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Scientists working with the CMS detector at the Large Hadron Collider searched for signs of new physics in events that produce one high‑energy photon and a large amount of missing transverse momentum. Missing transverse momentum is a way to spot particles that do not interact with the detector, such as hypothetical dark matter or invisible gravitons from extra dimensions. The search used proton–proton collision data at a center‑of‑mass energy of 13 TeV collected in 2017–2018 (101 fb−1) and combined those data with earlier 2016 results to reach a total of 137 fb−1.
The team selected events that passed a single‑photon trigger (photon transverse momentum pT>200 GeV with about 95% trigger efficiency) and then required a missing transverse momentum above 200 GeV and one photon with transverse energy E_T>225 GeV in the central part of the electromagnetic calorimeter (the barrel, with |η|<1.44). The central region was chosen because it gives the cleanest and most precise photon measurements. Major backgrounds from known Standard Model processes include Z bosons decaying to neutrinos plus a photon, W bosons plus a photon, and photon production with jets. The analysis used control regions and simulation to estimate these backgrounds.
The results show no significant excess of events above the Standard Model expectation. The collaboration therefore set limits at the 95% confidence level on parameters in several theoretical scenarios. These include simplified dark matter models where a heavy mediator particle produces dark matter, an effective electroweak dark matter interaction that couples dark matter to photons and Z bosons, and the Arkani‑Hamed, Dimopoulos and Dvali (ADD) model of large extra spatial dimensions that predicts invisible graviton emission.
Where possible the paper reports concrete improvements over the earlier 2016‑only CMS result. The combined search achieves up to a 14% better exclusion reach for mediator masses in the simplified dark matter models, and about 11% and 10% improvements in the limits on the effective field theory suppression scale and on the fundamental higher‑dimensional Planck scale, respectively. The paper also notes that these are the most stringent constraints on these particular parameters to date. For comparison, a recent ATLAS search with 139 fb−1 also saw no excess and reported lower limits on mediator masses of order 1.46–1.47 TeV under similar coupling assumptions.
These conclusions are model dependent and come with caveats. The dark matter limits assume specific coupling choices (quark coupling g_q=0.25, dark matter coupling g_DM=1.0, and no coupling to leptons) and the narrow‑width approximation for the mediator, which affects how the mediator is modeled. The effective interaction limits are expressed in terms of coupling ratios and a suppression scale Λ, and the ADD limits depend on the number of extra dimensions and the chosen interpretation of the higher‑dimensional Planck scale. Experimental uncertainties and background estimates also limit sensitivity, and the analysis is restricted to the ECAL barrel region, which reduces acceptance for some signal geometries.
In short, this CMS monophoton search found no sign of new invisible particles in the examined data. Instead it tightens constraints on several popular ideas for dark matter and extra dimensions, narrowing the range of parameters where such new physics could still hide.