Pseudoscalar Operators Research Articles

Renormalization of the pseudoscalar operator at four loops in QCD

We present the renormalization constant of the pseudoscalar operator defined with a non-anticommuting γ5 in dimensional regularization up to four-loop order in perturbative Quantum Chromodynamics (QCD). Furthermore, by virtue of renormalization-group invariance of the relation between the scalar and the pseudoscalar operator, we predict the MS¯ factor of the renormalization constant for the latter at five-loop order in QCD.

Exploring the lepton flavor violating decay modes b → sμ±τ∓ in SMEFT approach

We perform an analysis of the consequences of various new physics operators on the lepton flavor violating (LFV) decay modes mediated through b→sℓ1ℓ2 transitions. We scrutinize the imprints of the (pseudo)scalar and axial (vector) operators on the exclusive LFV decay channels B(s)→(ϕ,K⁎,K2⁎)ℓ1ℓ2 and Λb→Λℓ1ℓ2, where ℓ1,ℓ2 represent μ or τ. The new physics parameters are constrained by using the upper limits of the branching fractions of the Bs→τμ and B→Kτμ processes, assuming the new physics couplings to be real. We then explore the key observables such as the branching fractions, the forward-backward asymmetries, and the longitudinal polarization fractions of the B→(K⁎,ϕ,K2⁎)τ±μ∓ decays. In addition, we also investigate the impact of the new physics couplings on the baryonic Λb→Λτ±μ∓ decay channels mediated by the b→s quark level transition. With the experimental prospects at LHCb upgrade and Belle II, we also predict the upper limits of the above-discussed observables, which could intrigue the new physics search in these channels.

Time-reversal asymmetries in * *Supported in part by the National Key Research and Development Program of China (2020YFC2201501) and the National Natural Science Foundation of China (NSFC) (12147103)

We study the decays of with . We examine the full angular distributions with polarized , where the T-odd observables are identified. We discuss the possible effects of new physics (NP) and find that the T-odd observables are sensitive to them as they vanish in the standard model. Special attention is given to the interference of (pseudo)scalar operators with (axial)vector operators in polarized , which are studied for the first time. Their effects are proportional to the lepton masses and therefore may evade the constraint from at the LHCb naturally. As is uncontaminated by the charmonia resonance, it provides a clean background to probe NP. In addition, we show that the experimental central value of in at the LHCb can be explained by the NP case, which couples to the right-handed quarks and leptons. The polarization fraction of at the LHCb is found to be consistent with zero regardless of the NP scenarios.

CFT correlators and CP-violating trace anomalies

We analyze the parity-odd correlators langle JJOrangle _{odd},langle JJTrangle _{odd},langle TTOrangle _{odd} and langle TTTrangle _{odd} in momentum space, constrained by conformal Ward identities, extending our former investigation of the parity-odd chiral anomaly vertex. We investigate how the presence of parity-odd trace anomalies affect such correlators. Motivations for this study come from holography, early universe cosmology and from a recent debate on the chiral trace anomaly of a Weyl fermion. In the current CFT analysis, O can be either a scalar or a pseudoscalar operator and it can be identified with the trace of the stress–energy tensor. We find that the langle JJOrangle _{odd} and langle TTOrangle _{odd} can be different from zero in a CFT. This occurs when the conformal dimension of the scalar operator is Delta _3=4, as in the case of O=T^{mu }_{mu }. Moreover, if we assume the existence of parity-odd trace anomalies, the conformal langle JJTrangle _{odd} and langle TTTrangle _{odd} are nonzero. In particular, in the case of langle JJTrangle _{odd} the transverse–traceless component is constrained to vanish, and the correlator is determined only by the trace part with the anomaly pole.

Revisiting the fermionic dark matter absorption on electron target

We perform a systematic study of the fermionic DM absorption interactions on electron target in the context of effective field theory. The fermionic DM absorption is not just sensitive to sub-MeV DM with efficient energy release, but also gives a unique signature with clear peak in the electron recoil spectrum whose shape is largely determined by the atomic effects. Fitting with the Xenon1T and PandaX-II data prefers DM mass at mχ = 59 keV and 105 keV, respectively, while the cut-off scale is probed up to around 1 TeV. The DM overproduction in the early Universe, the invisible decay effect on the cosmological evolution, and the visible decay signal collected by the astrophysical X(gamma)-ray observations (Insight-HXMT, NuSTAR, HEAO-1, and INTEGRAL) are thoroughly explored to constrain the DM absorption interactions. With stringent bounds on the tensor and pseudo-scalar operators, the other fermionic DM operators are of particular interest at tonne-scale direct detection experiments such as PandaX-4T, XENONnT, and LZ.

Polarized Λb baryon decay to pπ and a dilepton pair

The recent anomalies in $b\to s\ell^+\ell^-$ transitions could originate from some New Physics beyond the Standard Model. Either to confirm or to rule out this assumption, more tests of $b\to s\ell^+\ell^-$ transition are needed. Polarized $\Lambda_b$ decay to a $\Lambda$ and a dilepton pair offers a plethora of observables that are suitable to discriminate New Physics from the Standard Model. In this paper, we present a full angular analysis of a polarized $\Lambda_b$ decay to a $\Lambda(\to p\pi)\ell^+\ell^-$ final state. The study is performed in a set of the operator where the Standard Model operator basis is supplemented with its chirality flipped counterparts, and new scalar and pseudo-scalar operators. The full angular distribution is calculated by retaining the mass of the final state leptons. At the low hadronic recoil, we use the Heavy Quark Effective Theory framework to relate the hadronic form factors which lead to simplified expression of the angular observables where short- and long-distance physics factorize. Using the factorized expressions of the observables, we construct a number of tests of short- and long-distance physics including null tests of the Standard Model and its chirality flipped counterparts that can be carried out using experimental data.

New physics in the angular distribution of {B}_c^{-} \u2192 J/\u03c8(\u2192 \u03bc+\u03bc\u2212)\u03c4\u2212(\u2192 \u03c0\u2212\u03bd\u03c4) {overline{nu}}_{tau } decay

In {B}_c^{-} → J/ψ(→ μ+μ−)τ− {overline{nu}}_{tau } decay, the three-momentum {boldsymbol{p}}_{tau^{-}} cannot be determined accurately due to the decay products of τ− inevitably include an undetected ντ. As a consequence, the angular distribution of this decay cannot be measured. In this work, we construct a measurable angular distribution by considering the subsequent decay τ− → π−ντ. The full cascade decay is {B}_c^{-} → J/ψ(→ μ+μ−)τ−(→ π−ντ) {overline{nu}}_{tau } , in which the three-momenta {boldsymbol{p}}_{mu^{+}},{boldsymbol{p}}_{mu^{-}} , and {boldsymbol{p}}_{pi^{-}} can be measured. The five-fold differential angular distribution containing all Lorentz structures of the new physics (NP) effective operators can be written in terms of twelve angular observables ℐi(q2, Eπ). Integrating over the energy of pion Eπ, we construct twelve normalized angular observables {hat{mathrm{mathcal{I}}}}_i (q2) and two lepton-flavor-universality ratios Rleft({P}_{L,T}^{J/psi}right) (q2). Based on the Bc → J/ψ form factors calculated by the latest lattice QCD and sum rule, we predict the q2 distribution of all {hat{mathrm{mathcal{I}}}}_i and Rleft({P}_{L,T}^{J/psi}right) both within the Standard Model and in eight NP benchmark points. We find that the benchmark BP2 (corresponding to the hypothesis of tensor operator) has the greatest effect on all ℐi and Rleft({P}_{L,T}^{J/psi}right) , except {hat{mathrm{mathcal{I}}}}_5 . The ratios Rleft({P}_{L,T}^{J/psi}right) are more sensitive to the NP with pseudo-scalar operators than the ℐi. Finally, we discuss the symmetries in the angular observables and present a model-independent method to determine the existence of tensor operators.

The {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil in HQET

In this paper we discuss the Standard Model and new physics sensitivity of the {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil, where ℓ± are massive leptons and Noverline{K}=left{{pK}^{-},n{overline{K}}^0right} . We provide a full angular distribution with a set of operators that includes the Standard Model operators and their chirality flipped counterparts, and new scalar and pseudo-scalar operators. The resulting angular distribution allows us to construct observables that we study in the Standard Model and in model-independent new physics scenarios. To reduce the hadronic effects emanating from the Λb→ Λ* (1520) transition form factors, we exploit the Heavy Quark Effective theory framework valid at low Λ∗(1520) recoil, i.e., large dilepton invariant mass squared {q}^2sim mathcal{O}left({m}_b^2right) . Working to the leading order in 1/mb and including mathcal{O}left({alpha}_sright) corrections, we compute the ‘improved Isgur-Wise relations’ between the form factors. The relations correlate the form factors and thereby allow the description of this decay at the low-recoil region with a smaller number of independent form factors.

Angular analysis of B¯→D2*(→Dπ)ℓν¯ decay and new physics

We derive the four-fold angular distribution for the semileptonic decay $\bar{B}\to D_2^*(\to D \pi)\,\ell \bar{\nu}$ where $D_2^*$(2460) is a tensor meson. We start with the most general beyond the Standard Model (SM) dimension-six effective Hamiltonian which comprises (axial)vector, (pseudo)scalar and tensor operators for both quark and lepton currents, and it also includes the right-handed neutrinos. The decay can be described by 16 transversity amplitudes and it provides a multitude of observables which can be extracted from data. We investigate the observables in the context of the SM and the new physics scenarios which can explain the intriguing discrepancies observed in the $b \to c \tau \bar{\nu}$ data.

Lepton flavor violating Lambda _brightarrow Lambda ell _1ell _2 decay

Inspired by the recent hints of lepton flavor universality violation in brightarrow sell ell and brightarrow cell nu transitions, we study lepton flavor violating exclusive Lambda _brightarrow Lambda ell _1^+ell _2^- (ell _1ne ell _2) decay, which is forbidden in the Standard Model. Starting from a general effective Hamiltonian for a brightarrow sell _1^+ell _2^- transition that includes vector and axial-vector operators, and scalar and pseudo-scalar operators, we derive a two-fold decay distribution of Lambda _brightarrow Lambda ell _1^+ell _2^-. The distribution helps us to construct the differential branching ratio and the lepton-side forward–backward asymmetry, which are studied in a vector leptoquark model. The parameter space of the vector leptoquark model is constrained by low energy observables.

Selecting μ → e conversion targets to distinguish lepton flavour-changing operators

The experimental sensitivity to μ→e conversion on nuclei is set to improve by four orders of magnitude in coming years. However, various operator coefficients add coherently in the amplitude for μ→econversion, weighted by nucleus-dependent functions, and therefore in the event of a detection, identifying the relevant new physics scenarios could be difficult. Using a representation of the nuclear targets as vectors in coefficient space, whose components are the weighting functions, we quantify the expectation that different nuclear targets could give different constraints. We show that all but two combinations of the 10 Spin-Independent (SI) coefficients could be constrained by future measurements, but discriminating among the axial, tensor and pseudoscalar operators that contribute to the Spin-Dependent (SD) process would require dedicated nuclear calculations. We anticipate that μ→econversion could constrain 10 to 14 combinations of coefficients; if μ→eγ and μ→ee¯e constrain eight more, that leaves 60 to 64 “flat directions” in the basis of QED × QCD-invariant operators which describe μ→e flavour change below mW.

Constraints on 2ℓ2q operators from μ↔e flavor-changing meson decays

We study lepton flavour violating two- and three-body decays of pseudoscalar mesons in Effective Field Theory (EFT). We give analytic formulae for the decay rates in the presence of a complete basis of QED and QCD-invariant operators. The constraints are obtained at the experimental scale, then translated to the weak scale via one-loop RGEs. The large RG-mixing between tensor and (pseudo)scalar operators weakens the constraints on scalar and pseudoscalar operators at the weak scale.

Hadronic and new physics contributions to b→s transitions

Assuming the source of the anomalies observed recently in $b \to s$ data to be new physics, there is a priori no reason to believe that - in the effective field theory language - only one type of operator is responsible for the tensions. We thus perform for the first time a global fit where all the Wilson coefficients which can effectively receive new physics contributions are considered, allowing for lepton flavour universality breaking effects as well as contributions from chirality flipped and scalar and pseudoscalar operators, and find the SM pull taking into account all effective parameters. As a result of the full fit to all available $b \to s$ data including all relevant Wilson coefficients, we obtain a total pull of 4.1$\sigma$ with the SM hypothesis assuming 10% error for the power corrections. Moreover, we make a statistical comparison to find whether the most favoured explanation of the anomalies is new physics or underestimated hadronic effects using the most general parameterisation which is fully consistent with the analyticity structure of the amplitudes. This Wilks' test will be a very useful tool to analyse the forthcoming $B\to K^* \mu^+ \mu^-$ data. Because the significance of the observed tensions in the angular observables in $B \to K^* \mu^+\mu^-$ is presently dependent on the theory estimation of the hadronic contributions to these decays, we briefly discuss the various available approaches for taking into account the long-distance hadronic effects and examine how the different estimations of these contributions result in distinct significance of the new physics interpretation of the observed anomalies.

On the angular distribution of \u039bb \u2192 \u039b(\u2192 N \u03c0)\u03c4 +\u03c4 \u2212 decay

We present a full angular distribution of the four body Λb → Λ(→ N π)ℓ+ℓ− decay where the leptons are massive and the Λb is unpolarized, in an operator basis which includes the Standard Model operators, new vector and axial-vector operators, and scalar and pseudo-scalar operators. The angular coefficients are expressed in terms of transversity amplitudes. We study several Λb → Λ(→ pπ)τ +τ − observables in the Standard Model and in the presence of the new operators. For our numerical analysis, we use the form factors from lattice QCD calculations.

RI/MOM and RI/SMOM renormalization of overlap quark bilinears on domain wall fermion configurations

Renormalization constants (RCs) of overlap quark bilinear operators on 2+1-flavor domain wall fermion configurations are calculated by using the RI/MOM and RI/SMOM schemes. The scale independent RC for the axial vector current is computed by using a Ward identity. Then the RCs for the quark field and the vector, tensor, scalar and pseudoscalar operators are calculated in both the RI/MOM and RI/SMOM schemes. The RCs are converted to the $\overline{\rm MS}$ scheme and we compare the numerical results from using the two intermediate schemes. The lattice size is $48^3\times96$ and the inverse spacing $1/a = 1.730(4) {\rm~GeV}$.

Model independent new physics analysis in Lambda _brightarrow Lambda mu ^+mu ^- decay

We study the rare Lambda _brightarrow Lambda mu ^+mu ^- decay in the Standard Model and beyond. Beyond the Standard Model we include new vector and axial-vector operators, scalar and pseudo-scalar operators, and tensor operators in the effective Hamiltonian. Working in the helicity basis and using appropriate parametrization of the Lambda _b rightarrow Lambda hadronic matrix elements, we give expressions of hadronic and leptonic helicity amplitudes and derive expression of double differential branching ratio with respect to dilepton invariant mass squared and cosine of lepton angle. Appropriately integrating the differential branching ratio over the lepton angle, we obtain the longitudinal polarization fraction and the leptonic forward-backward asymmetry and sequentially study the observables in the presence of the new couplings. To analyze the implications of the new vector and axial-vector couplings, we follow the current global fits to brightarrow smu ^+mu ^- data. While the impacts of scalar couplings can be significant, exclusive bar{B}rightarrow X_smu ^+mu ^- data imply stringent constraints on the tensor couplings and hence the effects on Lambda _brightarrow Lambda mu ^+mu ^- are negligible.

Probing new physics through Bs*→μ+μ− decay

We perform a model independent analysis of new physics in $B^*_s \rightarrow \mu^+ \mu^-$ decay. We intend to identify new physics operator(s) which can provide large enhancement in the branching ratio of $B^*_s \rightarrow \mu^+ \mu^-$ above its standard model prediction. For this, we consider new physics in the form of vector, axial-vector, scalar and pseudoscalar operators. We find that scalar and pseudoscalar operators do not contribute to the branching ratio of $B^*_s \rightarrow \mu^+ \mu^-$. We perform a global fit to all relevant $b \to s \mu^+ \mu^-$ data for different new physics scenarios. For each of these scenarios, we predict $Br(B^*_s \rightarrow \mu^+ \mu^-)$. We find that a significant enhancement in $Br(B^*_s \rightarrow \mu^+ \mu^-)$ is not allowed by any of these new physics operators. In fact, for all new physics scenarios providing a good fit to the data, the branching ratio of $B^*_s \rightarrow \mu^+ \mu^-$ is suppressed as compared to the SM value. Hence the present $b \to s \mu^+ \mu^-$ data indicates that the future measurements of $Br(B^*_s \rightarrow \mu^+ \mu^-)$ is expected to be suppressed in comparison to the standard model prediction.

\u201cSpin-dependent\u201d varvec{mu rightarrow e} conversion on light nuclei

The experimental sensitivity to mu rightarrow e conversion will improve by four or more orders of magnitude in coming years, making it interesting to consider the “spin-dependent” (SD) contribution to the rate. This process does not benefit from the atomic-number-squared enhancement of the spin-independent (SI) contribution, but probes different operators. We give details of our recent estimate of the spin-dependent rate, expressed as a function of operator coefficients at the experimental scale. Then we explore the prospects for distinguishing coefficients or models by using different targets, both in an EFT perspective, where a geometric representation of different targets as vectors in coefficient space is introduced, and also in three leptoquark models. It is found that comparing the rate on isotopes with and without spin could allow one to detect spin-dependent coefficients that are at least a factor of few larger than the spin-independent ones. Distinguishing among the axial, tensor and pseudoscalar operators that induce the SD rate would require calculating the nuclear matrix elements for the second two. Comparing the SD rate on nuclei with an odd proton vs. odd neutron could allow one to distinguish operators involving u quarks from those involving d quarks; this is interesting because the distinction is difficult to make for SI operators.

New methods for B decay constants and form factors from Lattice NRQCD

We determine the normalisation of scalar and pseudo scalar current operators made from NonRelativistic QCD (NRQCD) b quarks and Highly Improved Staggered (HISQ) light quarks through O(αs∧QCD/mb). We use matrix elements of these operators to extract B meson decay constants and form factors and compare to those obtained using the standard vector and axial vector operators. We work on MILC second-generation 2+1+1 gluon field configurations, including those with physical light quarks in the sea. This provides a test of systematic uncertainties in these calculations and we find agreement between the results to the 2% level of uncertainty previously quoted.

Exploring spin- 3/2 dark matter with effective Higgs couplings

We study an economical model of weakly-interacting massive particle dark matter (DM) which has spin 3/2 and interacts with the 125-GeV Higgs boson via effective scalar and pseudoscalar operators. We apply constraints on the model from the relic density data, LHC measurements of the Higgs boson, and direct and indirect searches for DM, taking into account the effective nature of the DM-Higgs couplings. We show that this DM is currently viable in most of the mass region from about 58 GeV to 2.3 TeV and will be probed more stringently by ongoing and upcoming experiments. Nevertheless, the presence of the DM-Higgs pseudoscalar coupling could make parts of the model parameter space elusive from future tests. We find that important aspects of this scenario are quite similar to those of its more popular spin-1/2 counterpart.