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John F. Donoghue

Publications and source records attributed to John F. Donoghue.

At least 91 records · Page 5Linked to original sources
arXiv2000

The Influence of Dimension Eight Operators on Weak Matrix Elements

I describe recent work with V. Cirigliano and E. Golowich on the effect of dimension eight operators on weak nonleptonic amplitudes. The basic message is that there is an inconsistency in the way that many calculations are traditionally performed. If one calculates matrix elements involving only physics below some scale μ, then one needs dimension eight operators explicitly in the weak OPE. On the other hand if one wants to use dimensional regularization throughout, then one needs all scales to be included within the matrix element, and this results in the same net effect. A numerical estimate indicates that this is important below μ= 2 GeV, and this calls into question many of the models that have been used to predict ε'/ε.

hep-ph↗
arXiv2000

K -> pi pi Phenomenology in the Presence of Electromagnetism

We describe the influence of electromagnetism on the phenomenology of K -> pi pi decays. This is required because the present data were analyzed without inclusion of electromagnetic radiative corrections, and hence contain several ambiguities and uncertainties which we describe in detail. Our presentation includes a full description of the infrared effects needed for a new experimental analysis. It also describes the general treatment of final state interaction phases, needed because Watson's theorem is no longer valid in the presence of electromagnetism. The phase of the isospin-two amplitude A_2 may be modified by 50% -> 100%. We provide a tentative analysis using present data in order to illustrate the sensitivity to electromagnetic effects, and also discuss how the standard treatment of epsilon'/epsilon is modified.

hep-ph↗
arXiv2000

Dimension-eight Operators in the Weak OPE

We expose a potential flaw in standard treatments of weak decay amplitudes, including that of epsilon'/epsilon. We show that (contrary to conventional wisdom) dimension-eight operators do contribute to weak amplitudes, at order G_F alpha_s and without 1/M_W^2 suppression. We demonstrate the existence of these operators with a simple weak hamiltonian and proceed to discuss their contribution in a general context. We estimate the effect of dimension-eight operators to be at the 100% level in a sum rule determination of the operator O_7 for mu = 1.5 GeV, suggesting presently available values of mu are too low to justify neglect of these effects.

hep-ph↗
arXiv2000

Random values of the cosmological constant

One way that an anthropic selection mechanism may be manifest in a physical theory involves multiple domains in the universe with different values of the physical parameters. If this mechanism is to be relevant for understanding the small observed value of the cosmological constant, it may involve a mechanism by which some contributions to the cosmological constant can be fixed at a continuous range of values in the different domains. I study the properties of four possible mechanisms, including the possibility of the Hubble damping of a scalar field with an extremely flat potential. Another interesting possibility involves fixed random values of non-dynamical form fields, and a cosmological mechanism is suggested. This case raises the possibility of anthropic selection of other parameters in addition. Further requirements needed for a consistent cosmology are discussed.

hep-ph↗
arXiv1999

Dispersive calculation of B_7^{3/2} and B_8^{3/2} in the chiral limit

We show how the isospin vector and axialvector current spectral functions rho_V and rho_A can be used to determine in leading chiral order the low energy constants B_7^{3/2} and B_8^{3/2}. This is accomplished by matching the Operator Product Expansion to the dispersive analysis of vacuum polarization functions. The data for the evaluation of these dispersive integrals has been recently enhanced by the ALEPH measurement of spectral functions in tau decay, and we update our previous phenomenological determination. Our calculation yields in the NDR renormalization scheme and at renormalization scale mu = 2 GeV the values B_7^{3/2} = 0.55 +- 0.07 +- 0.10 and B_8^{3/2} = 1.11 +- 0.16 +- 0.23 for the quark mass values m_s + m = 0.1 GeV.

hep-ph↗
arXiv1999

The leading chiral electromagnetic correction to the nonleptonic Delta I = 3/2 amplitude in kaon decays

In kaon decay, electromagnetic radiative corrections can generate shifts in the apparent Delta I = 3/2 amplitude of order alpha A_0/A_2 ~ 22 alpha. In order to know the true Delta I = 3/2 amplitude for comparison with lattice calculations and phenomenology, one needs to subtract off this electromagnetic effect. We provide a careful estimate of the electromagnetic shift in the amplitude, which shows that it is smaller than naive expectations, with a fractional shift of delta A_2/A_2 = -0.016 +- 0.01.

hep-ph↗
arXiv1998

SU(3) Baryon Chiral Perturbation Theory and Long Distance Regularization

The use of SU(3) chiral perturbation theory in the analysis of low energy meson-baryon interactions is discussed. It is emphasized that short distance effects, arising from propagation of Goldstone bosons over distances smaller than a typical hadronic size, are model-dependent and can lead to a lack of convergence in the SU(3) chiral expansion if they are included in loop diagrams. In this paper we demonstrate how to remove such effects in a chirally consistent fashion by use of a cutoff and demonstrate that such removal ameliorates problems which have arisen in previous calculations due to large loop effects.

hep-ph↗
arXiv1998

Chiral predictions for K_L -> pi0 gamma mu+ mu-

We have previously analysed the chiral predictions for the decay K_L -> pi0 gamma e+ e- and discussed how experimental measurements of this process can help our understanding of chiral theories and of CP tests. Motivated by the possibility that the analogous muonic process may soon be measurable, we extend this calculation to the decay K_L -> pi0 gamma mu+ mu-. Branching ratios and differential branching ratios are calculated and presented. Measurements of these will help shed some light on the puzzles in our understanding of the chiral loop effects which were raised by the rate for K_L -> pi0 gamma gamma.

hep-ph↗
arXiv1998

Improving the convergence of SU(3) baryon chiral perturbation theory

Baryon chiral perturbation theory as conventionally applied using dimensional regularization has a well-known problem with the convergence of the SU(3) chiral expansion. One can reformulate the theory equally rigorously using a momentum-space cutoff and we show that the convergence is thereby greatly improved for reasonable values of the cutoff. In effect, this is accomplished because the cutoff formalism removes the spurious physics of propagation at distance scales much smaller than the baryon size.

hep-ph↗
arXiv1998

Anthropic considerations in multiple domain theories and the scale of electroweak symmetry breaking

One of the puzzles of the Standard Model is why the mass parameter which determines the scale of the Weak interactions is closer to the scale of QCD than to the Grand Unification or Planck scales. We discuss a novel approach to this problem which is possible in theories in which different regions of the universe can have different values of the physical parameters. In such a situation, we would naturally find ourselves in a region which has parameters favorable for life. We explore the whole range of values of the mass parameter in the Higgs potential, $μ^2$, from $+M_P^2$ to $-M_P^2$ and find that there is only a narrow window, overlapping with the observed value, in which life is likely to be possible. The observed value of $μ^2$ is fairly typical of the values in this range. Thus multiple domain theories in which $μ^2$ varies among domains may give a promising approach to solving the fine tunign problem and explaining the closeness of the QCD scale and the Weak scale.

hep-ph↗
arXiv1997

Perturbative dynamics of quantum general relativity

The quantum theory of General Relativity at low energy exists and is of the form called "effective field theory". In this talk I describe the ideas of effective field theory and its application to General Relativity.

gr-qc↗
arXiv1997

The weight for random quark masses

In theories in which the parameters of the low energy theory are not unique, perhaps having different values in different domains of the universe as is possible in some inflationary models, the fermion masses would be distributed with respect to some weight. In such a situation the specifics of the fermion masses do not have a unique explanation, yet the weight provides the visible remnant of the structure of the underlying theory. This paper introduces this concept of a weight for the distribution of masses and provides a quantitative estimate of it from the observed quarks and leptons. The weight favors light quark masses and appears roughly scale invariant (rho ~ 1/m). Some relevant issues, such as the running of the weight with scale and the possible effects of anthropic constraints, are also discussed.

hep-ph↗
arXiv1997

K_L -> pi0 gamma e+ e- and its relation to CP and chiral tests

The decay K_L -> pi0 gamma e+ e- occurs at a it higher rate than the nonradiative process K_L -> pi0 e+ e-, and hence can be a background to CP violation studies using the latter reaction. It also has an interest in its own right in the context of chiral perturbation theory, through its relation to the decay K_L -> pi0 gamma gamma. The leading order chiral loop contribution to K_L -> pi0 gamma e+ e-, including the [q(e+) + q(e-)]^2/m^2(pi) dependence, is completely calculable. We present this result and also include the higher order modifications which are required in the analysis of K_L -> pi0 gamma gamma.

hep-ph↗
arXiv1996

Asymptotic Limits and Structure of the Pion Form Factor

We use dispersive techniques to address the behavior of the pion form factor as $Q^2 \to \infty$ and $Q^2 \to 0$. We perform the matching with the constraints of perturbative QCD and chiral perturbation theory in the high energy and low energy limits, leading to four sum rules. We present a version of the dispersive input which is consistent with the data and with all theoretical constraints. The results indicate that the asymptotic perturbative QCD limit is approached relatively slowly, and give a model independent determination of low energy chiral parameters.

hep-ph↗
arXiv1996

Non-perturbative Methods in Kaon Physics: Aside from the Lattice

I discuss the progress in the use of analytic techniques for low energy QCD, in particular as applied to kaon physics. These methods are becoming increasingly powerful and we have gained a good deal of control over the difficult hadronic interactions. There are continuing developments, and I speculate on the ways that these techniques may become yet more sophisticated in the future.

hep-ph↗
arXiv1996

The Electromagnetic Mass Differences of Pions and Kaons

We use the Cottingham method to calculate the pion and kaon electromagnetic mass differences with as few model dependent inputs as possible. The constraints of chiral symmetry at low energy, QCD at high energy and experimental data in between are used in the dispersion relation. We find excellent agreement with experiment for the pion mass difference. The kaon mass difference exhibits a strong violation of the lowest order prediction of Dashen's theorem, in qualitative agreement with several other recent calculations.

hep-ph↗
arXiv1996

Factorization in non-leptonic decays of heavy mesons

We propose a QCD-based model for calculation of the non-perturbative corrections to the factorization approximation in the decays of heavy mesons. In the framework of the model, factorization in pseudoscalar transitions holds exactly at the leading order leaving the opportunity to calculate non-leading corrections consistently.

hep-ph↗