Biomedical subjects

Urs Achim Wiedemann

Publications and source records attributed to Urs Achim Wiedemann.

7 recordsLinked to original sources

PubMed2006

Calculating the jet quenching parameter.

Models of medium-induced radiative parton energy loss account for the strong suppression of high-p(T) hadron spectra in square root of (S)NN=200 GeV Au-Au collisions at BNL RHIC in terms of a single "jet quenching parameter" q. We observe that q can be given a model-independent, nonperturbative, quantum field theoretic definition in terms of the short-distance behavior of a particular lightlike Wilson loop. We then use the anti-de Sitter/conformal-field-theory correspondence to obtain a strong-coupling calculation of q in hot N=4 supersymmetric QCD, finding q(SYM)=26.69 square root of alpha(SYM)N(c)T(3) in the limit in which both N(c) and 4pialpha(SYM)N(c) are large. Thus, at strong coupling q is not proportional to the entropy density s, or to some "number density of scatterers" since, unlike the number of degrees of freedom, q does not grow like N(c)(2).

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PubMed2005

Relating high-energy Lepton-Hadron, proton-nucleus, and nucleus-nucleus collisions through geometric scaling.

A characteristic feature of small-x lepton-proton data from HERA is geometric scaling: the fact that in the region of small Bjorken variable x, x less, similar 0.01, all data can be described by a single variable Q(2)/Q(2)(s,p)(x), with all x dependence encoded in the so-called saturation momentum Q(s,p)(x). Here, we observe that the same scaling ansatz accounts for nuclear photoabsorption cross sections and favors the nuclear dependence Q(2)(s,A) proportional, variant A(alpha)Q(2)(s,p), alpha approximately 4/9. We then make the empirical finding that the same A dependence accounts for the centrality evolution of the multiplicities measured in Au+Au collisions at RHIC. It also allows one to parametrize the high-p(t) particle suppression in d+Au collisions at forward rapidities. If these geometric scaling properties have a common dynamical origin, then this A dependence of Q(2)(s,A) should emerge as a consequence of the underlying dynamical model.

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PubMed2004

Measuring the collective flow with jets.

In nucleus-nucleus collisions, high-p(T) partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components can be disentangled by the measurement of particle production associated with high-p(T) trigger particles, jetlike correlations, and jets. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the eta x phi plane. We argue that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.

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PubMed2004

Medium modification of jet shapes and jet multiplicities.

Medium-induced parton energy loss is widely considered to underlie the suppression of high-pt leading hadron spectra in square root sNN = 200 GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Its description implies a characteristic kt broadening of the subleading hadronic fragments associated with the hard parton. However, this latter effect is more difficult to measure and has remained elusive so far. Here, we discuss how it affects genuine jet observables, which are accessible at the Large Hadron Collider and possibly at RHIC. We find that the kt broadening of jet multiplicity distributions provides a very sensitive probe of the properties of dense QCD matter, whereas the sensitivity of jet energy distributions is much weaker. In particular, the sensitive kinematic range of jet multiplicity distributions is almost unaffected by the high multiplicity background.

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PubMed2004

Energy dependence of the Cronin effect from nonlinear QCD evolution.

The nonlinear evolution of dense partonic systems has been suggested as a novel physics mechanism relevant for the dynamics of p-A and A-A collisions at collider energies. Here we study to what extent the description of Cronin enhancement in the framework of this nonlinear evolution is consistent with the recent observation in sqrt[s]=200 GeV d-Au collisions at the Relativistic Heavy Ion Collider. We solve the Balitsky-Kovchegov evolution equation numerically for several initial conditions encoding Cronin enhancement. We find that the properly normalized nuclear gluon distribution is suppressed at all momenta relative to that of a single nucleon. For the resulting spectrum of produced gluons in p-A and A-A collisions, the nonlinear QCD evolution is unable to generate a Cronin-type enhancement, and it quickly erases any such enhancement which may be present at lower energies.

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PubMed2003

Timing of network synchronization by refractory mechanisms.

Even without active pacemaker mechanisms, temporally patterned synchronization of neural network activity can emerge spontaneously and is involved in neural development and information processing. Generation of spontaneous synchronization is thought to arise as an alternating sequence between a state of elevated excitation followed by a period of quiescence associated with neuronal and/or synaptic refractoriness. However, the cellular factors controlling recruitment and timing of synchronized events have remained difficult to specify, although the specific temporal pattern of spontaneous rhythmogenesis determines its impact on developmental processes. We studied spontaneous synchronization in a model of 600-1,000 integrate-and-fire neurons interconnected with a probability of 5-30%. One-third of neurons generated spontaneous discharges and provided a background of intrinsic activity to the network. The heterogeneity and random coupling of these neurons maintained this background activity asynchronous. Refractoriness was modeled either by use-dependent synaptic depression or by cellular afterhyperpolarization. In both cases, the recruitment of neurons into spontaneous synchronized discharges was determined by the interplay of refractory mechanisms with stochastic fluctuations in background activity. Subgroups of easily recruitable neurons served as amplifiers of these fluctuations, thereby initiating a cascade-like recruitment of neurons ("avalanche effect"). In contrast, timing depended on the precise implementation of neuronal refractoriness and synaptic connectivity. With synaptic depression, neuronal synchronization always occurred stochastically, whereas with cellular afterhyperpolarization, stochastic turned into periodic behavior with increasing synaptic strength. These results associate the type of refractory mechanism with the temporal statistics and the mechanism of synchronization, thereby providing a framework for differentiating between cellular mechanisms of spontaneous rhythmogenesis.

Action Potentials↗

PubMed2002

Dynamical scaling law for jet tomography.

Medium modifications of parton fragmentation provide a novel tomographic tool for the study of the hot and dense matter created in ultrarelativistic nucleus-nucleus collisions. Their quantitative analysis, however, is complicated by the strong dynamical expansion of the collision region. Here we establish for the multiple scattering induced gluon radiation spectrum a scaling law which relates medium effects in a collision of arbitrary dynamical expansion to that in an equivalent static scenario. Based on this scaling, we calculate for typical kinematical values of the RHIC and LHC heavy ion programming medium-modified fragmentation functions for collisions with realistic dynamical expansion.

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