Direct observation by resonant tunneling of the B^+ level in a delta-doped silicon barrier

We observe a resonance in the conductance of silicon tunneling devices with a delta-doped barrier. The position of the resonance indicates that it arises from tunneling through the B^+ state of the boron atoms of the delta-layer. Since the emitter Fermi level in our devices is a field-independent reference energy, we are able to directly observe the diamagnetic shift of the B^+ level. This is contrary to the situation in magneto-optical spectroscopy, where the shift is absorbed in the measured ionization energy.Comment: submitted to PR

Energy funneling in a bent chain of Morse oscillators with long-range coupling

A bent chain of coupled Morse oscillators with long-range dispersive interaction is considered. Moving localized excitations may be trapped in the bending region. Thus chain geometry acts like an impurity. An energy funneling effect is observed in the case of random initial conditions.Comment: 6 pages, 12 figures. Submitted to Physical Review E, Oct. 13, 200

The New Physics at RHIC. From Transparency to High pt_t Suppression

Heavy ion collisions at RHIC energies (Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV) exhibit significant new features as compared to earlier experiments at lower energies. The reaction is characterized by a high degree of transparency of the collisions partners leading to the formation of a baryon-poor central region. In this zone, particle production occurs mainly from the stretching of the color field. The initial energy density is well above the one considered necessary for the formation of the Quark Gluon Plasma, QGP. The production of charged particles of various masses is consistent with chemical and thermal equilibrium. Recently, a suppression of the high transverse momentum component of hadron spectra has been observed in central Au+Au collisions. This can be explained by the energy loss experienced by leading partons in a medium with a high density of unscreened color charges. In contrast, such high $p_t$ jets are not suppressed in d+Au collisions suggesting that the high $p_t$ suppression is not due to initial state effects in the ultrarelativistic colliding nuclei.Comment: 15 pages, 11 figures. to appear in Nucl. Physics A. Invited talk at 'Nucleus-Nucleus Collisions 2003' conference, Mosco

Centrality dependence of charged-particle pseudorapidity distributions from d+Au collisions at sqrt(s_{NN})=200 GeV

Charged-particle pseudorapidity densities are presented for the d+Au reaction at sqrt{s_{NN}}=200 GeV with -4.2 <= eta <= 4.2$. The results, from the BRAHMS experiment at RHIC, are shown for minimum-bias events and 0-30%, 30-60%, and 60-80% centrality classes. Models incorporating both soft physics and hard, perturbative QCD-based scattering physics agree well with the experimental results. The data do not support predictions based on strong-coupling, semi-classical QCD. In the deuteron-fragmentation region the central 200 GeV data show behavior similar to full-overlap d+Au results at sqrt{s_{NN}}=19.4 GeV.Comment: 4 pages, 3figures; expanded discussion of uncertainties; added 60-80% centrality range; added additional discussion on centrality selection bia

Scanning the phases of QCD with BRAHMS

BRAHMS has the ability to study relativistic heavy ion collisions from the final freeze-out of hadrons all the way back to the initial wave-function of the gold nuclei. This is accomplished by studying hadrons with a very wide range of momenta and angles. In doing so we can scan various phases of QCD, from a hadron gas, to a quark gluon plasma and perhaps to a color glass condensate.Comment: 8 pages, 6 figures, proceedings of plenary talk at Quark Matter 2004 conferenc

D-Brane Propagation in Two-Dimensional Black Hole Geometries

We study propagation of D0-brane in two-dimensional Lorentzian black hole backgrounds by the method of boundary conformal field theory of SL(2,R)/U(1) supercoset at level k. Typically, such backgrounds arise as near-horizon geometries of k coincident non-extremal NS5-branes, where 1/k measures curvature of the backgrounds in string unit and hence size of string worldsheet effects. At classical level, string worldsheet effects are suppressed and D0-brane propagation in the Lorentzian black hole geometry is simply given by the Wick rotation of D1-brane contour in the Euclidean black hole geometry. Taking account of string worldsheet effects, boundary state of the Lorentzian D0-brane is formally constructible via Wick rotation from that of the Euclidean D1-brane. However, the construction is subject to ambiguities in boundary conditions. We propose exact boundary states describing the D0-brane, and clarify physical interpretations of various boundary states constructed from different boundary conditions. As it falls into the black hole, the D0-brane radiates off to the horizon and to the infinity. From the boundary states constructed, we compute physical observables of such radiative process. We find that part of the radiation to infinity is in effective thermal distribution at the Hawking temperature. We also find that part of the radiation to horizon is in the Hagedorn distribution, dominated by massive, highly non-relativistic closed string states, much like the tachyon matter. Remarkably, such distribution emerges only after string worldsheet effects are taken exactly into account. From these results, we observe that nature of the radiation distribution changes dramatically across the conifold geometry k=1 (k=3 for the bosonic case), exposing the `string - black hole transition' therein.Comment: 51 pages, 5 figures, v2: referece added, note added replying the comment made in hep-th/060206