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New results are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 52 kg of C_{3}F_{8} located in the SNOLAB underground laboratory. As in previous PICO bubble chambers, PICO-60 C_{3}F_{8} exhibits excellent electron recoil and alpha decay rejection, and the observed multiple-scattering neutron rate indicates a single-scatter neutron background of less than one event per month. A blind analysis of an efficiency-corrected 1167-kg day exposure at a 3.3-keV thermodynamic threshold reveals no single-scattering nuclear recoil candidates, consistent with the predicted background. These results set the most stringent direct-detection constraint to date on the weakly interacting massive particle (WIMP)-proton spin-dependent cross section at 3.4×10^{-41}  cm^{2} for a 30-GeV c^{-2} WIMP, more than 1 order of magnitude improvement from previous PICO results.

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The ultrahigh-energy (UHE) proton and neutrino spectra resulting from collapse or annihilations of topological defects surviving from the GUT era are calculated. Irrespective of the specific process under consideration (which determines the overall normalization of the spectrum), the UHE proton spectrum always 'recovers' at ~ 1.8 x 10 11 GeV after a partial Greisen-Zatsepin-Kuz'min 'cutoff' at ~ 5 x 10 10 GeV and continues to a GUT-scale energy with a universal shape determined by the physics of hadronic jet fragmentation. Implications of our results are discussed. Topological defects [l], e.g., monopoles, cosmic strings, domain walls, superconducting cosmic strings, etc., and various possible hybrid objects made of these, are likely to have i been formed in symmetry-breaking phase transitions in the early universe. Because of their topological stability the defects can survive indefinitely, until and unless physically destroyed due to collapse, annihilations or other reasons[2-5). When topological defects are destroyed the energy trapped in them is released in the form of massive quanta (hereafter generically referred to as X particles) of the various fields (gauge bosons, biggs bosons, superheavy fermions) that 'constitute' the defects. The X-particles can then decay into known quarks, gluons, leptons, etc, which ultimately materialize into, among other particles, nucleons, gamma rays and neutrinos with energies up to ~ m x , the mass of the X-particles released from the defects. If the defects were originally formed in a phase transition at the GUT-scale ~ 10 16 GeV, then we have here a possible natural mechanism of production of ultrahigh-energy (UHE) cosmic ray (CR) particles up to an energy of this order, without any acceleration mechanism. Below we give a general calculation of the expected evolved proton as well as neutrino spectra resulting from this kind of processes.We assume a fio = 1 "flat" universe and a Hubble constant HQ = 100.h Km.s~1.Mpc~l, with h = 0.75 throughout.The rate of release of X-particles due to destruction of topological defects can, in general, be effectively expressed in terms of two fundamental parameters entering in the problem, namely, the mass scale m x and the Hubble time ~ t, in the form (with ft = c = 'XV^t j( 1) where K and p are dimensionless constants whose values depend on the specific process involving specific kind of topological defect, (the 'amplitude' K may, in general, depend on p), and n x (t,-) denotes the number density of the X-particles released at the time t{. For example, p = 1 for a system of collapsing cosmic string loops [4,5] as well as for a system of collapsing monopolonia [2,7], while p = 0 for a system of saturated superconducting cosmic 2 string loops [3]. We shall see below that the observed UHE CR flux gives an upper limit to the possible value of « for any given value of p.We assume that each X decays into a quark and a lepton each carrying an energy m x J2.The quarks fragment and produce jets of hadrons. We use the following hadronic fr...

The rotation curve (RC) of our Galaxy, the Milky Way, is constructed starting from its very inner regions (few hundred pc) out to a large galactocentric distance of ∼ 200 kpc using kinematical data on a variety of tracer objects moving in the gravitational potential of the Galaxy, without assuming any theoretical models of the visible and dark matter components of the Galaxy. We study the effect on the RC due to the uncertainties in the values of the Galactic Constants (GCs) R 0 and V 0 (these being the sun's distance from and circular rotation speed around the Galactic center, respectively) and the velocity anisotropy parameter β of the halo tracer objects used for deriving the RC at large galactocentric distances. The resulting RC in the disk region is found to depend significantly on the choice of the GCs, while the dominant uncertainty in the RC at large distances beyond the stellar disk comes from the uncertainty in the value of β. In general we find that the mean RC steadily declines at distances beyond ∼ 60 kpc, independently of the value of β. Also, at a given radius, the circular speed is lower for larger values of β (i.e., for more radially biased velocity anisotropy). Considering that the largest possible value of β is unity, which corresponds to stellar orbits being purely radial, our results for the case of β = 1 give a lower limit to the total mass of the Galaxy within ∼ 200 kpc, M (200 kpc) > ∼ (6.8 ± 4.1) × 10 11 M ⊙ , independently of any model of the dark matter halo of the Galaxy.

New data are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 36.8 kg of CF3I and located in the SNOLAB underground laboratory. PICO-60 is the largest bubble chamber to search for dark matter to date. With an analyzed exposure of 92.8 live-days, PICO-60 exhibits the same excellent background rejection observed in smaller bubble chambers. Alpha decays in PICO-60 exhibit frequency-dependent acoustic calorimetry, similar but not identical to that reported recently in a C3F8 bubble chamber. PICO-60 also observes a large population of unknown background events, exhibiting acoustic, spatial, and timing behaviors inconsistent with those expected from a dark matter signal. These behaviors allow for analysis cuts to remove all background events while retaining 48.2% of the exposure. Stringent limits on WIMPs interacting via spin-dependent proton and spin-independent processes are set, and most interpretations of the DAMA/LIBRA modulation signal as dark matter interacting with iodine nuclei are ruled out.

New data are reported from a second run of the 2-liter PICO-2L C 3 F 8 bubble chamber with a total exposure of 129 kg-days at a thermodynamic threshold energy of 3.3 keV. These data show that measures taken to control particulate contamination in the superheated fluid resulted in the absence of the anomalous background events observed in the first run of this bubble chamber. One single nuclear-recoil event was observed in the data, consistent both with the predicted background rate from neutrons and with the observed rate of unambiguous multiple-bubble neutron scattering events. The chamber exhibits the same excellent electron-recoil and alpha decay rejection as was previously reported. These data provide the most stringent direct detection constraints on weakly interacting massive particle (WIMP)-proton spindependent scattering to date for WIMP masses < 50 GeV=c 2 .

New data are reported from the operation of a 2 liter C3F8 bubble chamber in the SNOLAB underground laboratory, with a total exposure of 211.5 kg days at four different energy thresholds below 10 keV. These data show that C3F8 provides excellent electron-recoil and alpha rejection capabilities at very low thresholds. The chamber exhibits an electron-recoil sensitivity of <3.5×10(-10) and an alpha rejection factor of >98.2%. These data also include the first observation of a dependence of acoustic signal on alpha energy. Twelve single nuclear recoil event candidates were observed during the run. The candidate events exhibit timing characteristics that are not consistent with the hypothesis of a uniform time distribution, and no evidence for a dark matter signal is claimed. These data provide the most sensitive direct detection constraints on WIMP-proton spin-dependent scattering to date, with significant sensitivity at low WIMP masses for spin-independent WIMP-nucleon scattering.