2,257 research outputs found
The exact tree-level calculation of the dark photon production in high-energy electron scattering at the CERN SPS
Dark photon () that couples to the standard model fermions via the
kinetic mixing with photons and serves as a mediator of dark matter production
could be observed in the high-energy electron scattering off nuclei followed by the decay. We have
performed the exact, tree-level calculations of the production cross
sections and implemented them in the program for the full simulation of such
events in the experiment NA64 at the CERN SPS. Using simulations results, we
study the missing energy signature for the bremsstrahlung
invisible decay that permits the determination of the mixing
strength in a wide, from sub-MeV to sub-GeV, mass range. We refine and
expand our earlier studies of this signature for discovering by including
corrections to the previously used calculations based on the improved
Weizsaker-Williams approximation, which turn out to be significant. We compare
our cross sections values with the results from other calculations and find a
good agreement between them. The possibility of future measurements with
high-energy electron beams and the sensitivity to are briefly discussed.Comment: 11 pages, 6 figures, revised version, improved cross-section
integrator is used, comparison with bremsstrahlung spectrum is added, final
conclusions remain unchange
Missing energy signature from invisible decays of dark photons at the CERN SPS
The dark photon () production through the mixing with the bremsstrahlung
photon from the electron scattering off nuclei can be accompanied by the
dominant invisible decay into dark-sector particles. In this work we
discuss the missing energy signature of this process in the experiment NA64
aiming at the search for decays with a high-energy electron
beam at the CERN SPS. We show the distinctive distributions of variables that
can be used to distinguish the signal from background. The
results of the detailed simulation of the detector response for the events with
and without emission are presented. The efficiency of the signal event
selection is estimated. It is used to evaluate the sensitivity of the
experiment and show that it allows to probe the still unexplored area of the
mixing strength and masses up to
GeV. The results obtained are compared with the results
from other calculations. In the case of the signal observation, a possibility
of extraction of the parameters and by using the missing
energy spectrum shape is discussed. We consider as an example the with the
mass 16.7 MeV and mixing , which can explain an
excess of events recently observed in nuclear transitions of an excited state
of Be. We show that if such exists its invisible decay can be observed
in NA64 within a month of running, while data accumulated during a few months
would allow also to determine the and parameters.Comment: 12 pages, 15 figures. Revised versio
Fully Geant4 compatible package for the simulation of Dark Matter in fixed target experiments
We present the package for the simulation of DM (Dark Matter) particles in
fixed target experiments. The most convenient way of this simulation (and the
only possible way in the case of beam-dump) is to simulate it in the framework
of the program for tracing particles in the experimental setup. One of the most
popular such programs is Geant4.
Specifically, the package includes the processes of DM particles production
via electron and muon bremsstrahlung off nuclei, resonant in-flight positron
annihilation on atomic electrons and gamma to ALP (axion-like particles)
conversion on nuclei. Four types of DM mediator particles are considered:
vector, scalar, pseudoscalar and axial vector.
In particular, for bremsstrahlung the total cross sections are calculated at
exact tree level (ETL). The code handles both the case of invisible DM mediator
decay and of visible decay into (or into in the case
of ALP).
The software consists of a collection of different classes, inheriting from
the Geant4 framework classes, thus the expected use of this package is to
include it in a Geant4-based code for the simulation of particles propagation
and interaction in the detector.
As an example of its usage, we discuss the results obtained from the
simulation of a typical active beam-dump experiment, considering 100 GeV electrons impinging on a lead/plastic scintillator active
thick target, showing the expected sensitivity for the four types of DM
mediator particles mentioned above.Comment: 10 pages, 4 figure
Electron microscopy analysis of ATP-independent nucleosome unfolding by FACT
FACT is a histone chaperone that participates in nucleosome removal and reassembly during transcription and replication. We used electron microscopy to study FACT, FACT:Nhp6 and FACT:Nhp6:nucleosome complexes, and found that all complexes adopt broad ranges of configurations, indicating high flexibility. We found unexpectedly that the DNA binding protein Nhp6 also binds to the C-terminal tails of FACT subunits, inducing more open geometries of FACT even in the absence of nucleosomes. Nhp6 therefore supports nucleosome unfolding by altering both the structure of FACT and the properties of nucleosomes. Complexes formed with FACT, Nhp6, and nucleosomes also produced a broad range of structures, revealing a large number of potential intermediates along a proposed unfolding pathway. The data suggest that Nhp6 has multiple roles before and during nucleosome unfolding by FACT, and that the process proceeds through a series of energetically similar intermediate structures, ultimately leading to an extensively unfolded form
Mistletoe lectin dissociates into catalytic and binding subunits before translocation across the membrane to the cytoplasm
AbstractHybridomas producing monoclonal antibodies (mAbs) against the mistletoe lectin A-chain (MLA) were obtained to investigate the intracellular routing and translocation of ribosome-inactivating proteins. Anti-MLA mAb MNA5 did not bind the holotoxin but interacted with isolated MLA. This epitope was not recognized upon MLA denaturation or conjugation of MLA with the ricin binding subunit (RTB). Furthermore, the mAbs did not appreciably react with a panel of MLA synthetic octapeptides linked to the surface of polyethylene pins. A study of the cytotoxicity of mistletoe lectin, ricin, and chimeric toxin MLA/RTB for the hybridomas revealed that interchain disulfide bond reduction and subunit dissociation are required for cytotoxic activity of mistletoe lectin
Ribonuclease from Bacillus thuringiensis var. subtoxicus: Gene structure and regulation of biosynthesis
The gene for extracellular guanyl-specific ribonuclease of Bacillus thuringiensis var. subtoxicus (RNase Bth), a close homologue of the B. intermedius RNase (binase), was completely sequenced. Analysis of nucleotide sequences in the regions adjoining RNase genes revealed an identical organization of the chromosomal loci of RNase Bth and binase. Growth characteristics of the Bacillus thuringiensis var. subtoxicus strain and its synthesis of RNase were studied. It was shown that the exogenous inorganic phosphate inhibits the biosynthesis of RNase. At the same time, actinomycin D in low doses stimulates the enzyme synthesis. Comparative analysis of the influence of inorganic phosphate and actinomycin D on the biosynthesis of RNAse Bth and binase suggests a possibility of coincidence of regulatory pathways of synthesis of these enzymes
Ribonuclease from Bacillus thuringiensis var. subtoxicus: Gene structure and regulation of biosynthesis
The gene for extracellular guanyl-specific ribonuclease of Bacillus thuringiensis var. subtoxicus (RNase Bth), a close homologue of the B. intermedius RNase (binase), was completely sequenced. Analysis of nucleotide sequences in the regions adjoining RNase genes revealed an identical organization of the chromosomal loci of RNase Bth and binase. Growth characteristics of the Bacillus thuringiensis var. subtoxicus strain and its synthesis of RNase were studied. It was shown that the exogenous inorganic phosphate inhibits the biosynthesis of RNase. At the same time, actinomycin D in low doses stimulates the enzyme synthesis. Comparative analysis of the influence of inorganic phosphate and actinomycin D on the biosynthesis of RNase Bth and binase suggests a possibility of coincidence of regulatory pathways of synthesis of these enzymes. © 2000 MAIK "Nauka/Interperiodica"
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