# Theory

The Manchester Particle Theory Group is home to around 30 researchers. As theoretical particle physicists, we develop and exploit powerful mathematical and computational tools to predict how subatomic particles behave. These predictions are tested against data from particle physics experiments, which probe nature on tiny length scales, and astronomical observations of our universe, which probe nature on its largest scales.

The behaviour of subatomic particles is determined by their intrinsic properties, like their mass, and the forces that they experience. These forces can be described by mathematical frameworks called quantum field theories (QFTs). The strong force, experienced by particles with colour charge like quarks and gluons, is one example, and the QFT that describes it is quantum chromodynamics (QCD). Electromagnetism and the weak force are two other examples, and these two forces are unified in the Standard Model (SM) of particle physics via the electroweak theory.

**QCD and Collider Physics**

Academic Staff: **Mrinal Dasgupta, Jeff Forshaw, Jonathan Gaunt, Michael Seymour, Eleni Vryonidou**

Researchers: ** Hesham El Faham**,

**Basem El-Menoufi, Rudi Rahn, Stefan Schacht**

A detailed understanding of the forces of Nature in conjunction with the simulation of particle collisions is vital for analysing data from facilities like CERN’s Large Hadron Collider (LHC). While we use the language of quarks and gluons (collectively called partons) to do so, these are not the states that propagate out of the collision and into particle detectors. Instead, quarks are confined into hadrons like protons and pions, and this makes understanding the underlying QFT fascinating.

One key aspect is to obtain a precise prediction of the highest energy “primary” parton-parton scattering. We might need to describe only the first few high-energy QCD corrections to this process (the “fixed-order” regime) or sum up the effects of many lower-energy parton emissions (the “resummation” regime). We are working on ways to improve the precision of theoretical predictions of proton collisions in both regimes. We are also studying the effects of additional parton-parton interactions beyond those in the primary process. Members of our group have developed the theoretical framework for describing two high-energy parton-parton collisions (“double parton scattering”), alongside phenomenological tools to make numerical predictions for processes with additional interactions.

We are experts in analytical all-orders calculations in QCD and their translation into “event generator” computer codes. In the first stage of a collision, the few partons involved can radiate additional partons and an extended system, called a parton shower, is produced. The properties of this shower can be calculated from perturbative QCD. Once the typical separation of the partons reaches the typical sizes of hadrons, non-perturbative effects become important and first-principles calculations are not available. Instead, phenomenological models of confinement are used. We are actively developing both the underlying theory and Monte Carlo event generators.

Herwig is one of three generators that simulate this process, and it is used by all collider experiments in the world. A limitation of existing generators is that they neglect some quantum colour interference effects. These can be included in a new approach at the level of a quantum mechanical density matrix. The CVolver project aims to achieve this and, with collaborators in Graz, we are leading the international effort to construct a “full colour” event generator. Parton showers also serve the key purpose of summing large logarithms for general multi-scale observables, which abound at hadron colliders like the LHC. Group members working on the PanScales project are contributing to achieving improved logarithmic accuracy of showers, which is needed for achieving a precise description of multi scale observables.

The output of these codes ultimately allows us to predict the jets of hadrons that are the experimental footprints of partons produced at colliders like the LHC. Through our work to develop a deep understanding of QCD radiation and jets, we can explore the TeV scale at the LHC and possible future colliders, with a view to discovering new physics beyond the Standard Model.

**Physics Beyond the Standard Model**

Academic Staff: **Richard Battye, Jeff Forshaw, Charanjit K. Khosa, Peter Millington, Apostolos Pilaftsis, Eleni Vryonidou**

Researchers: **Hesham El Faham, Dimitrios Karamitros,** **Alex Keshavarzi, Yannick Kluth, Andrei Lazanu, Rudi Rahn, Alejo Rossia**, **Stefan Schacht**

This new physics is needed to explain shortcomings of the Standard Model and resolve some of the most challenging questions in particle physics: e.g., the origin of neutrino masses, the origin of the matter-antimatter asymmetry of our universe, the natures of the dark matter and dark energy that together make up 95% of its energy budget, the huge discrepancy between the electroweak and quantum gravity scales, the apparent instability of the electroweak vacuum, and the absence of CP violation in QCD. There are also tantalising hints of deviations from the Standard Model in heavy flavour physics (the physics of particles like muons and bottom quarks).

**Quark Flavour Physics**

Flavour physics and charge parity (CP) violation provide powerful probes of the Standard Model. Current anomalies seen in quark flavor physics, such as lepton-flavor non-universality and the discovery of charm CP violation, herald a new era. Very soon, an abundance of additional unprecedented experimental data will become available. In order to completely exploit this new and more precise data, new and improved theoretical methods are crucial.

In our research in quark flavor physics, we test the Standard Model by applying QFT to decays of beauty, charm and strange hadrons. Our work includes especially the interplay of the symmetries of the strong interaction at low energies, i.e., in the non-perturbative regime of QCD, and the violation of the CP symmetry between matter and antimatter by the weak interaction. As CP is not a fundamental symmetry of nature, naturalness implies that BSM physics generically induces new order-one weak phases. This implies, e.g., that the recent discovery of CP violation in non-leptonic charm decays is an exciting starting signal for the search for BSM physics in a completely new sector.

Our research provides new ideas for more analyses that test the SM at LHCb, Belle II and future kaon facilities, and will be crucial in order to benefit from the new experimental data for the physics of strange, charm and b quarks.

**Lepton Flavour Physics**

In 2021, the Fermilab Muon g-2 experiment reported a value for the muon magnetic moment that disagrees with the SM prediction by 4.2 standard deviations (see the Lepton Flavour Group research page on this website). The potential to confirm any Beyond SM (BSM) muon interaction rests heavily on improving the SM predictions for the muon magnetic moment, which is entirely dependent on improving evaluations of the so-called hadronic vacuum polarization (HVP) of the photon, which results from the interactions of virtual particles as allowed by Heisenberg’s uncertainty principle. The same is true for the evaluation of the running coupling of quantum electrodynamics (QED) where, without improving the HVP, the large uncertainties of the hadronic contributions will render the precision of next generation measurements of electroweak (EW) precision observables redundant. At the University of Manchester, as part of the KNT analysis team (see links below), we are leading and providing the world’s most precise evaluations of the hadronic cross section and the HVP.

**Collider BSM**

New physics beyond the SM might consist of new particles. If these are heavier than the collider energy reach, their presence can be revealed by modifications of the interactions of the known particles. The Standard Model Effective Field Theory (SMEFT) parametrises such deviations, extending the sensitivity of colliders to energy scales beyond those being probed directly. Determining the parameters of the EFT, particularly if different from SM expectations, will shed light on the nature and scale of new physics, and hint at the answers to these important outstanding questions in particle physics. We are making theoretical contributions to the LHC SMEFT programme by providing precise Monte Carlo tools and using LHC data to constrain the SMEFT interactions.

Our efforts in the SMEFT programme are very broad. We are exploring new processes and devising new observables to exploit LHC data in new ways, hence maximising our knowledge of nature. We are particularly interested in top and Higgs physics, the two least known elementary particles. We collaborate with other groups to develop truly global fits of LHC data to the SMEFT parameter space, focusing on the addition of new observables, higher-precision predictions and matching the SMEFT to UV models of interest to give new interpretations of the data. Another line of research is the use of SMEFT to gauge the potential of future colliders, in particular muon and hadron colliders.

Complementing this work, we are also exploring how modern machine learning (ML) techniques can advance the search for physics beyond the standard model. In particular, we are investigating jet tagging efficiency using ML and novel approaches to perform model-independent searches.

**Cosmology** **and Astroparticle Physics**

The new particles could be much lighter than the collider energy reach, and examples include axions, which can solve the strong CP problem or provide a candidate for dark matter. Light new physics can also be related to how the Standard Model is coupled to gravity, and whether the correct theory of gravity is Einstein’s general relativity. Extended gravitational theories can lead to new long-range forces of nature, called fifth forces, and impact the evolution of our universe and the dynamics of astrophysical objects. They can also affect precision experiments based on atom interferometry and lead to violations of the universality of free fall. We are making precise predictions for fifth force models, which account for the screening mechanisms that allow them to evade local tests of gravity, while still impacting large-scale dynamics.

There are good reasons to believe that QFT is insufficient to fully describe quantum gravity. Recent developments in black hole physics and holography (the idea that all information about a system can be encoded in a lower dimensional projection of that system) suggest that spacetime and its dynamics are features that emerge from quantum entanglement. We are beginning to explore these ideas by exploiting our expertise in the theory of open quantum systems, and by making connections with the areas of quantum information and condensed matter physics.