Search Results (17 total)

We discuss 2d duality transformations in the classical AdS₅×S⁵ superstring and their effect on the integrable structure. T-duality along four directions in the Poincaré parametrization of AdS₅ maps the bosonic part of the superstring action into itself. On the bosonic level, this duality may be understood as a symmetry of the first-order (phase space) system of equations for the coset components of the current. The associated Lax connection is invariant modulo the action of an so(2,4)-automorphism. We then show that this symmetry extends to the full superstring, provided one supplements the transformation of the bosonic components of the current with a transformation on the fermionic ones. At the level of the action, this symmetry can be seen by combining the bosonic duality transformation with a similar one applied to part of the fermionic superstring coordinates. As a result, the full superstring action is mapped into itself, albeit in a different κ-symmetry gauge. One implication is that the dual model has the same superconformal symmetry group as the original one, and this may be seen as a consequence of the integrability of the superstring. The invariance of the Lax connection under the duality implies a map on the full set of conserved charges that should interchange some of the Noether (local) charges with hidden (nonlocal) ones and vice versa.

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

In this paper we study supersymmetric Wilson loops restricted to an S² submanifold of four-dimensional space in N=4 super Yang-Mills theory. We provide evidence from both perturbation theory and the AdS dual that those loops are equal to the analogous observables in two-dimensional Yang-Mills on S² (excluding nonperturbative contributions). This relates a subsector of N=4 SYM to a low-dimensional soluble model and also suggests that this subsector of N=4 SYM is invariant under area preserving diffeomorphisms.

We study how coupling to supergravity affects the phase structure of a system exhibiting dynamical supersymmetry breaking in a meta-stable vacuum. More precisely, we consider the Seiberg dual of SQCD coupled to supergravity at finite temperature. We show that the gravitational interactions decrease the critical temperature for the second order phase transition in the quark direction, that is also present in the global case. Furthermore, we find that, due to supergravity, a new second order phase transition occurs in the meson direction, whenever there is a nonvanishing constant term in the superpotential. Notably, this phase transition is a necessary condition for the fields to roll, as the system cools down, towards the meta-stable susy breaking vacuum, because of the supergravity-induced shift of the meta-stable minimum away from zero meson vevs. Finally, we comment on the phase structure of the KKLT model with uplifting sector given by the Seiberg dual of SQCD.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.

We present a large new family of Wilson loop operators in N=4 supersymmetric Yang-Mills theory. For an arbitrary curve on the three dimensional sphere one can add certain scalar couplings to the Wilson loop so it preserves at least two supercharges. Some previously known loops, notably the 1/2 BPS circle, belong to this class, but we point out many more special cases which were not known before and could provide further tests of the AdS/CFT correspondence.

The annihilation frequency of the reaction p̅ p→π⁰π⁰ at rest in liquid hydrogen has been measured by the Obelix experiment by using different apparatus configurations and trigger conditions. The value obtained is f(π⁰π⁰, LH)=(2.8±0.1_stat±0.4_syst)×10^-4. With the same data samples, the π⁰η annihilation frequency has been determined to be f(π⁰η, LH)=(0.9±0.2_stat±0.1_syst)×10^-4. The results are discussed within the frame of the present experimental situation.

Data about antiprotons slowing down in gaseous targets at very low energies (E<1 keV) show that the stopping power in D₂ is lower than in H₂; the right way to explain this behavior seems to be through a nuclear stopping power derived from the classical Rutherford formula. © 1996 The American Physical Society.

The systematics of strangeness enhancement is calculated using the hijing and venus models and compared to recent data on pp, pA, and AA collisions at CERN/SPS energies (200A GeV). The hijing model is used to perform a linear extrapolation from pp to AA. venus is used to estimate the effects of final state cascading and possible nonconventional production mechanisms. This comparison shows that the large enhancement of strangeness observed in S+Au collisions, interpreted previously as possible evidence for quark-gluon plasma formation, has its origins in nonequilibrium dynamics of few nucleon systems. A factor of 2 enhancement of Λ⁰ at midrapidity is indicated by recent pS data, where on the average one projectile nucleon interacts with only two target nucleons. There appears to be another factor of 2 enhancement in the light ion reaction SS relative to pS, when on the average only two projectile nucleons interact with two target ones.

The crystal blocking technique has been used to measure nuclear time delays in the ¹⁶O(135 MeV)+^natNi reaction. From the axial blocking patterns the time delays for the production of fragments with Z from 13 to 18 have been extracted. Times of the order of 10^-18 s for even Z and less than the sensitivity of the method for odd Z have been found. Two interpretations of these findings have been discussed. The first one seems to indicate that nucleus-nucleus dynamics has to be included in simple statistical models to understand the time scale of fusion-fission processes. In the second interpretation, an explicit model for dechanneling shows how the blocking technique can be extended to measure time delays in decay chains.

The simultaneous measurement of the spatial coordinates and times of p¯s annihilating at rest in a H₂ target at very low density ρ (ρ/ρ₀<10^-2, ρ₀ being the STP density) gives the possibility of evaluating the behavior of the p¯ stopping power in H₂ at low energies (below 120 keV). It is different from that of protons (the Barkas effect). Moreover, it is shown that a rise at low-energy values (≲1 keV) is needed to agree with experimental data.

Axial and planar blocking patterns in thin Si single crystals have been measured for the products of the reaction ²⁸Si(150 MeV)+²⁸Si at θ_L=15° and 12°. Reaction times of the order 10^-18–10^-17 s have been observed for fusion-evaporation and fusion-fission processes.

Transitional dissipative mechanisms in the fissionlike decay of medium mass composite systems have been studied systematically. Fragments produced in the collision of 4–7 MeV/nucleon, A=19–37 projectiles, with A=59–89 target nuclei, were identified by time-of-flight spectrometers. Mass, energy, and angular distributions of the fissionlike fragments have been reproduced by dynamical model calculations based on the transport theory. Systematics of lifetimes of the intermediate complex systems are given, indicating a clear dependence on the bombarding energy and entrance channel mass asymmetry.

The γ decay of energy relaxed fragments has been studied as a function of the mass asymmetry in the exit channel for the ³²S+ ⁵⁸Ni reaction at 143 MeV. γ-multiplicity and γ-anisotropy data as well as 〈Q〉 values from fragment inclusive measurements show the differences between the deep-inelastic and the ‘‘fission-like’’ regimes. The role of the deformation in the symmetric splitting is outlined for medium mass (A∼100) composite systems.

The mean lives of the three lower levels in ⁴³Ca have been measured with a new version of the Doppler-shift-attenuation method, using a gas as target and absorber. The reaction was ⁴⁰Ar(α,n)⁴³Ca at energies ranging from 5.5 to 8 MeV. The results for the mean lives are the following: τ_m=50±4, 116±5, and 66±4 psec for the 372-, 593-, and 990-keV levels, respectively. In addition, the relative intensity of γ rays from the decay of the ⁴³Ca levels have been obtained at E_α=8 MeV. Electron-conversion measurements have also been carried out for the transitions 372→0 and 593→372 keV, which turn out to be almost pure M1.