Search for high-mass dilepton resonances with the ATLAS experiment at sqrt(s) = 7 TeV

We present a search for high-mass l+l- resonances in pp collisions at a centre-of-mass energy of 7 TeV recorded by the ATLAS experiment in 2011. No statistically significant excess above the Standard Model expectation is observed in a dataset corresponding to an integrated luminosity of approximately 1/fb. Consequently, upper limits are set on the cross-section times branching ratio of resonances decaying to muon pairs as a function of the resonance mass. In particular, a Sequential Standard Model Z' is excluded for masses below 1.83 TeV, and a Randall-Sundrum Kaluza-Klein graviton with coupling k/M_Pl = 0.1 is excluded for masses below 1.63 TeV, both at the 95% C.L.Comment: 3 pages, 3 figures, 2 tables, XXXI Physics In Collision Proceeding

Strong MgII systems in quasar and gamma-ray burst spectra

The incidence of strong MgII systems in gamma-ray burst (GRB) spectra is a few times higher than in quasar (QSO) spectra. We investigate several possible explanations for this effect, including: dust obscuration bias, clustering of the absorbers, different beam sizes of the sources, multiband magnification bias of GRBs, association of the absorbers with the GRB event or the circumburst environment. We find that: i) the incidence rate of MgII systems in QSO spectra could be underestimated by a factor 1.3-2 due to dust obscuration; ii) the equivalent-width distribution of the MgII absorbers along GRBs is consistent with that observed along QSOs thus suggesting that the absorbers are more extended than the beam sizes of the sources; iii) on average, GRB afterglows showing more than one MgII system are a factor of 1.7 brighter than the others, suggesting a lensing origin of the observed discrepancy; iv) gravitational lensing (in different forms, from galaxy lensing to microlensing) can bias high the counts of MgII systems along GRBs if the luminosity functions of the prompt gamma-ray emission and of the optical afterglows have a mean faint-end slope approaching -5/3 -- -2; v) some of the absorbers can be associated with the circumburst environment or produced by supernova remnants unrelated to the GRB event itself but lying in the same star-forming region. With the possible exception of magnification bias, it is unlikely that one of these effects on its own can fully account for the observed counts. However, the combined action of some of them can substantially reduce the statistical significance of the discrepancy.Comment: 8 pages, 3 figures, ApJ in press, comments welcom

Constraining SuperWIMPy and Warm Subhalos with Future Submillilensing

We propose to observe QSO-galaxy strong lens systems to give a new constraint on the damping scale of the initial fluctuations. We find that the future observation of submilliarc scale astrometric shifts of the multiple lensed images of QSOs would find \sim 10^{(3-9)} M_{\odot} subhalos inside the macrolens halo. The superweakly interacting massive particles (superWIMPs) produced from a WIMP decay and the warm dark matter (WDM) particles that predict a comoving damping scale larger than \sim 2 kpc can be constrained if \sim 10^3 M_{\odot} subhalos are detected.Comment: 14 pages, 2 figures, revised version accepted for publication in PL

Discreteness effects in simulations of Hot/Warm dark matter

In Hot or Warm Dark Matter universes the density fluctuations at early times contain very little power below a characteristic wavelength related inversely to the particle mass. We study how discreteness noise influences the growth of nonlinear structures smaller than this coherence scale in N-body simulations of cosmic structure formation. It has been known for 20 years that HDM simulations in which the initial uniform particle load is a cubic lattice exhibit artifacts related to this lattice. In particular, the filaments which form in such simulations break up into regularly spaced clumps which reflect the initial grid pattern. We demonstrate that a similar artifact is present even when the initial uniform particle load is not a lattice, but rather a glass with no preferred directions and no long-range coherence. Such regular fragmentation also occurs in simulations of the collapse of idealised, uniform filaments, although not in simulations of the collapse of infinite uniform sheets. In HDM or WDM simulations all self-bound nonlinear structures with masses much smaller than the free streaming mass appear to originate through spurious fragmentation of filaments. These artificial fragments form below a characteristic mass which scales as $m_p^{1/3}k_{peak}^{-2}$, where $m_p$ is the $N$-body particle mass and $k_{peak}$ is the wavenumber at the maximum of $k^3 P(k)$ ($P(k)$ is the power spectrum). This has the unfortunate consequence that the effective mass resolution of such simulations improves only as the cube root of the number of particles employed.Comment: 14 pages, 13 figures, Accepted for publication in MNRA

Constraining DM properties with SPI

Using the high-resolution spectrometer SPI on board the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), we search for a spectral line produced by a dark matter(DM) particle with a mass in the range 40keV < M_DM < 14MeV, decaying in the DM halo of the Milky Way. To distinguish the DM decay line from numerous instrumental lines found in the SPI background spectrum, we study the dependence of the intensity of the line signal on the offset of the SPI pointing from the direction toward the Galactic Centre. After a critical analysis of the uncertainties of the DM density profile in the inner Galaxy, we find that the intensity of the DM decay line should decrease by at least a factor of 3 when the offset from the Galactic Centre increases from 0 to 180 degrees. We find that such a pronounced variation of the line flux across the sky is not observed for any line, detected with a significance higher than 3 sigma in the SPI background spectrum. Possible DM decay origin is not ruled out only for the unidentified spectral lines, having low (~3 sigma) significance or coinciding in position with the instrumental ones. In the energy interval from 20 keV to 7 MeV, we derive restrictions on the DM decay line flux, implied by the (non-)detection of the DM decay line. For a particular DM candidate, the sterile neutrino of mass MDM, we derive a bound on the mixing angle.Comment: Minor changes; v.2 - Final version appeared in MNRA

Constraints on decaying Dark Matter from XMM-Newton observations of M31

We derive constraints on parameters of the radiatively decaying Dark Matter (DM) particles, using XMM-Newton EPIC spectra of the Andromeda galaxy (M31). Using the observations of the outer (5'-13') parts of M31 we improve the existing constraints. For the case of sterile neutrino DM, combining our constraints with the latest computation of abundances of sterile neutrino in the Dodelson-Widrow (DW) scenario, we obtain the lower mass limit m_s < 4 keV, which is stronger than the previous one m_s < 6 kev, obtained recently by Asaka et al. (2007) [hep-ph/0612182]. Comparing this limit with the most recent results on Lyman-alpha forest analysis of Viel et al. (2007) [arXiv:0709.0131] (m_s > 5.6 kev), we argue that the scenario in which all the DM is produced via DW mechanism is ruled out. We discuss however other production mechanisms and note that the sterile neutrino remains a viable candidate of Dark Matter, either warm or cold.Comment: 13 pages, 12 figure

What is the (Dark) Matter with Dwarf Galaxies?

We present cosmological hydrodynamical simulations of the formation of dwarf galaxies in a representative sample of haloes extracted from the Millennium-II Simulation. Our six haloes have a z = 0 mass of ~10^10 solar masses and show different mass assembly histories which are reflected in different star formation histories. We find final stellar masses in the range 5 x 10^7 - 10^8 solar masses, consistent with other published simulations of galaxy formation in similar mass haloes. Our final objects have structures and stellar populations consistent with dwarf elliptical and dwarf irregular galaxies. However, in a Lambda CDM universe, 10^10 solar mass haloes must typically contain galaxies with much lower stellar mass than our simulated objects if they are to match observed galaxy abundances. The dwarf galaxies formed in our own and all other current hydrodynamical simulations are more than an order of magnitude more luminous than expected for haloes of this mass. We discuss the significance and possible implications of this result.Comment: 12 pages, 4 figures, MNRAS accepte

The velocity function of gas-rich galaxies

We measure the distribution function of rotational velocities phi(V_c) of late-type galaxies from the HIPASS galaxy catalogue. Previous measurements of the late-type velocity function are indirect, derived by converting the galaxy luminosity function using the relation between galaxy luminosity and rotation velocity (the Tully-Fisher relation). The advantage of HIPASS is that space densities and velocity widths are both derived from the same survey data. We find good agreement with earlier inferred measurements of phi(V_c), but we are able to define the space density of objects with V_c as low as 30 km/s. The measured velocity function is `flat' (power-law slope alpha ~ -1.0) below V_c = 100 km/s. We compare our results with predictions based on LCDM simulations and find good agreement for rotational velocities in excess of 100 km/s, but at lower velocities current models over-predict the space density of objects. At V_c=30 km/s this discrepancy is approximately a factor 20.Comment: 9 pages, 7 figures. Accepted for publication in MNRA

Precision Cosmology: Successes and Challenges

After briefly reviewing the good agreement between large-scale observations and the predictions of the now-standard CDM theory and problems with the MOND alternative, I summarize several of the main areas of possible disagreement between theory and observation: galaxy centers, dark matter substructure, angular momentum, and the sequence of cosmogony, updating earlier reviews [1]. All of these issues are sufficiently complicated that it is not yet clear how serious they are, but there is at least some reason to think that the problems will be resolved through a deeper understanding of the complicated "gastrophysics" of star formation and feedback from supernovae and AGN.Comment: 5 pages, 1 figure, to appear in proceedings of 7th UCLA Symposium on sources and detection of dark matter and dark energy in the universe, 22-24 Feb 2006, Marina de Rey, Californi

From dwarf spheroidals to cDs: Simulating the galaxy population in a LCDM cosmology

We apply updated semi-analytic galaxy formation models simultaneously to the stored halo/subhalo merger trees of the Millennium and Millennium-II simulations. These differ by a factor of 125 in mass resolution, allowing explicit testing of resolution effects on predicted galaxy properties. We have revised the treatments of the transition between the rapid infall and cooling flow regimes of gas accretion, of the sizes of bulges and of gaseous and stellar disks, of supernova feedback, of the transition between central and satellite status as galaxies fall into larger systems, and of gas and star stripping once they become satellites. Plausible values of efficiency and scaling parameters yield an excellent fit not only to the observed abundance of low-redshift galaxies over 5 orders of magnitude in stellar mass and 9 magnitudes in luminosity, but also to the observed abundance of Milky Way satellites. This suggests that reionisation effects may not be needed to solve the "missing satellite" problem except, perhaps, for the faintest objects. The same model matches the observed large-scale clustering of galaxies as a function of stellar mass and colour. The fit remains excellent down to ~30kpc for massive galaxies. For M* < 6 x 10^10Msun, however, the model overpredicts clustering at scales below 1 Mpc, suggesting that the sigma_8 adopted in the simulations (0.9) is too high. Galaxy distributions within rich clusters agree between the simulations and match those observed, but only if galaxies without dark matter subhalos (so-called orphans) are included. Our model predicts a larger passive fraction among low-mass galaxies than is observed, as well as an overabundance of ~10^10Msun galaxies beyond z~0.6, reflecting deficiencies in the way star-formation rates are modelled.Comment: Accepted for publication in MNRAS. SQL databases containing the full galaxy data at all redshifts and for both the Millennium and Millennium-II simulations are publicly released at http://www.mpa-garching.mpg.de/millenniu