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Jet-like correlations with neutral pion triggers in pp and central Pb-Pb collisions at 2.76 TeV. (arXiv:1608.07201v1 [nucl-ex])

We present measurements of two-particle correlations with neutral pion trigger particles of transverse momenta $8 < p_{mathrm{T}}^{rm trig} < 16~mathrm{GeV}/c$ and associated charged particles of $0.5 < p_{mathrm{T}}^{rm assoc} < 10~mathrm{GeV}/c$ versus the azimuthal angle difference $Deltavarphi$ at midrapidity in pp and central Pb-Pb collisions at $sqrt{s_{mathrm{NN}}}=2.76$ TeV with ALICE. The new measurements exploit associated charged hadrons down to $0.5~mathrm{GeV}/c$, which significantly extends our previous measurement that only used charged hadrons above $3~mathrm{GeV}/c$. After subtracting the contributions of the flow background, $v_2$ to $v_5$, the per-trigger yields are extracted for $|Deltavarphi|<0.7$ on the near and for $|Deltavarphi-pi| < 1.1$ on the away side. The ratio of per-trigger yields in Pb--Pb to those in pp collisions, $I_{mathrm{AA}}$, is measured on the near and away side for the $0$--$10$% most central Pb--Pb collisions. On the away side, the per-trigger yields in Pb--Pb are strongly suppressed to the level of $I_{mathrm{AA}} approx 0.6$ for $p_{mathrm{T}}^{rm assoc} > 3~mathrm{GeV}/c$, while with decreasing momenta an enhancement develops reaching about $5$ at low $p_{mathrm{T}}^{rm assoc}$. On the near side, an enhancement of $I_{mathrm{AA}}$ between $1.2$ at the highest to $1.8$ at the lowest $p_{mathrm{T}}^{rm assoc}$ is observed. The data are compared to parton-energy-loss predictions of the JEWEL and AMPT event generators, as well as to a perturbative QCD calculation with medium-modified fragmentation functions. All calculations qualitatively describe the away-side suppression at high $p_{mathrm{T}}^{rm assoc}$. Only AMPT captures the enhancement at low $p_{mathrm{T}}^{rm assoc}$, both on the near and away side. However, it also underpredicts $I_{mathrm{AA}}$ above $5$ GeV/$c$, in particular on the near-side.

Geometric Corroboration of the Earliest Lensed Galaxy at z~10.8 from Robust Free-Form Modelling. (arXiv:1608.06942v1 [astro-ph.CO])

A multiply-lensed galaxy, MACS0647-JD, with a probable photometric redshift of $zsimeq 10.7^{+0.6}_{-0.4}$ is claimed to constitute one of the very earliest known galaxies, formed well before reionization was completed. However, spectral evidence that MACS0647-JD lies at high redshift has proven infeasible and so here we seek an independent lensing based "geometric redshift" derived from the angles between the three lensed images of MACS0647-JD, using our free-form mass model (WSLAP+) for the lensing cluster MACSJ0647.7+7015 (at $z=0.591$). Our lens model uses the 9 sets of multiple images, including those of MACS0647-JD, identified by the CLASH survey towards this cluster. We convincingly exclude the low redshift regime of $z<3$, for which convoluted critical curves are generated by our method, as the solution bends to accommodate the wide angles of MACS0647-JD for this low redshift. Instead, a best fit to all sets of lensed galaxy positions and redshifts provides a geometric redshift of $zsimeq 10.8^{+0.3}_{-0.4}$ for MACS0647-JD, strongly supporting the higher photometric redshift solution. Importantly, we find a tight linear relation between the relative brightnesses of all 9 sets of multiply lensed images and their relative magnifications as predicted by our model. This agreement provides a benchmark for the quality of the lens model, and establishes the robustness of our free-form lensing method for measuring model-independent geometric source distances and for deriving objective central cluster mass distributions. After correcting for its magnification the luminosity of MACS0647-JD remains relatively high at $M_{UV}=-19.4$, which is within a factor of a few in flux of some surprisingly luminous $zsimeq 10$--$11$ candidates discovered recently in Hubble black field surveys.

The impact of baryonic physics on the subhalo mass function and implications for gravitational lensing. (arXiv:1608.06938v1 [astro-ph.GA])

We investigate the impact of baryonic physics on the subhalo population by analyzing the results of two recent hydrodynamical simulations (EAGLE and Illustris), which have very similar configuration, but a different model of baryonic physics. We concentrate on haloes with a mass between $10^{12.5}$ and $10^{14}M_{odot}h^{-1}$ and redshift between 0.2 and 0.5, comparing with observational results and subhalo detections in early-type galaxy lenses. We compare the number and the spatial distribution of subhaloes in the fully hydro runs and in their dark matter only counterparts, focusing on the differences between the two simulations. We find that the presence of baryons reduces the number of subhaloes, especially at the low mass end ($leq 10^{10}M_{odot}h^{-1}$), by different amounts depending on the model. The variations in the subhalo mass function are strongly dependent on those in the halo mass function, which is shifted by the effect of stellar and AGN feedback: a lower number of low mass haloes available for accretion in the first place; then additional differences can be attributed to the action of baryonic physics inside the halo. Finally, we search for analogues of the observed lenses (SLACS) in the simulations, doing a selection in velocity dispersion and dynamical properties. We use the selected galaxies to quantify detection expectations based on the subhalo populations in the different simulations, calculating the detection probability and the predicted values for the dark matter fraction in subhaloes $f_{DM}$ and the slope of the mass function $alpha$.

Hurricane Gaston Seen From Space

Composite image of Hurricane Gaston in the Atlantic at 06:00 UTC on Thursday, 25 August 2016.

Charting the future frontier(s) of particle production. (arXiv:1608.07196v1 [hep-ph])

This short note describes the long collaborative effort between Arizona and Krak'ow, showing some of the key strangeness signatures of quark-gluon plasma. It further presents an annotated catalog of foundational questions defining the research frontiers which I believe can be addressed in the foreseeable future in the context of relativistic heavy ion collision experiments. The list includes topics that are specific to the field, and ventures towards the known-to-be-unknown that may have a better chance with ions as compared to elementary interactions.

Why $Xi(1690)$ and $Xi(2120)$ are so narrow?. (arXiv:1608.07086v1 [nucl-th])

The $Xi$ baryons are expected to be naturally narrower as compared to their non-strange and strange counterparts since they have only one light quark and, thus, their decay involves producing either a light meson and doubly strange baryon or both meson and baryon with strangeness which involves, relatively, more energy. In fact, some $Xi$'s have full widths of the order of even 10-20 MeV when, in principle, they have a large phase space to decay to some open channels. Such is the case of $Xi (1690)$, for which the width has been found to be of the order of 10 MeV in the latest BABAR and BELLE data. In this manuscript we study why some $Xi$'s are so narrow. Based on a coupled channel calculation of the pseudoscalar meson-baryon and vector meson-baryon systems with chiral and hidden local symmetry Lagrangians, we find that the answer lies in the intricate hadron dynamics. We find that the known mass, width, spin-parity and branching ratios of $Xi (1690)$ can be naturally explained in terms of coupled channel meson-baryon dynamics. We find another narrow resonance which can be related to $Xi(2120)$. We also look for exotic states $Xi^{+}$ and $Xi^{--}$ but find none. In addition we provide the cross sections for $ bar K Lambda, bar K Sigma rightarrow pi Xi$ which can be useful for understanding the enhanced yield of $Xi$ reported in recent studies of heavy ion collisions.

Stopped nucleons in configuration space. (arXiv:1608.07041v1 [hep-ph])

In this note, using the colour string model, we study the configuration space distribution of stopped nucleons in heavy-ion collisions. We find that the stopped nucleons from the target and the projectile end up separated from each other by the distance increasing with the collision energy. In consequence, for the center of mass energies larger than 6 or 10 GeV (depending on the details of the model) it appears that the system created is not in thermal and chemical equilibrium, and the net baryon density reached is likely not much higher than that already present in the colliding nuclei.

Du Mont Blanc à l’Antarctique, une expédition pour stocker la mémoire des glaciers
D’ici deux à trois siècles, le Mont Blanc ne sera plus vraiment blanc. Le réchauffement climatique en a décidé autrement. Quand les glaciers fondent, c’est la mémoire de la Terre qui s’efface. Des scientifiques ont décidé d’agir pour sauvegarder l’histoire. Pour cela, des carottes de glace sont récupérées puis transportées vers l’Antarctique.
Spin mixing in Cs ultralong-range Rydberg molecules: a case study. (arXiv:1608.06939v1 [physics.atom-ph])

We calculate vibrational spectra of ultralong-range Cs(32p) Rydberg molecules which form in an ultracold gas of Cs atoms. We account for the partial-wave scattering of the Rydberg electrons from the ground Cs perturber atoms by including the full set of spin-resolved ${}^{1,3}S_J$ and ${}^{1,3}P_J$ scattering phase shifts, and allow for the mixing of singlet (S=0) and triplet (S=1) spin states through Rydberg electron spin-orbit and ground electron hyperfine interactions. Excellent agreement with observed data in Sa{ss}mannshausen et al. [Phys. Rev. Lett. 113, 133201(2015)] in line positions and profiles is obtained. We also determine the spin-dependent permanent electric dipole moments for these molecules. This is the first such calculation of ultralong-range Rydberg molecules in which all of the relativistic contributions are accounted for.

A simple setup to measure muon lifetime and electron energy spectrum of muon decay and its Monte Carlo simulation. (arXiv:1608.06936v1 [physics.ins-det])

We designed a simple setup to measure the muon lifetime and the electron energy spectra of muon decay. A low cost coincidental circuit was designed to select the signals of muon decay events detected by a plastic scintillator detector. It triggered a digital oscilloscope to record the signals of muon decay events for measuring the muon lifetime and electron energy spectrum. A Landau-distribution energy loss method was introduced to conduct the energy calibration of the system. The experimental results were well reproduced by the Monte Carlo simulation. The software and hardware of the system are completely open to students, thus more helpful for instruction and motivation.

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