Observation of deconfinement in a cold dense quark medium

In this paper we study the confinement/deconfinement transition in lattice $SU(2)$ QCD at finite quark density and zero temperature. The simulations are performed on an $32^4$ lattice with rooted staggered fermions at a lattice spacing $a = 0.044 \mathrm{~fm}$. This small lattice spacing allowed us to reach very large baryon density (up to quark chemical potential $\mu_q > 2000 \mathrm{~MeV}$) avoiding strong lattice artifacts. In the region $\mu_q\sim 1000 \mathrm{~MeV}$ we observe for the first time the confinement/deconfinement transition which manifests itself in rising of the Polyakov loop and vanishing of the string tension $\sigma$. After the deconfinement is achieved at $\mu_q > 1000 \mathrm{~MeV}$, we observe a monotonous decrease of the spatial string tension $\sigma_s$ which ends up with $\sigma_s$ vanishing at $\mu_q > 2000 \mathrm{~MeV}$. From this observation we draw the conclusion that the confinement/deconfinement transition at finite density and zero temperature is quite different from that at finite temperature and zero density. Our results indicate that in very dense matter the quark-gluon plasma is in essence a weakly interacting gas of quarks and gluons without a magnetic screening mass in the system, sharply different from a quark-gluon plasma at large temperature.Comment: 6 pages, 4 figure

Forming of the Optical Beam with the Rotating Polarization Vector

A method for the optical beam production with the rotating polarization vector based on the interference of two beams with the circular polarizations is proposed. The frequency shift between beams is implemented by means of acousto-optic (AO) diffraction. The method is used for the amplitude light modulation with the frequency nf where f is acoustic frequency and n is integer. AO modulators are fabricated from paratellurite crystal. Modulators allow modulating the optical radiation with wavelength of 0.63 mcm at the quadruple frequency of the acoustic wave. The modulation frequency achieves 180 MHz. Keywords: acousto-optic diffraction, Bragg regime, frequency shift, rotating polarization vector

Bound states of magnons in the S=1/2 quantum spin ladder

We study the excitation spectrum of the two-leg antiferromagnetic S=1/2 Heisenberg ladder. Our approach is based on the description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. We find singlet (S=0) and triplet (S=1) two-particle bound states of the elementary triplets. We argue that bound states generally exist in any dimerized quantum spin model.Comment: 4 REVTeX pages, 4 Postscript figure

Crystal experiments on efficient beam extraction

Silicon crystal was channeling and extracting 70-GeV protons from the U-70 accelerator with efficiency of 85.3+-2.8% as measured for a beam of 10^12 protons directed towards crystals of 2 mm length in spills of 1-2 s duration. The experimental data follow very well the prediction of Monte Carlo simulations. This success is important to devise a more efficient use of the U-70 accelerator in Protvino and provides a crucial support for implementation of crystal-assisted collimation of gold ion beam in RHIC and slow extraction from AGS onto E952, now in preparation at Brookhaven Nat'l Lab. Future applications, spanning in the energy from sub-GeV (medical) to order of 1 GeV (scraping in the SNS, extraction from COSY) to order of 1 TeV and beyond (scraping in the Tevatron, LHC, VLHC), can benefit from these studies.Comment: 12pp. Presented at 19-th Intern. Conference on Atomic Collisions in Solids (ICACS-19: Paris, July 29 - August 3, 2001

Absence of dynamical gap generation in suspended graphene

There is an interesting proposal that the long-range Coulomb interaction in suspended graphene can generate a dynamical gap, which leads to a semimetal-insulator phase transition. We revisit this problem by solving the self-consistent Dyson-Schwinger equations of wave function renormalization and fermion gap. In order to satisfy the Ward identity, a suitable vertex function is introduced. The impacts of singular velocity renormalization and dynamical screening on gap generation are both included in this formalism, and prove to be very important. We obtain a critical interaction strength, $3.2 < \alpha_{c} < 3.3$, which is larger than the physical value $\alpha = 2.16$ for suspended graphene. It therefore turns out that suspended graphene is a semimetal, rather than insulator, at zero temperature.Comment: 14 pages, 5 figures, 1 tabl

Gamma-radiation with E gamma 5 MeV detected from Seyfert galaxy 3C120 and region with 1" = 190 deg and b" = 20 deg

The observation of the Galaxy anticenter region in gamma-rays with E gamma = 5 / 100 MeV was made by gamma-telescope Natalya-1 in a balloon flight. The flight was performed at the ceiling 5.1 + or - 0.1 g/sq cm, magnetic cutoff being 17 GV. The description of the instrument and the analysis of the experiment conditions are given. The tracks of electron-positron pairs generated by gamma-quanta in the convertors were detected by wire spark chambers. The recorded events were classified manually by an operator using a graphic display into three classes: pairs, single and bad events. The arrival angle of gamma-quanta and their energy for selected gamma-ray events (pairs and singles) were determined through multiple scattering of pair components in the convertors. On the basis of the data obtained the celestial maps were made in gamma-rays for E sub gamma 5 MeV and E gamma 20 MeV energy ranges

Progress in crystal extraction and collimation

Recent IHEP Protvino experiments show efficiencies of crystal-assisted slow extraction and collimation of 85.3+-2.8%, at the intensities of the channeled beam on the order of 10^12 proton per spill of 2 s duration. The obtained experimental data well follows the theory predictions. We compare the measurements against theory and outline the theoretical potential for further improvement in the efficiency of the technique. This success is important for the efficient use of IHEP accelerator and for implementation of crystal-assisted collimation at RHIC and slow extraction from AGS onto E952, now in preparation. Future applications, spanning in the energy from order of 1 GeV (scraping in SNS, slow extraction from COSY and medical accelerators) to order of 1 TeV and beyond (scraping in Tevatron, LHC, VLHC), can benefit from these studies.Comment: 7pp. Presented at HEACC 2001 (Tsukuba, March 25-30

Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration

We have studied the low-energy excitation spectrum of a dimerized and frustrated antiferromagnetic Heisenberg chain. We use an analytic approach, based on a description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. Additional singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of the elementary triplets. We have also calculated the contributions of the elementary and collective excitations into the spin structure factor. Our results are in excellent agreement with exact diagonalizations and dimer series expansions data as long as the dimerization parameter $\delta$ is not too small ($\delta>0.1$), i.e. while the elementary triplets can be treated as localized objects.Comment: 18 pages, 13 figure