318 research outputs found
Factorization breaking in single-diffractive dijet production at the Tevatron
We perform a NLO QCD analysis of single-diffractive dijet production in
proton-antiproton collisions. By comparing the ratio of single- and
non-diffractive cross sections to data from the Tevatron, the rapidity-gap
survival probability is determined as a function of the momentum fraction of
the parton in the antiproton. Assuming Regge factorization, this probability
can be interpreted as a suppression factor for the diffractive structure
function measured in deep-inelastic scattering at HERA. In contrast to the
observations for photoproduction, the suppression factor in proton-antiproton
collisions depends on the momentum fraction of the parton in the Pomeron even
at NLO.Comment: 4 pages, 2 figures, contribution to the 15th international QCD
conference (QCD 10), Montpellier, France (2010
Three jet cross sections in photoproduction at HERA
We calculate three jet cross sections in photoproduction using exact matrix
elements for the direct and resolved contributions. Numerical distributions are
presented in a generic, irreducible set of variables that allows to disentangle
the dynamics of partonic QCD subprocesses from each other and from pure phase
space distributions. The results are compared to preliminary data from the ZEUS
collaboration at HERA. It is found that the largest contribution comes from
photon-gluon fusion in the mass range 36 GeV 80 GeV. The
measured leading jet scattering angle distribution is consistent with the
-channel exchange of a massless fermion in scattering, where the
third parton is assumed to arise from soft bremsstrahlung. The data are
inconsistent with pure phase space and Rutherford scattering distributions.Comment: 9 pages, 8 figure
Photon Scattering in Muon Collisions
We estimate the benefit of muon colliders for photon physics. We calculate
the rate at which photons are emitted from muon beams in different production
mechanisms. Bremsstrahlung is reduced, beamstrahlung disappears, and laser
backscattering suffers from a bad conversion of the incoming to the outgoing
photon beam in addition to requiring very short wavelengths. As a consequence,
the cross sections for jet photoproduction in and
collisions are reduced by factors of 2.2 and 5 compared to and
machines. However, the cross sections remain sizable and measurable giving
access to the photon and proton parton densities down to values of
to .Comment: Talk given at the Workshop on Physics at the First Muon Collider,
Fermilab, November 1997. To appear in the proceedings. 10 pages, 6 figures,
uses aipproc.cls and aipproc.st
Geography vs. Institutions at the Village Level
There is a well-known debate about the roles of geography versus institutions in explaining the long-term development of countries. These debates have usually been based on cross-country regressions where questions about parameter heterogeneity, unobserved heterogeneity, and endogeneity cannot easily be controlled for. The innovation of Acemoglu, Johnson and Robinson (2001) was to address this last point by using settler mortality as an instrument for geography-induced endogenous institutions and found that this supported their line of reasoning. We believe there is value-added to consider this debate at the micro level within a country as particularly questions of parameter heterogeneity and unobserved heterogeneity are likely to be smaller than between countries. Moreover, at the micro level it is possible to identify more precise transmission mechanisms from geography via institutions to economic development outcomes. In particular, we examine the determinants of economic development across villages on the Indonesian Island of Sulawesi and find that geography-induced endogenous emergence of land rights is the critical institutional link between geographic conditions and technological change. We therefore highlight and empirically validate a new transmission channel from endogenously generated institutions on economic development.geography, migration, land rights, institutions, technology adoption, agricultural development, Indonesia
Geography vs. Institutions at the Village Level
There is a well-known debate about the roles of geography versus institutions in explaining the long-term development of countries. These debates have usually been based on cross-country regressions where questions about parameter heterogeneity, unobserved heterogeneity, and endogeneity cannot easily be controlled for. The innovation of Acemoglu, Johnson and Robinson (2001) was to address this last point by using settler mortality as an instrument for geography-induced endogenous institutions and found that this supported their line of reasoning. We believe there is value-added to consider this debate at the micro level within a country as particularly questions of parameter heterogeneity and unobserved heterogeneity are likely to be smaller than between countries. Moreover, at the micro level it is possible to identify more precise transmission mechanisms from geography via institutions to economic development outcomes. In particular, we examine the determinants of economic development across villages on the Indonesian Island of Sulawesi and find that geography-induced endogenous emergence of land rights is the critical institutional link between geographic conditions and technological change. We therefore highlight and empirically validate a new transmission channel from endogenously generated institutions on economic development.geography, migration, land rights, institutions, technology adoption, agricultural development, Indonesia
Singlet-doublet fermion and triplet scalar dark matter with radiative neutrino masses
We present a detailed study of a combined singlet-doublet fermion and triplet
scalar model for dark matter. These models have only been studied separately in
the past. Together, they form a simple extension of the Standard Model that can
account for dark matter and explain the existence of neutrino masses, which are
generated radiatively. This holds even if singlet-doublet fermions and triplet
scalars never contribute simultaneously to the dark matter abundance. However,
this also implies the existence of lepton flavour violating processes. In
addition, this particular model allows for gauge coupling unification. The new
fields are odd under a new symmetry to stabilise the dark matter
candidate. We analyse the dark matter, neutrino mass and lepton flavour
violation aspects both separately and in conjunction, exploring the viable
parameter space of the model. This is done using a numerical random scan
imposing successively the neutrino mass and mixing, relic density, Higgs mass,
direct detection, collider and lepton flavour violation constraints. We find
that dark matter in this model is fermionic for masses below about 1 TeV and
scalar above. The narrow mass regions found previously for the two separate
models are enlarged by their coupling. While coannihilations of the weak
isospin partners are sizeable, this is not the case for fermions and scalars
despite their often similar masses due to the relatively small coupling of the
two sectors, imposed by the small neutrino masses. We observe a high degree of
complementarity between direct detection and lepton flavour violation
experiments, which should soon allow to fully probe the fermionic dark matter
sector and at least partially the scalar dark matter sector.Comment: 24 pages, 12 figures; version accepted by and published in JHE
Singlet-doublet/triplet dark matter and neutrino masses
In these proceedings, we present a study of a combined singlet--doublet
fermion and triplet scalar model for dark matter (DM). Together, these models
form a simple extension of the Standard Model (SM) that can account for DM and
explain the existence of neutrino masses, which are generated radiatively.
However, this also implies the existence of lepton flavour violating (LFV)
processes. In addition, this particular model allows for gauge coupling
unification. The new fields are odd under a new symmetry to
stabilise the DM candidate. We analyse the DM, neutrino mass and LFV aspects,
exploring the viable parameter space of the model. This is done using a
numerical random scan imposing successively the neutrino mass and mixing, relic
density, Higgs mass, direct detection, collider and LFV constraints. We find
that DM in this model is fermionic for masses below about 1 TeV and scalar
above. We observe a high degree of complementarity between direct detection and
LFV experiments, which should soon allow to fully probe the fermionic DM sector
and at least partially the scalar DM sector.Comment: 4 pages, 1 figure; contribution to the 2019 EW session of the 54th
Rencontres de Moriond (summary of arXiv:1812.11133
NNLO contributions to jet photoproduction and determination of \alpha_s
We present the first calculation of inclusive jet photoproduction with
next-to-next-to-leading order (NNLO) contributions, obtained from a unified
threshold resummation formalism. The leading coefficients for direct
photoproduction are computed analytically. Together with the coefficients
pertinent to parton-parton scattering, they are shown to agree with those
appearing in our full next-to-leading order calculations. For hadron-hadron
scattering, numerical agreement is found with a previous calculation of jet
production at the Tevatron. We show that the direct and resolved NNLO
contributions considerably improve the description of final ZEUS data on jet
photoproduction and that the error on the determination of the strong coupling
constant is significantly reduced.Comment: 4 pages, 3 figure
Tests of QCD: Summary of DIS 2000
This summary of the working group 2 of DIS 2000 encompasses experimental and
theoretical results of jet physics, open and bound state heavy flavour
production, prompt photon production, next-to-leading order QCD calculations
and beyond, instantons, fragmentation, event shapes, and power corrections,
primarily from deep-inelastic scattering and photoproduction at HERA, but also
from the LEP and Tevatron colliders.Comment: 16 pages, LaTeX, including 8 PostScript figures. Talk given at the
8th International Workshop on Deep-Inelastic Scattering (DIS2000), 25th-30th
April 2000, Liverpool, England, to appear in the proceeding
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