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Matthias Neubert

Publications and source records attributed to Matthias Neubert.

At least 145 records · Page 8Linked to original sources
arXiv2004

Soft-Collinear Factorization and the Calculation of the B->Xs+gamma Rate

Using results on soft-collinear factorization for inclusive B-meson decay distributions, a systematic study of the partial B->Xs+gamma decay rate with a cut E_gamma Xs+gamma branching ratio with E_gamma<1.8 GeV is Br(B->Xs+gamma)=(3.44+-0.53+-0.35)*10^{-4}, where the errors refer to perturbative and parameter uncertainties, respectively. The implications of larger theory uncertainties for New Physics searches are explored with the example of the type-II two-Higgs-doublet model.

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arXiv2004

The Physics Case for an e^+ e^- Super B-Factory

I share some personal reflections about the physics potential and the physics case that can be made for an e^+ e^- high-luminosity B-meson factory, as presented in my summary talk at the recent "Super B-Factory Workshop" jointly organized by the BaBar and Belle Collaborations (Honolulu, Hawaii, January 2004). These brief remarks will appear as part of a forthcoming, comprehensive report on the physics potential of such a ``10^{36} machine''.

hep-ph↗
arXiv2004

Proposal for a Precision Measurement of |Vub|

A new method for a precision measurement of the CKM matrix element |Vub| is discussed, which combines good theoretical control with high efficiency and a powerful discrimination against charm background. The resulting combined theoretical uncertainty on |Vub| is estimated to be 10%.

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arXiv2003

QCD factorization for B->PP and B->PV decays

A comprehensive study of exclusive hadronic B-meson decays into final states containing two pseudoscalar mesons (PP) or a pseudoscalar and a vector meson (PV) is presented. The decay amplitudes are calculated at leading power in Lambda_{QCD}/m_b and at next-to-leading order in alpha_s using the QCD factorization approach. The calculation of the relevant hard-scattering kernels is completed. Important classes of power corrections, including ``chirally-enhanced'' terms and weak annihilation contributions, are estimated and included in the phenomenological analysis. Predictions are presented for the branching ratios of the complete set of the 96 decays of B^-, B^0, and B_s mesons into PP and PV final states, and for most of the corresponding CP asymmetries. Several decays and observables of particular phenomenological interest are discussed in detail, including the magnitudes of the penguin amplitudes in PP and PV final states, an analysis of the pi-rho system, and the time-dependent CP asymmetry in the K phi and K eta' final states.

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arXiv2003

External Operators and Anomalous Dimensions in Soft-Collinear Effective Theory

It has recently been argued that soft-collinear effective theory for processes involving both soft and collinear partons contains a new soft-collinear mode, which can communicate between the soft and collinear sectors of the theory. The formalism incorporating the corresponding fields into the effective Lagrangian is extended to include external current and four-quark operators relevant to weak interactions. An explicit calculation of the anomalous dimensions of these operators reveals that soft-collinear modes are needed for correctly describing the ultraviolet behavior of the effective theory.

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arXiv2003

Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory

It is argued that soft-collinear effective theory for processes involving both soft and collinear partons, such as exclusive B-meson decays, should include a new mode in addition to soft and collinear fields. These "soft-collinear messengers" can interact with both soft and collinear particles without taking them far off-shell. They thus can communicate between the soft and collinear sectors of the theory. The relevance of the new mode is demonstrated with an explicit example, and the formalism incorporating the corresponding quark and gluon fields into the effective Lagrangian is developed.

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arXiv2003

A mixing-independent construction of the unitarity triangle

The study of charmless hadronic two-body decays of B mesons is one of the most fascinating topics in B physics. A construction of the unitarity triangle based on such decays is presented, which is independent of B-B(bar) and K-K(bar) mixing. It provides stringent tests of the Standard Model with small theoretical uncertainties.

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arXiv2003

Renormalization-Group Evolution of the B-Meson Light-Cone Distribution Amplitude

An integro-differential equation governing the evolution of the leading-order B-meson light-cone distribution amplitude is derived. The anomalous dimension in this equation contains a logarithm of the renormalization scale, whose coefficient is identified with the cusp anomalous dimension of Wilson loops. The exact analytic solution of the evolution equation is obtained, from which the asymptotic behavior of the distribution amplitude is derived. These results can be used to resum Sudakov logarithms entering the hard-scattering kernels in QCD factorization theorems for exclusive B decays.

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arXiv2003

Spectator Interactions in Soft-Collinear Effective Theory

Soft-collinear effective theory is generalized to include soft massless quarks in addition to collinear fields. This extension is necessary for the treatment of interactions with the soft spectator quark in a heavy meson. The power counting of the relevant fields and the construction of the effective Lagrangian are discussed at leading order in Lambda/m_b. Several novel effects occur in the matching of full-theory amplitudes onto effective-theory operators containing soft light quarks, such as the appearance of an intermediate mass scale and large non-localities of operators on scales of order 1/Lambda. Important examples of effective-theory operators with soft light quarks are studied and their renormalization properties explored. The formalism presented here forms the basis for a systematic analysis of factorization and power corrections for any exclusive B-meson decay into light particles.

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arXiv2002

Radiative B Decays: Standard Candles of Flavor Physics

Rare radiative decays based on b->s+gamma transitions are among the most prominent examples of flavor-changing neutral current processes. They benefit from good theoretical control and experimental accessibility, large sensitivity to physics beyond the Standard Model, and the availability of many observables. In this talk I summarize the status of the theoretical understanding of these decays and review how they may be used to constrain extensions of the Standard Model, with particular focus on supersymmetric models.

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arXiv2002

Flavor-Singlet B-Decay Amplitudes in QCD Factorization

Exclusive hadronic B-meson decays into two-body final states consisting of a light pseudoscalar or vector meson along with an eta or eta' meson are of great phenomenological interest. Their theoretical analysis involves decay mechanisms that are unique to flavor-singlet states, such as their coupling to gluons or their ``intrinsic charm'' content. These issues are studied systematically in the context of QCD factorization and the heavy-quark expansion. Theory can account for the experimental data on the B->K^{(*)} eta^{(')} branching fractions, albeit within large uncertainties.

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arXiv2002

Constraints on Light Bottom Squarks from Radiative B-Meson Decays

The presence of a light b-squark (with mass about 4 GeV) and gluino (with mass about 15 GeV) might explain the observed excess in b-quark production at the Tevatron. Though provocative, this model is not excluded by present data. The light supersymmetric particles can induce large flavor-changing effects in radiative decays of B mesons. We analyse the decays B->X_s gamma and B->X_{sg} in this scenario and derive restrictive bounds on the flavor-changing quark-squark-gluino couplings.

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arXiv2002

Subleading Shape Functions and the Determination of |V_{ub}|

It is argued that the dominant subleading shape-function contributions to the endpoint region of the charged-lepton energy spectrum in B -> X_u l nu decays can be related in a model-independent way to an integral over the B -> X_s gamma photon spectrum. The square root of the fraction of B -> X_u l nu events with charged-lepton energy above E_0=2.2 GeV can be calculated with a residual theoretical uncertainty from subleading shape-function effects that it safely below the 10% level. These effects have therefore a minor impact on the determination of |V_{ub}|.

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arXiv2002

Two-body modes of B mesons and the CP-b triangle

The study of charmless two-body decays of B mesons is currently one of the hottest topics in B physics. QCD factorization provides the theoretical framework for a systematic analysis of such decays. A global fit to B->pi K,pi pi branching fractions, combined with knowledge on |V(ub)|, establishes the existence of a CP-violating phase in the bottom sector of the CKM matrix and tends to favor values of gamma near 90 degrees, somewhat larger than those suggested by the standard analysis of the unitarity triangle. A novel construction of the unitarity triangle is presented, which is independent of B-B(bar) and K-K(bar) mixing. It can provide stringent tests of the Standard Model with small theoretical uncertainties.

hep-ph↗
arXiv2002

Isospin Breaking in B -> K^* gamma Decays

A calculation of the leading isospin-breaking contributions to the B -> K^* gamma decay amplitudes based on the QCD factorization approach is presented. They arise at order Lambda/m_b in the heavy-quark expansion and are due to annihilation contributions from 4-quark operators, the chromo-magnetic dipole operator, and charm penguins. In the Standard Model the decay rate for B^0 -> K^{*0} gamma is predicted to be about 10-20 % larger than that for B^- -> K^{*-} gamma. Isospin-breaking effects are a sensitive probe of the penguin sector of the effective weak Hamiltonian. New Physics models in which the hierarchy of B -> K^* gamma decay rates is either flipped or greatly enhanced could be ruled out with more precise data.

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arXiv2002

Higher-Order Corrections to QCD Factorization in B->pi K,pi pi Decays

The renormalon calculus is used to calculate the terms of order $β_0^{n-1}α_s^n$ in the perturbative expansions of the Wilson coefficients and hard-scattering kernels entering the QCD factorization formula for hadronic B-meson decays into two light pseudoscalar mesons. The asymptotic behavior of the expansions is analyzed, and a minimal model of power corrections arising from soft ``non-factorizable'' gluon exchange to the B->pi K,pi pi decay amplitudes is obtained, which takes into account the structure of the leading and subleading infrared renormalon singularities. Whereas the resulting power corrections are generally very small, some of the strong-interaction phases of the hard-scattering kernels receive sizeable two-loop corrections. The implications of these findings on CP asymmetries and branching ratios are investigated.

hep-ph↗
arXiv2001

Flavor-Change with Ultra-Light Sbottom and Gluinos

Implications of a 2 - 5.5 GeV sbottom and 12 - 16 GeV gluino masses for rare B decay phenomenology are discussed. An effective Hamiltonian is constructed in which the gluinos are integrated out and a \tilde{b} squark remains among the light flavor degrees of freedom. Restrictive constraints come from $b \to s γand b \to sg, but they allow a substantially enhanced inclusive b decay rate into charmless hadronic final states, and {\cal O}(10%) direct CP asymmetries in $B \to X_s γand B^\pm \to K^0 π^\pm decays, which are an order of magnitude larger than in the Standard Model. New contributions to B_s mixing are negligible but significant effects in B_d mixing may be possible.

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