Thanks to the collaboration between two experiments at the Large Hadron Collider (LHC), scientists have seen the first evidence of a rare behavior of the Higgs boson: the decay into Z bosons and photons. The result helps confirm the predictions of the Standard Model of particles, which describes the quantum world.
What is the Higgs boson?
The Higgs boson is the fundamental particle of the Higgs field and its discovery was officially announced in 2013, despite having already been predicted in 1964 by Peter Higgs and François Englet. Together, they explain how particles in the universe gain mass.
They also solved a great mystery about W and Z bosons, particles with a weak force that emerge without any mass, but soon have a mass almost 100 times equivalent to that of a proton. According to the theories of Higgs and Englet, this occurs thanks to the interaction of bosons with the Higgs field.
A useful analogy to understand the concept is the image of a pool playing the role of the Higgs field, while the particles are represented by a swimmer. To swim, the athlete needs to apply a certain amount of energy, as water offers resistance, and something similar happens with particles.
As Albert Einstein demonstrated in his famous formula E=mc², mass and energy are different manifestations of the same thing, that is, the energy applied by particles to “swim” in the Higgs field can be converted into mass — and that is exactly what it happens.
Electrons always need a certain energy to interact with the Higgs field, so they will always have the same mass (0.51 MeV/c², which is 1,836 times less than that of protons and neutrons). Protons, in turn, are formed by 2 up quarks (with a mass equal to 2.16 MeV/c²) and 1 down quark (67 MeV/c²). Photons, on the other hand, do not interact with the Higgs field, and therefore have no mass.
Decay into Z boson and photon
The ATLAS and CMS experiments, taking place at the LHC, have joined forces to try to find evidence of the Higgs boson decaying into a Z boson (the electrically neutral carrier of the weak force) and a photon. The new study used the datasets collected by both experiments between 2015 and 2018.
Particle decays are more or less like their “deaths”, but in the universe, nothing disappears, it transforms. In the case of particles, one decay gives rise to others, and the same goes for the Higgs boson. Standard Model theory predicts that it should decay into a Z boson and a photon, via an intermediate loop of “virtual” particles.
These virtual particles arise and destroy each other, leaving something behind. They do not exist in the literal sense of the word, and are “detected” only in the mathematics of decays. That is, they cannot be observed, although they occur whenever a particle arises through the decay of a “progenitor” — as is the case of the Z boson and the photon emitted by the decay of the Higgs.
According to the Standard Model’s prediction, if the Higgs boson has a mass of about 125 billion electron volts, 0.15% of them will decay into a Z boson and a photon. However, some studies suggest a different decay rate, so measuring the exact number is key to refining the Standard Model.
Despite this evidence observed by the authors of the new study, it is still early to say with conviction that the decay into a Z boson and a photon is even real. That’s because the observation has a lower degree of certainty than the 5 sigma level, science’s gold standard for determining the confidence of an observation’s success.
The level reached by the authors was sigma 3.4, so there is still some degree of uncertainty as to whether the observations are reliable or whether they could be the result of some noise in the data or some other unexpected behavior.
To refine this research and obtain a 5 sigma, new experiments will be needed at the LHC, which should occur soon, as soon as the data from its third run are collected by the ATLAS and CMS experiments. If everything is confirmed, including the 0.15% decay of the Higg into a Z boson and a photon, indirect evidence of the existence of unpublished particles can be obtained.
Source: CERN
