183 research outputs found

### $\Lambda_c \to N$ form factors from lattice QCD and phenomenology of $\Lambda_c \to n \ell^+ \nu_\ell$ and $\Lambda_c \to p \mu^+ \mu^-$ decays

A lattice QCD determination of the $\Lambda_c \to N$ vector, axial vector,
and tensor form factors is reported. The calculation was performed with $2+1$
flavors of domain wall fermions at lattice spacings of $a\approx 0.11\:{\rm
fm},\:0.085\:{\rm fm}$ and pion masses in the range $230\:{\rm MeV} \lesssim
m_\pi \lesssim 350$ MeV. The form factors are extrapolated to the continuum
limit and the physical pion mass using modified $z$ expansions. The rates of
the charged-current decays $\Lambda_c \to n\, e^+ \nu_e$ and $\Lambda_c \to n\,
\mu^+ \nu_\mu$ are predicted to be $\left( 0.405 \pm 0.016_{\,\rm stat} \pm
0.020_{\,\rm syst} \right)|V_{cd}|^2 \:{\rm ps}^{-1}$ and $\left( 0.396 \pm
0.016_{\,\rm stat} \pm 0.020_{\,\rm syst} \right)|V_{cd}|^2 \:{\rm ps}^{-1}$,
respectively. The phenomenology of the rare charm decay $\Lambda_c \to p\,
\mu^+ \mu^-$ is also studied. The differential branching fraction, the fraction
of longitudinally polarized dimuons, and the forward-backward asymmetry are
calculated in the Standard Model and in an illustrative new-physics scenario.Comment: 21 pages, 7 figures, form factor parameters included as ancillary
file

### $\Lambda_c \to \Lambda \ell^+ \nu_\ell$ form factors and decay rates from lattice QCD with physical quark masses

The first lattice QCD calculation of the form factors governing $\Lambda_c
\to \Lambda \ell^+ \nu_\ell$ decays is reported. The calculation was performed
with two different lattice spacings and includes one ensemble with a pion mass
of 139(2) MeV. The resulting predictions for the $\Lambda_c \to \Lambda e^+
\nu_e$ and $\Lambda_c \to \Lambda \mu^+ \nu_\mu$ decay rates divided by
$|V_{cs}|^2$ are $0.2007(71)(74)\:{\rm ps}^{-1}$ and $0.1945(69)(72)\:{\rm
ps}^{-1}$, respectively, where the two uncertainties are statistical and
systematic. Taking the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cs}|$ from
a global fit and the $\Lambda_c$ lifetime from experiments, this translates to
branching fractions of $\mathcal{B}(\Lambda_c\to\Lambda
e^+\nu_e)=0.0380(19)_{\rm LQCD\:\:}(11)_{\tau_{\Lambda_c}}$ and
$\mathcal{B}(\Lambda_c\to\Lambda \mu^+\nu_\mu)=0.0369(19)_{\rm
LQCD\:\:}(11)_{\tau_{\Lambda_c}}$. These results are consistent with, and two
times more precise than, the measurements performed recently by the BESIII
Collaboration. Using instead the measured branching fractions together with the
lattice calculation to determine the CKM matrix element gives $|V_{cs}|=
0.949(24)_{\rm LQCD\:\:}(14)_{\tau_{\Lambda_c}}(49)_{\mathcal{B}}$.Comment: 6 pages, 3 figures, form factor parameters included as ancillary
file

### Excited-state spectroscopy of triply-bottom baryons from lattice QCD

The spectrum of baryons containing three b quarks is calculated in
nonperturbative QCD, using the lattice regularization. The energies of ten
excited bbb states with J^P = 1/2^+, 3/2^+, 5/2^+, 7/2^+, 1/2^-, and 3/2^- are
determined with high precision. A domain-wall action is used for the up-, down-
and strange quarks, and the bottom quarks are implemented with NRQCD. The
computations are done at lattice spacings of a \approx 0.11 fm and a \approx
0.08 fm, and the results demonstrate the improvement of rotational symmetry as
a is reduced. A large lattice volume of (2.7 fm)^3 is used, and extrapolations
of the bbb spectrum to realistic values of the light sea-quark masses are
performed. All spin-dependent energy splittings are resolved with total
uncertainties of order 1 MeV, and the dependence of these splittings on the
couplings in the NRQCD action is analyzed.Comment: 26 pages, 15 figures; added uncertainty due to choice of fit range;
accepted by PR

### Omega_bbb excited-state spectroscopy from lattice QCD

Triply heavy baryons are very interesting systems analogous to heavy
quarkonia, but are difficult to access experimentally. Lattice QCD can provide
precise predictions for these systems, which can be compared to other
theoretical approaches. In this work, the spectrum of excited states of the
Omega_bbb baryon is calculated using lattice NRQCD for the b quarks, and using
a domain-wall action for the u, d and s sea quarks. The calculations are done
for multiple values of the sea-quark masses, and for two different lattice
spacings. The energies of states with angular momentum up to J=7/2 are
calculated, and the effects of rotational symmetry breaking by the lattice are
analyzed. Precise results are obtained even for the small spin-dependent energy
splittings, and the contributions of individual NRQCD interactions to these
energy splittings are studied. The results are compared to potential-model
calculations.Comment: 8 pages, 5 figure

### $\Lambda_b \to \Lambda \ell^+ \ell^-$ form factors, differential branching fraction, and angular observables from lattice QCD with relativistic $b$ quarks

Using $(2+1)$-flavor lattice QCD, we compute the 10 form factors describing
the $\Lambda_b \to \Lambda$ matrix elements of the $b \to s$ vector, axial
vector, and tensor currents. The calculation is based on gauge field ensembles
generated by the RBC and UKQCD Collaborations with a domain-wall action for the
$u$, $d$, and $s$ quarks and the Iwasaki gauge action. The $b$ quark is
implemented using an anisotropic clover action, tuned nonperturbatively to the
physical point, and the currents are renormalized with a mostly nonperturbative
method. We perform simultaneous chiral, continuum, and kinematic extrapolations
of the form factors through modified $z$ expansions. Using our form factor
results, we obtain precise predictions for the $\Lambda_b \to \Lambda(\to p^+
\pi^-) \mu^+ \mu^-$ differential branching fraction and angular observables in
the Standard Model.Comment: 33 pages, 9 figures, form factor parameters included as ancillary
file

### Using $\Lambda_b\to \Lambda\mu^+\mu^-$ data within a Bayesian analysis of $|\Delta B| = |\Delta S| = 1$ decays

We study the impact of including the baryonic decay $\Lambda_b\to \Lambda(\to
p \pi^-)\mu^+\mu^-$ in a Bayesian analysis of $|\Delta B | = |\Delta S| = 1$
transitions. We perform fits of the Wilson coefficients $C_{9}$, $C_{9'}$,
$C_{10}$ and $C_{10'}$, in addition to the relevant nuisance parameters. Our
analysis combines data for the differential branching fraction and three
angular observables of $\Lambda_b\to \Lambda(\to p \pi^-)\mu^+\mu^-$ with data
for the branching ratios of $B_s \to \mu^+\mu^-$ and inclusive $b \to
s\ell^+\ell^-$ decays. Newly available precise lattice QCD results for the full
set of $\Lambda_b \to \Lambda$ form factors are used to evaluate the
observables of the baryonic decay. Our fits prefer shifts to $C_{9}$ that are
opposite in sign compared to those found in global fits of only mesonic decays,
and the posterior odds show no evidence of physics beyond the Standard Model.
We investigate a possible hadronic origin of the observed tensions between
theory and experiment.Comment: 9 pages, 2 figures; v2 as published: added some clarifications,
changed setup for model comparisons, expanded conclusion

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### Heavy quark physics on the lattice with improved nonrelativistic actions

Hadrons containing heavy quarks, in particular b quarks, play an important role in high energy physics. Measurements of their electroweak interactions are
used to test the Standard Model and search for new physics. For the comparison of experimental results with theoretical predictions, nonperturbative calculations of hadronic matrix elements within the theory of quantum chromodymanics are required. Such calculations can be performed from first principles by formulating QCD on a Euclidean spacetime grid and computing the path integral numerically. Including b quarks in lattice QCD calculations requires special techniques as the lattice spacing in present computations usually can not be chosen fine enough to resolve their Compton wavelength. In this work, improved nonrelativistic lattice actions for heavy quarks are used to perform calculations of the bottom hadron mass spectrum and of form factors for heavy-to-light decays. In heavy-to-light decays, additional complications arise at high recoil, when the momentum of the light meson reaches a magnitude comparable to the cutoff imposed by the lattice. Discretisation errors at high recoil can be reduced by working in a frame of reference where the heavy and light mesons move in opposite directions. Using a formalism referred to as moving nonrelativistic QCD (mNRQCD), the nonrelativistic expansion for the heavy quark can be performed around a state with an arbitrary velocity. This dissertation begins with a review of the fundamentals of lattice QCD. Then, the construction of effective Lagrangians for heavy quarks in the continuum and on the lattice is discussed in detail. A highly improved lattice mNRQCD action is derived and its effectiveness is demonstrated by nonperturbative tests involving both heavy-heavy and heavy-light mesons at several frame velocities. This mNRQCD action is then used in combination with a staggered action for the light quarks to calculate hadronic matrix elements relevant for rare B decays, including B --> K* gamma and B --> K l l. A major contribution to the uncertainty of the results also comes from statistical errors. The effectiveness of random-wall sources to reduce these errors is studied. As another application of a nonrelativistic heavy quark action, the spectrum of bottomonium is calculated and masses of several bottom baryons are predicted. In these computations, the light quarks are implemented with a domain wall action.I thank St John's College Cambridge, the Cambridge European Trust and the Engineering and Physical Sciences Research Council for financial support. This work has made use of high performance computing resources provided by the Fermilab Lattice Gauge Theory Computational Facility (http://www.usqcd.org/fnal),
the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk), the National Energy Research Scientific Computing Center (http://www.nersc.gov/), the National Center for Supercomputing Applications (http://www.ncsa.illinois.edu/) and Teragrid (http://www.teragrid.org/)

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