The Invisible Caught in a Trap of Light express
At the Large Hadron Collider, protons collide at nearly the speed of light. Sometimes they don't smash apart but exchange photons — particles of light. These photons can produce a pair: one particle the detector sees, and another that flies off unnoticed. This process is like a game of billiards where after the strike, one ball stays on the table while the other disappears into a pocket without leaving a trace.
Scientists from the ATLAS collaboration used special detectors that catch protons deflected after the collision. From the energy of these protons, they reconstructed the total energy of the pair produced by the photons. Then they subtracted the energy of the visible particle to get the 'missing mass'—the mass of the invisible particle. It's like weighing a gift box without opening it: first weigh the whole box, then subtract the weight of the box itself.
In the mass range from 100 to 900 gigaelectronvolts (about 100 to 900 proton masses), physicists saw no significant excess over Standard Model expectations. This means that new particles, if they exist, are produced less frequently than would be noticeable in this data. Upper limits were set on the rate of such processes — from 128 to 2.5 femtobarns depending on the model.
This method is like listening for silence in a room where noise was expected. If the silence is too pure, you can say how loud a sound might have been that we didn't hear. Now physicists know that in this mass range, invisible particles are not produced too often, narrowing the field for future searches.
Original abstract
as written by the authors arXiv:2603.20837
A search is conducted in proton-proton collisions at the Large Hadron Collider for photon-induced production $$pp\rightarrow pp+ \gamma \gamma, (\gamma\gamma\rightarrow VX)$$ of a visible particle $$V$$ decaying into a pair of same-flavour charged leptons ($$e^+e^-$$ or $$\mu^+\mu^-$$) and an undetected invisible component $$X$$. Measurements of the outgoing proton energies by the ATLAS forward proton spectrometer allow the full photon-photon four-momentum to be reconstructed. By subtracting the visible four-momentum of the central system measured with the ATLAS detector, the 'missing mass' of any event components not detected in the central region can be reconstructed, enabling the reconstruction of $$X$$ without knowing its properties, thus allowing the search to be model-independent. A search for a narrow resonance is performed in the missing-mass spectrum between 100 GeV and 900 GeV. The analysis uses data collected in 2017 from proton-proton collisions at a centre-of-mass energy of $$\sqrt{s} =$$ 13 TeV, corresponding to an integrated luminosity of 14.7 fb$$^{-1}$$. No significant excess over the Standard Model expectation is observed and upper limits at 95% confidence level are set on the fiducial cross sections for three different signal models in the range between 128 and 2.5 fb. Additionally, model-independent limits are set on the visible cross section of BSM processes, for two sets of selection criteria. Both individual lepton flavour decay channels of the visible boson and a combination of the two channels are considered.
The collision energy at the Large Hadron Collider is 13 teraelectronvolts, millions of times greater than the energy of particles in the solar wind.
The idea of invisible particles carrying away energy echoes stories about dark matter, which interacts with our world only through gravity.