arXiv subjects

Iain W. Stewart

Publications and source records attributed to Iain W. Stewart.

At least 145 records · Page 8Linked to original sources

arXiv2004

Heavy Quark Symmetry in Isosinglet Nonleptonic B-Decays

We use a factorization theorem from the soft-collinear effective theory along with heavy quark symmetry to make model independent predictions for B-> D^(*)M where M={eta,eta',phi, omega}. Gluon production of these isosinglet mesons is included. We predict the equality of branching fractions in the B-> DM and B-> D*M channels, with corrections at order LambdaQCD/Q and alphas(Q) where Q=m_b,m_c, or E_M. We also predict that Br(B^0-> D eta')/Br(B^0-> D eta)=tan^2(theta) = 0.67 and Br(B-> D phi)/Br(B-> D omega) < 0.2, where here there are also alphas(sqrt{E*Lambda}) corrections. These results agree well with the available data. A test for SU(3) violation in these decays is constructed.

hep-ph↗

arXiv2004

B -> M1 M2: Factorization, Charming Penguins, Strong Phases, and Polarization

Using the soft-collinear effective theory we derive the factorization theorem for the decays B-> M1 M2 with M{1,2}= pi,K, rho,K*, at leading order in Lambda/E_M and Lambda/mb. The results derived here apply even if alpha_s(E_M Lambda) is not perturbative, and we prove that the physics sensitive to the E*Lambda scale is the same in B-> M1 M2 and B-> M form factors. We argue that c-cbar penguins could give long-distance effects at leading order. Decays to two transversely polarized vector mesons are discussed. Analyzing B-> pi pi we find predictions for B^0 -> pi^0 pi^0 and |V_{ub}| f_+(0) as a function of gamma.

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

Factorization and the Soft-Collinear Effective Theory: Color-Suppressed Decays

We discuss the soft-collinear effective theory (SCET) and kinematic expansions in B-decays, focusing on recent results for color suppressed B -> D^(*) X decays. In particular we discuss model independent predictions for B^0 -> D^0 pi^0 and B^0 -> D^*0 pi^0, and update the comparison using new experimental data. We show why HQET alone is insufficient to give these results. SCET predictions are also reviewed for other B and Lambda_b decay channels that are not yet tested by data.

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

Model Independent Results for SU(3) Violation in Light-Cone Distribution Functions

Using chiral symmetry we investigate the leading SU(3) violation in light-cone distribution functions phi_M(x) of the pion, kaon, and eta. It is shown that terms non-analytic in the quark masses do not affect the shape, and only appear in the decay constants. Predictive power is retained including the leading analytic m_q operators. With the symmetry violating corrections we derive useful model independent relations between phi_pi, phi_eta, and phi_K. Using the soft-collinear effective theory we discuss how factorization generates the subleading chiral coefficients.

hep-ph↗

arXiv2003

The phenomenology of rare and semileptonic B decays

We summarize recent progress in the theory of exclusive rare and semileptonic B decays, focusing on model-independent results. The heavy-to-light form factors parameterizing these decays admit a model-independent description in two distinct kinematical regions. In the large-energy limit of an energetic light meson, the Soft-Collinear Effective Theory can be used to prove factorization formulas for the form factors. We present factorization formulas for all B-> P,V form factors at leading order in Lambda/m_b. Near the zero-recoil point, Heavy Quark Effective Theory gives useful relations among the form factors of different currents.

hep-ph↗

arXiv2003

Theoretical Introduction to B Decays and the Soft-Collinear Effective Theory

In this talk I give an introduction to methods used to give a model independent description of QCD effects in B-decays. I discuss three useful expansions: mb/mW << 1 for the electroweak Hamiltonian, LambdaQCD/mb << 1 for Heavy Quark Effective theory, and LambdaQCD/Q << 1 for the Soft-Collinear Effective Theory where Q ~ mb is the large energy of light final state hadrons. I discuss predictions that can be made with each expansion.

hep-ph↗

arXiv2003

Strong Phases and Factorization for Color Suppressed Decays

We prove a factorization theorem in QCD for the color suppressed decays B0-> D0 M0 and B0-> D*0 M0 where M is a light meson. Both the color-suppressed and W-exchange/annihilation amplitudes contribute at lowest order in LambdaQCD/Q where Q={mb, mc, Epi}, so no power suppression of annihilation contributions is found. A new mechanism is given for generating non-perturbative strong phases in the factorization framework. Model independent predictions that follow from our results include the equality of the B0 -> D0 M0 and B0 -> D*0 M0 rates, and equality of non-perturbative strong phases between isospin amplitudes, delta(DM) = delta(D*M). Relations between amplitudes and phases for M=pi,rho are also derived. These results do not follow from large Nc factorization with heavy quark symmetry.

hep-ph↗

arXiv2003

On Power Suppressed Operators and Gauge Invariance in SCET

The form of collinear gauge invariance for power suppressed operators in the soft-collinear effective theory is discussed. Using a field redefinition we show that it is possible to make any power suppressed ultrasoft-collinear operators invariant under the original leading order gauge transformations. Our manipulations avoid gauge fixing. The Lagrangians to O(lambda^2) are given in terms of these new fields. We then give a simple procedure for constructing power suppressed soft-collinear operators in SCET_II by using an intermediate theory SCET_I.

hep-ph↗

arXiv2003

A Complete Basis for Power Suppressed Collinear-Ultrasoft Operators

We construct operators that describe power corrections in mixed collinear-ultrasoft processes in QCD. We treat the ultrasoft-collinear Lagrangian to O(lambda^2), and heavy-to-light currents involving collinear quarks to O(λ) including new three body currents. A complete gauge invariant basis is derived which has a full reduction in Dirac structures and is valid for matching at any order in alpha_s. The full set of reparameterization invariance (RPI) constraints are included, and are found to restrict the number of parameters appearing in Wilson coefficients and rule out some classes of operators. The QCD ultrasoft-collinear Lagrangian has two O(lambda^2) operators in its gauge invariant form. For the O(lambda) heavy-to-light currents there are (4,4,14,14,21) subleading (scalar, pseudo-scalar, vector, axial-vector, tensor) currents, where (1,1,4,4,7) have coefficients that are not determined by RPI. In a frame where v_perp=0 and n.v =1 the total number of currents reduces to (2,2,8,8,13), but the number of undetermined coefficients is the same. The role of these operators and universality of jet functions in the factorization theorem for heavy-to-light form factors is discussed.

hep-ph↗

arXiv2003

Factorization and Endpoint Singularities in Heavy-to-Light decays

We prove a factorization theorem for heavy-to-light form factors. Our result differs in several important ways from previous proposals. A proper separation of scales gives hard kernels that are free of endpoint singularities. A general procedure is described for including soft effects usually associated with the tail of wavefunctions in hard exclusive processes. We give an operator formulation of these soft effects using the soft-collinear effective theory, and show that they appear at the same order in the power counting as the hard spectator contribution.

hep-ph↗

arXiv2002

Factorization, Effective Field Theory, and B-> D^(*) X Decays

In this proceedings I review the soft-collinear effective theory (SCET), an effective theory for energetic particles. I also discuss factorization in exclusive and inclusive B-> D^(*)X decays, and tests which can help distinguish whether factorization is a result of a large energy limit, the large N_c limit, or a combination of the two.

hep-ph↗

arXiv2002

Ultrasoft Renormalization in Non-Relativistic QCD

For Non-Relativistic QCD the velocity renormalization group correlates the renormalization scales for ultrasoft, potential and soft degrees of freedom. Here we discuss the renormalization of operators by ultrasoft gluons. We show that renormalization of soft vertices can induce new operators, and also present a procedure for correctly subtracting divergences in mixed potential-ultrasoft graphs. Our results affect the running of the spin-independent potentials in QCD. The change for the NNLL t-tbar cross section near threshold is very small, being at the 1% level and essentially independent of the energy. We also discuss implications for analyzing situations where mv^2 ~ Lambda_QCD.

hep-ph↗

arXiv2002

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.

hep-ph↗

arXiv2002

The Soft-Collinear Effective Theory

In this talk I review an effective field theory that has been developed for processes with energetic hadrons, referred to as the Soft-Collinear Effective Theory. After outlining properties of the effective theory I briefly discuss its application to the processes B -> D pi and B -> Xs gamma.

hep-ph↗

arXiv2002

Hard Scattering Factorization from Effective Field Theory

In this paper we show how gauge symmetries in an effective theory can be used to simplify proofs of factorization formulae in highly energetic hadronic processes. We use the soft-collinear effective theory, generalized to deal with back-to-back jets of collinear particles. Our proofs do not depend on the choice of a particular gauge, and the formalism is applicable to both exclusive and inclusive factorization. As examples we treat the pi-gamma form factor (gamma gamma* -> pi^0), light meson form factors (gamma* M -> M), as well as deep inelastic scattering (e- p -> e- X), Drell-Yan (p pbar -> X l+ l-), and deeply virtual Compton scattering (gamma* p -> gamma(*) p).

hep-ph↗

arXiv2002

Reparameterization Invariance for Collinear Operators

We discuss restrictions on operators in the soft-collinear effective theory (SCET) which follow from the ambiguity in the decomposition of collinear momenta and the freedom in the choice of light-like basis vectors $n$ and $\bar n$. Invariance of SCET under small changes in $n$ and/or $\bar n$ implies a symmetry of the effective theory that constrains the form of allowed operators with collinear fields. The restrictions occur at a given order in the power counting as well as between different orders. As an example, we present the complete set of higher order operators that are related to the collinear quark kinetic term.

hep-ph↗