NIC Excellence Project 2016/2

Hadronic contributions to electroweak observables

NIC Excellence Projects 2016/2
Dr. Karl Jansen

John von Neumann Excellence Project 2016;
Dr. Karl Jansen, DESY Zeuthen

The "John von Neumann Excellence Project 2016" of Dr. Karl Jansen from the Deutsches Elektronen-Synchrotron (DESY) is embedded in the field of lattice gauge theory (LGT) computations. In LGT large scale numerical simulations are carried out on high performance supercomputers, in this case JUQUEEN at the supercomputer center in Julich. In the project up to 65536 cores were used simultaneously on JUQUEEN.

The particular project of Dr. Jansen is concerned with deviations from the classical value of the magnetic moment of the spin 1/2 muon, a lepton, very similar to the electron but about 200 times heavier. Such deviations are pure quantum effects, see fig.1, and can provide direct evidence for physics beyond the standard model (SM) of high energy physics since new particles can be responsible for significant corrections to the classical value.

The muon magnetic moment can be measured very precisely with 7 significant digits. Therefore, to find deviations from the SM the theory prediction needs to be equally accurate. Unfortunately, in the past, simulations within lattice gauge theory to compute the muon magnetic moment (or, to be more concrete, its hadronic contribution) have been rather imprecise making it seemingly impossible to reach the experimental precision. However, Dr. Jansen has achieved two important steps forward. He and his team developed a new technique to calculate the muon magnetic moment which led to a significant error reduction in the calculation. In addition, he could perform simulations directly in physical conditions, a step that has been missing in the past and which led to large systematic uncertainties.

NIC Excellence Projects 2016/2
Fig. 1: The spin of the muon. Left: on the classical level, the spin of the muon is 1/2. Right: when quantum corrections appear (here the generation of electron positron pairs), the muon is deformed and a deviation of the spin from the value 1/2 can be computed theoretically to a very high accuracy. This deviation has been indeed detected experimentally to a precision that matches the theoretical calculation.

In fig.2 physical quantities computed in this physical setup are compared to their experimentally measured counterparts and a very reassuring agreement is found.

Both achievements led to the fact that the theory error from the lattice calculation of Dr. Jansen and his team is now already of the order of the experimental one. This opens the very promising path to match the even more reduced error of soon to start new experiments at FermiLab and J-Parc and thus to possibly discover hints for physics beyond the SM.

NIC Excellence Projects 2016/2
Fig. 2: Ratios of basic physical results Qlat computed by ETMC using LQCD and phenomenological results Qphys published by the particle data group.

Last Modified: 23.06.2022