Preprint typeset in JHEP style - HYPER VERSIONDCPT-15/39 Contact interactions between particle worldlines James P. Edwards Centre for Particle Theory, Department of Mathematical Sciences, University of Durham, Durham DH1 3LE, UK Email: j.p.edwards@durham.ac.uk Abstract:We construct contact interactions for bosonic and fermionic point par- ticles. We first relate the resulting theories to classical electrostatics by taking func- tional averages over worldlines whose endpoints are fixed to charged particles. Count- ing those paths which pass through a space-time pointxµgives the static electric field at that point, provided we take the limit where the length measured along the worldlines is large.We also investigate corrections to the classical field that arise beyond leading order in this limit before constructing a theory of point particles that interact when their worldlines intersect. We quantise this theory and show that the partition function contains propagator couplings between the endpoints of the particles before discussing how this is related to the worldline formalism of quantum field theory and general action at a distance theories. Keywords:Effective field theories, Field theories in lower dimensions, Confinement. arXiv:1506.08130v2 [hep-th] 14 Dec 2015
Contents 1.Introduction1 2.Bosonic particles – the classical electric field4 3.Fermionic particles13 4.Analysis at finiteT20 4.1Corrections in the bosonic case20 4.2Corrections in the fermionic case26 5.Contact interactions between particles28 5.1Expansion to arbitrary order39 6.Discussion41 1. Introduction Classical electromagnetism is conventionally described by Maxwell’s field theory and there seems to be little room for debate about its formulation. In [1] and [2], however, building upon [3] an alternative approach to determining the electromagnetic field strength tensor for a pair of charged particles led directly to a novel interacting string theory.This theory contained contact interactions on the string worldsheet which served to produce expectation values of Wilson lines in Abelian quantum field theory. In the case of electrostatics, the description given in [3] was in terms of point particles whose worldlines have their endpoints fixed to the charged particles. The electric field at a position in space-time was arrived at via a weighted average over all such worldlines which also pass through the given point. The physical picture which motivated this approach (and the later interacting string theory) is of Faraday’s lines of force as fundamental objects which become the physical degrees of freedom of the electromagnetic field. To complement this work on contact interactions in string theory it seems ap- propriate to return to worldline theories to explore the consequences of allowing point particles to interact when their worldlines intersect. Such theories are of sig- nificant physical interest, since the so-called worldline formalism of quantum field – 1 –
theory [4, 5, 6] expresses physical quantities in a field theory in terms of the quan- tum mechanics of point particles which trace out worldlines in space-time [7, 8, 9]. This technique can also be extended to non-Abelian theories and chiral fermions [10] where it may provide insight to the unification of the fundamental forces [11]. The coupling between matter fields and the gauge field is described in the point particle theory as a Wilson-line interaction for the particle worldlines in the presence of a background field. For example, the partition function for a single scalar field mini- mally coupled to an Abelian gauge field,A, is expressed in the worldline formalism (which we derive in more detail later in the paper) by an integral over all closed curvesω Z= ∫∞ 0 dT T ∮ Dω e−Spoint[ω]W[A] ;W[A] =ei∫dω·A,(1.1) whereSpoint[ω] =m∫dτ√˙ω2is an action describing the dynamics of a point particle andW[A] is the Wilson loop describing the interaction of the particle with the gauge field (we have absorbed the coupling strength intoA). The right hand side is interpreted as quantum mechanics on the worldline of this particle and it is this first quantised theory which we propose to modify in this paper. Field theory is the conventional framework in which to introduce interactions and the local nature of this approach naturally leads to particles interacting upon contact. However, the worldline formalism can offer substantial calculational advantages over traditional approaches in field theory, especially since it represents a reorganisation of the usual perturbative expansion in Feynman diagrams and makes the local gauge invariance manifest [12].It is therefore important to develop worldline techniques further and one of the most basic modifications to the theory must be to introduce direct interactions between these particle worldlines.As we shall describe below, a modification to the worldline theory can be interpreted as inducing a change in the underlying field theory, so the results of this program may provide new tools to complement the conventional techniques familiar to field theorists. We comment on this in section 5. Direct inter-particle interactions can be found in many previous publications. One of the most well-known approaches is the action at a distance formulation of electrodynamics by Feynman and Wheeler [13, 14], originally proposed to ad- dress the problem of radiation reaction. This built upon earlier work in formulating a consistent theory involving direct inter-particle interactions by Tetrode [15] and Fokker [16], who described electromagnetic phenomena in terms of interactions be- tween particles with light-like separation. Ramond generalised this work and found a set of consistency constraints limiting the form of the interaction that can be introduced into worldline theories [17].This was further extended to include di- rect inter-string interactions as well as interactions between particles and strings [18, 19, 20, 21, 22]. A number of further theories involving action at a distance have – 2 –
been proposed to describe various other phenomena within this framework – see for example [23, 24, 25, 26, 27]. The general principle is to couple the worldlines of the particles together by adding extra terms to the free particle action,Spoint. An illustrative example would be to consider a theory of two particles whose worldlines are described byωµ a(τa) andωµ b(τb) and to introduce [28, 25] Sint=gagb ∫ ωa dτa ∫ ωb dτb˙ωa(τa)·˙ωb(τb)D(ωa−ωb)(1.2) as an interaction term in the action1. Here the functionDmust be symmetric and accounts for the relative strength of interaction as a function of particle separation. In the literature discussed aboveD(ωa−ωb) has been taken to be supported for light-like, time-like and space-like separations, the last of which is the relativistic generalisation of an instantaneous interaction. In Feynman-Wheeler theory, for ex- ample,D(ωa−ωb) is taken to be the sum of advanced and retarded Green functions of the (space-time) Laplacian. As is now well known, these action at a distance the- ories are often cast into a form reminiscent of a field theory, although the “fields” are not independent variables but are rather defined in terms of the particle trajectories and the choice ofD. For instance, if we take [29] D(ωa−ωb) =1 4π δ((ωa−ωb)2)−m 8π θ((ωa−ωb)2) √ (ωa−ωb)2 J1 ( m √ (ωa−ωb)2 ) (1.3) which is the time symmetric Green satisfying (∂µ∂µ−m2)D(ωa−ωb) =−δ4(ωa−ωb) then we may define Aµ(x) =∑ a ga ∫ dτa˙ωµ a(τa)D(ωa(τa)−x).(1.4) This satisfies the Maxwell equations and gauge condition (∂ν∂ν−m2)Aµ(x) =jµ(x) ;jµ(x) =−∑ a ga ∫ dτa˙ωµ a(τa)δ4(x−ωa), ∂µAµ(x) = 0(1.5) and in terms ofAthe interaction between the particles takes the form ∑ a ga ∫ ωa dτa˙ωa(τa)·A(ωa(τa))(1.6) which shows that the action at a distance formalism contains the same equations of motion and interactions as more traditional approaches using field theory. 1To be precise this provides vector-like interactions between point particles. Scalar interactions can be produced by replacing each˙ωiµby(˙ω2 i )1 2. – 3 –
The proposal we will make will follow the same form as (1.2) except that we shall chooseD(ωa−ωb) =δ4(ωa−ωb) so as to provide contact interactions between the worldlines. This also ensures that, although in principle (1.2) implies the interaction is non-local on the worldlines, the particles only communicate when they meet so that the theory is local in space-time. In other words we are no longer considering action at a distance but we allow particles to interact when they find themselves at the same space-time position. As stated above, we have previously considered such contact interactions between strings, where the theory found application to classical electromagnetism and quantum electrodynamics. We now intend to explore the same ideas for the case of point particles. This article revisits and extends the results of [3] and also generalises that work to the case of fermionic particles. It then goes beyond leading order in the coupling strength to demonstrate that in fact the full quantum theory of a set of interacting point particles is consistent and free of unwanted divergences. We develop the func- tional approach to one dimensional field theory for consistency with [1, 2] and for the generalisation to fermionic particles we will find it most natural to form the theory in superspace. We will first consider a single particle worldline with fixed endpoints that is constrained to pass through a given point in space and will relate it to clas- sical electrostatics and the well-known phenomenon of confinement. We then repeat a similar calculation for spin 1/2 particles to explore the fermionic version of the theory before considering an arbitrary set of interacting worldlines. We will see that the partition function of this theory is related to propagators of the Klein-Gordon operator. The structure of this article is as follows. In section 2 the bosonic theory pre- sented at lowest order in [3] is reviewed before we generalise it to include spin degrees of freedom in section 3. In section 4 we also carry out the first analysis of the theory beyond the classical limit to explore higher order corrections to the result in [3]. Fol- lowing this a full quantum theory of interacting worldlines is described and quantised in section 5. Some supporting calculations on our regularisation scheme are given in the Appendix. 2. Bosonic particles – the classical electric field We begin by working inDspatial dimensions and consider a static charged particle at positionaand an oppositely charged particle atb. The classical electric dipole field for this configuration is the well known solution to Maxwell’s equations in the presence of these point particles.In [3] an alternative proposal was made which generates the electric field by carrying out an average over a set of curves joining the two sources. This concept goes some way to reviving Faraday’s notion of electric flux lines and it is this calculation that we now review and extend. – 4 –
100%
Scan to connect with one of our mobile apps
Coinbase Wallet app
Connect with your self-custody wallet
Coinbase app
Connect with your Coinbase account
Open Coinbase Wallet app
Tap Scan
Or try the Coinbase Wallet browser extension
Connect with dapps with just one click on your desktop browser
Add an additional layer of security by using a supported Ledger hardware wallet