (anti) quark s) and they project two other particles: an electron-antielectron pair or a muon-antimuon pair. The K + particle, whose mass is half that of a proton, has a lifespan of just over one hundred millionth of a second. It doesn’t seem like much, but he’s ten thousand times longer than his mother, B +: strangeness is more enduring than beauty in the subatomic world.

The main cause of these decays is the weak interaction, one of the four known. According to the Standard Model, the previous two processes must occur with exactly the same number of times or, to be more precise, with the same probability (remember that the microscopic universe is a large timba). This democratic distribution of chances is pompously known as the lepton universality (since the electron and the muon are what we call leptons). So, if we could see how many times each of the previous decays occurs, the report between them should be 1.

It is easy to imagine that the experimental difficulties in making this comparison are innumerable. These are particles that live very little and appear at intermediate stages of complicated processes. To make matters worse, quantum mechanics imposes its probabilistic laws, so we have to experiment countless times to adjust this. report. The analysis of this vast data is extremely complex and cannot be done justice in this sense. Allow me to oversimplify.

Imagine that before you run any given ad what will come out and hit. Maybe I was lucky. I start again and it happens again. You start to look at me in surprise. I keep repeating it and surprise turns into suspicion: will the dice be loaded? I throw it again and history repeats itself: four consecutive hits! Would you dare bet against me on a fifth pitch? If the die is not loaded, the odds of you hitting will remain, as always, one in six, so you need to bet. However, you think that “four consecutive moves are too much of a coincidence: this die is rigged”. And I suspect you wouldn’t take the bet.

What has been observed with the decay of B + is that the report of the two preceding processes gives a number between 0.805 and 0.89, far from the leptonic universality. Far? Is it not possible that we got these numbers just because we did not observe enough? In the example above, if I kept rolling the dice and it wasn’t cheated, each number would end up rolling the same number of times, exposing my alleged divinatory gifts. As was the case with the octopus Paul at the World Cup in South Africa. This charge the B + meson and the probabilities of the two processes are not the same? Or is it a matter of continuing to observe?

Maybe the report as we see more and more decays in the future. However, if we look back we find the opposite trend: As more data was observed, the LHCb found the violation of lepton universality to be more and more robust. Hence the “cautious optimism” with which this result was presented. In order for it to be considered a proven fact, the report with many more observations. In the dice example, you would need four additional throws: eight consecutive random hits are implausible under the laws of chance. Any expert would consider, in this case, that the die is rigged.

If the announced result is consolidated in the next few years, we can categorically say that the standard model is incorrect. Although it sounds strange, we’ll celebrate in style: we need clues that allow us to understand what dark matter is, why there is more matter than antimatter, what is dark energy, among other issues that await any setbacks in known physics. . Perhaps it’s a consolation after the disappointment to realize that fleeting beauty, when it gets weird, isn’t fair when it comes to generating leptons.

* Theoretical physicist, IGFAE, University of Santiago de Compostela ([email protected])