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We present DES16C3cje, a low-luminosity, long-lived type II supernova (SN II) at redshift 0.0618, detected by the Dark Energy Survey (DES). DES16C3cje is a unique SN. The spectra are characterized by extremely narrow photospheric lines corresponding to very low expansion velocities of . 1500 km s−1 , and the light curve shows an initial peak that fades after 50 days before slowly rebrightening over a further 100 days to reach an absolute brightness of M푟 ∼ −15.5 mag. The decline rate of the late-time light curve is then slower than that expected from the powering by radioactive decay of 56Co, but is comparable to that expected from accretion power. Comparing the bolometric light curve with hydrodynamical models, we find that DES16C3cje can be explained by either i) a low explosion energy (0.11 foe) and relatively large 56Ni production of 0.075 M from a ∼ 15 M red supergiant progenitor typical of other SNe II, or ii) a relatively compact ∼ 40 M star, explosion energy of 1 foe, and 0.08 M of 56Ni. Both scenarios require additional energy input to explain the late-time light curve, which is consistent with fallback accretion at a rate of ∼ 0.5 × 10−8 M s−1., EU/FP7-ERC grant 615929, European Union (EU) 839090, Alexander von Humboldt Foundation, Science Foundation Ireland, Polish NCN MAESTRO 2014/14/A/ST9/00121, Royal Astronomical Society Research Fellowship, European Organisation for Astronomical Research in the Southern Hemisphere, Chile 299.D-5040(A) 299.D-5040(B) 0100.D-0461(A) 194.C-0207(I), PESSTO, (the Public ESO Spectroscopic Survey for Transient Objects Survey) ESO program 197.D1075 199.D-0143, Programme NOAO GS-2016B-Q-9, United States Department of Energy (DOE), National Science Foundation (NSF), Spanish Government, Science & Technology Facilities Council (STFC), Higher Education Funding Council for England, National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, Kavli Institute of Cosmological Physics at the University of Chicago, Ohio State University, Mitchell Institute for Fundamental Physics and Astronomy at Texas AM University, Ciencia Tecnologia e Inovacao (FINEP), Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro, National Council for Scientific and Technological Development (CNPq), Ministerio da Ciencia, Tecnologia e Inovacao, German Research Foundation (DFG), Collaborating Institutions in the Dark Energy Survey, United States Department of Energy (DOE) University of Chicago, University of California at Santa Cruz, University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, University of Chicago, University College London, DES-Brazil Consortium, University of Edinburgh, ETH Zurich, University of Illinois at Urbana-Champaign, Institut de Ciencies de l'Espai (IEEC/CSIC), Institut de Fisica d'Altes Energies, Ludwig-Maximilians Universitat Munchen and the associated Excellence Cluster Universe, University of Michigan System, National Science Foundation (NSF) NSF - Directorate for Mathematical & Physical Sciences (MPS), University of Nottingham, University of Pennsylvania, University of Portsmouth, Stanford University United States Department of Energy (DOE), Stanford University, University of Sussex, Texas AM University, OzDES Membership Consortium, German Research Foundation (DFG) HA 1850/28-1, European Union (EU) PGC2018-095317-B-C21, National Science Foundation (NSF) AST-1138766 AST-1536171, MINECO AYA2015-71825 ESP2015-66861 FPA2015-68048 SEV-2016-0588 SEV2016-0597 MDM-2015-0509, ERDF funds from the European Union, CERCA program of the Generalitat de Catalunya, European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013), European Research Council (ERC) 240672 291329 306478, National Council for Scientific and Technological Development (CNPq) 465376/2014-2, National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility DE-AC02-05CH11231, United States Department of Energy (DOE) DE-AC02-07CH11359

Prompt double-$J/\psi$ production at high-energy hadron colliders can be considered as a golden channel to probe double parton scatterings (DPS) --in particular to study gluon-gluon correlations inside the proton-- and, at the same time, to measure the distribution of linearly-polarised gluons inside the proton. Such studies however require a good control of both single and DPS in the respective regions where they are carried out. In this context, we have critically examined two mechanisms of single parton scatterings (SPS) that may be kinematically enhanced where DPS are thought to be dominant, even though they are either at higher orders in the strong-coupling or velocity expansion. First, we have considered a gauge-invariant and infrared-safe subset of the loop-induced contribution via Colour-Singlet (CS) transitions. We have found it to become the leading CS SPS contributions at large rapidity separation, yet too small to account for the data without invoking the presence of DPS yields. Second, we have surveyed the possible Colour-Octet (CO) contributions using both old and up-to-date non-perturbative long distance matrix elements (LDMEs). We have found that the pure CO yields crucially depend on the LDMEs. Among all the LDMEs we used, only two result into a visible modification of the NRQCD (CS+CO) yield, but only in two kinematical distributions measured by ATLAS, those of the rapidity separation and of the pair invariant mass. These modifications however do not impact the control region used for their DPS study., Comment: 32 pages, 21 figures, 3 tables

We present UV, optical, and NIR photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at $0.47$ days to $18.5$ days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy distribution (SED) inferred from this photometry at $0.6$ days is well described by a blackbody model with $T\approx 8300$ K, a radius of $R\approx 4.5\times 10^{14}$ cm (corresponding to an expansion velocity of $v\approx 0.3c$), and a bolometric luminosity of $L_{\rm bol}\approx 5\times10^{41}$ erg s$^{-1}$. At $1.5$ days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set we find that models with heating from radioactive decay of $^{56}$Ni, or those with only a single component of opacity from $r$-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data, the resulting "blue" component has $M_\mathrm{ej}^\mathrm{blue}\approx 0.01$ M$_\odot$ and $v_\mathrm{ej}^\mathrm{blue}\approx 0.3$c, and the "red" component has $M_\mathrm{ej}^\mathrm{red}\approx 0.04$ M$_\odot$ and $v_\mathrm{ej}^\mathrm{red}\approx 0.1$c. These ejecta masses are broadly consistent with the estimated $r$-process production rate required to explain the Milky Way $r$-process abundances, providing the first evidence that BNS mergers can be a dominant site of $r$-process enrichment., 13 Pages, 3 Figures, 2 Tables. ApJL, In Press. Keywords: GW170817, LVC