In 2012, the European Laboratory for Particle Physics (CERN) announced the discovery of the Higgs boson. This particle, which explains why matter has mass, confirmed predictions made in 1964 and is a fundamental piece of the Standard Model, the theory that explains the behavior of the particles that make up the world. The discovery was possible thanks to a huge infrastructure, the Large Hadron Collider (LHC), a ring 27 kilometers in diameter built near Geneva (Switzerland) that cost more than 7 billion euros. Inside, protons circulate directed by large magnets and collide with each other, recreating situations that have not existed in nature since the first moments after the Big Bang.
That milestone attracted extraordinary interest in fundamental physics that has since faded, but the machine continues to work in search of information with which to understand what the cosmos is made of. Carla Marín (Barcelona, 33 years old) is one of the scientists who continues to test the limits of the Standard Model. A professor at the University of Barcelona, she has just received the prize as a young researcher in Experimental Physics awarded by the Royal Spanish Society of Physics and the BBVA Foundation.
In Geneva, Marín collaborates on the LHCb detector, one of the four large ones where the LHC makes protons collide to test the limits of physics between the disintegrations that remain after the impact. “We are specialists in studying b-type quarks, the heaviest we know,” says Marín. “We measure very precisely how they are created, how they associate with other quarks or how they disintegrate, because they are unstable particles, and we compare that behavior with what the theory tells us, to see if at some point it breaks; we look for new physics indirectly,” she explains.
Ask. Can you remind us what you are doing at the LHC?
Answer. We accelerate protons at a high speed, giving them a lot of energy, and when they already have the highest energy we can give them, we make them collide. Hence, the fact of having so much energy is where these particles that have more mass are produced. They do not arise from nothing, we are transforming energy into matter.
Q. Have these particles that appear after these collisions ever existed in the universe?
R. We think so, but we don't know. It is believed that at the beginning of the universe, when it was very energetic, very hot and very dense, it was concentrated in a very small space. The Big Bang theory tells us that, at that moment, there was a state, a soup of quarks, where all these quarks that we know existed, the six quarks that the Standard Model predicts, which circulated freely, due to the amount of energy that they had. there was. We try to reproduce the conditions that existed at the origin of the universe.
Q. You have been awarded a Starting Grant from the European Research Council to carry out the CLIMB project. What are you going to look for?
R. We want to study the decays of a b quark, which is one of the most massive, to a d quark, which is one of the lightest, and two leptons. Because? This, in the Standard Model, is possible, but it happens very rarely. On the order of one every 100 million times a b quark decays it will do so. If there is another particle, another force that we do not know so far, that the standard model does not predict, but that exists in nature, it can affect these types of decays. The rarer this decay is in the Standard Model, the more sensitive it will be to any effect, no matter how small, that falls outside the model and that could indicate that there is a new particle or a new force interacting with the b quarks.
Q. Einstein was looking for a unifying theory that could explain the entire universe. Is it reasonable to continue searching for this type of theory or does reality work differently?
R. We don't really know. There are unified theories where we try to explain the forces that exist in nature from a common origin, although they are not very fashionable right now. The problem is how to check if what they predict happens in nature or not. We would need much more energy than we can now create in experiments to see its effects. There may be a point where the forces we study behave a little differently, but from what we know now, this will almost certainly happen at a very high energy, which we don't have access to right now.
There has sometimes been talk, in a somewhat science fiction idea, of creating an accelerator around the Earth. You would need something on this scale which, with the technology we have, is not feasible. What we are doing is looking for new ways to accelerate particles, beyond those we now use with superconducting magnets. Plasmas and other technologies are being studied to achieve greater accelerations in less space, but we do not know how long it may take to develop these new technologies.
Q. One of the particles that are sought, but not found, is the one that would make up dark matter.
R. We have quite clear evidence, especially on the scale of the universe, when we observe galaxies, that we need matter that we do not know to explain the gravitational effects that we see. We've looked in a lot of places and haven't found anything so far, so sometimes I wonder if we're looking for dark matter in the wrong places, if we have to turn the problem around. Variations on the theory of gravity have been proposed, for example, but they do not seem very promising. We are looking in all possible places, at very small masses that interact a lot or very large masses that interact very little, but we have not seen anything anywhere, so I wonder if we are not misapplying the knowledge we have.
Q. It seems that at the end of the 19th century and the beginning of the 20th century there were much more spectacular advances, with the atomic model, quantum theory, relativity or the Big Bang. Is progress slower now?
R. I think it's true, now progress is a little slower, but I think the difference is that precisely at the time you mention we overcame the technical barrier to be able to see, for example, quarks. You were able to see that level of small things that is related to the energy you need to put into your system. Since then, we have moved forward, but we have done so in a very linear way. We have come to be able to create and observe the Higgs boson in our collisions, but we are talking at the end of the same energy range, we are not talking about different orders of magnitude. We may have already seen everything there is to see in this range we are in and need a leap, not just a linear advance in technology to get to the next state. Without this technology, without being able to make this technological leap to have the new accelerators that I mentioned before, it will be very difficult for us to reach the next level.
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