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High-energy Physics: Predicting the Emergence of Jets

Thu, 06/19/2014 - 4:03pm
Amber Harmon, iSGTW

Sherpa collision simulation. Matrix element generators simulate 'hard' part of scattering (shown in red). Parton showers produce Bremsstrahlung (blue). Multiple interaction models simulate 'secondary' interactions (purple). Fragmentation models 'hadronize' QCD partons (green). Hadron decay packages simulate unstable hadron decay (dark green). YFS generators produce QED Bremsstrahlung (yellow). Courtesy of iSGTWJets resulting from particle collisions, like those taking place at the Large Hadron Collider (LHC) housed at CERN near Geneva, Switzerland, are quite possibly the single most important experimental signatures in high-energy physics. Virtually every final-state, high-energy particle produced will be part of a jet.

“It's actually very rare that these collisions produce a non-strongly interacting particle,” says Stefan Höche, a theoretical physicist at SLAC National Accelerator Laboratory at Stanford University in California, US. “So we have to describe the emergence of jets very precisely. Even if an interesting new particle is created, it will predominantly decay into jets, and we have to discern such decays from an overwhelming background.”

Höche and his colleagues developed and are refining Sherpa, a Monte Carlo event generator that simulates high-energy reactions resulting from particle collision events. Physicists use these simulations to compute how particles interact according to a given theory, and then compare simulated collisions with those measured by experiments at the LHC.

The scientists can make predictions for quarks and gluons, which are the elementary particles of quantum chromodynamics, but they cannot make predictions for jets — at least not based on first principles. That’s where the Monte Carlo comes in, relating the quarks and gluons to the jets in the LHC experiment. Each highly energetic quark or gluon becomes the seed for one or more jets.

In decades past, simple event generators were used, based on two particles in the final state which can describe two jets quite well. The massive amount of energy used to produce collisions today, however, results in hundreds or thousands of observed particles, which may form a dozen or more jets. Event generators must take into account many more quarks and gluons to describe this situation precisely, and thus perform much more complicated calculations.

Amber Harmon is the US desk editor of iSGTW. This article originally appeared in iSGTW on June 4, 2014. Read the full article: High-energy Physics: Predicting the Emergence of Jets

 

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