442 research outputs found
Fully-Unintegrated Parton Distribution and Fragmentation Functions at Perturbative k_T
We define and study the properties of generalized beam functions (BFs) and
fragmenting jet functions (FJFs), which are fully-unintegrated parton
distribution functions (PDFs) and fragmentation functions (FFs) for
perturbative k_T. We calculate at one loop the coefficients for matching them
onto standard PDFs and FFs, correcting previous results for the BFs in the
literature. Technical subtleties when measuring transverse momentum in
dimensional regularization are clarified, and this enables us to renormalize in
momentum space. Generalized BFs describe the distribution in the full
four-momentum k_mu of a colliding parton taken out of an initial-state hadron,
and therefore characterize the collinear initial-state radiation. We illustrate
their importance through a factorization theorem for pp -> l^+ l^- + 0 jets,
where the transverse momentum of the lepton pair is measured. Generalized FJFs
are relevant for the analysis of semi-inclusive processes where the full
momentum of a hadron, fragmenting from a jet with constrained invariant mass,
is measured. Their significance is shown for the example of e^+ e^- -> dijet+h,
where the perpendicular momentum of the fragmenting hadron with respect to the
thrust axis is measured.Comment: Journal versio
An effective theory for jet propagation in dense QCD matter: jet broadening and medium-induced bremsstrahlung
Two effects, jet broadening and gluon bremsstrahlung induced by the
propagation of a highly energetic quark in dense QCD matter, are reconsidered
from effective theory point of view. We modify the standard Soft Collinear
Effective Theory (SCET) Lagrangian to include Glauber modes, which are needed
to implement the interactions between the medium and the collinear fields. We
derive the Feynman rules for this Lagrangian and show that it is invariant
under soft and collinear gauge transformations. We find that the newly
constructed theory SCET recovers exactly the general result for the
transverse momentum broadening of jets. In the limit where the radiated gluons
are significantly less energetic than the parent quark, we obtain a jet
energy-loss kernel identical to the one discussed in the reaction operator
approach to parton propagation in matter. In the framework of SCET we
present results for the fully-differential bremsstrahlung spectrum for both the
incoherent and the Landau-Pomeranchunk-Migdal suppressed regimes beyond the
soft-gluon approximation. Gauge invariance of the physics results is
demonstrated explicitly by performing the calculations in both the light-cone
and covariant gauges. We also show how the process-dependent
medium-induced radiative corrections factorize from the jet production cross
section on the example of the quark jets considered here.Comment: 52 pages, 15 pdf figures, as published in JHE
Electroweak Gauge-Boson Production at Small q_T: Infrared Safety from the Collinear Anomaly
Using methods from effective field theory, we develop a novel, systematic
framework for the calculation of the cross sections for electroweak gauge-boson
production at small and very small transverse momentum q_T, in which large
logarithms of the scale ratio M_V/q_T are resummed to all orders. These cross
sections receive logarithmically enhanced corrections from two sources: the
running of the hard matching coefficient and the collinear factorization
anomaly. The anomaly leads to the dynamical generation of a non-perturbative
scale q_* ~ M_V e^{-const/\alpha_s(M_V)}, which protects the processes from
receiving large long-distance hadronic contributions. Expanding the cross
sections in either \alpha_s or q_T generates strongly divergent series, which
must be resummed. As a by-product, we obtain an explicit non-perturbative
expression for the intercept of the cross sections at q_T=0, including the
normalization and first-order \alpha_s(q_*) correction. We perform a detailed
numerical comparison of our predictions with the available data on the
transverse-momentum distribution in Z-boson production at the Tevatron and LHC.Comment: 34 pages, 9 figure
Factorization and NNLL Resummation for Higgs Production with a Jet Veto
Using methods of effective field theory, we derive the first all-order
factorization theorem for the Higgs-boson production cross section with a jet
veto, imposed by means of a standard sequential recombination jet algorithm.
Like in the case of small-q_T resummation in Drell-Yan and Higgs production,
the factorization is affected by a collinear anomaly. Our analysis provides the
basis for a systematic resummation of large logarithms log(m_H/p_T^veto) beyond
leading-logarithmic order. Specifically, we present predictions for the
resummed jet-veto cross section and efficiency at next-to-next-to-leading
logarithmic order. Our results have important implications for Higgs-boson
searches at the LHC, where a jet veto is required to suppress background
events.Comment: 28 pages, 5 figures; v2: published version; note added in proo
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
On the breaking of collinear factorization in QCD
We investigate the breakdown of collinear factorization for non-inclusive
observables in hadron-hadron collisions. For pure QCD processes, factorization
is violated at the three-loop level and it has a structure identical to that
encountered previously in the case of super-leading logarithms. In particular,
it is driven by the non-commutation of Coulomb/Glauber gluon exchanges with
other soft exchanges. Beyond QCD, factorization may be violated at the two-loop
level provided that the hard subprocess contains matrix element contributions
with phase differences between different colour topologies.Comment: Version 2: minor improvements for journal publicatio
Factorization Properties of Soft Graviton Amplitudes
We apply recently developed path integral resummation methods to perturbative
quantum gravity. In particular, we provide supporting evidence that eikonal
graviton amplitudes factorize into hard and soft parts, and confirm a recent
hypothesis that soft gravitons are modelled by vacuum expectation values of
products of certain Wilson line operators, which differ for massless and
massive particles. We also investigate terms which break this factorization,
and find that they are subleading with respect to the eikonal amplitude. The
results may help in understanding the connections between gravity and gauge
theories in more detail, as well as in studying gravitational radiation beyond
the eikonal approximation.Comment: 35 pages, 5 figure
The Quark Beam Function at NNLL
In hard collisions at a hadron collider the most appropriate description of
the initial state depends on what is measured in the final state. Parton
distribution functions (PDFs) evolved to the hard collision scale Q are
appropriate for inclusive observables, but not for measurements with a specific
number of hard jets, leptons, and photons. Here the incoming protons are probed
and lose their identity to an incoming jet at a scale \mu_B << Q, and the
initial state is described by universal beam functions. We discuss the
field-theoretic treatment of beam functions, and show that the beam function
has the same RG evolution as the jet function to all orders in perturbation
theory. In contrast to PDF evolution, the beam function evolution does not mix
quarks and gluons and changes the virtuality of the colliding parton at fixed
momentum fraction. At \mu_B, the incoming jet can be described perturbatively,
and we give a detailed derivation of the one-loop matching of the quark beam
function onto quark and gluon PDFs. We compute the associated NLO Wilson
coefficients and explicitly verify the cancellation of IR singularities. As an
application, we give an expression for the next-to-next-to-leading logarithmic
order (NNLL) resummed Drell-Yan beam thrust cross section.Comment: 54 pages, 9 figures; v2: notation simplified in a few places, typos
fixed; v3: journal versio
A general method for the resummation of event-shape distributions in e⁺ e− annihilation
We present a novel method for resummation of event shapes to next-to-next-to-leading-logarithmic (NNLL) accuracy. We discuss the technique and describe its implementation in a numerical program in the case of e + e − collisions where the resummed prediction is matched to NNLO. We reproduce all the existing predictions and present new results for oblateness and thrust major
Eikonal methods applied to gravitational scattering amplitudes
We apply factorization and eikonal methods from gauge theories to scattering
amplitudes in gravity. We hypothesize that these amplitudes factor into an
IR-divergent soft function and an IR-finite hard function, with the former
given by the expectation value of a product of gravitational Wilson line
operators. Using this approach, we show that the IR-divergent part of the
n-graviton scattering amplitude is given by the exponential of the one-loop IR
divergence, as originally discovered by Weinberg, with no additional subleading
IR-divergent contributions in dimensional regularization.Comment: 16 pages, 3 figures; v2: title change and minor rewording (published
version); v3: typos corrected in eqs.(3.2),(4.1
- …