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Stewart, Iain W

Publications and source records attributed to Stewart, Iain W.

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Workshop on Precision Measurements of $\alpha_s$

These are the proceedings of the Workshop on Precision Measurements of {alpha}{sub s} held at the Max-Planck-Institute for Physics, Munich, February 9-11, 2011. The workshop explored in depth the determination of {alpha}{sub s}(m{sub Z}) in the {ovr MS} scheme from the key categories where high precision measurements are currently being made, including DIS and global PDF fits, {tau}-decays, electro-weak precision observables and Z-decays, event-shapes, and lattice QCD. These proceedings contain a short summary contribution from the speakers, as well as the lists of authors, conveners, participants, and talks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Fragmentation inside an identified jet

Using Soft-Collinear Effective Theory (SCET) we derive factorization formulae for semi-inclusive processes where a light hadron h fragments from a jet whose invariant mass is measured. Our analysis yields a novel 'fragmenting jet function' G{sub i}{sup h}(s,z) that depends on the jet invariant mass {radical}(s), and on the fraction z of the large light-cone momentum components of the hadron and the parent parton i. We show that G{sub i}{sup h}(s,z) can be computed in terms of perturbatively calculable coefficients, J{sub ij}(s,z/x), integrated against standard non-perturbative fragmentation functions, D{sub j}{sup h}(x). Our analysis yields a simple replacement rule that allows any factorization theorem depending on a jet function J{sub i} to be converted to a semi-inclusive process with a fragmenting hadron h.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Beam Thrust Cross Section for Drell-Yan Production at Next-to-Next-to-Leading-Logarithmic Order

At the LHC and Tevatron strong initial-state radiation (ISR) plays an important role. It can significantly affect the partonic luminosity available to the hard interaction or contaminate a signal with additional jets and soft radiation. An ideal process to study ISR is isolated Drell-Yan production, pp{yields}Xl{sup +}l{sup -} without central jets, where the jet veto is provided by the hadronic event shape beam thrust {tau}{sub B}. Most hadron collider event shapes are designed to study central jets. In contrast, requiring {tau}{sub B}<<1 provides an inclusive veto of central jets and measures the spectrum of ISR. For {tau}{sub B}<<1 we carry out a resummation of {alpha}{sub s}{sup n}ln{sup m{tau}}{sub B} corrections at next-to-next-to-leading-logarithmic order. This is the first resummation at this order for a hadron-hadron collider event shape. Measurements of {tau}{sub B} at the Tevatron and LHC can provide crucial tests of our understanding of ISR and of {tau}{sub B}'s utility as a central jet veto.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

N Jettiness: An Inclusive Event Shape to Veto Jets

Jet vetoes are essential in many analyses at the LHC and Tevatron. Typical signals have a specific number of hard jets or leptons, while backgrounds have additional jets. Vetoing undesired jets efficiently discriminates signal and background. For a sample with {>=}N jets, the veto to give N energetic jets defines an 'exclusive' N-jet cross section. This strongly restricts the phase space and causes large double logarithms in perturbation theory that must be summed. Jet vetoes are typically implemented using jet algorithms, yielding complicated phase-space restrictions, and reliance on leading-log parton-shower Monte Carlo simulations. We introduce a global event shape 'N jettiness' {tau}{sub N}, which is defined for events with N signal jets. Requiring {tau}{sub N}<<1 constrains radiation between the signal jets and provides a theoretically well-controlled jet veto. N jettiness yields a factorization formula with inclusive jet and beam functions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

R evolution: Improving perturbative QCD

Perturbative QCD results in the MS scheme can be dramatically improved by switching to a scheme that accounts for the dominant power law dependence on the factorization scale in the operator product expansion. We introduce the ''MSR scheme'' which achieves this in a Lorentz and gauge invariant way and has a very simple relation to MS. Results in MSR depend on a cutoff parameter R, in addition to the {mu} of MS. R variations can be used to independently estimate (i.) the size of power corrections, and (ii.) higher-order perturbative corrections (much like {mu} in MS). We give two examples at three-loop order, the ratio of mass splittings in the B*-B and D*-D systems, and the Ellis-Jaffe sum rule as a function of momentum transfer Q in deep inelastic scattering. Comparing to data, the perturbative MSR results work well even for Q{approx}1 GeV, and power corrections are reduced compared to MS.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Factorization at the LHC: From parton distribution functions to initial state jets

We study proton-(anti)proton collisions at the LHC or Tevatron in the presence of experimental restrictions on the hadronic final state and for generic parton momentum fractions. At the scale Q of the hard interaction, factorization does not yield standard parton distribution functions (PDFs) for the initial state. The measurement restricting the hadronic final state introduces a new scale {mu}{sub B}<<Q and probes the proton prior to the hard collision. This corresponds to evaluating the PDFs at the scale {mu}{sub B}. After the proton is probed, the incoming hard parton is contained in an initial-state jet, and the hard collision occurs between partons inside these jets rather than inside protons. The proper description of such initial-state jets requires ''beam functions''. At the scale {mu}{sub B}, the beam function factorizes into a convolution of calculable Wilson coefficients and PDFs. Below {mu}{sub B}, the initial-state evolution is described by the usual PDF evolution which changes x, while above {mu}{sub B} it is governed by a different renormalization group evolution that sums double logarithms of {mu}{sub B}/Q and leaves x fixed. As an example, we prove a factorization theorem for 'isolated Drell-Yan', pp{yields}Xl{sup +}l{sup -} where X is restricted to have no central jets. We comment on the extension to cases where the hadronic final state contains a certain number of isolated central jets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗