Contact Terms Research Articles

Polarization of Λ(1405) in the γp → K+πΣ reaction

In this paper, we study the polarization of the Λ(1405) in the γp→K+πΣ reaction within an effective Lagrangian approach and isobar model. In our model, the Λ(1405) is excited through the t-channel K/K⁎ exchanges and u-channel hyperon exchange. Compared to previous studies, we also include the contribution from a contact term, which is necessary for our model to interpret the polarization of the Λ(1405). In addition, we also discuss the possibility to verify the proposed two-pole structure of the Λ(1405) using the polarization data. We find that the polarization of the Λ(1405) or the polarization of the final Σ in this reaction is sensitive to the invariant mass MπΣ. Thus the measurement of the dependence of the Λ(1405) polarization on the MπΣ can offer valuable information about the pole structure of the Λ(1405).

Muon flavor violation in two-Higgs-doublet model with extra Yukawa couplings

The new round of experiments, MEG II, COMET/Mu2e, and Mu3e, would soon start to push the $\mu \to e\gamma$, $\mu N \to eN$ conversion, and $\mu \to 3e$ frontier, while Belle II would probe $\tau \to \mu\gamma$ and $\tau \to 3\mu$. In the general two Higgs doublet model with extra Yukawa couplings, we show that all these processes probe the lepton flavor violating (LFV) dipole transition that arises from the two loop mechanism, with scalar-induced contact terms subdominant. This is because existing data suggest the extra Yukawa couplings $\rho_{\mu e},\, \rho_{ee} \lesssim \lambda_e$, while $\rho_{\tau\mu},\, \rho_{\tau\tau} \lesssim \lambda_\tau$ and $\rho_{tt} \lesssim \lambda_t$, with $\lambda_i$ the usual Yukawa coupling of the Standard Model (SM), where $\rho_{\mu e}\rho_{tt}$ and $\rho_{\tau\mu}\rho_{tt}$ enter the $\mu e\gamma$ and $\tau\mu\gamma$ two loop amplitudes, respectively. With the $B_s \to \mu\mu$ decay rate basically consistent with SM expectation, together with the $B_s$ mixing constraint, we show that $B_s \to \tau\tau$ would also be consistent with SM, while $B_s \to \tau\mu$ and $B \to K\tau\mu$ decays would be out of reach of projected sensitivities, in strong contrast with some models motivated by the B anomalies.

Gravitational contact interactions and the physical equivalence of Weyl transformations in effective field theory

Theories of scalars and gravity, with non-minimal interactions, $\sim (M_P^2 +F(\phi) )R +L(\phi)$, have graviton exchange induced contact terms. These terms arise in single particle reducible diagrams with vertices $\propto q^2$ that cancel the Feynman propagator denominator $1/q^2$ and are familiar in various other physical contexts. In gravity these lead to additional terms in the action such as $\sim F(\phi) T_\mu^\mu(\phi)/M_P^2$ and $F(\phi)\partial^2 F(\phi)/M_P^2$. The contact terms are equivalent to induced operators obtained by a Weyl transformation that removes the non-minimal interactions, leaving a minimal Einstein-Hilbert gravitational action. This demonstrates explicitly the equivalence of different representations of the action under Weyl transformations, both classically and quantum mechanically. To avoid such "hidden contact terms" one is compelled to go to the minimal Einstein-Hilbert representation.

Constructing massive on-shell contact terms

The purely on-shell approach to effective field theories requires the construction of independent contact terms. Employing the little-group-covariant massive-spinor formalism, we present the first systematic derivation of independent four-point contact terms involving massive scalars, spin-1/2 fermions, and vectors. Independent three-point amplitudes are also listed for massive particles up to spin-3. We make extensive use of the simple relations between massless and massive amplitudes in this formalism. Our general results are specialized to the (broken-phase) particle content of the electroweak sector of the standard model. The (anti)symmetrization among identical particles is then accounted for. This work opens the way for the on-shell computation of massive four-point amplitudes.

Comments on contact terms and conformal manifolds in the AdS/CFT correspondence

Abstract We study the contact terms that appear in the correlation functions of exactly marginal operators using the anti-de Sitter/conformal field theory (AdS/CFT) correspondence. It is known that CFT with an exactly marginal deformation requires the existence of the contact terms with their coefficients having a geometrical interpretation in the context of conformal manifolds. We show that the AdS/CFT correspondence captures properly the mathematical structure of the correlation functions that is expected from the CFT analysis. For this purpose, we employ a holographic renormalization group to formulate a most general setup in the bulk for describing an exactly marginal deformation. The resultant bulk equations of motion are nonlinear and solved perturbatively to obtain the on-shell action. We compute three- and four-point functions of the exactly marginal operators using the GKP–Witten prescription, and show that they match the expected results precisely. The cut-off surface prescription in the bulk serves as a regularization scheme for conformal perturbation theory in the boundary CFT. As an application, we examine a double OPE limit of the four-point functions. The anomalous dimensions of double trace operators are written in terms of the geometrical data of a conformal manifold.

Many-body factorization and position–momentum equivalence of nuclear short-range correlations

Ab-initio Quantum Monte Carlo (QMC) calculations of nuclei from deuterium to 40Ca, obtained using four different phenomenological and local chiral nuclear potentials, are analyzed using the Generalized Contact Formalism (GCF). We extract spin- and isospin-dependent "nuclear contact terms" for each interaction in both coordinate and momentum space. The extracted contact terms, that count the number of short-range correlated (SRC) pairs with different quantum numbers, are dependent on the nuclear interaction model used in the QMC calculation. However, the ratios of contact terms for a nucleus A to deuterium (for spin-1 pn pairs) or to 4He (for all NN pairs) are independent of the nuclear interaction model and are the same for both short-distance and high-momentum pairs. This implies that the relative abundance of short-range pairs in the nucleus is a long-range (mean-field) quantity that is insensitive to the short-distance nature of the nuclear force. Measurements of exclusive (e,e'NN) pair breakup processes are instead more sensitive to short-range dynamics

The unique Polyakov blocks

In this work we present a closed form expression for Polyakov blocks in Mellin space for arbitrary spin and scaling dimensions. We provide a prescription to fix the contact term ambiguity uniquely by reducing the problem to that of fixing the contact term ambiguity at the level of cyclic exchange amplitudes — defining cyclic Polyakov blocks — in terms of which any fully crossing symmetric correlator can be decomposed. We also give another, equivalent, prescription which does not rely on a decomposition into cyclic amplitudes. We extract the OPE data of double-twist operators in the direct channel expansion of the cyclic Polyakov blocks using and extending the analysis of [1, 2] to include contributions that are non-analytic in spin. The relation between cyclic Polyakov blocks and analytic Bootstrap functionals is underlined.

Novel regularization scheme for nucleon-nucleon lattice simulations with effective field theory

We propose a new regularization scheme to study the bound state of two-nucleon systems in Lattice Effective Field Theory. Inspired by continuum EFT calculation, we study an exponential regulator acting on the leading-order (LO) and next-to-leading order (NLO) interactions, consisting of local contact terms. By fitting the low-energy coefficients (LECs) to deuteron binding energy and the asymptotic normalization coefficient (ANC) on a lattice simulation, we extract the effective range expansion (ERE) parameters in the $^3S_1$ channel to order $p^2$. We explore the impact of different powers of the regulator on the extracted ERE parameters for the lattice spacing $a=1.97$ fm. Moreover, we investigate how the implementation of the regularization scheme improves the predicted ERE parameters on the lattice spacing in the range of $1.4 \le a \le 2.6$ fm. Our numerical analysis indicates that for lattice spacing greater than $2$ fm, the predicted observables are very close to the experimental data.

Photoproduction of D¯0Λc+ within the Regge-plus-resonance model

${\overline{D}}^{0}$-meson photoproduction off the proton is studied at energies from the process threshold up to 20 GeV. The background part of the process is modeled within the Regge model with the help of ${\overline{D}}^{0}$-meson trajectories that are being exchanged in the $t$ channel or ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}$-baryon trajectories exchanged in the $u$ channel. These contributions create cross sections which are smooth functions of energy. The introduction of exchanges of three hidden-charm pentaquarks with spin $1/2$ and $3/2$ in the $s$ channel then creates sharp peaks in the cross section predictions. In order to account for the finite size of the particles involved, we introduce hadron form factors. In analysis of model predictions with pseudoscalar and pseudovector couplings in the strong vertex, the contact term was shown to play a non-negligible role. A great merit of the proposed approach is its limited number of parameters that need to be adjusted as there are to this date no data available for this process. Being limited by the broken SU(4) symmetry and guided by other works that show their predictions of this process, we could adjust the parameters manually in order to provide at least qualitative results.

Conformal Contact Terms and Semi-local Terms

We study conformal properties of local terms such as contact terms and semi-local terms in correlation functions of a conformal field theory. Not all of them are universal observables but they do appear in physically important correlation functions such as (anomalous) Ward-Takahashi identities or Schwinger-Dynson equations. We develop some tools such as embedding space delta functions and effective action to examine conformal invariance of these local terms.

Interstitial defects in diamond: A quantum mechanical simulation of their EPR constants and vibrational spectra.

The local geometry, electronic structure, and vibrational features of three vicinal double interstitial defects in diamond, ICIC, ICIN, and ININ, are investigated and compared with those of three "simple" ⟨100⟩ interstitial defects, ICC, ICN, and INN, previously reported by Salustro et al. [Phys. Chem. Chem. Phys. 20, 16615 (2018)], using a similar quantum mechanical approach based on the B3LYP functional constructed from Gaussian-type basis sets, within a supercell scheme, as implemented in the CRYSTAL code. For the first time, the Fermi contact term and hyperfine coupling tensor B of the four open shell structures, ICIC, ICIN, ICC, and ICN, are evaluated and compared with the available experimental EPR data. For the two double interstitial defects, the agreement with experiment is good, whereas that for the single interstitials is found to be very poor, for which a likely reason is the incorrect attribution of the EPR spectra to uncertain atomic details of the micro-structure of the samples. The infrared spectra of the three double interstitial defects exhibit at least two peaks that can be used for their characterization.

Crossing symmetry, transcendentality and the Regge behaviour of 1d CFTs

We develop the technology for Polyakov-Mellin (PM) bootstrap in one- dimensional conformal field theories (CFT1). By adding appropriate contact terms, we bootstrap various effective field theories in AdS2 and analytically compute the CFT data to one loop. The computation can be extended to higher orders in perturbation theory, if we ignore mixing, for any external dimension. We develop PM bootstrap for O(N ) theories and derive the necessary contact terms for such theories (which also involves a new higher gradient contact term absent for N = 1). We perform cross-checks which include considering the diagonal limit of the 2d Ising model in terms of the 1d PM blocks. As an independent check of the validity of the results obtained with PM bootstrap, we propose a suitable basis of transcendental functions, which allows to fix the four-point correlators of identical scalar primaries completely, up to a finite number of ambiguities related to the number of contact terms in the PM basis. We perform this analysis both at tree level (with and without exchanges) and at one loop. We also derive expressions for the corresponding CFT data in terms of harmonic sums. Finally, we consider the Regge limit of one-dimensional correlators and derive a precise connection between the latter and the large-twist limit of CFT data. Exploiting this result, we study the crossing equation in the three OPE limits and derive some universal constraints for the large-twist limit of CFT data in Regge-bounded theories with a finite number of exchanges.

Photon induced KΛ production on the proton in the low energy region

The associated photoproduction of [Formula: see text] from the proton in the low energy region is studied using an isobar model in which the nonresonant contributions are obtained from the nonlinear sigma model with chiral SU(3) symmetry which predicts, in a natural way, the contact term with its coupling strength along with the coupling strengths of the various Born terms predicted by the nonlinear sigma model. The present model is an extension of the nonlinear sigma model with chiral SU(2) symmetry, used earlier to study the photo, electro, and neutrino productions of pions. In the resonance sector, the contributions from the well established nucleon resonances ([Formula: see text]) in the [Formula: see text] channel, the hyperon resonances ([Formula: see text]) in the [Formula: see text] channel, and the kaon resonances ([Formula: see text] and [Formula: see text]) in the [Formula: see text] channel having spin [Formula: see text] and mass [Formula: see text][Formula: see text]GeV with a significant branching ratio in [Formula: see text] decay mode, have been considered. The strong and electromagnetic couplings of the [Formula: see text] channel nucleon resonances are taken from experiments while the couplings for the resonances in the [Formula: see text] and [Formula: see text] channels are fitted to reproduce the current data on the associated photoproduction of [Formula: see text] in this energy region. The numerical results are presented for the total and differential cross-sections and are compared with the available experimental data from CLAS and SAPHIR as well as with some of the recent theoretical models.

Winding modes and the reflection symmetry in AdS3

We compute the contact term of the two-point function for the SL (2, ℝ)- WZNW model in the winding sector. After reviewing some generalities of the model and its Euclidean counterpart, we discuss the reflection symmetry for the spectral flowed sector. We define the corresponding intertwining operator and use it to find the explicit form of the contact term of the propagator for a vertex carrying an arbitrary amount of spectral flow. Finally, we enhance the already known integral expression of a singly flowed vertex in order to reproduce such contact term directly in the space-time picture.

The Effect of Various Three-Body Forces on Nuclear Matter and Neutron Stars Properties

The binding energy per nucleon for nuclear matter, i.e., equation of state (EOS), within the Brueckner–Hartree–Fock (BHF) approach with the consideration of various three-body forces (3BFs) like the phenomenological 3BF and by adding a contact term to the BHF calculations are considered at variance densities. The 3BF contribution turns out to be nonnegligible contribution and to have a substantial saturation effect. The calculations are done utilizing the CD-Bonn and Argonne V18 nucleon–nucleon (NN) potentials. These NN potentials give great fitting to the deuteron properties and are phase-shift equivalent. The resultant EOS is compatible with the phenomenological analysis on the saturation point. It is demonstrated that the 3BF influences significantly on the nuclear matter EOS at high densities. Moreover, it is necessary for reproducing the empirical saturation properties for symmetric nuclear matter. The pressure has been also calculated and the suggested approaches reproduce fairly well agreement with the empirical data. We also examined the maximum neutron star masses which are close to two solar masses, which is again compatible with recent observational data. Comparison with other microscopic EOS is presented and discussed.

Searching for New Physics with [formula omitted] contact interactions

We study the impact of contact interactions involving two leptons (electrons or muons) and two b-quarks (bb¯ℓ+ℓ−) on the high-mass di-lepton region at the LHC. We consider different selections of b-tagged jet multiplicities in the di-lepton final states: inclusive (no selection), 0, 1 and 2 b-tagged jets, and show that the single b-jet selection significantly improves the sensitivity to New Physics (NP) in the form of the bb¯ℓ+ℓ− contact term. We obtain a better sensitivity compared to the currently existing searches of NP in the di-lepton inclusive channel. In particular, the expected limits go beyond competitive bounds set by LEP (for electrons) on the scale of NP, Λ, by a factor of 1.2−3.1, depending on the chirality structure of the operator. In addition, the expected limits on Λ, set by using a non-resonant LHC di-lepton inclusive search, are expected to be improved by a factor of 1.3−1.4 for both electrons and muons.

D(*)N interaction and the structure of Σc(2800) and Λc(2940) in chiral effective field theory

We study the $DN$ and ${D}^{*}N$ interactions to probe the inner structure of ${\mathrm{\ensuremath{\Sigma}}}_{c}(2800)$ and ${\mathrm{\ensuremath{\Lambda}}}_{c}(2940)$ with the chiral effective field theory to the next-to-leading order. We consider the contact term, one-pion-exchange and two-pion-exchange contributions to characterize the short-, long-, and mid-range interactions of the ${D}^{(*)}N$ systems. The low energy constants of the ${D}^{(*)}N$ systems are related to those of the $N\overline{N}$ interaction with a quark level Lagrangian that was inspired by the resonance saturation model. The $\mathrm{\ensuremath{\Delta}}(1232)$ degree of freedom is also included in the loop diagrams. The attractive potential in the $[DN{]}_{J=1/2}^{I=1}$ channel is too weak to form a bound state, which indicates that the explanation of ${\mathrm{\ensuremath{\Sigma}}}_{c}(2800)$ as the compact charmed baryon is more reasonable. Meanwhile, the potentials of the isoscalar channels are deep enough to yield the molecular states. We obtain the masses of the $[DN{]}_{J=1/2}^{I=0}$, $[{D}^{*}N{]}_{J=1/2}^{I=0}$, and $[{D}^{*}N{]}_{J=3/2}^{I=0}$ systems to be 2792.0, 2943.6, and 2938.4 MeV, respectively. The ${\mathrm{\ensuremath{\Lambda}}}_{c}(2940)$ is probably the isoscalar ${D}^{*}N$ molecule considering its low mass puzzle. Besides, the ${\mathrm{\ensuremath{\Lambda}}}_{c}(2940)$ signal might contain the spin-$\frac{1}{2}$ and spin-$\frac{3}{2}$ two structures, which can qualitatively explain the significant decay ratio to ${D}^{0}p$ and ${\mathrm{\ensuremath{\Sigma}}}_{c}\ensuremath{\pi}$. We also study the ${\overline{B}}^{(*)}N$ systems and predict the possible molecular states in the isoscalar channels. We hope experimentalists could hunt for the open charmed molecular pentaquarks in the ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ final state.

Large- Nc analysis of magnetic and axial two-nucleon currents in pionless effective field theory

We analyze magnetic and axial two-nucleon contact terms in a combined large-$N_c$ and pionless effective field theory expansion. These terms play important roles in correctly describing, e.g., the low-energy cross section of radiative neutron capture and the deuteron magnetic moment. We show that the large-$N_c$ expansion hints towards a hierarchy between the two leading-order magnetic terms that matches that found in phenomenological fits. We also comment on the issue of naturalness in different Lagrangian bases.

One-pion-exchange potential with contact terms from lattice QCD simulations * * Supported by in part by the National Natural Science Foundation of China (11775119, 11675083, 11405090, 11405116), the Natural Science Foundation of Tianjin, and China Scholarship Council (201906205013, 201906255002)

Pion-mass-dependent nucleon-nucleon (NN) potentials are obtained in terms of the one-pion exchange and contact terms from the latest lattice QCD simulations of the two-nucleon system. They assume the forms of the leading order (LO) NN potential from the chiral effective field theory and thus are referred to as the LO chiral potential in this study. We extract the coefficients of contact terms and cut-off momenta in these potentials, for the first time, by fitting the phase shifts of and channels obtained from the HALQCD collaboration with various pion masses from 468.6 to 1170.9 MeV. The low-energy constants in the and channels become weaker and approach each other for larger pion masses. These LO chiral potentials are applied to symmetric nuclear and pure neutron matter within the Brueckner-Hartree-Fock method. Presently, however, we do not yet have the information of the P-wave NN interaction to be provided by the lattice QCD simulations for a complete description of nuclear matter. Our results enhance understanding of the development of nuclear structure and nuclear matter by controlling the contribution of the pionic effect and elucidate the role of chiral symmetry of the strong interaction in complex systems.

On the origin of divergences in time-dependent orbifolds

We consider time-dependent orbifolds in String Theory and we show that divergences are not associated with a gravitational backreaction since they appear in the open string sector too. They are related to the non existence of the underlying effective field theory as in several cases fourth and higher order contact terms do not exist. Since contact terms may arise from the exchange of string massive states, we investigate and show that some three points amplitudes with one massive state in the open string sector are divergent on the time-dependent orbifolds. To check that divergences are associated with the existence of a discrete zero eigenvalue of the Laplacian of the subspace with vanishing volume, we construct the Generalized Null Boost Orbifold where this phenomenon can be turned on and off.