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Precision jet physics from effective field theory

English title Precision jet physics from effective field theory
Applicant Becher Thomas
Number 182038
Funding scheme Project funding
Research institution Institut für Theoretische Physik Universität Bern
Institution of higher education University of Berne - BE
Main discipline Theoretical Physics
Start/End 01.10.2018 - 30.09.2022
Approved amount 699'693.00
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Keywords (8)

hadronic collisions; quantum chromodynamics; resummations of perturbation theory; effective field theory; soft-collinear effective theory; particle physics; LHC; jet physics

Lay Summary (German)

Lead
In einer Teilchenkollision am Large Hadron Collider (LHC) am CERN werden typischerweise hunderte von Teilchen produziert und detektiert. Um experimentelle Resultate zu interpretieren und darin nach Spuren neuer Physik zu suchen, braucht es genaue theoretische Vorhersagen für solche Prozesse. Eine Charakteristik von Kollisionen bei sehr hohen Energien ist die Produktion von Teilchenjets, in welchen eine grosse Anzahl von Teilchen in ähnliche Richtung fliegt. Die Anzahl solcher Jets und ihre Richtungen können mittels Störungsrechnung systematisch berechnet werden. Ein weiteres wichtiges Werkzeug zur Beschreibung der Kollisionen sind Parton-Shower, welche die Verteilung einzelner Teilchen im Jet simulieren. Anders als bei der Störungsrechnung, ist es aber bisher nicht klar, wie deren Genauigkeit systematisch verbessert werden kann.
Lay summary
Das vorliegende Projekt entwickelt neue Methoden, um genauere Vorhersagen für Jet Observablen zu erhalten, bei denen verschiedene physikalische Skalen eine Rolle spielen. In solchen Fällen sind Korrekturen durch Logarithmen des Verhältnisses der verschiedenen Skalen erhöht. Diese sogenannten Sudakov-Logarithmen können zu einem Zusammenbruch der Störungsrechnung führen. Um Vorhersagen auch für solche Fälle zu erhalten, wird im Rahmen des Projekts eine effektive Quantenfeldtheorie benutzt, um die Physik bei den verschiedenen Skalen zu trennen. Im Rahmen dieser Soft-Kollinearen Effektiven Theorie (SCET) kann man Sudakov-Logarithmen mittels Renormierungsgruppen-Methoden summieren. 

Interessanterweise haben die Gleichungen zur Resummation der Sudakov-Logarithmen in Jet-Prozessen die gleiche Form wie diejenigen, die Parton-Shower Programmen zugrunde liegen. Unsere Theorie erlaubt es aber, wie bei der Störungsrechnung die Genauigkeit systematisch zu erhöhen. Im Rahmen unseres Projekts werden wird die nötigen Bestandteile herleiten und damit zum ersten Mal höhere Logarithmen in Jet Observablen resummieren. Diese Arbeit sollte auch Hinweise liefern, wie man Parton-Shower erweitern muss, um diese systematisch zu verbessern. 
Direct link to Lay Summary Last update: 01.10.2018

Responsible applicant and co-applicants

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Associated projects

Number Title Start Funding scheme
165786 Precision jet physics from effective field theory 01.04.2016 Project funding

Abstract

During the year 2017 the Large Hadron Collider (LHC) delivered a record setting 50 fb^-1 of collision data to the ATLAS and CMS experiments and is scheduled to produce an even larger data set in 2018 to complete run 2 of the collider at a center-of-mass energy of 13 TeV. The LHC has performed numerous new measurements and has reached a precision below the per cent level for selected observables. To translate such measurements into physics results, they need to be matched by equally precise theoretical predictions. Such precision predictions and tests of the standard model are especially important since so far no new physics signals have been observed. Over the last years, there has been tremendous progress in fixed-order perturbative computations of collider observables. These computations work very well for inclusive observables, but the higher-order terms are enhanced by large logarithms for more exclusive quantities. An important tool to obtain more exclusive predictions are Monte Carlo parton showers. Modern parton showers include fixed-order predictions through matching and add soft and collinear emissions to capture the large logarithms. While there has been a lot of progress, in particular concerning the matching to fixed order, the parametric accuracy of these codes is the same as it was in the 1980s when they were introduced. It is now widely recognised that it will be important to systematically improve their logarithmic accuracy. For simple observables there are analytic methods such as Soft-Collinear Effective Theory (SCET) to perform resummations to high accuracy, but in the past it was not clear how to connect these results to shower codes.In a series of recent papers, we have derived factorization theorems, which enable higher-logarithmic resummation for a much broader class of cross sections (for non-global observables, which include e.g. jet cross sections). We furthermore find that the equations driving the resummation take the form of a parton shower. We are thus in the unique situation that we have a parton-shower framework with a clear field-theoretical definition of all ingredients, in which the resummation accuracy can be systematically increased. We recently implemented the leading-logarithmic parton shower and the main goal of the project is to compute all the necessary ingredients to increase the precision and to implement them into a shower code. This will result in the first higher-logarithmic resummation for non-global observables and the first systematically improved parton shower. In addition to going to higher-logarithmic accuracy, we also aim to go beyond the large-N_c limit inherent in current parton showers. While our code will be restricted to a specific set of observables, our project should also give important insight how more accurate general-purpose showers can be obtained.
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