New evidence for rare Higgs boson decay in ATLAS experiment

Visualisation of a candidate event for Higgs boson decay into two muons (red lines) and a photon (green area on the left) in the ATLAS experiment; source: https://cerncourier.com/

Scientists conducting research within the framework of the ATLAS experiment at the Large Hadron Collider (LHC) in CERN have announced evidence for one of the rarest Higgs boson decays observed so far at the LHC. It happens in the case of 1 in 10,000 decays. Doctor Mateusz Dyndał of the Faculty of Physics and Applied Computer Science was directly involved in the research project. 

Since the discovery of the Higgs boson in 2012, scientists in the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) have been hard at work characterising its properties and hunting down the diverse ways in which this ephemeral particle can decay. From the copious but experimentally challenging decay to b-quarks, to the exquisitely rare but low-background decay into four leptons, each offers a different avenue to study the properties of this new particle. Now, ATLAS has found first evidence of the Higgs boson decaying to two leptons (either an electron or a muon pair with opposite charge) and a photon. Known as “Dalitz decay”, this is one of the rarest Higgs boson decays yet seen at the LHC.

ATLAS physicists searched the full LHC Run 2 data set for collision events with a photon as well as two leptons whose combined mass was less than 30 GeV. In this region, decays with virtual photons should dominate over other processes that yield the same final state. ATLAS measured a Higgs boson signal rate in this decay channel that is 1.5 ± 0.5 times the expectation from the Standard Model. The chance that the observed signal was caused by a fluctuation in the background is 3.2 sigma – less than 1 in 1000.

With vast amounts of data expected from the upcoming High-Luminosity LHC programme, studying rare Higgs boson decays will become the new norm. This will allow physicists to progress from reporting evidence for their existence, to confirming their observation and conducting detailed studies of Higgs boson properties – leading to ever more stringent tests of the Standard Model.

Observing the Higgs boson decay to a photon and a lepton pair will make it possible for physicists to study charge parity (CP) symmetry. CP symmetry is a way of saying that the mirror image of interacting particles, where particles are replaced by their antiparticles, should look exactly the same as the original interaction. This was a natural assumption until 1964, when physicists studying kaon particles noticed – to their great surprise – that this is not the case in the particle physics world. Since then, physicists have learned that violation of CP symmetry is a signature of the electroweak interaction and have incorporated it into the Standard Model.

But with the Higgs boson decaying into three particles, two of which are charged, physicists will be able to examine whether decays have a preferred direction – allowing researchers to improve their understanding of the origins of CP symmetry violation and perhaps even leading to hints for new physics beyond the Standard Model.

During a scientific internship programme in CERN in the years 2018–2020, doctor Mateusz Dyndał was directly involved in the analysis of the experiment data. Since December 2020, the scientist has worked at the Faculty of Physics and Applied Computer Science, where he takes part in the prestigious “Polish Returns” programme held by the Polish National Agency for Academic Exchange. The scope of his research covers particle physics, experimental physics, and detector physics. 

A group of scientists at the Faculty of Physics and Applied Computer Science have been members of the ATLAS experiment since the very beginning, i.e. since 1991, and have played a leading role in the data analysis of heavy ion collisions and events at large rapidity intervals. The researchers from AGH UST are also involved in the current maintenance and modernisation of the trigger system and a subsystem of the tracking detector of the ATLAS experiment for the phase of collecting data from the High-Luminosity LHC programme (2030+). Research tasks conducted by the group also involve doctoral students, as well as first- and second-cycle students.