372 research outputs found
Characterization of bulk MgB2 synthesized by infiltration and growth
Superconducting MgB2 has been synthesized successfully by a modified infiltration and growth (IG) technique. The ambient pressure technique is relatively simple and scalable to complex shaped bulks. The extent of MgB 2 phase formation has been found to be influenced strongly by the IG process time and/or temperature, and this is found to reflect in the X-ray diffraction patterns, magnetization measurements, and microhardness. Scanning electron microscopy images show a bimodal particle size distribution with 20-50 nm sized fine precipitates in the inter particle region. A critical current density of 400 kA cm-2 was measured at 5 K.KACST-Cambridge Research Centre, Cambridge, U.K
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Numerical modelling and comparison of MgB<inf>2</inf> bulks fabricated by HIP and infiltration growth
MgB_2 in bulk form shows great promise as trapped field magnets (TFMs) as an alternative to bulk (RE)BCO materials to replace permanent magnets in applications such as rotating machines, magnetic bearings and magnetic separation, and the relative ease of fabrication of MgB_2 materials has enabled a number of different processing techniques to be developed. In this paper, a comparison is made between bulk MgB_2 samples fabricated by the hot isostatic pressing (HIP), with and without Ti-doping, and infiltration growth (IG) methods and the highest trapped field in an IG-processed bulk MgB_2 sample, B_z = 2.12 at 5 K and 1.66 T at 15 K, is reported. Since bulk MgB_2 has a more homogeneous J_c distribution than (RE)BCO bulks, studies on such systems are made somewhat easier because simplified assumptions regarding the geometry and J_c distribution can be made, and a numerical simulation technique based on the 2D axisymmetric H-formulation is introduced to model the complete process of field cooling (FC) magnetization. As input data for the model, the measured J_c(B,T) characteristics of a single, small specimen taken from each bulk sample are used, in addition to measured specific heat and thermal conductivity data for the materials. The results of the simulation reproduce the experimental results extremely well: (1) indicating the samples have excellent homogeneity, and (2) validating the numerical model as a fast, accurate and powerful tool to investigate the trapped field profile of bulk MgB_2 discs of any size accurately, under any specific operating conditions. Finally, the paper is concluded with a numerical analysis of the influence of the dimensions of the bulk sample on the trapped field.JZ would like to acknowledge the support of Churchill College, Cambridge, the China Scholarship Council and the Cambridge Commonwealth, European and International Trust. MA would like to acknowledge the support of a Royal Academy of Engineering Research Fellowship. HF would like to acknowledge support in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This research was also supported in part by a Royal Society International Exchanges Scheme grant, IE131084. J-FF would like to thank the Ministry of Higher Education through the Research Council of the University of Liege (Action de Recherches Concertées grant, ARC 11/16-03).This is the author accepted manuscript. The final version is available from IOP via http://dx.doi.org/10.1088/0953-2048/28/7/07500
High Trapped Fields in C-doped MgB2 Bulk Superconductors Fabricated by Infiltration and Growth Process.
The grain boundaries in superconducting MgB2 are known to form effective magnetic flux pinning sites and, consequently, bulk MgB2 containing a fine-grain microstructure fabricated from nanoscale Mg and B precursor powders exhibits good magnetic field-trapping performance below 20 K. We report here that the trapped field of MgB2 bulk superconductors fabricated by an infiltration and growth process to yield a dense, pore-free microstructure, can be enhanced significantly by carbon-doping, which increases intra-band scattering within the superconducting grains. A maximum trapped field of 4.15 T has been measured at 7.5 K at the centre of a five-sample stack of Mg(B1-xiCxi)2 bulk superconductors processed by infiltration and growth, which not only represents a ~40% increase in trapped field observed compared to undoped bulk MgB2, but also is the highest trapped field reported to date in MgB2 samples processed under ambient pressure. The trapped field is observed to decay at a rate of <2%/day at 10 K, which suggests that bulk MgB2 superconductors fabricated using the infiltration and growth technique can be used potentially to generate stable, high magnetic fields for a variety of engineering applications
Measuring V_ub and probing SUSY with double ratios of purely leptonic decays of B and D mesons
The experimental prospects for precise measurements of the leptonic decays
B_u -> tau nu / mu nu, B_s -> mu+ mu-, D -> mu nu and D_s -> mu nu / tau nu are
very promising. Double ratios involving four of these decays can be defined in
which the dependence on the values of the decay constants is essentially
eliminated, thus enabling complementary measurements of the CKM matrix element
V_ub with a small theoretical error. We quantify the experimental error in a
possible future measurement of |V_ub| using this approach, and show that it is
competitive with the anticipated precision from the conventional approaches.
Moreover, it is shown that such double ratios can be more effective than the
individual leptonic decays as a probe of the parameter space of supersymmetric
models. We emphasize that the double ratios have the advantage of using |V_ub|
as an input parameter (for which there is experimental information), while the
individual decays have an uncertainty from the decay constants (e.g. f_B_s),
and hence a reliance on theoretical techniques such as lattice QCD.Comment: 21 pages, 4 figure
Filarial Lymphedema Is Characterized by Antigen- Specific Th1 and Th17 Proinflammatory Responses and a Lack of Regulatory T Cells
Background: Lymphatic filariasis can be associated with development of serious pathology in the form of lymphedema,
hydrocele, and elephantiasis in a subset of infected patients.
Methods and Findings: To elucidate the role of CD4+ T cell subsets in the development of lymphatic pathology, we
examined specific sets of cytokines in individuals with filarial lymphedema in response to parasite antigen (BmA) and
compared them with responses from asymptomatic infected individuals. We also examined expression patterns of Toll-like
receptors (TLR1–10) and Nod-like receptors (Nod1, Nod2, and NALP3) in response to BmA. BmA induced significantly higher
production of Th1-type cytokines—IFN-c and TNF-a—in patients with lymphedema compared with asymptomatic
individuals. Notably, expression of the Th17 family of cytokines—IL-17A, IL-17F, IL-21, and IL-23—was also significantly
upregulated by BmA stimulation in lymphedema patients. In contrast, expression of Foxp3, GITR, TGFb, and CTLA-4, known
to be expressed by regulatory T cells, was significantly impaired in patients with lymphedema. BmA also induced
significantly higher expression of TLR2, 4, 7, and 9 as well Nod1 and 2 mRNA in patients with lymphedema compared with
asymptomatic controls.
Conclusion: Our findings implicate increased Th1/Th17 responses and decreased regulatory T cells as well as regulation of
Toll- and Nod-like receptors in pathogenesis of filarial lymphedema
Constraints from muon g-2 and LFV processes in the Higgs Triplet Model
Constraints from the muon anomalous magnetic dipole moment and lepton flavor
violating processes are translated into lower bounds on v_Delta*m_H++ in the
Higgs Triplet Model by considering correlations through the neutrino mass
matrix. The discrepancy of the sign of the contribution to the muon anomalous
magnetic dipole moment between the measurement and the prediction in the model
is clarified. It is shown that mu to e gamma, tau decays (especially, tau to mu
e e), and the muonium conversion can give a more stringent bound on
v_Delta*m_H++ than the bound from mu to eee which is expected naively to give
the most stringent one.Comment: 18 pages, 16 figure
Viability of MSSM scenarios at very large tan(beta)
We investigate the MSSM with very large tan(beta) > 50, where the fermion
masses are strongly affected by loop-induced couplings to the "wrong" Higgs,
imposing perturbative Yukawa couplings and constraints from flavour physics.
Performing a low-energy scan of the MSSM with flavour-blind soft terms, we find
that the branching ratio of B->tau nu and the anomalous magnetic moment of the
muon are the strongest constraints at very large tan(beta) and identify the
viable regions in parameter space. Furthermore we determine the scale at which
the perturbativity of the Yukawa sector breaks down, depending on the
low-energy MSSM parameters. Next, we analyse the very large tan(beta) regime of
General Gauge Mediation (GGM) with a low mediation scale. We investigate the
requirements on the parameter space and discuss the implied flavour
phenomenology. We point out that the possibility of a vanishing Bmu term at a
mediation scale M = 100 TeV is challenged by the experimental data on B->tau nu
and the anomalous magnetic moment of the muon.Comment: 29 pages, 7 figures. v2: discussion in sections 1 and 4 expanded,
conclusions unchanged. Matches version published in JHE
Direct CP violation in charm and flavor mixing beyond the SM
We analyze possible interpretations of the recent LHCb evidence for CP
violation in D meson decays in terms of physics beyond the Standard Model. On
general grounds, models in which the primary source of flavor violation is
linked to the breaking of chiral symmetry (left-right flavor mixing) are
natural candidates to explain this effect, via enhanced chromomagnetic
operators. In the case of supersymmetric models, we identify two motivated
scenarios: disoriented A-terms and split families. These structures predict
other non-standard signals, such as nuclear EDMs close to their present bounds
and, possibly, tiny but visible deviations in K and B physics, or even sizable
flavor-violating processes involving the top quark or the stops. Some of these
connections, especially the one with nuclear EDMs, hold beyond supersymmetry,
as illustrated with the help of prototype non-supersymmetric models.Comment: 30 pages, 6 figure
Single-Scale Natural SUSY
We consider the prospects for natural SUSY models consistent with current
data. Recent constraints make the standard paradigm unnatural so we consider
what could be a minimal extension consistent with what we now know. The most
promising such scenarios extend the MSSM with new tree-level Higgs interactions
that can lift its mass to at least 125 GeV and also allow for flavor-dependent
soft terms so that the third generation squarks are lighter than current bounds
on the first and second generation squarks. We argue that a common feature of
almost all such models is the need for a new scale near 10 TeV, such as a scale
of Higgsing or confinement of a new gauge group. We consider the question
whether such a model can naturally derive from a single mass scale associated
with supersymmetry breaking. Most such models simply postulate new scales,
leaving their proximity to the scale of MSSM soft terms a mystery. This
coincidence problem may be thought of as a mild tuning, analogous to the usual
mu problem. We find that a single mass scale origin is challenging, but suggest
that a more natural origin for such a new dynamical scale is the gravitino
mass, m_{3/2}, in theories where the MSSM soft terms are a loop factor below
m_{3/2}. As an example, we build a variant of the NMSSM where the singlet S is
composite, and the strong dynamics leading to compositeness is triggered by
masses of order m_{3/2} for some fields. Our focus is the Higgs sector, but our
model is compatible with a light stop (with the other generation squarks heavy,
or with R-parity violation or another mechanism to hide them from current
searches). All the interesting low-energy mass scales, including linear terms
for S playing a key role in EWSB, arise dynamically from the single scale
m_{3/2}. However, numerical coefficients from RG effects and wavefunction
factors in an extra dimension complicate the otherwise simple story.Comment: 32 pages, 3 figures; version accepted by JHE
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