ATLAS Monojet Search (2021): No Sign of Extra Dimensions at the LHC

In 2021 the ATLAS experiment at CERN’s Large Hadron Collider reported one of the most sensitive searches for particles that vanish into extra dimensions or escape as dark matter. Using the full 2015-2018 data set, the collaboration looked for collisions with a single energetic spray of particles and nothing visible to balance it. It found what the Standard Model predicts, and nothing more.

What the paper found

• No excess. In 139 inverse femtobarns of proton-proton collisions at 13 TeV, the number of “monojet” events matched Standard Model predictions in every signal region.
• Extra dimensions pushed back. For the large extra dimensions model of Arkani-Hamed, Dimopoulos and Dvali, the fundamental scale of gravity in 4 + n dimensions must be above 11.2 TeV for two extra dimensions and above 5.9 TeV for six, at 95% confidence. The previous ATLAS limits, from about a quarter of the data, were 7.7 TeV and 4.8 TeV.
• Dark matter limits. The same data exclude new ranges of weakly interacting dark matter candidates, supersymmetric particles in compressed scenarios, axion-like particles and new scalars in some dark-energy-inspired models.
• The Higgs boson does not vanish often. Decays of the Higgs into invisible particles must make up less than 34% of the total, at 95% confidence.

How they did it

When two protons collide, momentum across the beam line must balance. If a particle escapes undetected, whether a dark matter particle or a graviton slipping into extra dimensions, the visible debris looks lopsided: one energetic jet of particles on one side and “missing” momentum on the other. ATLAS selected events with at least one jet above 150 GeV of transverse momentum, no electrons, muons, taus or photons, and missing transverse momentum starting at 200 GeV.

The hard part is the background. Ordinary processes, such as a Z boson decaying to two neutrinos alongside a jet, produce the same signature. The collaboration estimated these backgrounds from control samples in the data and compared them with the signal regions.

Why it mattered

In the ADD picture, gravitons can be produced in collisions and immediately disappear into the extra dimensions, carrying energy away. Because a curled-up dimension turns the graviton into a whole tower of heavier states, the Kaluza-Klein tower first implied by Klein’s 1926 work, the expected signal is a broad excess of monojet events at high energy. The absence of that excess is one of the strongest direct tests of large extra dimensions.

The original motivation for ADD was a fundamental gravity scale near 1 TeV, close to the weak scale. A limit above 11 TeV for two extra dimensions does not kill the idea, but it removes its most natural version. Combined with tabletop gravity tests at 52 micrometers, it leaves less and less room.

What we know now

As of October 2026, no LHC experiment has reported evidence for extra dimensions or for dark matter production. Later data sets from the collider extend these searches. Dark matter remains detected only through gravity; see our dark matter guide and the Bullet Cluster evidence. For how this fits with the established four dimensions of spacetime, see our explainer on the fourth dimension.

Who wrote it

The paper is signed by the ATLAS Collaboration, with 2,928 authors listed in the publisher’s record, alphabetically starting with G. Aad. ATLAS is one of the two general-purpose detectors at the Large Hadron Collider at CERN, near Geneva.

Read the original

ATLAS Collaboration (2021). “Search for new phenomena in events with an energetic jet and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector.” Physical Review D 103, 112006. DOI: 10.1103/PhysRevD.103.112006. Open access under a CC BY 4.0 license, and free on arXiv (2102.10874).