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27 May, 2019
The CMS collaboration has searched for collision events in which the Higgs boson transforms into a photon and a hypothetical dark photon
Image above: A proton–proton collision event featuring a muon–antimuon pair (red), a photon (green), and large missing transverse momentum. (Image: CERN).
They know it’s there but they don’t know what it's made of. That pretty much sums up scientists’ knowledge of dark matter. This knowledge comes from observations of the universe, which indicate that the invisible form of matter is about five to six times more abundant than visible matter.
One idea is that dark matter comprises dark particles that interact with each other through a mediator particle called the dark photon, named in analogy with the ordinary photon that acts as a mediator between electrically charged particles. A dark photon would also interact weakly with the known particles described by the Standard Model of particle physics, including the Higgs boson.
At the Large Hadron Collider Physics (LHCP) conference, happening this week in Puebla, Mexico, the CMS collaboration reported the results of its latest search for dark photons.
The collaboration used a large proton–proton collision dataset, collected during the Large Hadron Collider’s second run, to search for instances in which the Higgs boson might transform, or “decay”, into a photon and a massless dark photon. They focused on cases in which the boson is produced together with a Z boson that itself decays into electrons or their heavier cousins known as muons.
Such instances are expected to be extremely rare, and finding them requires deducing the presence of the potential dark photon, which particle detectors won’t see. For this, researchers add up the momenta of the detected particles in the transverse direction – that is, at right angles to the colliding beams of protons – and identify any missing momentum needed to reach a total value of zero. Such missing transverse momentum indicates an undetected particle.
Large Hadron Collider (LHC). Animation Credit: CERN
But there’s another step to distinguish between a possible dark photon and known particles. This entails estimating the mass of the particle that decays into the detected photon and the undetected particle. If the missing transverse momentum is carried by a dark photon produced in the decay of the Higgs boson, that mass should correspond to the Higgs-boson mass.
The CMS collaboration followed this approach but found no signal of dark photons. However, the collaboration placed upper bounds on the likelihood that a signal would have been seen.
Another null result? Yes, but results such as these and the ATLAS results on supersymmetry also presented this week in Puebla, while not finding new particles or ruling out their existence, are much needed to guide future work, both experimental and theoretical.
For more details about this result, see the CMS website: https://cms.cern/news/no-sign-dark-light-higgs-boson
Note:
CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.
The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.
Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 23 Member States.
Related links:
Dark matter: https://home.cern/science/physics/dark-matter
Standard Model: https://home.cern/science/physics/standard-model
Higgs boson: https://home.cern/science/physics/higgs-boson
Z boson: https://home.cern/science/physics/z-boson
Large Hadron Collider Physics (LHCP): https://indico.cern.ch/event/687651/
Large Hadron Collider (LHC): https://home.cern/science/accelerators/large-hadron-collider
ATLAS results on supersymmetry: https://orbiterchspacenews.blogspot.com/2019/05/atlas-surveys-new-supersymmetry.html
For more information about European Organization for Nuclear Research (CERN), Visit: https://home.cern/
Image (mentioned), Animation (mentioned), Text, Credits: CERN/Ana Lopes.
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