# Theoretical Particle Physics

The research in theoretical physics focuses on theoretical physics research in elementary particle physics and astroparticle physics and establish the connection between abstract particle physics theories and reality by making predictions for experiments.

The research is on physics beyond and within the Standard Model, and we work on topics such as Higgs physics, particle cosmology, high-energy astroparticle and neutrino physics, and quantum chromodynamics.

Our field of research is theoretical physics with connection to experiment, both physics at particle colliders physics, and experiments in astroparticle physics and cosmology.

The approach is to develop theoretical models and computational tools for various physical processes and compare with experiment. Our research in this area involves development and analysis of models, numerical and analytical calculations, and Monte Carlo simulation. Several Monte Carlo event generators have been developed in the group.

## Quantum Chromodynamics

The birth of QCD dates back to the beginning of the 50s where a vast and ever growing amount of new particles were discovered (baryons and mesons). Researchers in those days were baffled at the amount of seemingly fundamental particles, and there was a feeling that not all of these could truly be fundamental. This feeling came to fruition with the discovery by Yuval Ne'eman and Gell-Mann, which showed that all these newly discovered particles could be described in terms of three flavors of smaller particles which came to be known as the quarks. Starting initially with three quark flavours (up, down, strange) it was later found that it was necessary to include three more quark flavours (charm, bottom, top), and in addition these quarks had to carry “color” charge from which the name QCD is derived.

The theory of QCD has been hugely successful in describing the rich structure that we see in nature. Some of the noteworthy properties of QCD is that of confinement which states that quarks can not be individually observed at low energies but can only exist as bound states (baryons, mesons), while at a high enough energy the quarks may interact by themselves, for instance in deep-inelastic scattering (DIS). This property of QCD has the remarkable consequence that for high energies we can describe nature in terms of individual quarks, while at low energies the proper degrees of freedom is baryons and mesons. The big challenge is how to connect these two separated scales i.e. how to connect the description of the high energies with that of low energies.

The aim of our research is to understand this transition from high to low energies. The approach is to develop models and computational tools for various physical processes and compare with experiment in order to gain new insights to the nature of this region of intermediate scales. Our research in this area involves development and analysis of models, numerical and analytical calculations, and Monte Carlo simulation. Several Monte Carlo event generators have been developed in the group.

## Higgs Physics

With the discovery of the Higgs boson by the LHC in 2012, a new era of theoretical electroweak physics was heralded. The discovered boson fits very well with the Standard Model prediction, but since we know that the Standard Model cannot be complete, it is worthwhile to ask the question if there in fact are more Higgs bosons out there.

One of the few ways to introduce more Higgs bosons at the electroweak scale, without contradicting earlier experiments, is by simply duplicating the structure of the Standard Model Higgs sector. These models are called Two Higgs Doublet Models, and they contain in total five different Higgs bosons. One reason these models are interesting is that they are able to explain the matter–antimatter asymmetry of our universe. Another reason is that to make the standard model supersymmetric, we would need two Higgs doublets.

In our research in Two Higgs Doublet Models we make predictions for particle colliders such as the LHC in conjunction with cosmological questions. For instance, is it possible to explain the matter–antimatter asymmetry with a Two Higgs Doublet Model while still explaining the data from LHC?

## Software

### Physics software developed by members of the group

- XQCAT: eXtra Quark Combined Analysis Tool

D. Barducci, A. Belyaev, M. Buchkremer, G. Cacciapaglia, A. Deandrea, S. De Curtis, J. Marrouche, S. Moretti and L. Panizzi (2014) - 2HDMC: Two-Higgs-Doublet Model Calculator

D. Eriksson, J. Rathsman, O. Stål (2010–) - BKsolver: Software for numerical solution of the Balitsky-Kovchegov equation

R. Enberg (2005) - MATCHIG: add-on to Pythia for matching of charged Higgs production

J. Alwall (2005) - HardCol: add-on to Pythia for hard color singlet exchange processes

R. Enberg (2002) - HERWIG 6.5, Hadron Emission Reactions With Interfering Gluons: A general-purpose Monte Carlo event generator

G. Corcella, I.G. Knowles, G. Marchesini, S. Moretti, K. Odagiri, P. Richardson, M.H. Seymour and B.R. Webber (2000–) - LEPTO: Monte Carlo event generator for deep inelastic scattering

G. Ingelman, A. Edin, J. Rathsman (1996) - AROMA: Monte Carlo event generator for heavy flavor in deep inelastic scattering

G. Ingelman, J. Rathsman, G. A. Schuler (1996) - MAJOR: Monte Carlo event generator for heavy Majorana neutrinos in ep collision

G. Ingelman, J. Rathsman (1996) - LUCIFER: Monte Carlo event generator for high-pT photoproduction

G. Ingelman, A. Weigend (1987) - TWISTER: Monte Carlo event generator for high-pT scattering in QCD

G. Ingelman (1986)

## Popular science description

Particle physics describes the constituents of Nature as a number of elementary particles called quarks and leptons. These quarks and leptons make up all known matter – protons and neutrons, nuclei and atoms – and are the most fundamental building blocks of the Universe known. In particle physics we study how these building blocks interact, and the forces that carry these interactions. These forces are called the electroweak and strong interactions. During the last forty years, a quantum mechanical theory known as the Standard Model was developed, which describes how the quarks and leptons interact by exchanging photons, W- and Z-bosons, and gluons. The Standard Model is an example of a quantum field theory.

The Standard Model is a very successful theory in predicting and explaining experimental observations, the input to the model are some 18 parameters that are fixed experimentally. With these input parameters specified, the Standard Model gives a large set of predictions which can be tested experimentally. Over the past decades a large set of experimental tests universally agree with the predictions of the Standard Model, solidifying it as our best understanding of nature.

Even with the success of the Standard Model there are nevertheless many reasons to expect new physics beyond what we already know. Some of the most prominent include the cosmological observation of dark matter and dark energy, which as to date remains an unknown. Additionally there exist various theoretical problems such as what happens with gravity at high energies. To explain these phenomena it is necessary to go **beyond **the Standard Model and include additional structure while at the same time not altering the successful experimental predictions.

Beyond the Standard Model theories come in a rich variety and flavour, some of the more popular ones include Supersymmetry which predict that each known particle has a heavy superpartner, some of which could act as a dark matter particle. Other popular possibilities include Grand Unified Theories which states that the known forces unify at a high energy scale, and what we see “down here” is really different aspects of the same force.

In order to judge if a particular beyond the Standard Model theory provides an adequate description of nature it is necessary to connect theory to experiment. This demands considerable calculations to predict signatures that might be seen, for example at the Large Hadron Collider. In our group we specialize on these kind of calculations and use them to study physics both within and beyond the Standard Model using theoretical and computational tools and methods.

## Contact

- Faculty members working on theoretical particle physics:
- Rikard Enberg
- Stefano Moretti