New results from one of the experiments installed at the LHC, the world’s largest particle accelerator, may provide clues to what is missing in our understanding of particle physics and, in addition, help explain why the Universe is made of matter, and not antimatter.
The work, which has strong Brazilian participation (centered at the Brazilian Center for Physics Research, in Rio de Janeiro), was presented during a seminar at CERN (European Center for Particle Physics, located in Geneva, Switzerland) last Tuesday (15 ), but has yet to yield a scientific paper published in a peer-reviewed journal — this is being delayed due to the Russian invasion of Ukraine.
The LHC (Large Hadron Collider) is a huge closed loop of 27 km in which superconductors, through magnetic fields, accelerate protons to very high speeds, and then make them collide head-on with each other.
For what? When these protons collide, their mass and all the accumulated motion energy (as they are accelerated to near-light speeds) are converted into a torrent of particles of all kinds, which are then analyzed by detectors installed at various points around the proton. ring.
Each of these points has a specific experiment, and this time we are talking about the LHCb, one of the four most important ones installed in the accelerator. Two others, CMS and Atlas, became famous in 2012, when the discovery of the Higgs boson was announced, celebrated as the missing piece to complete the album of the so-called standard model of particle physics.
This is the name given to the broad theoretical framework that brings together all known particles and their interactions. It is a summary of everything we understand about the components of the universe, supported by quantum mechanics (only the force of gravity excluded, for reasons that physicists still scratch their heads to understand).
Everyone celebrated the discovery of the Higgs, won the Nobel Prize and such, but after the joy, there was the anguish: there are many reasons to believe that the standard model is incomplete. Dark matter, for example, we know exists (because it has remarkable astrophysical gravitational effects), but it doesn’t match anything in the standard model. And, of course, there are fundamental questions yet to be answered, such as the composition of matter in the cosmos, that are not answered in the model.
The LHCb enters the scene, with a result that may begin to help overcome what physicists such as Rogério Rosenfeld, from Unesp (and not involved with the current work), call pHd: “post-Higgs depression”, or post- -Higgs. Its focus is to study the decay process of unstable particles known as B mesons. They are composed of a quark and an antiquark, and this antiquark is of type b (from bottom, or beauty).
If you’ve never heard of quarks, don’t feel bad. They do not exist in the universe, except in extreme situations. Their most stable and typical combination is in trios, where they form the well-known protons and neutrons. But the B mesons arise from the shrapnel of proton collisions at the LHC, and when they decay (which happens very quickly), what’s left of them can be analyzed by the LHCb’s detectors.
Watching this process tons of times, the LHCb collaboration team is noticing what the products of this decay are. And here comes the crucial revelation: the number of positively charged B mesons that decay into a specific product is different from the number of negatively charged B mesons that decay into that same product.
“That is, the particle disintegrates more often than its antiparticle (or vice versa)”, explains LaÃs Soares Lavra, a Brazilian researcher at CERN and a member of the LHCb collaboration. “What we observed was a 75% excess of B mesons over B antimesons.”
The process is known among physicists as a CP (load-parity) violation and is exciting in this case for two reasons. First, because violations of this kind are expected to explain why the Universe is made of matter and not antimatter. We know that the Big Bang (which can be seen as the ultimate LHC, performing the most energetic particle collision experiment possible, when the entire contents of the Universe were crammed into a space smaller than the head of a pin) must have produced, in principle, , equal amounts of matter and antimatter. That is, for each proton, with its positive charge, there would come an antiproton, with a negative charge. For each electron, with a negative charge, a positron, with a positive charge. And so on.
But that raises a problem. Particles and antiparticles, when they meet, annihilate each other, generating radiation as a by-product. At the beginning of the Universe, 13.8 billion years ago, with everything crammed in the way it was, there was no shortage of occasions for these collisions, so we might expect a cosmos of pure radiation, without matter — and without grace.
And yet it is not what we have. We end up, fortunately for us, with a universe that is all made of matter. Which means that somehow there was a surplus of particles over antiparticles when the collisions ended — and one way to explain this is to find CP asymmetries. Somehow, the laws of physics must favor matter over antimatter.
Does the new result resolve this issue? “It doesn’t work,” says Lavra. “We measure this asymmetry, but there are some models that predict the observation of this asymmetry — but in the order of 10%.”
And that’s where the result is really promising. If it were the 10%, everything would be “at home” with our current understanding of physics. As there was much more – 75% – new ideas will be needed to explain the huge disparity. “It’s the biggest asymmetry ever observed so far”, completes the researcher.
Experimental particle physics, however, is a tricky business. When you shatter protons with high energy and convert everything into more particles, much of what happens has a component of randomness introduced by quantum mechanics, so you can only confirm a specific finding after the same phenomenon is observed with enormous frequency.
The Higgs boson, for example, was only announced after systematic detection of the particle reached a 5-sigma confidence, scientific for “less than 1 in 3.5 million probability of a spurious result.”
Therefore, LHCb personnel remain cautious. “It’s still too early to talk about new physics,” says Lavra. “One of the models predicts 10%, so the other 65% could come from other unknown mechanisms, which may or may not be accommodated in the standard model. It could be a sign of new physics or not, we don’t know yet.”
How to deepen the investigation? More results. The LHC will resume the experiments, in the third data collection, on the 24th. “In particular, the LHCb experiment will have a new detector, capable of providing up to ten times more statistics than currently. This will allow a more detailed study of these processes.
