Gauge fixing using overrelaxation and simulated annealing on GPUs

We adopt CUDA-capable Graphic Processing Units (GPUs) for Coulomb, Landau and maximally Abelian gauge fixing in 3+1 dimensional SU(3) lattice gauge field theories. The local overrelaxation algorithm is perfectly suited for highly parallel architectures. Simulated annealing preconditioning strongly increases the probability to reach the global maximum of the gauge functional. We give performance results for single and double precision. To obtain our maximum performance of ~300 GFlops on NVIDIA's GTX 580 a very fine grained degree of parallelism is required due to the register limits of NVIDIA's Fermi GPUs: we use eight threads per lattice site, i.e., one thread per SU(3) matrix that is involved in the computation of a site update.Comment: 7 pages, 2 figures. Talk presented at the 30th International Symposium on Lattice Field Theory, June 24-29, 2012, Cairns, Australi

cuLGT: Lattice Gauge Fixing on GPUs

We adopt CUDA-capable Graphic Processing Units (GPUs) for Landau, Coulomb and maximally Abelian gauge fixing in 3+1 dimensional SU(3) and SU(2) lattice gauge field theories. A combination of simulated annealing and overrelaxation is used to aim for the global maximum of the gauge functional. We use a fine grained degree of parallelism to achieve the maximum performance: instead of the common 1 thread per site strategy we use 4 or 8 threads per lattice site. Here, we report on an improved version of our publicly available code (www.cuLGT.com and github.com/culgt) which again increases performance and is much easier to include in existing code. On the GeForce GTX 580 we achieve up to 470 GFlops (utilizing 80% of the theoretical peak bandwidth) for the Landau overrelaxation code.Comment: 6 pages, 2 figures, code available on cuLGT.com and github.com/culgt, talk presented at GPU Computing in High Energy Physics, September 10-12, 2014, Pisa, Ital

Strongly Coupled Theories in Lattice Coulomb Gauge

Quantum chromodynamics, despite its simple and elegant formulation at the Lagrangian level and numerous experimental verifications, still poses many interesting questions to particle physicists in the region where perturbation theory breaks down. The origin of confinement of quarks and gluons is one of these big puzzles. Analytic techniques, based on Dyson–Schwinger equations or the variational approach have proven to be useful tools to study the non-perturbative aspects of field theories. Of the latter, the Hamiltonian approach in Coulomb gauge offers an appealing physical interpretation of two-point func- tions of the theory. In recent years, as numerical algorithms improved and more and more compute power became available to the physics community, lattice gauge theory, a fully numerical approach, has become established as the main tool for studies in the non- perturbative sector of field theories. A verification of these different approaches against each other is of great interest to learn about their limitations. In the first part of this work we will study the correlation functions of pure SU(2) Yang– Mills theory at zero and finite temperature. After an introduction to QCD and lattice gauge theory, we will discuss the Gribov problem and investigate a recent proposal to resolve it. Then we will turn on temperature to study the deconfinement phase transition. Based on the center vortex picture of confinement, we will propose an answer to the question why the correlators from lattice gauge theory in Coulomb gauge fail to detect the phase transition. Afterwards we will leave pure Yang–Mills theory and apply our knowledge to the so-called Minimal Walking Technicolor theory, a possible extension to the Standard Model. Finally we discuss how lattice gauge theory applications can be implemented efficiently on graphics processing units used nowadays in high performance computing.Obwohl die Quantenchromodynamik durch eine einfache und elegante Lagrangedichte beschrieben wird und auch experimentell sehr gut bestätigt ist, sind einige interessan- te Fragen, im Energiebereich, der nicht-störungstheoretisch zugänglich ist, noch immer unbeantwortet. Insbesondere die Frage nach dem Ursprung des Farbeinschlusses (Confinement) beschäftigt die theoretische Teilchenphysik seit mehreren Jahrzehnten. Analytische Zugänge, basierend auf Dyson–Schwinger-Gleichungen oder dem Variationsprinzip, sind wichtige Hilfsmittel, um die nicht-störungstheoretischen Eigenschaften von Feldtheorien zu untersuchen. Aus der zweiten Kategorie bietet insbesondere der Hamiltonzugang in Coulombeichung eine schlüssige physikalische Interpretation der Zweipunktfunktionen der Theorie. Auch beflügelt durch die enorme Leistungssteigerung der Computer sowie Fortschritte in numerischen Algorithmen hat sich die Gitterfeldtheorie zur bedeutendsten Technik zur Erforschung des nicht-störungtheoretischen Sektors von Feldtheorien entwickelt. Eine Überprüfung der Resultate aus den verschiedenen Zugängen ist von großem Interesse, um etwaige Beschränkungen der Methoden zu verstehen. Im ersten Teil dieser Arbeit beschäftigen wir uns mit den Korrelationsfunktionen der reinen SU(2) Yang–Mills-Theorie bei Nulltemperatur und bei endlichen Temperaturen. Nach einer Einführung zur QCD und der Gitterfeldtheorie werden wir uns zuerst mit dem Gribovproblem beschäftigen, sowie einem neuen Vorschlag dieses zu beheben. Anschließend werden wir die Theorie bei endlicher Temperatur betrachten, um den Deconfinementphasenübergang zu untersuchen. Basierend auf dem Bild der Zentrumsvortices, einem Modell zur Beschreibung des Confinements, werden wir eine Erklärung finden wieso, der Phasenübergang in Korrelatoren in Coulombeichung auf dem Gitter nicht sichtbar ist. Dann werden wir die reine Eichtheorie verlassen und die sogenannte Minimal Walking Technicolor Theorie betrachten, die eine mögliche Erweiterung des Standardmodells darstellt. Abschließen werden wir diese Arbeit mit einem Kapitel über die effiziente Implementierung von Algorithmen der Gitterfeldtheorie auf Grafikkarten, welche heutzutage als Rechenbeschleuniger im High Performance Computing zur Anwendung kommen

Composite graft replacement of the aortic root in acute dissection

Objective: In acute type A dissection the indication for composite graft replacement of the aortic root and the optimal implantation technique are a matter of debate. In this study early and late results of root replacement in acute dissection are determined and compared with supracoronary graft replacement. Two implantation techniques (open vs. inclusion) are evaluated. Methods: Between 1985 and 1995, 207 consecutive patients (mean age 58±12 years, 78% men) were operated for acute type A dissection of the aorta. Root replacement in 50 patients (inclusion technique in 34/50 patients with Cabrol shunt in 15/34 patients, open technique in 16/50 patients) was compared with more conservative procedures in 157 patients: supracoronary graft replacement in 143 patients (with aortic valve replacement in 23 patients) and local repair without graft interposition in 14 patients. Preoperative risk factors, like hemodynamic instability, renal failure, neurologic disorder and coronary artery disease did not differ in the two treatment groups. Results: Early results, survival and reoperation-free survival after 5 years were insignificantly better after root replacement; mortality 10/50 (20%) vs. 38/157 (24%) P=n.s.; hemorrhage 10/50 (20%) vs. 39/157 (25%) P=n.s.; stroke 5/50 (10%) vs. 27/157 (17%) P=n.s.; survival 70±7% vs. 63±4%, reoperation free survival 92±6% vs. 78±5% P=0.0815). For the open technique, early mortality was 18.8 vs. 20.6%, P=n.s. and reoperation free survival at 5 years was 80.7 vs. 65.2%, P=n.s. Perioperative complications did not differ in the two technical groups and a single pseudoaneurysm occurred in the Bentall group. Conclusion: In acute dissection composite graft replacement of the aortic root can be carried out with good early and late results not inferior to more conservative procedures. The open technique is the implantation method of choice and the modified Bentall technique is indicated in situations with increased risk of bleedin

Surgery for acute type a aortic dissection: comparison of techniques

Objective: In order to determine the optimal surgical strategy for acute ascending aortic dissection, the graft inclusion technique was compared with the open resection technique. Methods: Between 1985 and 1995 a consecutive series of 193 patients (77% male, mean age 58 years) had emergency surgery during a mean interval of 13.2 h after onset of symptoms. Graft replacement of the ascending aorta was performed in all patients (supracoronary graft 143/193=74%, aortic root replacement 50/193=26%, aortic valve replacement 73/193=38%, arch replacement 44/193=20%) The open resection technique was applied in 93 patients and the inclusion technique in 100 patients with a Cabrol-shunt in 26%. Preoperative risk factors were equally distributed between groups (inclusion technique vs. open technique): left ventricular ejection fraction≪45% (13 vs. 2%, not significant (n.s.)), neurological deficit (31 vs. 25%; n.s.), systolic blood pressure≪90 mmHg (20 vs. 15%, n.s.) pericardial tamponade (25 vs. 9%, n.s.), renal failure (6 vs. 4%; n.s.). Results: The overall early mortality was 24%. Following graft inclusion it was 31% compared with 16% in the open technique group (P=0.0154). Postoperative complications (graft inclusion vs. open technique): myocardial infarction (9 vs. 12%, n.s.), low cardiac output (40 vs. 32%, n.s.), reexploration for hemorrhage (23 vs. 25%, n.s.). Survival at 8 years was significantly increased in the open technique group (P=0.0300). Pseudoaneurysm formation occurred in 3% of patients and only after graft inclusion. Freedom from reoperation was 80% at 8 years and did not differ between groups. Graft inclusion was an independent significant predictor of early (P=0.0069; relative risk=2.3673) and late mortality (P=0.0119; relative risk=2.0981). Conclusions: Surgery of acute ascending aortic dissection still carries a considerable early mortality whereas the late outcome is satisfactory. The open resection technique is the method of choice showing superior early and late results and avoiding pseudoaneurysm formation. The inclusion technique may be indicated in situations with increased risk of bleeding. A consequent decompression of the perigraft-space could reduce the rate of pseudoaneurysm