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Register a new userCharm-baryon semileptonic decays and the strange $Lambda^*$ resonances: New insights from lattice QCD
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Understanding the properties of the strange $Lambda^*$ baryon resonances is a long-standing and fascinating problem. $Lambda_c$ charm-baryon semileptonic weak decays to these resonances are highly sensitive to their internal structure and can be used to test theoretical models. We have performed the first lattice-QCD computation of the form factors governing $Lambda_c$ semileptonic decays to a $Lambda^*$ resonance: the $Lambda^*(1520)$, which has negative parity and spin $3/2$. Here we present the resulting Standard-Model predictions of the $Lambda_ctoLambda^*(1520)ell^+ u_ell$ differential and integrated decay rates as well as angular observables. Furthermore, by combining the recent BESIII measurement of the $Lambda_c to X e^+ u_e$ inclusive semipositronic branching fraction [Phys. Rev. Lett. 121, 251801 (2018)] with lattice-QCD predictions of the $Lambda_c to Lambda e^+ u_e$, $Lambda_c to n e^+ u_e$, and $Lambda_c to Lambda^*(1520) e^+ u_e$ decay rates, we obtain an upper limit on the sum of the branching fractions to all other semipositronic final states. In particular, this upper limit constrains the $Lambda_ctoLambda^*(1405)e^+ u_e$ branching fraction to be very small, which may be another hint for a molecular structure of the $Lambda^*(1405)$.
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We study the exclusive semileptonic $B$-meson decays $Bto K(pi)ell^+ell^-$, $Bto K(pi) ubar u$, and $Btopitau u$, computing observables in the Standard model using the recent lattice-QCD results for the underlying form factors from the Fermilab Lattice and MILC Collaborations. These processes provide theoretically clean windows into physics beyond the Standard Model because the hadronic uncertainties are now under good control for suitably binned observables. For example, the resulting partially integrated branching fractions for $Btopimu^+mu^-$ and $Bto Kmu^+mu^-$ outside the charmonium resonance region are 1-2$sigma$ higher than the LHCb Collaborations recent measurements, where the theoretical and experimental errors are commensurate. The combined tension is 1.7$sigma$. Combining the Standard-Model rates with LHCbs measurements yields values for the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements $|V_{td}|=7.45{(69)}times10^{-3}$, $|V_{ts}|=35.7(1.5)times10^{-3}$, and $|V_{td}/V_{ts}|=0.201{(20)}$, which are compatible with the values obtained from neutral $B_{(s)}$-meson oscillations and have competitive uncertainties. Alternatively, taking the CKM matrix elements from unitarity, we constrain new-physics contributions at the electroweak scale. The constraints on the Wilson coefficients ${rm Re}(C_9)$ and ${rm Re}(C_{10})$ from $Btopimu^+mu^-$ and $Bto Kmu^+mu^-$ are competitive with those from $Bto K^* mu^+mu^-$, and display a 2.0$sigma$ tension with the Standard Model. Our predictions for $Bto K(pi) ubar u$ and $Btopitau u$ are close to the current experimental limits.
We demonstrate that a nonzero strangeness contribution to the spacelike electromagnetic form factor of the nucleon is evidence for a strange-antistrange asymmetry in the nucleons light-front wave function, thus implying different nonperturbative contributions to the strange and antistrange quark distribution functions. A recent lattice QCD calculation of the nucleon strange quark form factor predicts that the strange quark distribution is more centralized in coordinate space than the antistrange quark distribution, and thus the strange quark distribution is more spread out in light-front momentum space. We show that the lattice prediction implies that the difference between the strange and antistrange parton distribution functions, $s(x)-bar{s}(x)$, is negative at small-$x$ and positive at large-$x$. We also evaluate the strange quark form factor and $s(x)-bar{s}(x)$ using a baryon-meson fluctuation model and a novel nonperturbative model based on light-front holographic QCD. This procedure leads to a Veneziano-like expression of the form factor, which depends exclusively on the twist of the hadron and the properties of the Regge trajectory of the vector meson which couples to the quark current in the hadron. The holographic structure of the model allows us to introduce unambiguously quark masses in the form factors and quark distributions preserving the hard scattering counting rule at large-$Q^2$ and the inclusive counting rule at large-$x$. Quark masses modify the Regge intercept which governs the small-$x$ behavior of quark distributions, therefore modifying their small-$x$ singular behavior. Both nonperturbative approaches provide descriptions of the strange-antistrange asymmetry and intrinsic strangeness in the nucleon consistent with the lattice QCD result.
The strong decays of charm-strange baryons up to N=2 shell are studied in a chiral quark model. The theoretical predictions for the well determined charm-strange baryons, $Xi_c^*(2645)$, $Xi_c(2790)$ and $Xi_c(2815)$, are in good agreement with the experimental data. This model is also extended to analyze the strong decays of the other newly observed charm-strange baryons $Xi_c(2930)$, $Xi_c(2980)$, $Xi_c(3055)$, $Xi_c(3080)$ and $Xi_c(3123)$. Our predictions are given as follows. (i) $Xi_c(2930)$ might be the first $P$-wave excitation of $Xi_c$ with $J^P=1/2^-$, favors the $|Xi_c ^2P_lambda 1/2^->$ or $|Xi_c ^4P_lambda 1/2^->$ state. (ii) $Xi_c(2980)$ might correspond to two overlapping $P$-wave states $|Xi_c ^2P_rho 1/2^->$ and $|Xi_c ^2P_rho 3/2^->$, respectively. The $Xi_c(2980)$ observed in the $Lambda_c^+bar{K}pi$ final state is most likely to be the $|Xi_c ^2P_rho 1/2^->$ state, while the narrower resonance with a mass $msimeq 2.97$ GeV observed in the $Xi_c^*(2645)pi$ channel favors to be assigned to the $|Xi_c ^2P_rho 3/2^->$ state. (iii) $Xi_c(3080)$ favors to be classified as the $|Xi_c S_{rhorho} 1/2^+>$ state, i.e., the first radial excitation (2S) of $Xi_c$. (iv) $Xi_c(3055)$ is most likely to be the first $D$-wave excitation of $Xi_c$ with $J^P=3/2^+$, favors the $|Xi_c ^2D_{lambdalambda} 3/2^+>$ state. (v) $Xi_c(3123)$ might be assigned to the $|Xi_c ^4D_{lambdalambda} 3/2^+>$, $|Xi_c ^4D_{lambdalambda} 5/2^+>$, or $|Xi_c ^2D_{rhorho} 5/2^+>$ state. As a by-product, we calculate the strong decays of the bottom baryons $Sigma_b^{pm}$, $Sigma_b^{*pm}$ and $Xi_b^*$, which are in good agreement with the recent observations as well.
In this work we discuss in detail the non-perturbative determination of the momentum dependence of the form factors entering in semileptonic decays using unitarity and analyticity constraints. The method contains several new elements with respect to previous proposals and allows to extract, using suitable two-point functions computed non-perturbatively, the form factors at low momentum transfer $q^2$ from those computed explicitly on the lattice at large $q^2$, without any assumption about their $q^2$-dependence. The approach will be very useful for exclusive semileptonic $B$-meson decays, where the direct calculation of the form factors at low $q^2$ is particularly difficult due to large statistical fluctuations and discretisation effects. As a testing ground we apply our approach to the semileptonic $D to K ell u_ell$ decay, where we can compare the results of the unitarity approach to the explicit direct lattice calculation of the form factors in the full $q^2$-range. We show that the method is very effective and that it allows to compute the form factors with rather good precision.
We present first lattice QCD results for semileptonic form factors for the decays $B_c to eta_c l u$ and $B_c to J/psi l u$ over the full $q^2$ range, using both improved non-relativistic QCD (NRQCD) and fully relativistic (HISQ) formalisms. These can be viewed as prototype calculations for pseudoscalar to pseudoscalar and pseudoscalar to vector decays involving a $b to c$ transition. In particular we can use information from the relativistic computations to fix the NRQCD current normalisations, which can then be used in improved computations of decays such as $B to D l u$ and $B to D^* l u$.
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