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Vertical Integration as a Source of Hold-Up
No full textWhile vertical integration is traditionally seen as a solution to the hold-up problem, this article highlights instead that it can generate hold-up problems-for rivals. We consider a successive duopoly where downstream firms invest and then secure support from an upstream supplier. We first show that vertical integration generates ex ante incentives to create hold-up problems: an integrated supplier is willing to pre-commit itself to appropriating or dissipating part of its customer's profits, to expose the independent rival to being held-up by the other supplier, and discourage in this way the rival's investment. We then show that, even in the absence of any pre-commitment, vertical integration also creates hold-up problems ex post when degrading the quality of the support provided to one downstream firm benefits its rival. We also provide illustrations in terms of standard industrial organization models and of antitrust cases, and discuss the robustness of the insights
Transverse energy production and charged-particle multiplicity at midrapidity in various systems from to 200 GeV
No full text706 authors, 32 pages, 20 figures, 34 tables, 2004, 2005, 2008, 2010, 2011, and 2012 data.International audienceMeasurements of midrapidity charged particle multiplicity distributions, , and midrapidity transverse-energy distributions, , are presented for a variety of collision systems and energies. Included are distributions for AuAu collisions at , 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, CuCu collisions at and 62.4 GeV, CuAu collisions at GeV, UU collisions at GeV, Au collisions at GeV, HeAu collisions at GeV, and collisions at GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, , and the number of constituent quark participants, . For all collisions down to GeV, it is observed that the midrapidity data are better described by scaling with than scaling with . Also presented are estimates of the Bjorken energy density, , and the ratio of to , the latter of which is seen to be constant as a function of centrality for all systems
Photoionization time delays
No full textInternational audienceThe material presented in this chapter is based on important advances realized in " attophysics " which make feasible to follow the motion of electrons in atoms and molecules with attosecond-level time resolution. In this context, time-delays have been recently determined in the process of photoionization by extreme-ultraviolet (xuv) pulses and the question of the significance of these measured delays arises. As we shall outline here, numerical experiments show that they are intimately related to the structure of the ionized species' continuous spectrum. Another point addressed here is that, in experiments, the measurements have the common characteristic to be performed in the presence of an auxiliary infra-red (IR) field, used to " clock " the timing of the process. This implies to adapt the theory treatment to handle such " two-color " photoionization processes. We review a systematic analysis of these features that are characteristic of this class of electronic transitions, when viewed in the time domain
Search for resonant ttbar production in proton-proton collisions at sqrt(s)=8 TeV
No full textsee paper for full list of authorsInternational audienceA search is performed for the production of heavy resonances decaying into top-antitop quark pairs in proton-proton collisions at sqrt(s)=8 TeV. Data used for the analyses were collected with the CMS detector and correspond to an integrated luminosity of 19.7 inverse femtobarns. The search is performed using events with three different final states, defined by the number of leptons (electrons and muons) from the ttbar to W b W b decay. The analyses are optimized for reconstruction of top quarks with high Lorentz boosts, where jet substructure techniques are used to enhance the sensitivity. Results are presented for all channels and a combination is performed. No significant excess of events relative to the expected yield from standard model processes is observed. Upper limits on the production cross section of heavy resonances decaying to ttbar are calculated. A narrow leptophobic topcolor Z' resonance with a mass below 2.4 TeV is excluded at 95% confidence level. Limits are also derived for a broad Z' resonance with a 10% width relative to the resonance mass, and a Kaluza-Klein excitation of the gluon in the Randall-Sundrum model. These are the most stringent limits to date on heavy resonances decaying into top-antitop quark pairs
Search for narrow resonances in dijet final states at sqrt(s)=8 TeV with the novel CMS technique of data scouting
No full textSubmitted to Phys. Rev. Lett, see paper for full list of authorsInternational audienceA search for narrow resonances decaying into dijet final states is performed on data from proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 18.8 inverse femtobarns. The data were collected with the CMS detector using a novel technique called data scouting, in which the information associated with these selected events is much reduced, permitting collection of larger data samples. This technique enables CMS to record events containing jets at a rate of 1 kHz, by collecting the data from the high-level-trigger system. In this way, the sensitivity to low-mass resonances is increased significantly, allowing previously inaccessible couplings of new resonances to quarks and gluons to be probed. The resulting dijet mass distribution yields no evidence of narrow resonances. Upper limits are presented on the resonance cross sections as a function of mass, and compared with a variety of models predicting narrow resonances. The limits are translated into upper limits on the coupling of a leptophobic resonance Z'[B] to quarks, improving on the results obtained by previous experiments for the mass range from 500 to 800 GeV
Search for pair production of first and second generation leptoquarks in proton-proton collisions at sqrt(s) = 8 TeV
No full textSubmitted to Phys. Rev. D ; see paper for full list of authorsInternational audienceA search for pair production of first and second generation leptoquarks is performed in final states containing either two charged leptons and two jets, or one charged lepton, one neutrino and two jets, using proton-proton collision data at sqrt(s) = 8 TeV. The data, corresponding to an integrated luminosity of 19.7 inverse-femtobarns were recorded with the CMS detector at the LHC. First-generation scalar leptoquarks with masses less than 1010 (850) GeV are excluded for beta = 1.0 (0.5), where beta is the branching fraction of a leptoquark decaying to a charged lepton and a quark. Similarly, second-generation scalar leptoquarks with masses less than 1080 (760) GeV are excluded for beta = 1.0 (0.5). Mass limits are also set for vector leptoquark production scenarios with anomalous vector couplings, and for R-parity violating supersymmetric scenarios of top squark pair production resulting in similar final-state signatures. These are the most stringent limits placed on the masses of leptoquarks and RPV top squarks to date
Untangling trigonal diagrams
No full text10p., 24 figsInternational audienceLet be a link of Conway's normal form , , or with , and let be a trigonal diagram of We show that it is possible to transform into an alternating trigonal diagram, so that all intermediate diagrams remain trigonal, and the number of crossings never increases
Prices vs. quantities in presence of a second, unpriced, externality
No full textWe study a situation in which two goods jointly generate an externality but onlyone of them is regulated. Unilateral regulation of greenhouse gas emissions and relatedcarbon leakage is a well known example. We compare tax and quantity instrumentsunder uncertainty à la Weitzman (1974). Because of the uncertainty surroundingthe unregulated good, the external cost is stochastic with both instruments. Whetherthe unregulated good quantity is more or less variable under a tax or under a quotadepends on the degree of substitutability and the correlation between uncertaintieson private valuations. In case of a positive correlation and imperfect substitution, atax better stabilize the unregulated good quantity and can therefore dominate a quotawhen the slope of the external cost associated to the unregulated good is large. In aspecification, relevant for leakage, it is shown that if uncertainty about the unregulatedgood (imports) is large, a tax might be preferable to a quota, regardless of the convexityof the external cost
COP21 : Pour rejoindre rapidement une trajectoire limitant le réchauffement à 2 °C
No full textNational audiencePour un futur sans émission de gaz à effet de serre et un changement climatique contrôlé, il est nécessaire de créer les outils adéquats. Les « Contributions prévues déterminées au niveau national » créées par la COP21 en font-elles partie, s’interroge un groupe de chercheurs
A framework for proof certificates in finite state exploration
No full textInternational audienceModel checkers use automated state exploration in order to prove various properties such as reach-ability, non-reachability, and bisimulation over state transition systems. While model checkers have proved valuable for locating errors in computer models and specifications, they can also be used to prove properties that might be consumed by other computational logic systems, such as theorem provers. In such a situation, a prover must be able to trust that the model checker is correct. Instead of attempting to prove the correctness of a model checker, we ask that it outputs its " proof evidence " as a formally defined document—a proof certificate—and that this document is checked by a trusted proof checker. We describe a framework for defining and checking proof certificates for a range of model checking problems. The core of this framework is a (focused) proof system that is augmented with premises that involve " clerk and expert " predicates. This framework is designed so that soundness can be guaranteed independently of any concerns for the correctness of the clerk and expert specifications. To illustrate the flexibility of this framework, we define and formally check proof certificates for reachability and non-reachability in graphs, as well as bisimulation and non-bisimulation for labeled transition systems. Finally, we describe briefly a reference checker that we have implemented for this framework