B026 $R_{D^*}$
LFU ratio in B $ \to D* \tau \nu$ over B $ \to D*$ light-lepton $\nu$ Status REVIEWED VERIFIED High Code: NO Priority Low
PDG / equivalent values
| Observable | Value | Year | Experiment / source | Provenance |
|---|---|---|---|---|
| R(D*) = B(B $ \to D* \tau \nu_\tau$) / B(B $ \to D*$ l $\nu_l$), $l=e,\mu$ | 0.281 dimensionless ratio | 2025 | HFLAV semileptonic B averages, CKM 2025 | source ↑ |
Why this constrains the RS scan
\(R_{D^*}\) is not a neutral-meson \(\Delta F=2\) bound and does not constrain
the current quark-scan backend directly. It becomes relevant in warped or
anarchic extensions with non-universal charged currents, third-generation
enhanced lepton couplings, charged-scalar exchange, \(W'\)-like vectors, or
leptoquark-motivated matching. Because the observable is a ratio, many CKM and
normalization uncertainties cancel, making it a sensitive diagnostic once a
model predicts the \(b\to c\tau\nu\) Wilson coefficients.
What's changed since the original paper
The 2008 CFW/Perez-Randall-era baseline
(
cfw2008\_arxiv0804\_1954) predates the \(R_D^{(*)}\) anomaly
program. The post-2008 experimental sequence starts with BaBar's 2012 and
2013 excess in \(B\to D^{(*)}\tau\nu\)
(babar2012\_arxiv1205\_5442;
babar2013\_arxiv1303\_0571), followed by Belle hadronic-tag,
tau-polarization, and semileptonic-tag measurements, LHCb muonic-tau and
three-prong-tau measurements, and Belle II inputs recorded in the HFLAV CKM
2025 average. HFLAV now performs a correlated average rather than comparing
one experiment to one SM number (hflav\_ckm2025\_rdrds;
hflav\_ckm2025\_logfile). On the SM side,
\(B\to D^*\ell\nu\) form-factor theory changed materially after 2008: HFLAV
cites benchmark, FNAL/MILC, HPQCD, JLQCD, and FLAG inputs, with the local FLAG
snapshot stored as flag2024\_arxiv2411\_04268. The residual tension
is therefore tied to correlated experimental averaging and form-factor
conventions, not to a missing basic SM prediction.Validity and model dependence
The HFLAV average is a robust experimental summary, but interpreting it as an
RS constraint is highly model-dependent. A live likelihood needs the
\((\bar c\,\Gamma b)(\bar\tau\,\Gamma\nu)\) charged-current operator basis,
neutrino-flavor assumptions, and the integrated \(B\to D^*\) response to
vector, scalar, pseudoscalar, and tensor operators. A standalone Gaussian for
\(R_{D^*}\) would discard the documented correlation with \(R_D\); any serious
implementation should use the two-dimensional HFLAV covariance or a dedicated
likelihood.
Code coverage in this repo
NO. The author reran the required greps over
quarkConstraints/, qcd/, flavorConstraints/,
neutrinos/, yukawa/, warpConfig/,
solvers/, scanParams/, and tests/. The broad
searches recover existing neutral-meson \(\Delta F=2\) and
\(\mu\to e\gamma\) code, but targeted searches for \(R(D^*)\), \(D^*\tau\),
\(B\to D^*\), \(b\to c\), charged-current, charged-Higgs, and leptoquark
observable code returned no implementation hits.
Linked evidence (opens GitHub blob at flavor-catalog-website/2026q2):
Implementation difficulty
HIGH. Promotion to a live constraint requires new charged-current
semileptonic matching, \(B\to D^*\) form-factor integration, and a correlated
\(R_D\)-\(R_{D^*}\) likelihood. This is beyond the existing \(\Delta F=2\)
and \(\mu\to e\gamma\) surfaces and should target
quarkConstraints/modern/
only if the PI later approves implementation.Reason: A live R(D*) constraint needs charged-current semileptonic matching, $B \to D*$ form-factor integration, and the correlated R(D)-R(D*) HFLAV likelihood; none is present in the current $\Delta F = 2$ or $\mu \to e \gamma$ code.
Key references
Process-local reference keys before bibliography consolidation:
hflav\_ckm2025\_rdrds,
hflav\_ckm2025\_logfile,
babar2012\_arxiv1205\_5442,
babar2013\_arxiv1303\_0571,
belle2015\_arxiv1507\_03233,
belle2017\_arxiv1612\_00529,
belle2018\_arxiv1709\_00129,
belle2019\_arxiv1910\_05864,
lhcb2023\_arxiv2302\_02886,
lhcb2023\_arxiv2305\_01463,
lhcb2025\_arxiv2406\_03387,
belleii2024\_arxiv2401\_02840,
belleii2025\_arxiv2504\_11220,
flag2024\_arxiv2411\_04268, and
cfw2008\_arxiv0804\_1954.