1,661 research outputs found
Lattice QCD calculation of the ${{B}_{(s)}\to D_{(s)}^{*}\ell{\nu}}$ form factors at zero recoil and implications for ${V_{cb}}$
We present results of a lattice QCD calculation of $B\to D^*$ and $B_s\to
D_s^*$ axial vector matrix elements with both states at rest. These zero recoil
matrix elements provide the normalization necessary to infer a value for the
CKM matrix element $V_{cb}$ from experimental measurements of $\bar{B}^0\to
D^{*+}\ell^\bar{\nu}$ and $\bar{B}^0_s\to D_s^{*+}\ell^\bar{\nu}$ decay.
Results are derived from correlation functions computed with highly improved
staggered quarks (HISQ) for light, strange, and charm quark propagators, and
nonrelativistic QCD for the bottom quark propagator. The calculation of
correlation functions employs MILC Collaboration ensembles over a range of
three lattice spacings. These gauge field configurations include sea quark
effects of charm, strange, and equalmass up and down quarks. We use ensembles
with physically light up and down quarks, as well as heavier values. Our main
results are $\mathcal{F}^{B\to D^*}(1)= 0.895\pm
0.010_{\mathrm{stat}}\pm{{0.024}_{\mathrm{sys}}}$ and $\mathcal{F}^{B_s\to
D_s^*}(1)= 0.883\pm 0.010_{\mathrm{stat}}\pm{0.028_{\mathrm{sys}}}$. We discuss
the consequences for $V_{cb}$ in light of recent investigations into the
extrapolation of experimental data to zero recoil.Comment: 23 pages. v3: Typos corrected. v2: Improved treatment of finite
volume effects. Small change to some results (but smaller than the quoted
uncertainties). Version accepted for publication in Phys. Rev.
Improving the theoretical prediction for the $B_s\bar{B}_s$ width difference: matrix elements of nexttoleading order $\Delta B=2$ operators
We present lattice QCD results for the matrix elements of $R_2$ and other
dimension7, $\Delta B = 2$ operators relevant for calculations of $\Delta
\Gamma_s$, the $B_s\bar{B}_s$ width difference. We have computed correlation
functions using 5 ensembles of the MILC Collaboration's 2+1+1flavour gauge
field configurations, spanning 3 lattice spacings and light sea quarks masses
down to the physical point. The HISQ action is used for the valence strange
quarks, and the NRQCD action is used for the bottom quarks. Once our analysis
is complete, the theoretical uncertainty in the Standard Model prediction for
$\Delta \Gamma_s$ will be substantially reduced.Comment: 8 pages. To appear in the Proceedings of the 35th International
Symposium on Lattice Field Theory, 1824 June 2017, Granada, Spai
$B \rightarrow D^*$ vector, axialvector and tensor form factors for the full $q^2$ range from lattice QCD
We compute the complete set of SM and tensor $B_{(s)}\to
D_{(s)}^*\ell\bar{\nu}$ semileptonic form factors across the full kinematic
range of the decay using second generation MILC $n_f=2+1+1$ HISQ gluon field
configurations and HISQ valence quarks, with the heavyHISQ method. Lattice
spacings range from $0.09\mathrm{fm}$ to $0.044\mathrm{fm}$ with pion masses
from $\approx 300\mathrm{MeV}$ down to the physical value and heavy quark
masses ranging between $\approx 1.5 m_c$ and $4.1 m_c \approx 0.9 m_b$;
currents are normalised nonperturbatively. Using the recent $B_{(s)}\to
D^*_{(s)}\ell\bar{\nu}_\ell$ data from Belle and LHCb together with our form
factors we determine a model independent value of
$V_{cb}=39.03(56)_\mathrm{exp}(67)_\mathrm{latt}\times 10^{3}$, in agreement
with previous exclusive determinations and in tension with the inclusive result
at the level of $3.6\sigma$. We observe a $\approx 1\sigma$ tension between the
shape of the differential decay rates computed using our form factors and those
measured by Belle. We compute a latticeonly SM value for the ratio of
semitauonic and semimuonic decay rates, $R(D^*)=0.273(15)$, which we find to be
closer to the recent Belle measurement and HFLAV average than theory
predictions using fits to experimental differential rate data for $B\to
D^*\ell\bar{\nu}_\ell$. Determining $V_{cb}$ using the total rate for $B\to
D^*\ell\nu$ gives a value in agreement with inclusive results. We compute the
longitudinal polarisation fraction for the semitauonic mode,
$F_L^{D^*}=0.395(24)$, which is in tension at the level of $2.2\sigma$ with the
recent Belle measurement. Our calculation combines $B\to D^*$ and $B_s\to
D_s^*$ lattice results, and we provide an update which supersedes our previous
lattice computation of the $B_s\to D_s^*$ form factors. We also give the chiral
perturbation theory needed to analyse the tensor form factors.Comment: 49 pages, 27 figure
Initial distribution spread: A density forecasting approach
Ensemble forecasting of nonlinear systems involves the use of a model to run forward a discrete ensemble (or set) of initial states. Data assimilation techniques tend to focus on estimating the true state of the system, even though model error limits the value of such efforts. This paper argues for choosing the initial ensemble in order to optimise forecasting performance rather than estimate the true state of the system. Density forecasting and choosing the initial ensemble are treated as one problem. Forecasting performance can be quantified by some scoring rule. In the case of the logarithmic scoring rule, theoretical arguments and empirical results are presented. It turns out that, if the underlying noise dominates model error, we can diagnose the noise spread
$B_c \rightarrow J/\psi$ Form Factors for the full $q^2$ range from Lattice QCD
We present the first lattice QCD determination of the $B_c \rightarrow
J/\psi$ vector and axialvector form factors. These will enable experimental
information on the rate for $B_c$ semileptonic decays to $J/\psi$ to be
converted into a value for $V_{cb}$. Our calculation covers the full physical
$q^2$ range of the decay and uses nonperturbatively renormalised lattice
currents. We use the Highly Improved Staggered Quark (HISQ) action for all
valence quarks on the second generation MILC ensembles of gluon field
configurations including $u$, $d$, $s$ and $c$ HISQ sea quarks. Our HISQ heavy
quarks have masses ranging upwards from that of $c$; we are able to reach that
of the $b$ on our finest lattices. This enables us to map out the dependence on
heavy quark mass and determine results in the continuum limit at the $b$. We
use our form factors to construct the differential rates for $B_c^ \rightarrow
J/\psi \mu^ \bar{\nu}_\mu$ and obtain a total rate with $7\%$ uncertainty:
$\Gamma(B_c^\rightarrow J/\psi
\mu^\bar{\nu}_{\mu})/\eta_{\mathrm{EW}}V_{cb}^2 = 1.73(12)\times 10^{13}
~\mathrm{s}^{1}$. Including values for $V_{cb}$, $\eta_{\mathrm{EW}}$ and
$\tau_{B_c}$ yields a branching fraction for this decay mode of
0.0150(11)(10)(3) ~with uncertainties from lattice QCD,
$\eta_\mathrm{EW}V_{cb}$ and $\tau_{B_c}$ respectively.Comment: 23 pages, 14 Figures, Version accepted for publication in Phys. Rev.
Recommended from our members
Lattice QCD determination of weak decays of B mesons
This thesis uses a variety of numerical and statistical techniques to perform high precision calculations in high energy physics using quantum field theory. It introduces the experimental motivation for the calculation of B meson form factors and includes a discussion of previous work. It then describes the modern theoretical framework describing these phenomena, outlining quantum chromodynamics and electroweak theory, and then illustrating the procedure of gauge fixing, the quantum effective action and background field gauge which is required for subsequent perturbative work. Details of the basic methodology of lattice quantum field theory are given as well as the specific formulation of the relativistic theory and nonrelativistic approximations used in this work to describe quantum chromodynamics. A comprehensive calculation of the zero recoil B to D* form factor is then presented, using state of the art lattice techniques with relativistic charm
sea quarks and light sea quarks with correct physical masses, leading to a discussion of the dominant sources of uncertainty and possible resolutions of experimental tensions. Also included is preliminary work towards the full calculation of nonzero recoil matrix elements, with the aim of outlining possible future work. Finally, this thesis presents the computation of parameters correcting for radiative one loop phenomena and corrections to the kinetic coupling parameters in nonrelativistic quantum chromodynamics in order to achieve a desirable level of precision in future calculations. This is done using MonteCarlo integration to evaluate integrals from diagrams generated using automated lattice perturbation theory in background field gauge in order to match the coefficients of the effective action between the lattice and the continuum
Bs â†’ D*s Form Factors for the full qÂ² range from lattice QCD
We compute the Standard Model semileptonic vector and axialvector form factors for
B
s
â†’
D
âˆ—
s
decay across the full
q
2
range using lattice QCD. We use the highly improved staggered quark (HISQ) action for all valence quarks, enabling us to normalize weak currents nonperturbatively. Working on secondgeneration MILC ensembles of gluon field configurations which include
u
,
d
,
s
, and
c
HISQ sea quarks and HISQ heavy quarks with masses from that of the
c
mass up to that of the
b
on the ensemble with the smallest lattice spacing, allows us to map out the heavy quark mass dependence of the form factors, and to constrain the associated discretization effects. We can then determine the physical form factors at the
b
mass. We use these to construct the differential and total rates for
Î“
(
B
0
s
â†’
D
*
âˆ’
s
â„“
+
Î½
â„“
)
and find
Î“
â„“
=
e
/

Î·
EW
V
c
b

2
=
2.07
(
17
)
latt
(
2
)
EM
Ã—
10
13
â€‰
â€‰
s
âˆ’
1
,
Î“
â„“
=
Î¼
/

Î·
EW
V
c
b

2
=
2.06
(
16
)
latt
(
2
)
EM
Ã—
10
13
â€‰
â€‰
s
âˆ’
1
, and
Î“
â„“
=
Ï„
/

Î·
EW
V
c
b

2
=
5.14
(
37
)
latt
(
5
)
EM
Ã—
10
12
â€‰
â€‰
s
âˆ’
1
, where
Î·
EW
contains the shortdistance electroweak correction to
G
F
, the first uncertainty is from our lattice calculation, and the second allows for longdistance QED effects. The ratio
R
(
D
*
âˆ’
s
)
â‰¡
Î“
â„“
=
Ï„
/
Î“
â„“
=
Î¼
=
0.2490
(
60
)
latt
(
35
)
EM
. We also obtain a value for the ratio of decay rates
Î“
â„“
=
Î¼
(
B
s
â†’
D
s
)
/
Î“
â„“
=
Î¼
(
B
s
â†’
D
âˆ—
s
)
=
0.443
(
40
)
latt
(
4
)
EM
, which agrees well with recent LHCb results. We can determine
V
c
b
by combining our lattice results across the full kinematic range of the decay with experimental results from LHCb and obtain

V
c
b

=
42.2
(
1.5
)
latt
(
1.7
)
exp
(
0.4
)
EM
Ã—
10
âˆ’
3
. A comparison of our lattice results for the shape of the differential decay rate to the binned, normalized differential decay rate from LHCb shows good agreement. We also test the impact of new physics couplings on angular observables and ratios which are sensitive to lepton flavor universality violation
The impact of behavioural risk factors on communicable diseases: a systematic review of reviews
Abstract Background The coronavirus (COVID19) pandemic has highlighted that individuals with behavioural risk factors commonly associated with noncommunicable diseases (NCDs), such as smoking, harmful alcohol use, obesity, and physical inactivity, are more likely to experience severe symptoms from COVID19. These risk factors have been shown to increase the risk of NCDs, but less is known about their broader influence on communicable diseases. Taking a wide focus on a range of common communicable diseases, this review aimed to synthesise research examining the impact of behavioural risk factors commonly associated with NCDs on risks of contracting, or having more severe outcomes from, communicable diseases. Methods Literature searches identified systematic reviews and metaanalyses that examined the association between behavioural risk factors (alcohol, smoking, illicit drug use, physical inactivity, obesity and poor diet) and the contraction/severity of common communicable diseases, including infection or associated pathogens. An a priori, prospectively registered protocol was followed (PROSPERO; registration number CRD42020223890). Results Fiftythree systematic reviews were included, of which 36 were also metaanalyses. Reviews focused on: tuberculosis, human immunodeficiency virus, hepatitis C virus, hepatitis B virus, invasive bacterial diseases, pneumonia, influenza, and COVID19. Twentyone reviews examined the association between behavioural risk factors and communicable disease contraction and 35 examined their association with communicable disease outcomes (three examined their association with both contraction and outcomes). Fifty out of 53 reviews (94%) concluded that at least one of the behavioural risk factors studied increased the risk of contracting or experiencing worse health outcomes from a communicable disease. Across all reviews, effect sizes, where calculated, ranged from 0.83 to 8.22. Conclusions Behavioural risk factors play a significant role in the risk of contracting and experiencing more severe outcomes from communicable diseases. Prevention of communicable diseases is likely to be most successful if it involves the prevention of behavioural risk factors commonly associated with NCDs. These findings are important for understanding risks associated with communicable disease, and timely, given the COVID19 pandemic and the need for improvements in future pandemic preparedness. Addressing behavioural risk factors should be an important part of work to build resilience against any emerging and future epidemics and pandemics
R(J/Ïˆ) and Bâˆ’c â†’ J/ Ïˆâ„“âˆ’Â¯Î½l lepton flavor universality violating observables from lattice QCD
We use our lattice QCD computation of the Bcâ†’J/Ïˆ form factors to determine the differential decay rate for the semitauonic decay channel and construct the ratio of branching fractions R(J/Ïˆ)=B(Bâˆ’câ†’J/ÏˆÏ„âˆ’Î½Ï„)/B(Bâˆ’câ†’J/ÏˆÎ¼âˆ’Î½Î¼). We find R(J/Ïˆ)=\rjpsi and give an error budget. We also extend the relevant angular observables, which were recently suggested for the study of lepton flavor universality violating effects in
Bâ†’Dâˆ—â„“Î½, to Bcâ†’J/Ïˆâ„“Î½ and make predictions for their values under different new physics scenarios
 â€¦