Short time growth of a KPZ interface with flat initial conditions

The short time behavior of the 1+1 dimensional KPZ growth equation with a flat initial condition is obtained from the exact expressions of the moments of the partition function of a directed polymer with one endpoint free and the other fixed. From these expressions, the short time expansions of the lowest cumulants of the KPZ height field are exactly derived. The results for these two classes of cumulants are checked in high precision lattice numerical simulations. The short time limit considered here is relevant for the study of the interface growth in the large diffusivity/weak noise limit, and describes the universal crossover between the Edwards-Wilkinson and KPZ universality classes for an initially flat interface.Comment: 9 pages, 7 figure

Imaging fractional incompressible stripes in integer quantum Hall systems

Transport experiments provide conflicting evidence on the possible existence of fractional order within integer quantum Hall systems. In fact integer edge states sometimes behave as monolithic objects with no inner structure, while other experiments clearly highlight the role of fractional substructures. Recently developed low-temperature scanning probe techniques offer today an opportunity for a deeper-than-ever investigation of spatial features of such edge systems. Here we use scanning gate microscopy and demonstrate that fractional features were unambiguously observed in every integer quantum Hall constriction studied. We present also an experimental estimate of the width of the fractional incompressible stripes corresponding to filling factors 1/3, 2/5, 3/5, and 2/3. Our results compare well with predictions of the edge-reconstruction theory

Unificaxion

Dark matter, gauge coupling unification, and the strong CP problem find a common and simple solution (in the absence of naturalness) within axion models. We show that such solution, even without specifying the details of the model implementation, makes testable predictions for the experimentally measurable axion parameters: the axion mass and its coupling to photons.Comment: 16 pages, 5 figure

X-ray Polarimetry: a new window on the high energy sky

Polarimetry is widely considered a powerful observational technique in X-ray astronomy, useful to enhance our understanding of the emission mechanisms, geometry and magnetic field arrangement of many compact objects. However, the lack of suitable sensitive instrumentation in the X-ray energy band has been the limiting factor for its development in the last three decades. Up to now, polarization measurements have been made exclusively with Bragg diffraction at 45 degrees or Compton scattering at 90 degrees and the only unambiguous detection of X-ray polarization has been obtained for one of the brightest object in the X-ray sky, the Crab Nebula. Only recently, with the development of a new class of high sensitivity imaging detectors, the possibility to exploit the photoemission process to measure the photon polarization has become a reality. We will report on the performance of an imaging X-ray polarimeter based on photoelectric effect. The device derives the polarization information from the track of the photoelectrons imaged by a finely subdivided Gas Pixel Detector. It has a great sensitivity even with telescopes of modest area and can perform simultaneously good imaging, moderate spectroscopy and high rate timing. Being truly 2D it is non-dispersive and does not require any rotation. This device is included in the scientific payload of many proposals of satellite mission which have the potential to unveil polarimetry also in X-rays in a few years.Comment: Accepted for publication by NIMA. Proceeding of the 1st International Conference on "Frontiers in Diagnostic Technologies", November 25-27 2009, Frascati (Italy). 11 pages, 4 figures, 1 table

Photometric determination of the mass accretion rates of pre-main sequence stars. II. NGC346 in the Small Magellanic Cloud

[Abridged] We have studied the properties of the stellar populations in the field of the NGC346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and combines broad-band V and I photometry with narrow-band Halpha imaging to identify all stars with excess Halpha emission and derive the accretion luminosity Lacc and mass accretion rate Macc for all of them. The application of this method to existing HST/ACS photometry of the NGC346 field has allowed us to identify and study 680 bona-fide PMS stars with masses from ~0.4 to ~4 Msolar and ages in the range from ~1 to ~30 Myr. This is the first study to reveal that, besides a young population of PMS stars (~ 1 Myr old), in this field there is also an older population of PMS objects with a median age of ~20 Myr. We provide for all of them accurate physical parameters. We study the evolution of the mass accretion rate as a function of stellar parameters and find that logMacc ~ -0.6 Log t + Log m + c, where t is the age of the star, m its mass and c a quantity that is higher at lower metallicity. The high mass accretion rates that we find suggest that a considerable fraction of the stellar mass is accreted during the PMS phase and that PMS evolutionary models that do not account for this effect will systematically underestimate the true age when compared with the observations.Comment: Accepted for publication in the ApJ. 14 pages, 11 figures. Corrected typos and reference

Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV

The performance of muon reconstruction, identification, and triggering in CMS has been studied using 40 inverse picobarns of data collected in pp collisions at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection criteria covering a wide range of physics analysis needs have been examined. For all considered selections, the efficiency to reconstruct and identify a muon with a transverse momentum pT larger than a few GeV is above 95% over the whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4, while the probability to misidentify a hadron as a muon is well below 1%. The efficiency to trigger on single muons with pT above a few GeV is higher than 90% over the full eta range, and typically substantially better. The overall momentum scale is measured to a precision of 0.2% with muons from Z decays. The transverse momentum resolution varies from 1% to 6% depending on pseudorapidity for muons with pT below 100 GeV and, using cosmic rays, it is shown to be better than 10% in the central region up to pT = 1 TeV. Observed distributions of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO

Which solar neutrino data favour the LMA solution?

Assuming neutrino oscillations, global analyses of solar data find that the LOW solution is significantly disfavoured, leaving LMA as the best solution. But the preference for LMA rests on three weak hints: the spectrum of earth matter effects (Super-Kamiokande sees an overall day/night asymmetry only at 1 sigma), the Cl rate (but LMA and LOW predictions are both above the measured value), the Ga rate (newer data decrease towards the LOW predictions both in GNO and SAGE). Only new data will tell us if LMA is the true solution.Comment: 4 pages, 2 figure