Magnetic phase diagram of MnSi inferred from magnetization and ac susceptibility

We report simultaneous measurements of the magnetization and the ac susceptibility across the magnetic phase diagram of single-crystal MnSi. In our study we explore the importance of the excitation frequency, excitation amplitude, sample shape, and crystallographic orientation. The susceptibility, dM/dH, calculated from the magnetization, is dominated by pronounced maxima at the transition from the helical to the conical and the conical to the skyrmion lattice phase. The maxima in dM/dH are not tracked by the ac susceptibility, which in addition varies sensitively with the excitation amplitude and frequency at the transition from the conical to the skyrmion lattice phase. The same differences between dM/dH and the ac susceptibility exist for Mn1-xFexSi (x=0.04) and Fe1-xCoxSi (x=0.20). Taken together our study establishes consistently for all major crystallographic directions the existence of a single pocket of the skyrmion lattice phase in MnSi, suggestive of a universal characteristic of all B20 transition metal compounds with helimagnetic order.Comment: 19 pages, 20 figure

Testing factorization in B -> D(*)X decays

In QCD the amplitude for B0 -> D(*)+pi- factorizes in the large Nc limit or in the large energy limit Q >> Lambda_QCD where Q = {m_b, m_c, m_b-m_c}. Data also suggests factorization in exclusive processes B-> D* pi+ pi- pi- pi0 and B-> D* omega pi-, however by themselves neither large Nc nor large Q can account for this. Noting that the condition for large energy release in B0-> D+ pi- is enforced by the SV limit, m_b, m_c >> m_b-m_c >> Lambda, we propose that the combined large Nc and SV limits justify factorization in B -> D(*) X. This combined limit is tested with the inclusive decay spectrum measured by CLEO. We also give exact large Nc relations among isospin amplitudes for B -> D(*)X and B -> D(*) D-bar(*)X, which can be used to test factorization through exclusive or inclusive measurements. Predictions for the modes B-> D(*) pi pi, B-> D(*)K K-bar and B-> D(*) D-bar(*) K are discussed using available data.Comment: 15 pages, 3 included .eps figures, minor change

Magnetic Field Effects on Quasiparticles in Strongly Correlated Local Systems

We show that quasiparticles in a magnetic field of arbitrary strength $H$ can be described by field dependent parameters. We illustrate this approach in the case of an Anderson impurity model and use the numerical renormalization group (NRG) to calculate the renormalized parameters for the levels with spin $\sigma$, $\tilde\epsilon_{\mathrm{d},\sigma}(H)$, resonance width $\tilde\Delta(H)$ and the effective local quasiparticle interaction $\tilde U(H)$. In the Kondo or strong correlation limit of the model the progressive de-renormalization of the quasiparticles can be followed as the magnetic field is increased. The low temperature behaviour, including the conductivity, in arbitrary magnetic field can be calculated in terms of the field dependent parameters using the renormalized perturbation expansion. Using the NRG the field dependence of the spectral density on higher scales is also calculated.Comment: 15 pages, 17 figure

Power Counting in the Soft-Collinear Effective Theory

We describe in some detail the derivation of a power counting formula for the soft-collinear effective theory (SCET). This formula constrains which operators are required to correctly describe the infrared at any order in the Lambda_QCD/Q expansion (lambda expansion). The result assigns a unique lambda-dimension to graphs in SCET solely from vertices, is gauge independent, and can be applied independent of the process. For processes with an OPE the lambda-dimension has a correspondence with dynamical twist.Comment: 12 pages, 1 fig, journal versio

Pilot Open Case Series of Voice over Internet Protocol-Delivered Assessment and Behavior Therapy for Chronic Tic Disorders

Comprehensive Behavioral Intervention for Tics (CBIT) is an efficacious treatment for children with chronic tic disorders (CTDs). Nevertheless, many families of children with CTDs are unable to access CBIT due to a lack of adequately trained treatment providers, time commitment, and travel distance. This study established the interrater reliability between in-person and Voice over Internet Protocol (VoIP) administrations of the Yale Global Tic Severity Scale (YGTSS), and examined the preliminary efficacy, feasibility, and acceptability of VoIP-delivered CBIT for reducing tics in children with CTDs in an open case series. Across in-person and VoIP administrations of the YGTSS, results showed mean agreement of 91%, 96%, and 95% for motor, phonic, and total tic severity subscales. In the pilot feasibility study, 4 children received 8 weekly sessions of CBIT via VoIP and were assessed at pre- and posttreatment by an independent evaluator. Results showed a 29.44% decrease in clinician-rated tic severity from pre- to posttreatment on the YGTSS. Two of the 4 patients were considered treatment responders at posttreatment, using Clinical Global Impressions–Improvement ratings. Therapeutic alliance, parent and child treatment satisfaction, and videoconferencing satisfaction ratings were high. CBIT was considered feasible to implement via VoIP, although further testing is recommended

Semileptonic B Decays and Determination of |Vub|

Semileptonic decays of the B mesons provide an excellent probe for the weak and strong interactions of the bottom quark. The large data samples collected at the B Factories have pushed the experimental studies of the semileptonic B decays to a new height and stimulated significant theoretical developments. I review recent progresses in this fast-evolving field, with an emphasis on the determination of the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element |Vub|.Comment: 16 pages, 4 figures, accepted by Mod. Phys. Lett.

Rare radiative exclusive B decays in soft-collinear effective theory

We consider rare radiative B decays such as B -> K^* gamma or B -> rho gamma in soft-collinear effective theory, and show that the decay amplitudes are factorized to all orders in alpha_s and at leading order in Lambda/m_b.By employing two-step matching, we classify the operators for radiative B decays in powers of a small parameter lambda(~ \sqrt{Lambda/m_b}) and obtain the relevant operators to order lambda in SCET_I. These operators are constructed with or without spectator quarks including the four-quark operators contributing to annihilation and W-exchange channels. And we employ SCET_II where the small parameter becomes of order Lambda/m_b, and evolve the operators in order to compute the decay amplitudes for rare radiative decays in soft-collinear effective theory. We show explictly that the contributions from the annihilation channels and the W-exchange channels vanish at leading order in SCET. We present the factorized result for the decay amplitudes in rare radiative B decays at leading order in SCET, and at next-to-leading order in alpha_s.Comment: v2: 31 pages, 11 figures. An appendix is added about the quark mass effects on radiative B decay

Diffeomorphic random sampling using optimal information transport

In this article we explore an algorithm for diffeomorphic random sampling of nonuniform probability distributions on Riemannian manifolds. The algorithm is based on optimal information transport (OIT)---an analogue of optimal mass transport (OMT). Our framework uses the deep geometric connections between the Fisher-Rao metric on the space of probability densities and the right-invariant information metric on the group of diffeomorphisms. The resulting sampling algorithm is a promising alternative to OMT, in particular as our formulation is semi-explicit, free of the nonlinear Monge--Ampere equation. Compared to Markov Chain Monte Carlo methods, we expect our algorithm to stand up well when a large number of samples from a low dimensional nonuniform distribution is needed.Comment: 8 pages, 3 figure

Shape-Function Effects and Split Matching in B-> Xs l+ l-

We derive the triply differential spectrum for the inclusive rare decay B -> Xs l+ l- in the shape function region, in which Xs is jet-like with $mX^2 \lesssim mb \Lambda_QCD$. Experimental cuts make this a relevant region. The perturbative and non-perturbative parts of the matrix elements can be defined with the Soft-Collinear Effective Theory, which is used to incorporate alphas corrections consistently. We show that, with a suitable power counting for the dilepton invariant mass, the same universal jet and shape functions appear as in B-> Xs gamma and B-> Xu l nu decays. Parts of the usual alphas(m_b) corrections go into the jet function at a lower scale, and parts go into the non-perturbative shape function. For B -> Xs l+ l-, the perturbative series in alphas are of a different character above and below mu=mb. We introduce a ``split matching'' method that allows the series in these regions to be treated independently.Comment: 33 pages; journal versio