TOPICAL REVIEW: Event shapes in e+e- annihilation and deep inelastic scattering

Journal of Physics G Nuclear and Particle Physics (Impact Factor: 2.84). 01/2004; 30(5). DOI: 10.1088/0954-3899/30/5/R01
Source: arXiv

ABSTRACT This review examines the status of event-shape studies in e+e- annihilation and DIS. It includes discussions of perturbative calculations, of various approaches to modelling hadronization and of comparisons to data.

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    ABSTRACT: We present an analytic $\mathcal O(\alpha_s)$ calculation of cross sections in deep inelastic scattering (DIS) dependent on an event shape, 1-jettiness, that probes final states with one jet plus initial state radiation. This is the first entirely analytic calculation for a DIS event shape cross section at this order. We present results for the differential and cumulative 1-jettiness cross sections, and express both in terms of structure functions dependent not only on the usual DIS variables $x$, $Q^2$ but also on the 1-jettiness $\tau$. Combined with previous results for log resummation, predictions are obtained over the entire range of the 1-jettiness distribution.
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    ABSTRACT: We compare methods to resum logarithms in event shape distributions as they have been used in perturbative QCD directly and in effective field theory. We demonstrate that they are equivalent. In showing this equivalence, we are able to put standard soft-collinear effective theory (SCET) formulae for cross sections in momentum space in a form that resums a larger number of terms than normally written, and endow direct QCD formulae with dependence on separated hard, jet, and soft scales, providing potential ways to improve estimates of theoretical uncertainty. We show how to compute cross sections in momentum space to keep them as accurate as the corresponding expressions in Laplace space. In particular, we point out how certain existing formulas for resummed differential distributions in momentum space are susceptible to being evaluated in such a way that their accuracy does not match the corresponding accuracy of the integrated distribution (cumulant) or of the Laplace transform computed using the same methods. We provide an improved formula for the differential distribution that avoids this complication.
    Journal of High Energy Physics 01/2014; 2014(4). · 6.22 Impact Factor
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    ABSTRACT: In collider physics, jet algorithms are a ubiquitous tool for clustering particles into discrete jet objects. Event shapes offer an alternative way to characterize jets, and one can define a jet multiplicity event shape, which can take on fractional values, using the framework of "jets without jets". In this paper, we perform the first analytic studies of fractional jet multiplicity $\tilde{N}_{\rm jet}$ in the context of $e^+e^-$ collisions. We use fixed-order QCD to understand the $\tilde{N}_{\rm jet}$ cross section at order $\alpha_s^2$, and we introduce a candidate factorization theorem to capture certain higher-order effects. The resulting distributions have a hybrid jet algorithm/event shape behavior which agrees with parton shower Monte Carlo generators. The $\tilde{N}_{\rm jet}$ observable does not satisfy ordinary soft-collinear factorization, and the $\tilde{N}_{\rm jet}$ cross section exhibits a number of unique features, including the absence of collinear logarithms and the presence of soft logarithms that are purely non-global. Additionally, we find novel divergences connected to the energy sharing between emissions, which are reminiscent of rapidity divergences encountered in other applications. Given these interesting properties of fractional jet multiplicity, we advocate for future measurements and calculations of $\tilde{N}_{\rm jet}$ at hadron colliders like the LHC.

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