**Principal Investigator:**

Zoltán Fodor

**Affiliation:**

Institut für Theoretische Physik, FB-C, Universität Münster (Germany)

**Local Project ID:**

hwu10

**HPC Platform used:**

JUQUEEN of JSC

**Date published:**

The ratio of a particle’s magnetic moment to angular momentum gives the dimensionless constant *g*. A spinning ball with uniform charge has *g* = 1. Relativistic quantum mechanics predicts the muon has intrinsic spin which is is twice as effective at generating a magnetic moment as classical angular momentum, i.e., *g _{µ}* = 2.

A more complete description, quantum field theory, allows for the creation and annihilation of particle-antiparticle pairs. Interactions between the muon and a cloud of such pairs and force-carriers around it make *g _{µ}* = 2.002331 that we call the

Since the 1970s, the SM has explained experimental results with great success. However, the measured value of *a _{µ}* and the SM prediction differ

The SM prediction *a _{µ}*

Fig. 4 shows the most important interaction involving QCD. The internal forming the loop in Fig. 3 interrupts its journey to briefly become a foam of gluons and quark-antiquark pairs, represented by the red blob. This interaction contributes only ~ 0.006% of *a _{µ}*, but more than 85% of the theoretical uncertainty Best estimates are phenomenological calculations using data from e

With lattice QCD we calculate the importance of the QCD foam by inverting a large, sparse matrix (dimension ~ 10^{8}). The matrix encodes simulated quark and gluon fields on a discretized block of spacetime. Its inverse describes the propagation of quarks or antiquarks in every permissible way from one end of the blob to the other.

Simulations at different quark masses and discretization scales help control systematic errors. Preliminary results2 (Fig. 5) are consistent with the e^{+} e^{−} data.

The goal is to further reduce the uncertainty to resolve if ∆*a _{µ}* is a signal of physics beyond the SM.

HPC platform used for this project: System JUQUEEN of JSC Jülich.

*© for all images: Bergische Universität Wuppertal, Fachbereich C - Theoretische Physik*

References:

1. J. Beringer, et al..(Particle Data Group), PR**D86**, 010001 (2012) and 2013 update for the 2014 edition (http://pdg.lbl.gov)

2. E. B. Gregory, et al., Leading-order hadronic contributions to g_{µ} − 2, PoS(LATTICE 2013)302, arXiv 1311.4446.

**Scientific Contact:**

Eric B. Gregory

Theoretische Physik, Fachbereich C - Bergische Universität Wuppertal

D-42097 Wuppertal/Germany

e-mail: gregory@uni-wuppertal.de