Contact Terms Research Articles

Implications of Efimov physics for the description of three and four nucleons in chiral effective field theory

In chiral effective field theory the leading order (LO) nucleon-nucleon potential includes two contact terms, in the two spin channels $S=0,1$, and the one-pion-exchange potential. When the pion degrees of freedom are integrated out, as in the pionless effective field theory, the LO potential includes two contact terms only. In the three-nucleon system, the pionless theory includes a three-nucleon contact term interaction at LO whereas the chiral effective theory does not. Accordingly arbitrary differences could be observed in the LO description of three- and four-nucleon binding energies. We analyze the two theories at LO and conclude that a three-nucleon contact term is necessary at this order in both theories. In turn this implies that subleading three-nucleon contact terms should be promoted to lower orders. Furthermore this analysis shows that one single low energy constant might be sufficient to explain the large values of the singlet and triplet scattering lengths.

Superfluid helium II as the QCD vacuum

We study the winding number susceptibility 〈I2〉 in a superfluid system and the topological susceptibility 〈Q2〉 in QCD. We argue that both correlation functions exhibit similar structures, including the generation of the contact terms. We discuss the nature of the contact term in superfluid system and argue that it has exactly the same origin as in QCD, and it is related to the long distance physics which cannot be associated with conventional microscopical degrees of freedom such as phonons and rotons. We emphasize that the conceptual similarities between superfluid system and QCD may lead, hopefully, to a deeper understanding of the topological features of a superfluid system as well as the QCD vacuum.

On WZ and RR couplings of BPS branes and their all order α ′ corrections in IIB, IIA

We compute all three and four point couplings of BPS $D_{p}$-branes for all different nonzero $p$-values on the entire world volume and transverse directions. We start finding out all four point function supersymmetric Wess-Zumino (WZ) actions of one closed string Ramond-Ramond (RR) field with two fermions, either with the same (IIB) or different chirality (IIA) as well as their all order $\alpha'$ corrections. The closed form of S-matrices of two closed string RR in both IIB, IIA, including their all order $\alpha'$ corrections have also been addressed. Our results confirm that, not only the structures of $\alpha'$ corrections but also their coefficients of IIB are quite different from their IIA ones. The S-matrix of an RR and two gauge (scalar) fields and their all order corrections in antisymmetric picture of RR have been carried out as well. Various remarks on the restricted Bianchi identities as well as all order $\alpha'$ corrections to all different supersymmetric WZ couplings in both type IIA and IIB superstring theory are also released. Lastly, different singularity structures as well as all order contact terms for all non-vanishing traces in type II have also been constructed.

Computational Study of Ga NMR Shielding in Metallic Gallides

We present first-principles calculations of the isotropic NMR Ga shielding in metallic MGa2 with M = Ca, Sr, Ba and MGa4 with M = Na, Ca, Sr and Ba. We show that the experimentally observed trend of Ga NMR shifts is as expected driven mainly by the spin part of the response, but the orbital contribution must not be neglected. For all analyzed compounds the spin contact term constitute the major component of the response, except for BaGa2, where the spin-dipolar contribution is unusually large. This spin-dipolar contribution is related to the difference of the Ga-4pz and 4px,y partial density of states (PDOS) at the Fermi level, which is large only for BaGa2. It is related to the honeycomb-like Ga-lattice and the distances between Ga atoms. The spin-contact term is determined to a large extend by Ga-4s PDOS at the Fermi level, because the magnetic field leads to a small spin-splitting and a reoccupation of spin-up and spin-down states. This Ga-4s PDOS is related to the local atomic structure around the Ga ...

Revisiting AdS/CFT at a finite radial cut-off

We revisit AdS/CFT at finite radial cut-off, specifically in the context of double trace perturbations, $\mathbb{O}_n$= $\mathbb{O}(x) (\partial^2)^{n} {\mathcal O}(x)$, with arbitrary $n$. As well-known, the standard GKPW prescription, applied to a finite radial cut-off, leads to contact terms in correlators. de Haro et al (arXiv:hep-th/0002230) introduced bulk counterterms to remove these. However, this prescription yields additional terms in the correlator corresponding to spurious double trace deformations. Further, if we view GKPW prescription coupled with the prescription in arXiv:hep-th/0002230, in terms of a boundary wavefunction, we find that it is incompatible with radial Schrodinger evolution (in the spirit of holographic Wilsonian RG). We consider a more general wavefunction satisfying the Schrodinger equation, and find that generically such wavefunctions generate both (a) double trace deformations and (b) contact terms. However, we find that there exist special choices of these wavefunctions, amounting to a new AdS/CFT prescription at a finite cut-off, so that both (a) and (b) are removed and we obtain a pure power law behaviour for the correlator. We compare these special wavefunctions with a specific RG scheme in field theory. We give a geometric interpretation of these wavefunctions; these correspond to some specific smearing of boundary points in the Witten diagrams. We present a comprehensive calculation of exact double-trace beta-functions for all couplings $\mathbb{O}_n$ and match with a holographic computation using the method described above. The matching works with a mapping between the field theory and bulk couplings; such a map is highly constrained because the beta-functions are quadratic and exact on both sides. We generalize standard double-trace Wilson-Fisher flow to the space of the infinite number of couplings.

Photoproduction of hidden-charm states in the reaction near threshold**Supported by Major State Basic Research Development Program in China (2014CB845400), National Natural Science Foundation of China (11475227, 11275235, 11035006) and Chinese Academy of Sciences (Knowledge Innovation Project (KJCX2-EW-N01), Youth Innovation Promotion Association CAS (2016367), Open Project Program of State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of

We report on a theoretical study of the hidden charm states in the reaction near threshold within an effective Lagrangian approach. In addition to the contributions from the s-channel nucleon pole, the t-channel D0 exchange, the u-channel exchange and the contact term, we study the contributions from the states with spin-parity JP = 1/2− and 3/2−. The total and differential cross sections of the reaction are predicted. It is found that the contributions of these states give clear peak structures in the total cross sections. Thus, this reaction is another new platform to study the hidden-charm states. It is expected that our model calculation may be tested by future experiments.

Symmetry constraints in inflation, α-vacua, and the three point function

The Ward identities for conformal symmetries in single field models of inflation are studied in more detail in momentum space. For a class of generalized single field models, where the inflaton action contains arbitrary powers of the scalar and its first derivative, we find that the Ward identities are valid. We also study a one-parameter family of vacua, called $\alpha$-vacua, which preserve conformal invariance in de Sitter space. We find that the Ward identities, upto contact terms, are met for the three point function of a scalar field in the probe approximation in these vacua. Interestingly, the corresponding non-Gaussian term in the wave function does not satisfy the operator product expansion. For scalar perturbations in inflation, in the $\alpha$-vacua, we find that the Ward identities are not satisfied. We argue that this is because the back-reaction on the metric of the full quantum stress tensor has not been self-consistently incorporated. We also present a calculation, drawing on techniques from the AdS/CFT correspondence, for the three point function of scalar perturbations in inflation in the Bunch-Davies vacuum.

Geometric entropy and edge modes of the electromagnetic field

We calculate the vacuum entanglement entropy of Maxwell theory in a class of curved spacetimes by Kaluza-Klein reduction of the theory onto a two-dimensional base manifold. Using two-dimensional duality, we express the geometric entropy of the electromagnetic field as the entropy of a tower of scalar fields, constant electric and magnetic fluxes, and a contact term, whose leading-order divergence was discovered by Kabat. The complete contact term takes the form of one negative scalar degree of freedom confined to the entangling surface. We show that the geometric entropy agrees with a statistical definition of entanglement entropy that includes edge modes: classical solutions determined by their boundary values on the entangling surface. This resolves a long-standing puzzle about the statistical interpretation of the contact term in the entanglement entropy. We discuss the implications of this negative term for black hole thermodynamics and the renormalization of Newton's constant.

Mirror and Triplet Displacement Energies Within Nuclear DFT: Numerical Stability

Isospin-symmetry-violating class II and III contact terms are introduced into the Skyrme energy density functional to account for charge dependence of the strong nuclear interaction. The two new coupling constants are adjusted to available experimental data on triplet and mirror displacement energies, respectively. We present preliminary results of the fit, focusing on its numerical stability with respect to the basis size.

On the hidden charm pentaquarks in $$\Lambda _b \rightarrow J/\psi K^- p$$ Λ b → J / ψ K - p decay

In a previous work we presented a theoretical analysis of the $$\Lambda _b \rightarrow J/\psi K^- p$$ reaction based on which a recent experiment by the LHCb collaboration at CERN claimed the existence of two hidden charm pentaquarks, $$P_c(4380)^+$$ and $$P_c(4450)^+$$ . In that work we focused only on the $$\Lambda (1405)$$ and $$P_c(4450)^+$$ signals and discussed the possible explanation of this pentaquark state within the picture of a dynamical meson–baryon molecule made up mostly from $$\bar{D}^* \Sigma _c$$ and $$\bar{D}^* \Sigma ^*_c$$ components. In the present work we improve upon the previous one by considering the total $$K^- p$$ and $$J/\psi p$$ data including all the relevant resonances contributing to the spectra, and discuss the possible nature of both $$P_c(4380)^+$$ and $$P_c(4450)^+$$ . We also discuss several important topics, like the effect of the contact term in the reaction, the viability of reproducing the data without the $$P_c(4380)^+$$ and the possible quantum number assignment to these pentaquarks.

Higher derivative corrections to the Wess-Zumino action

In the first part of this paper, we calculate the disk-level S-matrix elements of one RR, one NSNS and one NS vertex operators, and show that they are consistent with the amplitudes that have been recently found by applying various Ward identities. We show that the massless poles of the amplitude at low energy are fully consistent with the known D-brane couplings at order $\alpha'^2$ which involve one RR or NSNS and two NS fields. Subtracting the massless poles, we then find the contact terms of one RR, one NSNS and one NS fields at order $\alpha'^2$. Some of these terms are reproduced by the Taylor expansion and the pull-back of two closed string couplings, some other couplings are reproduced by linear graviton in the second fundamental form and by the B-field in the gauge field extension $F\rightarrow F+B$, in one closed and two open string couplings. In the second part, we write all independent covariant contractions of one RR, one NSNS and one NS fields with unknown coefficients. We then constrain the couplings to be consistent with the linear T-duality and with the above contact terms. Interestingly, we have found that up to total derivative terms and Bianchi identities, these constraints uniquely fix all the unknown coefficients.

Physical effects of the gravitational Θ-parameter

We describe the effect of the gravitational [Formula: see text]-parameter on the behavior of the stretched horizon of a black hole in [Formula: see text]-dimensions. The gravitational [Formula: see text]-term is a total derivative, however, it affects the transport properties of the horizon. In particular, the horizon acquires a third-order parity violating, dimensionless transport coefficient which affects the way localized perturbations scramble on the horizon. In the context of the gauge/gravity duality, the [Formula: see text]-term induces a nontrivial contact term in the energy–momentum tensor of a [Formula: see text]-dimensional large-N gauge theory, which admits a dual gravity description. As a consequence, the dual gauge theory in the presence of the [Formula: see text]-term acquires the same third-order parity violating transport coefficient.

Late-time structure of the Bunch-Davies FRW wavefunction

In this short note we organize a perturbation theory for the Bunch-Davies wavefunction in flat, accelerating cosmologies. The calculational technique avoids the in-in formalism and instead uses an analytic continuation from Euclidean signature. We will consider both massless and conformally coupled self-interacting scalars. These calculations explicitly illustrate two facts. The first is that IR divergences get sharper as the acceleration slows. The second is that UV-divergent contact terms in the Euclidean computation can contribute to the absolute value of the wavefunction in Lorentzian signature. Here UV divergent refers to terms involving inverse powers of the radial cutoff in the Euclidean computation. In Lorentzian signature such terms encode physical time dependence of the wavefunction.

Density-dependent effective baryon–baryon interaction from chiral three-baryon forces

A density-dependent effective potential for the baryon–baryon interaction in the presence of the (hyper)nuclear medium is constructed, based on the leading (irreducible) three-baryon forces derived within SU(3) chiral effective field theory. We evaluate the contributions from three classes: contact terms, one-pion exchange and two-pion exchange. In the strangeness-zero sector we recover the known result for the in-medium nucleon–nucleon interaction. Explicit expressions for the ΛN in-medium potential in (asymmetric) nuclear matter are presented. Our results are suitable for implementation into calculations of (hyper)nuclear matter. In order to estimate the low-energy constants of the leading three-baryon forces we introduce the decuplet baryons as explicit degrees of freedom and construct the relevant terms in the minimal non-relativistic Lagrangian. With these, the constants are estimated through decuplet saturation. Utilizing this approximation we provide numerical results for the effect of the three-body force in symmetric nuclear matter and pure neutron matter on the ΛN interaction. A moderate repulsion that increases with density is found in comparison to the free ΛN interaction.

Contact interactions in Higgs-vector boson associated production at the ILC

We explore new physics (NP) effects in Higgs-Vector boson associated production at a future International Linear Collider (ILC) via e+e- -> Zh,Zhh, using effective field theory (EFT) techniques. In particular, we focus on a certain class of dimension 6 operators, which are generated by tree-level exchanges of a new heavy vector field in the underlying theory. These operators induce new contact terms of the form \psi\psi\phi D\phi, involving the Standard Model (SM) fermions (\psi), gauge-bosons (D is the covariant derivative) and the SM Higgs field (\phi). We investigate the high-energy behaviour of these new effective interactions in e+e- -> Zh,Zhh, imposing bounds from electroweak precision measurements, and show that the ILC is an excellent testing ground for probing this type of NP via e+e- -> Zh,Zhh. We also address the validity of the EFT expansion and we study the correlation between the hZ and hhZ signals, which can be utilized in future searches for NP in these channels.

On bulk singularity structures and all order α ′ contact terms of BPS string amplitudes

The entire form of the amplitude of three SYM ( involving two transverse scalar fields, a gauge field) and a potential $C_{n-1}$ Ramond-Ramond (RR) form field is found out. We first derive $<V_{C^{-2}} V_{A^{0}} V_{\phi ^{0}} V_{\phi ^{0}}>$ and then start constructing an infinite number of $t,s$ channel bulk singularity structures by means of all order $\alpha'$ corrections to pull-back of brane in an Effective Field Theory (EFT). Due to presence of the complete form of S-matrix, several new contact interactions as well as new couplings are explored. It is also shown that these couplings can be verified at the level of EFT by either the combinations of Myers terms, pull-back, Taylor expanded of scalar fields or the mixed combination of the couplings of this paper as well as employed Bianchi identities. For the first time, we also derive the algebraic and the complete form of the integrations for some arbitrary combinations of Mandelstam variables and for the most general case $\int d^2z |1-z|^{a} |z|^{b} (z - \bar{z})^{c} (z + \bar{z})^{3}$ on upper half plane as well.

Many-body effects on graphene conductivity: Quantum Monte Carlo calculations

Optical conductivity of graphene is studied using Quantum Monte Carlo calculations. We start from Euclidean current-current correlator and extract $\sigma (\omega)$ from Green-Kubo relations using Backus-Gilbert method. Calculations were performed both for long-range interactions and taking into account only contact term. In both cases we vary interaction strength and study its influence on optical conductivity. We compare our results with previous theoretical calculations choosing $\omega \approx \kappa$ thus working in the region of the plateau in $\sigma(\omega)$ which corresponds to optical conductivity of Dirac quasiparticles. No dependence of optical conductivity on interaction strength is observed unless we approach antiferromagnetic phase transition in case of artificially enhanced contact term. Our results strongly support previous theoretical studies claimed very weak regularization of graphene conductivity.

No contact terms for the magnetic field in Lorentz- and CPT-violating electrodynamics

In a Lorentz- and CPT-violating modification of electrodynamics, the fields of a moving charge are known to have unusual singularities. This raises the question of whether the singular behavior may include δ-function contact terms, similar to those that appear in the fields of idealized dipoles. However, by calculating the magnetic field of an infinite straight wire in this theory, we demonstrate that there are no such contact terms in the magnetic field of a moving point charge.

Pseudo-Contact Shift in Periodic Paramagnetic Solids Via Density Functional Theory

Nuclear Magnetic Resonance (NMR) spectroscopy of paramagnetic solids represents an exceptional means for getting insight into structural and electronic properties of the systems, giving on the other hand a particularly involved response to analyse. Specifically, the overall isotropic shift is given by a combination of the Fermi contact term, dependent on the electronic structure of the paramagnetic site and on the strength and covalency of its bond with the observed centre, and on the pseudo-contact term, a relativistic dipolar contribution that gives information regarding the structure and the spatial arrangements of the sites in the system. In this work we present a protocol to extend the computation of the paramagnetic shielding in the presence of spin-orbit coupling towards solid systems, with which to calculate these two contributions explicitly via periodic solid-state Density Functional Theory (DFT) so to unravel detailed information hidden in the NMR spectrum. We describe how the involved properties such as g- and hyperfine tensors behave in paramagnetic solids where the spin-bearing ions are a major constituent of the crystal lattice. Finally it is clarified how to combine these quantities in order to separate the various terms contributing to the paramagnetic shielding, σs : this method enables to get deeper insight into the overall isotropic shift and shift anisotropy observed experimentally, and to identify information regarding electronic structure and geometry of the system. The method is applied to a series of solid-state LiTMPO4 (with TM=Mn, Fe, Co and Ni) powders and the corresponding 7Li and 31P NMR shifts are computed. From these results we outline differences in the extent of spin-orbit coupling effects and electron-nuclear magnetic interaction, and the various tensorial products are combined together to explicitly calculate all the terms involved in the description of σs . A detailed comparison is presented between contact and dipolar interactions within the same system and across the series, to untangle the dual nature of the overall isotropic shift and shift anisotropy.

Light-front representation of chiral dynamics with Δ isobar and large-N c relations

Transverse densities describe the spatial distribution of electromagnetic current in the nucleon at fixed light-front time. At peripheral distances b = O(M_pi^{-1}) the densities are governed by chiral dynamics and can be calculated model-independently using chiral effective field theory (EFT). Recent work has shown that the EFT results can be represented in first-quantized form, as overlap integrals of chiral light-front wave functions describing the transition of the nucleon to soft-pion-nucleon intermediate states, resulting in a quantum-mechanical picture of the peripheral transverse densities. We now extend this representation to include intermediate states with Delta isobars and implement relations based on the large-N_c limit of QCD. We derive the wave function overlap formulas for the Delta contributions to the peripheral transverse densities by way of a three-dimensional reduction of relativistic chiral EFT expressions. Our procedure effectively maintains rotational invariance and avoids the ambiguities with higher-spin particles in the light-front time-ordered approach. We study the interplay of pi-N and pi-Delta intermediate states in the quantum-mechanical picture of the densities in a transversely polarized nucleon. We show that the correct N_c-scaling of the charge and magnetization densities emerges as the result of the particular combination of currents generated by intermediate states with degenerate N and Delta. The off-shell behavior of the chiral EFT is summarized in contact terms and can be studied easily. The methods developed here can be applied to other peripheral densities and to moments of the nucleon's generalized parton distributions.