Helicity Selection Rules Research Articles

Promising TMDC-like optical and excitonic properties of the TiBr2 2H monolayer.

The presented simulation protocol provides a solid foundation for exploring two-dimensional materials. Taking the TiBr2 2H monolayer as an example, this material displays promising TMDC-like optical and excitonic properties, making it an excellent candidate for optoelectronic and valleytronic applications. The direct band gap semiconductor (1.19 eV) is both structurally and thermodynamically stable, with spin-orbit coupling effects revealing a broken mirror symmetry in the K and K' valleys of the band structure, as confirmed by opposite values of the Berry curvature. A direct and bright exciton ground state was found, with an exciton binding energy of 0.56 eV. The study also revealed an optical helicity selection rule, suggesting selectivity in the valley excitation by specific circular light polarizations.

Selective excitation of four-wave mixing by helicity in gated graphene.

Gapless Dirac fermions in monolayer graphene give rise to an abundance of peculiar physical properties, including exceptional broadband nonlinear optical responses. By tuning the chemical potential, stacking order, and photonic structures, the effective modulation of nonlinear optical phenomena in graphene has been demonstrated in recent years. Here, we demonstrate that optical helicity can be used as an extra tuning knob for four-wave mixing in gated graphene. Our results reveal the helicity selection rule for four-wave mixing in monolayer graphene, revealing nearly perfect circular polarization. Corresponding theoretical interpretations of the helicity selection rule that are also applicable to other nonlinear optical processes and materials are presented.

Helicity‐resolved resonant Raman spectroscopy of layered WS2

AbstractWhen excited by circularly polarized light, the Raman scattered light may have the same or opposite helicity as the incident light determined by the helicity selection rule. For two‐dimensional (2D) transition metal dichalcogonides (TMDCs), the helicity selection rule can be broken down due to the strong Fröhlich exciton–phonon interaction. However, how the helicity selection rule changes with excitation energy has not been reported yet. Here, we study the helicity‐resolved Raman scattering of layered WS2 excited by circularly polarized light for excitations off‐excitonic resonance and near resonance to different excitons. We find that for off‐resonance excitation, the helicity of mode of WS2 obeys the helicity selection rule determined by the symmetry of crystal structure and vibration modes. When excited near resonance to the B exciton, the breakdown of the helicity selection rule is observed, which is attributed to the appearance of nonzero diagonal elements of the Raman tensor resulted from the Fröhlich exciton–phonon interaction. The layer number dependence of the helicity polarization ratio of the mode shows that the proportion of helicity‐conserved component decreases with the increase of layer number. When the excitation energy is near resonance to the A exciton, the different helicity polarization ratio is found and remains unchanged with increasing layer number, which may be attributed to the different coupling strengths of exciton–phonon interactions.

Loops and trees in generic EFTs

We consider aspects of tree and one-loop behavior in a generic 4d EFT of massless scalars, fermions, and vectors, with a particular eye to the high-energy limit of the Standard Model EFT at operator dimensions 6 and 8. First, we classify the possible Lorentz structures of operators and the subset of these that can arise at tree-level in a weakly coupled UV completion, extending the tree/loop classification through dimension 8 using functional methods. Second, we investigate how operators contribute to tree and one-loop helicity amplitudes, exploring the impact of non-renormalization theorems through dimension 8. We further observe that many dimension 6 contributions to helicity amplitudes, including rational parts, vanish exactly at one-loop level. This suggests the impact of helicity selection rules extends beyond one loop in non-supersymmetric EFTs.

Polarization in χcJ→ΛΛ¯ decays

To understand the enhanced partial decay width for the decay of ${\ensuremath{\chi}}_{c1}$ to the vector-meson pair $\ensuremath{\phi}\ensuremath{\phi}$ recently observed by the BESIII collaboration, we suggest measuring the polarization parameter of the $\ensuremath{\phi}$ meson by analyzing the jointed angular distribution in the cascade decay ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\psi}(2S)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{cJ}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\phi}\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{\gamma}2({K}^{+}{K}^{\ensuremath{-}})$. The formulas to measure polarization parameters are presented, and they are estimated in the framework of perturbative QCD (pQCD) and the quark-pair-creation model. In pQCD we obtain results consistent with the expectation of the helicity selection rule. In the quark-pair-creation model, the parameter associated with the violation of the helicity selection rule is enhanced relative to that estimated in pQCD.

Nonrenormalization and Operator Mixing via On-Shell Methods

Using on-shell methods, we present a new perturbative nonrenormalization theorem for operator mixing in massless four-dimensional quantum field theories. By examining how unitarity cuts of form factors encode anomalous dimensions, we show that longer operators are often restricted from renormalizing shorter operators at the first order where Feynman diagrams exist. The theorem applies quite generally and depends only on the field content of the operators involved. We apply our theorem to operators of dimension five through seven in the standard model effective field theory, including examples of nontrivial zeros in the anomalous-dimension matrix at one through four loops. The zeros at two and higher loops go beyond those previously explained using helicity selection rules. We also include explicit sample calculations at two loops.

Jet substructure measurements of interference in non-interfering SMEFT effects

The tails of diboson production at the LHC are sensitive to the interference between Standard Model and higher dimension operators parameterizing the effects of heavy new physics. However, helicity selection rules for the diboson scattering amplitudes set an obstruction to the na¨ıve interference contributions of dimension six operators, causing the total diboson rate correction’s leading contribution to cancel. In this case, carefully measuring the azimuthal decay angles “resurrects” the interference, recouping sensitivity to the “non-interfering” operators. We explore these signatures in detail, and find that the EFT uncertainties associated with higher-dimensional operators are uniquely well-suppressed by the construction of an asymmetry variable which is only generated by these non-interfering operators, relegating the effects of higher-dimensional, interfering operators to the same status as statistical errors in this observable. We perform a complete analysis of this azimuthal interference pattern in hadronic decays of W bosons using jet substructure techniques to tag the bosons and measure their azimuthal decay angles. This technique provides a valuable cross-check to purely-leptonic measurements of interference resurrection in diboson production.

A subleading power operator basis for the scalar quark current

Factorization theorems play a crucial role in our understanding of the strong interaction. For collider processes they are typically formulated at leading power and much less is known about power corrections in the λ ≪ 1 expansion. Here we present a complete basis of power suppressed operators for a scalar quark current at mathcal{O}left({lambda}^2right) in the amplitude level power expansion in the Soft Collinear Effective Theory, demonstrating that helicity selection rules significantly simplify the construction. This basis applies for the production of any color singlet scalar in qoverline{q} annihilation (such as boverline{b}to H ). We also classify all operators which contribute to the cross section at mathcal{O}left({lambda}^2right) and perform matching calculations to determine their tree level Wilson coefficients. These results can be exploited to study power corrections in both resummed and fixed order perturbation theory, and for analyzing the factorization properties of gauge theory amplitudes and cross sections at subleading power.

Interplay of valley selection and helicity exchange of light in Raman scattering for graphene and MoS2

Raman spectra of graphene and ${\mathrm{MoS}}_{2}$ are calculated for incident and scattered circularly polarized light. In the case of graphene, the well known $G$-band Raman spectra have a not well known property that the helicity of the incident circularly polarized light changes to another helicity in the scattered light. Using the electron-photon and electron-phonon matrix elements by first-principles calculation, we calculate resonant Raman spectra of graphene and ${\mathrm{MoS}}_{2}$ for circularly polarized light which are compared with recent experiments. The Raman intensity for circularly polarized light is relevant to optical valley polarization in the case of ${\mathrm{MoS}}_{2}$. We also discuss how the helicity-selection rule can be modified by applying stress to graphene.

A subleading operator basis and matching for gg \u2192 H

The Soft Collinear Effective Theory (SCET) is a powerful framework for studying factorization of amplitudes and cross sections in QCD. While factorization at leading power has been well studied, much less is known at subleading powers in the λ ≪ 1 expansion. In SCET subleading soft and collinear corrections to a hard scattering process are described by power suppressed operators, which must be fixed case by case, and by well established power suppressed Lagrangians, which correct the leading power dynamics of soft and collinear radiation. Here we present a complete basis of power suppressed operators for gg → H, classifying all operators which contribute to the cross section at mathcal{O}left({lambda}^2right) , and showing how helicity selection rules significantly simplify the construction of the operator basis. We perform matching calculations to determine the tree level Wilson coefficients of our operators. These results are useful for studies of power corrections in both resummed and fixed order perturbation theory, and for understanding the factorization properties of gauge theory amplitudes and cross sections at subleading power. As one example, our basis of operators can be used to analytically compute power corrections for N -jettiness subtractions for gg induced color singlet production at the LHC.

Helicity selection rules and noninterference for BSM amplitudes

Precision studies of scattering processes at colliders provide powerful indirect constraints on new physics. We study the helicity structure of scattering amplitudes in the SM and in the context of an effective Lagrangian description of BSM dynamics. Our analysis reveals a novel set of helicity selection rules according to which, in the majority of 2 to 2 scattering processes at high energy, the SM and the leading BSM effects do not interfere. In such situations, the naive expectation that dimension-6 operators represent the leading BSM contribution is compromised, as corrections from dimension-8 operators can become equally (if not more) important well within the validity of the effective field theory approach.

Optical selection rules for excitonic Rydberg series in the massive Dirac cones of hexagonal two-dimensional materials

We investigate the optical transition selection rules for excitonic Rydberg series formed in massive Dirac cones. The entanglement of the exciton envelop function with the pseudospin texture leads to anomalous selection rules for one-photon generation of excitons, where $d$-orbitals can be excited with the opposite helicity selection rule from the $s$-orbitals in a given valley. The trigonal warping effects in realistic hexagonal lattices further renders more excited states bright, where $p$-orbitals can also be accessed by one-photon excitation with the opposite valley selection rules to the $s$-orbitals. The one-photon generation of exciton in the various states and the intra-excitonic transition between these states are both dictated by the discrete in-plane rotational symmetry of the lattices, and our results show that in hexagonal 2D materials the symmetry allowed transitions are enabled when trigonal warping effects are included in the massive Dirac fermion model. In monolayer transition metal dichalcogenides where excitons can be generated by visible light and intra-excitonic transitions can be induced by infrared light, we give the strength of these optical transitions, estimated using modified hydrogen-like envelope functions combined with the optical transition matrix elements between the Bloch states calculated at various $k$ points.

Anomalous triple gauge couplings in the effective field theory approach at the LHC

We discuss how to perform consistent extractions of anomalous triple gauge couplings (aTGC) from electroweak boson pair production at the LHC in the Standard Model Effective Field Theory (SMEFT). After recasting recent ATLAS and CMS searches in pp → W Z(W W ) → ℓ′νℓ+ℓ−(νℓ) channels, we find that: (a) working consistently at order Λ−2 in the SMEFT expansion the existing aTGC bounds from Higgs and LEP-2 data are not improved, (b) the strong limits quoted by the experimental collaborations are due to the partial Λ−4 corrections (dimension-6 squared contributions). Using helicity selection rule arguments we are able to explain the suppression in some of the interference terms, and discuss conditions on New Physics (NP) models that can benefit from such LHC analyses. Furthermore, standard analyses assume implicitly a quite large NP scale, an assumption that can be relaxed by imposing cuts on the underlying scale of the process ( sqrt{widehat{s}} ). In practice, we find almost no correlation between sqrt{widehat{s}} and the experimentally accessible quantities, which complicates the SMEFT interpretation. Nevertheless, we provide a method to set (conservative) aTGC bounds in this situation, and recast the present searches accordingly. Finally, we introduce a simple NP model for aTGC to compare the bounds obtained directly in the model with those from the SMEFT analysis.

Evasion of HSR in S-wave charmonium decaying to P-wave light hadrons

The S-wave charmonium decaying to a P-wave and S-wave light hadron pairs are supposed to be suppressed by the helicity selection rule in the perturbative QCD framework. With an effective Lagrangian method, we show that the intermediate charmed meson loops can provide a possible mechanism for the evasion of the helicity selection rule, and result in sizeable decay branching ratios in some of those channels. The theoretical predictions can be examined by the forthcoming BES-III data in the near future.

Polarization inχcJ→ϕϕdecays

To understand the enhanced partial decay width for the decay of chi(c1) to the vector-meson pair phi phi recently observed by the BESIII collaboration, we suggest measuring the polarization parameter of the phi meson by analyzing the jointed angular distribution in the cascade decay e(+)e(-) -> psi(2S) -> gamma chi(cJ) -> gamma phi phi -> gamma 2(K+K-). The formulas to measure polarization parameters are presented, and they are estimated in the framework of perturbative QCD (pQCD) and the quark-pair-creation model. In pQCD we obtain results consistent with the expectation of the helicity selection rule. In the quark-pair-creation model, the parameter associated with the violation of the helicity selection rule is enhanced relative to that estimated in pQCD.

Observation ofηcdecay intoΣ+Σ¯−andΞ−Ξ¯+final states

Using a data sample of 2.25×108 J/ψ events collected with the BESIII detector, we present the first observation of the decays of ηc mesons to Σ+Σ¯− and Ξ−Ξ¯+. The branching fractions are measured to be (2.11±0.28stat±0.18syst±0.50PDG)×10−3 and (0.89±0.16stat±0.08syst±0.21PDG)×10−3 for ηc→Σ+Σ¯− and Ξ−Ξ¯+, respectively. These branching fractions provide important information on the helicity selection rule in charmonium-decay processes.Received 10 October 2012DOI:https://doi.org/10.1103/PhysRevD.87.012003© 2013 American Physical Society

P-wave quarkonium decays to meson pairs

The processes of p-wave Quarkonium exclusive decays to two mesons are investigated, in which the final state vector mesons with various polarizations are considered separately. In the calculation, the initial heavy quarkonia are treated in the framework of non-relativistic quantum chromodynamics, whereas for light mesons, the light cone distribution amplitudes up to twist-3 are employed. It turns out that the higher twist contribution is significant and provides a possible explanation for the observation of the hadron helicity selection rule violated processes χ c1 → φφ, ωω by the BESIII collaboration in recently. We also evaluate the χ b1 → J/ψJ/ψ process and find that its branching ratio is big enough to be measured at the B-factories.

Experimental study ofψ′decays toK+K−π0andK+K−η

Using $(106\pm4)\times 10^6$ $\psip$ events accumulated with the BESIII detector at the BEPCII $e^+e^-$ collider, we present measurements of the branching fractions for psi' decays to $K^{+}K^{-}\pi^{0}$ and $K^{+}K^{-}\eta$. In these final states, the decay $\psi'\to K_2^{*}(1430)^+K^-+c.c.$ is observed for the first time, and its branching fraction is measured to be $(7.12\pm{0.62}{\rm (stat.)}^{+1.13}_{-0.61}{\rm (syst.)})\times 10^{-5}$, which indicates a violation of the helicity selection rule in $\psip$ decays. The branching fractions of $\psi'\to K^*(892)^+K^-+c.c., \phi\eta, \phi\pi^0$ are also measured. The measurements are used to test the QCD predictions on charmonium decays.

Updated study of the [formula omitted] and [formula omitted] decays into light vector mesons

We re-investigate the exclusive decays of ηc and ηc′ to a pair of light vector mesons, i.e. ηc(ηc′)→VV. The long-distance intermediate meson loop (IML) effects are evaluated as a non-perturbative mechanism in addition to the short-distance cc¯ annihilation contributions. We show that both processes can be reasonably well constrained with the help of the available experimental data. Since ηc and ηc′ are the spin-0 partners of J/ψ and ψ′, respectively, our study is useful for gaining insights into the pQCD helicity selection rule violations in charmonium decays and the long-standing “ρπ puzzle”.

Open charm effects in the explanation of the long-standing “ρπpuzzle”

A detailed analysis of the open charm effects on the decays of $J/\psi(\psi^\prime)\to VP$ is presented, where $V$ stands for light vector meson and $P$ for light pseudoscalar meson. These are the channels that the so-called "12% rule" of perturbative QCD (pQCD) is obviously violated. Nevertheless, they are also the channels that violate the pQCD helicity selection rule (HSR) at leading order. In this work, we put constraints on the electromagnetic (EM) contribution, short-distance contribution from the $c\bar{c}$ annihilation at the wavefunction origin, and long-distance contribution from the open charm threshold effects on these two decays. We show that interferences among these amplitudes, in particular, the destructive interferences between the short-distance and long-distance strong amplitudes play a key role to evade the HSR and cause the significant deviations from the pQCD expected "12% rule".