Physicists from the ATLAS and CMS collaborations at CERN’s Large Hadron Collider (LHC) have independently conducted extensive searches for the rare Higgs boson decay into a Z boson and a photon.
Event display of a candidate H→ Zγ event with the Z boson decaying μ+μ-. The transverse momenta of the two muon candidates, shown in red, are 72 GeV and 20 GeV. The photon candidate is reconstructed as an unconverted photon with a transverse momentum of 32.5 GeV. Two jets (R=0.4) are represented by light blue cones, with mjj of 965 GeV. The green boxes correspond to energy deposits in cells of the electromagnetic calorimeter, while yellow boxes correspond to energy deposits in cells of the hadronic calorimeter. The invariant mass of the Z boson candidate is 91.1 GeV and the invariant mass of Zγ system is 125.4 GeV. Image credit: ATLAS Collaboration / CERN.
The Standard Model predicts that, if the Higgs boson has a mass of around 125 billion electronvolts (GeV), approximately 0.15% of Higgs bosons will decay into a Z boson and a photon. But some theories that extend the Standard Model predict a different decay rate.
Measuring the decay rate therefore provides valuable insights into both physics beyond the Standard Model and the nature of the Higgs boson.
Previously, using data from proton-proton collisions at the LHC, ATLAS and CMS independently conducted extensive searches for the decay of the Higgs boson into a Z boson and a photon.
Both searches used similar strategies, identifying the Z boson through its decays into pairs of electrons or muons — heavier versions of electrons. These Z boson decays occur in about 6.6% of the cases.
In these searches, collision events associated with this Higgs boson decay would be identified as a narrow peak, over a smooth background of events, in the distribution of the combined mass of the decay products.
To enhance the sensitivity to the decay, ATLAS and CMS exploited the most frequent modes in which the Higgs boson is produced and categorized events based on the characteristics of these production processes.
They also used advanced machine-learning techniques to further distinguish between signal and background events.
“The discovery of the Higgs boson in 2012 by the ATLAS and CMS collaborations has been followed by a detailed program of measurements that have confirmed its couplings and other properties are consistent with those predicted in the Standard Model,” they said.
“However, there are several Higgs boson decay channels, including H → Zγ, that have small predicted branching fractions and have not yet been observed.”
“These channels also provide probes for possible contributions arising from physics beyond the Standard Model.”
“In the Standard Model, the H → Zγ decay is expected to have a small branching fraction, of about 1.5*10−3 for a Higgs boson mass close to 125 GeV.”
“As the H → Zγ decay proceeds via loop diagrams, it is sensitive to modifications in several beyond Standard Model scenarios that would cause the branching fraction to differ from the Standard Model value.”
“Examples include models where the Higgs boson is a neutral scalar of different origin, a composite state, or a pseudo Nambu-Goldstone boson.”
“Different branching fractions are also expected for models with additional colourless charged scalars, leptons or vector bosons that couple to the Higgs boson, due to their contributions via loop corrections.”
A proton-proton collision event recorded by the CMS detector in 2018 at a center of mass energy of 13 TeV, consistent with the decay of a Higgs boson to a Z boson and a photon. The Z decays into a pair of muons (red lines), with transverse momenta of 85 and 11 GeV, which extend all the way to the muon detection system. An energy deposit in the electromagnetic calorimeter (represented by the green boxes) with no associated track is indicative of a photon (yellow dashed line), with transverse momentum of 19 GeV. The invariant mass of the Zγ system is 126.0 GeV. Image credit: CMS Collaboration / CERN.
In the new research, ATLAS and CMS teams have now joined forces to maximise the outcome of their search.
By combining the data sets collected by both experiments during the second run of the LHC, which took place between 2015 and 2018, they have significantly increased the statistical precision and reach of their searches.
This collaborative effort resulted in the first evidence of the Higgs boson decay into a Z boson and a photon.
The result has a statistical significance of 3.4 standard deviations, which is below the conventional requirement of 5 standard deviations to claim an observation.
The measured signal rate is 1.9 standard deviations above the Standard Model prediction.
“Each particle has a special relationship with the Higgs boson, making the search for rare Higgs decays a high priority,” said ATLAS physics coordinator Pamela Ferrari.
“Through a meticulous combination of the individual results of ATLAS and CMS, we have made a step forward towards unravelling yet another riddle of the Higgs boson.”
“The existence of new particles could have very significant effects on rare Higgs decay modes,” added CMS physics coordinator Florencia Canelli.
“This study is a powerful test of the Standard Model. With the ongoing third run of the LHC and the future High-Luminosity LHC, we will be able to improve the precision of this test and probe ever rarer Higgs decays.”
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The ATLAS and CMS Collaborations. 2023. Evidence for the Higgs boson decay to a Z boson and a photon at the LHC. ATLAS-CONF-2023-025
