6,041 research outputs found
Simplified dark matter top-quark interactions at the LHC
Stringent limits on the interactions between dark matter (DM) and the
standard model can be set by studying how initial-state or final-state
particles recoil against missing transverse energy (MET). In this work, we
improve, extend and correct LHC constraints on the interactions between DM and
top quarks that are mediated by the exchange of spin-0 s-channel resonances. A
comparison of the LHC run-1 sensitivity of the two main search channels is
presented, which shows that mono-jet searches are typically more restrictive
than the MET + tbar t searches. We furthermore explore the reach of the 14 TeV
LHC. The collider constraints are compared to the restrictions arising from
direct and indirect detection as well as the DM relic abundance, and we also
reflect on effective field theory interpretations of the LHC exclusions.Comment: 29 pages, 11 figures; v4: conflict with CERN-PH-TH number fixe
QCD effects in mono-jet searches for dark matter
LHC searches for missing transverse energy in association with a jet allow to
place strong bounds on the interactions between dark matter and quarks. In this
article, we present an extension of the POWHEG BOX capable of calculating the
underlying cross sections at the next-to-leading order level. This approach
enables us to consistently include the effects of parton showering and to apply
realistic experimental cuts. We find significant differences from a fixed-order
analysis that neglects parton showering effects. In particular, next-to-leading
order corrections do not lead to a significant enhancement of the mono-jet
cross section once a veto on additional jets is imposed. Nevertheless, these
corrections reduce the theoretical uncertainties of the signal prediction and
therefore improve the reliability of the derived bounds. We present our results
in terms of simple rescaling factors, which can be directly applied to existing
experimental analyses and discuss the impact of changing experimental cuts.Comment: Added KITP affiliations, fixed 2 very minor typos, matches version
published in JHE
Determining the structure of dark-matter couplings at the LHC
The latest LHC mono-jet searches place stringent bounds on the pp -> chibar
chi cross section of dark matter. Further properties such as the dark matter
mass or the precise structure of the interactions between dark matter and the
standard model can however not be determined in this manner. We point out that
measurements of the azimuthal angle correlations between the two jets in 2 j +
chibar chi events may be used to disentangle whether dark matter pair
production proceeds dominantly through tree or loop diagrams. Our general
observation is illustrated by considering theories in which dark matter
interacts predominantly with the top quark. We show explicitly that in this
case the jet-jet azimuthal angle difference is a gold-plated observable to
probe the Lorentz structure of the couplings of dark matter to top quarks, thus
testing the CP nature of the particle mediating these interactions.Comment: 8 pages, 5 figures; v2: discussion of signal and background cross
sections added, labelling of plots improved, typos fixed and references
updated; matches version published in PR
Constraining Light-Quark Yukawa Couplings from Higgs Distributions
We propose a novel strategy to constrain the bottom and charm Yukawa
couplings by exploiting LHC measurements of transverse momentum distributions
in Higgs production. Our method does not rely on the reconstruction of
exclusive final states or heavy-flavour tagging. Compared to other proposals it
leads to an enhanced sensitivity to the Yukawa couplings due to distortions of
the differential Higgs spectra from emissions which either probe quark loops or
are associated to quark-initiated production. We derive constraints using data
from LHC Run I, and we explore the prospects of our method at future LHC runs.
Finally, we comment on the possibility of bounding the strange Yukawa coupling.Comment: Added analysis of the Higgs transverse momentum distribution. Version
published in Physical Review Letter
Mechanistic analysis of PCNA poly-ubiquitylation by the ubiquitin protein ligases Rad18 and Rad5
Poly-ubiquitylation is a common post-translational modification that can impart various functions to a target protein. Several distinct mechanisms have been reported for the assembly of poly-ubiquitin chains, involving either stepwise transfer of ubiquitin monomers or attachment of a preformed poly-ubiquitin chain and requiring either a single pair of ubiquitin-conjugating enzyme (E2) and ubiquitin ligase (E3), or alternatively combinations of different E2s and E3s. We have analysed the mechanism of poly-ubiquitylation of the replication clamp PCNA by two cooperating E2âE3 pairs, Rad6âRad18 and Ubc13âMms2âRad5. We find that the two complexes act sequentially and independently in chain initiation and stepwise elongation, respectively. While loading of PCNA onto DNA is essential for recognition by Rad6âRad18, chain extension by Ubc13âMms2âRad5 is only slightly enhanced by loading. Moreover, in contrast to initiation, chain extension is tolerant to variations in the attachment site of the proximal ubiquitin moiety. Our results provide information about a unique conjugation mechanism that appears to be specialised for a regulatable pattern of dual modification
Selective inhibition of T suppressor-cell function by a monosaccharide
Interactions between regulatory T lymphocytes and other cells are assumed to occur at the level of the cell surface. T cells which suppress the generation of specifically effector cells have been described as having antigenic, idiotypic, allotypic and I-region specificity1â4. Other T suppressor cells generated by in vitro cultivation with or without mitogenic stimulation5,6 have suppressive activity for T and B cells but no specificity can be assigned to them. These T suppressor cells (Ts) inhibit various lymphoid functionsâthis either reflects their polyclonal origin or indicates that the structures recognized by the Ts receptors must be common for many cell types. Carbohydrates on cell membrane-inserted glycoproteins or glycolipids might function as specific ligands for recognition by cellular receptors or soluble factors. Almost all cell-surface proteins of mammalian cells are glycosylated. There is evidence for lectin-like carbohydrate binding proteins not only in plants7 but also in toxins8, viruses9, prokaryotic cells10 and even mammalian cells, including T cells11. A functional role for these lectin-like proteins has been described for slime moulds and suggested for the selective association of embryonic cells12,13. We report here that addition of a monosaccharide can counteract the effect of T suppressor cells during the generation of alloreactive cytotoxic T cells (CTLs) in vitro
The lower mass function of the young open cluster Blanco 1: from 30 Mjup to 3 Mo
We performed a deep wide field optical survey of the young (~100-150 Myr)
open cluster Blanco1 to study its low mass population well down into the brown
dwarf regime and estimate its mass function over the whole cluster mass
range.The survey covers 2.3 square degrees in the I and z-bands down to I ~ z ~
24 with the CFH12K camera. Considering two different cluster ages (100 and 150
Myr), we selected cluster member candidates on the basis of their location in
the (I,I-z) CMD relative to the isochrones, and estimated the contamination by
foreground late-type field dwarfs using statistical arguments, infrared
photometry and low-resolution optical spectroscopy. We find that our survey
should contain about 57% of the cluster members in the 0.03-0.6 Mo mass range,
including 30-40 brown dwarfs. The candidate's radial distribution presents
evidence that mass segregation has already occured in the cluster. We took it
into account to estimate the cluster mass function across the
stellar/substellar boundary. We find that, between 0.03Mo and 0.6Mo, the
cluster mass distribution does not depend much on its exact age, and is well
represented by a single power-law, with an index alpha=0.69 +/- 0.15. Over the
whole mass domain, from 0.03Mo to 3Mo, the mass function is better fitted by a
log-normal function with m0=0.36 +/- 0.07Mo and sigma=0.58 +/- 0.06. Comparison
between the Blanco1 mass function, other young open clusters' MF, and the
galactic disc MF suggests that the IMF, from the substellar domain to the
higher mass part, does not depend much on initial conditions. We discuss the
implications of this result on theories developed to date to explain the origin
of the mass distribution.Comment: 18 pages, 15 figures and 5 tables accepted in A&
Effects of rotation and thermohaline mixing in red giant stars
Thermohaline mixing has been recently identified as the probable dominating
process that governs the photospheric composition of low-mass bright giant
stars. Here we present the predictions of stellar models computed with the code
STAREVOL including this mechanism together with rotational mixing. We compare
our theoretical predictions with recent observations.Comment: Proc. of the workshop "Red Giants as Probes of the Structure and
Evolution of the Milky Way" (Roma, 15-17 Nov 2010), Astrophysics and Space
Science Proceedings, ISBN 978-3-642-18417-8 (eds. A. Miglio, J. Montalban, A.
Noels). Part of RedGiantsMilkyWay/2011/ proceedings available at
http://arxiv.org/html/1108.4406v
HIGGS PHENOMENOLOGY OF THE SUPERSYMMETRIC MODEL WITH A GAUGE SINGLET
We discuss the Higgs sector of the supersymmetric standard model extended by
a gauge singlet for the range of parameters, which is compatible with universal
soft supersymmetry breaking terms at the GUT scale. We present results for the
masses, couplings and decay properties of the lightest Higgs bosons, in
particular with regard to Higgs boson searches at LEP. The prospects differ
significantly from the ones within the MSSM.Comment: 12 pages (Plain Tex), 7 fig
Sialic Acid Glycobiology Unveils Trypanosoma cruzi Trypomastigote Membrane Physiology.
Trypanosoma cruzi, the flagellate protozoan agent of Chagas disease or American trypanosomiasis, is unable to synthesize sialic acids de novo. Mucins and trans-sialidase (TS) are substrate and enzyme, respectively, of the glycobiological system that scavenges sialic acid from the host in a crucial interplay for T. cruzi life cycle. The acquisition of the sialyl residue allows the parasite to avoid lysis by serum factors and to interact with the host cell. A major drawback to studying the sialylation kinetics and turnover of the trypomastigote glycoconjugates is the difficulty to identify and follow the recently acquired sialyl residues. To tackle this issue, we followed an unnatural sugar approach as bioorthogonal chemical reporters, where the use of azidosialyl residues allowed identifying the acquired sugar. Advanced microscopy techniques, together with biochemical methods, were used to study the trypomastigote membrane from its glycobiological perspective. Main sialyl acceptors were identified as mucins by biochemical procedures and protein markers. Together with determining their shedding and turnover rates, we also report that several membrane proteins, including TS and its substrates, both glycosylphosphatidylinositol-anchored proteins, are separately distributed on parasite surface and contained in different and highly stable membrane microdomains. Notably, labeling for α(1,3)Galactosyl residues only partially colocalize with sialylated mucins, indicating that two species of glycosylated mucins do exist, which are segregated at the parasite surface. Moreover, sialylated mucins were included in lipid-raft-domains, whereas TS molecules are not. The location of the surface-anchored TS resulted too far off as to be capable to sialylate mucins, a role played by the shed TS instead. Phosphatidylinositol-phospholipase-C activity is actually not present in trypomastigotes. Therefore, shedding of TS occurs via microvesicles instead of as a fully soluble form
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