[Show abstract][Hide abstract]ABSTRACT: Revisiting the strong couplings of three mesons, each of which involves at least one charm quark, proves clear disaccord between quark-model and QCD sum-rule results.
[Show abstract][Hide abstract]ABSTRACT: We review lattice results related to pion, kaon, D- and B-meson physics with the aim of making them easily accessible to the particle physics community. More specifically, we report on the determination of the light-quark masses, the form factor f+(0), arising in the semileptonic K -> pi transition at zero momentum transfer, as well as the decay constant ratio fK/fpi and its consequences for the CKM matrix elements Vus and Vud. Furthermore, we describe the results obtained on the lattice for some of the low-energy constants of SU(2)LxSU(2)R and SU(3)LxSU(3)R Chiral Perturbation Theory. We review the determination of the BK parameter of neutral kaon mixing as well as the additional four B parameters that arise in theories of physics beyond the Standard Model. The latter quantities are an addition compared to the previous review. For the heavy-quark sector, we provide results for mc and mb (also new compared to the previous review), as well as those for D- and B-meson decay constants, form factors, and mixing parameters. These are the heavy-quark quantities most relevant for the determination of CKM matrix elements and the global CKM unitarity-triangle fit. Finally, we review the status of lattice determinations of the strong coupling constant alpha_s.
[Show abstract][Hide abstract]ABSTRACT: We present precise lattice computations for the b-quark mass, the quark mass ratios mb/mc and mb/ms as well as the leptonic B-decay constants. We employ gauge configurations with four dynamical quark flavors, up-down, strange and charm, at three values of the lattice spacing (a∼0.06–0.09 fm) and for pion masses as low as 210 MeV. Interpolation in the heavy quark mass to the bottom quark point is performed using ratios of physical quantities computed at nearby quark masses exploiting the fact that these ratios are exactly known in the static quark mass limit. Our results are also extrapolated to the physical pion mass and to the continuum limit and read mb(MS¯,mb)=4.26(10) GeV, mb/mc=4.42(8), mb/ms=51.4(1.4), fBs=229(5) MeV, fB=193(6) MeV, fBs/fB=1.184(25) and (fBs/fB)/(fK/fπ)=0.997(17).
[Show abstract][Hide abstract]ABSTRACT: We present precise lattice computations for the b-quark mass, the quark mass ratios mb/mc and mb/ms as well as the leptonic B-decay constants. We employ gauge configurations with four dynamical quark flavors, up/down, strange and charm, at three values of the lattice spacing (a ~ 0.06 - 0.09 fm) and for pion masses as low as 210 MeV. Interpolation in the heavy quark mass to the bottom quark point is performed using ratios of physical quantities computed at nearby quark masses exploiting the fact that these ratios are exactly known in the static quark mass limit. Our results are also extrapolated to the physical pion mass and to the continuum limit and read: mb(MSbar, mb) = 4.26(10) GeV, mb/mc = 4.42(8), mb/ms = 51.4(1.4), fBs = 229(5) MeV, fB = 193(6) MeV, fBs/fB = 1.184(25) and (fBs/fB)/(fK/fpi) = 0.997(17).
[Show abstract][Hide abstract]ABSTRACT: We present a lattice QCD determination of the vector and scalar form factors of the semileptonic $K \to \pi \ell \nu$ decay which are relevant for the extraction of the CKM matrix element $|V_{us}|$ from experimental data. Our results are based on the gauge configurations produced by the European Twisted Mass Collaboration with $N_f = 2+1+1$ dynamical fermions, which include in the sea, besides two light mass degenerate quarks, also the strange and the charm quarks. We use data simulated at three different values of the lattice spacing and with pion masses as small as 210 MeV. Our final result for the vector form factor at zero momentum transfer is $f_+(0) = 0.9709 (46)$, where the uncertainty is both statistical and systematic combined in quadrature. Using the latest experimental value of $f_+(0) |V_{us}|$ from $K_{\ell 3}$ decays, we obtain $|V_{us}| = 0.2230 (11)$, which allows to test the unitarity constraint of the Standard Model below the permille level once the determination of $|V_{ud}|$ from superallowed nuclear $\beta$ decays is adopted. A slight tension with unitarity at the level of $\sim 2$ standard deviations is observed. Moreover we present our results for the semileptonic scalar $f_0(q^2)$ and vector $f_+(q^2)$ form factors in the whole range of values of the squared four-momentum transfer $q^2$ measured in $K_{\ell 3}$ decays, obtaining a very good agreement with the momentum dependence of the experimental data. We provide a set of synthetic data points representing our results for the vector and scalar form factors at the physical point for several selected values of $q^2$.
[Show abstract][Hide abstract]ABSTRACT: We discuss the strong couplings gPPV and gVVP for vector (V) and pseudoscalar (P) mesons, at least one of which is a charmonium state J/ψ or ηc. The strong couplings are obtained as residues at the poles of suitable form factors, calculated in a broad range of momentum transfers using a dispersion formulation of the relativistic constituent quark model. The form factors obtained in this approach satisfy all constraints known for these quantities in the heavy-quark limit. Our results suggest sizably higher values for the strong meson couplings than those reported in the literature from QCD sum rules.
[Show abstract][Hide abstract]ABSTRACT: We present a lattice QCD determination of the vector and scalar form factors
of the semileptonic decays D->\pi l \nu and D -> K l \nu which are relevant for
the extraction of the CKM matrix elements |Vcd| and |Vcs| from experimental
data. Our analysis is based on the gauge configurations produced by the
European Twisted Mass Collaboration with Nf = 2+1+1 dynamical fermions. We
simulated at three different values of the lattice spacing and with pion masses
as small as 210 MeV. Our preliminary estimates for the vector form factor at
zero 4-momentum transfer are f+(D -> \pi)(0) = 0.610 (23) and f+(D -> K)(0) =
0.747 (22), where the uncertainties are only statistical. By combining our
results with the experimental values of f+(D -> \pi)(0) |Vcd| and f+(D -> K)(0)
|Vcs| we obtain |Vcd| = 0.2336 (93) and |Vcs| = 0.975 (30), which together with
the PDG determination of |Vcb| are in agreement with the unitarity constraint
of the Standard Model.
[Show abstract][Hide abstract]ABSTRACT: We present a lattice QCD determination of the vector and scalar form factors
of the kaon semileptonic decay $K \to \pi \ell \nu$, which is relevant for the
determination of the CKM matrix element $|V_{us}|$ from experimental data. Our
results are based on the gauge configurations produced by the European Twisted
Mass Collaboration with Nf = 2+1+1 dynamical fermions. We simulated at three
different values of the lattice spacing and with pion masses as small as 210
MeV. Our estimate for the vector form factor at zero 4-momentum transfer is
$f_+(0) = 0.9709 (46)$, where the uncertainty is both statistical and
systematic. By combining our result with the latest experimental value of
$f_+(0)|V_{us}|$ we obtain $|V_{us}| = 0.2230 (11)$, which satisfies the
unitarity constraint of the Standard Model at the permille level using the
updated determination of $|V_{ud}|$ coming from superallowed nuclear $\beta$
decays. We present also the momentum dependence of the vector and scalar form
factors in the whole range of values of the squared 4-momentum transfer
measured in $K_{\ell 3}$ decays, obtaining a good agreement with the
experimental data.
[Show abstract][Hide abstract]ABSTRACT: We present the implementation of twisted mass fermion operators for the QPhiX
library. We analyze the performance on the Intel Xeon Phi (Knights Corner)
coprocessor as well as on Intel Xeon Haswell CPUs. In particular, we
demonstrate that on the Xeon Phi 7120P the Dslash kernel is able to reach 80\%
of the theoretical peak bandwidth, while on a Xeon Haswell E5-2630 CPU our
generated code for the Dslash operator with AVX2 instructions outperforms the
corresponding implementation in the tmLQCD library by a factor of $\sim
5\times$ in single precision. We strong scale the code up to 6.8 (14.1) Tflops
in single (half) precision on 64 Xeon Haswell CPUs.
[Show abstract][Hide abstract]ABSTRACT: We extract the strong coupling constants of three mesons, each of which is
composed of either two charm quarks or one charm quark and one light (i.e.,
$u$, $d$, or $s$) quark, from the matrix elements for the transitions of two of
these mesons induced by appropriate quark currents within the framework of a
relativistic dispersion approach to the constituent-quark picture of mesons.
Among others, we also analyse the impact of the violation of the SU(3) flavour
symmetry by the quark masses. In the case of mesons containing one light quark,
we observe, in two respects, discrepancies between our findings and the
predictions of QCD sum rules: our strong couplings exceed considerably the ones
emerging from QCD sum rules, and, in our approach, the replacement of a light
quark by a strange quark entails, in contrast to QCD sum rules, a reduction of
the magnitudes of the strong couplings.
[Show abstract][Hide abstract]ABSTRACT: The relative magnitude of the decay constants of the pseudoscalar and vector
beauty mesons poses (in contrast to the case of charmed mesons) a long-standing
puzzle. We revisit this problem within the framework of our recent improvements
of the QCD sum-rule formalism for relating observable properties of mesons to
quantum chromodynamics and are led to conclude that the decay constants of the
beauty vector mesons are undoubtedly smaller than those of their pseudoscalar
counterparts.
[Show abstract][Hide abstract]ABSTRACT: We discuss the strong couplings $g_{PPV}$ and $g_{VVP}$ for vector ($V$) and
pseudoscalar ($P$) mesons, at least one of which is a charmonium state $J/\psi$
or $\eta_c$. The strong couplings are obtained as residues at the poles of
suitable form factors, calculated in a broad range of momentum transfers by a
relativistic dispersion approach relying on the constituent quark picture. The
spectral representations for the couplings under discussion satisfy all
constraints known for these quantities in the heavy-quark limit. Our results
suggest sizeably higher values than those reported in the literature from QCD
sum rules.
[Show abstract][Hide abstract]ABSTRACT: We present unquenched lattice QCD results for the matrix elements of
four-fermion operators relevant to the description of the neutral K and D
mixing in the Standard Model and its extensions. We have employed simulations
with Nf = 2 + 1 + 1 dynamical sea quarks at three values of the lattice
spacings in the interval 0.06 - 0.09 fm and pseudoscalar meson masses in the
range 210 - 450 MeV. Our results are extrapolated to the continuum limit and to
the physical pion mass. Renormalization constants have been determined
non-perturbatively in the RI-MOM scheme. In particular, for the Kaon
bag-parameter, which is relevant for the \overline{K}^0-K^0 mixing in the
Standard Model, we obtain B_K^{RGI} = 0.717(24).
[Show abstract][Hide abstract]ABSTRACT: We present our analysis of the decay constants of the beauty vector mesons
$B^*$ and $B^*_s$ within the framework of dispersive sum rules for the
two-point correlator of vector currents in QCD. While the decay constants of
the vector mesons $f_{B^*}$ and $f_{B_s^*}$ $-$ similar to the decay constants
of the pseudoscalar mesons $f_B$ and $f_{B_s}$ $-$ individually have large
uncertainties induced by theory parameters not known with a satisfactory
precision, these uncertainties almost entirely cancel out in the ratios of
vector over pseudoscalar decay constants. These ratios may be thus predicted
with very high accuracy due to the good control over the systematic
uncertainties of the decay constants gained upon application of our
hadron-parameter extraction algorithm. Our final results read
$f_{B^*}/f_B=0.944\pm0.011_{\rm OPE}\pm0.018_{\rm syst}$ and
$f_{B_s^*}/f_{B_s}=0.947\pm0.023_{\rm OPE}\pm0.020_{\rm syst}$. Thus, both
$f_{B^*}/f_B$ and $f_{B_s^*}/f_{B_s}$ are less than unity at 2.5$\sigma$ and
2$\sigma$ level, respectively.
[Show abstract][Hide abstract]ABSTRACT: We study matrix elements of the "chromomagnetic" operator on the lattice. This operator is contained in the strangeness-changing effective Hamiltonian which describes electroweak effects in the Standard Model and beyond. Having dimension 5, the chromomagnetic operator is characterized by a rich pattern of mixing with other operators of equal and lower dimensionality, including also non gauge invariant quantities; it is thus quite a challenge to extract from lattice simulations a clear signal for the hadronic matrix elements of this operator. We compute all relevant mixing coefficients to one loop in lattice perturbation theory; this necessitates calculating both 2-point (quark-antiquark) and 3-point (gluon-quark-antiquark) Green's functions at nonzero quark masses. We use the twisted mass lattice formulation, with Symanzik improved gluon action. We also provide a nonperturbative method to compute mixing with lower dimensional operators; we use this method in numerical simulations, and extract the mixing with the 3-dimensional scalar density, finding good agreement with one-loop results.
Full-text Article · Dec 2014 · Journal of Physics Conference Series
[Show abstract][Hide abstract]ABSTRACT: The Chromomagnetic operator (CMO) mixes with a large number of operators
under renormalization. We identify which operators can mix with the CMO, at the
quantum level. Even in dimensional regularization (DR), which has the simplest
mixing pattern, the CMO mixes with a total of 9 other operators, forming a
basis of dimension-five, Lorentz scalar operators with the same flavor content
as the CMO. Among them, there are also gauge noninvariant operators; these are
BRST invariant and vanish by the equations of motion, as required by
renormalization theory. On the other hand using a lattice regularization
further operators with $d \leq 5$ will mix; choosing the lattice action in a
manner as to preserve certain discrete symmetries, a minimul set of 3
additional operators (all with $d<5$) will appear. In order to compute all
relevant mixing coefficients, we calculate the quark-antiquark (2-pt) and the
quark-antiquark-gluon (3-pt) Green's functions of the CMO at nonzero quark
masses. These calculations were performed in the continuum (dimensional
regularization) and on the lattice using the maximally twisted mass fermion
action and the Symanzik improved gluon action. In parallel, non-perturbative
measurements of the $K-\pi$ matrix element are being performed in simulations
with 4 dynamical ($N_f = 2+1+1$) twisted mass fermions and the Iwasaki improved
gluon action.
[Show abstract][Hide abstract]ABSTRACT: We present preliminary results of the first lattice QCD calculation of the K
-> pi matrix elements of the chromomagnetic operator O_{CM}=g sbar sigma_{munu}
G_{munu} d, which appears in the effective Hamiltonian describing Delta S=1
transitions in and beyond the Standard Model. Having dimension 5, the
chromomagnetic operator is characterized by a rich pattern of mixing with
operators of equal and lower dimensionality. The multiplicative renormalization
factor as well as the mixing coefficients with the operators of equal dimension
have been computed at one-loop in perturbation theory. The power divergent
coefficients controlling the mixing with operators of lower dimension have been
computed non-perturbatively, by imposing suitable subtraction conditions. The
numerical simulations have been carried out using the gauge field
configurations produced by the European Twisted Mass Collaboration with
N_f=2+1+1 dynamical quarks at three values of the lattice spacing. Our
preliminary result for the B-parameter of the chromomagnetic operator is
B_{CMO}=0.29(11), which can be compared with the estimate B_{CMO}~1-4 currently
used in phenomenological analyses.