269 research outputs found
Fakeons, quantum gravity and the correspondence principle
The correspondence principle made of unitarity, locality and
renormalizability has been very successful in quantum field theory. Among the
other things, it helped us build the standard model. However, it also showed
important limitations. For example, it failed to restrict the gauge group and
the matter sector in a powerful way. After discussing its effectiveness, we
upgrade it to make room for quantum gravity. The unitarity assumption is better
understood, since it allows for the presence of physical particles as well as
fake particles (fakeons). The locality assumption is applied to an interim
classical action, since the true classical action is nonlocal and emerges from
the quantization and a later process of classicization. The renormalizability
assumption is refined to single out the special role of the gauge couplings. We
show that the upgraded principle leads to an essentially unique theory of
quantum gravity. In particular, in four dimensions, a fakeon of spin 2,
together with a scalar field, is able to make the theory renormalizable while
preserving unitarity. We offer an overview of quantum field theories of
particles and fakeons in various dimensions, with and without gravity.Comment: Proceedings of the conference "Progress and Visions in Quantum Theory
in View of Gravity: Bridging foundations of physics and mathematics", Max
Planck Institute for Mathematics in the Sciences, Leipzig, October 2018 - to
appear in a book with the same title edited by F. Finster, D. Giulini, J.
Kleiner and J. Tolksdorf - 21 page
Dual Identities inside the Gluon and the Graviton Scattering Amplitudes
Recently, Bern, Carrasco and Johansson conjectured dual identities inside the
gluon tree scattering amplitudes. In this paper, we use the properties of the
heterotic string and open string tree scattering amplitudes to refine and
derive these dual identities. These identities can be carried over to loop
amplitudes using the unitarity method. Furthermore, given the -gluon (as
well as gluon-gluino) tree amplitudes, -graviton (as well as
graviton-gravitino) tree scattering amplitudes can be written down immediately,
avoiding the derivation of Feynman rules and the evaluation of Feynman diagrams
for graviton scattering amplitudes.Comment: 43 pages, 3 figures; typos corrected, a few points clarified
Spin 3 cubic vertices in a frame-like formalism
Till now most of the results on interaction vertices for massless higher spin
fields were obtained in a metric-like formalism using completely symmetric
(spin-)tensors. In this, the Lagrangians turn out to be very complicated and
the main reason is that the higher the spin one want to consider the more
derivatives one has to introduce. In this paper we show that such
investigations can be greatly simplified if one works in a frame-like
formalism. As an illustration we consider massless spin 3 particle and
reconstruct a number of vertices describing its interactions with lower spin 2,
1 and 0 ones. In all cases considered we give explicit expressions for the
Lagrangians and gauge transformations and check that the algebra of gauge
transformations is indeed closed.Comment: 17 pades, no figure
Spinor Helicity and Dual Conformal Symmetry in Ten Dimensions
The spinor helicity formalism in four dimensions has become a very useful
tool both for understanding the structure of amplitudes and also for practical
numerical computation of amplitudes. Recently, there has been some discussion
of an extension of this formalism to higher dimensions. We describe a
particular implementation of the spinor-helicity method in ten dimensions.
Using this tool, we study the tree-level S-matrix of ten dimensional super
Yang-Mills theory, and prove that the theory enjoys a dual conformal symmetry.
Implications for four-dimensional computations are discussed.Comment: 24 pages, 1 figure
Optimal use of Information for Measuring in Lepton+jets Events
We present a novel approach that is being developed at DZero for extracting
information from data through a direct comparison of all measured variables in
an event with a matrix element that describes the entire production process.
The method is exemplified in the extraction of the mass of the top quark in
top-antitop events in the lepton+jets final state. Monte Carlo studies suggest
that an improvement of about a factor of two in statistical uncertainty on the
mass of the top quark can be achieved relative to previously published work for
the same channel. Preliminary results from the re-analysis provide a reduction
in the statistical uncertainty of almost a factor of 1.6, corresponding to an
effective factor of 2.4 increase in the size of the data sample.Comment: presented at HCP200
Boundary Contributions Using Fermion Pair Deformation
Continuing the study of boundary BCFW recursion relation of tree level
amplitudes initiated in \cite{Feng:2009ei}, we consider boundary contributions
coming from fermion pair deformation. We present the general strategy for these
boundary contributions and demonstrate calculations using two examples, i.e,
the standard QCD and deformed QCD with anomalous magnetic momentum coupling. As
a by-product, we have extended BCFW recursion relation to off-shell gluon
current, where because off-shell gluon current is not gauge invariant, a new
feature must be cooperated.Comment: 26 pages, 4 figure
Leptogenesis from loop effects in curved spacetime
We describe a new mechanism -- radiatively-induced gravitational leptogenesis -- for generating the matter-antimatter asymmetry of the Universe. We show how quantum loop effects in C and CP violating theories cause matter and antimatter to propagate differently in the presence of gravity, and prove this is forbidden in flat space by CPT and translation symmetry. This generates a curvature-dependent chemical potential for leptons, allowing a matter-antimatter asymmetry to be generated in thermal equilibrium in the early Universe. The time-dependent dynamics necessary for leptogenesis is provided by the interaction of the virtual self-energy cloud of the leptons with the expanding curved spacetime background, which violates the strong equivalence principle and allows a distinction between matter and antimatter. We show here how this mechanism is realised in a particular BSM theory, the see-saw model, where the quantum loops involve the heavy sterile neutrinos responsible for light neutrino masses. We demonstrate by explicit computation of the relevant two-loop Feynman diagrams how the size of the radiative corrections relevant for leptogenesis becomes enhanced by increasing the mass hierarchy of the sterile neutrinos, and show that for realistic phenomenological parameters this mechanism can generate the observed baryon-to-photon ratio of the Universe
Analyticity, Unitarity and One-loop Graviton Corrections to Compton Scattering
We compute spin-flip cross section for graviton photoproduction on a spin-1/2
target of finite mass. Using this tree-level result, we find one-loop graviton
correction to the spin-flip low-energy forward Compton scattering amplitude by
using Gerasimov-Drell-Hearn sum rule. We show that this result agrees with the
corresponding perturbative computations, implying the validity of the sum rule
at one-loop level, contrary to the previous claims. We discuss possible effects
from the black hole production and string Regge trajectory exchange at very
high energies. These effects seem to soften the UV divergence present at
one-loop graviton level. Finally, we discuss the relation of these observations
with the models that involve extra dimensions.Comment: 15 pages, 3 figure
Using gamma+jets Production to Calibrate the Standard Model Z(nunu)+jets Background to New Physics Processes at the LHC
The irreducible background from Z(nunu)+jets, to beyond the Standard Model
searches at the LHC, can be calibrated using gamma+jets data. The method
utilises the fact that at high vector boson pT, the event kinematics are the
same for the two processes and the cross sections differ mainly due to the
boson-quark couplings. The method relies on a precise prediction from theory of
the Z/gamma cross section ratio at high pT, which should be insensitive to
effects from full event simulation. We study the Z/gamma ratio for final states
involving 1, 2 and 3 hadronic jets, using both the leading-order parton shower
Monte Carlo program Pythia8 and a leading-order matrix element program Gambos.
This enables us both to understand the underlying parton dynamics in both
processes, and to quantify the theoretical systematic uncertainties in the
ratio predictions. Using a typical set of experimental cuts, we estimate the
net theoretical uncertainty in the ratio to be of order 7%, when obtained from
a Monte Carlo program using multiparton matrix-elements for the hard process.
Uncertainties associated with full event simulation are found to be small. The
results indicate that an overall accuracy of the method, excluding statistical
errors, of order 10% should be possible.Comment: 22 pages, 14 figures; Accepted for publication by JHE
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