Periodic Reporting for period 1 - BCFS (Beyond Colours and Flavours on Supercomputers)
Reporting period: 2020-10-01 to 2022-09-30
The overarching goal of particle physics is the discovery of new fundamental particles in order to
advance our understanding of the universe. Our current best knowledge of the interactions of fundamental particles is
summarised in the Standard Model of Particle Physics (SM). Searches for "New Physics" (NP)
beyond the Standard Model (BSM) are often categorised as direct and
indirect searches where the former aim to directly observe a new particle and
the latter aim to infer the existence of some unknown particle as an
explanation of observational inconsistencies. NP searches can only be
successful, through the interplay between high-precision experimental
measurements (for example performed by the experiments at the Large Hadron
Collider at CERN) and high-precision theoretical predictions. This project
contributes to direct and indirect NP searches by providing first-principle
theory predictions which can be compared to experimental measurements.
The strong force (Quantum Chromodynamics or QCD) governs the interactions of
quarks and gluons (the fundamental constituents of protons and neutrons) into
bound states, called hadrons. The project "Beyond Colours and Flavours on
Supercomputers" aims to predict hadronic observables with control over all
sources of uncertainty. We use the well established framework of Lattice Quantum
Chromodynamics (LQCD) to provide such first-principle predictions, which cannot
be computed with analytical methods. LQCD numerically simulates a finite
space-time grid on which a representative sample of field configurations is
generated via Markov Chain Monte Carlo methods. This is achieved via the use of
large scale numerical simulations on supercomputers. On these configurations we
can compute certain objects from which we can extract hadron masses and other
relevant observables such as matrix elements.
Our work focuses on the computation of observables which are currently
displaying some degree of disagreement ("tension") between theory and
experiment, with the goal to substantiate whether this originates from
statistical fluctuations or is indeed a sign of NP. To achieve this, it is
paramount to make pure theory predictions from first principles, to avoid the
introduction of any un-quantifiable uncertainties.
We aim to compute "semi-leptonic form factors" which parameterise decays of a
heavy quark (such as a "charm" or "bottom" quark) to a lighter quark (for
example "up", "down" or "strange"). Since the target precision for many
state-of-the-art observables is below the percent-level accuracy, it is
important to consider and quantify all effects that contribute at this level of
precision. We therefore aim to predict isospin breaking effects arising from the
mass difference of the up and the down quark (strong isospin breaking) and from
the electric charge of quarks (weak isospin breaking). This is achieved in the
theoretically sound framework of massive QED.
advance our understanding of the universe. Our current best knowledge of the interactions of fundamental particles is
summarised in the Standard Model of Particle Physics (SM). Searches for "New Physics" (NP)
beyond the Standard Model (BSM) are often categorised as direct and
indirect searches where the former aim to directly observe a new particle and
the latter aim to infer the existence of some unknown particle as an
explanation of observational inconsistencies. NP searches can only be
successful, through the interplay between high-precision experimental
measurements (for example performed by the experiments at the Large Hadron
Collider at CERN) and high-precision theoretical predictions. This project
contributes to direct and indirect NP searches by providing first-principle
theory predictions which can be compared to experimental measurements.
The strong force (Quantum Chromodynamics or QCD) governs the interactions of
quarks and gluons (the fundamental constituents of protons and neutrons) into
bound states, called hadrons. The project "Beyond Colours and Flavours on
Supercomputers" aims to predict hadronic observables with control over all
sources of uncertainty. We use the well established framework of Lattice Quantum
Chromodynamics (LQCD) to provide such first-principle predictions, which cannot
be computed with analytical methods. LQCD numerically simulates a finite
space-time grid on which a representative sample of field configurations is
generated via Markov Chain Monte Carlo methods. This is achieved via the use of
large scale numerical simulations on supercomputers. On these configurations we
can compute certain objects from which we can extract hadron masses and other
relevant observables such as matrix elements.
Our work focuses on the computation of observables which are currently
displaying some degree of disagreement ("tension") between theory and
experiment, with the goal to substantiate whether this originates from
statistical fluctuations or is indeed a sign of NP. To achieve this, it is
paramount to make pure theory predictions from first principles, to avoid the
introduction of any un-quantifiable uncertainties.
We aim to compute "semi-leptonic form factors" which parameterise decays of a
heavy quark (such as a "charm" or "bottom" quark) to a lighter quark (for
example "up", "down" or "strange"). Since the target precision for many
state-of-the-art observables is below the percent-level accuracy, it is
important to consider and quantify all effects that contribute at this level of
precision. We therefore aim to predict isospin breaking effects arising from the
mass difference of the up and the down quark (strong isospin breaking) and from
the electric charge of quarks (weak isospin breaking). This is achieved in the
theoretically sound framework of massive QED.
The project started with a pilot study, designed to aid a direct search for NP within a class of super-symmetric models. We predicted certain mass-ratios within theories with a large number of "colours" and one "quark flavour". In the event of one new particle being discovered at an experiment, these ratios predict further experimentally detectable particles. Depending on whether or not the experiment subsequently finds another particle with the correct mass ratio, our work can be used to test whether the discovered new particle can be explained by this type of theory or not. We explored the relevant parameter space and established a computational set-up that we will use to refine this study in the future as more computing resources become available. First results have been presented at several conferences (Lattice 2021 and 2022, 1st and 2nd Nordic Lattice Meeting) and workshops, have been published in proceedings and will soon be submitted for a journal publication.
Through the pilot study we gained familiarity with the code base we are using for the heavy quark part of the project. Based on this experience we explored and optimised a computational set-up which allows us to make the best use of the existing gauge field configurations to which we have access. These results have been presented at Lattice 2022 and the 2nd Nordic Lattice Meeting and will soon be published as conference proceedings. We are now starting large scale simulations for several quark masses, space-time volumes and lattice spacings (smallest distance in the space-time grid) that will allow for the controlled prediction of a wide range of observables.
Simultaneously, we explored the numerical implementation of isospin breaking effects in the formulation of massive QED. This formulation has the advantage of being a valid causal Quantum Field Theory and computationally affordable and usable on pre-existing publicly available configurations. In order to scan the accessible parameter space of this formulation, we explored a wide range of choices, such as simulated photon masses and space-time volumes on existing gauge field configurations. We confronted our numerical computations with effective field theory based expectations, in order to confirm the viability of this method. Results of this feasibility study have been presented at Lattice 2021 and Lattice 2022 as well as the 1st and 2nd Nordic Lattice meeting and several seminars and workshops. First results have been published as conference proceedings and a journal publication is forthcoming. Based on this feasibility study, we have extended our computation to a wide range of ensembles with different quark masses, volumes and lattice spacings enabling the extrapolation of our results to physical parameters in a controlled fashion. We anticipate timely predictions of the experimentally known mass splittings. With data production already underway, we are currently extending our computation of isospin breaking effects to further highly relevant observables such as the anomalous magnetic moment of the muon, leptonic and semi-leptonic decay rates and the nucleon axial charge gA.
Through the pilot study we gained familiarity with the code base we are using for the heavy quark part of the project. Based on this experience we explored and optimised a computational set-up which allows us to make the best use of the existing gauge field configurations to which we have access. These results have been presented at Lattice 2022 and the 2nd Nordic Lattice Meeting and will soon be published as conference proceedings. We are now starting large scale simulations for several quark masses, space-time volumes and lattice spacings (smallest distance in the space-time grid) that will allow for the controlled prediction of a wide range of observables.
Simultaneously, we explored the numerical implementation of isospin breaking effects in the formulation of massive QED. This formulation has the advantage of being a valid causal Quantum Field Theory and computationally affordable and usable on pre-existing publicly available configurations. In order to scan the accessible parameter space of this formulation, we explored a wide range of choices, such as simulated photon masses and space-time volumes on existing gauge field configurations. We confronted our numerical computations with effective field theory based expectations, in order to confirm the viability of this method. Results of this feasibility study have been presented at Lattice 2021 and Lattice 2022 as well as the 1st and 2nd Nordic Lattice meeting and several seminars and workshops. First results have been published as conference proceedings and a journal publication is forthcoming. Based on this feasibility study, we have extended our computation to a wide range of ensembles with different quark masses, volumes and lattice spacings enabling the extrapolation of our results to physical parameters in a controlled fashion. We anticipate timely predictions of the experimentally known mass splittings. With data production already underway, we are currently extending our computation of isospin breaking effects to further highly relevant observables such as the anomalous magnetic moment of the muon, leptonic and semi-leptonic decay rates and the nucleon axial charge gA.
We performed state-of-the art computations using modern codes and novel approaches for the computation of theoretical predictions of SM and BSM observables. We numerically predicted the ratios of masses in a class of BSM theories which can be further refined and explored in the future. We determined an optimal set-up for the computation of heavy-to-light quark transitions which we will exploit in large scale simulations for various observables. We numerically established the method of massive photons as a feasible, versatile, cost-efficient and theoretically clean method to predict isospin breaking corrections and have extended this method to the computation of several observables. We anticipate that it will be used for many more observables in the future.
All of the results obtained through this project are vital ingredients required for the search of NP and are indispensable for the success of the experimental programme pursued at flavour physics experiments, for example at the Large Hadron Collider at CERN. Furthermore, once the existence of NP is clearly established, they will help to discern between different candidate theories trying to explain the NP observations.
This research has contributed to the scientific standing of the University of Southern Denmark and fostered significant scientific exchange and collaboration within the Nordic Countries and beyond.
All of the results obtained through this project are vital ingredients required for the search of NP and are indispensable for the success of the experimental programme pursued at flavour physics experiments, for example at the Large Hadron Collider at CERN. Furthermore, once the existence of NP is clearly established, they will help to discern between different candidate theories trying to explain the NP observations.
This research has contributed to the scientific standing of the University of Southern Denmark and fostered significant scientific exchange and collaboration within the Nordic Countries and beyond.