Measurement of an excess in the yield of J/ψ\psi at very low-pTp_{\rm T} in Pb--Pb collisions with the ALICE detector

We report on the measurement of J/$\psi$ production at very low transverse momentum ($p_{\rm T} <$ 300 MeV/$c$) in Pb--Pb collisions performed with the ALICE detector at the LHC. We find an excess in the yield of J/$\psi$ with respect to expectations from hadronic production. Coherent photo-production of J/$\psi$ is proposed as a plausible origin of this excess. We show the nuclear modification factor of very low-$p_{\rm T}$ J/$\psi$ as a function of centrality. Then we measure the J/$\psi$ coherent photoproduction cross section in peripheral events assuming that it is the mechanism at the origin of the measured excess. It's worth noting that the observation of J/$\psi$ coherent photoproduction in Pb--Pb collisions at impact parameters smaller than twice the nuclear radius has never been observed so far and would open new theoretical challenges.Comment: Proceeding of EDS Blois Conference, 29th June - 4th July 2015, Borgo, Corsic

Vector meson production in the dimuon channel in the ALICE experiment at the LHC

The purpose of the ALICE experiment at the LHC is the study of the Quark Gluon Plasma (QGP) formed in ultra-relativistic heavy-ion collisions, a state of matter in which quarks and gluons are deconfined. The properties of this state of strongly-interacting matter can be accessed through the study of light vector mesons ($\rho$, $\omega$ and $\phi$). Indeed, the strange quark content ($s\bar{s}$) of the $\phi$ meson makes its study interesting in connection with the strangeness enhancement observed in heavy-ion collisions. Moreover, $\rho$ and $\omega$ spectral function studies give information on chiral symmetry restoration. Vector meson production in pp collisions is important as a baseline for heavy-ion studies and for constraining hadronic models. We present results on light vector meson production obtained with the muon spectrometer of the ALICE experiment in pp collisions at $\sqrt{s}$=7 TeV. Production ratios, integrated and differential cross sections for $\phi$ and $\omega$ are presented. Those results are extracted for $p_{\rm T} > 1$ GeV/$c$ and $2.5 < y < 4$

B70/B7-2 is identical to CD86 and is the major functional ligand for CD28 expressed on human dendritic cells.

Dendritic cells comprise a system of highly efficient antigen-presenting cells involved in the initiation of T cell responses. Herein, we investigated the role of the CD28 pathway during alloreactive T cell proliferation induced by dendritic-Langerhans cells (D-Lc) generated by culturing human cord blood CD34+ progenitor cells with granulocyte/macrophage colony-stimulating factor and tumor necrosis factor alpha. In addition to expressing CD80 (B7/BB1), a subset of D-Lc expressed B70/B7-2. Binding of the CTLA4-Ig fusion protein was completely inhibited by a combination of monoclonal antibodies (mAbs) against CD80 and B70/B7-2, indicating the absence of expression of a third ligand for CD28/CTLA-4. It is interesting to note that mAbs against CD86 completely prevented the binding of CTLA4-Ig in the presence of mAbs against CD80 and bound to a B70/B7-2-transfected fibroblast cell line, demonstrating that the B70/B7-2 antigen is identical to CD86. CD28 triggering was essential during D-Lc-induced alloreaction as it was inhibited by mAbs against CD28 (9 out of 11 tested). However, none of six anti-CD80 mAbs demonstrated any activity on the D-Lc-induced alloreaction, though some were previously described as inhibitory in assays using CD80-transfected cell lines. In contrast, a mAb against CD86 (IT-2) was found to suppress the D-Lc-dependent alloreaction by 70%. This inhibitory effect was enhanced to &gt; or = 90% when a combination of anti-CD80 and anti-CD86 mAbs was used. The present results demonstrate that D-Lc express, in addition to CD80, the other ligand for CTLA-4, CD86 (B70/B7-2), which plays a primordial role during D-Lc-induced alloreaction

Feasibility studies for quarkonium production at a fixed-target experiment using the LHC proton and lead beams (AFTER@LHC)

Used in the fixed-target mode, the multi-TeV LHC proton and lead beams allow for studies of heavy-flavour hadroproduction with unprecedented precision at backward rapidities - far negative Feyman-x - using conventional detection techniques. At the nominal LHC energies, quarkonia can be studies in detail in p+p, p+d and p+A collisions at sqrt(s_NN) ~ 115 GeV as well as in Pb+p and Pb+A collisions at sqrt(s_NN) ~ 72 GeV with luminosities roughly equivalent to that of the collider mode, i.e. up to 20 fb-1 yr-1 in p+p and p+d collisions, up to 0.6 fb-1 yr-1 in p+A collisions and up to 10 nb-1 yr-1 in Pb+A collisions. In this paper, we assess the feasibility of such studies by performing fast simulations using the performance of a LHCb-like detector.Comment: 12 pages, 14 figure

Analysis of the phenomenon of speculative trading in one of its basic manifestations: postage stamp bubbles

We document and analyze the empirical facts concerning one of the clearest evidence of speculation in financial trading as observed in the postage collection stamp market. We unravel some of the mechanisms of speculative behavior which emphasize the role of fancy and collective behavior. In our conclusion, we propose a classification of speculative markets based on two parameters, namely the amplitude of the price peak and a second parameter that measures its ``sharpness''. This study is offered to anchor modeling efforts to realistic market constraints and observations.Comment: 9 pages, 5 figures and 2 tables, in press in Int. J. Mod. Phys.

Source blending effects on microlensing time-histograms and optical depth determination

Source blending in microlensing experiments is known to modify the Einstein time of the observed events. In this paper, we have conducted Monte-Carlo calculations, using the analytical relationships derived by Han (1999) to quantify the effect of blending on the observed event time distribution and optical depth. We show that short-time events are affected significantly by source blending and that, for moderately blended sources, the optical depth $\tau$ is globally overestimated, because of an underestimation of the exposure. For high blending situations, on the opposite, blending leads to an {\it under}estimation of the optical depth. Our results are in agreement with the most recent optical depth determinations toward the Galactic Center of the MACHO collaboration (Popowski et al. 2004) and the OGLE-II collaboration (Sumi et al. 2005) that use clump giants (less affected by the blending effect) as sources. The blending-corrected, lower optical depth toward the Galactic Bulge is now in good agreement with the value inferred from galactic models, reconciling theoretical and observational determinations.Comment: Accepted in Astronomy Astrophysics. Note that these calculations were conducted in 2001, prior to the recent DIA analyses mentioned in the references (see Alibert, Y. SF2A-conference, 2001

Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2 in dense clouds

Aim: To determine whether or not gas-phase chemical models with homogeneous and time-independent physical conditions explain the many observed molecular abundances in astrophysical sources, it is crucial to estimate the uncertainties in the calculated abundances and compare them with the observed abundances and their uncertainties. Non linear amplification of the error and bifurcation may limit the applicability of chemical models. Here we study such effects on dense cloud chemistry. Method: Using a previously studied approach to uncertainties based on the representation of rate coefficient errors as log normal distributions, we attempted to apply our approach using as input a variety of different elemental abundances from those studied previously. In this approach, all rate coefficients are varied randomly within their log normal (Gaussian) distribution, and the time-dependent chemistry calculated anew many times so as to obtain good statistics for the uncertainties in the calculated abundances. Results: Starting with so-called ``high-metal'' elemental abundances, we found bimodal rather than Gaussian like distributions for the abundances of many species and traced these strange distributions to an extreme sensitivity of the system to changes in the ratio of the cosmic ray ionization rate zeta\_He for He and that for molecular hydrogen zeta\_H2. The sensitivity can be so extreme as to cause a region of bistability, which was subsequently found to be more extensive for another choice of elemental abundances. To the best of our knowledge, the bistable solutions found in this way are the same as found previously by other authors, but it is best to think of the ratio zeta\_He/zeta\_H2 as a control parameter perpendicular to the ''standard'' control parameter zeta/n\_H.Comment: Accepted for publicatio

Heavy-ion Physics at a Fixed-Target Experiment Using the LHC Proton and Lead Beams (AFTER@LHC): Feasibility Studies for Quarkonium and Drell-Yan Production

We outline the case for heavy-ion-physics studies using the multi-TeV lead LHC beams in the fixed-target mode. After a brief contextual reminder, we detail the possible contributions of AFTER@LHC to heavy-ion physics with a specific emphasis on quarkonia. We then present performance simulations for a selection of observables. These show that $\Upsilon(nS)$, $J/\psi$ and $\psi(2S)$ production in heavy-ion collisions can be studied in new energy and rapidity domains with the LHCb and ALICE detectors. We also discuss the relevance to analyse the Drell-Yan pair production in asymmetric nucleus-nucleus collisions to study the factorisation of the nuclear modification of partonic densities and of further quarkonia to restore their status of golden probes of the quark-gluon plasma formation.Comment: 18 pages, 7 figure

Magnetic fields from reionisation

We present a complementary study to a new model for generating magnetic fields of cosmological interest. The driving mechanism is the photoionisation process by photons provided by the first luminous sources. Investigating the transient regime at the onset of inhomogeneous reionisation, we show that magnetic field amplitudes as high as $2 \times 10^{-16}$ Gauss can be obtained within a source lifetime. Photons with energies above the ionisation threshold accelerate electrons, inducing magnetic fields outside the Stroemgren spheres which surround the ionising sources. Thanks to their mean free path, photons with higher energies propagate further and lead to magnetic field generation deeper in the neutral medium. We find that soft X-ray photons could contribute to a significant premagnetisation of the intergalactic medium at a redshift of z=15.Comment: accepted for publication in A&