Balsiger, Marcel (2020). Resummation of nonglobal logarithms. (Thesis). Universität Bern, Bern

Text
20balsiger_m.pdf  Thesis Available under License Creative Commons: AttributionNoncommercialNo Derivative Works (CCBYNCND 4.0). Download (57MB)  Preview 
Abstract
Most of what we know about the laws of physics at the subnuclear length scale is derived from analyzing the outcomes of highenergy collisions of particles. Size and sophistication of such experiments have steadily increased, producing more accurate and precise measurements. To interpret these and to understand the underlying physics we need to develop and improve stateoftheart theoretical tools to predict collider observables in a more refined way. Such a tool is SoftCollinear Effective Theory, which is a theoretical framework for calculations that involve particles at widely separated energy scales. In a collider experiment, such hierarchies are very common. In the analysis of detector patterns after a collision event, one observes regions where a lot of energetic particles were measured close together, and a small amount of energy that is uniformly distributed in the entire detector. This namegiving collinear and soft radiation is captured by this specific effective theory. When calculating the cross sections of such multiscale problems, corrections that are normally suppressed by orders of the coupling constant are multiplied by a logarithm of the ratio of the scales. In the case of the interjet energy flow, for example, we have particles of high energy Ein ~ Q produced at the hard interaction inside the jets and soft particles of energy Eout ~ βQ outside the jets. If these scales are far separated, the logarithms ln(β) become big and can ruin the perturbative expansion in the coupling constants. One needs to resum these logarithmically enhanced terms to all orders. The socalled nonglobal logarithms that arise in the presence of hard phasespace cuts are especially difficult to resum. A simple exponentiation of the logarithmically enhanced terms is in this case not possible. Starting from a factorization theorem in SoftCollinear Effective Theory, we derive a parton shower equation for the resummation of nonglobal logarithms. We implement the shower in a dedicated, easytouse computer code NGL_RESUM to resum such logarithms at leading logarithmic accuracy in the largeNc limit. We use treelevel event files in the common Les Houches format as input and explicitly calculate observables such as the interjet energy flow and photon isolation cone cross sections at leadinglogarithmic accuracy. Since our parton shower is derived from first principles and based on renormalization group evolution, it is clear what ingredients we have to include to perform the resummation at subleading logarithmic accuracy. After adding the observabledependent nexttoleading order corrections for dijet processes, we are able to take a first step towards the higherlogarithmic resummation for the interjet energy flow and the jet mass. To reach full nexttoleading logarithmic accuracy, one would also have to include the twoloop anomalous dimension. This is indeed very challenging and has not been reached thus far. In a final step, we extend the framework to get away from the highenergy limit and include massive quarks. At this point, NGL_RESUM becomes a generalpurpose parton shower to resum nonglobal logarithms at leading logarithmic accuracy for a plethora of observables. We apply it to compute the interjet energy flow in tt̅ production.
Item Type:  Thesis 

Dissertation Type:  Cumulative 
Date of Defense:  24 September 2020 
Subjects:  500 Science > 530 Physics 
Institute / Center:  08 Faculty of Science > Institute of Theoretical Physics 
Depositing User:  Hammer Igor 
Date Deposited:  05 Nov 2020 10:42 
Last Modified:  24 Sep 2021 00:30 
URI:  https://boristheses.unibe.ch/id/eprint/2294 
Actions (login required)
View Item 