arXiv subjects
Publications and source records attributed to Michael Murray.
Ultra-peripheral collisions (UPCs) involving heavy ions and protons are the energy frontier for photon-mediated interactions. UPC photons can be used for many purposes, including probing low-$x$ gluons via photoproduction of dijets and vector mesons, probes of beyond-standard-model processes, such as those enabled by light-by-light scattering, and studies of two-photon production of the Higgs.
Robots navigating in human environments should use language to ask for assistance and be able to understand human responses. To study this challenge, we introduce Cooperative Vision-and-Dialog Navigation, a dataset of over 2k embodied, human-human dialogs situated in simulated, photorealistic home environments. The Navigator asks questions to their partner, the Oracle, who has privileged access to the best next steps the Navigator should take according to a shortest path planner. To train agents that search an environment for a goal location, we define the Navigation from Dialog History task. An agent, given a target object and a dialog history between humans cooperating to find that object, must infer navigation actions towards the goal in unexplored environments. We establish an initial, multi-modal sequence-to-sequence model and demonstrate that looking farther back in the dialog history improves performance. Sourcecode and a live interface demo can be found at https://cvdn.dev/
In this paper, we derive the sharp lower and upper bounds of nodal lengths of Laplacian eigenfunctions in the disc. Furthermore, we observe a geometric property of the eigenfunctions whose nodal curves maximize the nodal length.
In this paper we address the question of the existence of a model for the string 2-group as a strict Lie-2-group using the free loop group $LSpin$ (or more generally $LG$ for compact simple simply-connected Lie groups $G$). Baez-Crans-Stevenson-Schreiber constructed a model for the string 2-group using a based loop group. This has the deficiency that it does not admit an action of the circle group $S^1$, which is of crucial importance, for instance in the construction of a (hypothetical) $S^1$-equivariant index of (higher) differential operators. The present paper shows that there are in fact obstructions for constructing a strict model for the string 2-group using $LG$. We show that a certain infinite-dimensional manifold of smooth paths admits no Lie group structure, and that there are no nontrivial Lie crossed modules analogous to the BCSS model using the universal central extension of the free loop group. Afterwards, we construct the next best thing, namely a coherent model for the string 2-group using the free loop group, with explicit formulas for all structure. This is in particular important for the expected representation theory of the string group that we discuss briefly in the end.
The almost hermetic coverage of CMS is used to measure the distribution of transverse energy as a function of pseudo-rapidity for pPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV. For minimum bias collisions $(1/N)~dE_T/dη$ reaches 23 GeV which implies an $E_T$ per participant pair comparable to that of peripheral PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV. The centrality dependence of transverse energy production has been studied using centrality measures defined in three different angular regions. There is a strong auto-correlation between $(1/N)~dE_T/dη$ and the $η$ range used to define centrality %both for data and the EPOS-LHC and HIJING event generators. The centrality dependence of the data is much stronger for $η$ values on the lead side than the proton side and shows significant differences from that predicted by either event generator.
We consider the existence of bibundles, in other words locally trivial principal $G$ spaces with commuting left and right $G$ actions. We show that their existence is closely related to the structure of the group $\Out(G)$ of outer automorphisms of $G$. We also develop a classifying theory for bibundles. The theory is developed in full generality for $(H, G)$ bibundles for a crossed-module $(H, G)$ and we show with examples the close links with loop group bundles.
Let $(P, Y)$ be a bundle gerbe over a fibre bundle $Y \to M$. We show that if $M$ is simply-connected and the fibres of $Y \to M$ are connected and finite-dimensional then the Dixmier-Douady class of $(P, Y)$ is torsion. This corrects and extends an earlier result of the first author.
We present the capabilities of the CMS experiment to explore the heavy-ion physics program offered by the CERN Large Hadron Collider (LHC). The prime goal of this research is to test the fundamental theory of the strong interaction (QCD) in extreme conditions of temperature, density and parton momentum fraction by colliding nuclei at energies of sqrt(s_NN) = 5.5 TeV. This presentation will give the overview of the potential of the CMS to carry out a full set of representative Pb-Pb measurements both in "soft" and "hard" regimes. Measurements include "bulk" observables -- charged hadron multiplicity, low pT inclusive hadron identified spectra and elliptic flow -- which provide information on the collective properties of the system; as well as perturbative processes - such as quarkonia, heavy-quarks, jets, gamma-jet, and high pT hadrons -- which yield "tomographic" information of the hottest and densest phases of the reaction.
The purpose of BRAHMS is to survey the dynamics of relativistic heavy ion (as well as pp and d-A) collisions over a very wide range of rapidity and transverse momentum. The sum of these data may give us a glimpse of the initial state of the system, its transverse and longitudinal evolution and how the nature of the system changes with time. Here I will concentrate on the origin and dynamics of the light flavors, i.e. the creation and transport of the up, down and strange quarks. The results presented here are certainly not the end of the story. It is my hope that in a few years new detectors will reveal the rapidity dependence of the charm and bottom quarks.
When heavy ions collide at ultra-relativistic energy, thousands of particles are emitted and it is reasonable to attempt to use hydrodynamic descriptions, with suitable initial conditions, to describe the time evolution of the collisons. In the longitudinal direction pions seem to exhibit Landau flow. This simple model assumes that all the entropy in the collisions is created the instant the two Lorentz contracted nuclei overlap and that the system then expands adiabatically. The system also displays radial and elliptic flow. Radial flow is manifested as a broadening of the pT distributions with respect to pp collisions. It is typically thought to result from multiple scattering of partons or hadrons before dynamic freeze-out. Elliptic flow occurs when heavy ions do not collide exactly head on. The initial geometrical asymetry is translated into a momentum asymetry via pressure gradiants. Since these gradients are self quenching, strong elliptic flow is thought to be linked to early thermalization and a large initial pressure. Using the concept of limiting fragmentation we attempt to sketch a link between the initial and final states of relativistic heavy ion collisions using new preliminary data from the BRAHMS collaboration on elliptic and radial flow.
Thermal models have been used to successfully describe the hadron yields from heavy ion collisions at a variety of energies. For root(S)<17 GeV this has usually been done using yields integrated over 4pi but at the higher energies available at RHIC, yields measured at central rapidity have been used. Recent BRAHMS data allows us to test whether thermal models can be generalized to describe the rapidity dependence of particle ratios. We have used the THERMUS package to fit BRAHMS data for the 5% most central Au+Au collisions for several rapidities at root(S) = 62 and 200 GeV. We have found a relationship between the strange and light quark chemical potentials, muS = 0.21 +-0.01muB. Using this relation we are able to describe the energy dependence of Lambda, Xsi and Omega ratios from other experiments. We also find that the chemical potentials are consistent with limiting fragmentation.
The CMS heavy ion program can study quark matter over an unprecedented range of Bjorken x and mass. CMS is equipped with excellent detectors to exploit the new physics probes available at_/Snn = 5.5 TeV. The high rate capability and wide rapidity coverage will allow us to study the correlations between low x and high mass phenomena.
BRAHMS has the ability to study relativistic heavy ion collisions over a wide range of pT and rapidity. This allows us to test whether thermal models can be generalized to describe the rapidity dependence of particle ratios. This appears to work with the baryo-chemical potential changing more rapidly than the temperature. Using fits to BRAHMS data for the 5% most central Au+Au collisions we are able to describe Xi and Omega ratios from other experiments. This paper is dedicated to Julia Thompson who worked to bring South African teachers into physics.
BRAHMS has the ability to study relativistic heavy ion collisions from the final freeze-out of hadrons all the way back to the initial wave-function of the gold nuclei. This is accomplished by studying hadrons with a very wide range of momenta and angles. In doing so we can scan various phases of QCD, from a hadron gas, to a quark gluon plasma and perhaps to a color glass condensate.
The first measurement of kaon phase space densities are presented as a function of transverse mass, center of mass energy and the number of participants. The kaon phase space density increases with the number of participants from e+e- to Pb+Pb collisions. However the ratio of the kaon and pion phase space densities at low transverse momentum is independent of the number of participants for sqrt{s}=17GeV/nucleon This paper is dedicated to Francis Riccardelli, engineer for the Port Authority, who died on September 11th 2001 while evacuating others.
We report on the first measurement of Mutual Coulomb Dissociation in heavy ion collisions. We employ forward calorimeters to measure neutron multiplicity at beam rapidity in peripheral collisions. The cross-section for simultaneous electromagnetic breakup of Au nuclei at $\sqrt{s_{NN}}=130$ GeV is $σ_{MCD}=3.67\pm 0.25$ barns in good agreement with calculations.
For relativistic heavy ion collisions the energy flow in the collision reveals information on the equation of state of matter at high density. The BRAHMS experiment has studied the relation between multipliciity in the sidewards direction and zero degree energy using our Tile Multiplicity Array and Zero Degree Calorimeters. To understand this spectrum requires a knowledge of the coalesence of nucleons that are close in phase space. We have also studied electromagnetic collisions and compared our results to lower energy data. As the center of mass energy increases, each nucleus sees a stronger electromagnetic field resulting in more energy being absorbed. This in turn causes an increase in the neutron multiplicity.
A summary of current interferometry data in relativistic heavy ions is presented. At sqrt{s}=17GeV a sudden increase in the pion source volume is observed for central PbPb collisions. This seems to imply that the pion phase density has reached a limit. The source size of different particles decreases with mass when the transverse velocity is held constant but increases with mass when the transverse mass is held constant. The antiproton source radius is larger than the proton source radius. So far no long lived source has been seen. The pion source size varies slowly with rapidity but more rapidly with transverse mass implying strong transverse flow. There is very slow increase of pion radii with sqrt{s}.