87 research outputs found
Non-perturbative studies of gauge theories: their renormalisation and hierarchies of scales
No full textTwo aspects of gauge theories are studied in the non-perturbative regime; firstly, using a set of pre-determined, approximate renormalised Feynman rules, the divergent parts of the O(α(_8)) virtual graphs of the process e(^+)e" → qq are determined to explicitly test whether multiplicative renormalisation is preserved by these rules. The calculation is performed using dimensional regularisation in 2(2 - ɛ) dimensional Euclidean space, where the divergences appear as 1/ɛ(^n) poles as ɛ → 0 Though the corrections to both the fermion-photon vertex and to the final state self energy are shown to have 1/ɛ singularities, the coefficients of these are quite different. This mis-match in singular behaviour signals the breakdown of multiplicative renormalisation, which, in turn, implies that the physical process is not guaranteed to be finite and the rules used are in admissable as a set of consistent Feynman rules. The second investigation is to solve numerically the Schwinger-Dyson equation for the fermion propagator in QED in three (Euclidean) dimensions. The aim being to study the scale of dynamical mass generation. To control infrared divergences the 1/N (flavour) expansion is used and to close the equation vertex and gauge propagator are approximated by their lowest order forms in 1/N. Numerical solutions for the fermion self energy and wavefunction renormalisation are determined. The latter is found not to be suppressed by O(1/N), contrary to the expectation of Appelquist et al, and the coupled equation for these functions has to be solved. It is then found that a mass scale is dynamically generated and that a scale hierarchy between it and the dimensionful coupling, α, of many orders of magnitude exists (typically m/a ~ 10(^-7) for N(_F)=5). Thus showing, albeit in a simplified 'toy' model, how large scale hierarchies can 'naturally' occur in gauge theories with spontaneously broken symmetries
A nonperturbative study of three dimensional quantum electrodynamics with N flavours of fermion
No full textThis work is concerned with the breaking of chiral symmetry in gauge theories and the associated generation of a dynamical mass scale. We investigate this phenomenon in the context of a simple model, three dimensional QED, where the complicating factor of infinite renormalisations is absent. This model possesses an intrinsic scale, set by the coupling [e(^2)] = M, and it is the relationship between this and the dynamically generated mass scale that is of interest. The chiral symmetry breaking mechanism is investigated using the Schwinger Dyson equations which are then truncated in a nonperturbative manner using the Ball-Chiu vertex ansatz. The complexity of the resulting coupled fermion-photon system means that the photon is initially replaced by its perturbative form. Numerical investigations of this simplified system then reveal the existence of an exponential relationship, in terms of the dimensionless parameter N, between the intrinsic and dynamical mass scales, m ~ e(^2) exp(-cN). Contrary to the assertions of Appelquist et al the wavefunction renormalisation was found to be nonperturbative and crucial in determining this behaviour. The sensitivity of this mechanism to the nonperturbative behaviour of the photon is investigated. A simple analysis shows it to be far stronger than previously expected. This is confirmed by a numerical analysis of the coupled photon-fermion system which suggest the relationship between the two scales in the theory is of the form m ~ e(^2) exp(-cN(^2)). This model therefore illustrates how a large hierarchy of scales may naturally occur in a gauge theory, for instance N=3 m/a ~ 10(^-5). Finally an investigation of the gauge dependence of this approach is initiated. The softening of the photon in the low momentum region is shown to amplify automatically any inadequacy of the vertex ansatz by factors of O(a/m) in all but the Landau gauge. It is therefore expected that any incomplete vertex form will result in the generation of a "critical gauge", ɛ(_e), below which chiral symmetry breaking solutions will not exist. A path of further investigation is suggested
Probes of universal extra dimensions at colliders
No full textIn the Universal Extra Dimensions model of Appelquist, Cheng and Dobrescu, all of the Standard Model fields are placed in the bulk and thus have Kaluza-Klein(KK) excitations. These KK states can only be pair produced at colliders due to the tree-level conservation of KK number, with the lightest of them being stable and possibly having a mass as low as ’ 350 − 400 GeV. After calculating the contribution to g − 2 in this model we investigate the production cross sections and signatures for these particles at both hadron and lepton colliders. We demonstrate that these signatures critically depend upon whether the lightest KK states remain stable or are allowed to decay by any of a number of new physics mechanisms. These mechanisms which induce KK decays are studied in detail. ∗Work supported by the Department of Energy, Contract DE-AC03-76SF00515
Establishing clinical supervision in prison health care settings: Phase Three. A report for Offender Health, Department of Health
No full text'What she told us made the world of difference': Carers perspectives of a hospice at home service
No full textCopyright @ 2013 Institut universitaire de gériatrie de Montréal. Posted with publisher's permission.This paper explores the expectations and needs of current and bereaved carers whose relatives
received care at home from a palliative care team. Aim: A hospice at home service was established in
2006 to provide patients with care in their own homes. We examined whether this model of care was helpful in
mitigating carers’ burden and in enabling terminally ill patients to be cared for and die at home. Methods: This
study utilized a survey and interviews. Participants were carers in the midwest of Ireland. Survey responses from 122 carers were analyzed using SPSS 18.0 (SPSS Inc., 2009); interviews with 15 carers were also conducted. Results: Carers’ expectations of the service were often exceeded, and quality of care dimensions were rated highly. Future improvements could include facilitating discussions on place of death and offering bereavement support. Conclusion: The service is supporting carers in facilitating their relatives’ choice to die at home.Atlantic Philanthropies and the Health Service Executiv
Renormalization Group Summation with Heavy Fields
No full textThe summation of logarithmic contributions to perturbative radiative corrections in physical processes through use of the renormalization group equation has proved to be a useful way of enhancing the information one can obtain from explicit calculation. However, it has proved difficult to perform this summation when massive fields are present. In this note we point out that if the masses involved are quite large, the decoupling theorem of Symanzik and of Appelquist and Carazzone can be used to make the summation of logarithms possible.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
Traces of a fixed point: Unravelling the phase diagram at large Nf
No full textWith a sufficiently high number of fundamental fermionic flavours present, Yang-Mills theory develops an infrared fixed point and becomes (quasi-)conformal in nature. The range of flavour numbers for which this occurs defines the conformal window, the lower limit of which has yet to be determined. We studied the phase diagram of SU(3) Yang-Mills theory with twelve flavours of staggered fermions. Here we present evidence for the occurrence of a bulk transition towards a chirally broken phase and the existence of a Coulomb phase on the symmetric side of this transition, using results from the measurements of the chiral condensate and spectrum, leading to the determination of a positive sign of the beta function. Assuming the validity of the Appelquist-Miransky-Yamawaki scenario, this implies the existence of a conformal window that comprises the theory under investigation
A new constraint on strongly coupled field theories
No full textWe propose a new constraint on the structure of strongly coupled, asymptotically free field theories. The constraint takes the form of an inequality limiting the number of degrees of freedom in the infrared description of a theory relative to the number of underlying, ultraviolet degrees of freedom. We apply the inequality to a variety of theories (both supersymmetric and nonsupersymmetric), where it agrees with all known results and leads to interesting new constraints on low energy spectra. We discuss the relation of this constraint to renormalization group c theorems.</p
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