Higgs Boson Properties
The discovery of the Higgs boson on July 4, 2012 by the CMS experiment and the ATLAS experiment at CERN marked a historic milestone in physics. It confirmed the mechanism responsible for giving mass to fundamental particles—but it was also the beginning of a new scientific program. Today, our goal is to precisely measure the Higgs boson’s properties and test whether they match the predictions of the Standard Model or hint at new physics.
A central prediction is that the Higgs boson interacts more strongly with heavier particles. Testing this relationship across different particle types, including very light particles and even invisible ones, provides powerful ways to search for cracks in our current theory.
So far, experiments have measured the Higgs coupling to heavy gauge bosons (Z, W bosons [3, 4, 5]), to 3rd generation massive fermions (tau lepton [6, 7] and top, bottom [8, 9] quarks), and to the 2nd generation lepton (muon). Recent efforts have extended to the 2nd generation charm quark [10, 11]. However, as the mass of the fermions become smaller, the chances of observing their decays become increasingly sparse.
Higgs → μμ: probing second-generation particles
One of our primary efforts is the measurement of the Higgs boson decay into two muons (H→μμ). This process is extremely rare but critically important, as it provides direct access to the Higgs boson’s interaction with second-generation fermions, allowing us to test a key prediction of the Standard Model.
This measurement is particularly challenging for two reasons. First, the decay is intrinsically rare: only about one in 5,000 Higgs bosons decays into two muons. Second, the background is enormous: for every Higgs boson signal event, roughly 1,000 standard processes produce identical pairs of muons, making the signal difficult to isolate.
Despite these challenges, the CMS experiment reported the first evidence for this decay in 2020, with a significance of 3 standard deviations. Our group is now working toward the first definitive observation and increasingly precise measurements using the latest Run 3 data. This effort relies on advances in detector calibration, improved analysis techniques, and modern machine learning methods to maximize sensitivity.
Higgs → invisible: searching for dark matter
Another key question is whether the Higgs boson can decay into invisible particles. These particles would leave no direct signal in the detector but could reveal themselves through missing energy.
Such invisible decays are predicted in many theories of dark matter. Observing them would revolutionize our understanding of the universe by linking particle physics with cosmology.
So far, experiments have found no evidence for invisible Higgs decays, but current measurements still allow room for discovery. Our group is actively working to improve this sensitivity using new data.
Search for Rare Higgs Boson Decays
We also search for rare Higgs decays involving a photon and a vector light meson. These processes yield clearer experimental signatures.
These decays provide a unique window into the Higgs interaction with the lightest quarks (up, down, and strange), which are otherwise extremely difficult to probe.
These measurements are sensitive to new physics that could modify the Higgs couplings. Even small deviations from predictions could point toward new particles or forces.
With a larger dataset and better triggers available from Run3, we aim to push down the limits.