Precision tests of first-row unitarity of the Cabibbo–Kobayashi–Maskawa matrix currently display two intriguing tensions, both at the 3σ level. First, combining determinations of Vud from ...superallowed β decays with Vus from kaon decays suggests a deficit in the unitarity relation. At the same time, a tension of similar significance has emerged between Kℓ2 and Kℓ3 decays. In this Letter, we point out that a measurement of the Kμ3/Kμ2 branching fraction at the level of 0.2% would have considerable impact on clarifying the experimental situation in the kaon sector, especially in view of tensions in the global fit to kaon data as well as the fact that the Kμ2 channel is currently dominated by a single experiment. Such a measurement, as possible for example at NA62, would further provide important constraints on physics beyond the Standard Model, most notably on the role of right-handed vector currents.
Charged lepton flavor violation at the EIC Cirigliano, Vincenzo; Fuyuto, Kaori; Lee, Christopher ...
The journal of high energy physics,
03/2021, Volume:
2021, Issue:
3
Journal Article
Peer reviewed
Open access
A
bstract
We present a comprehensive analysis of the potential sensitivity of the Electron-Ion Collider (EIC) to charged lepton flavor violation (CLFV) in the channel
ep
→
τX
, within the ...model-independent framework of the Standard Model Effective Field Theory (SMEFT). We compute the relevant cross sections to leading order in QCD and electroweak corrections and perform simulations of signal and SM background events in various
τ
decay channels, suggesting simple cuts to enhance the associated estimated efficiencies. To assess the discovery potential of the EIC in
τ
-
e
transitions, we study the sensitivity of other probes of this physics across a broad range of energy scales, from
pp
→
eτX
at the Large Hadron Collider to decays of
B
mesons and
τ
leptons, such as
τ
→
eγ
,
τ
→
eℓ
+
ℓ
−
, and crucially the hadronic modes
τ
→
eY
with
Y
∈
{
π, K, ππ, Kπ,
…}. We find that electroweak dipole and four-fermion semi-leptonic operators involving light quarks are already strongly constrained by
τ
decays, while operators involving the
c
and
b
quarks present more promising discovery potential for the EIC. An analysis of three models of leptoquarks confirms the expectations based on the SMEFT results. We also identify future directions needed to maximize the reach of the EIC in CLFV searches: these include an optimization of the
τ
tagger in hadronic channels, an exploration of background suppression through tagging
b
and
c
jets in the final state, and a global fit by turning on all SMEFT couplings, which will likely reveal new discovery windows for the EIC.
A
bstract
We present a method to determine the leading-order (LO) contact term contributing to the
nn → ppe
−
e
−
amplitude through the exchange of light Majorana neutrinos. Our approach is based on ...the representation of the amplitude as the momentum integral of a known kernel (proportional to the neutrino propagator) times the generalized forward Compton scattering amplitude
n
(
p
1
)
n
(
p
2
)
W
+
(
k
)
→
p
p
1
′
p
p
2
′
W
−
k
, in analogy to the Cottingham formula for the electromagnetic contribution to hadron masses. We construct model-independent representations of the integrand in the low- and high-momentum regions, through chiral EFT and the operator product expansion, respectively. We then construct a model for the full amplitude by interpolating between these two regions, using appropriate nucleon factors for the weak currents and information on nucleon-nucleon (
NN
) scattering in the
1
S
0
channel away from threshold. By matching the amplitude obtained in this way to the LO chiral EFT amplitude we obtain the relevant LO contact term and discuss various sources of uncertainty. We validate the approach by computing the analog
I
= 2
NN
contact term and by reproducing, within uncertainties, the charge-independence-breaking contribution to the
1
S
0
NN
scattering lengths. While our analysis is performed in the
MS
¯
scheme, we express our final result in terms of the scheme-independent renormalized amplitude
A
ν
p
p
′
at a set of kinematic points near threshold. We illustrate for two cutoff schemes how, using our synthetic data for
A
ν
, one can determine the contact-term contribution in any regularization scheme, in particular the ones employed in nuclear-structure calculations for isotopes of experimental interest.
The experimental sensitivity to μ→e conversion on nuclei is expected to improve by four orders of magnitude in coming years. We consider the impact of μ→e flavour-changing tensor and axial-vector ...four-fermion operators which couple to the spin of nucleons. Such operators, which have not previously been considered, contribute to μ→e conversion in three ways: in nuclei with spin they mediate a spin-dependent transition; in all nuclei they contribute to the coherent (A2-enhanced) spin-independent conversion via finite recoil effects and via loop mixing with dipole, scalar, and vector operators. We estimate the spin-dependent rate in Aluminium (the target of the upcoming COMET and Mu2e experiments), show that the loop effects give the greatest sensitivity to tensor and axial-vector operators involving first-generation quarks, and discuss the complementarity of the spin-dependent and independent contributions to μ→e conversion.
Leptonic anomalous magnetic moments in ν SMEFT Cirigliano, Vincenzo; Dekens, Wouter; de Vries, Jordy ...
The journal of high energy physics,
08/2021, Volume:
2021, Issue:
8
Journal Article
Peer reviewed
Open access
A
bstract
We investigate contributions to the anomalous magnetic moments of charged leptons in the neutrino-extended Standard Model Effective Field Theory (
ν
SMEFT). We discuss how
ν
SMEFT operators ...can contribute to a lepton’s magnetic moment at one- and two-loop order. We show that only one operator can account for existing electronic and muonic discrepancies, assuming new physics appears above 1 TeV. In particular, we find that a right-handed charged current in combination with minimal sterile-active mixing can explain the discrepancy for sterile neutrino masses of
O
(100) GeV while avoiding direct and indirect constraints. We discuss how searches for sterile neutrino production at the (HL-)LHC, measurements of
h
→
μ
+
μ
−
and searches for
h
→
e
+
e
−
, neutrinoless double beta decay experiments, and improved unitarity tests of the CKM matrix can further probe the relevant parameter space.
We present lattice QCD results on the neutron tensor charges including, for the first time, a simultaneous extrapolation in the lattice spacing, volume, and light quark masses to the physical point ...in the continuum limit. We find that the "disconnected" contribution is smaller than the statistical error in the "connected" contribution. Our estimates in the modified minimal subtraction scheme at 2 GeV, including all systematics, are g_{T}^{d-u}=1.020(76), g_{T}^{d}=0.774(66), g_{T}^{u}=-0.233(28), and g_{T}^{s}=0.008(9). The flavor diagonal charges determine the size of the neutron electric dipole moment (EDM) induced by quark EDMs that are generated in many new scenarios of CP violation beyond the standard model. We use our results to derive model-independent bounds on the EDMs of light quarks and update the EDM phenomenology in split supersymmetry with gaugino mass unification, finding a stringent upper bound of d_{n}<4×10^{-28} e cm for the neutron EDM in this scenario.
A new spin on neutrino quantum kinetics Cirigliano, Vincenzo; Fuller, George M.; Vlasenko, Alexey
Physics letters. B,
07/2015, Volume:
747, Issue:
C
Journal Article
Peer reviewed
Open access
Recent studies have demonstrated that in anisotropic environments a coherent spin-flip term arises in the Quantum Kinetic Equations (QKEs) which govern the evolution of neutrino flavor and spin in ...hot and dense media. This term can mediate neutrino–antineutrino transformation for Majorana neutrinos and active-sterile transformation for Dirac neutrinos. We discuss the physical origin of the coherent spin-flip term and provide explicit expressions for the QKEs in a two-flavor model with spherical geometry. In this context, we demonstrate that coherent neutrino spin transformation depends on the absolute neutrino mass and Majorana phases.
We present state-of-the-art results for the matrix elements of flavor diagonal tensor operators within the nucleon state. The calculation of the dominant connected contribution is done using 11 ...ensembles of gauge configurations generated by the MILC Collaboration using the highly improved staggered quark action with 2+1+1 dynamical flavors. The calculation of the disconnected contributions is done using seven (six) ensembles for the strange (light) quarks. These high-statistics simulations allowed us to address various systematic uncertainties. A simultaneous fit in the lattice spacing and the light-quark mass is used to extract the tensor charges in the continuum limit and at Mπ=135 MeV. Results for the proton in the MS¯ scheme at 2 GeV are gTu=0.784(28)(10), gTd=−0.204(11)(10) and gTs=−0.0027(16). Implications of these results for constraining the quark electric dipole moments and their contributions to the neutron electric dipole moment are discussed.
PDF
