Noncommutativity Parameter Theta Research Articles

Noncommutative D=5 Chern–Simons gravity: Kaluza–Klein reduction and chiral gravitational anomaly

Actions for noncommutative (NC) gauge field theories can be expanded perturbatively in powers of the noncommutativity parameter theta using the Seiberg–Witten map between ordinary classical fields and their NC counterparts. The leading order term represents classical (theta =0) action while higher-order terms give us theta -dependent NC corrections that ought to capture some aspects of quantum gravity. Building on previous work of Aschieri and Castellani on NC Chern–Simons (CS) gauge and gravity theories, showing that non-trivial theta -dependence exists only for spacetime dimensions Dge 5, we investigate a correlated effect of these extra spatial dimensions and noncommutativity on four-dimensional physics, up to first-order in theta . Assuming that one spatial dimension is compactified into a circle, we apply the Kaluza–Klein reduction procedure on the NC D=5 CS theory for the conformal gauge group SO(4, 2), to obtain an effective, theta -dependent four-dimensional theory of gravity that has Einstein–Hilbert gravity with negative cosmological constant as its commutative limit. We derive field equations for this modified theory of gravity and study the effect of NC interactions on some classical geometries, such as the AdS-Schwarzschild black hole. We find that this NC background spacetime gives rise to chiral gravitational anomaly due to the nonvanishing theta -dependent Pontryagin density.

Noncommutative Schwarzschild geometry and generalized uncertainty principle

We discuss a possible link between the deformation parameter Theta ^{mu nu } arising in the framework of noncommutative geometry and the parameter beta of the generalized uncertainty principle (GUP). We compute the shift of the Hawking temperature induced by the Theta ^{mu nu }-deformed Schwarzschild geometry, and then we relate it to one obtained by GUP. Results suggest a granular structure of specetime at the Planck scales. The current bounds on beta allow to constraint the noncommutative parameter Theta ^{mu nu }.

Inferring type and scale of noncommutativity from the PTOLEMY experiment

If neutrinos are Dirac particles and their right-handed components can be copiously produced in the early universe, then they could influence a direct observation of the cosmic neutrino background, which, most likely, will come about with the recently proposed PTOLEMY experiment. For coupling of photons to the right-handed neutrinos we use a state-of-the-art version of gauge field theory deformed by the spacetime noncommutativity, to disclose by it not only the decoupling temperature for the said neutrino component, but also the otherwise hidden coupling temperature. Considering two relevant processes, the plasmon decay and the neutrino elastic scattering, we study the interplay between the structure of the noncommutativity parameter theta ^{mu nu } (type of noncommutativity) and the reheating temperature after inflation to obtain otherwise elusive upper bound on the scale of noncommutativity Lambda _mathrm{NC}. If PTOLEMY enhanced capture rate is due to spacetime noncommutativity, we verify that a nontrivial maximum upper bound on Lambda _mathrm{NC} (way below the Planck scale) emerges for a space-like theta ^{mu nu } and sufficiently high reheating temperature.

Neutrino propagation in noncommutative spacetimes

One-loop theta-exact quantum corrections to the neutrino propagator are computed in noncommutative U*(1) gauge-theory based on Seiberg-Witten maps. Our closed form results show that the one-loop correction contains a hard 1/epsilon UV divergence, as well as a logarithmic IR-divergent term of the type ln sqrt(theta p)^2, thus considerably softening the usual UV/IR mixing phenomenon. We show that both of these problematic terms vanish for a certain choice of the noncommutative parameter theta which preserves unitarity. We find non-perturbative modifications of the neutrino dispersion relations which are assymptotically independent of the scale of noncommutativity in both the low and high energy limits and may allow superluminal propagation. Finally, we demonstrate how the prodigious freedom in Seiberg-Witten maps may be used to affect neutrino propagation in a profound way.

On UV/IR mixing in noncommutative gauge field theories

In formulating gauge field theories on noncommutative (NC) spaces it is suggested that particles carrying gauge invariant quantities should not be viewed as pointlike, but rather as extended objects whose sizes grow linearly with their momenta. This and other generic properties deriving from the nonlocal character of interactions (showing thus unambiguously their quantum-gravity origin) lead to a specific form of UV/IR mixing as well as to a pathological behavior at the quantum level when the noncommutativity parameter theta is set to be arbitrarily small. In spite of previous suggestions that in a NC gauge theory based on the theta-expanded Seiberg-Witten (SW) maps UV/IR mixing effects may be under control, a fairly recent study of photon self-energy within a SW theta-exact approach has shown that UV/IR mixing is still present. We study the self-energy contribution for neutral fermions in the theta-exact approach of NC QED, and show by explicit calculation that all but one divergence can be eliminated for a generic choice of the noncommutativity parameter theta. The remaining divergence is linked to the pointlike limit of an extended object.

Renormalisability of noncommutative GUT inspired field theories with anomaly safe groups

We consider noncommutative GUT inspired field theories formulated within the enveloping-algebra formalism for anomaly safe compact simple gauge groups. Our theories have only gauge fields and fermions, and we compute the UV divergent part of the one-loop background-field effective action involving two fermionic fields at first order in the noncommutativity parameter theta. We show that, if the second-degree Casimir has the same value for all the irreps furnished by the fermionic multiplets of the model, then, that UV divergent part can be renormalised by carrying out multiplicative renormalisations of the coupling constant, theta and the fields, along with the inclusion of theta-dependent counterterms which vanish upon imposing the equations of motion. These theta-dependent counterterms have no physical effect since they vanish on-shell. This result along with the vanishing of the UV divergent part of the fermionic four-point functions leads to the unexpected conclusion that the one-loop matter sector of the background-field effective action of these theories is one-loop multiplicatively renormalisable on-shell. We also show that the background-field effective action of the gauge sector of the theories considered here receives no theta-dependent UV divergent contributions at one-loop. We thus conclude that these theories are on-shell one-loop multiplicatively renormalisable at first order in theta.

Quantum corrections to the noncommutative kink

We calculate quantum corrections to the mass of noncommutative phi^4 kink in (1+1) dimensions for intermediate and large values of the noncommutativity parameter theta. All one-loop divergences are removed by a mass renormalization (which is different from the one required in the topologically trivial sector). For large theta quantum corrections to the mass grow linearly with theta signaling about possible break down of the perturbative expansion.

On consistency of noncommutative Chern-Simons theory

We consider the noncommutative extension of Chern-Simons theory. We show the the theory can be fully expanded in power series of the noncommutative parameter theta and that no non-analytical sector exists. The theory appears to be unstable under radiative corrections, but we show that the infinite set of instabilities, to all orders in \hbar and in theta, is confined to a BRS exact cocycle. We show also that the theory is anomaly free. The quantum theory cannot be written in terms of the Groenewald-Moyal star product, and hence doubts arise on the interpretation of the noncommutative nature of the underlying spacetime. Nonetheless, the deformed theory is well defined as a quantum field theory, and the beta function of the Chern-Simons coupling constant vanishes, as in the ordinary Chern-Simons theory.

Noncommutative isotropic harmonic oscillator

Energy spectrum of isotropic oscillator as a function of noncommutativity parameter theta is studied. It is shown that for a dense set of values of theta the spectrum is degenerated and the algebra responsible for degeneracy can be always chosen to be sU(2). The generators of the algebra are constructed explicitely.

UV divergence-free QFT on noncommutative plane

We formulate Noncommutative Qauntum Field Theory in terms of fields defined as mean value over coherent states of the noncommutative plane. No *-product is needed in this formulation and noncommutativity is carried by a modified Fourier transform of fields. As a result the theory is UV finite and the cutoff is provided by the noncommutative parameter theta.

The one-loop effective action for quantum electrodynamics on noncommutative space

In this paper we calculate the divergent part of the one loop effective action for QED on noncommutative space using the background field method. The effective action is obtained up to the second order in the noncommutative parameter theta and in the classical fields.

Hamiltonian formulation of Noncommutative D3–Brane

Lagrangians of the Abelian Gauge Theory and its dual are related in terms of a shifted action. We show that in d=4 constrained Hamiltonian formulation of the shifted action yields Hamiltonian description of the dual theory, without referring to its Lagrangian. We apply this method, at the first order in the noncommutativity parameter theta, to the noncommutative U(1) gauge theory possessing spatial noncommutativity. Its dual theory is effectively a space--time noncommutative U(1) gauge theory. However, we obtain a Hamiltonian formulation where time is commuting. Space-time noncommutative D3--brane worldvolume Hamiltonian is derived as the dual of space noncommutative U(1) gauge theory. We show that a BPS like bound can be obtained and it is saturated for configurations which are the same with the ordinary D3-brane BIon and dyon solutions.

Towards an explicit expression of the Seiberg-Witten map at all orders

The Seiberg-Witten map links noncommutative gauge theories to ordinary gauge theories, and allows to express the noncommutative variables in terms of the commutative ones. Its explicit form can be found order by order in the noncommutative parameter theta and the gauge potential A by the requirement that gauge orbits are mapped on gauge orbits. This of course leaves ambiguities, corresponding to gauge transformations, and there is an infinity of solutions. Is there one better, clearer than the others ? In the abelian case, we were able to find a solution, linked by a gauge transformation to already known formulas, which has the property of admitting a recursive formulation, uncovering some pattern in the map. In the special case of a pure gauge, both abelian and non-abelian, these expressions can be summed up, and the transformation is expressed using the parametrisation in terms of the gauge group.

Noncommutative multi-solitons in 2+1 dimensions

The study of noncommutative solitons is greatly facilitated if the field equations are integrable, i.e. result from a linear system. For the example of a modified but integrable U(n) sigma model in 2+1 dimensions we employ the dressing method to construct explicit multi-soliton configurations on noncommutative R^{2,1}. These solutions, abelian and nonabelian, feature exact time-dependence for any value of the noncommutativity parameter theta and describe various lumps of finite energy in relative motion. We discuss their scattering properties and prove asymptotic factorization for large times.

Singularities of the Seiberg-Witten map

We construct an explicit solution of the Seiberg-Witten map for a linear gauge field on the non-commutative plane. We observe that this solution as well as the solution for a constant curvature diverge when the non-commutativity parameter theta reaches certain event horizon in the theta-space. This implies that an ordinary Yang-Mills theory can be continuously deformed by the Seiberg-Witten map into a non-commutative theory only within one connected component of the theta-space.

The IR/UV connection in the non-commutative gauge theories

Quantum field theory on non-commutative spaces does not enjoy the usual ultraviolet-infrared decoupling that forms the basis for conventional renormalization. The high momentum contributions to loop integrations can lead to unfamiliar long distance behavior which can potentially undermine naive expectations for the IR behavior of the theory. These "anomalies" involve non-analytic behavior in the noncommutativity parameter Theta making the limit Theta goes to zero singular. In this paper we will analyze such effects in the one loop approximation to gauge theories on non-commutative space. We will see that contrary to expectations poles in Theta do occur and lead to large discrepancies between the expected and actual infrared behavior. We find that poles in Theta are absent in supersymmetric theories. The "anomalies" are generally still present, but only at the logarithmic level. A notable exception is non-commutative super Yang Mills theory with 16 real supercharges in which anomalous effects seem to be absent altogether.

Brane configuration from monopole solution in non-commutative super Yang-Mills theory

We study the structure of the monopole configuration in U(2) non-commutative super Yang-Mills theory. Our analysis consists of two steps: solving the BPS equation and then the eigenvalue equation in the non-commutative space. Calculation to the first non-trivial order in the non-commutativity parameter theta shows that the monopole exhibits a certain non-locality. This structure is precisely the one expected from the recent predictions by the brane-configuration technique.