Higgs Boson Decay: A Remarkable Chapter in Particle Physics
The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 marked a profound advancement in particle physics. The ATLAS and CMS collaborations have since been engrossed in probing the characteristics of this extraordinary particle. Their focus is on discerning the diverse methods in which it is created and decays into other particles, a process offering valuable insight into our understanding of the universe at the subatomic level.
Unveiling a Rare Process: The Higgs Boson’s Unexpected Transformation
During the Large Hadron Collider Physics (LHCP) conference this week, it was reported that ATLAS and CMS had combined their expertise to uncover the first evidence of a unique and rare process: the Higgs boson’s decay into a Z boson (the electrically neutral carrier of the weak force) and a photon (the carrier of the electromagnetic force). This unusual decay mode of the Higgs boson could supply indirect evidence for the existence of particles beyond those anticipated by the Standard Model of particle physics, thus potentially expanding our knowledge of the subatomic world.
A Quantum Leap: Understanding the Decay Mechanism
The decay of the Higgs boson into a Z boson and a photon echoes the decay process into two photons. In these processes, the Higgs boson doesn’t directly decay into these particle pairs. Instead, the decays proceed via an intermediary “loop” of “virtual” particles that momentarily flicker into and out of existence and can’t be directly detected. Interestingly, these fleeting, virtual particles might include new, undiscovered particles that interact with the Higgs boson, which could have broad implications for particle physics.
Predictions and Reality: Contrasting the Standard Model with Other Theories
According to the Standard Model, assuming the Higgs boson has a mass of about 125 billion electronvolts, roughly 0.15% of Higgs bosons will decay into a Z boson and a photon. However, some alternative theories predict different decay rates, making the decay rate a crucial parameter to investigate. Thus, accurate measurement of this decay rate furnishes valuable insights into not only physics beyond the Standard Model but also the nature of the Higgs boson itself.
An Intensive Hunt: Searching for the Higgs Boson Decay
Previously, using data from proton-proton collisions at the LHC, ATLAS and CMS independently carried out exhaustive searches for the decay of the Higgs boson into a Z boson and a photon. Both used comparable strategies, identifying the Z boson through its decays into pairs of electrons or muons — heavier versions of electrons, a phenomenon occurring in about 6.6% of cases. Advanced machine-learning techniques were employed to further differentiate between signal and background events, thereby improving the detection accuracy.
Joining Forces: Maximizing the Outcome of the Search
Recently, ATLAS and CMS collaborated to maximize the search outcomes. By amalgamating the data sets collected by both experiments during the second LHC run (2015-2018), they significantly increased the statistical precision and reach of their searches.
The Moment of Truth: The First Evidence of Higgs Boson Decay
The collaboration yielded the first evidence of the Higgs boson decaying into a Z boson and a photon. The result has a statistical significance of 3.4 standard deviations, falling short of the conventional 5 standard deviations required to claim an observation. However, the measured signal rate was 1.9 standard deviations above the Standard Model’s prediction, showcasing the potential of the combined efforts of ATLAS and CMS.
A New Horizon in Particle Physics
Physics coordinators from both ATLAS and CMS highlight the significance of this discovery and the potential of the LHC in exploring even rarer Higgs decays in the future. This study forms a powerful test of the Standard Model, and the continuing runs of the LHC hold the promise of refining the precision of this test.
