Spectra and nuclear modification factor of charged hadrons produced in lead-lead collisions at sqrt(s_NN)=2.76TeV with the ATLAS detector at the LHC

The measurement of charged particle spectra in heavy ion collisions is a way to study properties of hot and dense matter created in these interactions. The centrality dependence of the spectral shape is an important tool to understand the energy loss mechanism. The ATLAS detector at the LHC accumulated integrated luminosity equal to 0.15nb^{-1} of lead-lead data at 2.76TeV per nucleon-nucleon pair. Due to the excellent capabilities of the ATLAS detector, and its stable operation in 2010 and 2011 heavy ion physics runs, these data allow measurements of the charged particle spectra and their ratios in different centrality bins over a wide range of transverse momenta (0.5-150GeV) and pseudorapidity (|eta|<2.5). The measured ratio central to peripheral events shows a suppresion by a factor of 5 at pT=7GeV. At higher pT the ratio increases

Measurement of the nuclear modification factor for high-pTp_{\mathrm{T}} charged hadrons in pp+Pb collisions with the ATLAS detector

The charged hadron spectra in $p$+Pb and $pp$ collisions at $\sqrt{s_{NN}}=\sqrt{s}=5.02$TeV are measured with the ATLAS experiment at the LHC. The measurements are performed with $p$+Pb data recorded in 2013 with an integrated luminosity of 25nb${}^{-1}$ and $pp$ data recorded in 2015 with an integrated luminosity of 28pb${}^{-1}$. The $p$+Pb results are compared to $pp$ spectra, presented as a ratio of transverse momentum distributions in the two systems scaled by the number of binary nucleon-nucleon collisions, the nuclear modification factor $R_{p \mathrm{Pb}}$. The study of $R_\mathrm{pPb}$ allows a detailed comparison of the collision systems in different centrality intervals and in a wide range of transverse momentum. It is shown that the nuclear modification factor does not have any significant deviation from unity in the high transverse momentum region.Comment: 4 pages, 4 figures, presented at Hard Probes 201

Effect of radiation-like solid on CMB anisotropies

We compute the power in the lowest multipoles of CMB anisotropies in the presence of radiation-like solid, a hypothetical new kind of radiation with nonzero shear modulus. If only the ordinary Sachs-Wolfe effect is taken into account, the shear modulus to energy density ratio must be in absolute value of order $10^{-5}$ or less for the theory to be consistent with observations within cosmic variance. With the integrated Sachs-Wolfe effect switched on, the constraint is relaxed almost by two orders of magnitude.Comment: 24 pages, 3 figure

Euclidean action for vacuum decay in a de Sitter universe

The behavior of the action of the instantons describing vacuum decay in a de Sitter is investigated. For a near-to-limit instanton (a Coleman-de Luccia instanton close to some Hawking-Moss instanton) we find approximate formulas for the Euclidean action by expanding the scalar field and the metric of the instanton in the powers of the scalar field amplitude. The order of the magnitude of the correction to the Hawking-Moss action depends on the order of the instanton (the number of crossings of the barrier by the scalar field): for instantons of odd and even orders the correction is of the fourth and third order in the scalar field amplitude, respectively. If a near-to-limit instanton of the first order exists in a potential with the curvature at the top of the barrier greater than 4 $\times$ (Hubble constant)$^2$, which is the case if the fourth derivative of the potential at the top of the barrier is greater than some negative limit value, the action of the instanton is less than the Hawking-Moss action and, consequently, the instanton determines the outcome of the vacuum decay if no other Coleman-de Luccia instanton is admitted by the potential. A numerical study shows that for the quartic potential the physical mode of the vacuum decay is given by the Coleman-de Luccia instanton of the first order also in the region of parameters in which the potential admits two instantons of the second order.Comment: 16 pages, 3 figures, references adde