2,339 research outputs found
Lower Redshift Analogues of the Sources of Reionization
Known populations of QSOs appear to fall short of producing the ionizing flux
required for re-ionizing the universe. The alternative, galaxies as sources of
ionizing photons, suffers from the problem that known types of galaxies are
almost completely opaque to ionizing photons. For reionization to happen,
either large numbers of (largely undiscovered) sources are required, or the
known populations of galaxies need to have had a much larger escape fraction
for ionizing radiation in the past. We discuss recent discoveries of faint z~3
Lyman alpha emitters with asymmetric, extended Lyman alpha emission regions,
which apparently are related to interacting galaxies. The unusually shaped line
profiles and the underlying stellar populations of these objects suggest the
presence of damaged gaseous halos, infall of gas, tidal or stripped stellar
features and young populations of hot stars, that would all be conducive to the
release of ionizing radiation. As galaxy interactions and mergers increase with
redshift, these effects can only become more important at earlier times, and so
these interacting z~3 objects may be late, lower redshift analogues of the
sources of reionization.Comment: 6 pages, 4 figures; contribution to the meeting First Stars IV,
Kyoto, May 21-25, 201
Higgs-Boson Production in Association with Heavy Quarks
Associated production of a Higgs boson with a heavy, i.e. top or bottom,
quark-anti-quark pair provide observation channels for Higgs bosons at the LHC
which can be used to measure the respective Yukawa couplings. For the light
supersymmetric Higgs boson we present SUSY-QCD corrections at the one-loop
level, which constitute a significant contribution to the cross section.Comment: 4 pages, 1 table; to appear in Proceedings of SUSY06, the 14th
International Conference on Supersymmetry and the Unification of Fundamental
Interactions, UC Irvine, California, 12-17 June 200
Higgs Spin Determination in the WW channel and beyond
After the discovery of the 126 GeV resonance at the LHC, the determination of
its features, including its spin, is a very important ongoing task. In order to
distinguish the two most likely spin hypotheses, spin-0 or spin-2, we study the
phenomenology of a light Higgs-like spin-2 resonance produced in different
gluon-fusion and vector-boson-fusion processes at the LHC. Starting from an
effective model for the interaction of a spin-2 particle with the SM gauge
bosons, we calculate cross sections and differential distributions within the
Monte Carlo program Vbfnlo. We find that with specific model parameters such a
spin-2 resonance can mimic SM Higgs rates and transverse-momentum distributions
in , and decays, whereas several distributions allow
to separate spin-2 from spin-0, independently of the spin-2 model parameters.Comment: 14 pages, 7 figure
ZZ production at high transverse momenta beyond NLO QCD
We study the production of the four-lepton final state ,
predominantly produced by a pair of electroweak Z bosons, ZZ. Using the LoopSim
method, we merge NLO QCD results for ZZ and ZZ+jet and obtain approximate NNLO
predictions for ZZ production. The exact gluon-fusion loop-squared contribution
to the ZZ process is also included. On top of that, we add to our merged sample
the gluon-fusion ZZ+jet contributions from the gluon-gluon channel, which is
formally of N^3LO and provides approximate results at NLO for the gluon-fusion
mechanism. The predictions are obtained with the VBFNLO package and include the
leptonic decays of the Z bosons with all off-shell and spin-correlation
effects, as well as virtual photon contributions. We compare our predictions
with existing results for the total inclusive cross section at NNLO and find a
very good agreement. Then, we present results for differential distributions
for two experimental setups, one used in searches for anomalous triple gauge
boson couplings, the other in Higgs analyses in the four charged-lepton final
state channel. We find that the approximate NNLO corrections are large,
reaching up to 20% at high transverse momentum of the Z boson or the leading
lepton, and are not covered by the NLO scale uncertainties. Distributions of
the four-lepton invariant mass are, however, stable with respect to QCD
corrections at this order.Comment: 21 pages, 2 tables, 9 figure
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