125350297
COMPASS results on gluon polarisation
117242696
Measurement of The Gluon Polarisation Through High $p_T$ Hadron Production in COMPASS
One of the missing keys in the present understanding of the spin structure of the nucleon is the contribution from the gluons: the so called gluon polarisation. This quantity can be determined in DIS through the Photon-Gluon Fusion (PGF) process, in which two analysis methods may be used: (i) identifying open charm events or (ii) selecting events with high pT hadrons. The data used in the present work were collected by the COMPASS Experiment, where a naturally polarised muon beam of 160 GeV, impinging on a polarised nucleon fixed target, is used. The results for the gluon polarisation from high pT are presented. The gluon polarisation result for high pT hadrons is divided, for the first time, into three independent xG bins at leading order (LO). A new weighted method based on a neural network approach is used.
121546704
Experimental Program of the Future COMPASS-II Experiment at CERN
Over the last decade the COMPASS experiment has successfully obtained precise results on nucleon structure and hadron spectroscopy, with statistical errors much lower than previously reported. Recently, the new COMPASS-II program, focused on the study of a more complete nucleon description, was approved by CERN. The goal of the program is to access the nucleon structure beyond the collinear approxi-mation, including the quark intrinsic transverse momentum distributions, which are described by the Transverse Momentum Dependent (TMD) Parton Distribution Functions (PDFs) and by Generalised Parton Distributions (GPDs). The COMPASS collaboration proposes to measure the TMD PDFs, for the first time, via the polarised Drell-Yan process. The results will be complementary to those already obtained in polarised Semi-Inclusive Deep Inelastic Scattering (SIDIS). The GPDs can be accessed using deeply virtual Compton scattering and hard exclusive meson production using an unpolarised hydrogen target in a kinematic region not yet covered by any existing experiment. Also unpolarised SIDIS will be studied, by improving the knowledge of the strange quark PDF and obtaining the kaon fragmentation functions. Another topic covered in this COMPASS-II program is the measurement of the pion and kaon polarisabilities using the Primakoff reaction, which corresponds to a test of the chiral perturbative theory.
118204757
Gluon polarisation from high transverse momentum hadron pairs production (COMPASS)
A new preliminary result of a gluon polarisation \Delta G/G obtained selecting high transverse momentum hadron pairs in DIS events with Q^2>1 (GeV/c)^2 is presented. Data has been collected by COMPASS at CERN during the 2002-2004 years. In the extraction of $\Delta G/G$ contributions coming from the leading order $\gamma q$ and QCD processes are taken into account. A new weighting method based on a neural network approach is used. Also a preliminary result of \Delta G/G for events with Q^2<1 (GeV/c)^2 is presented.
119297394
Fast radiative transfer of dust reprocessing in semi-analytic models with artificial neural networks
A serious concern for semi-analytical galaxy formation models, aiming to simulate multiwavelength surveys and to thoroughly explore the model parameter space, is the extremely time-consuming numerical solution of the radiative transfer of stellar radiation through dusty media. To overcome this problem, we have implemented an artificial neural network (ANN) algorithm in the radiative transfer code grasil, in order to significantly speed up the computation of the infrared (IR) spectral energy distribution (SED). The ANN we have implemented is of general use, in that its input neurons are defined as those quantities effectively determining the shape of the IR SED. Therefore, the training of the ANN can be performed with any model and then applied to other models. We made a blind test to check the algorithm, by applying a net trained with a standard chemical evolution model (i.e. che_evo) to a mock catalogue extracted from the semi-analytic model morgana, and compared galaxy counts and evolution of the luminosity functions in several near-IR to sub-millimetre (sub-mm) bands, and also the spectral differences for a large subset of randomly extracted models. The ANN is able to excellently approximate the full computation, but with a gain in CPU time by ∼2 orders of magnitude. It is only advisable that the training covers reasonably well the range of values of the input neurons in the application. Indeed in the sub-mm at high redshift, a tiny fraction of models with some sensible input neurons out of the range of the trained net cause wrong answer by the ANN. These are extreme starbursting models with high optical depths, favourably selected by sub-mm observations, and are difficult to predict a priori.
117873386
Dust and Nebular Emission in Star Forming Galaxies
Star forming galaxies exhibit a variety of physical conditions, from quiescent normal spirals to the most powerful dusty starbursts. In order to study these complex systems, we need a suitable tool able to analyze the information available from observations at all wavelengths. We present a new spectro-photometric model which considers in a consistent way starlight as reprocessed by gas and dust. We discuss preliminary results of its application to a sample of local VLIRGs.
15518099
Faint Lyman-break galaxies as a crucial test for galaxy formation models
It has recently been shown that galaxy formation models within theCDM cosmology predict that, compared to the observed population, small galaxies (with stellar masses < 10 11 M⊙) form too early, are too passive since z � 3 and host too old stellar populations at z = 0. We then expect an overproduction of small galaxies at z & 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the MORGANA galaxy formation model and GRASIL spectro-photometric + radiative transfer code to gener- ate mock catalogues of deep fields observed with HST-ACS. We a dd observational noise and the effect of Lyman-alpha emission, and perform color-color selections to identify Lyman- break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i-dropouts), but uncertainties and degeneracies in dust abs orption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z850 � 27 V -dropouts at z � 5. We quantify the properties of these "excess" galaxies and discuss the implications: these galaxies are hosted in dark matter halos with circular velocities in excess of 100 km s −1 , and their suppression may require a deep re-thinking of stellar feedback processes taking place in galaxy formation.
10824988
Chemical evolution in a model for the joint formation of quasars and spheroids
Direct and indirect pieces of observational evidence point to a strong connection between high-redshift quasars and their host galaxies. In the framework of a model where the shining of the quasar is the episode that stops the formation of the galactic spheroid inside a virialized halo, it has been proven possible to explain the submillimetre source counts together with their related statistics and the local luminosity function of spheroidal galaxies. The time delay between the virialization and the quasar manifestation required to fit the counts is short and incresing with decresing the host galaxy mass. In this paper we compute the detailed chemical evolution of gas and stars inside virialized haloes in the framework of the same model, taking into account the combined effects of cooling and stellar feedback. Under the assumption of negligible angular momentum, we are able to reproduce the main observed chemical properties of local ellipticals. In particular, by using the same duration of the bursts which are required in order to fit the submillimetre source counts, we recover the observed increase of the Mg/Fe ratio with galactic mass. Since for the most massive objects the assumed duration of the burst is Tburst < 0.6 Gyr, we end up with a picture for elliptical galaxy formation in which massive spheroids complete their assembly at early times, thus resembling a monolithic collapse, whereas smaller galaxies are allowed for a more prolonged star formation, thus allowing for a more complicated evolutionary history. In the framework of the adopted scenario, only quasar activity can provide energies large enough to stop the star formation very soon after virialization in the most massive galactic haloes. The chemical abundance of the gas that we estimate at the end of the burst matches well the metallicity inferred from the quasar spectra. Therefore, the assumption that quasar activity interrupts the main episode of star formation in elliptical galaxies turns out to be quite reasonable. In this scenario, we also point out that non-dusty extremely red objects are the best targets for searching for high-redshift Type Ia supernovae.