1,115 research outputs found
SCET approach to top quark decay
In this work we study QCD corrections to the top quark doubly decay rate with
a detected hadron containing a quark. We focus on the regime among
which the emitted boson nearly carries its maxim energy. The tool that we
use here is the soft-collinear effective theory (SCET). The factorization
theorem based on SCET indicates a novel fragmenting jet function. We calculate
this function to next-to-leading order in . Large logarithms due to
several well separated scale are summed up using the renormalization group
equation (RGE). Finally we reach an analytic formula for the distribution which
could easily be generalized to other heavy hadron decay.Comment: 13 pages, 4 figure
Consistent Factorization of Jet Observables in Exclusive Multijet Cross-Sections
We demonstrate the consistency at the next-to-leading-logarithmic (NLL) level
of a factorization theorem based on Soft-Collinear Effective Theory (SCET) for
jet shapes in e+e- collisions. We consider measuring jet observables in
exclusive multijet final states defined with cone and k_T-type jet algorithms.
Consistency of the factorization theorem requires that the renormalization
group evolution of hard, jet, and soft functions is such that the physical
cross-section is independent of the factorization scale mu. The anomalous
dimensions of the various factorized pieces, however, depend on the color
representation of jets, choice of jet observable, the number of jets whose
shapes are measured, and the jet algorithm, making it highly nontrivial to
satisfy the consistency condition. We demonstrate the intricate cancellations
between anomalous dimensions that occur at the NLL level, so that, up to power
corrections that we identify, our factorization of the jet shape distributions
is consistent for any number of quark and gluon jets, for any number of jets
whose shapes are measured or unmeasured, for any angular size R of the jets,
and for any of the algorithms we consider. Corrections to these results are
suppressed by the SCET expansion parameter lambda (the ratio of soft to
collinear or collinear to hard scales) and in the jet separation measure 1/t^2
= tan^2(R/2)/tan^2(psi/2), where psi is the angular separation between jets.
Our results can be used to calculate a wide variety of jet observables in
multijet final states to NLL accuracy.Comment: 8 pages, 1 figure, uses elsarticle.cls; v2: minor edits, added
reference
Designing Gapped Soft Functions for Jet Production
Distributions in jet production often depend on a soft function, S, which
describes hadronic radiation between the jets. Near kinematic thresholds S
encodes nonperturbative information, while far from thresholds S can be
computed with an operator product expansion (OPE). We design soft functions for
jets that serve this dual purpose, reducing to the perturbative result in the
OPE region and to a consistent model in the nonperturbative region. We use the
MSbar scheme, and in both regions S displays the appropriate renormalization
group scale dependence. We point out that viable soft function models should
have a gap associated with the minimum hadronic energy deposit. This gap is
connected to the leading O(Lambda_QCD) renormalon ambiguity in jet event
shapes. By defining the gap in a suitable scheme we demonstrate that the
leading renormalon can be eliminated. This improves the convergence of
perturbative results, and also the stability by which non-perturbative
parameters encode the underlying soft physics.Comment: 17 pages, 5 figure
Factorization of e+e- Event Shape Distributions with Hadronic Final States in Soft Collinear Effective Theory
We present a new analysis of two-jet event shape distributions in soft
collinear effective theory. Extending previous results, we observe that a large
class of such distributions can be expressed in terms of vacuum matrix elements
of operators in the effective theory. We match these matrix elements to the
full theory in the two-jet limit without assuming factorization of the complete
set of hadronic final states into independent sums over partonic collinear and
soft states. We also briefly discuss the relationship of this approach to
diagrammatic factorization in the full theory.Comment: 21 pages. Journal version. Defined an explicit thrust axis operator;
clarified meaning of a delta function operato
The Resummed Photon Spectrum in Radiative Upsilon Decays
We present a theoretical prediction for the photon spectrum in radiative
Upsilon decay including the effects of resumming the endpoint region, E_\gamma
-> M_\Upsilon/2. Our approach is based on NRQCD and the soft collinear
effective theory. We find that our results give much better agreement with data
than the leading order NRQCD prediction.Comment: 4 pages, 6 figure
Leading power SCET analysis of
Recently, Belle and BaBar Collaborations observed surprising suppression in
the endpoint spectrum, which stimulates us to examine the endpoint
behaviors of the production. We calculate the
momentum and angular distributions for this process within the framework of the
soft-collinear effective theory (SCET). The decreasing spectrum in the endpoint
region is obtained by summing the Sudakov logarithms. We also find a large
discrepancy between the NRQCD and SCET spectrum in the endpoint region even
before the large logarithms are summed, which is probably due to the fact that
only the scalar structure of the two-gluon system is picked out in the leading
power expansion. A comparison with the process is
made.Comment: 12 pages, 3 figures, one reference added and some minor changes,
version to appear in Phys. Lett.
Medium-induced parton splitting kernels from Soft Collinear Effective Theory with Glauber gluons
We derive the splitting kernels for partons produced in large
scattering processes that subsequently traverse a region of
strongly-interacting matter using a recently-developed effective theory \SCETG.
We include all corrections beyond the small- approximation, consistent with
the power counting of \SCETG. We demonstrate how medium recoil, geometry and
expansion scenarios, and phase space cuts can be implemented numerically for
phenomenological applications. For the simplified case of infinite transverse
momentum kinematics and a uniform medium, we provide closed-form analytic
results that can be used to validate the numerical simulations.Comment: 9 pages, 3 figure
Parton Fragmentation within an Identified Jet at NNLL
The fragmentation of a light parton i to a jet containing a light energetic
hadron h, where the momentum fraction of this hadron as well as the invariant
mass of the jet is measured, is described by "fragmenting jet functions". We
calculate the one-loop matching coefficients J_{ij} that relate the fragmenting
jet functions G_i^h to the standard, unpolarized fragmentation functions D_j^h
for quark and gluon jets. We perform this calculation using various IR
regulators and show explicitly how the IR divergences cancel in the matching.
We derive the relationship between the coefficients J_{ij} and the quark and
gluon jet functions. This provides a cross-check of our results. As an
application we study the process e+ e- to X pi+ on the Upsilon(4S) resonance
where we measure the momentum fraction of the pi+ and restrict to the dijet
limit by imposing a cut on thrust T. In our analysis we sum the logarithms of
tau=1-T in the cross section to next-to-next-to-leading-logarithmic accuracy
(NNLL). We find that including contributions up to NNLL (or NLO) can have a
large impact on extracting fragmentation functions from e+ e- to dijet + h.Comment: expanded introduction, typos fixed, journal versio
On Glauber modes in Soft-Collinear Effective Theory
Gluon interactions involving spectator partons in collisions at hadronic
machines are investigated. We find a class of examples in which a mode, called
Glauber gluons, must be introduced to the effective theory for consistency.Comment: 19 pages, three figures. Uses JHEP3.cl
Infrared Safety in Factorized Hard Scattering Cross-Sections
The rules of soft-collinear effective theory can be used naively to write
hard scattering cross-sections as convolutions of separate hard, jet, and soft
functions. One condition required to guarantee the validity of such a
factorization is the infrared safety of these functions in perturbation theory.
Using e+e- angularity distributions as an example, we propose and illustrate an
intuitive method to test this infrared safety at one loop. We look for regions
of integration in the sum of Feynman diagrams contributing to the jet and soft
functions where the integrals become infrared divergent. Our analysis is
independent of an explicit infrared regulator, clarifies how to distinguish
infrared and ultraviolet singularities in pure dimensional regularization, and
demonstrates the necessity of taking zero-bins into account to obtain
infrared-safe jet functions.Comment: 6 pages, 7 figures, uses elsarticle.cls. v2: extended introduction
and clarified discussion of ingredients necessary for proving factorizatio
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