The First Traces Of A New Variant Of The Higgs Boson Decay Discovered In The LHC Data
Physicists have received the first evidence that Higgs bosons can break up into pairs of muons, heavy particles with properties similar to electrons. The results of their work are available by the electronic scientific library arXiv.
"We searched for these decays by analyzing the entire data set that the ATLAS detector collected during the second cycle of the Large hadron Collider from April 2015 to December 2018. These data indicated a small peak in the number of muons that generate particles with a mass of 125 gigaelectronvolts (GeV)," the scientists write.
The Large Hadron Collider (LHC) solved its main task – the search for the Higgs boson – during the first cycle of work, which ended in February 2013. This is what scientists call a special particle with a mass of 125 GeV, which is responsible for the mass of all objects in the Universe. Scientists found traces of the Higgs boson decays in pairs of photons and two other bosons that are responsible for the transport of weak and electromagnetic interactions.
The study of the properties of the Higgs boson did not end there. There are at least eight other decay variants of this particle, some of which (for example, its transformation into pairs of b-quarks), presumably, occur much more often than the decay of the Higgs boson into photons or W-bosons. Their search and study are complicated by the fact that other particles also decay similarly. This prevents scientists from seeing a clear signal associated with the Higgs boson.
You can solve this problem if you accumulate a large enough amount of data on collisions and decays of particles. This makes it possible to separate the traces of the Higgs boson decays from other events based on differences in energy, separation angles, and other properties. For example, in August 2018, physicists from the ATLAS and CMS projects proved that the Higgs boson decays into pairs of b-quarks.
In the footsteps of the Higgs boson
Recently, participants in the ATLAS project, which also includes physicists from Russia's leading scientific centers, received the first hints that the LHC will be able to detect another type of Higgs boson decay. In this case, the particle turns into pairs of muons-heavy analogs of electrons.
These decays, as scientists note, occur even less frequently than the five already known and studied variants. Therefore, even in the huge data set that the LHC has received for almost three years of continuous operation, it is quite difficult to search for them. Besides, in the mass range associated with the Higgs boson, muon pairs can occur in 20 different ways. This made analyzing the collected data even more difficult.
The signal associated with the decay of Higgs bosons was "extracted" by scientists, who calculated in detail all two dozen variants of the appearance of muon pairs and compared the results of theoretical calculations with the ATLAS data. Thanks to this, physicists have noticed a fairly clear, but still small excess in the number of muon pairs that result from the decays of particles with a mass like the Higgs boson.
So far, the level of statistical significance of these measurements (95%), as noted by scientists, does not reach completely reliable (99.99995%). However, they expect to quickly get the missing data after the LHC restarts in May 2021.
The new cycle of the Collider, scientists expect, will help them discover another type of decay of the Higgs boson, in which it turns into a pair of Z-boson and photon. The discovery of even rarer events associated with the transformation of this particle into pairs of c-quarks will be possible only after the construction of the so-called "Higgs boson factory," physicists conclude. Its construction has recently become a top priority for CERN.