Global Fits for Parton Distributions and Implications for Hadron Collider Physics

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

In order to reach the highest possible energies, the current particle colliders, the DESY electron-positron collider at Hamburg and the Tevatron proton-antiproton collider near Chicago, have protons as at least one of their colliding particles. Protons are particles which interact via the strong force, and are composite particles because the strong force binds the fundamental constituents, partons - which may be quarks or gluons, into these composite particles. The collider currently under construction at CERN, the Large Hadron Collider (LHC) will be a proton-proton collider. The LHC will be the largest energy particle collider ever created by a factor of more than 7, and will enable us to see if the missing particle within the Standard Model of particle physics, the Higgs boson exists - as well as to detect the first signs of physics beyond the Standard Model, for example, Supersymmetry, where each Standard Model particle has a supersymmetric partner. At low energies we think of the partons being bound within the proton, but at very high energies the strong coupling becomes weaker and the interactions of colliding protons can be thought of as interactions between the partons in each proton, which may be calculated as an expansion in the strong coupling constant. Hence, in order to understand the results of any hadron collider experiments, one must first understand how the proton is made up out of its partonic constituents. To a certain degree this can be calculated, but strong coupling makes some expansions badly defined, and some of the information can be determined from experiment. Therefore one must perform enough independent experiments, and use the theoretical calculations within the theory of the strong force (Quantum Chromodynamics - QCD) as accurately as possible, to determine the composition of the proton in terms of the gluons and the six flavours of quarks (up, down, strange, charm, bottom and top). This requires use of data from a variety of experiments, and it must be checked that all pieces of data are consistent with the partons and QCD theory, hence testing QCD to great accuracy, and measuring the strong coupling. Once consistency is determined these partons may be used to predict any other process using the protons. This can be the production of beyomd the Standard Model particles, or for Standard Model processes, where the latter often mask the former. The project proposed is to improve the determination of parton distributions and their consequences for collider physics, by incorporating new theoretical calculations, e.g. higher order in the coupling, more precise inclusion of heavy particle corrections, and by the inclusion of new processes, both from the existing colliders and from the forthcoming LHC. These new theoretical developments and the new influx of data will mean we require a lot of work to obtain the best partons. However, this is essential if the data are to be interpreted properly, and hence, if we are to increase our understanding of the Standard Model, and very importantly, try to search for the physics beyond it.

Publications

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Martin A (2007) Update of parton distributions at NNLO in Physics Letters B

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Martin A (2009) Parton distributions for the LHC in The European Physical Journal C

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Sherstnev A (2008) Parton distributions for LO generators in The European Physical Journal C

 
Description This grant resulted in a new set of parton distributions, which describe the manner in which the proton is made up of constituent quarks and gluons, generically known as partons. This is necessary to understand how the partons interact to form other particles when protons are collided at high energy, e.g. at the LHC (Large Hadron Collider) in CERN. Work mainly from this grant resulted in a new set of parton distributions which increased our knowledge of the proton and enabled much of the study at the LHC to proceed optimally.
Exploitation Route The determination of parton distributions is always improving, so we, and others have developed the work in this grant to take things further and improve our understanding of partons. The parton distributions themselves, called MSTW2008, have been used extensively at the LHC and other experiments, and by the theory community, and with the exception of the papers for the recent discovery of the Higgs boson, the MSTW2008 article is the most cited paper in particle physics in the past five years.
Sectors Education

URL https://mstwpdf.hepforge.org/
 
Description The determination of parton distributions is always improving, so we, and others have developed the work in this grant to take things further and improve our understanding of partons. The parton distributions themselves, first the 2006 PDF sets and then an improvement, called MSTW2008, have been used extensively at the LHC and other experiments, and by the theory community, and with the exception of the papers for the recent discovery of the Higgs boson, the MSTW2008 article is the most cited paper in particle physics in the past five years.
First Year Of Impact 2007
Sector Education
Impact Types Societal

 
Description MSTW 
Organisation Durham University
Department Department of Physics
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution The main coordinater and one of the most active working participants in a four-person collaboration.
Collaborator Contribution Part of a four person collaboration on a project.
Impact Publications of MSTW parton distributions which are a default in analyses at the LHC and Teavtron particle colliders.
Start Year 2006
 
Description MSTW 
Organisation European Organization for Nuclear Research (CERN)
Department Theoretical Physics Unit
Country Switzerland, Swiss Confederation 
Sector Academic/University 
PI Contribution The main coordinater and one of the most active working participants in a four-person collaboration.
Collaborator Contribution Part of a four person collaboration on a project.
Impact Publications of MSTW parton distributions which are a default in analyses at the LHC and Teavtron particle colliders.
Start Year 2006
 
Description MSTW 
Organisation University of Cambridge
Department Department of Physics
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution The main coordinater and one of the most active working participants in a four-person collaboration.
Collaborator Contribution Part of a four person collaboration on a project.
Impact Publications of MSTW parton distributions which are a default in analyses at the LHC and Teavtron particle colliders.
Start Year 2006