In particular, what has been observed at the Large Hadron Collider (LHC) is a disintegration of the Higgs boson into a muon and an antimuon, that is to say, into a pair composed of a particle and its antiparticle. The interesting thing about this observation is the very low probability that it will happen: the H process μ+μ- has only a 0.022% chance of happening (according to the standard model). The study of this very rare decay is particularly interesting because it opens a new window to see how the Higgs interacts with second-generation fermions (a group of particles that includes muons, some types of quarks and the muon neutrino).
This mode of decay is the rarest of the possible modes of Higgs decay that have been observed so far. Previously, the ATLAS collaboration had studied the coupling of the Higgs boson with fermions (known as Yukawa coupling), but with heavier fermions. To be able to observe the coupling of Yukawa with the muons, it has been necessary to analyze jointly the data of Run2 and those of Run3 of the LHC (the Runs of the LHC are its active periods, where it collects collision data, followed by technical stops). This analysis has been a challenge since the detector conditions have not remained stable throughout the data acquisition. In particular, the energy (13 and 13.6 TeV) and flux intensity have changed.
"This result represents a further step to better understand the Yukawa coupling in the standard model and better understand the structure of matter", Salvador Martí, IFIC researcher
On the other hand, the identification of such a rare phenomenon is extremely complex. A weak signal that is buried by a huge background noise, which comes from other processes that give a similar final state, has to be extracted. Salvador Martí, scientific researcher at the CSIC and member of the team of the Institute of Corpuscular Physics (IFIC), research center of the University of Valencia Science Park (PCUV), who has participated and led part of the analysis clarifies that: «as the H signal μ+μ- is so weak, and the background of the standard model so large, we had to combine all production processes to reach a significance level of 3.4 σ, and study each process carefully». An important ingredient of the analysis has been the calibration of the muons recorded by ATLAS. Knowing very precisely the energies of these particles has made it easier to identify the phenomenon, since it has made it possible to discern in the distribution of the invariant mass of the muons a small peak corresponding to the signal of H μ+μ-, which stands out against the large background.
Tamar Zakareishvili, also from IFIC, comments that in order to achieve a good resolution at muon time: «we have greatly refined our calibration by studying the bias we had initially with the disintegration of Z and J/ψ to μ+μ-». This result, Salvador Martí comments again, «represents a further step forward to better understand the coupling of Yukawa in the standard model and better understand the structure of matter».
Source: IFIC