arXiv subjects

Zoltan Ligeti

Publications and source records attributed to Zoltan Ligeti.

125 records · Page 7Linked to original sources

arXiv1997

Comment on nonperturbative effects in $B\to X_sγ$

Uncertainties in the theoretical prediction for the inclusive $B\to X_sγ$ decay rate are discussed. We emphasize that there is no operator product expansion for this process. Nonetheless, some nonperturbative effects involving a virtual $c\bar c$ loop are calculable using the operator product expansion. They give a contribution to the decay rate that involves the B meson matrix element of an infinite tower of operators. The higher dimension operators give effects that are only suppressed by powers of $m_bΛ_{QCD}/m_c^2 \sim 0.6$, but come with small coefficients.

hep-ph↗

arXiv1997

Model independent results for $B\to D_1(2420)\ell\barν$ and $B\to D_2^*(2460)\ell\barν$ at order $Λ_{QCD}/m_{c,b}$

Exclusive semileptonic B decays into $D_1$ and $D_2^*$ mesons are investigated including order $Λ_{QCD}/m_{c,b}$ corrections using the heavy quark effective theory. At zero recoil, the $Λ_{QCD}/m_{c,b}$ corrections can be written in terms of the leading Isgur-Wise function for these transitions, $τ$, and known meson mass splittings. We obtain an almost model independent prediction for the shape of the spectrum near zero recoil, including order $Λ_{QCD}/m_{c,b}$ corrections. We determine $τ(1)$ from the measured $B\to D_1\ell\barν$ branching ratio. Implications for B decay sum rules are discussed.

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arXiv1996

Corrections to the Bjorken and Voloshin sum rules

We calculate near zero recoil the order $α_s$ corrections to the Bjorken and Voloshin sum rules that bound the $B\to D^{(*)}\ell\barν$ form factors. These bounds are derived by relating the result of inserting a complete set of physical states in a time ordered product of weak currents to the operator product expansion. The sum rules sum over physical states with excitation energies less than a scale $Δ$. We find that the corrections to the Bjorken bound are moderate, while the Voloshin bound receives sizable corrections enhanced by $Δ/Λ_{QCD}$. With some assumptions, we find that the slope parameter for the form factor $h_{A_1}$ in $B\to D^*\ell\barν$ decay satisfies $0.4\lesssimρ_{A_1}^2 \lesssim 1.3$.

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arXiv1996

$B \toτ^+ τ^- (X)$ decays: first constraints and phenomenological implications

The study of $B \to τ^+ τ^- (X)$ decays can provide us with a better understanding of the third generation, and can be a useful probe of physics beyond the standard model. We present a model independent analysis of these decays. We classify new physics that can largely enhance the decay rates and we discuss the constraints implied by other processes. Experimentally, flavor changing neutral current $B$ decays into final state $τ$'s are still unconstrained. Searches for $B$ decays with large missing energy at LEP provide the first limits. We estimate that existing data already imply bounds on the $B_d \to τ^+ τ^-$, $B_s \to τ^+ τ^-$, and $B \to X τ^+ τ^-$ decay rates at the few percent level. Although these bounds are over four orders of magnitude above the standard model predictions, they provide the first constraints on some leptoquarks, and on some R-parity violating couplings.

hep-ph↗

arXiv1996

First limit on inclusive $B \to X_s ν\barν$ decay and constraints on new physics

The inclusive $B \to X_s ν\barν$ decay rate, on which no experimental bound exists to date, can be constrained by searching for large missing energy events in $B$ decays. Carefully examining the experimental and theoretical aspects of such an analysis, we argue that the published ALEPH limit on $\BR(B \to τ\barν)$ implies, conservatively, the bound $\BR(B \to X_s ν\barν)<3.9\times10^{-4}$, which is less than one order of magnitude above the standard model prediction. The LEP collaborations could significantly improve this bound by a dedicated experimental analysis. We study the constraints this new limit imposes on various extensions of the standard model. We derive new bounds on the couplings of third generation fermions in models with leptoquarks, and in supersymmetric models without R-parity. We also constrain models where new gauge bosons are coupled dominantly to the third generation, such as TopColor models and models based on horizontal gauge symmetries. For models which predict an enhanced effective $bsZ$ vertex, the constraint from $B \to X_s ν\barν$ is competitive with the limits from inclusive and exclusive $B \to X_s \ell^+ \ell^-$ decays.

hep-ph↗

arXiv1996

Implications of the $B \to X \ell \barν_\ell$ lepton spectrum for heavy quark physics

The shape of the lepton spectrum in inclusive semileptonic $B\to X\,\ell\,\barν_\ell$ decay is sensitive to matrix elements of the heavy quark effective theory, $\barΛ$ and $λ_1$. From CLEO data we extract $\barΛ=0.39\pm0.11\,$GeV and $λ_1=-0.19\pm0.10\,{\rm GeV}^2$, where the uncertainty is the $1σ$ statistical error only. Systematic uncertainties are discussed. These values for $\barΛ$ and $λ_1$ are used to determine $|V_{cb}|$ and the $\overline{\rm MS}$ bottom and charm quark masses. We discuss the theoretical uncertainties related to order $(Λ_{\rm QCD}/m_b)^3$ effects and higher orders in the perturbative expansion.

hep-ph↗

arXiv1996

Perturbative corrections to zero recoil inclusive $B$ decay sum rules

Comparing the result of inserting a complete set of physical states in a time ordered product of $b$ decay currents with the operator product expansion gives a class of zero recoil sum rules. They sum over physical states with excitation energies less than $Δ$, where $Δ$ is much greater than the QCD scale and much less than the heavy charm and bottom quark masses. These sum rules have been used to derive an upper bound on the zero recoil limit of the $B\to D^*$ form-factor, and on the matrix element of the kinetic energy operator between $B$ meson states. Perturbative corrections to the sum rules of order $α_s(Δ) Δ^2/m_{c,b}^2$ have previously been computed. We calculate the corrections of order $α_s(Δ)$ and $α_s^2(Δ) β_0$ keeping all orders in $Δ/m_{c,b}$, and show that these perturbative QCD corrections suppressed by powers of $Δ/m_{c,b}$ significantly weaken the upper bound on the zero recoil $B\to D^*$ form-factor, and also on the kinetic energy operator's matrix element.

hep-ph↗

arXiv1995

$|V_{ub}|$ from exclusive $B$ and $D$ decays

We propose a model-independent method to determine the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ from exclusive $B$ and $D$ decays. Combining information obtainable from $B\to ρ\ell\barν_\ell$, $B\to K^*ν\barν$, $D\to ρ\bar\ellν_\ell$ and $D\to K^*\bar\ellν_\ell$, a determination of $|V_{ub}|$ is possible, with an uncertainty from theory of around 10\%. Theoretical uncertainties in the $B\to K^*\ell\bar\ell$ decay rate are discussed.

hep-ph↗

arXiv1995

Leading logarithms of the $b$ quark mass in inclusive $B\to X_sγ$ decay

Part of the order $α_s$ correction to the inclusive $B\to X_s\,γ$ photon spectrum is enhanced by $\log(m_b^2/m_s^2)$. We discuss its origin and sum the corrections proportional to $[α_s\log(m_b^2/μ^2)]^n$ to all orders. These are the calculable leading logarithms in the parton fragmentation functions of a quark or gluon into a photon. Although the gluon fragmentation into a photon starts only at order $α_s^2$, its contribution is of the same order as the $s$ quark's in the leading log sum. For not too small values of the photon energy, the resummation yields a moderate suppression. In the standard model, the coefficient of the operator whose matrix element gives rise to such terms is small. A measurement of the photon spectrum around $1\,$GeV would provide a theoretically clean determination of $C_8$, the Wilson coefficient of the $b\to s\,g$ operator.

hep-ph↗

arXiv1995

Moments of the photon spectrum in the inclusive $B\to X_sγ$ decay

The moments of the photon spectrum in the inclusive $B\to X_s\,γ$ decay can be calculated to order $α_s$ accuracy at present, without knowing the $α_s$ corrections to the effective Hamiltonian. We discuss the standard model predictions, and how the moments of the photon spectrum are related to certain matrix elements of the heavy quark effective theory. The sensitivity of these moments to new physics is small, and they provide a model-independent determination of the $b$ quark pole mass (at order $α_s$), or equivalently, the matrix elements $\barΛ$ and $λ_1$ of the effective theory.

hep-ph↗

arXiv1995

Renormalons and confinement

We compute the renormalon ambiguity of the static potential, in the limit of a large number of flavors. An extrapolation of the QED result to QCD implies that the large distance behavior of the quark potential is arbitrary in perturbation theory, as there are an infinite number of prescriptions to assign. The shape of the potential at large distances is not only affected by the renormalon pole closest to the origin of the Borel plane, but a resummation of all renormalon contributions is required. In particular, confinement can be accommodated, but it is not explained. At short distances there is no indication of a linear term in the potential.

hep-ph↗

arXiv1994

Transverse tau polarization in inclusive $\bar B\to τ\barνX$ decays

We calculate, in the framework of multi Higgs doublet models, the $CP$ violating transverse tau polarization, $P_\perp$, in inclusive semileptonic $B$ meson decays. We find that $P_\perp$ diverges at ${\cal O}(1/m_b^2)$ of the heavy quark expansion. We discuss the physical reasons for this divergence and show how to regularize it. We find large $1/m_b^2$ corrections that can suppress the $CP$ asymmetry by as much as 30\%. The maximal allowed asymmetry is $a_{CP}\sim0.34$. We discuss how the allowed range is expected to change in the future.

hep-ph↗

arXiv1994

The Inclusive $\bar B\to τ\barνX$ Decay in Two Higgs Doublet Models

We calculate in the framework of two Higgs doublet models the differential decay rate for inclusive $\bar B\toτ\,\barν\,X$ transitions to order $1/m_b^2$ in the heavy quark expansion, for both polarized and unpolarized tau leptons. In contrast to the situation in the standard model, we find sizeable $1/m_b^2$ corrections. A systematic heavy quark expansion significantly reduces the theoretical uncertainties in the calculation compared to the existing free quark decay model results. From the experimental measurement of the branching ratio, we derive the bound $\tanβ< 0.51\, m_{H^\pm}\,[{\rm GeV}]$. We point out that the tau polarization is potentially a more sensitive probe of multi scalar models than the branching ratio itself.

hep-ph↗

arXiv1994

Phenomenological Constraints on $\barΛ$ and $λ_1$

Combining the experimental data on the inclusive decays $D\to X\,e\,ν$, $B\to X\,e\,ν$ and $B\to X\,τ\,ν$, we find severe constraints on the $\barΛ$ and $λ_1$ parameters of the Heavy Quark Effective Theory. In particular, we get $\barΛ<0.7\,$GeV. Our constraints further imply $m_c\geq1.43\,$GeV and $m_b\geq4.66\,$GeV. We discuss future prospects of improving these bounds and their phenomenological implications.

hep-ph↗

arXiv1994

Heavy Quark Expansion for the Inclusive Decay $\bar B\toτ\,\barν\,X$

We calculate the differential decay rate for inclusive $\bar B\toτ\,\barν\,X$ transitions to order $1/m_b^2$ in the heavy quark expansion, for both polarized and unpolarized tau leptons. We show that using a systematic $1/m_b$ expansion significantly reduces the theoretical uncertainties in the calculation. We obtain for the total branching ratio ${\rm BR}(\bar B\toτ\,\barν\,X)=2.30\pm 0.25\%$, and for the tau polarization $A_{\rm pol}=-0.706\pm0.006$. {}From the experimental measurement of the branching ratio at LEP, we derive the upper bound $\lo\leq 0.8\gev^2$ for one of the parameters of the heavy quark effective theory.

hep-ph↗

arXiv1993

The Determination of $|\Vcb|$ and QCD Sum Rules in Hqet

I review recent developments in Heavy Quark Effective Theory (HQET) that lead to an almost model--independent determination of the $|\Vcb|$ element of the Cabibbo--Kobayashi--Maskawa matrix from exclusive semileptonic $B\ra D^{(*)}$ decays. In particular, I compare the theoretical uncertainties in the $B\ra D^*\ell\,\barν$ and the $B\ra D\,\ell\,\barν$ decay modes. I discuss the applications of QCD sum rules within HQET to semileptonic heavy meson decays and give predictions for the form factors measurable in $B\ra D^{(*)}\ell\,\barν$ decays.

hep-ph↗

arXiv1993

The Decay $B\toπ\ellν$ in Heavy Quark Effective Theory

We present a systematic analysis of the $B^{(*)}\toπ\,\ell\,ν$ weak decay form factors to order $1/m_b$ in the heavy quark effective theory, including a discussion of renormalization group effects. These processes are described by a set of ten universal functions (two at leading order, and eight at order $1/m_b$), which are defined in terms of matrix elements of operators in the effective theory. In the soft pion limit, the effective theory yields normalization conditions for these functions, which generalize the well-known current algebra relations derived from the combination of heavy quark and chiral symmetries to next-to-leading order in $1/m_b$. In particular, the effects of the nearby $B^*$-pole are correctly contained in the form factors of the effective theory. We discuss the prospects for a model independent determination of $|V_{ub}|$ and the $B B^*π$ coupling constant from these processes.

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