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2+1+1 Lattice QCD Calculations With HEX Smeared Clover Fermions

Principal Investigator: Kálmán Szabó, Forschungszentrum Jülich, Institute for Advanced Simulation, Jülich Supercomputing Centre
HPC Platform: JUQUEEN of JSC

Dark Matter (DM) is a type of matter that has not yet been identified in laboratory experiments. Their interaction with ordinary matter must be extremely small so that they evaded detection so far. There are many experiments looking for a conversion of a DM particle to ordinary protons and neutrons. The theories of DM however are formulated in terms of couplings between DM particles and quarks, the elementary particles that build up protons and neutrons (aka. nucleons). To calculate the prediction of these theories and aid the experimental searches, one has to know the "quark content" of the nucleon, which is roughly speaking the probability of finding quarks in the nucleon.

An international team from Wuppertal, Jülich and Marseille has performed computations for the nucleon quark content, the Jülich contribution was coordinated by Kalman Szabo. The results appeared in Physical Review Letters (Phys.Rev.Lett. 116 (2016) no.17, 172001).

Since the equations describing the relation between the quarks and the proton have a very large number of unknowns, order of 10 millions, the use of supercomputers becomes unavoidable. Fortunately the problem is of embarrassingly parallel type, and thus massively parallel supercomputers can be used very efficiently. Among others, the research group used the supercomputing resources of JUQUEEN.

There has been several previous calculations on the quark content of the proton before, even using the same technique (lattice QCD). The calculation performed now is the first where all systematic effects are taken into account. In particular, simulations are carried out with the correct values of the quark masses, a task which is usually regarded time consuming. Previous calculations used higher than physical quarks masses which, albeit less expensive, introduces an extrapolation whose systematics is hard to get under control. The quark contents are calculated for three variants (aka. flavors) of quarks: up, down and the strange.

The previous results and the group's recent finding for the up/down quark content can be seen in Figure 1. The number on the figure can be used to calculate conversion rates of DM particles to nucleons starting from theories based on DM particles and quarks.

2+1+1 Lattice QCD Calculations With HEX Smeared Clover FermionsCopyright: Forschungszentrum Jülich

Scientific Contact:
Prof. Dr. Kálmán Szabó
Forschungszentrum Jülich GmbH
Institute for Advanced Simulation (IAS), Jülich Supercomputing Centre (JSC)
Wilhelm-Johnen-Straße, D-52425 Jülich (Germany)
e-mail: szaboka [@]

October 2016