Looking at the Dark side of Matter

A search for dark matter particles

19 February 2015 | By

The search continues for dark matter, a new kind of matter that doesn’t emit or absorb light. It is assumed to account for the missing amount of mass in our Universe. The total mass in our Universe can be inferred from the observation of gravitational effects of stars in galaxies, and galaxies in clusters of galaxies. However the amount of mass calculated from the observed distribution of light is much less. It is proposed that dark matter makes up the discrepancy as it does not emit light.

Other observations that support the presence of dark matter in the Universe include gravitational lensing of objects by galaxy clusters, the study of cluster-cluster collisions, and the pattern of anisotropies in the cosmic microwave background. These observations indicate that our galaxy is surrounded by a large halo of dark matter, and that dark matter makes up around 85% of the Universe’s mass.

The favourite candidates for dark matter are weakly interacting massive particles or WIMPs. They are relatively heavy particles with masses usually above 1 GeV (roughly the mass of a proton). Several direct detection experiments try to catch these elusive particles with very sensitive detectors that are capable of measuring the very low level of vibration produced by rare collision of WIMP particles from our galaxy’s halo with the nuclei in the detector.

Physicists at the ATLAS experiment have carried out a new search for dark matter in events with energetic jets and large missing transverse energy using all the proton–proton collision data at 8 TeV accumulated by the experiment in 2012.

At the Large Hadron Collider (LHC), dark matter particles could be produced directly in the proton-proton collisions. As weakly interacting particles, dark matter would not interact much in the detector, leaving the experiment undetected. Hence, physicists at the LHC try to identify the presence of WIMPs when they are produced together with an energetic gluon or a photon emitted before the hard collision takes place (referred as to initial-state radiation). This kind of events leads to very distinctive signatures of monojet (one jet stemming from the energetic gluon) and monophoton final states in the detector, with large missing transverse momentum, as measured in the plane perpendicular to the collision line. Recently, those classical “mono-X” studies have been extended to include also W or Z bosons, bottom or top quarks recoiling against missing transverse momentum. Altogether, this complete picture allows the physicists to explore different models for the interaction between dark and ordinary matter.

A team of physicists at the ATLAS experiment has carried out a new search for dark matter in events with energetic jets and large missing transverse energy using all the proton–proton collision data at 8 TeV accumulated by the experiment in 2012. Such an analysis required a careful evaluation of the physics backgrounds that mimic the monojet signature, of the potential loss of jets in partially instrumented regions in the detector, and an adequate rejection of fake jets from cosmic rays and proton beam-induced backgrounds. The team profited from the full potential of the ATLAS detector.

ATLAS,physics briefing,updates
Measured distribution of the missing transverse momentum in the ATLAS detector for monojet events. The data, shown in black, are in agreement with the Standard Model prediction (histogram). For illustration purposes, the distributions of signals from WIMPs production, large extra spatial dimensions, and light gravitino production are also shown. (Image: CERN)

The results show agreement with Standard Model predictions; no signature of dark matter production has been observed. The ATLAS physicists have used the results to put strong constraints on the production of dark matter particles that complement those obtained by direct detection experiments. In addition, the results have been translated into constraints on other models for new physics with extra spatial dimensions, or the production of a light gravitino (the partner of the graviton in supersymmetric theories).