field-theory.org
the blog of wolfram schroers

Research interests

Table of contents:

Research interests and activities
Collaborations and affiliates
Scientific software downloads

Research interests and activities

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My primary field of study is hadron structure, with an emphasis on generalized parton distributions (GPDs) and form factors. As a member of the QCDSF and LHPC collaborations I have pioneered the first calculation of moments of GPDs on the lattice.

I am also interested in algorithmic research and have been working on several problems in these fields.

My previous activities have concentrated both on hadron structure using phenomenological methods and on selected topics of computational science to benefit from developments which might turn out to be valuable to my primary research interests.

Generalized parton distributions

GPDs express several important aspects of hadron structure, like the transverse shape, spin content, and a variety of exclusive reactions. They reduce to form factors and moments of parton distributions in certain limits. While the latter quantities are accessible to contemporary experiments, it is unlikely that a similarly complete experimental study of GPDs is possible due to their complex structure. Lattice simulations, on the other hand, provide a model-independent way to study these functions and for the first time put lattice theorists in a position to genuinely predict rather than postdict essential properties of hadron structure.

I am currently involved in the two major studies aiming to compute moments of GPDs in lattice simulations. One is being conducted by the QCDSF collaboration, the other by the LHPC collaboration as a part of the SciDAC initiative.

Nucleon electromagnetic form factors

A special case application of the techniques for determining generalized parton distributions are form factors. They are of special interest both to phenomenology and experiment and are among the most basic quantities describing a given hadron. A lot is already known about the form factors of the proton and the neutron. But even in these cases there have been surprising new insights — like their asymptotic scaling behavior — in the recent years. These activities have been triggered by new experiments at Jefferson Lab. With the spin-polarization transfer measurement the ratio of the two nucleon form factor has been extracted with previously unmatched accuracy. Together with the QCDSF and the LHPC collaborations I am working on unraveling this mystery using lattice techniques.

Electromagnetic transition form-factors

While GPDs are able to provide insight into the transverse structure of the nucleon, they do not teach us about a potential aspherical structure. To assess such properties, we must also extend our studies to the case of transition form factors which involve higher states like the Delta resonance.

In a series of publications under the leadership of C. Alexandrou at Cyprus University, other members of the LHPC collaboration and myself have found that the nucleon-Delta system is indeed deformed. Our lattice calculations find the same sign and order of magnitude of the quadrupole form factors that experimentalists observe. A remaining discrepancy might be due to chiral symmetry or unquenching. This should be resolved in the near future.

Hadron distribution amplitudes

Hadronic wave functions are of crucial importance when describing exclusive and semi-exclusive reactions. However, only very limited information is available from experiment and our knowledge mostly comes from model calculations. Even where experimental data is available, reasonable accuracy can only be achieved by taking recourse to models of the strong interaction.

Recently, together with the QCDSF collaboration I have succeeded in computing the first two moments of the distribution amplitudes of the pion and the kaon. For the first time, the first moment of the kaon distribution amplitude has been obtained without model assumptions.

Algorithms for unquenched QCD simulations

The topic of my PhD thesis was the implementation and investigation of a new flavor of simulation scheme for dynamical fermion simulations, the multiboson algorithm (MBA). The MBA relies on local updating schemes rather than global updates like the standard algorithm used so far in unquenched simulations, the hybrid Monte-Carlo algorithms (HMC).

A major finding was that indeed the MBA becomes superior to the HMC when going to relatively light quark masses. This feature could turn out to be useful in future simulations.

Analytic coupling and sudakov effects

To enlarge my understanding of hadronic physics I have also engaged in a phenomenological project to predict electromagnetic form factors using a factorization ansatz. The particular feature of this calculation was that the coupling entering the sudakov contribution was chosen to be analytic instead of singular at ΛQCD. The major result was that the analytic coupling did indeed preserve the factorization ansatz while providing a superior agreement with the data and excellent stability in the infrared region.

Hypersystolic algorithms on parallel computers

For the implementation of O(n2) problems, i.e., problems with mutual interactions of n bodies, the use of parallel computers is an absolute necessity. These problems are dominated by computation if the number of bodies is large compared to the number of nodes available to a given machine. If the number of nodes is large, however, the problem becomes dominated by communication. Naively, the communication between the nodes is an O(n) problem. However, it can be reduced to only logarithmic complexity by the use of hypersystolic routing schemes. I have developed a standard benchmark which is used for evaluating architectures beyond the specific requirements of lattice simulations.

It can be downloaded from the software section.

Instantons on the lattice

There is an ongoing debate on whether gauge field configurations dominated by instantons, i.e., solutions of the classical field equations, contribute substantially to the path integral. The lattice is an ideal testing ground for this idea. We scrutinize one particular prediction of the instanton model against lattice data and find the prediction to be confirmed.

Spectroscopy of light hadrons

The current challenge of lattice simulations is the usage of light quarks. I have co-authored an exploratory investigation of the light hadron spectrum with three dynamical light quarks which utilizes the MBA algorithm as a starting point. This algorithm was also the subject of my PhD thesis.

With light quark masses, the influence of virtual quark loops also becomes a concern. This influence is most dominant in the mass of the η′ meson when compared to the light pseudo-scalar meson octet. I have also co-authored an investigation of this quantity.

Anomalous dimension spectrum of trilinear quark operators

The anomalous dimensions of field operators are crucial for determining the scale dependence of hadronic wave functions in QCD. Together with two collaborators from Bochum University I have succeeded in computing the dimensions of trilinear twist-3 quark operators of the leading order one-gluon exchange kernel without simplifying assumptions. It is of particular interest that the large-order behavior could be extracted and a gap in the spectrum could indicate the presence of a scalar diquark in the system. However, this feature may not survive once higher order corrections are included.

Influence of self-consistency on the electroweak sphaleron configuration

The topic of my diploma thesis was the investigation of the electroweak sphaleron configuration. Particular emphasis was placed on the influence of the fermion determinant. It turned out that after a careful investigation including correct renormalization that the effect of the fermion determinant is small and the effect of self-consistency is almost negligible. The absolute value of the sphaleron barrier is important for a prediction of an upper limit of the Higgs mass based on the matter-antimatter asymmetry in our universe.

Collaborations and affiliates

I am a senior leading member of the LHPC collaboration as a part of the US National SciDAC initiative. Furthermore, I have been a member of the QCDSF collaboration.

I have received support by the following organizations and companies:

DFG AvH Foundation Abitz.com

Scientific software downloads

Scientific software can be downloaded on the software section of this site.

Note: The software on this site is distributed according to either the GNU Public License or the proprietary licensing conditions (English) and Lizenzbedingungen (Deutsch) for the remainder of this web site. In particular, there are NO WARRANTIES of any kind — neither explicit nor implied! If such regulations are in conflict with your local laws, you are denied the usage of this software.