arXiv subjects

Mark B. Wise

Publications and source records attributed to Mark B. Wise.

At least 91 records · Page 5Linked to original sources

arXiv2004

Anthropic Distribution for Cosmological Constant and Primordial Density Perturbations

The anthropic principle has been proposed as an explanation for the observed value of the cosmological constant. Here we revisit this proposal by allowing for variation between universes in the amplitude of the scale-invariant primordial cosmological density perturbations. We derive a priori probability distributions for this amplitude from toy inflationary models in which the parameter of the inflaton potential is smoothly distributed over possible universes. We find that for such probability distributions, the likelihood that we live in a typical, anthropically-allowed universe is generally quite small.

hep-th↗

arXiv2004

Gradient instability for w < -1

We show that in single scalar field models of the dark energy with equations of state satisfying $w \equiv p / ρ< -1$, the effective Lagrangian for fluctuations about the homogeneous background has a wrong sign spatial kinetic term. In most cases, spatial gradients are ruled out by microwave background observations. The sign of $w+1$ is not connected to the sign of the time derivative kinetic term in the effective Lagrangian.

astro-ph↗

arXiv2004

Enhanced Nonperturbative Effects in Z Decays to Hadrons

We use soft collinear effective field theory (SCET) to study nonperturbative strong interaction effects in Z decays to hadronic final states that are enhanced in corners of phase space. These occur, for example, in the jet energy distribution for two jet events near E_J=M_Z/2, the thrust distribution near unity and the jet invariant mass distribution near zero. The extent to which such nonperturbative effects for different observables are related is discussed.

hep-ph↗

arXiv2004

Stable Heavy Pentaquark States

If the Theta^+(1540) is interpreted as a bound state of a $\bar s$ quark and two (ud) diquarks in a relative P-wave, then it is very likely that there exist pentaquark states with a heavy antiquark, $\bar b$ or $\bar c$, and two ``light'' diquarks in a relative S-wave which are stable against strong decays. We make a mass estimate for exotic states of this type and discuss their weak decays. Isospin relations are constructed which test their flavor quantum numbers.

hep-ph↗

arXiv2004

Predictions for nonleptonic Lambda_b and Theta_b decays

We study nonleptonic Lambda_b -> Lambda_c pi, Sigma_c pi and Sigma_c^* pi decays in the limit m_b, m_c, E_pi >> Lambda_{QCD} using the soft-collinear effective theory. Here Sigma_c = Sigma_c(2455) and Sigma_c^* = Sigma_c(2520). At leading order the Lambda_b -> Sigma_c^{(*)} pi rates vanish, while the Lambda_b -> Lambda_c pi rate is related to Lambda_b -> Lambda_c\ell\barν, and is expected to be larger than Gamma(B -> D^{(*)} pi). The dominant contributions to the Lambda_b -> Sigma_c^{(*)} pi rates are suppressed by Lambda_{QCD}^2/E_pi^2. We predict Gamma(Lambda_b -> Sigma_c^* pi) / Gamma(Lambda_b -> Sigma_c pi) = 2 + O[Lambda_{QCD}/m_Q, alpha_s(m_Q)], and the same ratio for Lambda_b -> Sigma_c^{(*)} rho and for Lambda_b -> Xi_c^{(',*)}K. ``Bow tie'' diagrams are shown to be suppressed. We comment on possible discovery channels for weakly decaying pentaquarks, Theta_{b,c} and their nearby heavy quark spin symmetry partners, Theta_{b,c}^*.

hep-ph↗

arXiv2003

Comment on Quark Masses in SCET

Quark masses are included in the SCET Lagrangian. Treating the strange quark mass as order Lambda_{QCD}, we find that strange quark mass terms are suppressed in SCET1, but are leading order in SCET2. This is relevant for B decays to K^* and K. Strange quark mass effects in semileptonic and weak radiative form factors are studied. They give corrections to the form factors that are not suppressed by powers of the bottom quark mass, or, equivalently, by the large recoil energy of the final state meson, and and preserve the heavy to light form factor relations that follow from using the leading order current.

hep-ph↗

arXiv2003

Enhanced nonperturbative effects in jet distributions

We consider the triple differential distribution dΓ/(dE_J)(dm_J^2)(dΩ_J) for two-jet events at center of mass energy M, smeared over the endpoint region m_J^2 << M^2, |2 E_J -M| ~ Δ, \lqcd << Δ<< M. The leading nonperturbative correction, suppressed by \lqcd/Δ, is given by the matrix element of a single operator. A similar analysis is performed for three jet events, and the generalization to any number of jets is discussed. At order \lqcd/Δ, non-perturbative effects in four or more jet events are completely determined in terms of two matrix elements which can be measured in two and three jet events.

hep-ph↗

arXiv2003

Debt Subordination and The Pricing of Credit Default Swaps

First passage models, where corporate assets undergo a random walk and default occurs if the assets fall below a threshold, provide an attractive framework for modeling the default process. Recently such models have been generalized to allow a fluctuating default threshold or equivalently a fluctuating total recovery fraction $R$. For a given company a particular type of debt has a recovery fraction $R_i$ that is greater or less than $R$ depending on its level of subordination. In general the $R_i$ are functions of $R$ and since, in models with a fluctuating default threshold, the probability of default depends on $R$ there are correlations between the recovery fractions $R_i$ and the probability of default. We find, using a simple scenario where debt of type $i$ is subordinate to debt of type $i-1$, the functional dependence $R_i(R)$ and explore how correlations between the default probability and the recovery fractions $R_i(R)$ influence the par spreads for credit default swaps. This scenario captures the effect of debt cushion on recovery fractions.

cond-mat↗

arXiv2002

Diversification and Gerneralized Tracking Errors For Correlated Non-Normal Returns

The probability distribution for the relative return of a portfolio constructed from a subset n of the assets from a benchmark, consisting of N assets whose returns are multivariate normal, is completely characterized by its tracking error. However, if the benchmark asset returns are not multivariate normal then higher moments of the probability distribution for the portfolio's relative return are not related to its tracking error. We discuss the convergence of generalized tracking error measures as the size of the subset of benchmark assets increases. Assuming that the joint probability distribution for the returns of the assets is symmetric under their permutations we show that increasing n makes these generalized tracking errors small (even though n< >1 the probability distribution for the portfolio's relative return is approximately symmetric and strongly peaked about the origin. The results of this paper generalize the conclusions of Dynkin et. al. (2002) to more general underlying asset distributions.

cond-mat↗

arXiv2002

Implications of Correlated Default For Portfolio Allocation To Corporate Bonds

This article deals with the problem of optimal allocation of capital to corporate bonds in fixed income portfolios when there is the possibility of correlated defaults. Using a multivariate normal Copula function for the joint default probabilities we show that retaining the first few moments of the portfolio default loss distribution gives an extremely good approximation to the full solution of the asset allocation problem. We provide detailed results on the convergence of the moment expansion and explore how the optimal portfolio allocation depends on recovery fractions, level of diversification and investment time horizon. Numerous numerical illustrations exhibit the results for simple portfolios and utility functions.

nlin.AO↗

arXiv2002

Portfolio Allocation to Corporate Bonds with Correlated Defaults

This article deals with the problem of optimal allocation of capital to corporate bonds in fixed income portfolios when there is the possibility of correlated defaults. Under fairly general assumptions for the distribution of the total net assets of a set of firms we show that retaining the first few moments of the portfolio default loss distribution gives an extremely good approximation to the full solution of the asset allocation problem. We provide detailed results on the convergence of the moment expansion. We also provide explicit results for the inverse problem, i.e. for a given allocation to the set of risky bonds, what is the average risk premium required to make the portfolio optimal. Numerous numerical illustrations exhibit the results for realistic portfolios and utility functions.

nlin.AO↗

arXiv2002

Enhanced subleading structure functions in semileptonic B decay

The charged lepton spectrum in semileptonic $B\to X_u \ell\barν$ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of $Λ_{QCD}/m_b$ but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width $ΔE_\ell \sim Λ_{QCD}$ near the endpoint the contributions proportional to $δ(E_\ell - m_b/2)$ times (i) the matrix element of the chromomagnetic operator, and (ii) four-quark operators. These contribute to the total rate at the few percent level, but affect the endpoint region much more significantly. Implications for the determination of $|V_{ub}|$ are discussed.

hep-ph↗

arXiv2002

Hadronic Light-by-Light Contribution to Muon g-2 in Chiral Perturbation Theory

We compute the hadronic light-by-light scattering contributions to the muon anomalous magnetic moment, $\amulbl$, in chiral perturbation theory that are enhanced by large logarithms and a factor of $N_C$. They depend on a low-energy constant entering pseudoscalar meson decay into a charged lepton pair. The uncertainty introduced by this constant is $\pm 60\times 10^{-11}$, which is comparable in magnitude to the present uncertainty entering the leading-order vacuum polarization contributions to the anomalous moment. It may be reduced to some extent through an improved measurement of the $π^0\to e^+ e^-$ branching ratio. However, the dependence of $\amulbl$ on non-logarithmically enhanced effects cannot be constrained except through the measurement of the anomalous moment itself. The extraction of information on new physics would require a future experimental value for the anomalous moment differing significantly from the 2001 result reported by the E821 collaboration.

hep-ph↗

arXiv2001

Recent Progress in Heavy Quark Physics

Some of the recent progress in heavy quark physics is reviewed. Special attention is paid to inclusive methods for determining Vub and factorization in nonleptonic B decays. Theoretical predictions for top-antitop production near threshold are also discussed.

hep-ph↗

arXiv2001

The Long Range Gravitational Potential Energy Between Strings

We calculate the gravitational potential energy between infinitely long parallel strings with tensions τ_1 and τ_2. Classically, it vanishes, but at one loop, we find that the long range gravitational potential energy per unit length is U/L = 24G_N^2τ_1τ_2/(5 πa^2) + ..., where a is the separation between the strings, G_N is Newton's constant, and we set \hbar = c =1. The ellipses represent terms suppressed by more powers of G_N τ_i. Typically, massless bulk fields give rise at one loop to a long range potential between p-branes in space-times of dimension p+2+1. The contribution to this potential from bulk scalars is computed for arbitrary p (strings correspond to p=1) and in the case of three-branes its possible relevance for cosmological quintessence is commented on.

hep-th↗

arXiv2001

Forecasting Portfolio Risk in Normal and Stressed Markets

The instability of historical risk factor correlations renders their use in estimating portfolio risk extremely questionable. In periods of market stress correlations of risk factors have a tendency to quickly go well beyond estimated values. For instance, in times of severe market stress, one would expect with certainty to see the correlation of yield levels and credit spreads go to -1, even though historical estimates will miss this region of correlation. This event might lead to realized portfolio risk profile substantially different from what was initially estimated. The purpose of this paper is to explore the correlation driven effects on fixed income portfolio risks. To achieve this, we propose a methodology to estimate portfolio risks in both normal and stressed times using confidence weighted forecast correlations.

nlin.AO↗

arXiv2001

Renormalization Group Flows for Brane Couplings

Field theories in the presence of branes encounter localized divergences that renormalize brane couplings. The sources of these brane-localized divergences are understood as arising either from broken translation invariance, or from short distance singularities as the brane thickness vanishes. While the former are generated only by quantum corrections, the latter can appear even at the classical level. Using as an example six-dimensional scalar field theory in the background of a 3-brane, we show how to interpret such classical divergences by the usual regularization and renormalization procedure of quantum field theory. In our example, the zero thickness divergences are logarithmic, and lead classically to non-trivial renormalization group flows for the brane couplings. We construct the tree level renormalization group equations for these couplings as well as the one-loop corrections to these flows from bulk-to-brane renormalization effects.

hep-th↗

arXiv2001

Supersymmetric Correction to Top Quark Pair Production near Threshold

We studied the leading supersymmetric contribution to top-antitop threshold production using the NRQCD framework. The one-loop matching to the potential and the Wilson coefficient of the leading production current were considered. We point out that the leading correction to the potential is zero due to SU(3)_c gauge invariance. This is true in general for any new physics that enters above the electroweak scale. The shape of the top quark pair production cross section is therefore almost unaffected near threshold, allowing a precise determination of the top quark mass based on the Standard Model calculations. The supersymmetric correction to the Wilson coefficient c_1 of the production current decouples for heavy super particles. Its contribution is smaller than the Standard Model next-to-next-leading-log results.

hep-ph↗