Periodic Reporting for period 3 - SIMDAMA (Strong-interaction matter coupled to electroweak probes and dark matter candidates) Reporting period: 2021-04-01 to 2022-09-30 Summary of the context and overall objectives of the project After the discovery of the Higgs boson and the non-observation ofparticles beyond the Standard Model at the Large Hadron Collider, manyresearch avenues in particle physics are currently being pursued inparallel. These include the search for dark matter particles, thequest to complete the determination of neutrino oscillation parametersand their absolute mass scale, as well as the intensified search fordeviations between Standard Model predictions and experimentalmeasurements of precision observables. The current situation andplans for the future are described in the Physics Briefing Book(arXiv:1910.11775) which serves as input for the European Strategyfor Particle Physics. While the Standard Model of particle physicshas been enormously successful at predicting observables measured incollider experiments, it fails to account for gravity, for neutrinooscillations and for dark matter, which makes up 27 percent of theenergy in the universe.Of the fields of precision tests of the Standard Model, dark mattersearches and neutrino oscillation experiments, it is presently an openrace as to which one will lead to the most decisive progress in ourunderstanding of fundamental physics. Project SIMDAMA, based on the`lattice QCD' framework, aims at enabling a more stringent test of theStandard Model (SM), contributing to narrowing down the list ofdark-matter candidate particles, and reducing uncertainties inneutrino detection. In all cases, the complexity of the stronginteraction is a bottleneck in pursuing these research avenues. Thestrong-interaction physics is addressed within SIMDAMA using the abinitio method of lattice QCD. The basic idea of this computationalframework is to discretize space and time and to formulate a latticefield theory whose renormalized correlation functions converge to theQCD correlation functions in the limit of zero lattice spacing. Inprinciple, all physics observables can be extracted from thesecorrelation functions. The method is numerically very demanding andrequires massively parallel computing resources. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far The specific task chosen of testing the SM more stringentlyconsists in improving its prediction for the anomalous magnetic moment(g-2) of the muon, which is measured experimentally to sub-ppm leveland has served as a precision observable for decades. The uncertaintyof the SM prediction is entirely dominated by the hadroniccontributions, and SIMDAMA aims at reducing the uncertainty on thevirtual effect of hadronic light-by-light scattering. At this point, alattice calculation carried out within SIMDAMA at larger-than-physicalquark masses has been published. It has yielded a result with acompetitive overall precision of 13 percent. The publication [Chao et al, Eur. Phys. J. C 80 (2020) no.9 869] also contains aphenomenological estimate of the correction needed to reach aprediction at physical quark masses. Lattice calculations are ongoingto directly compute this hadronic contribution at or near physical quarkmasses. In addition to this direct calculation of hadroniclight-by-light scattering in (g-2), its single largest contribution,namely the pion-pole exchange, can be computed separately. WithinSIMDAMA, this contribution was computed with a state-of-the-artprecision of six percent [Gerardin et al, Phys.Rev. D100 (2019) no.3 034520]. A significant amount of computing time was allocated to thisSIMDAMA project within the PRACE programme to further reduce this uncertainty.In the area of neutrino detection, lattice calculations are underwayto provide a high-quality determination of the nucleon axial formfactor. This is a central input in nuclear calculations of theneutrino-nucleus interaction, which are needed in order to determineneutrino fluxes in long-baseline oscillation experiments. New methodshave been developed to perform this calculation (H.B. Meyer et al, PoSLATTICE2018 (2018), 062). Next, inelastic contributions toneutrino-nucleon scattering will be studied, which become the dominantcomponent of the cross-section at the neutrino energies of several GeVrelevant for the DUNE experiment.In the area of WIMP dark matter detection -- WIMP stands for weaklyinteracting massive particle, providing more accurate scalar matrixelements of the nucleon enables one to reliably translate theexperimental measurement of a WIMP-nucleus cross-section or anexclusion limit thereof into a value of the coupling of the WIMPparticle to the SM Higgs particle, assuming the former is a scalarparticle. Here too, significant computing resources have beenallocated to perform this challenging calculation (see the onlinereporthttps://www.gauss-centre.eu/results/elementaryparticlephysics/article/baryon-structure-from-lattice-qcd-with-2-1-flavours-of-wilson-quarks/).A further goal of SIMDAMA is to make decisive progress in determiningthe spectral functions of the quark-gluon plasma (QGP), thehigh-temperature phase of strong-interaction matter. The spectralfunctions associated with the electromagnetic current are directlyproportional to the photon emissivity and the rate of dileptonproduction. The direct photon and dilepton spectra are centralobservables in heavy-ion collision experiments, which study theproperties of the QGP in the lab. Thus, combined with the hydrodynamicdescription of the reaction, the spectral functions allow one to makepredictions for these spectra and compare them to experimentalmeasurements. A second motivation comes from sterile neutrinos as adark matter candidate. Through their oscillation into activeneutrinos, their interactions with the QGP are governed by closelyrelated spectral functions. Hence, under certain conditions therate of sterile-neutrino production and therefore their abundance inthe universe are determined by the QGP spectral functions at atemperature around 200\,MeV.In the past year, a new quality of lattice calculations has beenachieved to probe the photon emissivity of the QGP (Ce et al,Phys. Rev. D 102 (2020) no.9 091501). It is the first calculation inQCD with dynamical quarks to take the continuum limit of the relevantcorrelation function, before analyzing it in terms of spectralfunctions. The results obtained are in good agreement with thepredictions of weak-coupling calculations, in spite of the temperatureconsidered being only 250 MeV.As part of SIMDAMA, a new method has been developed to probe thephoton emission rate (H.B. Meyer, Eur. Phys. J. A 54 (2018) no.11 192). Thismethod has been implemented and the project has obtained computingtime to be carried out at J\"ulich Supercomputing Centre. The photonemissivity is now accessible in a physically more transparent way viaa dispersion relation at fixed virtuality. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) Concerning the anomalous magnetic moment of the muon, in the past yearthe SIMDAMA calculation of the hadronic light-by-light contributionwas only the second lattice QCD calculation to be published, andachieved a relatively high degree of statistical precision. These twocalculations are consistent with each other and also confirm withinthe uncertainties the result obtained by dispersive methods. Untilthe end of the SIMDAMA project, further increase in precision andreliability is expected, mainly by performing the calculation closerto physical quark masses. In addition, the calculation of thepion-pole contribution is expected to improve significantly via acalculation directly at the physical point.Several publications on nucleon structure, in particular on the axialform factor and the scalar form factor, relevant respectively forneutrino detection and scalar WIMP detection, are planned for 2021. Anew analysis technique of lattice data will be used. The axial formfactor will be provided in terms of the coefficients of the so-calledz-expansion, a model-independent way of parametrizing the form factorat spacelike virtualities. The vector and axial-vector transition formfactors between the nucleon and the delta resonance will be calculatedwith a publication expected towards the end of SIMDAMA.On the front of spectral functions at finite temperature, the plan isto publish a calculation of the dilepton production rate at atemperature of 250 MeV, with direct relevance to heavy-ion collisions.The calculation will be based, similarly to the case of the photonemissivity published this year, on continuum-limit extrapolatedlattice data in QCD with dynamical quarks. This will also enable across-check of the results obtained this year for the photonemissivity. Secondly, a global constraint on the photon emissivitywill be obtained in the framework of dispersion relations at fixedvirtuality, which will test for the first time resummed weak-couplingpredictions. Thirdly, a publication is planned in the near future onthe physics of the spectral functions at spacelike virtualities, andthe associated question of the resolution scale at which the quarksbecome `visible' inside the quark-gluon plasma.