6,725 research outputs found

### 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

### Cosmological and astrophysical parameters from the SDSS flux power spectrum and hydrodynamical simulations of the Lyman-alpha forest

(abridged) The flux power spectrum of the Lyman-alpha forest in quasar (QSO)
absorption spectra is sensitive to a wide range of cosmological and
astrophysical parameters and instrumental effects. Modelling the flux power
spectrum in this large parameter space to an accuracy comparable to the
statistical uncertainty of large samples of QSO spectra is very challenging. We
use here a coarse grid of hydrodynamical simulations run with GADGET-2 to
obtain a ``best guess'' model around which we calculate a finer grid of flux
power spectra using a Taylor expansion of the flux power spectrum to first
order. We find that the SDSS flux power spectrum alone is able to constrain a
wide range of parameters including the amplitude of the matter power spectrum
sigma_8, the matter density Omega_m, the spectral index of primordial density
fluctuations n, the effective optical depth tau_eff and its evolution. The
thermal history of the Intergalactic Medium (IGM) is, however, poorly
constrained and the SDSS data favour either an unplausibly large temperature or
an unplausibly steep temperature-density relation. By enforcing a thermal
history of the IGM consistent with that inferred from high-resolution QSO
spectra, we find the following values for the best fitting model (assuming a
flat Universe with a cosmological constant and zero neutrino mass): Omega_
m=0.28 \pm 0.03, n=0.95\pm0.04, \sigma_8=0.91\pm0.07 (1\sigma error bars).We
argue that the major uncertainties in this measurement are still systematic
rather than statistical.Comment: 16 pages, 7 figures, 3 tables. Minor changes to match the accepted
version. MNRAS, in pres

### Feshbach resonances and weakly bound molecular states of boson-boson and boson-fermion NaK pairs

We study theoretically magnetically induced Feshbach resonances and
near-threshold bound states in isotopic NaK pairs. Our calculations accurately
reproduce Feshbach spectroscopy data on Na$^{40}$K and explain the origin of
the observed multiplets in the p-wave [Phys. Rev. A 85, 051602(R) (2012)]. We
apply the model to predict scattering and bound state threshold properties of
the boson-boson Na$^{39}$K and Na$^{41}$K systems. We find that the Na$^{39}$K
isotopic pair presents broad magnetic Feshbach resonances and favorable
ground-state features for producing non-reactive polar molecules by two-photon
association. Broad s-wave resonances are also predicted for Na$^{41}$K
collisions.Comment: 39 pages, 15 figures. Phys. Rev. A in pres

### Is the Concentration of Dark Matter Halos at Virialization Universal ?

Several recent studies suggest a correlation between dark matter halo mass
and the shape of the density profile. We re-analyze simulations from Ricotti
(2003) in which such a correlation was proposed. We use a standard analysis of
the halo density profiles and compare the old simulations to new ones performed
with Gadget2, including higher resolution runs. We confirm Ricotti's result
that, at virialization, the central log slopes alpha, at 5%-10% of the virial
radius are correlated with the halo mass and that the halo concentration is a
universal constant. Our results do not contradict the majority of published
papers: when using a split power law to fit the density profiles, due to the
alpha-concentration degeneracy, the fits are consistent with halos having a
universal shape with alpha=1 or 1.5 and concentrations that depend on the mass,
in agreement with results published elsewhere.
Recently, several groups have found no evidence for convergence of the inner
halo profile to a constant power law. The choice of a split power law
parameterization used in this letter is motivated by the need to compare our
results to previous ones and is formally valid because we are not able to
resolve regions where the slope of the fitting function reaches its asymptotic
constant value. Using a non-parameterized technique, we also show that the
density profiles of dwarf galaxies at z ~ 10 have a log slope shallower than
0.5 within 5% of the virial radius.Comment: Accepted for Publication in the Astrophysical Journal Lette

### The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon

Spatial inhomogeneities in the spectral shape of the ultra-violet background
(UVB) at the tail-end of HeII reionisation are thought to be the primary cause
of the large fluctuations observed in the HeII to HI Ly-a forest optical depth
ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will
also influence the ionisation balance of intergalactic metals; we extract
realistic quasar absorption spectra from a large hydrodynamical simulation to
examine their impact on intergalactic SiIV and CIV absorbers. Using a variety
of toy UVB models, we find that while the predicted spatial inhomogeneities in
spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean
free path for photons with frequencies above and below the HeII ionisation edge
means these fluctuations have less effect on the SiIV and CIV ionisation
balance. Furthermore, UVB models which produce the largest fluctuations in
specific intensity at the HeII ionisation edge also have the softest ionising
spectra, and thus result in photo-ionisation rates which are too low to produce
significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find
spatial variations in the IGM metallicity will dominate any scatter in
tau_SiIV/tau_CIV. Our results suggest that observational evidence for
homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the
possibility of significant fluctuations in the UVB spectral shape at z~2-3. On
the other hand, the scatter in metallicity inferred from observations of
intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform
ionisation corrections is likely intrinsic, and therefore provides a valuable
constraint on intergalactic metal enrichment scenarios at these redshifts.Comment: 13 pages, 7 figures, accepted to MNRA

### Minimally Parametric Power Spectrum Reconstruction from the Lyman-alpha Forest

Current results from the Lyman alpha forest assume that the primordial power
spectrum of density perturbations follows a simple power law form, with
running. We present the first analysis of Lyman alpha data to study the effect
of relaxing this strong assumption on primordial and astrophysical constraints.
We perform a large suite of numerical simulations, using them to calibrate a
minimally parametric framework for describing the power spectrum. Combined with
cross-validation, a statistical technique which prevents over-fitting of the
data, this framework allows us to reconstruct the power spectrum shape without
strong prior assumptions. We find no evidence for deviation from
scale-invariance; our analysis also shows that current Lyman alpha data do not
have sufficient statistical power to robustly probe the shape of the power
spectrum at these scales. In contrast, the ongoing Baryon Oscillation Sky
Survey (BOSS) will be able to do so with high precision. Furthermore, this
near-future data will be able to break degeneracies between the power spectrum
shape and astrophysical parameters.Comment: 11 pages plus appendices, 8 figures. v2: matches version published in
MNRAS. Some clarifications to discussion and exposition, updated reference

### The Statistics of Cosmological Lyman-alpha Absorption

We study the effect of the non-Gaussianity induced by gravitational evolution
upon the statistical properties of absorption in quasar (QSO) spectra. Using
the generic hierarchical ansatz and the lognormal approximation we derive the
analytical expressions for the one-point PDF as well as for the joint two-point
probability distribution (2PDF) of transmitted fluxes in two neighbouring QSOs.
These flux PDFs are constructed in 3D as well as in projection (i.e. in 2D).
The PDFs are constructed by relating the lower-order moments, i.e. cumulants
and cumulant correlators, of the fluxes to the 3D neutral hydrogen distribution
which is, in turn, expressed as a function of the underlying dark matter
distribution. The lower-order moments are next modelled using a generating
function formalism in the context of a {\em minimal tree-model} for the
higher-order correlation hierarchy. These different approximations give nearly
identical results for the range of redshifts probed, and we also find a very
good agreement between our predictions and outputs of hydrodynamical
simulations. The formalism developed here for the joint statistics of
flux-decrements concerning two lines of sight can be extended to multiple lines
of sight, which could be particularly important for the 3D reconstruction of
the cosmic web from QSO spectra (e.g. in the BOSS survey). These statistics
probe the underlying projected neutral hydrogen field and are thus linked to
"hot-spots" of absorption. The results for the PDF and the bias presented here
use the same functional forms of scaling functions that have previously been
employed for the modelling of other cosmological observation such as the
Sunyaev-Zel'dovich effect.Comment: 17 pages, 8 figure

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