294,722 research outputs found
Prompt photon processes in photoproduction at HERA
We present results for the photoproduction of inclusive prompt photons and
for prompt photons accompanied by jets, measured with the ZEUS detector at
HERA. Cross sections as a function of pseudorapidity and transverse energy are
presented for 5 5 GeV in the centre of mass
energy range 120-270 GeV. Comparisons are made with predictions from leading
logarithm parton shower Monte Carlos and next-to-leading order QCD calculations
using currently available models of the photon structure. NLO QCD calculations
describe the shape and magnitude of the measurements reasonably well.Comment: Talk given at Photon99 Conference, Freiburg, German
Worldsheet Instantons and the amplitude for string pair production in an external field as a WKB exact functional integral
We revisit the problem of charged string pair creation in a constant external
electric field. The string states are massive and creation of pairs from the
vacuum is a tunnelling process, analogous to the Schwinger process where
charged particle-anti-particle pairs are created by an electric field. We find
the instantons in the worldsheet sigma model which are responsible for the
tunnelling events. We evaluate the sigma model partition function in the
multi-instanton sector in the WKB approximation which keeps the classical
action and integrates the quadratic fluctuations about the solution. We find
that the summation of the result over all multi-instanton sectors reproduces
the known amplitude. This suggests that corrections to the WKB limit must
cancel. To show that they indeed cancel, we identify a fermionic symmetry of
the sigma model which occurs in the instanton sectors and which is associated
with collective coordinates. We demonstrate that the action is symmetric and
that the interaction action is an exact form. These conditions are sufficient
for localization of the worldsheet functional integral onto its WKB limit.Comment: 40 pages; Expanded discussion section, added reference
Quiver Varieties, Category O for Rational Cherednik Algebras, and Hecke Algebras
We relate the representations of the rational Cherednik algebras associated with the complex reflection group Āµā ā Sn to sheaves on Nakajima quiver varieties associated with extended Dynkin graphs via a Z-algebra construction. This is done so that as the parameters defining the Cherednik algebra vary, the stability conditions defining the quiver variety change. This construction motivates us to use the geometry of the quiver varieties to interpret the ordering function (the c-function) used to define a highest weight structure on category O of the Cherednik algebra. This interpretation provides a natural partial ordering on O which we expect will respect the highest weight structure. This partial ordering has appeared in a conjecture of Yvonne on the composition factors in O and so our results provide a small step towards a geometric picture for that. We also interpret geometrically another ordering function (the a-function) used in the study of Hecke algebras. (The connection between Cherednik algebras and Hecke algebras is provided by the KZ-functor.) This is related to a conjecture of BonnafĆ© and Geck on equivalence classes of weight functions for Hecke algebras with unequal parameters since the classes should (and do for type B) correspond to the G.I.T. chambers defining the quiver varieties. As a result anything that can be defined via the quiver varieties
Diversifying academic and professional identities in higher education: some management challenges
This paper draws on an international study of the management challenges arising from diversifying academic and professional identities in higher education. These challenges include, for instance, the introduction of practice-based disciplines with different traditions such as health and social care, the changing aspirations and expectations of younger generations of staff, a diffusion of management responsibilities and structures, and imperatives for a more holistic approach to the "employment package", including new forms of recognition and reward. It is suggested that while academic and professional identities have become increasingly dynamic and multi-faceted, change is occurring at different rates in different contexts. A model is offered, therefore, that relates approaches to "people management" to different organisational environments, against the general background of increasing resource constraint arising from the global economic downturn
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