213 research outputs found
A simple shower and matching algorithm
We present a simple formalism for parton-shower Markov chains. As a first
step towards more complete uncertainty bands, we incorporate a comprehensive
exploration of the ambiguities inherent in such calculations. To reduce this
uncertainty, we then introduce a matching formalism which allows a generated
event sample to simultaneously reproduce any infrared safe distribution
calculated at leading or next-to-leading order in perturbation theory, up to
sub-leading corrections. To enable a more universal definition of perturbative
calculations, we also propose a more general definition of the hadronization
cutoff. Finally, we present an implementation of some of these ideas for
final-state gluon showers, in a code dubbed VINCIA.Comment: 32 pages, 6 figure
Bloch-Nordsieck violating electroweak corrections to inclusive TeV scale hard processes
We point out that, since the colliders initial states (e+ e-,p p, p pbar, ...
) carry a definite nonabelian flavor, electroweak radiative corrections to
inclusive hard cross sections at the TeV scale are affected by peculiar
Bloch-Nordsieck violating double logs. We recall the setup of soft cancellation
theorems, and we analyze the magnitude of the noncancelling terms in the
example of electron - positron annihilation into hadrons.Comment: Minor typos corrected, references added. Final version to appear on
Phys. Rev. Let
The role of universal and non universal Sudakov logarithms in four fermion processes at TeV energies: the one-loop approximation revisited
We consider the separate effects on four fermion processes, in the TeV energy
range, produced at one loop by Sudakov logarithms of universal and not
universal kind, working in the 't Hooft xi=1 gauge. Summing the various vertex
and box contributions allows to isolate two quite different terms.The first one
is a combination of vertex and box quadratic and linear logarithms that are
partially universal and partially not universal and independent of the
scattering angle theta. The second one is theta-dependent, not universal,
linearly logarithmic and only produced by weak boxes. We show that for several
observables, measurable at future linear e+e- colliders (LC, CLIC), the role of
the latter term is dominant and we discuss the implications of this fact for
what concerns the reliability of a one-loop approximation.Comment: 22 pages and 13 figures; version to appear in Phys.Rev.D. e-mail:
[email protected]
Top Quark Production at TeV Energies as a Potential SUSY Detector
We consider the process of top-antitop production from electron-positron
annihilation, for c. m. energies in the few TeV regime, in the MSSM theoretical
framework. We show that, at the one loop level, the \underline{slopes} of a
number of observable quantities in an energy region around 3 TeV are only
dependent on . Under optimal experimental conditions, a combined
measurement of slopes might identify values in a range , with acceptable precision.Comment: 14 pages, 6 Encapsulated PostScript figure
Top quark production at future lepton colliders in the asymptotic regime
The production of a tt(bar) pair from lepton-antilepton annihilation is
considered for values of the center of mass energy much larger than the top
mass, typically of the few TeV size. In this regime a number of simplifications
occurs that allows to derive the leading asymptotic terms of various
observables using the same theoretical description that was used for light
quark production. Explicit examples are shown for the Standard Model and the
Minimal Supersymmetric Standard Model cases.Comment: 20 pages and 13 figures. e-mail: [email protected]
Maximum union-free subfamilies
An old problem of Moser asks: how large of a union-free subfamily does every
family of m sets have? A family of sets is called union-free if there are no
three distinct sets in the family such that the union of two of the sets is
equal to the third set. We show that every family of m sets contains a
union-free subfamily of size at least \lfloor \sqrt{4m+1}\rfloor - 1 and that
this bound is tight. This solves Moser's problem and proves a conjecture of
Erd\H{o}s and Shelah from 1972. More generally, a family of sets is
a-union-free if there are no a+1 distinct sets in the family such that one of
them is equal to the union of a others. We determine up to an absolute
multiplicative constant factor the size of the largest guaranteed a-union-free
subfamily of a family of m sets. Our result verifies in a strong form a
conjecture of Barat, F\"{u}redi, Kantor, Kim and Patkos.Comment: 10 page
Logarithmic SUSY electroweak effects on four-fermion processes at TeV energies
We compute the MSSM one-loop contributions to the asymptotic energy behaviour
of fermion-antifermion pair production at future lepton-antilepton colliders.
Besides the conventional logarithms of Renormalization Group origin, extra SUSY
linear logarithmic terms appear of "Sudakov-type". In the TeV range their
overall effect on a variety of observables can be quite relevant and
drastically different from that obtained in the SM case.Comment: 19 pages and 14 figures, corrected version. e-mail:
[email protected]
The early evolution of the H-free process
The H-free process, for some fixed graph H, is the random graph process
defined by starting with an empty graph on n vertices and then adding edges one
at a time, chosen uniformly at random subject to the constraint that no H
subgraph is formed. Let G be the random maximal H-free graph obtained at the
end of the process. When H is strictly 2-balanced, we show that for some c>0,
with high probability as , the minimum degree in G is at least
. This gives new lower bounds for
the Tur\'an numbers of certain bipartite graphs, such as the complete bipartite
graphs with . When H is a complete graph with we show that for some C>0, with high probability the independence number of
G is at most . This gives new lower bounds
for Ramsey numbers R(s,t) for fixed and t large. We also obtain new
bounds for the independence number of G for other graphs H, including the case
when H is a cycle. Our proofs use the differential equations method for random
graph processes to analyse the evolution of the process, and give further
information about the structure of the graphs obtained, including asymptotic
formulae for a broad class of subgraph extension variables.Comment: 36 page
The central limit problem for random vectors with symmetries
Motivated by the central limit problem for convex bodies, we study normal
approximation of linear functionals of high-dimensional random vectors with
various types of symmetries. In particular, we obtain results for distributions
which are coordinatewise symmetric, uniform in a regular simplex, or
spherically symmetric. Our proofs are based on Stein's method of exchangeable
pairs; as far as we know, this approach has not previously been used in convex
geometry and we give a brief introduction to the classical method. The
spherically symmetric case is treated by a variation of Stein's method which is
adapted for continuous symmetries.Comment: AMS-LaTeX, uses xy-pic, 23 pages; v3: added new corollary to Theorem
High energy behaviour of gamma gamma to f f(bar) processes in SM and MSSM
We compute the leading logarithms electroweak contributions to gamma gamma to
f f(bar) processes in SM and MSSM. Several interesting properties are pointed
out, such as the importance of the angular dependent terms, of the Yukawa
terms, and especially of the dependence in the SUSY
contributions. These properties are complementary to those found in e+e- to f
f(bar). These radiative correction effects should be largely observable at
future high energy gamma gamma colliders. Polarized beams would bring
interesting checks of the structure of the one loop corrections. We finally
discuss the need for two-loop calculations and resummation.Comment: 22 pages and 12 figures. e-mail: [email protected]
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