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Zoltan Ligeti

Publications and source records attributed to Zoltan Ligeti.

At least 109 records · Page 6Linked to original sources
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.

hep-ph↗
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

Lessons from CLEO and FOCUS Measurements of D0-anti-D0 Mixing Parameters

If the true values of the D0-anti-D0 mixing parameters lie within the one sigma ranges of recent measurements, then there is strong evidence for a large width difference, y > 0.01, and large SU(3) breaking effects in strong phases, δ> pi/4. These constraints are model independent, and would become stronger if M_{12}/Gamma_{12} << 1 in the D0-anti-D0 system. The interesting fact that the FOCUS result cannot be explained by a large mass difference is not trivial and depends on the small D0-anti-D0 production asymmetry in FOCUS and the bounds on CP violating effects from CLEO. The large value of δmight help explain why y ~ sin^2(theta_c).

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.

hep-ph↗
arXiv1999

B Decays in the Upsilon Expansion

Theoretical predictions for B decay rates are rewritten in terms of the Upsilon meson mass instead of the b quark mass, using a modified perturbation expansion. The theoretical consistency is shown both at low and high orders. This method improves the behavior of the perturbation series for inclusive and exclusive decay rates, and the largest theoretical error in the predictions coming from the uncertainty in the quark mass is eliminated. Applications to the determination of CKM matrix elements, moments of inclusive decay distributions, and the B -> X_s γphoton spectrum are discussed.

hep-ph↗
arXiv1999

B -> K^* gamma from D -> K^* l nu

The B -> K^* gamma branching fraction is predicted using heavy quark spin symmetry at large recoil to relate the tensor and (axial-)vector form factors, using heavy quark flavor symmetry to relate the B decay form factors to the measured D -> K^* l nu form form factors, and extrapolating the semileptonic B decay form factors to large recoil assuming nearest pole dominance. This prediction agrees with data surprisingly well, and we comment on its implications for the extraction of |Vub| from B -> rho l nu.

hep-ph↗
arXiv1999

Theoretical Developments in Inclusive B Decays

Some recent theoretical work on inclusive B decays relevant for the model independent determination of |V_{ub}| and |V_{cb}| is summarized. The theoretical predictions and their reliability for several differential decay distributions in B -> X_{c,u}e nu and B -> X_s gamma are reviewed. These can be used to determine certain important HQET matrix elements. The upsilon expansion and ways of testing it are discussed.

hep-ph↗
arXiv1999

The B -> X_s gamma Photon Spectrum

The photon energy spectrum in inclusive weak radiative B -> X_s gamma decay is computed to order alpha_s^2 beta_0. This result is used to extract a value for the HQET parameter $\barΛ$ from the average <1-2E_γ/m_B>, and a value of the parameter $λ_1$ from <(1-2E_γ/m_B)^2>. An accurate measurement of <1-2E_γ/m_B> can determine the size of the nonperturbative contributions to the Upsilon(1S) mass which cannot be absorbed into the b quark pole mass.

hep-ph↗
arXiv1998

B decays in the upsilon expansion

Theoretical predictions for B decay rates are rewritten in terms of the Upsilon(1S) meson mass instead of the b quark mass, using a modified perturbation expansion. The theoretical consistency of this expansion is shown both at low and high orders. Our method improves the behavior of the perturbation series for semileptonic and nonleptonic inclusive decay modes, as well as for exclusive decay form factors. The results are applied to the determination of the semileptonic B branching ratio, charm counting, the ratio of B -> X tau nu and B -> X e nu decay rates, and form factor ratios in B -> D* e nu decay. We also comment on why it is not possible to separate perturbative and nonperturbative effects in QCD.

hep-ph↗
arXiv1998

B decay and the Upsilon mass

Theoretical predictions for inclusive semileptonic B decay rates are rewritten in terms of the Upsilon(1S) meson mass instead of the b quark mass, using a modified perturbation expansion. This method gives theoretically consistent and phenomenologically useful results. Perturbation theory is well behaved, and the largest theoretical error in the predictions coming from the uncertainty in the quark mass is eliminated. The results are applied to the determination of $|V_{cb}|$, $|V_{ub}|$, and $λ_1$.

hep-ph↗
arXiv1998

Local duality and nonleptonic B decay

In the large $N_c$ limit, the nonleptonic width of the B meson is determined by the lepton mass spectrum in semileptonic B decay and the hadron mass spectrum in $τ$ decay. This result can be compared with the nonleptonic width computed using the operator product expansion and local duality, to estimate the violation of local duality in B decay. In the absence of the required data, we use experimental Monte Carlo predictions and find that the difference is small, at the few percent level. We estimate the theoretical uncertainty in our results from higher order corrections (including $1/N_c$ effects). We also identify a new contribution to the $Λ_b-B$ lifetime difference.

hep-ph↗
arXiv1998

Comment on $V_{ub}$ from Exclusive Semileptonic B and D Decays

The prospects for determining $|V_{ub}|$ from exclusive B semileptonic decay are discussed. The double ratio of form factors $(f^{B\toρ}/f^{B\to K^*})/ (f^{D\toρ}/f^{D\to K^*})$ is calculated using chiral perturbation theory. Its deviation from unity due to contributions that are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from (B->rho l nu), (B->K* l l) and (D->rho l nu) can lead to a model independent determination of $|V_{ub}|$ with an uncertainty from theory of about 10%.

hep-ph↗
arXiv1997

Semileptonic B decays to excited charm mesons

Exclusive semileptonic B decays to the lightest excited charmed mesons are investigated at order $Λ_{QCD}/m_Q$ in the heavy quark effective theory. At zero recoil, $Λ_{QCD}/m_Q$ corrections to the matrix elements of the weak currents can be written in terms of the leading Isgur-Wise functions for the corresponding transition and meson mass splittings. The differential decay rates are predicted, including $Λ_{QCD}/m_Q$ corrections with some model dependence away from zero recoil. Applications to B decay sum rules and factorization are presented.

hep-ph↗
arXiv1997

Semileptonic B decays to excited charmed mesons

Exclusive semileptonic B decays into excited charmed mesons are investigated at order $Λ_{QCD}/m_Q$ in the heavy quark effective theory. Differential decay rates for each helicity state of the four lightest excited $D$ mesons ($D_1$, $D_2^*$, $D_0^*$, and $D_1^*$) are examined. At zero recoil, $Λ_{QCD}/m_Q$ corrections to the matrix elements of the weak currents can be written in terms of the leading Isgur-Wise functions for the corresponding transition and meson mass splittings. A model independent prediction is found for the slope parameter of the decay rate into helicity zero $D_1$ at zero recoil. The differential decay rates are predicted, including $Λ_{QCD}/m_Q$ corrections with some model dependence away from zero recoil and including order $α_s$ corrections. Ratios of various exclusive branching ratios are computed. Matrix elements of the weak currents between $B$ mesons and other excited charmed mesons are discussed at zero recoil to order $Λ_{QCD}/m_Q$. These amplitudes vanish at leading order, and can be written at order $Λ_{QCD}/m_Q$ in terms of local matrix elements. Applications to $B$ decay sum rules and factorization are presented.

hep-ph↗
arXiv1997

V_{ub} from the Hadronic Invariant Mass Spectrum in Semileptonic B Decay

The hadronic invariant mass spectrum for the inclusive charmless semileptonic decay $B\to X_u e \barν_e$ is studied. Particular attention is paid to the region $s_H<m_D^2$, which may be useful for extracting the value of $|V_{ub}|$. The sensitivity of the spectrum to the parameter $\barΛ\equiv m_B - m_b$ is explored. Perturbative QCD corrections to $dΓ/ds_H$ of order $α_s^2β_0$ are calculated. For $s_H \sim \barΛm_b$ nonperturbative QCD effects are important and the shape of the invariant mass spectrum is controlled by the B meson matrix element of an infinite sum of local operators. The utility of the hadronic mass spectrum for extracting $|V_{ub}|$ is explored.

hep-ph↗
arXiv1997

Nonperturbative effects in inclusive $B\to X_sγ$

Uncertainties in the theoretical prediction for the inclusive $\bar B\to X_sγ$ decay rate are examined. Certain nonperturbative effects involving a virtual $c\bar c$ loop, which are calculable using the operator product expansion, are discussed.

hep-ph↗