In the last 30 days

• Open/close PYTHIA 6.4 physics and manual

  Torbjörn Sjöstrand et al JHEP05(2006)026 Tag this article

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  The Pythia program can be used to generate high-energy-physics `events', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, within and beyond the Standard Model, with emphasis on those where strong interactions play a rôle, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics input is summarized here, for areas such as hard subprocesses, initial- and final-state parton showers, underlying events and beam remnants, fragmentation and decays, and much more. Furthermore, extensive information is provided on all program elements: subroutines and functions, switches and parameters, and particle and process data. This should allow the user to tailor the generation task to the topics of interest.

  The code and further information may be found on the Pythia web page: http://www.thep.lu.se/~torbjorn/Pythia.html .

• Open/close SL(2,)-invariant IIB brane actions

  Eric A. Bergshoeff et al JHEP02(2007)007 Tag this article

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  We give a universal SL(2, )-invariant expression for all IIB p-brane actions with p = −1,1,3,5,7,9. The Wess-Zumino terms in the brane actions are determined by requiring (i) target space gauge invariance and (ii) the presence of a single Born-Infeld vector. We find that for p = 7 ( p = 9) brane actions with these properties only exist for orbits that contain the standard D7-brane (D9-brane). We comment about the actions for the other orbits.
• Open/close The anti-k_t jet clustering algorithm

  Matteo Cacciari et al JHEP04(2008)063 Tag this article

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  The k _t and Cambridge/Aachen inclusive jet finding algorithms for hadron-hadron collisions can be seen as belonging to a broader class of sequential recombination jet algorithms, parametrised by the power of the energy scale in the distance measure. We examine some properties of a new member of this class, for which the power is negative. This ``anti- k <sub>t</sub> '' algorithm essentially behaves like an idealised cone algorithm, in that jets with only soft fragmentation are conical, active and passive areas are equal, the area anomalous dimensions are zero, the non-global logarithms are those of a rigid boundary and the Milan factor is universal. None of these properties hold for existing sequential recombination algorithms, nor for cone algorithms with split-merge steps, such as SISCone. They are however the identifying characteristics of the collinear unsafe plain ``iterative cone'' algorithm, for which the anti- k _t algorithm provides a natural, fast, infrared and collinear safe replacement.
• Open/close An Approach to the Equivalence Theorem by the Slavnov-Taylor Identities

  Ruggero Ferrari et al JHEP04(2002)033 Tag this article

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  We discus the Equivalence Theorem (ET) in the BRST formalism. In particular the Slavnov-Taylor (ST) identities, derived at the formal level in the path-integral approach, are considered at the quantum level and are shown to be always anomaly free. Some discussion is devoted to the transformation of the fields; in fact the existence of a local inverse (at least as a formal power expansion) suggests a formulation of the ET, which allows a nilpotent BRST symmetry. This strategy cannot be implemented at the quantum level if the inverse is non-local. In this case we propose an alternative formulation of the ET, where, by using Faddeev-Popov fields, this difficulty is circumvented. In fact this approach allows the loop expansion both in the original and in the transformed theory. In this case the algebraic formulation of the problem can be simplified by introducing some auxiliary fields. The auxiliary fields can be eliminated by using the method of Batalin-Vilkovisky. We study the quantum deformation of the associated ST identity and show that a selected set of Green functions, which in some cases can be identified with the physical observables of the model, does not depend on the choice of the transformation of the fields. The computation of the cohomology for the classical linearized ST operator is performed by purely algebraic methods. We do not rely on power-counting arguments. In general the transformation of the fields yields a non-renormalizable theory. When the equivalence is established between a renormalizable and a non-renormalizable theory, the ET provides a way to give a meaning to the last one by using the resulting ST identity. In this case the Quantum Action Principle cannot be of any help in the discussion of the ET. We assume and discuss the validity of a Quasi Classical Action Principle, which turns out to be sufficient for the present work. As an example we study the renormalizability and unitarity of massive QED in Proca's gauge by starting from a linear Lorentz-covariant gauge.

• Open/close Non-singular solutions to Einstein-Klein-Gordon equations with a phantom scalar field

  Vladimir Dzhunushaliev et al JHEP07(2008)094 Tag this article

  Article References Full text PDF (214 KB)

  It is shown that the 4D Einstein-Klein-Gordon equations with a phantom scalar field (a scalar field with a negative sign in front of the kinetic energy term of its Lagrange density) has non-singular, spherically symmetry solutions. These solutions have a combination of features found in other spherically symmetric gravity plus field solutions. A stability analysis on these solutions indicates they are unstable.
• Open/close String theory and noncommutative geometry

  Nathan Seiberg and Edward Witten JHEP09(1999)032 Tag this article

  Article References Full text PDF (995 KB)

  We extend earlier ideas about the appearance of noncommutative geometry in string theory with a nonzero B-field. We identify a limit in which the entire string dynamics is described by a minimally coupled (supersymmetric) gauge theory on a noncommutative space, and discuss the corrections away from this limit. Our analysis leads us to an equivalence between ordinary gauge fields and noncommutative gauge fields, which is realized by a change of variables that can be described explicitly. This change of variables is checked by comparing the ordinary Dirac-Born-Infeld theory with its noncommutative counterpart. We obtain a new perspective on noncommutative gauge theory on a torus, its T-duality, and Morita equivalence. We also discuss the D0/ D4 system, the relation to M-theory in DLCQ, and a possible noncommutative version of the six-dimensional (2,0) theory.

• Open/close Recursion relations, generating functions, and unitarity sums in = 4 SYM theory

  Henriette Elvang et al JHEP04(2009)009 Tag this article

  Article References Full text PDF (560 KB)

  We prove that the MHV vertex expansion is valid for any NMHV tree amplitude of = 4 SYM. The proof uses induction to show that there always exists a complex deformation of three external momenta such that the amplitude falls off at least as fast as 1/ z for large z. This validates the generating function for n-point NMHV tree amplitudes. We also develop generating functions for anti-MHV and anti-NMHV amplitudes. As an application, we use these generating functions to evaluate several examples of intermediate state sums on unitarity cuts of 1-, 2-, 3- and 4-loop amplitudes. In a separate analysis, we extend the recent results of arXiv:0808.0504 to prove that there exists a valid 2-line shift for any n-point tree amplitude of = 4 SYM. This implies that there is a BCFW recursion relation for any tree amplitude of the theory.
• Open/close Recursive numerical calculus of one-loop tensor integrals

  Francisco del Aguila and Roberto Pittau JHEP07(2004)017 Tag this article

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  A numerical approach to compute tensor integrals in one-loop calculations is presented. The algorithm is based on a recursion relation which allows to express high rank tensor integrals as a function of lower rank ones. At each level of iteration only inverse square roots of Gram determinants appear. For the phase-space regions where Gram determinants are so small that numerical problems are expected, we give general prescriptions on how to construct reliable approximations to the exact result without performing Taylor expansions. Working in 4+ dimensions does not require an analytic separation of ultraviolet and infrared/collinear divergences, and, apart from trivial integrals that we compute explicitly, no additional ones besides the standard set of scalar one-loop integrals are needed.

• Open/close Recursion relations in CFT and N=2 SYM theory

  Rubik Poghossian JHEP12(2009)038 Tag this article

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  Based on prototypical example of Al. Zamolodchikov's recursion relations for the four point conformal block and using recently proposed Alday-Gaiotto-Tachikawa (AGT) conjecture, recursion relations are derived for the generalized prepotential of = 2 SYM with f = 0,1,2,3,4 (anti) fundamental or an adjoint hypermultiplets. In all cases the large expectation value limit is derived explicitly. A precise relationship between generic 1-point conformal block on torus and specific 4-point conformal block on sphere is established. In view of AGT conjecture this translates into a relation between partition functions with an adjoint and 4 fundamental hypermultiplets.
• Open/close = 6 superconformal Chern-Simons-matter theories, M2-branes and their gravity duals

  Ofer Aharony et al JHEP10(2008)091 Tag this article

  Article References Full text PDF (467 KB)

  We construct three dimensional Chern-Simons-matter theories with gauge groups U( N) × U( N) and SU( N) × SU( N) which have explicit = 6 superconformal symmetry. Using brane constructions we argue that the U( N) × U( N) theory at level k describes the low energy limit of N M2-branes probing a C ⁴/ Z _k singularity. At large N the theory is then dual to M-theory on AdS ₄ × S ⁷/ Z _k . The theory also has a 't Hooft limit (of large N with a fixed ratio N/ k) which is dual to type IIA string theory on AdS ₄ × CP ³. For k = 1 the theory is conjectured to describe N M2-branes in flat space, although our construction realizes explicitly only six of the eight supersymmetries. We give some evidence for this conjecture, which is similar to the evidence for mirror symmetry in d = 3 gauge theories. When the gauge group is SU(2) × SU(2) our theory has extra symmetries and becomes identical to the Bagger-Lambert theory.