Eight collisions that revealed a new process express
In the ATLAS detector at the Large Hadron Collider, protons collide billions of times per second. From this stream, physicists fished out eight events in which the collision produced two Z particles and one photon. The Z particle is a heavy carrier of the weak force; it lives less than a billionth of a second and decays into a pair of charged particles. In this case, each Z particle decayed into an electron or a muon, while the photon remained in the detector as a distinct clump of energy.
To isolate these rare events, physicists selected collisions where the Z decays left four charged particles (electrons or muons) and one photon. Such a signature is like four tracks from debris and one flash of light. Over four years of collider operation, from 2015 to 2018, only eight such patterns accumulated. The background from other processes that could mimic a similar signature was estimated at less than one event. This means the signal is real.
The measured rate of producing two Z particles and a photon matches the calculation from the Standard Model, the most precise theory of elementary particles. The agreement between the number and the calculation strengthens confidence in the model. But it is precisely in such rare processes that deviations could appear if new particles or forces exist. So far, no deviations are visible, but physicists now have a tool to keep looking.
Original abstract
as written by the authors arXiv:2602.17165
This paper presents the first evidence of the simultaneous production of two $$Z$$ bosons and one photon with the ATLAS detector at the Large Hadron Collider. The measurement is performed using the full Run-2 dataset, recorded from 2015 to 2018, of proton-proton collisions at a center-of-mass energy of $$13$$ TeV, corresponding to an integrated luminosity of $$140$$ fb$$^{-1}$$. The fully leptonic final state with four leptons and one photon is analyzed, $$pp\rightarrow ZZ\gamma\rightarrow\ell^+\ell^-\ell'^+\ell'^-\gamma$$ with $$\ell, \ell' = e$$ or $$\mu$$. This final state is measured in a fiducial region where photon final-state radiation is minimized, and the photon has a transverse momentum of $$p_{\mathrm{T}}^\gamma > 20$$ GeV. Eight events are selected, with a background estimate of $$0.92\pm0.15$$. This results in an observed (expected) significance of the $$ZZ\gamma$$ final state of $$4.4\sigma$$ ($$4.4\sigma$$). The measured cross-section for $$pp\rightarrow ZZ\gamma\rightarrow\ell^+\ell^-\ell'^+\ell'^-\gamma$$ in the fiducial region is $$\sigma_{ZZ\gamma} = 0.144 _{-0.051}^{+0.064} \text{ (stat.)} _{-0.005}^{+0.007} \text{ (syst.) fb}$$, in agreement with the predicted Standard Model one, $$\sigma_{\textrm{fid.}}^{\textrm{SM}} = 0.143 _{-0.004}^{+0.007}$$ fb.
Two Z particles and a photon are produced on average once in every 7 trillion proton collisions, like finding one special grain of sand on a beach several kilometers long.