arXiv subjects

John F. Donoghue

Publications and source records attributed to John F. Donoghue.

At least 55 records · Page 3Linked to original sources

arXiv2012

Self-healing of unitarity in effective field theories and the onset of new physics

In effective field theories it is common to identify the onset of new physics with the violation of tree-level unitarity. However, we show that this is parametrically incorrect in the case of chiral perturbation theory, and is probably theoretically incorrect in general. In the chiral theory, we explore perturbative unitarity violation as a function of the number of colors and the number of flavors, holding the scale of the "new physics" (i.e. QCD) fixed. This demonstrates that the onset of new physics is parametrically uncorrelated with tree-unitarity violation. When the latter scale is lower than that of new physics, the effective theory must heal its unitarity violation itself, which is expected because the field theory satisfies the requirements of unitarity. In the chiral theory, the self-healing results in a resonant structure with scalar quantum numbers. In the electroweak variant of this argument, the structure must have the properties of the Higgs and must couple proportional to the mass in both gauge boson and fermion scattering. A similar example can be seen in the case of general relativity coupled to multiple matter fields, where iteration of the vacuum polarization diagram restores unitarity. We present arguments that suggest the correct identification should be connected to the onset of inelasticity rather than unitarity violation. We describe how the onset of inelasticity can occur in the effective theory, although it does not appear possible to predict the onset reliably.

hep-ph↗

arXiv2012

On the running of the gravitational constant

We show that there is no useful and universal definition of a running gravitational constant, G(E), in the perturbative regime below the Planck scale. By consideration of the loop corrections to several physical processes, we show that the quantum corrections vary greatly, in both magnitude and sign, and do not exhibit the required properties of a running coupling constant. We comment on the potential challenges of these results for the Asymptotic Safety program.

hep-th↗

arXiv2011

Limiting velocities as running parameters and superluminal neutrinos

In the context of theories where particles can have different limiting velocities, we review the running of particle speeds towards a common limiting velocity at low energy. Motivated by the recent OPERA experimental results, we describe a model where the neutrinos would deviate from the common velocity by more than do other particles in the theory, because their running is slower due to weaker interactions.

hep-ph↗

arXiv2011

Spatial variation of fundamental couplings and Lunar Laser Ranging

If the fundamental constants of nature have a cosmic spatial variation, there will in general be extra forces with a preferred direction in space which violate the equivalence principle. We show that the millimeter-precision Apache Point Observatory Lunar Laser-ranging Operation provides a very sensitive probe of such variation that has the capability of detecting a cosmic gradient of the ratio between the quark masses and the strong interaction scale at the level with the gradient of ln (m_quark/Lambda_QCD) ~ 2.6 x 10^-6 Glyr^-1, which is comparable to the cosmic gradients suggested by the recently reported measurements of Webb et al. We also point out the capability of presently planned improved equivalence principle tests, at the Delta g/g < 10^-17 level, to probe similar cosmic gradients.

gr-qc↗

arXiv2011

The emergence of a universal limiting speed

We display several examples of how fields with different limiting velocities (the "speed of light") at a high energy scale can nevertheless have a common limiting velocity at low energies due to the effects of interactions. We evaluate the interplay of the velocities through the self-energy diagrams and use the renormalization group to evolve the system to low energy. The differences normally vanish only logarithmically, so that an exponentially large energy trajectory is required in order to satisfy experimental constraints. However, we also display a model in which the running is power-law, which could be more phenomenologically useful. The largest velocity difference should be in system with the weakest interaction, which suggests that the study of the speed of gravitational waves would be the most stringent test of this phenomenon.

hep-th↗

arXiv2010

Running couplings and operator mixing in the gravitational corrections to coupling constants

The use of a running coupling constant in renormalizable theories is well known, but the implementation of this idea for effective field theories with a dimensional coupling constant is in general less useful. Nevertheless there are multiple attempts to define running couplings including the effects of gravity, with varying conclusions. We sort through many of the issues involved, most particularly the idea of operator mixing and also the kinematics of crossing, using calculations in Yukawa and lambda phi^4 theory as illustrative examples. We remain in the perturbative regime. In some theories with a high permutation symmetry, such as lambda phi^4, a reasonable running coupling can be defined. However in most cases, such as Yukawa and gauge theories, a running coupling fails to correctly account for the energy dependence of the interaction strength. As a byproduct we also contrast on-shell and off-shell renormalizaton schemes and show that operators which are normally discarded, such as those that vanish by the equations of motion, are required for off-shell renormalization of effective field theories. Our results suggest that the inclusion of gravity in the running of couplings is not useful or universal in the description of physical processes.

hep-th↗

arXiv2010

Gauge non-invariance as tests of emergent gauge symmetry

We motivate the concept of emergent gauge symmetry and discuss ways that this concept can be tested. The key idea is that if a symmetry is emergent, one should look for small violations of this symmetry because the underlying fundamental theory does not contain the symmetry. We describe our recent work implementing this idea in the gravity sector. We also describe the reasons why violations of gauge symmetry may well be linked to violations of Lorentz invariance.

hep-ph↗

arXiv2010

Phenomenology of the Equivalence Principle with Light Scalars

Light scalar particles with couplings of sub-gravitational strength, which can generically be called 'dilatons', can produce violations of the equivalence principle. However, in order to understand experimental sensitivities one must know the coupling of these scalars to atomic systems. We report here on a study of the required couplings. We give a general Lagrangian with five independent dilaton parameters and calculate the "dilaton charge" of atomic systems for each of these. Two combinations are particularly important. One is due to the variations in the nuclear binding energy, with a sensitivity scaling with the atomic number as $A^{-1/3}$. The other is due to electromagnetism. We compare limits on the dilaton parameters from existing experiments.

gr-qc↗

arXiv2010

Equivalence Principle Violations and Couplings of a Light Dilaton

We consider possible violations of the equivalence principle through the exchange of a light `dilaton-like' scalar field. Using recent work on the quark-mass dependence of nuclear binding, we find that the dilaton-quark-mass coupling induces significant equivalence-principle-violating effects varying like the inverse cubic root of the atomic number - A^{-1/3}. We provide a general parameterization of the scalar couplings, but argue that two parameters are likely to dominate the equivalence-principle phenomenology. We indicate the implications of this framework for comparing the sensitivities of current and planned experimental tests of the equivalence principle.

gr-qc↗

arXiv2010

Likely values of the Higgs vev

We make an estimate of the likelihood function for the Higgs vacuum expectation value by imposing anthropic constraints on the existence of atoms while allowing the other parameters of the Standard Model to also be variable. We argue that the most important extra ingredients are the Yukawa couplings, and for the intrinsic distribution of Yukawa couplings we use the scale invariant distribution which is favored phenomenologically. The result is successful phenomenologically, favoring values close to the observed vev. We also discuss modifications that could change these conclusions. Our work supports the hypothesis that the anthropic constraints could be the origin of the small value of the Higgs vev.

hep-ph↗

arXiv2009

Breaking Diffeomorphism Invariance and Tests for the Emergence of Gravity

If general relativity is an emergent phenomenon, there may be small violations of diffeomorphism invariance. We propose a phenomenology of perturbatively small violations of general relativity by the inclusion of terms which break general covariance. These can be tested by matching to the Parametrized Post Newtonian (PPN) formalism. The most sensitive tests involve pulsar timing and provide an extremely strong bound, with a dimensionless constraint of order 10^{-20} relative to gravitational strength.

gr-qc↗

arXiv2009

When Effective Field Theories Fail

In this talk, I describe and defend four non-standard claims about four effective field theories, and try to extract some lessons about the limits of effective field theory. The four theses (and a capsule diagnosis given in parentheses) are: 1) Kaon loops are not a reliable part of chiral perturbation theory (dimensional regularization does not know about the chiral scale), 2) Regge physics is inappropriately missing from SCET (an infinite set of scales are needed) 3) There is likely a barrier in the use of EFT in general relativity in the extreme infrared (curvature effects build up) and 4) Gauge non-invariant operators should be included in describing physics beyond the Standard Model (as they could probe the idea of emergent gauge symmetry and falsify string theory).

hep-ph↗

arXiv2009

On Regge kinematics in SCET

We discuss the kinematics of the particles that make up a Reggeon in field theory, using the terminology of the Soft Collinear Effective Theory (SCET). Reggeization sums a series of strongly-ordered collinear emissions resulting in an overall Reggeon exchange that falls in the Glauber or Coulomb kinematic region. This is an extremely multi-scale problem and appears to fall outside of the usual organizing scheme of SCET.

hep-ph↗

arXiv2009

Comments on the Minimal Vectorial Standard Model

We explore the available parameter space of the minimal vectorial Standard Model. In this theory, the gauge currents are initially vectorial but the Higgs sector produces chiral mass eigenstates, leading to a set of heavy right-handed mirror particles. We describe the phenomenology of the residual parameter space and suggest that the model will be readily tested at the LHC.

hep-ph↗

arXiv2009

Non-isotropy in the CMB power spectrum in single field inflation

Contaldi et al. [1] have suggested that an initial period of kinetic energy domination in single field inflation may explain the lack of CMB power at large angular scales. We note that in this situation it is natural that there also be a spatial gradient in the initial value of the inflaton field, and that this can provide a spatial asymmetry in the observed CMB power spectrum, manifest at low multipoles. We investigate the nature of this asymmetry and comment on its relation to possible anomalies at low multipoles.

astro-ph↗

arXiv2009

Gauge federation as an alternative to unification

We motivate and explore the possibility that extra SU(N) gauge groups may exist independently of the Standard Model groups, yet not be subgroups of some grand unified group. We study the running of the coupling constants as a potential evidence for a common origin of all the gauge theories. Several different example are displayed. Some of the multiple options involve physics at the TeV scale.

hep-ph↗

arXiv2007

Constraints on the variability of quark masses from nuclear binding

Based on recent work on nuclear binding, we update and extend the anthropic constraints on the light quark masses, with results that are more tightly constrained than previously obtained. We find that heavy nuclei would fall apart (because the attractive nuclear central potential becomes too weak) if the sum of the light quark masses m_u+m_d would exceed their physical values by 64% (at 95% confidence level). We summarize the anthropic constraints that follow from requiring the existence both of heavy atoms and of hydrogen. With the additional assumption that the quark Yukawa couplings do not vary, these constraints provide a remarkably tight anthropic window for the Higgs vacuum expectation value: 0.39 < v/v_physical < 1.64.

hep-ph↗

arXiv2007

The fine-tuning problems of particle physics and anthropic mechanisms

Many of the classic problems of particle physics appear in a very different light when viewed from the perspective of the multiverse. Most importantly the two great ``fine tuning'' problems that motivate the field are far less serious when one accounts for the required anthropic constraints which exist in a multiverse. However, the challenge then becomes to construct a realistic physical theory of the multiverse and test it. I describe some phenomenology of the quark and lepton masses that may provide a window to the theory that underlies the multiverse.

hep-ph↗