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Christian W. Bauer

Publications and source records attributed to Christian W. Bauer.

101 records · Page 6Linked to original sources
arXiv2001

A proof of factorization for B -> D pi

We prove that the matrix elements of four fermion operators mediating the decay B^0 -> D^+ π^- and B^- -> D^0 π^- factor into the product of a form factor describing the B -> D transition and a convolution of a short distance coefficient with the nonperturbative pion light-cone wave function. This is shown to all orders in alpha_s, up to corrections suppressed by factors of 1/mb, 1/mc, and 1/E_pi. It is not necessary to assume that the pion state is dominated by the q-qbar Fock state.

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arXiv2001

Invariant Operators in Collinear Effective Theory

We consider processes which produce final state hadrons whose energy is much greater than their mass. In this limit interactions involving collinear fermions and gluons are constrained by a symmetry, and we give a general set of rules for constructing leading and subleading invariant operators. Wilson coefficients C(mu,P) are functions of a label operator P, and do not commute with collinear fields. The symmetry is used to reproduce a two-loop result for factorization in B -> D pi in a simple way.

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arXiv2001

An effective field theory for collinear and soft gluons: heavy to light decays

We construct the Lagrangian for an effective theory of highly energetic quarks with energy Q, interacting with collinear and soft gluons. This theory has two low energy scales, the transverse momentum of the collinear particles, p_perp, and the scale p_perp^2/Q. The heavy to light currents are matched onto operators in the effective theory at one-loop and the renormalization group equations for the corresponding Wilson coefficients are solved. This running is used to sum Sudakov logarithms in inclusive B -> X_s gamma and B -> X_u \ell nu decays. We also show that the interactions with collinear gluons preserve the relations for the soft part of the form factor for heavy to light decays found by Charles et al., establishing these relations in the large energy limit of QCD.

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arXiv2001

Precision determination of |V_{ub}| from inclusive decays

We propose determining |V_{ub}| from inclusive semileptonic B decay using combined cuts on the leptonic and hadronic invariant masses to eliminate the b->c background. Compared to a pure dilepton invariant mass cut, the uncertainty from unknown order (Λ_{QCD}/m_b)^3 terms in the OPE is significantly reduced and the fraction of b->u events is roughly doubled. Compared to a pure hadronic invariant mass cut, the uncertainty from the unknown light-cone distribution function of the b quark is significantly reduced. We find that |V_{ub}| can be determined with theoretical uncertainty at the 5-10% level.

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arXiv2001

Resumming the color-octet contribution to radiative Upsilon decay

At the upper endpoint of the photon energy spectrum in Upsilon -> X gamma, the standard NRQCD power counting breaks down and the OPE gives rise to color-octet structure functions. Furthermore, in this kinematic regime large Sudakov logarithms appear in the octet Wilson coefficients. The endpoint spectrum can be treated consistently within the framework of a recently developed effective field theory of collinear and soft particles. Here we show that within this approach the octet structure functions arise naturally and that Sudakov logarithms can be summed using the renormalization group equations. We derive an expression for the resummed energy spectrum and, using a model lightcone structure function, investigate the phenomenological importance of the resummation.

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arXiv2001

Light-cone distribution functions for B decays at subleading order in 1/mb

We construct the higher twist structure functions that describe inclusive b hadron decays in regions of phase space where the hadronic decay products carry high energy but have low invariant mass. We show that, for B meson decays, there are four new non-vanishing matrix elements of non-local operators. We show that to subleading twist these decays are parametrized in terms of four functions. We compute the tree-level matching for a general heavy-to-light current and apply it to B -> X_s gamma. Using a simple model for these functions we estimate the subleading twist contributions to this decay.

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arXiv2001

Determining $|V_{ub}|$ from the $\bar B\to X_u\ell\barν$ dilepton invariant mass spectrum

The invariant mass spectrum of the lepton pair in inclusive semileptonic $\bar B\to X_u \ell\barν$ decay yields a model independent determination of $|V_{ub}|$. Unlike the lepton energy and hadronic invariant mass spectra, nonperturbative effects are only important in the resonance region, and play a parametrically suppressed role when $dΓ/ dq^2$ is integrated over $q^2 > (m_B-m_D)^2$, which is required to eliminate the $\bar B\to X_c \ell\barν$ background. We discuss these backgrounds for $q^2$ slightly below $(m_B-m_D)^2$, and point out that instead of $q^2 > (m_B-m_D)^2 = 11.6 GeV^2$, the cut can be lowered to $q^2 \gtrsim 10.5 GeV^2$. This is important experimentally, particularly when effects of a finite neutrino reconstruction resolution are included.

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arXiv2000

Nonperturbative corrections to moments of the decay B -> X_s l^+ l^-

We study nonperturbative corrections to the inclusive rare decay B -> X_s l^+ l^- by performing an operator product expansion (OPE) to O(1/m_b^3). The values of the matrix elements entering at this order are unknown and introduce uncertainties into physical quantities. We study uncertainties introduced into the partially integrated rate, moments of the hadronic spectrum, as well as the forward-backward asymmetry. We find that for large dilepton invariant mass q^2 > M_{ψ'}^2 these uncertainties are large. We also assess the possibility of extracting the HQET parameters λ_1 and \barΛ using data from this process.

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arXiv2000

On |V_{ub}| from the B -> X_u l νdilepton invariant mass spectrum

The invariant mass spectrum of the lepton pair in inclusive semileptonic $\bar B\to X_u \ell\barν$ decay yields a model independent determination of $|V_{ub}|$. Unlike the lepton energy and hadronic invariant mass spectra, nonperturbative effects are only important in the resonance region, and play a parametrically suppressed role when $dΓ/ dq^2$ is integrated over $q^2 > (m_B-m_D)^2$, which is required to eliminate the $\bar B\to X_c \ell\barν$ background. We discuss these backgrounds for $q^2$ slightly below $(m_B-m_D)^2$, and point out that instead of $q^2 > (m_B-m_D)^2 = 11.6 GeV^2$, the cut can be lowered to $q^2 \gtrsim 10.5 GeV^2$. This is important experimentally, particularly when effects of a finite neutrino reconstruction resolution are included.

hep-ph↗
arXiv2000

A Model Independent Determination of |V_{ub}|

It is shown that measuring the lepton invariant mass spectrum in inclusive semileptonic B -> X_u l nu decay yields a model independent determination of |V_{ub}|. Unlike the lepton energy and hadronic invariant mass spectra, nonperturbative effects are only important in the resonance region, and play a parametrically suppressed role when dΓ/dq^2 is integrated over q^2 > (m_B-m_D)^2, which is required to eliminate the charm background. Perturbative and nonperturbative corrections are presented to order α_s^2β_0 and Λ_{QCD}^2/m_b^2, and the Λ_{QCD}^3/m_b^3 corrections are used to estimate the uncertainty in our results. The utility of the B -> X_s l^+ l^- decay rate above the ψ(2S) resonance is discussed.

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arXiv1999

Nonperturbative corrections to B -> X_s l^+ l^- with phase space restrictions

We study nonperturbative corrections up to O(1/m_b^3) in the inclusive rare B decay B -> X_s l^+ l^- by performing an operator product expansion. The values of the matrix elements entering at this order are unknown and introduce uncertainties into physical quantities. Imposing a phase space cut to eliminate the $c \bar{c}$ resonances we find that the O(1/m_b^3) corrections introduce an O(10%) uncertainty in the measured rate. We also find that the contributions arising at O(1/m_b^3) are comparable to the ones arising at O(1/m_b^2) over the entire region of phase space.

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