118594965
The Missing Massive Satellites of the Milky Way
Recent studies suggest that only three of the 12 brightest satellites of the Milky Way (MW) inhabit dark matter haloes with maximum circular velocity, Vmax, exceeding ∼30 km s−1. This is in apparent contradiction with the Λ cold dark matter (CDM) simulations of the Aquarius Project, which suggest that MW-sized haloes should have at least eight subhaloes with Vmax > 30 km s−1. The absence of luminous satellites in such massive subhaloes is thus puzzling and may present a challenge to the ΛCDM paradigm. We note, however, that the number of massive subhaloes depends sensitively on the (poorly known) virial mass of the MW, and that their scarcity makes estimates of their abundance from a small simulation set like Aquarius uncertain. We use the Millennium Simulation series and the invariance of the scaled subhalo velocity function (i.e. the number of subhaloes as a function of ν, the ratio of the subhalo Vmax to the host halo virial velocity, V200) to secure improved estimates of the abundance of rare massive subsystems. In the range 0.1 ν) is approximately Poisson distributed about an average given by 〈Nsub〉 = 10.2 (ν/0.15)−3.11. This is slightly lower than that in Aquarius haloes, but consistent with recent results from the Phoenix Project. The probability that a ΛCDM halo has three or fewer subhaloes with Vmax above some threshold value, Vth, is then straightforward to compute. It decreases steeply both with decreasing Vth and with increasing halo mass. For Vth = 30 km s−1, ∼40 per cent of Mhalo = 1012 M⊙ haloes pass the test; fewer than ∼5 per cent do so for Mhalo ≳ 2 × 1012 M⊙ and the probability effectively vanishes for Mhalo ≳ 3 × 1012 M⊙. Rather than a failure of ΛCDM, the absence of massive subhaloes might simply indicate that the MW is less massive than is commonly thought.
119239648
Substructure lensing: Effects of galaxies, globular clusters and satellite streams
Lensing flux-ratio anomalies have been frequently observed and taken as evidence for the presence of abundant dark matter substructures in lensing galaxies, as predicted by the cold dark matter (CDM) model of cosmogony. In previous work, we examined the cusp-caustic relations of the multiple images of background quasars lensed by galaxy-scale dark matter haloes, using a suite of high-resolution N-body simulations (the Aquarius simulations). In this work, we extend our previous calculations to incorporate both the baryonic and diffuse dark components in lensing haloes. We include in each lensing simulation: (1) a satellite galaxy population derived from a semi-analytic model applied to the Aquarius haloes; (2) an empirical Milky Way globular cluster population and (3) satellite streams (diffuse dark component) identified in the simulations. Accounting for these extra components, we confirm our earlier conclusion that the abundance of intrinsic substructures (dark or bright, bound or diffuse) is not sufficient to explain the observed frequency of cusp-caustic violations in the Cosmic Lens All-Sky Survey (CLASS). We conclude that the observed effect could be the result of the small number statistics of CLASS, or intergalactic haloes along the line of sight acting as additional sources of lensing flux anomalies. Another possibility is that this discrepancy signals a failure of the CDM model.
17078771
Phase‐space structure in the local dark matter distribution and its signature in direct detection experiments
We study predictions for dark matter phase-space structure near the Sun based on highresolution simulations of six galaxy halos taken from the Aquarius Project. The local DM density distribution is predicted to be remarkably smooth; the density at the Sun differs from the mean over a best-fit ellipsoidal equidensity contour by less than 15% at the 99.9% confidence level. The local velocity distribution is also very smooth, but it differs systematically from a (multivariate) Gaussian distribution. This is not due to th e presence of individual clumps or streams, but to broad features in the velocity modulus and energy distributions that are stable both in space and time and reflect the detailed assembly histo ry of each halo. These features have a significant impact on the signals predicted for WIMP an d axion searches. For example, WIMP recoil rates can deviate by � 10% from those expected from the best-fit multivariate Gaussian models. The axion spectra in our simulations typically peak at lower frequencies than in the case of multivariate Gaussian velocity distribu tions. Also in this case, the spectra show significant imprints of the formation of the halo. Th is implies that once direct DM detection has become routine, features in the detector sign al will allow us to study the dark matter assembly history of the Milky Way. A new field, “dark ma tter astronomy”, will then emerge.
17433293
Are mergers responsible for universal halo properties
N-body simulations of cold dark matter (CDM) have shown that, in this hierarchical structure formation model, dark matter halo properties, such as the density profile, the phase-space density profile, the distribution of axial ratio, the distribution of spin parameter and the distribution of internal specific angular momentum, follow 'universal' laws or distributions. Here, we study the properties of the first generation of haloes in a hot dark matter (HDM) dominated universe, as an example of halo formation through monolithic collapse. We find all these universalities to be present in this case also. Halo density profiles are very well fit by the Navarro, Frenk & White profile over two orders of magnitude in mass. The concentration parameter depends on mass as c α M 0.2 , reversing the dependence found in a hierarchical CDM universe. However, the concentration-formation time relation is similar in the two cases: earlier forming haloes tend to be more concentrated than their later forming counterparts. Halo formation histories are also characterized by two phases in the HDM case: an early phase of rapid accretion followed by slower growth. Furthermore, there is no significant difference between the HDM and CDM cases concerning the statistics of other halo properties: the phase-space density profile; the velocity anisotropy profile; the distribution of shape parameters; the distribution of spin parameter and the distribution of internal specific angular momentum are all similar in the two cases. Only substructure content differs dramatically. These results indicate that mergers do not play a pivotal role in establishing the universalities, thus contradicting models which explain them as consequences of mergers.
124150385
Variational Treatment with the Third Order Corrections for SU(2) Lattice Gauge Theory
The variational method in Lagrangian formalism in lattice gauge theory in cumulant expansion is discussed. For SU(2) pure gauge Wi1son model to the third order corrections we show that there is no phase transition in four dimensions, but there exists the first order phase transition in five dimensions at βC=0.838 in agreement with the Monte Carlo results. The physical meaning of higher order corrections is discussed.
119516023
Lattice SU(2) gauge-Higgs system in analytic treatment
The phase diagrams of SU(2) gauge-Higgs model with Higgs field of arbitrary length in the fundamental representation in four and five dimension are studied analytically. A method for determining phase transition lines is proposed. All qualitative features of the phase diagram obtained by Monte Carlo simulations are obtained analytically, and the order of phase transitions can be determined definitely.
118274966
Search for electroweak production of charginos and neutralinos using leptonic final states in pp collisions at $ \sqrt{s}=7 $ TeV
A bstractThe 2011 dataset of the CMS experiment, consisting of an integrated luminosity of 4.98 fb−1 of pp collisions at $ \sqrt{s}=7 $ TeV, enables expanded searches for direct electroweak pair production of charginos and neutralinos in supersymmetric models as well as their analogs in other models of new physics. Searches sensitive to such processes, with decays to final states that contain two or more leptons, are presented. Final states with three leptons, with a same-sign lepton pair, and with an opposite-sign lepton pair in conjunction with two jets, are examined. No excesses above the standard model expectations are observed. The results are used in conjunction with previous results on four-lepton final states to exclude a range of chargino and neutralino masses from approximately 200 to 500 GeV in the context of models that assume large branching fractions of charginos and neutralinos to leptons and vector bosons.
14096992
Constraints on the star formation histories of galaxies from z∼ 1 to 0
We present a new method to estimate the average star formation rate per unit stellar mass (SSFR) of a stacked population of galaxies. We combine the spectra of 600-1000 galaxies with similar stellar masses and parametrize the star formation history of this stacked population using a set of exponentially declining functions. The strength of the hydrogen Balmer absorption-line series in the rest-frame wavelength range 3750-4150 A is used to constrain the SSFR by comparing with a library of models generated using the BC03 stellar population code. Our method, based on a principal component analysis, can be applied in a consistent way to spectra drawn from local galaxy surveys and from surveys at z ∼ 1, and is only weakly influenced by attenuation due to dust. We apply our method to galaxy samples drawn from Sloan Digital Sky Survey and DEEP2 to study the mass-dependent growth of galaxies from z ∼ 1 to 0. We find that (i) high-mass galaxies have lower SSFRs than low-mass galaxies and (ii) the average SSFR has decreased from z = 1 to 0 by a factor of ∼3-4, independent of galaxy mass. Additionally, at z ∼ 1, our average SSFRs are a factor of 2-2.5 lower than those derived from multiwavelength photometry using similar data sets. We then compute the average time [in units of the Hubble time, t H (z)] needed by galaxies of a given mass to form their stars at their current rate. At both z = 0 and 1, this time-scale decreases strongly with stellar mass from values close to unity for galaxies with masses ∼10 10 M ⊙ , to more than 10 for galaxies more massive than 10 11 M ⊙ . Our results are in good agreement with models in which active galactic nuclei feedback is more efficient at preventing gas from cooling and forming stars in high-mass galaxies.
18867535
Clustering of Lyman alpha emitters at z ~ 4.5
We present the clustering properties of 151 Lyα-emitting galaxies at z ≈ 4.5 selected from the Large Area Lyman Alpha (LALA) survey. Our catalog covers an area of 36' × 36' observed with five narrowband filters. We assume that the angular correlation function w(θ) is well represented by a power law Awθ-β with slope β = 0.8, and we find Aw = 6.73 ± 1.80. We then calculate the correlation length r0 of the real-space two-point correlation function ξ(r) = (r/r0)-1.8 from Aw through the Limber transformation, assuming a flat, Λ-dominated universe. Neglecting contamination, we find r0 = 3.20 ± 0.42 h-1 Mpc. Taking into account a possible 28% contamination by randomly distributed sources, we find r0 = 4.61 ± 0.6 h-1 Mpc. We compare these results with the expectations for the clustering of dark matter halos at this redshift in a cold dark matter model, and find that the measured clustering strength can be reproduced if these objects reside in halos with a minimum mass of (1-2) × 1011 h-1 M☉. Our estimated correlation length implies a bias of b ~ 3.7, similar to that of Lyman break galaxies (LBGs) at z ~ 3.8-4.9. However, Lyα emitters are a factor of ~2-16 rarer than LBGs with a similar bias value and implied host halo mass. Therefore, one plausible scenario seems to be that Lyα emitters occupy host halos of roughly the same mass as LBGs, but shine with a relatively low duty cycle of 6%-50%.
118594336
Classical stability of the Galileon
A bstractWe consider the classical equations of motion for a single Galileon field with generic parameters in the presence of non-relativistic sources. We introduce the concept of absolute stability of a theory: if one can show that a field at a single point — like infinity for instance — in spacetime is stable, then stability of the field over the rest of spacetime is guaranteed for any positive energy source configuration. The Dvali-Gabadadze-Porrati (DGP) model is stable in this manner, and previous studies of spherically symmetric solutions suggest that certain classes of the single field Galileon (of which the DGP model is a subclass) may have this property as well. We find, however, that when general solutions are considered this is not the case. In fact, when considering generic solutions there are no choices of free parameters in the Galileon theory that will lead to absolute stability except the DGP choice. Our analysis indicates that the DGP model is an exceptional choice among the large class of possible single field Galileon theories. This implies that if general solutions (non-spherically symmetric) exist they may be unstable. Given astrophysical motivation for the Galileon, further investigation into these unstable solutions may prove fruitful.
118456421
Searching for the signal of dark matter and photon associated production at the LHC beyond leading order
We study the signal of dark matter and photon associated production induced by the vector and axial-vector operators at the LHC, including the QCD next-to-leading order (NLO) effects. We find that the QCD NLO corrections reduce the dependence of the total cross sections on the factorization and renormalization scales, and the $K$ factors increase with the increasing of the dark matter mass, which can be as large as about 1.3 for both the vector and axial-vector operators. Using our QCD NLO results, we improve the constraints on the new physics scale from the results of the recent CMS experiment. Moreover, we show the Monte Carlo simulation results for detecting the $\gamma+\Slash{E}_{T}$ signal at the QCD NLO level, and present the integrated luminosity needed for a $5\sigma$ discovery at the 14 TeV LHC . If the signal is not observed, the lower limit on the new physics scale can be set.