arXiv subjects

Dan Pirjol

Publications and source records attributed to Dan Pirjol.

At least 109 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

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

Rare radiative B decays in perturbative QCD

We report on recent progress on perturbative QCD calculations of certain exclusive rare weak $B$ meson decays involving hard photons. In the limit of a photon energy $E_γ$ much larger than $Λ_{QCD}$, the amplitudes for such processes can be analyzed in a twist expansion in powers of $Λ/E_γ$. The leading twist amplitude is given by the convolution of a hard scattering amplitude with the $B$ meson light-cone wavefunction. This approach is applied to a calculation of the leptonic radiative $B\to γ\ell ν_\ell$ formfactors and to an estimate of the weak annihilation contribution to the penguin decays $B\to ρ(ω)γ$. As an application we discuss a few methods for constraining the unitarity triangle with exclusive radiative B decays.

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arXiv2000

Long-distance effects in $B\to Vγ$ radiative weak decays

A systematic approach to long-distance effects in exclusive radiative weak $B$ decays is presented, based on a combination of the heavy quark limit with perturbative QCD. The dominant long-distance effects, connected with weak annihilation and $W$-exchange topologies, can be computed in a model-independent way using experimental data on $B$ radiative leptonic decays. Nonfactorizable corrections vanish in the chiral limit and to leading twist. The left-handed photon amplitudes are shown to be enhanced relative to the right-handed ones, both in the long- and short-distance parts of the $\bar B$ decay amplitudes. We discuss the implications of our results for the extraction of $|V_{td}|$.

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arXiv2000

Bound on $\cosα$ from exclusive weak radiative B decays

We present a bound on the weak phase $α$ from isospin-breaking effects in weak radiative decays, which requires the CP-averaged branching ratios for the weak radiative decays $B^\pm\to ρ^\pm γ$, $B^0\to ρ^0/ωγ$, $B\to K^* γ$ and the photon energy spectrum in $B\toγ\ellν_\ell$. We carefully identify all sources of isospin breaking, which could possibly mask information about the CKM parameters. They are introduced by diagrams with photon bremsstrahlung off the spectator quark and diagrams with annihilation penguin topologies. The former can be eliminated by combining $B\toργ$ and $B\to K^{*}γ$ data, whereas the latter effects are OZI-suppressed and can be controlled by measuring also the modes $B_s\to ρ(ω)γ$. The resulting bound excludes values of $α$ around $90^\circ$, provided that the combined ratio ${\cal B}(B^\pm \to ρ^\pmγ)/{\cal B}(B^0 \to ρ^0/ωγ) \times {\cal B}(B^0 \to K^{*0}γ)/{\cal B}(B^\pm \to K^{*\pm}γ)$ is found to be different from 1.

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arXiv2000

Extracting and using photon polarization information in radiative B decays

We discuss the uses of conversion electron pairs for extracting photon polarization information in weak radiative B decays. Both cases of leptons produced through a virtual and real photon are considered. Measurements of the angular correlation between the $(Kπ)$ and $(e^+e^-)$ decay planes in $B\to K^*(\to Kπ)γ^{(*)}(\to e^+e^-)$ decays can be used to determine the helicity amplitudes in the radiative $B\to K^*γ$ decay. A large right-handed helicity amplitude in $\bar B$ decays is a signal of new physics. The time-dependent CP asymmetry in the $B^0$ decay angular correlation is shown to measure $\sin 2β$ and $\cos 2β$ with little hadronic uncertainty.

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arXiv2000

Extrapolating SU(3) breaking effects from D to B decays

We consider two SU(3) breaking parameters, $R_1(m_B)$ and $R_2(m_B)$, appearing in a relation between $B^+\to Kπ$ and $B^+\toππ$ amplitudes, which plays an important role in determining the weak phase $γ$. We identify an isospin-related quantity $R_2(m_D)$ measured in $D$ decays, exhibiting large SU(3) breaking which is likely due to nonfactorizable effects. With a cautious remark about possible nonfactorizable SU(3) breaking in $B$ decays, we proceed to calculate factorizable SU(3) breaking corrections. Applying heavy quark symmetry to semileptonic $D$ and B decay form factors, we find that SU(3) breaking in $R_2(m_B)/R_1(m_B)$ may be significantly larger than estimated from certain model calculations of form factors.

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arXiv1999

Radiative leptonic decays of B mesons in QCD

We compute the form factors parametrizing radiative leptonic decays of heavy mesons B^+\toγe^+νfor photon energies much larger than Λ_{QCD}, where perturbative QCD methods for exclusive processes can be combined with the heavy quark effective theory. The form factors can be reliably obtained in this region in an expansion in powers of Λ/E_γ. The leading term in this expansion displays an additional spin symmetry manifested in the equality of form factors of vector and axial currents. The leading twist form factors can be written as the convolution of the B meson light-cone wave function with a hard scattering amplitude, which is explicitly calculated to one-loop order. The Sudakov double logarithms of the form (\frac{α_s}π\log^2\frac{2E_γ}Λ)^n are resummed to all orders. As an application we present a method for determining the CKM matrix element |V_{ub}| from a comparison of photon spectra in B and D radiative leptonic decays.

hep-ph↗

arXiv1999

Rescattering and electroweak penguin effects in determinations of the weak phase $γ$

Determinations of the CKM phase $γ$ from weak nonleptonic $B$ decays are affected by electroweak (EW) penguins and rescattering effects. In this talk it is explained how the EW penguin effects can be controlled with the help of SU(3) symmetry, by relating them to tree-level amplitudes. The impact of the final-state interactions on the determination of $γ$ from $B^+\to Kπ$ decays is studied numerically, showing that they can be important. A few alternative methods are discussed which use additional decays to eliminate their effects.

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arXiv1999

Spin-Momentum Correlations in Inclusive Semileptonic Decays of Polarized $Λ_b$ Baryons

We consider spin-momentum correlations between the spin of the bottom baryon $Λ_{\mathrm{b}}$ and the momenta of its decay products in its inclusive semileptonic decay. We define several polar and azimuthal spin-momentum correlation measures in different event coordinate systems. The values of the spin-momentum correlation measures are calculated up to ${\cal O}(1/m_b^2)$ using the standard OPE und HQET methods. Some of the measures turn out to be sufficiently large to make them good candidates for a determination of the polarization of the $Λ_{\mathrm{b}}$ in e.g. $\mathrm{Z}$ decays.

hep-ph↗

arXiv1999

Bounding the penguin effects on determinations of $α$ from $B^0(t)\toπ^+π^-$ decays

In the absence of the QCD penguin contributions a measurement of the time-dependent asymmetry in the decay $B^0(t)\to π^+π^-$ gives directly the weak angle $α$. Several bounds have been proposed in the literature on the magnitude of the penguin effects on this determination, the prototype of which is the isospin bound of Grossman and Quinn. It is pointed out that large strong final state interactions could cause these bounds to overestimate the real penguin effect. A new flavor SU(3) bound is proposed, requiring only the charge-averaged branching ratios for $B^0\to π^+π^-$ and $B_s\to K^+K^-$, which exactly takes into account all relevant amplitudes and electroweak penguin effects. This bound on the penguin-induced error on the determination of the weak phase $α$ holds even without a knowledge of the direct CP asymmetry in the $π^+π^-$ channel.

hep-ph↗

arXiv1999

A Critical Look at Rescattering Effects on $γ$ from $B^+\to Kπ$

Three ways are compared of dealing with rescattering effects in $B^{\pm}\to K^0π^{\pm}$, in order to determine the weak phase $γ$ from these processes and $B^{\pm}\to K^{\pm}π^0$. We find that neglecting these contributions altogether may involve sizable errors in $γ$, depending on the rescattering amplitude and on the value of a certain measurable strong phase. We show that an attempt to eliminate these effects by using the charge-averaged rate of $B^{\pm}\to K^{\pm}K^0$ suffers from a large theoretical error due to SU(3) breaking, which may be resolved when using also the processes $B^{\pm}\to π^{\pm}η_8$.

hep-ph↗

arXiv1998

Weak phases $γ$ and $α$ from $B^+$, or $B^0$ and $B_s$ decays

An improved flavor SU(3) method is presented for determining the weak angle $γ$ of the unitarity triangle using decay rates for $B^+\to Kπ, B^+\to K^+\bar K^0$ and $B^+\to π^+η$ (or $B^0\to Kπ$ and $B_s\to K π$), their CP-conjugate modes and the CP-averaged rate for $B^\pm\toπ^\pmπ^{0}$. Rescattering (color-suppressed) contribution in $B^+(B^0)\to Kπ$, for which an improved bound is obtained, is subtracted away. The only significant SU(3) breaking effects are accounted for in the factorization approximation of tree amplitudes. The weak angle $α$ is obtained as a byproduct.

hep-ph↗

arXiv1998

Model-independent electroweak penguins in B decays to two pseudoscalars

We study the effects of electroweak penguin (EWP) amplitudes in $B$ meson decays into two charmless pseudoscalars in the approximation of retaining only the dominant EWP operators $Q_9$ and $Q_{10}$. Using flavor SU(3) symmetry, we derive a set of model-independent relations between EWP contributions and tree-level decay amplitudes one of which was noted recently by Neubert and Rosner. Two new applications of these relations are demonstrated in which uncertainties due to EWP corrections are eliminated in order to determine a weak phase. Whereas the weak angle $α$ can be obtained from $B\toππ$ free of hadronic uncertainties, a determination of $γ$ from $B^{0,\pm}\to Kπ^{\pm}$ requires the knowledge of a ratio of certain tree-level hadronic matrix elements. The smallness of this ratio implies a useful constraint on $γ$ if rescattering can be neglected.

hep-ph↗

arXiv1998

Hybrid Baryons in Large-N_c QCD

We study the properties and couplings of hybrid baryons in the large-$N_c$ expansion. These are color-neutral baryon states which contain in addition to $N_c$ quarks also one constituent gluon. Hybrid baryons with both symmetric and mixed symmetric orbital wave functions are considered. We introduce a Hartree description for these states, similar to the one used by Witten for ordinary baryons. It is shown that the Hartree equations for $N_c (N_c-1)$ quarks for symmetric (mixed symmetric) states in these states coincide with those in ordinary baryons in the large-$N_c$ limit. The energy due to the gluon field is of order $Λ_{QCD}$. Under the assumption of color confinement, our results prove the existence of hybrid baryons made up of heavy quarks in the large $N_c$ limit and provides a justification for the constituent gluon picture of these states. The couplings of the hybrid baryons to mesons of arbitrary spin are computed in the quark model. Using constraints from the large $N_c$ scaling laws for the meson-baryon scattering amplitudes, we write down consistency conditions for the meson couplings of the hybrid baryons. These consistency conditions are solved explicitly with results in agreement with those in the quark model for the respective couplings.

hep-ph↗

arXiv1998

Four-fermion heavy quark operators and light current amplitudes in heavy flavor hadrons

We introduce and study the properties of the "color-straight" four-quark operators containing heavy and light quark fields. They are of the form (\bar bΓ_b b)(\bar qΓ_q q) where both brackets are color singlets. Their expectation values include the bulk of the nonfactorizable contributions to the nonleptonic decay widths of heavy hadrons. The expectation values of the color-straight operators in the heavy hadrons are related to the momentum integrals of the elastic light-quark formfactors of the respective heavy hadron. We calculate the asymptotic behavior of the light-current formfactors of heavy hadrons and show that the actual decrease is 1/(q^2)^3/2 rather than 1/q^4. The two-loop hybrid anomalous dimensions of the four-quark operators and their mixing (absent in the first loop) are obtained. Using plausible models for the elastic formfactors, we estimate the expectation values of the color-straight operators in the heavy mesons and baryons. Improved estimates will be possible in the future with new data on the radiative decays of heavy hadrons. We give the Wilson coefficients of the four-fermion operators in the 1/m_b expansion of the inclusive widths and discuss the numerical predictions. Estimates of the nonfactorizable expectation values are given.

hep-ph↗

arXiv1997

Excited Baryons Phenomenology from Large-$N_c$ QCD

We present a phenomenological analysis of the strong couplings of the negative-parity L=1 baryons from the perspective of the large-$N_c$ expansion. In the large-$N_c$ limit the mass spectrum and mixing pattern of these states are constrained in a very specific way. The mixing angles are completely determined in this limit, with predictions in good agreement with experiment. In the combined large-$N_c$ and SU(3) limits the pion couplings of the five negative-parity octets to the ground state baryons are given in terms of only 3 independent couplings. The large-$N_c$ predictions for the ratios of strong couplings are tested against experimental data.

hep-ph↗

arXiv1997

$1/N_c$ Expansion for Excited Baryons

We derive consistency conditions which constrain the possible form of the strong couplings of the excited baryons to the pions. The consistency conditions follow from requiring the pion-excited baryon scattering amplitudes to satisfy the large-N_c Witten counting rules and are analogous to consistency conditions used by Dashen, Jenkins and Manohar and others for s-wave baryons. The consistency conditions are explicitly solved, giving the most general allowed form of the strong vertices for excited baryons in the large-N_c limit. We show that the solutions to the large-N_c consistency conditions coincide with the predictions of the nonrelativistic quark model for these states, extending the results previously obtained for the s-wave baryons. The 1/N_c corrections to these predictions are studied in the quark model with arbitrary number of colors N_c.

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