37 research outputs found
Variable Flavor Number Scheme for Final State Jets
We discuss a variable flavor number scheme (VFNS) for final state jets which
can account for the effects of arbitrary finite quark masses in inclusive jet
observables. The scheme is a generalization of the VFNS scheme for PDFs applied
to setups with additional dynamical scales and relies on appropriate
renormalization conditions for the matrix elements in the factorization
theorem. We illustrate general properties by means of the example of
deep-inelastic scattering (DIS) in the endpoint region and
event shapes in the dijet limit, in particular the calculations of threshold
corrections, consistency conditions and relations to mass singularities found
in fixed-order massive calculations.Comment: 7 pages, 4 figures, Proceedings of the XXII. International Workshop
on Deep-Inelastic Scattering and Related Subjects, 28 April - 2 May 2014,
Warsaw, Polan
Factorization and Resummation for Massive Quark Effects in Exclusive Drell-Yan
Exclusive differential spectra in color-singlet processes at hadron colliders
are benchmark observables that have been studied to high precision in theory
and experiment. We present an effective-theory framework utilizing
soft-collinear effective theory to incorporate massive (bottom) quark effects
into resummed differential distributions, accounting for both heavy-quark
initiated primary contributions to the hard scattering process as well as
secondary effects from gluons splitting into heavy-quark pairs. To be specific,
we focus on the Drell-Yan process and consider the vector-boson transverse
momentum, , and beam thrust, , as examples of exclusive
observables. The theoretical description depends on the hierarchy between the
hard, mass, and the (or ) scales, ranging from the decoupling
limit to the massless limit . The phenomenologically
relevant intermediate regime requires in particular quark-mass
dependent beam and soft functions. We calculate all ingredients for the
description of primary and secondary mass effects required at NNLL
resummation order (combining NNLL evolution with NNLO boundary conditions) for
and in all relevant hierarchies. For the distribution
the rapidity divergences are different from the massless case and we discuss
features of the resulting rapidity evolution. Our results will allow for a
detailed investigation of quark-mass effects in the ratio of and boson
spectra at small , which is important for the precision measurement of the
-boson mass at the LHC.Comment: 42 pages + appendices, 21 figures; v2: journal versio
Stochastic simulation algorithm for the quantum linear Boltzmann equation
We develop a Monte Carlo wave function algorithm for the quantum linear
Boltzmann equation, a Markovian master equation describing the quantum motion
of a test particle interacting with the particles of an environmental
background gas. The algorithm leads to a numerically efficient stochastic
simulation procedure for the most general form of this integro-differential
equation, which involves a five-dimensional integral over microscopically
defined scattering amplitudes that account for the gas interactions in a
non-perturbative fashion. The simulation technique is used to assess various
limiting forms of the quantum linear Boltzmann equation, such as the limits of
pure collisional decoherence and quantum Brownian motion, the Born
approximation and the classical limit. Moreover, we extend the method to allow
for the simulation of the dissipative and decohering dynamics of superpositions
of spatially localized wave packets, which enables the study of many physically
relevant quantum phenomena, occurring e.g. in the interferometry of massive
particles.Comment: 21 pages, 9 figures; v2: corresponds to published versio
Hard Matching for Boosted Tops at Two Loops
Cross sections for top quarks provide very interesting physics opportunities,
being both sensitive to new physics and also perturbatively tractable due to
the large top quark mass. Rigorous factorization theorems for top cross
sections can be derived in several kinematic scenarios, including the boosted
regime in the peak region that we consider here. In the context of the
corresponding factorization theorem for collisions we extract the last
missing ingredient that is needed to evaluate the cross section differential in
the jet-mass at two-loop order, namely the matching coefficient at the scale
. Our extraction also yields the final ingredients needed to
carry out logarithmic resummation at next-to-next-to-leading logarithmic order
(or NLL if we ignore the missing 4-loop cusp anomalous dimension). This
coefficient exhibits an amplitude level rapidity logarithm starting at
due to virtual top quark loops, which we treat using
rapidity renormalization group (RG) evolution. Interestingly, this rapidity RG
evolution appears in the matching coefficient between two effective theories
around the heavy quark mass scale .Comment: 35 pages, 3 figures, v2: added extraction of 3-loop anon. dimension,
journal versio
Secondary Production of Massive Quarks in Thrust
We present a factorization framework that takes into account the production
of heavy quarks through gluon splitting in the thrust distribution for e+ e-
--> hadrons. The explicit factorization theorems and some numerical results are
displayed in the dijet region where the kinematic scales are widely separated,
which can be extended systematically to the whole spectrum. We account for the
necessary two-loop matrix elements, threshold corrections, and include
resummation up to N3LL order. We include nonperturbative power corrections
through a field theoretical shape function, and remove the O(Lambda_QCD)
renormalon in the partonic soft function by appropriate mass-dependent
subtractions. Our results hold for any value of the quark mass, from an
infinitesimally small (merging to the known massless result) to an infinitely
large one (achieving the decoupling limit). This is the first example of an
application of a variable flavor number scheme to final state jets.Comment: 6 pages, 1 figure. Presented at the XIth International Conference on
Quark Confinement and the Hadron Spectrum, Saint Petersburg, Russia,
September 8-12, 201