Quantum entanglement – where the state of one particle cannot be described independently of another – is one of the most mind-bending features of quantum mechanics. Traditionally studied in low-energy experiments, entanglement has recently been put to the test at the LHC's high-energy scales. In 2023, the ATLAS Collaboration achieved the first-ever observation of quantum entanglement between quarks, detecting it in top-antitop pairs at their production threshold. Now, the ATLAS experiment’s exceptional sensitivity to the H → ZZ* → 4l decay channel has enabled physicists to explore this frontier in pairs of massive vector bosons.
The Higgs boson is unique for having a spin of zero. The Z boson, by contrast, is a spin-1 particle, meaning it can exist in three possible polarisation states, corresponding to spin projections of +1, 0, or −1 along a chosen direction. The polarisation of two Z bosons describes how their spin states are correlated. When a Higgs boson decays into two Z bosons, the pair must conserve the Higgs boson’s total spin of zero. By studying these polarisation correlations, physicists can perform a precise test for both electroweak interactions and quantum entanglement.
The new analysis looked at combined data from LHC Run 2 at 13 TeV together with the three years of 13.6 TeV data. Researchers measured two parameters which describe correlations between the spin states of the Z-boson pair. A non-zero value for either parameter would signal quantum entanglement. The ATLAS team also compared their data to two possible scenarios: the Standard Model prediction, in which the Z bosons are entangled, and an alternative non-entangled scenario, where the entanglement-sensitive parameters would be zero.
We propose a Master's project exploring quantum entanglement in decays of the Higgs boson into ZZ*, the two Z-bosons leading to two pairs of lepton-antilepton (e+e- or μ+μ-). The Higgs decay H→ZZ*→4l (the "golden channel" that helped discover the Higgs in 2012) is a uniquely clean quantum system: a single spin-0 particle decaying into a pair of spin-1 bosons whose polarisation states are locked together by angular-momentum conservation.
Courses relevant from the bachelor's degree:
Recommended courses for theoretical curriculum in the master's programme:
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