arXiv subjects

Zoltan Ligeti

Publications and source records attributed to Zoltan Ligeti.

At least 73 records · Page 4Linked to original sources

arXiv2009

Disentangling Neutrino Oscillations

The theory underlying neutrino oscillations has been described at length in the literature. The neutrino state produced by a weak decay is usually portrayed as a linear superposition of mass eigenstates with, variously, equal energies or equal momenta. We point out that such a description is incomplete, that in fact, the neutrino is entangled with the other particle or particles emerging from the decay. We offer an analysis of oscillation phenomena involving neutrinos (applying equally well to neutral mesons) that takes entanglement into account. Thereby we present a theoretically sound proof of the universal validity of the oscillation formulae ordinarily used. In so doing, we show that the departures from exponential decay reported by the GSI experiment cannot be attributed to neutrino mixing. Furthermore, we demonstrate that the `Mossbauer' neutrino oscillation experiment proposed by Raghavan, while technically challenging, is correctly and unambiguously describable by means of the usual oscillation formalae.

hep-ph↗

arXiv2008

Treating the b quark distribution function with reliable uncertainties

The parton distribution function for a b quark in the B meson (called the shape function) plays an important role in the analysis of the B -> X_s gamma and B -> X_u l nu data, and gives one of the dominant uncertainties in the determination of |Vub|. We introduce a new framework to treat the shape function, which consistently incorporates its renormalization group evolution and all constraints on its shape and moments in any short distance mass scheme. At the same time it allows a reliable treatment of the uncertainties. We develop an expansion in a suitable complete set of orthonormal basis functions, which provides a procedure for systematically controlling the uncertainties due to the unknown functional form of the shape function. This is a significant improvement over fits to model functions. Given any model for the shape function, our construction gives an orthonormal basis in which the model occurs as the first term, and corrections to it can be studied. We introduce a new short distance scheme, the "invisible scheme", for the kinetic energy matrix element, lambda_1. We obtain closed form results for the differential rates that incorporate perturbative corrections and a summations of logarithms at any order in perturbation theory, and present results using known next-to-next-to-leading order expressions. The experimental implementation of our framework is straightforward.

hep-ph↗

arXiv2008

Deciphering top flavor violation at the LHC with B factories

The LHC will have unprecedented sensitivity to flavor-changing neutral current (FCNC) top quark decays, whose observation would be a clear sign of physics beyond the standard model. Although many details of top flavor violation are model dependent, the standard model gauge symmetries relate top FCNCs to other processes, which are strongly constrained by existing data. We study these constraints in a model independent way, using a low energy effective theory from which the new physics is integrated out. We consider the most important operators which contribute to top FCNCs and analyze the current constraints on them. We find that the data rule out top FCNCs at a level observable at the LHC due to most of the operators comprising left-handed first or second generation quark fields, while there remains a substantial window for top decays mediated by operators with right-handed charm or up quarks. If FCNC top decays are observed at the LHC, such an analysis may help decipher the underlying physics.

hep-ph↗

arXiv2007

Power Corrections in Charmless Nonleptonic B-Decays: Annihilation is Factorizable and Real

We classify LambdaQCD/mb power corrections to nonleptonic B-> M1 M2 decays, where M1 and M2 are charmless non-isosinglet mesons. Using recent developments in soft-collinear effective theory, we prove that the leading contributions to annihilation amplitudes of O[alphas(mb) LambdaQCD/mb] are real. The leading annihilation amplitudes depend on twist-2 and twist-3 three parton distributions. A complex nonperturbative parameter from annihilation first appears at O[alphas^2(sqrt{Lambda mb}) LambdaQCD/mb]. ``Chirally enhanced'' contributions are also factorizable and real at lowest order. Thus, incalculable strong phases are suppressed in annihilation amplitudes, unless the alphas(sqrt{Lambda mb}) expansion breaks down. Modeling the distribution functions, we find that (11 +- 9)% and (15 +- 11)% of the absolute value of the measured B-> K- pi+ and B-> K- K0 penguin amplitudes come from annihilation. This is consistent with the expected size of power corrections.

hep-ph↗

arXiv2007

Precise predictions for B -> Xs l+ l- in the large q^2 region

The inclusive B -> Xs l+ l- decay rate in the large q^2 region (q^2 > m_{ψ'}^2) receives significant nonperturbative corrections. The resulting uncertainties can be drastically reduced by normalizing the rate to the B -> Xu l nu rate with the same q^2 cut, which allows for much improved tests of short distance physics. We calculate this ratio, including the order 1/m_b^3 nonperturbative corrections and the analytically known NNLO perturbative corrections. Since in the large q^2 region an inclusive measurement may be feasible via a sum over exclusive states, our results could be useful for measurements at LHCb and possibly for studies of B -> Xd l+ l-.

hep-ph↗

arXiv2007

Mixing and CP violation in the D0 and Bs systems

Recent developments for mixing and CP violation in the D0 and Bs systems are reviewed, including (i) the recently emerging evidence for D0-D0bar mixing and the interpretations of the measurements; (ii) the theoretical status of the calculations of Delta(Gamma_D) and Delta(m_D); (iii) some implications of the measurement of Bs mixing for new physics.

hep-ph↗

arXiv2007

Extracting short distance information from b -> s l+ l- effectively

We point out that in inclusive B -> Xs l+ l- decay an angular decomposition provides a third (q^2 dependent) observable sensitive to a different combination of Wilson coefficients than the rate and the forward-backward asymmetry. Since a precise measurement of q^2 dependence requires large data sets, it is important to consider the data integrated over regions of q^2. We develop a strategy to extract all measurable Wilson coefficients in B -> Xs l+ l- from a few simple integrated rates in the low q^2 region. A similar decomposition in B -> K* l+ l-, together with the B -> K* γrate, also provides a determination of the Wilson coefficients, without reliance on form factor models and without having to measure the zero of the forward-backward asymmetry.

hep-ph↗

arXiv2006

Universality and m_X cut effects in B -> Xs l+ l-

The most precise comparison between theory and experiment for the B -> Xs l+ l- rate is in the low q^2 region, but the hadronic uncertainties associated with an experimentally required cut on m_X potentially spoil the search for new physics in these decays. We show that a 10-30% reduction of dΓ(B -> Xs l+ l-) / dq^2 due to the m_X cut can be accurately computed using the B -> X_s gamma shape function. The effect is universal for all short distance contributions in the limit m_X^2 << m_B^2, and this universality is spoiled neither by realistic values of the m_X cut nor by alpha_s corrections. Both the differential decay rate and forward-backward asymmetry with an m_X cut are computed.

hep-ph↗

arXiv2006

Implications of the measurement of the B^0_s-\bar B^0_s mass difference

We analyze the significant new model independent constraints on extensions of the standard model (SM) that follow from the recent measurements of the B^0_s-\bar B^0_s mass difference. The time-dependent CP asymmetry in B_s -> ψϕ, S_{ψϕ}, will be measured with good precision in the first year of LHC data taking, which will further constrain the parameter space of many extensions of the SM, in particular, next-to-minimal flavor violation. The CP asymmetry in semileptonic B_s decay, A_{SL}^s, is also important to constrain these frameworks, and could give further clues to our understanding the flavor sector in the LHC era. We point out a strong correlation between S_{ψϕ} and A_{SL}^s in a very broad class of new physics models.

hep-ph↗

arXiv2006

CP violation and the CKM matrix

Our knowledge of quark-flavor physics and CP violation increased tremendously over the past five years. It is confirmed that the Standard Model correctly describes the dominant parts of the observed CP-violating and flavor-changing phenomena. Not only does CP violation provide some of the most precise constraints on the flavor sector, but several measurements performed at the B-factories achieved much better precision than had been expected. We review the present status of the Cabibbo-Kobayashi-Maskawa matrix and CP violation, recollect the relevant experimental and theoretical inputs, display the results from the global CKM fit, and discuss their implications for the Standard Model and some of its extensions.

hep-ph↗

arXiv2006

The CKM matrix and CP violation (in the continuum approximation)

The first part of this talk reviews recent developments in flavor physics that can be made without detailed understanding of hadronic physics, driven by the data. The error of sin(2beta) has shrunk below 5%, and the measurements of alpha and gamma have reached interesting precisions. For the first time, there are significant constraints on the deviations from the standard model in B-Bbar mixing and in b->s and b->d transitions. In the second part, I review some theoretical developments for exclusive semileptonic and nonleptonic B decays that have become possible using the soft-collinear effective theory. I concentrate on topics where the recent progress has model independent implications for interpreting the data.

hep-lat↗

arXiv2005

Testing the dynamics of B -> ππand constraints on α

In charmless nonleptonic B decays to ππor ρρ, the "color allowed" and "color suppressed" tree amplitudes can be studied in a systematic expansion in \alphas(mb) and Λ/mb. At leading order in this expansion their relative strong phase vanishes. The implications of this prediction are obscured by penguin contributions. We propose to use this prediction to test the relative importance of the various penguin amplitudes using experimental data. The present B->ππdata suggest that there are large corrections to the heavy quark limit, which can be due to power corrections to the tree amplitudes, large up-quark penguin amplitude, or enhanced weak annihilation. Because the penguin contributions are smaller, the heavy quark limit is more consistent with the B->ρρdata, and its implications may become important for the extraction of αfrom this mode in the future.

hep-ph↗

arXiv2005

Perturbative corrections to the determination of Vub from the P+ spectrum in B->X_u l nu

We investigate the relation between the E_gamma spectrum in B->X_s gamma decay and the P+ spectrum in semileptonic B->X_u l nu decay (P+ is the hadronic energy minus the absolute value of the hadronic three-momentum), which provides in principle the theoretically simplest determination of Vub from any of the "shape function regions" of B->X_u l nu spectra. We calculate analytically the P+ spectrum to order alpha_s^2 beta_0, and study its relation to the B->X_s gamma photon spectrum to eliminate the leading dependence on nonperturbative effects. We compare the result of fixed order perturbation theory to the next-to-leading log renormalization group improved calculation, and argue that fixed order perturbation theory is likely to be a more appropriate expansion. Implications for the perturbative uncertainties in the determination of Vub from the P+ spectrum are discussed.

hep-ph↗

arXiv2004

The photon polarization in B -> X gamma in the standard model

The standard model prediction for the $B\to X_{s,d}γ$ decay amplitude with a right-handed photon is believed to be tiny, suppressed by $m_{s,d}/m_b$, compared to the amplitude with a left-handed photon. We show that this suppression is fictitious: in inclusive decays, the ratio of these two amplitudes is only suppressed by $g_s/(4π)$, and in exclusive decays by $Λ_{QCD}/m_b$. The suppression is not stronger in $B\to X_dγ$ decays than it is in $B\to X_sγ$. We estimate that the time dependent CP asymmetries in $B\to K^*γ$, $ργ$, $K_Sπ^0γ$, and $π^+π^-γ$ are of order 0.1 and that they have significant uncertainties.

hep-ph↗

arXiv2004

Global analysis of inclusive B decays

In light of the large amount of new experimental data, we revisit the determination of V_{cb} and m_b from inclusive semileptonic and radiative B decays. We study shape variables to order lqcd^3/m_b^3 and alpha_s^2β_0, and include the order alpha_s, lqcd/m_b correction to the hadron mass spectrum in semileptonic decay, which improves the agreement with the data. We focus on the 1S and kinetic mass schemes for the b quark, with and without expanding m_b-m_c in HQET. We perform fits to all available data from BABAR, BELLE, CDF, CLEO, and DELPHI, discuss the theoretical uncertainties, and compare with earlier results. We find V_{cb} = (41.4 +- 0.6 +- 0.1) x 10^{-3} and m_b^{1S} = 4.68 +- 0.03 GeV, including our estimate of the theoretical uncertainty in the fit.

hep-ph↗

arXiv2004

The CKM matrix and CP Violation

The status of CP violation and the CKM matrix is reviewed. Direct CP violation in B decay has been established and the measurement of sin(2beta) in ψK modes reached 5% accuracy. I discuss the implications of these, and of the possible deviations of the CP asymmetries in b->s modes from that in ψK. The first meaningful measurements of alpha and gamma are explained, together with their significance for constraining both the SM and new physics in B-Bbar mixing. I also discuss implications of recent developments in the theory of nonleptonic decays for B->pi K rates and CP asymmetries, and for the polarization in charmless B decays to two vector mesons.

hep-ph↗

arXiv2004

The $D^0 - \bar D^0$ mass difference from a dispersion relation

We study the Standard Model prediction for the mass difference between the two neutral D meson mass eigenstates, $Δm$. We derive a dispersion relation based on heavy quark effective theory that relates $Δm$ to an integral of the width difference of heavy mesons, $ΔΓ$, over varying values of the heavy meson mass. Modeling the $m_D$-dependence of certain D decay partial widths, we investigate the effects of SU(3) breaking from phase space on the mass difference. We find that $Δm$ may be comparable in magnitude to $ΔΓ$ in the Standard Model.

hep-ph↗

arXiv2004

Heavy quark symmetry in B -> D(*) ell nu spectra

We calculate heavy quark symmetry breaking in the slopes and curvatures of the $B\to D^{(*)}\ell\barν$ spectra at zero recoil, including the order $α_s^2β_0$ corrections. We point out that the theoretical uncertainties in the differences between $B\to D$ and $B\to D^*$ slopes and curvatures are smaller than in the deviations of the slopes and curvatures themselves from their infinite mass limits. We find that the central values of the current experimental results for the difference of the slopes differ from our calculations when QCD sum rules are used to estimate subleading Isgur-Wise functions. A better understanding of the shapes of the $B\to D^{(*)}\ell\barν$ spectra may also help to reduce the error of $|V_{cb}|$ extracted from the zero recoil limit of $B\to D^*\ell\barν$. We argue that heavy quark symmetry requires that the same fitting procedure be used in the experimental determinations of the shape parameters and $|V_{cb}|$ from the $B\to D\ell\barν$ and $B\to D^{*}\ell\barν$ spectra.

hep-ph↗