Searcharxiv⌕ Search

arXiv subjects

Radu Roiban

Publications and source records attributed to Radu Roiban.

99 records · Page 6Linked to original sources
arXiv2001

Breaking CPT by mixed non-commutativity

The mixed component of the non-commutative parameter θ_{μM}, where μ= 0,1,2,3 and M is an extra dimensional index may violate four-dimensional CPT invariance. We calculate one and two-loop induced couplings of θ_{μ5} with the four-dimensional axial vector current and with the CPT odd dim=6 operators starting from five-dimensional Yukawa and U(1) theories. The resulting bounds from clock comparison experiments place a stringent constraint on θ_{μ5}, |θ_{μ5}|^{-1/2} > 5\times 10^{11} GeV. The orbifold projection and/or localization of fermions on a 3-brane lead to CPT-conserving physics, in which case the constraints on θ{μ5} are softened.

hep-ph↗
arXiv2001

Brane-Bulk Duality and Non-conformal Gauge Theories

We discuss non-conformal gauge theories from type IIB D3-branes embedded in orbifolded space-times. Such theories can be obtained by allowing some non-vanishing logarithmic twisted tadpoles. In certain cases with N=0,1 supersymmetry correlation functions in the planar limit are the same as in the parent N=2 supersymmetric theories. In particular, the effective action in such theories perturbatively is not renormalized beyond one loop in the planar limit. In the N=2 as well as such N=0,1 theories quantum corrections in the D3-brane gauge theories are encoded in the corresponding classical higher dimensional field theories whose actions contain the twisted fields with non-vanishing tadpoles. We argue that this duality can be extended to the non-perturbative level in the N=2 theories. We give some evidence that this might also be the case for N=0,1 theories as well.

hep-th↗
arXiv2000

T-duality of the Green-Schwarz superstring

We study T-duality in the Green-Schwarz formalism to all orders in superspace coordinates. We find two analogs of Buscher rules for the supervielbein and clarify their meaning from the superstring point of view. The transformation rules for the dilaton, spin 1/2 fermions and Ramond-Ramond superfields are also derived.

hep-th↗
arXiv2000

Superstrings on AdS_5 x S^5 supertwistor space

We derive the Green-Schwarz action on AdS_5 x S^5 using an alternate version of the coset superspace construction. By Wick rotations and Lie algebra identifications we bring the coset to GL(4|4)/(Sp(4) x GL(1))^2, which allows us to represent the conformal transformations on unconstrained matrices. The derivation is more streamlined even for the bosonic sector, and conformal symmetry is manifest at every step. Kappa-symmetry gauge fixing is more transparent.

hep-th↗
arXiv2000

Convergence Theorem for Non-commutative Feynman Graphs and Renormalization

We present a rigorous proof of the convergence theorem for the Feynman graphs in arbitrary massive Euclidean quantum field theories on non-commutative R^d (NQFT). We give a detailed classification of divergent graphs in some massive NQFT and demonstrate the renormalizability of some of these theories.

hep-th↗
arXiv2000

Renormalization of Quantum Field Theories on Noncommutative R^d, I. Scalars

A noncommutative Feynman graph is a ribbon graph and can be drawn on a genus $g$ 2-surface with a boundary. We formulate a general convergence theorem for the noncommutative Feynman graphs in topological terms and prove it for some classes of diagrams in the scalar field theories. We propose a noncommutative analog of Bogoliubov-Parasiuk's recursive subtraction formula and show that the subtracted graphs from a class $Ω_d$ satisfy the conditions of the convergence theorem. For a generic scalar noncommutative quantum field theory on $\re^d$, the class $Ω_d$ is smaller than the class of all diagrams in the theory. This leaves open the question of perturbative renormalizability of noncommutative field theories. We comment on how the supersymmetry can improve the situation and suggest that a noncommutative analog of Wess-Zumino model is renormalizable.

hep-th↗
arXiv2000

Low-energy Limits on the Antisymmetric Tensor Field Background on the Brane and on the Non-commutative Scale

A non-vanishing vacuum expectation value for an antisymmetric tensor field leads to the violation of Lorentz invariance on the brane. This violation is controlled by the $θ_{μν}$ parameter, which has a dimension of inverse mass squared. We assume that the zeroth order term in theta-expansion represents the Standard Model and study the effects induced by linear terms in theta_{μν}. Low-energy precision experiments place the limit on the possible size of this background at the level of $1/\sqrtθ > 5 \times 10^{14}$ GeV. This poses certain difficulty for the TeV-range string scale models, in which the antisymmetric field has to acquire a relatively large mass to ensure that $θ_{μν}=0$ and avoid cosmological moduli problem.

hep-ph↗
arXiv1999

MSSM Predictions for the Electric Dipole Moment of the $^{199}$Hg Atom

The Minimal Supersymmetric Standard Model can possess several CP-violating phases beyond the conventional Cabibo-Kobayashi-Maskawa phase. We calculate the contribution of these phases to T-violating nuclear forces. These forces induce a Schiff moment in the $^{199}$Hg nucleus, which is strongly limited by experiments aimed at the detection of the electric dipole moment of the mercury atom. The result for $d_{Hg}$ is found to be very sensitive to the CP-violating phases of the MSSM and the calculation carries far fewer QCD uncertainties than the corresponding calculation of the neutron EDM. In certain regions of the MSSM parameter space, the limit from the mercury EDM is stronger than previous constraints based on either the neutron or electron EDMs. We present combined constraints from the mercury and electron EDMs to limit both CP-violating phases of the MSSM. We also present limits in mSUGRA models with unified gaugino and scalar masses at the GUT scale.

hep-ph↗