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Collider Phenomenology and Event Generators

Periodic Reporting for period 4 - MorePheno (Collider Phenomenology and Event Generators)

Reporting period: 2020-05-01 to 2020-10-31

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, and is the field of this grant.

As such it is a purely curiosity-driven endeavour, with no promise of immediate financial return. But most students and postdocs do go on to positions outside academia, bringin with them advanced skills they have acquired in mathematics, physics, statistical methods, the handling of big data and deep learning, and more, which makes them very attractive and productive.

Nowhere is the gap between theory and experiment 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 35 years and vital for physics studies at the LHC and other acccelerators. 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. During the course of this project, the PYTHIA code has been significantly extended to be relevant in new contexts, and to provide more sophisticated predictions for existing ones.

The event generator activities are embedded in a broader phenomenology context, also comprising studies of low-energy processes (hadronic physics) and of scenaros for physics beyond the Standard Model, where several studies have been made. In addition to the PI, the members of the theoretical high energy physics group in Lund and of the PYTHIA collaboration 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, colour reconnection, and more.

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 relate to this area, including colour ropes, repulsion of overlapping strings (""shoving""), the space-time structure of hadronization, hadronic rescattering, and a new model for heavy-ion collisions.

The release of several new PYTHIA versions has been a most visible outcome, and especially the new 8.3 series brought many new improvements, both technical and physical. On the technical side, the program has been transitioned from the C++98 standard to the C++11 one. 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.

Partly related to event generators, there have also been improvements in calculational techniques for matrix elements, in subleading-colour corrections, and in the combination of parton showers and matrix elements.

In addition to the event-generator-related activities, many studies have also been related to physics Beyond the Standard Model (BSM), 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. Other studied BSM scenarios include several Grand Unification scenarios.

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 have been 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. Generally, several chiral perturbation theory results have been obtained to higher order than achieved previously.

In addition to the main themes listed above, some further smaller studies have also been performed on various topics.

Information is disseminated by journal articles and presentations at conferences and workshops. PYTHIA, in particular, is picked up by the experimental particle physics community and used by thousands of people across the globe."
Several new physics aspect have been added to the PYTHIA event generator, that add to the unique capabilities only available in it. That notably includes the heavy-ion framework, which brings together many individual developments described elsewhere. It is not the only model on the market, but offers a unique scenario.

There have also been the introduction of new calculational techniques for matrix elements, of new calculations related to the muon anomalous moment with special emphasis on integrating lattice QCD results into the overall picture, and much more. Since the project has contributed to the authoring of 110 articles so far, with a handful more to be expected in the next few months, it is not meaningful to cover all that has been done. A fuller - but still incomplete - description can be found in the Projects Achievements section.
Schematic illustration of the several components involved in the modelling of collisions in PYTHIA.