arXiv subjects

Zoltan Ligeti

Publications and source records attributed to Zoltan Ligeti.

At least 91 records · Page 5Linked to original sources

arXiv2004

Predictions for nonleptonic Lambda_b and Theta_b decays

We study nonleptonic Lambda_b -> Lambda_c pi, Sigma_c pi and Sigma_c^* pi decays in the limit m_b, m_c, E_pi >> Lambda_{QCD} using the soft-collinear effective theory. Here Sigma_c = Sigma_c(2455) and Sigma_c^* = Sigma_c(2520). At leading order the Lambda_b -> Sigma_c^{(*)} pi rates vanish, while the Lambda_b -> Lambda_c pi rate is related to Lambda_b -> Lambda_c\ell\barν, and is expected to be larger than Gamma(B -> D^{(*)} pi). The dominant contributions to the Lambda_b -> Sigma_c^{(*)} pi rates are suppressed by Lambda_{QCD}^2/E_pi^2. We predict Gamma(Lambda_b -> Sigma_c^* pi) / Gamma(Lambda_b -> Sigma_c pi) = 2 + O[Lambda_{QCD}/m_Q, alpha_s(m_Q)], and the same ratio for Lambda_b -> Sigma_c^{(*)} rho and for Lambda_b -> Xi_c^{(',*)}K. ``Bow tie'' diagrams are shown to be suppressed. We comment on possible discovery channels for weakly decaying pentaquarks, Theta_{b,c} and their nearby heavy quark spin symmetry partners, Theta_{b,c}^*.

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arXiv2003

Comment on extracting alpha from B -> rho rho

Recent experimental results on $B\toρρ$ decays indicate that the CP asymmetry $S_{ρ^+ρ^-}$ will give an interesting determination of $α=\arg[-(V_{td}V_{tb}^*)/(V_{ud}V_{ub}^*)]$. In the limit when the $ρ$ width is neglected, the $B\toππ$ isospin analysis can also be applied to $B\toρρ$, once an angular analysis is used to separate transversity modes. The present bound on the shift of $S_{ρ^+ρ^-}$ from the true $\sin2α$ is already stronger than it is for $S_{π^+π^-}$. We point out a subtle violation of the isospin relations when the two $ρ$ mesons are observed with different invariant masses, and how to constrain this effect experimentally.

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arXiv2003

|Vcb| and |Vub|: Theoretical Developments

The determinations of |Vcb| and |Vub| from semileptonic B decays are reviewed with a critical discussion of the theoretical uncertainties and concentrating on recent developments. Future prospects and limitations are also discussed.

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arXiv2003

SU(3) Relations and the CP Asymmetries in B Decays to $η' K_S$, $ϕK_S$ and $K^+ K^- K_S$

We consider CP asymmetries in neutral $B$ meson decays to $η' K_S$, $ϕK_S$, and $K^+ K^- K_S$. We use SU(3) relations to estimate or bound the contributions to these amplitudes proportional to $V_{ub}^*V_{us}$. Such contributions induce a deviation of the $S_f$ terms measured in these time dependent CP asymmetries from that measured for $ψK_S$. For the $K^+ K^- K_S$ mode, we estimate the deviation to be of order 0.1. For the $η' K_S$ mode, we obtain an upper bound on this deviation of order 0.3. For the $ϕK_S$ mode, we have to add a mild dynamical assumption to the SU(3) analysis due to insufficient available data, yielding an upper bound of order 0.25. These bounds may improve significantly with future data. While they are large at present compared to the usually assumed Standard Model contribution, they are obtained with minimal assumptions and hence provide more rigorous tests for new physics. If measurements yield $|S_f-S_{ψK}|$ that are much larger than our bounds, it would make a convincing case for new physics.

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arXiv2003

Comment on Quark Masses in SCET

Quark masses are included in the SCET Lagrangian. Treating the strange quark mass as order Lambda_{QCD}, we find that strange quark mass terms are suppressed in SCET1, but are leading order in SCET2. This is relevant for B decays to K^* and K. Strange quark mass effects in semileptonic and weak radiative form factors are studied. They give corrections to the form factors that are not suppressed by powers of the bottom quark mass, or, equivalently, by the large recoil energy of the final state meson, and and preserve the heavy to light form factor relations that follow from using the leading order current.

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arXiv2003

Measuring $γ$ in $B^\pm \to K^\pm (K K^*)_D$ decays

We develop a method to measure the CKM angle $γ$ without hadronic uncertainties from the analysis of $B^\pm \to K^\pm D^0$ and $K^\pm \D0bar$ followed by singly Cabibbo-suppressed $D$ decays to non CP-eigenstates, such as $K^\pm K^{*\mp}$. This method utilizes the interference between $b\to c\bar u s$ and $b\to u\bar c s$ decays, and we point out several attractive features of it. All the modes that need to be measured for this method are accessible in the present data.

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arXiv2003

Introduction to Heavy Meson Decays and CP Asymmetries

These lectures are intended to provide an introduction to heavy meson decays and CP violation. The first lecture contains a brief review of the standard model and how the CKM matrix and CP violation arise, mixing and CP violation in neutral meson systems, and explanation of the cleanliness of the sin(2beta) measurement. The second lecture deals with the heavy quark limit, some applications of heavy quark symmetry and the operator product expansion for exclusive and inclusive semileptonic B decays. The third lecture concerns with theoretically clean CP violation measurements that may become possible in the future, and some developments toward a better understanding of nonleptonic B decays. The conclusions include a subjective best buy list for the near future.

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arXiv2003

B decay shape variables and the precision determination of |Vcb| and mb

We present expressions for shape variables of B decay distributions in several different mass schemes, to order $α_s^2β_0$ and (Lambda_{QCD}/mb)^3. Such observables are sensitive to the b quark mass and matrix elements in the heavy quark effective theory, and recent measurements allow precision determinations of some of these parameters. We perform a combined fit to recent experimental results from CLEO, BABAR, and DELPHI, and discuss the theoretical uncertainties due to nonperturbative and perturbative effects. We discuss the possible discrepancy between the OPE prediction, recent BABAR results and the measured branching fraction to D and D* states. We find |Vcb| = (40.8 +- 0.9) x 10^{-3} and mb^{1S} = 4.74 +- 0.10 GeV, where the errors are dominated by experimental uncertainties.

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arXiv2002

Can the CP asymmetries in $B\to ψK_S$ and $B\to ψK_L$ differ?

In the standard model the CP asymmetries in $B\to ψK_S$ and $B\to ψK_L$ are equal in magnitude and opposite in sign to very good approximation. We compute the order $ε_K$ corrections to each of these CP asymmetries and find that they give a deviation from $\sin 2β$ at the half percent level, which may eventually be measurable. However, the correction to $a_{CP}(B\to ψK_S)+a_{CP}(B\to ψK_L)$ due to $ε_K$ is further suppressed. The dominant corrections to this sum, at the few times $10^{-3}$ level, come from the $B$ lifetime difference, and CP violation in $B- \bar B$ mixing and $B \to ψK$ decay. New physics could induce a significant difference in the $\sin(Δm_B t)$ time dependence in the asymmetries if and only if the "wrong-flavor" amplitudes $B \to ψ\bar K$ or $\bar B \to ψK$ are generated. A scale of new physics that lies well below the weak scale would be required. Potential scenarios are therefore highly constrained, and do not appear feasible. A direct test is proposed to set bounds on such effects.

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arXiv2002

D0-D0bar Mixing

The main challenge in the Standard Model calculation of the mass and width difference in $D^0 - \bar D^0$ mixing is to estimate the size of SU(3) breaking. We prove that mixing occurs in the Standard Model only at second order in SU(3) violation. We calculate SU(3) breaking due to phase space effects, and find that it can naturally give rise to a width difference $ΔΓ/ 2Γ\sim 1%$, potentially reducing the sensitivity of D mixing to new physics.

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arXiv2002

Enhanced subleading structure functions in semileptonic B decay

The charged lepton spectrum in semileptonic $B\to X_u \ell\barν$ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of $Λ_{QCD}/m_b$ but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width $ΔE_\ell \sim Λ_{QCD}$ near the endpoint the contributions proportional to $δ(E_\ell - m_b/2)$ times (i) the matrix element of the chromomagnetic operator, and (ii) four-quark operators. These contribute to the total rate at the few percent level, but affect the endpoint region much more significantly. Implications for the determination of $|V_{ub}|$ are discussed.

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arXiv2002

Implications of the CP asymmetry in semileptonic B decay

Recent experimental searches for $A_{SL}$, the CP asymmetry in semileptonic B decay, have reached an accuracy of order one percent. Consequently, they give meaningful constraints on new physics. We find that cancellations between the Standard Model (SM) and new physics contributions to $B^0 - \bar B^0$ mixing cannot be as strong as was allowed prior to these measurements. The predictions for this asymmetry within the SM and within models of minimal flavor violation (MFV) are below the reach of present and near future measurements. Including order $m_c^2/m_b^2$ and $Λ_{QCD}/m_b$ corrections we obtain the SM prediction: $-1.3 \times 10^{-3} < A_{SL} < -0.5 \times 10^{-3}$. Future measurements can exclude not only the SM, but MFV as well, if the sign of the asymmetry is opposite to the SM or if it is same-sign but much enhanced. We also comment on the CP asymmetry in semileptonic $B_s$ decay, and update the range of the angle $β_s$ in the SM: $0.026 < \sin2β_s < 0.048$.

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arXiv2002

New Physics and Future B Factories

Further experimental and theoretical studies of the physics of flavor and CP violation are well motivated. Within the supersymmetric framework, higher precision measurements will allow to explore classes of models with stronger degree of universality: first, models with no universality, such as alignment or heavy first two squark generations; second, models with approximate universality, such as dilaton dominance or AMSB; and finally models of exact universality, such as GMSB. A broad program, including various rare processes or CP asymmetries in B, D and K decays, will provide detailed information about viable extensions of the Standard Model. Some highlights of future B-physics experiments (the present B-factories with integrated luminosity of 0.5 ab^{-1}, hadron machines, and future high-luminosity B-factories) are described.

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arXiv2001

Precision determination of Vub

We review how to determine |Vub| from inclusive semileptonic B decay using combined cuts on the leptonic and hadronic invariant masses to eliminate the b -> c background. This leads to a determination of |Vub| with theoretical uncertainty at the 5 -10% level.

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arXiv2001

New Results on Flavor Physics

Recent progress in flavor physics is discussed. In particular, I review theoretical and experimental developments relevant for semileptonic B decays and the determination of |Vcb| and |Vub|, for exclusive rare decays, for nonleptonic b->c decays and tests of factorization, and for D meson mixing.

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arXiv2001

SU(3) Breaking and D0-D0bar Mixing

The main challenge in the Standard Model calculation of the mass and width difference in the D0-D0bar system is to estimate the size of SU(3) breaking effects. We prove that D meson mixing occurs in the Standard Model only at second order in SU(3) violation. We consider the possibility that phase space effects may be the dominant source of SU(3) breaking. We find that y=(Delta Gamma)/(2Gamma) of the order of one percent is natural in the Standard Model, potentially reducing the sensitivity to new physics of measurements of D meson mixing.

hep-ph↗

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

Comment on studying the corrections to factorization in B -> D(*) X

We propose studying the mechanism of factorization in exclusive decays of the form B->D(*)X by examining the differential decay rate as a function of the invariant mass of the light hadronic state X. If factorization works primarily due to the large N_c limit then its accuracy is not expected to decrease as the X invariant mass increases. However, if factorization is mostly a consequence of perturbative QCD then the corrections should grow with the X invariant mass. Combining data for hadronic tau decays and semileptonic B decays allows tests of factorization to be made for a variety of final states. We discuss the examples of B->D^*π^+π^-π^-π^0 and B->D^*ωπ^-. The mode B->D^*ωπ^- will allow a precision study of the dependence of the corrections to factorization on the invariant mass of the light hadronic state.

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