At CERN, scientists resume exploring the origins of the Universe

On July 4, 2012, the world learned of the discovery of the Higgs boson. Exactly ten years after this historic event, the LHC facilities - which have just completed a three-year makeover - are preparing for a new data collection, twice as large as the previous ones. Enough to lift the veil a little more on the secrets of elementary matter.

This time it's there, after three years of shutdown for maintenance, the LHC ( Large Hadron Collider ), the giant particle collider at Cern, near Geneva, has restarted. One hundred meters underground, in its ring of 27 kilometers in circumference, protons are again accelerated in both directions to a speed close to that of light, before being projected against each other within the four detectors – Alice, Atlas, CMS, LHCb – placed along their trajectory. The result is a concentrate of kinetic energy which, by virtue of the equivalence between energy and mass, transforms into sheaves of particles which bear witness to the processes at work in the intimacy of matter.

The Higgs boson, 10 years old already!

After an extensive makeover of all the LHC facilities, the energy and frequency of collisions have been increased, as has the capacity of the detectors to harvest the products. Thus, at the end of this new period or “run” lasting four years, the scientists who operate the Geneva colossus will see the mass of data at their disposal multiplied by three. Enough to allow them to explore a little further the fundamental laws of our Universe, on which the history of the cosmos and that of all the matter it contains depends.

More precisely, run 3 of the LHC constitutes the continuation of a formidable scientific adventure that began twelve years ago with the commissioning of the accelerator. At the time, the objective of scientists was mainly to demonstrate the existence of the Higgs boson. Higgs boson? According to the current prevailing theory of the infinitesimally small, the Standard Model, elementary matter is composed of particles of matter that interact by exchanging particles of force. Developed between the 1960s and 1970s, this “model” subsequently saw all of its predictions verified. All, except one: that of the existence of an additional particle, the famous boson, supposed to confer their mass to particles which have one. The colossal energy involved in the LHC collisions should finally make it possible to extricate the Higgs from the corners of space-time where it was hiding. And indeed, in 2012, forty-eight years after a handful of theorists had predicted its existence, the experimentalists of the Atlas and CMS experiments highlighted the famous Higgs boson. The discovery, rewarded with a Nobel Prize in physics in 2013, completed the construction of the standard model.

Towards a new continent of physics

However, from the design of the LHC, specialists in the field assigned it another mission: that of putting them on the trail of a new physics, or “non-standard” physics, not described by the said model. In fact, between dark matter, absence of antimatter in the Universe, acceleration of cosmic expansion, formal inconsistencies... they have known for decades that their representation of the laws of the infinitely small as well as the infinitely large is incomplete. And that's the whole point of run 3: allowing physicists to finally sketch the contours of a new physical continent that has remained completely unexplored until today. “  We are entering a phase of exploring the unknown ,” says Marie-Hélène Genest, researcher at the Laboratory of Subatomic Physics and Cosmology 1  (LPSC), in Grenoble, and member of the Atlas experiment. We are not sure of anything, except that run 3 will allow us to take very good measurements with unrivaled precision.  »

Concretely, this exploration will take multiple complementary paths, in particular with the Atlas and CMS experiments known as “generalist” experiments. The first consists of looking for traces of unknown particles among those appearing during collisions between protons. No such observations were made during previous runs. “  Due to the slightly higher energy engaged during collisions (from 13 to 13.6 TeV), the probability of producing rare and interesting events will increase from 20% to 250% depending on the mass of the particles  ,” explains Marie- Hélène Genest. Furthermore, thanks to the greater statistics available to physicists at the end of the campaign which begins, it is not impossible that particles more massive and/or at lower production rates than those hitherto accessible , occur.

Furthermore, via renewed detection and analysis strategies, scientists are considering the possibility of very light particles which would have remained “under their radar”. Finally, they will look for long-lived particles which, therefore disintegrating far from the point of impact, make their products more difficult to identify.

Track small deviations and exotic particles

According to theoretical speculation, some of these putative particles are candidates for solving the enigma of dark matter, which has remained invisible, but whose gravitational effects astrophysicists believe they observe at all scales of the Universe. Others would be the sign of new symmetries to which the most fundamental laws would obey, or of the existence of additional dimensions of space. In any case, as Marie-Hélène Schune, at the Laboratoire des 2 infinis Irène Joliot-Curie 2  (IJCLab) and member of the LHCb experiment, explains, “  in a period where the experiment must serve as a guide for sorting through the many approaches proposed beyond the standard model, our strategy will be to look in all possible directions without reference to a particular theory. »

In particular, beyond the observation of new particles, scientists will closely monitor possible deviations from what the standard model predicts in different particle decay channels. As Marie-Hélène Genest details, “  these differences could in fact be a sign of the existence of non-standard particles too massive to materialize at the LHC, due to lack of sufficient energy, but nevertheless making their virtual influence felt in the processes. quantum waves accompanying collisions.  »

In this regard, all eyes will be on the LHCb experiment designed to study the decay of particles called B mesons, which contain a beautiful quark or its antiparticle. In fact, data recorded during previous “runs” revealed several “ anomalies ” in the way certain beautiful mesons decay into either electrons or muons. According to the standard model, the two processes should be perfectly equivalent. However, it is not the case. Likewise, “non-standard” angular distributions in the manner in which certain decay products of these particles are emitted have been observed. “  At this time, these differences are not yet statistically significant enough to conclude, but they could strengthen and end up drawing a coherent landscape in favor of unknown physics  ,” enthuses Marie-Hélène Schune.

Go even deeper into the secrets of the Higgs 

On the program for run 3, the precise study of the properties of the Higgs boson will also be one of the priorities. During runs 1 and 2, the experiments revealed how the particle interacts with the top and beau quarks, as well as the tau lepton, and the W and Z bosons which carry the weak interaction. Now, in particular, scientists hope to begin comparing its rate of decay into a pair of tau leptons to that of decay into a pair of muons.

As explained by Yves Sirois, at the Leprince-Ringuet 3 Laboratory (LLR) and member of the CMS experiment, this measurement touches on one of the greatest mysteries of matter: “  We observe that the particles of matter organize themselves into three perfectly identical families without us knowing why. More precisely, the particles of the different families are only distinguished by their mass, that is to say via the way in which they interact with the Higgs. Thus, a possible discrepancy between the way in which the Higgs decays into muons or taus, two particles which play the same role within their respective families, could put us on a path to understanding why the material Universe seems to have three copies identical to itself.  »

On the Higgs field, run 3 will also allow the first measurements of the particle's interaction with itself. In doing so, physicists will begin the study of the properties of the Higgs field, this quantum entity from which the boson of the same name is extracted during collisions within the accelerator, and which bathes the entire Universe, giving its structure in vacuum and their mass in particles. “  There is a deep link there with the history of the Universe ,” explains Yves Sirois. Indeed, we know that just after the Big Bang, the particles had no mass, which means that the Higgs field had a different structure. Did it evolve everywhere at the same time in the cosmos, or by the growth of increasingly vast regions? We don't know anything about it, just as we don't know whether the Higgs field, in its current structure, is definitively stabilized or not.  » Possible start of response at the end of run 3.

A plasma of quarks and gluons

The links between the infinitely small and the infinitely large will also be in the crosshairs of the Alice experiment, whose detector is dedicated to the study of quark and gluon plasma (PQG). Obtained by colliding lead nuclei, it is a sort of “soup” of ultra-hot and dense matter within which the quarks and gluons, usually bound to each other, are free to move. move. However, essential for studying in detail the properties of the strong interaction, the one which structures atomic nuclei in particular, the PQG also corresponds to the state in which cosmologists believe the Universe was a few microseconds after the Big Bang.

Impossible to observe directly, the PQG reveals itself through the myriads of particles that it ejects within the detector at the same time as it cools and the elementary matter that composes it reorganizes. However, as Stefano Panebianco, from the Institute for Research on the Fundamental Laws of the Universe (CEA Paris-Saclay) and member of Alice, explains, “  during runs 1 and 2, we focused on the most more commonly emitted by plasma, such as protons and neutrons, as well as some characteristic resonances. » Thus, the researchers obtained several spectacular results, notably demonstrating that the quark and gluon plasma behaves like a liquid with almost zero viscosity. But with the developments made over the last three years, “  we will have access to probe particles containing heavy quarks, both produced in low numbers and with exotic decay modes, which will finally provide access to a more detailed and selective description of the properties of these probes  ,” continues the researcher.

This should in particular make it possible to provide an answer to the question of knowing what is the deep nature of the transition which sees the burning plasma transform into ordinary matter when the temperature decreases. “  This is very important for constraining models of the history of the Universe and extrapolating them back to the Planck era during which the cosmos, barely emerging from the Big Bang, was still just a mush of quantum particles.  », describes Stefano Panebianco.

Just like their colleagues in the LHCb experiment, whose experiment is sensitive to the asymmetry between matter and antimatter, the Alice scientists also imagine providing clues to help understand why the Universe is completely empty of antimatter. For a century, this enigma has remained nagging, while the equations of the standard model indicate an almost perfect symmetry between matter and its Nemesis. Enough to make run 3 a fantastic opportunity to challenge the LHC with the greatest mysteries of the infinitely small as well as the infinitely large. ♦