Showing papers in "Journal of High Energy Physics in 2010"

A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX

Abstract: In this work we illustrate the POWHEG BOX, a general computer code framework for implementing NLO calculations in shower Monte Carlo programs according to the POWHEG method. Aim of this work is to provide an illustration of the needed theoretical ingredients, a view of how the code is organized and a description of what a user should provide in order to use it.

Dimension-Six Terms in the Standard Model Lagrangian

Abstract: When the Standard Model is considered as an effective low-energy theory, higher dimensional interaction terms appear in the Lagrangian. Dimension-six terms have been enumerated in the classical article by Buchmuller and Wyler [3]. Although redundance of some of those operators has been already noted in the literature, no updated complete list has been published to date. Here we perform their classification once again from the outset. Assuming baryon number conservation, we find 15 + 19 + 25 = 59 independent operators (barring flavour structure and Hermitian conjugations), as compared to 16 + 35 + 29 = 80 in ref. [3]. The three summed numbers refer to operators containing 0, 2 and 4 fermion fields. If the assumption of baryon number conservation is relaxed, 5 new operators arise in the four-fermion sector.

Exact results for Wilson loops in superconformal Chern-Simons theories with matter

Abstract: We use localization techniques to compute the expectation values of supersymmetric Wilson loops in Chern-Simons theories with matter. We find the path-integral reduces to a non-Gaussian matrix model. The Wilson loops we consider preserve a single complex supersymmetry, and exist in any N = 2 theory, though the localization requires superconformal symmetry. We present explicit results for the cases of pure Chern-Simons theory with gauge group U(N), showing agreement with the known results, and ABJM, showing agreement with perturbative calculations. Our method applies to other theories, such as Gaiotto-Witten theories, BLG, and their variants.

Freeze-in production of FIMP dark matter

Abstract: We propose an alternate, calculable mechanism of dark matter genesis, “thermal freeze-in”, involving a Feebly Interacting Massive Particle (FIMP) interacting so feebly with the thermal bath that it never attains thermal equilibrium. As with the conventional “thermal freeze-out” production mechanism, the relic abundance reflects a combination of initial thermal distributions together with particle masses and couplings that can be measured in the laboratory or astrophysically. The freeze-in yield is IR dominated by low temperatures near the FIMP mass and is independent of unknown UV physics, such as the reheat temperature after inflation. Moduli and modulinos of string theory compactifications that receive mass from weak-scale supersymmetry breaking provide implementations of the freeze-in mechanism, as do models that employ Dirac neutrino masses or GUT-scale-suppressed interactions. Experimental signals of freeze-in and FIMPs can be spectacular, including the production of new metastable coloured or charged particles at the LHC as well as the alteration of big bang nucleosynthesis.

Generalized metric formulation of double field theory

Abstract: The generalized metric is a T-duality covariant symmetric matrix constructed from the metric and two-form gauge field and arises in generalized geometry. We view it here as a metric on the doubled spacetime and use it to give a simple formulation with manifest T-duality of the double field theory that describes the massless sector of closed strings. The gauge transformations are written in terms of a generalized Lie derivative whose commutator algebra is defined by a double field theory extension of the Courant bracket.

Is there still any Tc mystery in lattice QCD? Results with physical masses in the continuum limit III

Abstract: The present paper concludes our investigations on the QCD cross-over transi- tion temperatures with 2+1 staggered avours and one-link stout improvement. We extend our previous two studies (Phys. Lett. B643 (2006) 46, JHEP 0906:088 (2009)) by choosing even ner lattices ( Nt=16) and we work again with physical quark masses. The new results on this broad cross-over are in complete agreement with our earlier ones. We compare our ndings with the published results of the hotQCD collaboration. All these results are con- fronted with the predictions of the Hadron Resonance Gas model and Chiral Perturbation Theory for temperatures below the transition region. Our results can be reproduced by using the physical spectrum in these analytic calculations. The ndings of the hotQCD collaboration can be recovered by using a distorted spectrum which takes into account lat- tice discretization artifacts and heavier than physical quark masses. This analysis provides a simple explanation for the observed discrepancy in the transition temperatures between our and the hotQCD collaborations.

Observation of long-range, near-side angular correlations in proton-proton collisions at the LHC

Abstract: This is the pre-print version of the Published Article, which can be accessed from the link below - Copyright @ 2010 Springer Verlag

Properties and uses of the Wilson flow in lattice QCD

Abstract: Theoretical and numerical studies of the Wilson flow in lattice QCD suggest that the gauge field obtained at flow time t > 0 is a smooth renormalized field. The expectation values of local gauge-invariant expressions in this field are thus well-defined physical quantities that probe the theory at length scales on the order of \( \sqrt {t} \). Moreover, by transforming the QCD functional integral to an integral over the gauge field at a specified flow time, the emergence of the topological (instanton) sectors in the continuum limit becomes transparent and is seen to be caused by a dynamical effect that rapidly separates the sectors when the lattice spacing is reduced from 0.1fm to smaller values.

The QCD equation of state with dynamical quarks

Abstract: The present paper concludes our investigation on the QCD equation of state with 2+1 staggered flavors and one-link stout improvement. We extend our previous study (JHEP 0601:089 (2006)) by choosing even finer lattices. Lattices with Nt = 6,8 and 10 are used, and the continuum limit is approached by checking the results at Nt = 12. A Symanzik improved gauge and a stout-link improved staggered fermion action is utilized. We use physical quark masses, that is, for the lightest staggered pions and kaons we fix the m�/fK and mK/fK ratios to their experimental values. The pressure, the interaction measure, the energy and entropy density and the speed of sound are presented as functions of the temperature in the range 100...1000MeV. We give estimates for the pion mass dependence and for the contribution of the charm quark. We compare our data to the equation of state obtained by the "hotQCD" collaboration.

Aspects of the BMS/CFT correspondence

Abstract: After a review of symmetries and classical solutions involved in the AdS3/CFT2 correspondence, we apply a similar analysis to asymptotically flat spacetimes at null infinity in 3 and 4 dimensions. In the spirit of two dimensional conformal field theory, the symmetry algebra of asymptotically flat spacetimes at null infinity in 4 dimensions is taken to be the semi-direct sum of supertranslations with infinitesimal local conformal transformations and not, as usually done, with the Lorentz algebra. As a first application, we derive how the symmetry algebra is realized on solution space. In particular, we work out the behavior of Bondi’s news tensor, mass and angular momentum aspects under local conformal transformations.

Effective Holographic Theories for low-temperature condensed matter systems

Abstract: The IR dynamics of effective holographic theories capturing the interplay between charge density and the leading relevant scalar operator at strong coupling are analyzed. Such theories are parameterized by two real exponents (γ, δ) that control the IR dynamics. By studying the thermodynamics, spectra and conductivities of several classes of charged dilatonic black hole solutions that include the charge density back reaction fully, the landscape of such theories in view of condensed matter applications is characterized. Several regions of the (γ, δ) plane can be excluded as the extremal solutions have unacceptable singularities. The classical solutions have generically zero entropy at zero temperature, except when γ = δ where the entropy at extremality is finite. The general scaling of DC resistivity with temperature at low temperature, and AC conductivity at low frequency and temperature across the whole (γ, δ) plane, is found. There is a codimension-one region where the DC resistivity is linear in the temperature. For massive carriers, it is shown that when the scalar operator is not the dilaton, the DC resistivity scales as the heat capacity (and entropy) for planar (3d) systems. Regions are identified where the theory at finite density is a Mott-like insulator at T = 0. We also find that at low enough temperatures the entropy due to the charge carriers is generically larger than at zero charge density.

Asymptotic symmetries of three-dimensional gravity coupled to higher-spin fields

Abstract: We discuss the emergence of $ \mathcal{W} \mbox{-algebras}$ as asymptotic symmetries of higher-spin gauge theories coupled to three-dimensional Einstein gravity with a negative cosmological constant. We focus on models involving a finite number of bosonic higher-spin fields, and especially on the example provided by the coupling of a spin-3 field to gravity. It is described by a SL(3) × SL(3) Chern-Simons theory and its asymptotic symmetry algebra is given by two copies of the classical $ {\mathcal{W}_3}\mbox{-algebra} $ with central charge the one computed by Brown and Henneaux in pure gravity with negative cosmological constant.

Combined Measurement and QCD Analysis of the Inclusive e± p Scattering Cross Sections at HERA

Abstract: A combination is presented of the inclusive deep inelastic cross sections measured by the H1 and ZEUS Collaborations in neutral and charged current unpolarised e(+/-)p scattering at HERA during the period 1994-2000. The data span six orders of magnitude in negative four-momentum-transfer squared, Q(2), and in Bjorken x. The combination method used takes the correlations of systematic uncertainties into account, resulting in an improved accuracy. The combined data are the sole input in a NLO QCD analysis which determines a new set of parton distributions, HERAPDF1.0, with small experimental uncertainties. This set includes an estimate of the model and parametrisation uncertainties of the fit result.

What is the simplest quantum field theory

Abstract: Conventional wisdom says that the simpler the Lagrangian of a theory the simpler its perturbation theory. An ever-increasing understanding of the structure of scattering amplitudes has however been pointing to the opposite conclusion. At tree level, the BCFW recursion relations that completely determine the S-matrix are valid not for scalar theories but for gauge theories and gravity, with gravitational amplitudes exhibiting the best UV behavior at infinite complex momentum. At 1-loop, amplitudes in $ \mathcal{N} = 4 $ SYM only have scalar box integrals, and it was recently conjectured that the same property holds for $ \mathcal{N} = 8 $ SUGRA, which plays an important role in the suspicion that this theory may be finite. In this paper we explore and extend the S-matrix paradigm, and suggest that $ \mathcal{N} = 8 $ SUGRA has the simplest scattering amplitudes in four dimensions. Labeling external states by supercharge eigenstates-Grassmann coherent states-allows the amplitudes to be exposed as completely smooth objects, with the action of SUSY manifest. We show that under the natural supersymmetric extension of the BCFW deformation of momenta, all tree amplitudes in $ \mathcal{N} = 4 $ SYM and $ \mathcal{N} = 8 $ SUGRA vanish at infinite complex momentum, and can therefore be determined by recursion relations. An important difference between $ \mathcal{N} = 8 $ SUGRA and $ \mathcal{N} = 4 $ SYM is that the massless S-matrix is defined everywhere on moduli space, and is acted on by a non-linearly realized E 7(7) symmetry. We elucidate how non-linearly realized symmetries are reflected in the more familiar setting of pion scattering amplitudes, and go on to identify the action of E 7(7) on amplitudes in $ \mathcal{N} = 8 $ SUGRA. Moving beyond tree level, we give a simple general discussion of the structure of 1-loop amplitudes in any QFT, in close parallel to recent work of Forde, showing that the coefficients of scalar “triangle” and “bubble” integrals are determined by the “pole at infinite momentum” of products of tree amplitudes appearing in cuts. In $ \mathcal{N} = 4 $ SYM and $ \mathcal{N} = 8 $ SUGRA, the on-shell superspace makes it easy to compute the multiplet sums that arise in these cuts by relating them to the best behaved tree amplitudes of highest spin, leading to a straightforward proof of the absence of triangles and bubbles at 1-loop. We also argue that rational terms are absent. This establishes that 1-loop amplitudes in $ \mathcal{N} = 8 $ SUGRA only have scalar box integrals. We give an explicit expression for 1-loop amplitudes for both $ \mathcal{N} = 4 $ SYM and $ \mathcal{N} = 8 $ SUGRA in terms of tree amplitudes that can be determined recursively. These amplitudes satisfy further relations in $ \mathcal{N} = 8 $ SUGRA that are absent in $ \mathcal{N} = 4 $ SYM. Since both tree and 1-loop amplitudes for maximally supersymmetric theories can be completely determined by their leading singularities, it is natural to conjecture that this property holds to all orders of perturbation theory. This is the nicest analytic structure amplitudes could possibly have, and if true, would directly imply the perturbative finiteness of $ \mathcal{N} = 8 $ SUGRA. All these remarkable properties of scattering amplitudes call for an explanation in terms of a “weak-weak” dual formulation of QFT, a holographic dual of flat space.

Quantum gravity partition functions in three dimensions

Abstract: We consider pure three-dimensional quantum gravity with a negative cosmological constant. The sum of known contributions to the partition function from classical geometries can be computed exactly, including quantum corrections. However, the result is not physically sensible, and if the model does exist, there are some additional contributions. One possibility is that the theory may have long strings and a continuous spectrum. Another possibility is that complex geometries need to be included, possibly leading to a holomorphically factorized partition function. We analyze the subleading corrections to the Bekenstein-Hawking entropy and show that these can be correctly reproduced in such a holomorphically factorized theory. We also consider the Hawking-Page phase transition between a thermal gas and a black hole and show that it is a phase transition of Lee-Yang type, associated with a condensation of zeros in the complex temperature plane. Finally, we analyze pure three-dimensional supergravity, with similar results.

Towards Strange Metallic Holography

Abstract: We initiate a holographic model building approach to `strange metallic' phenomenology. Our model couples a neutral Lifshitz-invariant quantum critical theory, dual to a bulk gravitational background, to a finite density of gapped probe charge carriers, dually described by D-branes. In the physical regime of temperature much lower than the charge density and gap, we exhibit anomalous scalings of the temperature and frequency dependent conductivity. Choosing the dynamical critical exponent z appropriately we can match the non-Fermi liquid scalings, such as linear resistivity, observed in strange metal regimes. As part of our investigation we outline three distinct string theory realizations of Lifshitz geometries: from F theory, from polarised branes, and from a gravitating charged Fermi gas. We also identify general features of renormalisation group ow in Lifshitz theories, such as the appearance of relevant charge-charge interactions when z ≥ 2. We outline a program to extend this model building approach to other anomalous observables of interest such as the Hall conductivity.

Higher spin gauge theory and holography: the three-point functions

Abstract: In this paper we calculate the tree level three-point functions of Vasiliev's higher spin gauge theory in AdS4 and find agreement with the correlators of the free field theory of N massless scalars in three dimensions in the O(N) singlet sector. This provides substantial evidence that Vasiliev theory is dual to the free field theory, thus verifying a conjecture of Klebanov and Polyakov. We also find agreement with the critical O(N) vector model, when the bulk scalar field is subject to the alternative boundary condition such that its dual operator has classical dimension 2.

Loop and surface operators in N=2 gauge theory and Liouville modular geometry

Abstract: Recently, a duality between Liouville theory and four dimensional N = 2 gauge theory has been uncovered by some of the authors. We consider the role of extended objects in gauge theory, surface operators and line operators, under this correspondence. We map such objects to specific operators in Liouville theory. We employ this connection to compute the expectation value of general supersymmetric ’t Hooft-Wilson line operators in a variety of N = 2 gauge theories.

Background independent action for double field theory

Abstract: Double field theory describes a massless subsector of closed string theory with both momentum and winding excitations. The gauge algebra is governed by the Courant bracket in certain subsectors of this double field theory. We construct the associated nonlinear background-independent action that is T-duality invariant and realizes the Courant gauge algebra. The action is the sum of a standard action for gravity, antisymmetric tensor, and dilaton fields written with ordinary derivatives, a similar action for dual fields with dual derivatives, and a mixed term that is needed for gauge invariance.

Nonlinear W ∞ as asymptotic symmetry of three-dimensional higher spin AdS gravity

Abstract: We investigate the asymptotic symmetry algebra of (2+1)-dimensional higher spin, anti-de Sitter gravity. We use the formulation of the theory as a Chern-Simons gauge theory based on the higher spin algebra hs(1, 1). Expanding the gauge connection around asymptotically anti-de Sitter spacetime, we specify consistent boundary conditions on the higher spin gauge fields. We then study residual gauge transformation, the corresponding surface terms and their Poisson bracket algebra. We find that the asymptotic symmetry algebra is a nonlinearly realized W ∞ algebra with classical central charges. We discuss implications of our results to quantum gravity and to various situations in string theory.

A duality for the S matrix

Abstract: We propose a dual formulation for the S Matrix of $$ \mathcal N $$ = 4 SYM. The dual provides a basis for the “leading singularities” of scattering amplitudes to all orders in perturbation theory, which are sharply defined, IR safe data that uniquely determine the full amplitudes at tree level and 1-loop, and are conjectured to do so at all loop orders. The scattering amplitude for n particles in the sector with k negative helicity gluons is associated with a simple integral over the space of k planes in n dimensions, with the action of parity and cyclic symmetries manifest. The residues of the integrand compute a basis for the leading singularities. A given leading singularity is associated with a particular choice of integration contour, which we explicitly identify at tree level and 1-loop for all NMHV amplitudes as well as the 8 particle N2MHV amplitude. We also identify a number of 2-loop leading singularities for up to 8 particles. There are a large number of relations among residues which follow from the multi-variable generalization of Cauchy’s theorem known as the “global residue theorem”. These relations imply highly non-trivial identities guaranteeing the equivalence of many different representations of the same amplitude. They also enforce the cancellation of non-local poles as well as consistent infrared structure at loop level. Our conjecture connects the physics of scattering amplitudes to a particular subvariety in a Grassmannian; space-time locality is reflected in the topological properties of this space.

Erratum: NLO single-top production matched with shower in POWHEG: s - and t -channel contributions

Abstract: We present a next-to-leading order calculation of single-top production interfaced to Shower Monte Carlo programs, implemented according to the POWHEG method. A detailed comparison with MC@NLO and PYTHIA is carried out for several observables, for the Tevatron and LHC colliders.

Janus configurations, Chern-Simons couplings, and The θ-Angle in \mathcal{N} = 4 super Yang-Mills theory

Abstract: We generalize the half-BPS Janus configuration of four-dimensional \( \mathcal{N} = 4 \) super Yang-Mills theory to allow the theta-angle, as well as the gauge coupling, to vary with position. We show that the existence of this generalization is closely related to the existence of novel three-dimensional Chern-Simons theories with \( \mathcal{N} = 4 \) supersymmetry. Another closely related problem, which we also elucidate, is the D3-NS5 system in the presence of a four-dimensional theta-angle.

Holographic evolution of entanglement entropy

Abstract: We study the evolution of entanglement entropy in a 2-dimensional equilibration process that has a holographic description in terms of a Vaidya geometry. It models a unitary evolution in which the field theory starts in a pure state, its vacuum, and undergoes a perturbation that brings it far from equilibrium. The entanglement entropy in this set up provides a measurement of the quantum entanglement in the system. Using holographic techniques we recover the same result obtained before from the study of processes triggered by a sudden change in a parameter of the hamiltonian, known as quantum quenches. Namely, entanglement in 2-dimensional conformal field theories propagates with velocity v2 = 1 [1]. Both in quantum quenches and in the Vaidya model equilibration is only achieved at the local level. Remarkably, the holographic derivation of this last fact requires information from behind the apparent horizon generated in the process of gravitational collapse described by the Vaidya geometry. In the early stages of the evolution the apparent horizon seems however to play no relevant role with regard to the entanglement entropy. We speculate on the possibility of deriving a thermalization time for occupation numbers from our analysis.

The Tevatron at the frontier of dark matter direct detection

Abstract: Direct detection of dark matter (DM) requires an interaction of dark matter particles with nucleons. The same interaction can lead to dark matter pair production at a hadron collider, and with the addition of initial state radiation this may lead to mono-jet signals. Mono-jet searches at the Tevatron can thus place limits on DM direct detection rates. We study these bounds both in the case where there is a contact interaction between DM and the standard model and where there is a mediator kinematically accessible at the Tevatron. We find that in many cases the Tevatron provides the current best limit, particularly for light dark matter, below ∼5 GeV, a and for spin dependent interactions. Non-standard dark matter candidates are also constrained. The introduction of a light mediator significantly weakens the collider bound. A direct detection discovery that is in apparent conflict with mono-jet limits will thus point to a new light state coupling the standard model to the dark sector. Mono-jet searches with more luminosity and including the spectrum shape in the analysis can improve the constraints on DM-nucleon scattering cross section.

Holography of Charged Dilaton Black Holes

Abstract: We study charged dilaton black branes in AdS 4. Our system involves a dilaton ϕ coupled to a Maxwell field F μν with dilaton-dependent gauge coupling, $ \frac{1}{{{g^2}}} = {f^2}\left( \phi \right) $ . First, we find the solutions for extremal and near extremal branes through a combination of analytical and numerical techniques. The near horizon geometries in the simplest cases, where f(ϕ) = e αϕ , are Lifshitz-like, with a dynamical exponent z determined by α. The black hole thermodynamics varies in an interesting way with α, but in all cases the entropy is vanishing and the specific heat is positive for the near extremal solutions. We then compute conductivity in these backgrounds. We find that somewhat surprisingly, the AC conductivity vanishes like ω 2 at T = 0 independent of α. We also explore the charged black brane physics of several other classes of gauge-coupling functions f(ϕ). In addition to possible applications in AdS/CMT, the extremal black branes are of interest from the point of view of the attractor mechanism. The near horizon geometries for these branes are universal, independent of the asymptotic values of the moduli, and describe generic classes of endpoints for attractor flows which are different from AdS 2 × R 2.

Holographic GB gravity in arbitrary dimensions

Abstract: We study the properties of the holographic CFT dual to Gauss-Bonnet gravity in general D(≥ 5) dimensions. We establish the AdS/CFT dictionary and in particular relate the couplings of the gravitational theory to the universal couplings arising in correlators of the stress tensor of the dual CFT. This allows us to examine constraints on the gravitational couplings by demanding consistency of the CFT. In particular, one can demand positive energy fluxes in scattering processes or the causal propagation of fluctuations. We also examine the holographic hydrodynamics, commenting on the shear viscosity as well as the relaxation time. The latter allows us to consider causality constraints arising from the second-order truncated theory of hydrodynamics.

Charm-loop effect in B → K(*)ℓ+ℓ− and B → K*γ

Abstract: We calculate the long-distance effect generated by the four-quark operators with c-quarks in the B → K (*) l + l − decays. At the lepton-pair invariant masses far below the $ \bar{c}c $ -threshold, q 2 ≪ 4m c 2, we use OPE near the light-cone. The nonfactorizable soft gluon emission from c-quarks is cast in the form of a nonlocal effective operator. The B → K (∗) matrix elements of this operator are calculated from the QCD light-cone sum rules with the B-meson distribution amplitudes. As a byproduct, we also predict the charm-loop contribution to B → K * γ beyond the local-operator approximation. To describe the charm-loop effect at large q 2, we employ the hadronic dispersion relation with ψ = J/ψ, ψ(2S),... contributions, where the measured B → K (∗) ψ amplitudes are used as inputs. Matching this relation to the result of QCD calculation reveals a destructive interference between the J/ψ and ψ(2S) contributions. The resulting charm-loop effect is represented as a q 2-dependent correction ΔC 9(q 2) to the Wilson coefficient C 9. Within uncertainties of our calculation, at q 2 below the charmonium region the predicted ratio ΔC 9(q 2)/C 9 is ≤5% for B → Kl + l −, but can reach as much as 20% for B → K * l + l −, the difference being mainly caused by the soft-gluon contribution.

Nekrasov functions and exact Bohr-Sommerfeld integrals

Abstract: In the case of SU(2), associated by the AGT relation to the 2d Liouville theory, the Seiberg-Witten prepotential is constructed from the Bohr-Sommerfeld periods of 1d sine-Gordon model. If the same construction is literally applied to monodromies of exact wave functions, the prepotential turns into the one-parametric Nekrasov prepotential $$\mathcal{F} (a, \epsilon_{1}) $$ with the other epsilon parameter vanishing, ϵ2 = 0, and ϵ1 playing the role of the Planck constant in the sine-Gordon Shrodinger equation, ℏ = ϵ1. This seems to be in accordance with the recent claim in [1] and poses a problem of describing the full Nekrasov function as a seemingly straightforward double-parametric quantization of sine-Gordon model. This also provides a new link between the Liouville and sine-Gordon theories.

NLO Higgs boson production via vector-boson fusion matched with shower in POWHEG

Abstract: We present a next-to-leading order calculation of Higgs boson production in vector-boson fusion processes interfaced to shower Monte Carlo programs, implemented according to the POWHEG method.