Spectator Quark Research Articles

Light-cone sum rules for the heavy-to-light transition in the effective theory

Heavy-to-light weak form factor is calculated using the light-cone sum rule (LCSR) in the framework of soft-collinear effective theory (SCET). There are spin-symmetric and spin-nonsymmetric contributions. Leading order spin-symmetric contribution corresponds to the “soft overlap” where some of the partons carry very small momentum. The next-to-leading order spin-symmetric and spin-nonsymmetric parts are characterized by a collinear gluon exchange with the spectator quark. We reproduce the full theory LCSR results and give comments on recent LCSR in SCET.

Estimating the [formula omitted]Kπ interaction in D decay

Heavy flavour decay to light hadrons is the key to understanding many aspects of the Standard Model from CP violation to strong dynamics. It is often presumed in line with the simple quark spectator model of D decay to Kππ that the Kπ system has only I=1/2. E791 have recently presented an analysis of their results on D+→(K−π+)π+ using a generalised isobar picture of two body interactions. While higher Kπ waves are described by sums of known resonances, the S-wave amplitude and phase are determined bin-by-bin in Kπ mass. The phase variation is found not to be that of K−π+ elastic scattering. This hints at a different mixture of I=1/2 and I=3/2S-wave interactions than in elastic scattering. Applying Watson's theorem to this generalised isobar model allows us to estimate the I=3/2Kπ S-wave component. We indeed find that this is larger than in hadronic scattering or semileptonic processes.

Parity-violating asymmetry of W bosons produced in p– p collisions

The parity-violating asymmetry is an ideal tool to study the quark helicity distribution in the proton. We study the parity-violating asymmetry of W ± bosons produced by longitudinally polarized p– p collision in RHIC, based on predictions of quark distributions of the proton in the SU ( 6 ) quark–spectator–diquark model and a perturbative QCD based counting rule analysis. We find that the two models give nearly equal asymmetry for W + but that for W − quite different. Therefore future experiments on such quantity can help to clarify different predictions of the value Δ d ( x ) / d ( x ) at x → 1 in the proton.

The Femto-experiment for the LHC: The W-boson beams and their targets*

The LHC has been designed as a collider of proton and ion beams. However, in its experimental program, which is focused mainly on studies of high energy transfer collisions of Standard Model point-like particles, protons and ions will play a backstage role. For the majority of the LHC experimentalists, their role will be confined to providing standardized, acceleration-process-stable envelopes for tunable-density and tunable-isospin bunches of Standard Model constituents: quarks and gluons. The inter-bunch environment of collisions of these Standard Model particles specific to hadronic colliders and absent in the leptonic ones, has always been considered as a burden - an annoying but unavoidable price to pay for increasing the collision-energy of point-like particles in the storage rings. In this paper we shall argue that such a burden can be converted into an important merit of the high-energy hadronic colliders - a corner-stone for a fermi-length-scale ``collision-experiment'' employing the bunches of spectator quarks and gluons as tunable ``femtoscopic'' targets for the beams of short-living electroweak bosons.

B-meson wavefunction in the Wandzura–Wilczek approximation

The B-meson wavefunction has been studied with the help of the vacuum-to-meson matrix element of the non-local operators in the heavy quark effective theory. In order to obtain the Wandzura–Wilczek-type B-meson wavefunction, we solve the equations which are derived from the equation of motion of the light spectator quark in the B meson by using two different assumptions. Under the condition that ω0=2Λ¯, the solutions for the B-meson wavefunction in this paper agree well with the one obtained by directly taking the heavy quark limit mb→∞. Our results show that the equation of motion of the light spectator quark in the B meson can impose a strong constraint on the B-meson wavefunctions Ψ±(ω,z2). Based on the obtained results, we claim that both its distribution amplitudes ϕB(ω) and ϕ¯B(ω) are important for calculating the B meson decays.

CharmlessB→VPdecays using flavor SU(3) symmetry

The decays of $B$ mesons to a charmless vector ($V$) and pseudoscalar ($P$) meson are analyzed within a framework of flavor SU(3) in which symmetry breaking is taken into account through ratios of decay constants in tree ($T$) amplitudes. The magnitudes and relative phases of tree and penguin amplitudes are extracted from data; the symmetry assumption is tested; and predictions are made for rates and CP asymmetries in as-yet-unseen decay modes. A key assumption for which we perform some tests and suggest others is a relation between penguin amplitudes in which the spectator quark is incorporated into either a pseudoscalar meson or a vector meson. Values of $\gamma$ slightly restricting the range currently allowed by fits to other data are favored, but outside this range there remain acceptable solutions which cannot be excluded solely on the basis of present $B \to VP$ experiments.

Rare radiative exclusiveBdecays in soft-collinear effective theory

We consider rare radiative B decays such as B -> K^* gamma or B -> rho gamma in soft-collinear effective theory, and show that the decay amplitudes are factorized to all orders in alpha_s and at leading order in Lambda/m_b.By employing two-step matching, we classify the operators for radiative B decays in powers of a small parameter lambda(~ \sqrt{Lambda/m_b}) and obtain the relevant operators to order lambda in SCET_I. These operators are constructed with or without spectator quarks including the four-quark operators contributing to annihilation and W-exchange channels. And we employ SCET_II where the small parameter becomes of order Lambda/m_b, and evolve the operators in order to compute the decay amplitudes for rare radiative decays in soft-collinear effective theory. We show explictly that the contributions from the annihilation channels and the W-exchange channels vanish at leading order in SCET. We present the factorized result for the decay amplitudes in rare radiative B decays at leading order in SCET, and at next-to-leading order in alpha_s.

Final-state phases inB→baryon-antibaryon decays

The recent observation of the decay ${B}^{0}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}_{c}^{+}\overline{p}$ suggests that related decays may soon be visible at ${e}^{+}{e}^{\ensuremath{-}}$ colliders. It is shown how these decays can shed light on strong final-state phases and amplitudes involving the spectator quark, both of which are normally expected to be small in B decays.

The [formula omitted] form at Zero Recoil and the determination of | Vcb|

We described a model independent lattice QCD method for determining the zero recoil form factor, h A 1 (1), for the semileptonic decay B → D ∗ℓν . We find h A 1 (1) = 0.913 −0.017 +0.024 ± 0.016 −0.014−0.016−0.014 +0.003+0.000+0.006, where uncertainties, respectively, are from statistics and fit procedures, matching of lattice theory to QCD, residual lattice spacing dependence, spectator quark mass effects and effects due to working in quenched lattice QCD.

Spectator interactions in soft-collinear effective theory

Soft-collinear effective theory is generalized to include soft massless quarks in addition to collinear fields. This extension is necessary for the treatment of interactions with the soft spectator quark in a heavy meson. The power counting of the relevant fields and the construction of the effective Lagrangian are discussed at leading order in Λ/ m b . Several novel effects occur in the matching of full-theory amplitudes onto effective-theory operators containing soft light quarks, such as the appearance of an intermediate mass scale and large non-localities of operators on scales of order 1/ Λ. Important examples of effective-theory operators with soft light quarks are studied and their renormalization properties explored. The formalism presented here forms the basis for a systematic analysis of factorization and power corrections for any exclusive B-meson decay into light particles.

Axial vector form factor of nucleons in a light-cone diquark model

The nucleon axial vector form factor is investigated in a light-cone quark spectator diquark model, in which Melosh rotations are applied to both the quark and vector diquark. It is found that this model gives a very good description of available experimental data and the results have very little dependence on the parameters of the model. The relation between the nucleon axial constant and the anomalous magnetic moment of nucleons is also discussed.

Factorization and hadronic B decays in the heavy quark limit

Recent theoretical investigations (Beneke M et al 1999 Phys. Rev. Lett. 83 1914) of two-body hadronic B decays have provided justification, at least to order αs, for the use of the factorization ansatz to evaluate the B decay matrix elements provided the decay meson containing the spectator quark is not heavy. Although data is now available on many charmless two-body B decay channels, it is so far not very precise with systematic and statistical errors in total of the order of 25% or greater. Analyses have been made on ππ and πK channels with somewhat contradictory results (Beneke M et al 2001 Nucl. Phys. B 606 245; Ciuchini M et al 2001 Preprint hep-ph/0110022). We take an overview of these channels and of other channels containing vector mesons. Because factorization involves many poorly known soft QCD parameters, and because of the imprecision of the current data, we present simplified formulae for a wide range of B decays into the lowest mass pseudoscalar and vector mesons. These formulae, valid in the heavy quark limit, involve a reduced set of soft QCD parameters and, although resulting in some loss of accuracy, should still provide an adequate and transparent tool with which to confront data for some time to come. Finally we confront these formulae with the data on 19 channels. We find a plausible set of soft QCD parameters that, apart from three pseudoscalar vector channels, fit the branching ratios and the recently measured value of sin(2β) quite well.

Spin transfers for baryon production in polarized pp collisions at RHIC-BNL

We consider the inclusive production of longitudinally polarized baryons in p → p collisions at RHIC-BNL, with one longitudinally polarized proton. We study the spin transfer between the initial proton and the produced baryon as a function of its rapidity and we elucidate its sensitivity to the quark helicity distributions of the proton and to the polarized fragmentation functions of the quark into the baryon. We make predictions using an SU(6) quark spectator model and a perturbative QCD (pQCD) based model. We discuss these different predictions, and what can be learned from them, in view of the forthcoming experiments at RHIC-BNL.

Simple relations for two body B decays to charmonium and tests for η–η′ mixing

The two body decays of $B_d$ and $B_s$ decays into $J/\psi M$, where $M$ is a light meson, is studied under the very simple assumptions that the spectator quark does not play a role in the decay of the weak heavy quark or antiquark. This hypothesis leads to interesting relations between decay amplitudes. The assumption of SU(3) symmetry leads to additional relations between the decay amplitudes and in particular, the eight CP eigenstates $J/\psi K_S$, $J/\psi \eta$, $J/\psi \eta^{\prime}$ and $J/\psi \pi^o$ are all given in terms of 0. If agreement with experiment validates these assumptions the parameters over determined by the results will give information about the ratio of penguin to tree contributions to the "golden channel" $B^o \to J/\psi K_S$ decay and will provide tests for the standard $\eta-\eta^{\prime}$ mixing, which assumes that this mixing is determined by a single mixing angle, as well as determine the value of the mixing angle. We also present tests of the standard $\eta-\eta^{\prime}$ mixing involving semileptonic $D$ decays.

Preliminaries on a lattice analysis of the pion light-cone wave function: A partonic signal?

We present the first attempt of a new method to compute the pion light-cone wave function (LCWF) on the lattice. We compute the matrix element between the pion and the vacuum of a nonlocal operator: the propagator of a ``scalar quark'' (named, for short a ``squark''). A theoretical analysis shows that for some kinematical conditions (an energetic pion and hard squark) this matrix element depends dominantly on the LCWF ${\ensuremath{\Phi}}_{\ensuremath{\pi}}(u),u\ensuremath{\in}[0,1].$ On the lattice, the discretization of the parton momenta imposes further constraints on the pion momentum. The two-point Green functions made of squark-quark and squark-squark fields show a hadronlike bound-state behavior and verify the standard energy spectrum. We show some indications that during a short time, after being created, the system of the spectator quark and the squark behave like partons, before they form a hadronlike bound state. This short time is the place where the partonic wave function has to be looked for.

Exclusive and inclusive semileptonic Λ b-decays

In this talk we present theoretical evidence that the exclusive/inclusive ratio of semileptonic Λ b -decays exceeds that of semileptonic B-decays, where the experimental exclusive/inclusive ratio amounts to about 66%. We start from the observation that the spectator quark model provides a lower bound on the leading order Isgur-Wise function of the Λ b → Λ c transition in terms of the corresponding B → D,D * mesonic Isgur-Wise function. Using experimental data for the B → D,D * mesonic Isgur-Wise functions this bound is established. Applying a QCD sum rule estimate of the Λ b → Λ c transition form factor which satisfy the spectator quark model bound we predict the exclusive/inclusive ratio of semileptonic Λ b decay rates to lie in a range between 0.81 and 0.89. We also provide an upper bound on the baryonic Isgur-Wise function which is determined from the requirement that the exclusive rate should not exceed the inclusive rate. Our pre-Osaka results are discussed in the light of new recent preliminary experimental results on the pertinent mesonic and baryonic form factors presented at the Osaka ICHEP 2000 Conference.

Measuring|Vub|withB→Ds+Xutransitions

We propose the determination of the CKM matrix element $|V_{ub}|$ by the measurement of the spectrum of $B \to D_s^+ X_u$, dominated by the spectator quark model mechanism $\bar{b} \to D_s^{(*)+} \bar{u}$. The interest of considering $B \to D_s^+X_u$ versus the semileptonic decay is that more than 50 % of the spectrum for $B \to D_s^+ X_u$ occurs above the kinematical limit for $B \to D_s^+ X_c$, while most of the spectrum $B \to l \nu X_u$ occurs below the $B \to l \nu X_c$ one. Furthermore, the measure of the hadronic mass $M_X$ is easier in the presence of an identified $D_s$ than when a $\nu$ has been produced. As a consistency check, we point out that the rate $\bar{b} \to D_s^{(*)+} \bar{c}$ (including QCD corrections that we present elsewhere) is consistent with the measured $BR (B \to D_s^{\pm} X)$. Although the hadronic complications may be more severe in the mode that we propose than in the semileptonic inclusive decay, the end of the spectrum in $B \to l \nu X_u$ is not well understood on theoretical grounds. We argue that, in our case, the excited $D_s^{**}$, decaying into $D K$, do not contribute and, if there is tagging of the $B$ meson, the other mechanisms to produce a $D_s$ of the right sign are presumably small, of $O(10^{-2})$ relative to the spectator amplitude, or can be controlled by kinematical cuts. In the absence of tagging, other hadronic backgrounds deserve careful study. We present a feasability study with the BaBar detector.

Exclusive/inclusive ratio of semileptonicΛbdecays

We present theoretical evidence that the exclusive/inclusive ratio of semileptonic $\Lambda_b$-decays exceeds that of semileptonic B-decays where the experimental exclusive/inclusive ratio amounts to about 66%. We start from the observation that the spectator quark model provides a lower bound on the leading order Isgur-Wise function of the $\Lambda_b \to \Lambda_c$ transition in terms of the corresponding $B \to D,D^*$ mesonic Isgur-Wise function. Using experimental data for the $B \to D,D^*$ mesonic Isgur-Wise functions this bound is established. Applying a Bethe-Salpeter model including spectator quark interactions and a QCD sum rule estimate of the $\Lambda_b \to \Lambda_c$ transition form factor which satisfy the spectator quark model bound we predict the exclusive/inclusive ratio of semileptonic $\Lambda_b$ decay rates to lie in a range between 0.81 and 0.92. We also provide an upper bound on the baryonic Isgur-Wise function which is determined from the requirement that the exclusive rate should not exceed the inclusive rate.

Bound on cos α 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 ±→ ρ ± γ, B 0→ ρ 0/ ωγ, B→K ∗γ and the photon energy spectrum in B→ γℓ ν ℓ. We carefully identify all sources of isospin-violating effects, 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→ ργ and B→K ∗γ data, whereas the latter effects are OZI-suppressed and can be controlled by measuring also the weak radiative modes B s → ρ( ω) γ. The resulting bound excludes values of α around 90°, provided that the combined ratio B(B ±→ρ ±γ)/ B(B 0→ρ 0/ωγ)× B(B 0→K ∗0γ)/ B(B ±→K ∗±γ) is found to be different from 1.

New top-flavor models with a seesaw mechanism

A new class of models is constructed in which the third family quarks, but not leptons, experience a new $\mathrm{SU}(2)$ or $U(1)$ gauge force. Anomaly cancellation enforces the introduction of spectator quarks so that top and bottom quark masses are naturally generated via a seesaw mechanism. We find the new contributions to the $(S,T,U)$ parameters and $\mathrm{Zb}\overline{b}$ vertex to be generically small. We analyze how the reasonable flavor mixing pattern can be generated to ensure the top-seesaw mechanism and sufficiently suppress flavor-changing effects for light quarks. Collider signatures for the light Higgs boson and top quark are also discussed.