22,640 research outputs found
Boundary of Nuclear Physics and QCD
Recent progress in lattice QCD, combined with the imminent advent of a new
generation of dedicated supercomputers and advances in chiral extrapolation
mean that the next few years will bring quite novel insights into hadron
structure. We review some of the recent highlights in this field, the questions
which might be addressed and the experiments which may be expected to stretch
that understanding to its limits. Only with a sound understanding of hadron
structure can one hope to explore the fundamental issue of how that structure
may change at finite density (or temperature). We explore potential future
insights from lattice QCD into the phenomenon of nuclear saturation and a very
important hint from recent data of a change in the structure of a bound
nucleon.Comment: Invited talk presented at INPC2001, Berkeley, August 200
The spin structure function of the neutron
The neutron spin structure function, , has been of considerable
interest recently in connection with the Bjorken sum rule and the proton spin
crisis. Work on this problem has concentrated on measurements at low-. We
recall the important, non-perturbative physics to be learnt by going instead to
larger values of and especially from a determination of the place where the
expected sign change occurs. Of course, in order to obtain neutron data one
must use nuclear targets and apply appropriate corrections. In this regard, we
review recent progress concerning the various nuclear corrections that must be
applied to measurements on polarised He.Comment: Invited presentation at the Workshop on the Spin Structure of the
Proton and Polarized Collider Physics, ECT* Trento, July 23-28, 200
Bottom quark contribution to spin-dependent dark matter detection
We investigate a previously overlooked bottom quark contribution to the
spin-dependent cross section for Dark Matter(DM) scattering from the nucleon.
While the mechanism is relevant to any supersymmetric extension of the Standard
Model, for illustrative purposes we explore the consequences within the
framework of the Minimal Supersymmetric Standard Model(MSSM). We study two
cases, namely those where the DM is predominantly Gaugino or Higgsino. In both
cases, there is a substantial, viable region in parameter space ( GeV) in which the bottom contribution
becomes important. We show that a relatively large contribution from the bottom
quark is consistent with constraints from spin-independent DM searches, as well
as some incidental model dependent constraints.Comment: 11 pages, 10 figures, version published in NP
Endogenous Entry in Markets with Adverse Selection
Since Akerlof's (1970) seminal paper the existence of adverse selection due to asymmetric information about quality is well-understood. Yet two questions remain. First, given the negative implications for trading and welfare, how do such markets come into existence? And second, why have many studies failed to find direct or indirect evidence of adverse selection? In addressing the first question directly we shed some light on the second. We consider a market in which firms make an observable investment that generates products of a quality that becomes known only to the firm. Entry has the tendency to lower prices, which may lead to adverse selection. The implied price collapse limits the amount of entry so that high prices are supported in the market equilibrium, which results in above normal profits. While contributing to our understanding of markets with asymmetric information and adverse selection, the model also provides insight into the question of why markets with adverse selection are empirically hard to identify. The analysis suggests that rather than observing the canonical market collapse, such markets are instead characterized by less entry than would be empirically predicted and above normal profts even in markets with low measures of concentration.adverse selection, asymmetric information, entry, entry barriers, investment
Necessary Conditions for the Generic Global Rigidity of Frameworks on Surfaces
A result due in its various parts to Hendrickson, Connelly, and Jackson and
Jord\'an, provides a purely combinatorial characterisation of global rigidity
for generic bar-joint frameworks in . The analogous conditions
are known to be insufficient to characterise generic global rigidity in higher
dimensions. Recently Laman-type characterisations of rigidity have been
obtained for generic frameworks in when the vertices are
constrained to lie on various surfaces, such as the cylinder and the cone. In
this paper we obtain analogues of Hendrickson's necessary conditions for the
global rigidity of generic frameworks on the cylinder, cone and ellipsoid.Comment: 13 page
Electromagnetic Gauge Invariance of the Cloudy Bag Model
We examine the question of the gauge invariance of electromagnetic form
factors calculated within the cloudy bag model. One of the assumptions of the
model is that electromagnetic form factors are most accurately evaluated in the
Breit frame. This feature is used to show that gauge invariance is respected in
this frame.Comment: 8 pages, RevTex, 1 figure, to be published in Phys. Rev.
Thinking Fast and Slow with Deep Learning and Tree Search
Sequential decision making problems, such as structured prediction, robotic
control, and game playing, require a combination of planning policies and
generalisation of those plans. In this paper, we present Expert Iteration
(ExIt), a novel reinforcement learning algorithm which decomposes the problem
into separate planning and generalisation tasks. Planning new policies is
performed by tree search, while a deep neural network generalises those plans.
Subsequently, tree search is improved by using the neural network policy to
guide search, increasing the strength of new plans. In contrast, standard deep
Reinforcement Learning algorithms rely on a neural network not only to
generalise plans, but to discover them too. We show that ExIt outperforms
REINFORCE for training a neural network to play the board game Hex, and our
final tree search agent, trained tabula rasa, defeats MoHex 1.0, the most
recent Olympiad Champion player to be publicly released.Comment: v1 to v2: - Add a value function in MCTS - Some MCTS hyper-parameters
changed - Repetition of experiments: improved accuracy and errors shown.
(note the reduction in effect size for the tpt/cat experiment) - Results from
a longer training run, including changes in expert strength in training -
Comparison to MoHex. v3: clarify independence of ExIt and AG0. v4: see
appendix
Overview of Issues Surrounding Strangeness in the Nucleon
The calculation of the strangeness content of the nucleon and its
experimental verification is a fundamental step in establishing
non-perturbative QCD as the correct theory describing the structure of hadrons.
It holds a role in QCD analogous to the correct calculation of the Lamb shift
in QED. We review the latest developments in the vector and scalar matrix
elements of the strange quarks in the proton, where there has recently been
considerable progress.Comment: Invited presentation at the 10th Conference on the Intersection of
Nuclear and Particle Physics, San Diego, May 26-May 30, 200
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