Atlas goes under the hood of the Higgs mechanism

The detection of the production of longitudinally polarized W bosons at the level of the great collision of Hadrons is an important step towards understanding the rupture of primordial electro-electrowe symmetry, which gives birth to elementary particles.

In 2012, the discovery of the Higgs boson by Atlas and CMS collaborations in CERN opened a new window on the most interior operation of the universe. He revealed the existence of a mysterious and ancient field with which elementary particles interact to acquire their very important masses.

This process is governed by a delicate mechanism called rupture of electro-fair symmetry, which was proposed for the first time in 1964 but remains among the least included phenomena of the standard model of particle physics. To probe this critical mechanism in the evolution of the universe, physicists need a very large set of data from high -energy particle collisions.

Last week, during the Dating Conference in MoriDE, the collaboration of the Atlas brought the physicists closer to the nature of the symmetry revolution mechanism. Using the complete LHC Run 2 collision -pro -protein collision data set, which was collected at an energy from 13 TEV from 2015 to 2018, the team presented the first proof of a key process involving the Boson W – One of the mediators of the weak force.

The document is published on the arxiv pre -printed server.

In the standard model of particle physics, electromagnetic and weak interactions are two sides of the same room, unified as the electro-Fowe interaction. It is believed that the Electrweak interaction prevailed in the aftermath of the Big Bang, when the universe was extremely hot. But the symmetry between the two interactions has been broken in a way, because the carriers of the weak interaction, the W and Z bosons, are massive; While the photon, which intervenes in the electromagnetic interaction, is without mass.

The rupture of this symmetry is carried out in the standard model through the BROUT-ENGLET-HIGGS (BEH) mechanism. The discovery of the Higgs boson provided the first experimental confirmation of this mechanism. The next step is to measure the properties of the new particle, in particular how it interacts with other elementary particles. These measures are currently underway, in order to confirm that the masses of elementary materials are also the result of their interaction with the Beal field.

But the mechanism of Bel also has other predictions. Two in particular processes must be measured to confirm that the mechanism is indeed like predicting the standard model: the interaction between the longitudinally polarized W or Z bosons and the interaction of the Higgs boson with itself.

While studies on HIGGS self -interaction should be possible as soon as possible with high luminosity LHC – to start the operation in 2030 – and will require a future collision to be pinned in detail, the first studies on the diffusion of longitudinal polarized gauge bosons should be possible earlier.

For particles, polarization refers to the way their spin is oriented in space. The longitudinally polarized particles have their rotation perpendicular to the direction of their momentum, which is only possible for the particles which have a mass. The existence of longitudinally polarized W and Z bosons (WL and ZL) is a direct consequence of the BEA mechanism

The study of this interaction should allow physicists to discern whether the rupture of symmetry is carried out via the minimum Bea mechanism or if a new physics beyond the standard model is involved. The new Atlas result offers a first overview of this elusive process.

The WL-WL interaction can be surveyed experimentally in Proton-Protton collisions by studying a process called Vector-Boson (VBS) diffusion. The VBS process can be visualized as a quark in each of the incoming protons emitting a Boson W and these two W bosons interacting with each other, producing a pair of W or Z bosons. The VB can be identified by looking for collisions containing the disintegration products of the two bosons with the two quarks that participated in the interaction, forming two particles opposite directions.

The new analysis of the Atlas targets the collisions in which the two W bosons decompose in an electron or a respective muon and their neutrinos. In order to remove the backgrounds, mainly from processes involving the production of quarter-level pairs, the two leptons must be of the same electrical load. The experimental signature is therefore a pair of leptons of the same charge (electron – electron, muon – muon or electron – muon), two “jets” of particles with opposite directions produced by the disintegrations of quarks and the missing energy from undetectable neutrinos.

Once the candidates for the VBS process are selected, the polarization of the W bosons must be determined. This is very difficult and can only be done via an in -depth analysis of the correlations between the directions of the electrons and the reconstructed muons and the properties of other particles produced in the interaction.

Dedicated neural networks have been trained to distinguish between transversal and longitudinal polarization and have made it possible to extract the final result: evidence with the statistical significance of 3.3 SIGMA only at least one of the two W bosons was polarized longitudinally.

“This measure is an important step in the studies of the value of basic physics via polarized boson interactions in the Vector-Boson diffusion processes,” explains Yusheng Wu, the group of standard Atlas models. “It marks a path to the possible study of the dissemination of polarized bosons longitudinally using data LHC Run-3 and HL-LHC.”