Nikhef Theory Group

Amsterdam, Netherlands

Nikhef Theory Group

Amsterdam, Netherlands
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Tentyukov M.,Karlsruhe Institute of Technology | Vermaseren J.A.M.,NIKHEF Theory Group
Computer Physics Communications | Year: 2010

We present TFORM, the version of the symbolic manipulation system FORM that can make simultaneous use of several processors in a shared memory architecture. The implementation uses Posix threads, also called pthreads, and is therefore easily portable between various operating systems. Most existing FORM programs will be able to take advantage of the increased processing power, without the need for modifications. In some cases some minor additions may be needed. For a computer with two processors a typical improvement factor in the running time is 1.7 when compared to the traditional version of FORM. In the case of computers with 4 processors a typical improvement factor in the execution time is slightly above 3. © 2010 Elsevier B.V. All rights reserved.


Aybat S.M.,Nikhef Theory Group | Prokudin A.,Jefferson Lab | Rogers T.C.,State University of New York at Stony Brook
Physical Review Letters | Year: 2012

The Sivers transverse single spin asymmetry (TSSA) is calculated and compared at different scales using the transverse-momentum-dependent (TMD) evolution equations applied to previously existing extractions. We apply the Collins-Soper-Sterman (CSS) formalism, using the version recently developed by Collins. Our calculations rely on the universality properties of TMD functions that follow from the TMD-factorization theorem. Accordingly, the nonperturbative input is fixed by earlier experimental measurements, including both polarized semi-inclusive deep inelastic scattering (SIDIS) and unpolarized Drell-Yan (DY) scattering. It is shown that recent preliminary COMPASS measurements are consistent with the suppression prescribed by TMD evolution. © 2012 American Physical Society.


de Adelhart Toorop R.,Nikhef Theory Group | Feruglio F.,University of Padua | Hagedorn C.,University of Padua
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2011

We show that a non-vanishing angle θ13 of order 0.1 can be predicted in the framework of discrete flavour symmetries. We assume that left-handed leptons transform as triplets under a group Gf which is broken in such a way that neutrino and charged lepton sectors remain invariant under the subgroups Gν and Ge of Gf, respectively. In this limit mixing angles and the Dirac CP violating phase δ CP are determined. By choosing G f=δ(6n 2) (n=4,8), Gν=Z 2×Z 2 and G e=Z 3 we find sin2θ13=0.045(0.011) for n=4(8). At the same time θ23 and θ12 remain close to their experimental best fit values, particularly in the case n=8, where sin2θ23θ0.424 and sin2θ12θ0.337. δCP is predicted to be 0 or Π so that CP is conserved in our examples. © 2011 Elsevier B.V.


De Adelhart Toorop R.,Nikhef Theory Group | Feruglio F.,University of Padua | Hagedorn C.,University of Padua
Nuclear Physics B | Year: 2012

We study lepton mixing patterns which are derived from finite modular groups Γ N, requiring subgroups G ν and G e to be preserved in the neutrino and charged lepton sectors, respectively. We show that only six groups Γ N with N=3, 4, 5, 7, 8, 16 are relevant. A comprehensive analysis is presented for G e arbitrary and G ν=Z 2×Z 2, as demanded if neutrinos are Majorana particles. We discuss interesting patterns arising from both groups G e and G ν being arbitrary. Several of the most promising patterns are specific deviations from tri-bimaximal mixing, all predicting θ 13 non-zero as favoured by the latest experimental data. We also comment on prospects to extend this idea to the quark sector. © 2012 Elsevier B.V.


Gupta R.K.,NIKHEF Theory Group | Lal S.,Harish Chandra Research Institute
Journal of High Energy Physics | Year: 2012

We calculate the one-loop partition function for a massless arbitrary-spin field on quotients of a general dimensional AdS background using the results of arXiv:1103.3627. We use these results to compute the one-loop partition function for a Vasiliev theory in AdS5. An interesting form of the answer, suggestive of a vacuum character of an enhanced symmetry algebra is obtained. We also observe a close connection between the partition function for this Vasiliev theory and the d-dimensional MacMahon function. © SISSA 2012.


Aybat S.M.,Nikhef Theory Group | Aybat S.M.,VU University Amsterdam | Rogers T.C.,VU University Amsterdam
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We assess the current phenomenological status of transverse momentum dependent (TMD) parton distribution functions (PDFs) and fragmentation functions (FFs) and study the effect of consistently including perturbative QCD (pQCD) evolution. Our goal is to initiate the process of establishing reliable, QCD-evolved parametrizations for the TMD PDFs and TMD FFs that can be used both to test TMD factorization and to search for evidence of the breakdown of TMD factorization that is expected for certain processes. In this article, we focus on spin-independent processes because they provide the simplest illustration of the basic steps and can already be used in direct tests of TMD factorization. Our calculations are based on the Collins-Soper-Sterman (CSS) formalism, supplemented by recent theoretical developments which have clarified the precise definitions of the TMD PDFs and TMD FFs needed for a valid TMD-factorization theorem. Starting with these definitions, we numerically generate evolved TMD PDFs and TMD FFs using as input existing parametrizations for the collinear PDFs, collinear FFs, nonperturbative factors in the CSS factorization formalism, and recent fixed-scale fits. We confirm that evolution has important consequences, both qualitatively and quantitatively, and argue that it should be included in future phenomenological studies of TMD functions. Our analysis is also suggestive of extensions to processes that involve spin-dependent functions such as the Boer-Mulders, Sivers, or Collins functions, which we intend to pursue in future publications. At our website, we have made available the tables and calculations needed to obtain the TMD parametrizations presented herein. © 2011 American Physical Society.


Butter D.,Nikhef Theory Group
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

We present a new formulation of curved projective superspace. The 4D N=2 supermanifold M4|8 (four bosonic and eight Grassmann coordinates) is extended by an auxiliary SU(2) manifold, which involves introducing a vielbein and related connections on the full M7|8=M4|8×SU(2). Constraints are chosen so that it is always possible to return to the central basis where the auxiliary SU(2) manifold largely decouples from the curved manifold M4|8 describing 4D N=2 conformal supergravity. We introduce the relevant projective superspace action principle in the analytic subspace of M7|8 and construct its component reduction in terms of a five-form J living on M4×C, with C a contour in SU(2). This approach is inspired by and generalizes the original approach, which can be identified with a complexified version of the central gauge of the formulation presented here. © 2015 American Physical Society.


Butter D.,Nikhef Theory Group
Journal of High Energy Physics | Year: 2015

Abstract: Projective superspace provides a natural framework for the construction of actions coupling hypermultiplets to conformal supergravity. We review how the off-shell actions are formulated in superspace and then discuss how to eliminate the infinite number of auxiliary fields to produce an on-shell N=2$$ \mathcal{N}=2 $$ supersymmetric sigma model, with the target space corresponding to a generic 4n-dimensional hyperkähler cone. We show how the component action coupling the hypermultiplets to conformal supergravity may be constructed starting from curved superspace. The superspace origin of the geometric data — the hyperkähler potential, complex structures, and any gauged isometries — is also addressed. © 2015, The Author(s).


Gupta R.K.,NIKHEF Theory Group | Murthy S.,NIKHEF Theory Group
Journal of High Energy Physics | Year: 2013

We find the most general bosonic solution to the localization equations describing the contributions to the quantum entropy of supersymmetric black holes in four dimensional N = 2 supergravity coupled to nv vector multiplets. This requires the analysis of the BPS equations of the corresponding off-shell supergravity (including fluctuations of the auxiliary fields) with AdS2 × S2 attractor boundary conditions. Our work completes and extends the results of arXiv:1012.0265 that were obtained for the vector multiplet sector, to include the fluctuations of all the fields of the off-shell supergravity. We find that, when the auxiliary SU(2) gauge field strength vanishes, the most general supersymmetric configuration preserving four supercharges is labelled by nv + 1 real parameters corresponding to the excitations of the conformal mode of the graviton and the scalars of the nv vector multiplets. In the general case, the localization manifold is labelled by an additional SU(2) triplet of one-forms and a scalar function. © 2013 SISSA.


George D.P.,Nikhef Theory Group
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

Models with an extra dimension generally contain background scalar fields in a nontrivial configuration, whose stability must be ensured. With gravity present, the extra dimension is warped by the scalars, and the spin-0 degrees of freedom in the metric mix with the scalar perturbations. Where possible, we formally solve the coupled Schrödinger equations for the zero modes of these spin-0 perturbations. When specializing to the case of two scalars with a potential generated by a superpotential, we are able to fully solve the system. We show how these zero modes can be used to construct a solution matrix, whose eigenvalues tell whether a normalizable zero mode exists, and how many negative mass modes exist. These facts are crucial in determining stability of the corresponding background configuration. We provide examples of the general analysis for domain-wall models of an infinite extra dimension and domain-wall soft-wall models. For five-dimensional models with two scalars constructed using a superpotential, we show that a normalizable zero mode survives, even in the presence of warped gravity. Such models, which are widely used in the literature, are therefore phenomenologically unacceptable. © 2011 American Physical Society.

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