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170
result(s) for
"Straub, David M."
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Constraining new physics in b → cℓν transitions
A
bstract
B
decays proceeding via
b
→
cℓν
transitions with
ℓ
=
e
or
μ
are tree-level processes in the Standard Model. They are used to measure the CKM element
V
cb
, as such forming an important ingredient in the determination of e.g. the unitarity triangle; hence the question to which extent they can be affected by new physics contributions is important, specifically given the long-standing tension between
V
cb
determinations from inclusive and exclusive decays and the significant hints for lepton flavour universality violation in
b
→
cτ ν
and
b
→
sℓℓ
decays. We perform a comprehensive model-independent analysis of new physics in
b
→
cℓν
, considering all combinations of scalar, vector and tensor interactions occuring in single-mediator scenarios. We include for the first time differential distributions of
B
→
D
∗
ℓν
angular observables for this purpose. We show that these are valuable in constraining non-standard interactions. Specifically, the zero-recoil endpoint of the
B
→
Dℓν
spectrum is extremely sensitive to scalar currents, while the maximum-recoil endpoint of the
B
→
D
∗
ℓν
spectrum with transversely polarized
D
∗
is extremely sensitive to tensor currents. We also quantify the room for
e
-
μ
universality violation in
b
→
cℓν
transitions, predicted by some models suggested to solve the
b
→
cτ ν
anomalies, from a global fit to
B
→
Dℓν
and
B
→
D
∗
ℓν
for the first time. Specific new physics models, corresponding to all possible tree-level mediators, are also discussed. As a side effect, we present
V
cb
determinations from exclusive
B
decays, both with frequentist and Bayesian statistics, leading to compatible results. The entire numerical analysis is based on open source code, allowing it to be easily adapted once new data or new form factors become available.
Journal Article
InlineMediaObject not available: see fulltext. : a Python package for the running and matching of Wilson coefficients above and below the electroweak scale
wilson is a Python library for matching and running Wilson coefficients of higher-dimensional operators beyond the Standard Model. Provided with the numerical values of the Wilson coefficients at a high new physics scale, it automatically performs the renormalization group evolution within the Standard Model effective field theory (SMEFT), matching onto the weak effective theory (WET) at the electroweak scale, and QCD/QED renormalization group evolution below the electroweak scale down to hadronic scales relevant for low-energy precision tests. The matching and running encompasses the complete set of dimension-six operators in both SMEFT and WET. The program builds on the Wilson coefficient exchange format (WCxf) and can thus be easily combined with a number of existing public codes.
Journal Article
A global likelihood for precision constraints and flavour anomalies
We present a global likelihood function in the space of dimension-six Wilson coefficients in the Standard Model Effective Field Theory. The likelihood includes contributions from flavour-changing neutral current B decays, lepton flavour universality tests in charged- and neutral-current B and K decays, meson-antimeson mixing observables in the K, B, and D systems, direct CP violation in \\[K\\rightarrow \\pi \\pi \\], charged lepton flavour violating B, tau, and muon decays, electroweak precision tests on the Z and W poles, the anomalous magnetic moments of the electron, muon, and tau, and several other precision observables, 265 in total. The Wilson coefficients can be specified at any scale, with the one-loop running above and below the electroweak scale automatically taken care of. The implementation of the likelihood function is based on the open source tools flavio and wilson as well as the open Wilson coefficient exchange format (WCxf) and can be installed as a Python package. It can serve as a basis either for model-independent fits or for testing dynamical models, in particular models built to address the anomalies in B physics. We discuss a number of example applications, reproducing results from the EFT and model building literature.
Journal Article
Constraints on new physics from radiative B decays
A
bstract
A new phase for the measurements of radiative decay modes in
b
→
s
transitions has started with new measurements of exclusive modes by LHCb and with Belle-II showing distinctive promises in both inclusive and exclusive channels. After critically reviewing the hadronic uncertainties in exclusive radiative decays, we analyze the impact of recent measurements of the branching ratio and mass-eigenstate rate asymmetry in
B
s
→
ϕγ
and of the angular distribution of
B
→
K
∗
e
+
e
−
at low
q
2
on new physics in the
b
→
s
γ transition.
Journal Article
B-decay discrepancies after Moriond 2019
Following the updated measurement of the lepton flavour universality (LFU) ratio
R
K
in
B
→
K
ℓ
ℓ
decays by LHCb, as well as a number of further measurements, e.g.
R
K
∗
by Belle and
B
s
→
μ
μ
by ATLAS, we analyse the global status of new physics in
b
→
s
transitions in the weak effective theory at the
b
-quark scale, in the Standard Model effective theory above the electroweak scale, and in simplified models of new physics. We find that the data continues to strongly prefer a solution with new physics in semi-leptonic Wilson coefficients. A purely muonic contribution to the combination
C
9
=
-
C
10
, well suited to UV-complete interpretations, is now favoured with respect to a muonic contribution to
C
9
only. An even better fit is obtained by allowing an additional LFU shift in
C
9
. Such a shift can be renormalization-group induced from four-fermion operators above the electroweak scale, in particular from semi-tauonic operators, able to account for the potential discrepancies in
b
→
c
transitions. This scenario is naturally realized in the simplified
U
1
leptoquark model. We also analyse simplified models where a LFU effect in
b
→
s
ℓ
ℓ
is induced radiatively from four-quark operators and show that such a setup is on the brink of exclusion by LHC di-jet resonance searches.
Journal Article
Anthropic solution to the magnetic muon anomaly: the charged see-saw
A
bstract
We present models of new physics that can explain the muon
g
− 2 anomaly in accord with the assumption that the only scalar existing at the weak scale is the Higgs, as suggested by anthropic selection. Such models are dubbed “charged see-saw” because the muon mass term is mediated by heavy leptons. The electroweak contribution to the
g
− 2 gets modified by order one factors, giving an anomaly of the same order as the observed hint, which is strongly correlated with a modification of the Higgs coupling to the muon.
Journal Article
Electroweak symmetry breaking and collider signatures in the next-to-minimal composite Higgs model
A
bstract
We conduct a detailed numerical analysis of the composite pseudo-Nambu-Goldstone Higgs model based on the next-to-minimal coset SO(6)/SO(5) ≅ SU(4)/Sp(4), featuring an additional SM singlet scalar in the spectrum, which we allow to mix with the Higgs boson. We identify regions in parameter space compatible with all current exper-imental constraints, including radiative electroweak symmetry breaking, flavour physics, and direct searches at colliders. We find the additional scalar, with a mass predicted to be below a TeV, to be virtually unconstrained by current LHC data, but potentially in reach of run 2 searches. Promising indirect searches include rare semi-leptonic
B
decays,
CP
violation in
B
s
mixing, and the electric dipole moment of the neutron.
Journal Article
B →Vℓ+ℓ− in the Standard Model from light-cone sum rules
A
bstract
We present
B
q
→
ρ
,
B
q
→
ω
,
B
q
→
K
∗
,
B
s
→
K
∗
and
B
s
→
ϕ
form factors from light-cone sum rules (LCSR) at
O
α
s
for twist-2 and 3 and
O
α
s
0
for twist-4 with updated hadronic input parameters. Three asymptotic light-cone distribution amplitudes of twist-4 (and 5) are determined, necessary for the form factors to obey the equations of motion. It is argued that the latter constrain the uncertainty of tensor-to-vector form factor ratios thereby improving the prediction of zeros of helicity amplitudes of major importance for
B
→
K
∗
ℓℓ angular observables. We provide easy-to-use fits to the LCSR results, including the full error correlation matrix, in all modes at low
q
2
as well as combined fits to LCSR and lattice results covering the entire kinematic range for
B
q
→
K
∗
,
B
s
→
K
∗
and
B
s
→
ϕ
. The error correlation matrix avoids the problem of overestimating the uncertainty in phenomenological applications. Using the new form factors and recent computations of non-factorisable contributions we provide Standard Model predictions for
B
→
K
∗
γ as well as
B
→
K
∗
ℓ
+
ℓ
−
and
B
s
→
ϕμ
+
μ
−
at low dilepton invariant mass. Employing our
B
→ (
ρ,ω
) form factor results we extract the CKM element |
V
ub
| from the semileptonic decays
B
→ (
ρ, ω
)ℓ
ν
and find good agreement with other exclusive determinations.
Journal Article
Dipole operator constraints on composite Higgs models
Flavour- and CP-violating electromagnetic or chromomagnetic dipole operators in the quark sector are generated in a large class of new physics models and are strongly constrained by measurements of the neutron electric dipole moment and observables sensitive to flavour-changing neutral currents, such as the
B
→
X
s
γ
branching ratio and
ϵ
′
/
ϵ
. After a model-independent discussion of the relevant constraints, we analyze these effects in models with partial compositeness, where the quarks get their masses by mixing with vector-like composite fermions. These scenarios can be seen as the low-energy limit of composite Higgs or warped extra dimensional models. We study different choices for the electroweak representations of the composite fermions motivated by electroweak precision tests as well as different flavour structures, including flavour anarchy and
U
(
3
)
3
or
U
(
2
)
3
flavour symmetries in the strong sector. In models with “wrong-chirality” Yukawa couplings, we find a strong bound from the neutron electric dipole moment, irrespective of the flavour structure. In the case of flavour anarchy, we also find strong bounds from flavour-violating dipoles, while these constraints are mild in the flavour-symmetric models.
Journal Article