Skip to main content

Collider Phenomenology and Event Generators

Periodic Reporting for period 3 - MorePheno (Collider Phenomenology and Event Generators)

Reporting period: 2018-11-01 to 2020-04-30

Collider physics is about exploring the smallest constituents of matter, and unravelling the basic laws of the Universe. Unfortunately there can be a huge gap between a one-line formula of a fundamental theory and the experimental reality it implies. Phenomenology is intended to fill that gap, e.g. to explore the consequences of a theory such that it can be directly compared with data.

Nowhere is the gap more striking than for QCD, the theory of strong interactions, which dominates in most high-energy collisions, like at the LHC (Large Hadron Collider) at CERN. And yet, when such collisions produce hundreds of outgoing particles, calculational complexity is insurmountable. Instead ingenious but approximate QCD-inspired models have to be invented.

Such models are especially powerful if they can be cast in the form of computer code, and combined to provide a complete description of the collision process. An event generator is such a code, where random numbers are used to emulate the quantum mechanical uncertainty that leads to no two collision events being quite identical.

The Principal Investigator is the main author of PYTHIA, the most widely used event generator of the last 30 years and vital for physics studies at the LHC. It is in a state of continuous extension: new concepts are invented, new models developed, new code written, to provide an increasingly accurate understanding of collider physics. But precise LHC data has put a demand on far more precise descriptions, and have also shown that some models need to be rethought from the ground up.

This project, at its core, is about conducting more frontline research with direct implications for event generators, embedded in a broader phenomenology context. In addition to the PI, the members of the theoretical high energy physics group in Lund and of the PYTHIA collaboration will participate in this project, as well as graduate students and postdocs.
"The key component of the project is the continued development of the PYTHIA event generator. Here information exchange with other similar projects is essential for the development of the field as a whole, in a combination of a collaborative and a competitive spirit. Interaction with the experimental community, especially the one at the LHC at CERN, is also crucial to make advances relevant for a greater community, and to test new ideas that may or may not work out. In this context we have studied a number of areas, such as diffraction (soft and hard), different fragmentation models, gamma-gamma collisions and photoproduction, parton showers, and more. On the technical side, the program has been transitioned from the C++98 standard to the C++11 one, with the release of PYTHIA 8.3. This has also allowed a cleaner administrative structure. On the physics side, the 8.3 version integrates the previously free-standing VINCIA and DIRE parton showers, thereby allowing a richer view on the transition from perturbative to nonperturbative physics. Currently a major new article is being written, to explain all the physics encompassed by PYTHIA.

One of the most unexpected discoveries at the LHC in recent years has been that high-multiplicity pp events in many respects behave like heavy-ion ones, and unlike low-multiplicity pp ones. This is reflected in the ""ridge effect"", in signs of collective flow and, above all, by the increased rate of strange baryon production (but not of nonstrange ones). This was unexpected both in the context of standard pp models like PYTHIA and in models for quark-gluon plasma formation, where pp collisions are not expected to generate sufficiently large volumes and long timescales for quark-gluon plasma formation to take place. Several of the studies undertaken or planned relate to this area, including colour ropes, repulsion of overlapping strings (""shoving""), the space-time structure of hadronization, hadronic rescattering, and in particular a new model for heavy-ion collisions. This model, Angantyr, has already attracted widespread attention in the experimental community.

In addition to the event-generator-related activities, many studies have also been related to physics Beyond the Standard Model, not least in the Higgs sector. With the discovery of the Higgs at the LHC the Standard Model was complete in many respects, but unexplained phenomena remain, that require new physics mechanisms. It is natural to associate some of these with the existence of more Higgs states than the one discovered, and therefore to explore different aspects of such physics.

One challenge to the Standard Model is the muon anomalous magnetic moment, where experiment and theory show a small but tantalizing discrepancy. Here more detailed calculations of the least well understood contributions are performed to reduce the error on the theoretical prediction. This uses techniques from chiral perturbation theory, e.g. to study the finite-volume effects of results obtained from lattice QCD. Such techniques are used also in other contexts, e.g. for the prospects of neutron-antineutron oscillations at the ESS.

In addition to the main themes listed above, some further smaller studies have also been performed on various topics."
Research is open-ended, fortunately. We are making and will continue to make progress in a number of respects. The end of this project is not the end of progress. It is also not possible to promise or predict the outcome of research, only to promise to strive to make progress within the topics listed in the applications.

Thus activities in all the areas above continue, and new ones will be added. The unexpected behaviour of high-multiplicity pp events and its relation to heavy-ion collisions will continue to attract special attention. And if the searches for new physics will dig up some unexpected signal then we will be active in its interpretation.

With the arrival of a new staff member, Stefan Prestel, studies of next-to-leading-logarithmic-accurate showers and of matching and merging with next-to-next-to-leading-order matrix elements will also play a significant role in the activities of the Lund group.