ATLAS experiment sets strong constraints on supersymmetric dark matter

ATLAS Experiment sets strong constraints on Supersymmetric Dark Matter
Figure 1: A comparison of the significance for the signal plus background hypothesis (vertical axis) of a chosen supersymmetric model obtained by selecting events using the new object-based ETmiss significance variable (black line), compared to the previous approximation (ETmiss/ET, cyan) or to selecting events using only the measured missing transverse energy (ETmiss, mauve). Higher significance is found for the new variable. Credit: ATLAS Collaboration/CERN

Dark matter is an unknown type of matter present in the universe that could be of particle origin. One of the most complete theoretical frameworks that includes a dark matter candidate is supersymmetry. Many supersymmetric models predict the existence of a new stable, invisible particle called the lightest supersymmetric particle (LSP), which has the right properties to be a dark matter particle. 

The ATLAS Collaboration at CERN has recently reported two new results on searches for an LSP that exploited the experiment's full Run 2 data sample taken at 13 TeV proton-proton collision energy. The analyses looked for the pair production of two heavy supersymmetric particles, each of which decays to observable Standard Model particles and an LSP in the detector.

Identifying missing energy

A central challenge of these searches is that dark candidate particles would escape the ATLAS detector without leaving a visible signal. Their presence can only be inferred through the magnitude of the collision's missing transverse momentum (ETmiss) – an imbalance in the momenta of detected particles in the plane perpendicular to the colliding protons. In the dense environment of numerous overlapping collisions generated by the Large Hadron Collider (LHC), it can be difficult to separate genuine ETmiss from fake ETmiss originating from mis-measurement of the visible collision debris in the detector.

To resolve this difficulty, ATLAS developed a new ETmiss significance variable that quantifies the likelihood that the observed ETmiss originates from undetectable particles rather than from mis-measured objects. Unlike previous calculations based entirely on the reconstructed event kinematics, the new variable also considers the resolution and misidentification probability of each of the reconstructed particles used in the calculation. This helps discriminate more effectively between events with genuine and fake ETmiss, respectively, as shown in Figure 1, thus improving ATLAS' ability to identify and partially reconstruct particles.

ATLAS Experiment sets strong constraints on Supersymmetric Dark Matter
Figure 2: 95% exclusion limits on chargino pair production. The grey shaded region shows the results from Run 1 of the LHC. The new results substantially extend previous limits. Credit: ATLAS Collaboration/CERN

Applying new reconstruction techniques

Both of the new ATLAS searches implement this new reconstruction technique to the full Run 2 dataset. One search looks for the pair production of charginos (the charged superpartners of bosons) and sleptons (superpartners of leptons), respectively, which decay to either two electrons or muons and give rise to large ETmiss due to the escaping LSPs. These signals are very challenging to extract as they look similar to Standard Model diboson processes, where some (although less) ETmiss is produced from invisible neutrinos. Events were selected at high ETmiss significance together with several other variables that help discriminate signal from background. In absence of a significant excess in the data over the background expectation, strong limits were placed on the considered supersymmetric scenarios, as shown in Figure 2.

The second new search targets the pair production of supersymmetric bottom squarks (superpartners of bottom quarks), which both decay to a final state involving a Higgs boson and an LSP (plus an additional b-quark). Then – targeting Higgs boson decays to two b-quarks, as it is predicted to occur 58 percent of the time – the final state measured in the ATLAS detector would have a unique signature: large ETmiss associated with up to six jets of hadronic , originating from b-quarks. Again, no significant excess in data was found in this search.

Both results place strong constraints on important supersymmetric scenarios, which will guide future ATLAS searches. Further, they provide an example how novel reconstruction techniques can help improve the sensitivity of new physics searches at the LHC.

Explore further

Chasing invisible particles at the ATLAS Experiment

More information: Search for bottom-squark pair production with the ATLAS detector in final states containing Higgs bosons, b-jets and missing transverse momentum: … ATLAS-CONF-2019-011/

Search for electroweak production of charginos and sleptons decaying in final states with two leptons and missing transverse momentum in 13 TeV proton-proton collisions using the ATLAS detector: … ATLAS-CONF-2019-008/

Provided by ATLAS Experiment
Citation: ATLAS experiment sets strong constraints on supersymmetric dark matter (2019, May 14) retrieved 17 September 2019 from
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May 14, 2019
Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

Displaced supersolid dark matter is curved spacetime.

In the Bullet Cluster collision the dark matter has not separated from the ordinary matter. The collision is analogous to two boats that collide, the boats slow down and their bow waves continue to propagate. The water has not separated from the boats, the bow waves have. In the Bullet Cluster collision the galaxy's associated dark matter displacement waves have separated from the colliding galaxies, causing the light to lense

May 14, 2019
'Scientists Thought All Galaxies Had Dark Matter, but They Just Found One Without It'

> "Analysis shows the ultra-diffuse DF2 lies about 6.5 million light years away and is roughly the same size as our own Milky Way galaxy, but contains 200 times fewer stars."

The reason for the mistaken notion the galaxy is missing dark matter is that the galaxy is so diffuse that it doesn't displace the supersolid dark matter outward and away from it to the degree that the dark matter is able to push back and cause the stars far away from the galactic center to speed up.

It's not that there is no dark matter connected to and neighboring the visible matter. It's that the galaxy has not coalesced enough to displace the supersolid dark matter to such an extent that it forms a halo around the galaxy.

A galaxy's halo is not a clump of dark matter traveling with the galaxy. A galaxy's halo is displaced supersolid dark matter.

May 14, 2019

'Astronomers Discover New Galaxy That Is 99.99% Dark Matter'

> "A relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue."

The more compact the cluster the greater the displacement of the supersolid dark matter connected to and neighboring the cluster, the greater the displaced supersolid dark matter pushes back and exerts pressure toward the cluster, the faster the stars in the cluster move.

May 14, 2019
Recently two papers have been published. The first one deals with the measurement of the speed of rotation of galaxies and, in our view, closes the issue of the existence of dark matter. The second one argues that the expansion of the universe is not accelerating. However, this fact does not answer the question as to what in general is the cause of the universe's expansion and does not address the widespread opinion that 70% of the universe consists of dark energy.

May 14, 2019
The second one argues that the expansion of the universe is not accelerating. However, this fact does not answer the question...

A scientific argument of the kind you describe does not equate to a "fact". It is probably at best an hypothesis. Although comments here are typically particularly informal, please be careful in your terminology around such matters as this can otherwise be misleading, and in the worst case might create a completely incorrect view of an issue by a reader.

May 14, 2019
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