Periodic Reporting for period 3 - DenseMatter (High-density QCD matter from first principles) Reporting period: 2020-07-01 to 2021-12-31 Summary of the context and overall objectives of the project The project "Dense QCD matter from first principles" aims to apply the ab-initio machinery of perturbative quantum field theory to the quantitative description of ultra-high-density quark matter (QM). This line of work is extremely topical, as recent years have witnessed major breakthroughs in the observational study of neutron stars (NSs) - the only systems in our present-day Universe where this kind of exotic matter may reside. In particular, the first detection of gravitational waves (GWs) from the merger of two distant neutron stars, recorded in August 2017, marked the birth of a new era of multimessenger astronomy, which will have a lasting impact on both astrophysics and the physics of the strong nuclear interaction.The two most important concrete objectives of the DenseMatter project were the derivation of the four-loop order in the weak coupling expansion of the Equation of State (EoS) of unpaired QM, and obtaining the most accurate NS matter EoS utilizing the former result. Here, the former project naturally splits into three distinct parts, as the four-loop EoS contains terms of order g^6ln^2(g), g^6ln(g) and g^6 in the gauge coupling g of QCD, and the computations of these three terms are somewhat independent. Similarly, the NS matter EoS calculation is something that can be updated whenever new observational or theoretical results emerge, so this part of the project in a natural way splits to several subprojects that can be addressed during the course of the ERC grant.The final, extremely ambitious goal of the project is to apply the above results in determining the inner structure of NSs, and firmly determine whether deconfined QM resides inside them. If successful, this would be a momentous breakthrough in the field of nuclear astrophysics. 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 project got off to a flying start when just a few months after its beginning the LIGO/Virgo GW discovery was announced. Due to the preparatory work my group had been doing for the NS EoS subproject, we were able to apply the new results to EoS determination very efficiently, and managed to publish the most accurate model-independent EoS for NS matter just a month after the publication of the GW results. This paper, which was later published in Physical Review Letters, became the most popular particle/nuclear theory paper written on the subject and has by now gathered almost 300 citations in less than 2.5 years. On the QM front, my group finished the first task, i.e. the determination of the g^6ln^2(g) term in the QM EoS, which required developing an effective description for the soft sector of QCD at high density. This work was finished in the summer of 2018, and was subsequently also published in PRL in the fall of the same year. 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) Ever since the publication of the two above-mentioned articles, we have kept pushing the state-of-the-art even further on both research directions.In attempts to further constrain the NS matter EoS, my group has teamed up with Joonas Nättilä - a leading expert on NS radius measurements - to build extensive Bayesian machinery to take into account all relevant NS measurements in determining the most likely EoS in yet unpublished work. Prior to finishing this project, we however also tackled a somewhat different challenge, namely trying to find model-independent constraints for the existence of QM inside the cores of the most massive NSs. This line of work turned out to be much more fruitful than we anticipated, and a manuscript of ours featuring evidence for the presence of QM is currently under review in Nature Physics. We expect very useful results to follow from continuations of both of these lines of woek.After the determination of the g^6ln^2(g) term in the QM EoS, the next natural goal in this subproject is clearly the determination of the subleading logarithm of order g^6ln(g). To this end, we have continued work on the soft contributions to the EoS, and are at the moment very close to finishing this very demanding contribution. At the same time, we have already begun work towards the eventual determination of the full g^6 term, which will however take considerable time and effort considering the magnitude and demanding nature of the corresponding computation.In addition to the two main goals above, we have naturally been occupied with several independent challenges - and will continue to do so in the foreseeable future. Neutron star matter Equation of State taken from article accepted for publication in Nature Physics
