arXiv subjects

Ph. Brax

Publications and source records attributed to Ph. Brax.

50 records · Page 3Linked to original sources

arXiv2000

Models with Inverse Sfermion Mass Hierarchy and Decoupling of the SUSY FCNC Effects

We study the decoupling of the first two squark and slepton families in order to lower the flavour changing neutral current effects. Models with inverse sfermion mass hierarchy based upon gauged U(1) flavour symmetries provide a natural framework where decoupling can be implemented. Decoupling requires a large gap between the Fermi scale and the supersymmetry breaking scale. Maintaining the electroweak symmetry breaking at the Fermi scale requires some fine-tuning that we investigate by solving the two-loop renormalization group equations. We show that the two-loop effects are governed by the anomaly compensated by the Green-Schwarz mechanism and can be determined from the quark and lepton masses. The electroweak breaking constraints lead to a small $μ$ scenario where the LSP is Higgsino-like.

hep-ph↗

arXiv1999

The Supermoduli Space of Matrix String Theory

We study matrix string scattering amplitudes and matrix string instantons on a marked Riemann surface in the limit of a vanishing string coupling constant. We give an explicit parameterization of the moduli space of such instantons. We also give a description of the set of fermionic supermoduli. The integration over the supermoduli leads to the inclusion of picture changing operators at the interaction points. Finally we investigate the large N limit of the measure on the instanton moduli space and show its convergence to the Weil-Petersson measure on the moduli space of marked Riemann surfaces.

hep-th↗

arXiv1999

Anomaly Matching and Syzygies in N=1 Gauge Theories

We investigate the connection between the moduli space of N=1 supersymmetric gauge theories and the set of polynomial gauge invariants constrained by classical/quantum relations called syzygies. We examine the existence of a superpotential reproducing these syzygies and the link with the 't Hooft anomaly matching between the fundamental fields at high energy and the gauge invariant degrees of freedom at low energy for the flavour symmetry group. We show that the anomaly matching is equivalent to the vanishing of the flavour anomaly on the normal space to the manifold defined by the syzygies. For normal spaces in a real representation of the flavour group we strengthen the connection between the 't Hooft anomaly matching and the existence of a superpotential by constructing a flavour invariant polynomial whose gradient vanishes at least on the solutions of the syzygies. This corroborates a recent definition of confining theories. We illustrate our general result by considering two examples based on the SU(Nc) and Spin(7) gauge theories. We also examine the role of syzygies in the context of non-Abelian duality. We emphasize the relevance of non-perturbative effects in the dual magnetic theories in proving the equivalence of the electric and magnetic syzygies.

hep-ph↗

arXiv1998

Limits of Matrix Theory in Curved Space

We study curved space versions of matrix string theory taking as a definition of the theory a gauged matrix sigma model. By analyzing the divergent terms in the loop expansion for the effective action we reduce the problem to a simple matrix generalization of the standard string theory beta function calculation. It is then demonstrated that the model can only be consistent for Ricci flat manifolds with vanishing six-dimensional Euler density.

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arXiv1997

Open Supermembranes Coupled to M-Theory Five-Branes

We consider open supermembranes in eleven dimensions in the presence of closed M-Theory five-branes. It has been shown that, in a flat space-time, the world-volume action is kappa invariant and preserves a fraction of the eleven dimensional supersymmetries if the boundaries of the membranes lie on the five-branes. We calculate the reparametrisation anomalies due to the chiral fermions on the boundaries of the membrane and examine their cancellation mechanism. We show that these anomalies cancel with the aid of a classical term in the world-volume action, provided that the tensions of the five-brane and the membrane are related to the eleven dimensional gravitational constant in a way already noticed in M-Theory.

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arXiv1997

Exponentially Small Couplings and the Hierarchy Problem

We propose a stringy mechanism whereby a large hierarchy between symmetry breaking scales is generated. This mechanism is based upon the existence of a fifth dimension compactified on a segment. We focus on a simple supersymmetric model with one massless Higgs field in the 3 of SU(3) and another one in the $\bar 3$ on each four dimensional end-point of the fifth dimension. Along supersymmetric flat directions the gauge symmetry is broken down to SU(2) due to the vacuum expectation value of the Higgs fields on one of the end points. The remaining massless mode on the other end point acquires a potential due to a massive five dimensional state propagating between the end points. This potential breaks the SU(2) symmetry at an exponentially suppressed scale compared to the SU(3) breaking scale. The suppression factor depends exponentially on the mass M of the massive state and the length $πR$ of the fifth dimension. For reasonably large values of the length scale R one can achieve a factor of order $M_{W}/M_GUT}$.

hep-th↗

arXiv1997

Exponentially Small Couplings between Twisted Fields of Orbifold String Theories

We investigate the natural occurence of exponentially small couplings in effective field theories deduced from higher dimensional models. We calculate the coupling between twisted fields of the Z_3 Abelian orbifold compactification of the heterotic string. Due to the propagation of massive Kaluza-Klein modes between the fixed points of the orbifold, the massless twisted fields located at these singular points become weakly coupled. The resulting small couplings have an exponential dependence on the mass of the intermediate states and the distance between the fixed points.

hep-th↗

arXiv1997

Open supermembranes in eleven dimensions

We consider open supermembranes in an eleven dimensional background. We show that, in a flat space-time, the world-volume action is kappa-symmetric and has global space-time supersymmetry if space-time has even dimensional topological defects where the membrane can end. An example of such topological defects is provided by the space-time with boundaries considered by Horava and Witten. In that case the world-volume action has reparametrisation anomalies whose cancellation requires the inclusion of a current algebra on the boundaries of the membrane. The role of kappa-anomalies in a general background is discussed. The tension of the membrane is related to the eleven dimensional gravitational constant with the aid of the Green-Schwarz mechanism allowing a consistency check of M-theory.

hep-th↗

arXiv1996

A Renormalisation Group Study of Three Dimensional Turbulence

We study the three dimensional Navier-Stokes equation with a random Gaussian force acting on large wavelengths. Our work has been inspired by Polyakov's analysis of steady states of two dimensional turbulence. We investigate the time evolution of the probability law of the velocity potential. Assuming that this probability law is initially defined by a statistical field theory in the basin of attraction of a renormalisation fixed point, we show that its time evolution is obtained by averaging over small scale features of the velocity potential. The probability law of the velocity potential converges to the fixed point in the long time regime. At the fixed point, the scaling dimension of the velocity potential is determined to be ${-{4\over 3}}$. We give conditions for the existence of such a fixed point of the renormalisation group describing the long time behaviour of the velocity potential. At this fixed point, the energy spectrum of three dimensional turbulence coincides with a Kolmogorov spectrum.

hep-th↗

arXiv1996

A Renormalisation Group Analysis of 2d Freely Decaying Magnetohydrodynamic Turbulence

We study two dimensional freely decaying magnetohydrodynamic turbulence. We investigate the time evolution of the probability law of the gauge field and the stream function. Assuming that this probability law is initially defined by a statistical field theory in the basin of attraction of a renormalisation group fixed point, we show that its time evolution is generated by renormalisation transformations. In the long time regime, the probability law is described by non-unitary conformal field theories. In that case, we prove that the kinetic and magnetic energy spectra are proportional. We then construct a family of fixed points using the $(p,p+2)$ non-unitary minimal models of conformal field theories.

hep-th↗

arXiv1995

The Coulomb Gas Behaviour of Two Dimensional Turbulence

The long-time large-distance behaviour of free decaying two dimensional turbulence is studied. Stochastic solutions of the Navier-Stokes equation are explicitly shown to follow renormalisation group trajectories. It is proven that solutions of the Navier-Stokes equation asymptotically converge to fixed points which are conformal field theories. A particular fixed point is given by the free Gaussian field with a charge at infinity. The stream function is identified with a vertex operator. It happens that this solution also admits constant $n$-enstrophy fluxes in the asymptotic regime, therefore fulfilling all requirements to represent an asymptotic state of two-dimensional turbulence. The renormalisation basin of attraction of this fixed point consists of a charged Coulomb gas. This Coulomb gas gives an effective description of turbulence.

hep-th↗

arXiv1995

Flavour Changing Neutral Current Effects From Flavour Dependent Supergravity Couplings

We investigate the impact on low energy flavour mixing phenomena of the renormalisation group running of parameters in supersymmetric theories. We have explicitly chosen flavour dependent supergravity couplings. The renormalisation group equations are displayed in general matrix form. We work in a scale dependent basis such that the hierarchy in the quark masses can be more easily utilised. Simple analytic solutions are provided in matrix form (for general boundary conditions) when only the third generation Yukawa couplings are retained in the renormalisation group equations. We solve the evolution equations of the KM matrix and the supersymmetry breaking soft terms in the squark sector. We suggest a general parametrisation of the soft analytic terms, more suitable for non-universal supergravity couplings. We point out that for flavour dependent soft terms, there are new contributions to the neutron electric dipole moment. Finally, we consider a model with maximal flavour mixing in order to estimate the effects due to the renormalisation scale dependence.

hep-ph↗

arXiv1994

Supersymmetric Models with a Gauge Singlet and Non-Universal Soft Terms from Orbifold String Theory

The particle spectrum of the supersymmetric extension of the standard model with a gauge singlet is studied. Soft supersymmetry breaking terms are explicitly chosen to be non-universal according to orbifold string theory. they depend on modular weights of chiral fields and on an angle $θ$ specifying the supersymmetry breaking sector. Imposing radiative weak symmetry breaking and requiring that standard model Yukawa couplings should be allowed yield constraints on modular weights and almost specifies the angle $θ$. We then perform a numerical anlysis of the low energy spectrum. It turns out that the spectrum is very constrained, revealing salient features such as light Higgses and neutralinos. The latter turns out to be essentially gaugino-like.

hep-ph↗

arXiv1993

Angular Intermittency in QCD Jets

Using two methods, via fluctuations and correlations, an analytical formula is derived for the factorial multiplicity moments in a QCD jet at the Double Leading Logarithm accuracy. The resulting self-similar dependence on the solid-angle cell size is characteristic of an intermittency behaviour in angular variables. The intermittency indices depend on the diffusion angle through the running of $ α_ s. $ Physical features of jet fluctuations such as collimation at large angles and saturation at small angles are well described in the perturbative framework. A parameter-free prediction of angular intermittency is proposed for $ {\cal Z}^0 $ decays into hadrons, assuming hadron-parton duality.

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