62 research outputs found
Light Stop Decays: Implications for LHC Searches
We investigate the flavour-changing neutral current decay of the lightest
stop into a charm quark and the lightest neutralino and its four-body decay
into the lightest neutralino, a down-type quark and a fermion pair. These are
the relevant stop search channels in the low-mass region. The SUSY-QCD
corrections to the two-body decay have been calculated for the first time and
turn out to be sizeable. In the four-body decay both the contributions from
diagrams with flavour-changing neutral current (FCNC) couplings and the mass
effects of final state bottom quarks and leptons have been taken into
account, which are not available in the literature so far. The resulting
branching ratios are investigated in detail. We find that in either of the
decay channels the branching ratios can deviate significantly from one in large
parts of the allowed parameter range. Taking this into account, the
experimental exclusion limits on the stop, which are based on the assumption of
branching ratios equal to one, are considerably weakened. This should be taken
into account in future searches for light stops at the next run of the LHC,
where the probed low stop mass region will be extended
Gluino Polarization at the LHC
Gluinos are produced pairwise at the LHC in quark-antiquark and gluon-gluon
collisions: . While the individual
polarization of gluinos vanishes in the limit in which the small mass
difference between L and R squarks of the first two generations is neglected,
non-zero spin-spin correlations are predicted within gluino pairs. If the
squark/quark charges in Majorana gluino decays are tagged, the spin
correlations have an impact on the energy and angular distributions in
reconstructed final states. On the other hand, the gluino polarization in
single gluino production in the supersymmetric Compton process is predicted to be non-zero, and the polarization
affects the final-state distributions in super-Compton events.Comment: 15 pages, 8 figure
Dissociative dynamics of spin-triplet and spin-singlet O2 on Ag(100)
10 páginas, 8 figuras.We study the dissociative dynamics of O2 molecules on the Ag(100) surface. Initially, the impinging molecules are either in the spin-triplet ground state or in the spin-singlet excited state. The molecule-surface interaction is obtained in each case by constructing the six-dimensional potential energy surface (PES) from the interpolation of the energies calculated with spin-polarized and non-spin-polarized density functional theories, respectively. Classical trajectory calculations performed in both PESs show that O2 molecules initially in the spin-triplet ground state only dissociate for incidence energies above 1.05 eV. This result is consistent with molecular beam experiments performed in this system. Interestingly, our results also suggest that for the spin-singlet O2 dissociation occurs even for incidence energies as low as 50 meV. We propose the use of spin-singlet excited O2 molecules to improve the otherwise low dissociative reactivity of O2 at clean Ag(100).We acknowledge partial support by the Spanish MEC (Grant
No. FIS2007-66711-C02-02), the Basque Government
(Grant No. CTP07-P02), and the Basque Departamento de
Educación, Universidades e Investigación, the University of
the Basque Country UPV/EHU (Grant No. IT-366-07). Computational
resources were provided by the SGI/IZO-SGIker
at the UPV/EHU (supported by the Spanish Ministry of Education
and Science and the European Social Fund) and the
DIPC computer center.Peer reviewe
Color-octet scalars at the LHC
Elements of the phenomenology of color-octet scalars (sgluons), as predicted
in the hybrid N=1/N=2 supersymmetric model, are discussed in the light of
forthcoming experiments at the CERN Large Hadron Collider.Comment: LaTeX, 10 pages, 4 figures. Talk at the Epiphany Conference on Hadron
Interactions at the Dawn of the LHC, 5-7 January 2009, Cracow, Polan
Light stop decays into Wb x ~ 10 near the kinematic threshold
We investigate the decays of the light stop in scenarios with the lightest neutralino x~10 being the lightest supersymmetric particle, including flavour-violating (FV) effects. We analyse the region where the three-body decay t~1->;Wb x ~10 is kinematically allowed and provide a proper description of the transition region between the three-body decay and the four-body decay t~1->;Wb x ~10bff\u27. The improved treatment has been implemented in the Fortran package SUSY-HIT and is used for the analysis of this region. A scan over the parameter range including all relevant experimental constraints reveals that the FV two-body decay into charm and x~10 can be as important as the three-, respectively, four-body decays if not dominant and therefore should be taken into account in order to complete the experimental searches for the light stop
Light Stop Decay in the MSSM with Minimal Flavour Violation
In supersymmetric scenarios with a light stop particle and a
small mass difference to the lightest supersymmetric particle (LSP) assumed to
be the lightest neutralino, the flavour changing neutral current decay
can be the dominant decay channel and can
exceed the four-body stop decay for certain parameter values. In the framework
of Minimal Flavour Violation (MFV) this decay is CKM-suppressed, thus inducing
long stop lifetimes. Stop decay length measurements at the LHC can then be
exploited to test models with minimal flavour breaking through Standard Model
Yukawa couplings. The decay width has been given some time ago by an
approximate formula, which takes into account the leading logarithms of the MFV
scale. In this paper we calculate the exact one-loop decay width in the
framework of MFV. The comparison with the approximate result exhibits
deviations of the order of 10% for large MFV scales due to the neglected
non-logarithmic terms in the approximate decay formula. The difference in the
branching ratios is negligible. The large logarithms have to be resummed. The
resummation is performed by the solution of the renormalization group
equations. The comparison of the exact one-loop result and the tree level
flavour changing neutral current decay, which incorporates the resummed
logarithms, demonstrates that the resummation effects are important and should
be taken into account.Comment: 29 page
Selectivity of stop codon recognition in translation termination is modulated by multiple conformations of GTS loop in eRF1
Translation termination in eukaryotes is catalyzed by two release factors eRF1 and eRF3 in a cooperative manner. The precise mechanism of stop codon discrimination by eRF1 remains obscure, hindering drug development targeting aberrations at translation termination. By solving the solution structures of the wild-type N-domain of human eRF1 exhibited omnipotent specificity, i.e. recognition of all three stop codons, and its unipotent mutant with UGA-only specificity, we found the conserved GTS loop adopting alternate conformations. We propose that structural variability in the GTS loop may underline the switching between omnipotency and unipotency of eRF1, implying the direct access of the GTS loop to the stop codon. To explore such feasibility, we positioned N-domain in a pre-termination ribosomal complex using the binding interface between N-domain and model RNA oligonucleotides mimicking Helix 44 of 18S rRNA. NMR analysis revealed that those duplex RNA containing 2-nt internal loops interact specifically with helix α1 of N-domain, and displace C-domain from a non-covalent complex of N-domain and C-domain, suggesting domain rearrangement in eRF1 that accompanies N-domain accommodation into the ribosomal A site
Color-Octet Scalars of N=2 Supersymmetry at the LHC
The color gauge hyper-multiplet in N=2 supersymmetry consists of the usual
N=1 gauge vector/gaugino super-multiplet, joined with a novel gaugino/scalar
super-multiplet. Large cross sections are predicted for the production of pairs
of the color-octet scalars [sgluons] at the LHC: . Single production is possible at one-loop level,
but the amplitude vanishes in the limit of degenerate and
squarks. When kinematically allowed, decays predominantly into two
gluinos, whose cascade decays give rise to a burst of eight or more jets
together with four LSP's as signature for pair events at the LHC.
can also decay into a squark-antisquark pair at tree level. At
one-loop level decays into gluons or a pair are predicted,
generating exciting resonance signatures in the final states. The corresponding
partial widths are very roughly comparable to that for three body final states
mediated by one virtual squark at tree level.Comment: LaTeX, 12 pages, 5 embedded figures. Added a couple of references;
version to appear in Phys. Lett.
- …