arXiv subjects

Maxwell T. Hansen

Publications and source records attributed to Maxwell T. Hansen.

85 records · Page 5Linked to original sources

arXiv2015

Extracting three-body observables from finite-volume quantities

Scattering and transition amplitudes with three-hadron final states play an important role in nuclear and particle physics. However, predicting such quantities using numerical Lattice QCD is very difficult, in part because of the effects of Euclidean time and finite volume. In this review we highlight recent formal developments that work towards overcoming these issues. We organize the presentation into three parts: large volume expansions, non-relativistic nonperturbative analyses, and nonperturbative studies based in relativistic field theory. In the first part we discuss results for ground state energies and matrix elements given by expanding in inverse box length, $1/L$. We describe complications that arise at $\mathcal O(1/L^6)$ and include a table summarizing the results of different calculations. In the second part we summarize three recent non-relativistic non-perturbative studies and highlight the main conclusions of these works. This includes demonstrating that the three-particle finite-volume spectrum is determined, up to exponentially suppressed effects, by on-shell amplitudes, as well as recovering a finite-volume quantization condition for scattering a stable particle off a two-particle bound state. In this part we also highlight recent work concerning a three-particle bound state in a finite volume. In the third and final part, we review recent work based in non-perturbative relativistic field theory. Here the finite-volume spectrum has been related to an intermediate infinite-volume quantity which itself is related via a known integral equation to the relativistic, model-independent three-particle scattering amplitude. We motivate the appearance of the intermediate quantity, explain how it is related to the standard amplitude, and discuss prospects for using the result to constrain three-particle observables.

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arXiv2015

Relativistic, model-independent, multichannel $2\to2$ transition amplitudes in a finite volume

We derive formalism for determining $\textbf{2} + \mathcal J \to \textbf{2}$ infinite-volume transition amplitudes from finite-volume matrix elements. Specifically, we present a relativistic, model-independent relation between finite-volume matrix elements of external currents and the physically observable infinite-volume matrix elements involving two-particle asymptotic states. The result presented holds for states composed of two scalar bosons. These can be identical or non-identical and, in the latter case, can be either degenerate or non-degenerate. We further accommodate any number of strongly-coupled two-scalar channels. This formalism will, for example, allow future lattice QCD calculations of the $ρ$-meson form factor, in which the unstable nature of the $ρ$ is rigorously accommodated.

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arXiv2015

Perturbative results for two and three particle threshold energies in finite volume

We calculate the energy of the state closest to threshold for two and three identical, spinless particles confined to a cubic spatial volume with periodic boundary conditions and with zero total momentum in the finite-volume frame. The calculation is performed in relativistic quantum field theory with particles coupled via a $λϕ^4$ interaction, and we work through order $λ^3$. The energy shifts begin at ${\cal O}(1/L^3)$, and we keep subleading terms proportional to $1/L^4$, $1/L^5$ and $1/L^6$. These terms allow a non-trivial check of the results obtained from quantization conditions that hold for arbitrary interactions, namely that of Lüscher for two particles and our recently developed formalism for three particles. We also compare to previously obtained results based on non-relativistic quantum mechanics.

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arXiv2015

Multichannel 0-to-2 and 1-to-2 transition amplitudes for arbitrary spin particles in a finite volume

We present a model-independent, non-perturbative relation between finite-volume matrix elements and infinite-volume $\textbf{0}\rightarrow\textbf{2}$ and $\textbf{1}\rightarrow\textbf{2}$ transition amplitudes. Our result accommodates theories in which the final two-particle state is coupled to any number of other two-body channels, with all angular momentum states included. The derivation uses generic, fully relativistic field theory, and is exact up to exponentially suppressed corrections in the lightest particle mass times the box size. This work distinguishes itself from previous studies by accommodating particles with any intrinsic spin. To illustrate the utility of our general result, we discuss how it can be implemented for studies of $N+\mathcal{J}~\rightarrow~(Nπ,Nη,Nη',ΣK,ΛK)$ transitions, where $\mathcal{J}$ is a generic external current. The reduction of rotational symmetry, due to the cubic finite volume, manifests in this example through the mixing of S- and P-waves when the system has nonzero total momentum.

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arXiv2015

Multichannel one-to-two transition amplitudes in a finite volume

We derive a model-independent expression for finite-volume matrix elements. Specifically, we present a relativistic, non-perturbative analysis of the matrix element of an external current between a one-scalar in-state and a two-scalar out-state. Our result, which is valid for energies below higher-particle inelastic thresholds, generalizes the Lellouch-Luscher formula in two ways: we allow the external current to inject arbitrary momentum into the system and we allow for the final state to be composed an arbitrary number of strongly coupled two-particle states with arbitrary partial waves (including partial-wave mixing induced by the volume). We also illustrate how our general result can be applied to some key examples, such as heavy meson decays and meson photo production. Finally, we point out complications that arise involving unstable resonance states, such as $B\rightarrow K^*\ell^+\ell^-$ when staggered or mixed-action/partially-quenched calculations are performed.

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arXiv2015

Multichannel one-to-two transition amplitudes in a finite volume

We perform a model-independent, non-perturbative investigation of two-point and three-point finite-volume correlation functions in the energy regime where two-particle states can go on-shell. We study three-point functions involving a single incoming particle and an outgoing two-particle state, relevant, for example, for studies of meson decays (e.g., B-to-pi Kll) or meson photo production (e.g., pi gamma-to-pi pi). We observe that, while the spectrum solely depends on the on-shell scattering amplitude, the correlation functions also depend on off-shell amplitudes. The main result of this work is a generalization of the Lellouch-Luscher formula relating matrix elements of currents in finite and infinite spatial volumes. We extend that work by considering a theory with multiple, strongly-coupled channels and by accommodating external currents which inject arbitrary four-momentum as well as arbitrary angular momentum. The result is exact up to exponentially suppressed corrections governed by the pion mass times the box size. We also apply our master equation to various examples, including the two processes mentioned above as well as examples where the final state is an admixture of two open channels.

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arXiv2014

A relativistic, model-independent, three-particle quantization condition

We present a generalization of Luescher's relation between the finite-volume spectrum and scattering amplitudes to the case of three particles. We consider a relativistic scalar field theory in which the couplings are arbitrary aside from a Z2 symmetry that removes vertices with an odd number of particles. The theory is assumed to have two-particle phase shifts that are bounded by π/2 in the regime of elastic scattering. We determine the spectrum of the finite-volume theory from the poles in the odd-particle-number finite-volume correlator, which we analyze to all orders in perturbation theory. We show that it depends on the infinite-volume two-to-two K-matrix as well as a nonstandard infinite-volume three-to-three K-matrix. A key feature of our result is the need to subtract physical singularities in the three-to-three amplitude and thus deal with a divergence-free quantity. This allows our initial, formal result to be truncated to a finite dimensional determinant equation. At present, the relation of the three-to-three K-matrix to the corresponding scattering amplitude is not known, although previous results in the non-relativistic limit suggest that such a relation exists.

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arXiv2014

Three-particle quantization condition: an update

We give an update on our derivation of a quantization condition relating the finite-volume spectrum of three particles in a cubic box to infinite-volume scattering quantities. We have discovered and fixed technical problems in the derivation sketched in the proceedings of last year's lattice conference [arXiv:1311.4848], and have presented a detailed description of the corrected derivation in Ref. [arXiv:1408.5933]. Here we give an overview of the problems and their solutions, and describe open questions.

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arXiv2013

Relativistic, model-independent, three-particle quantization condition

This is a combined write-up for two talks which were given consecutively and which described different aspects of the same topic. We present a generalization of Lüscher's relation between the finite-volume spectrum and S-matrix to three particles. Specifically, we consider a scalar field theory, which has a $\mathbb{Z}_2$ symmetry that prevents even/odd coupling. The theory is assumed to have no two-particle bound states and to have two-particle phase shifts that are bounded by $π/2$ in the regime of elastic scattering. Considering center of mass energies between one and five particle masses, we evaluate a three-to-three finite-volume correlator to all orders in perturbation theory. Only terms which are exponentially suppressed in volume are neglected. From poles in the correlator we then determine the relation between finite-volume spectrum and scattering quantities. In this analysis one must carefully treat the unitary cusp at two-particle threshold. This point, which was neglected in the conference talks, is described in some detail here. We also describe an important check on our main result by reproducing the large volume expansion of the energy shift from the three-particle threshold. This is found to be consistent with previous work through four non-trivial orders.

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arXiv2012

Multiple-channel generalization of Lellouch-Lüscher formula: Lattice 2012 conference proceedings

We describe a generalization of the Lellouch-Lüscher formula to the case of multiple strongly-coupled decay channels. As in the original formula, our final result is a relation between weak matrix elements in finite and infinite volumes. Our extension is limited to final states with two scalar particles, with center of mass energies below the lowest three- or four-particle threshold. Otherwise the extension is general, accommodating any number of channels, arbitrary strong coupling between channels, as well as any form of weak decay operators in the matrix elements. Among many possible applications, we emphasize that this is a necessary first step on the way to a lattice-QCD calculation of weak decay rates for D -> pi pi and D -> K K-bar. Our results allow for arbitrary total momentum and hold for degenerate or non-degenerate particles.

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arXiv2012

Multiple-channel generalization of Lellouch-Luscher formula

We generalize the Lellouch-Luscher formula, relating weak matrix elements in finite and infinite volumes, to the case of multiple strongly-coupled decay channels into two scalar particles. This is a necessary first step on the way to a lattice QCD calculation of weak decay rates for processes such as D -> pi pi and D -> KK. We also present a field theoretic derivation of the generalization of Luscher's finite volume quantization condition to multiple two-particle channels. We give fully explicit results for the case of two channels, including a form of the generalized Lellouch-Luscher formula expressed in terms of derivatives of the energies of finite volume states with respect to the box size. Our results hold for arbitrary total momentum and for degenerate or non-degenerate particles.

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arXiv2011

Determining low-energy constants in partially quenched Wilson chiral perturbation theory

In the low energy effective theory describing the partially quenched extension of two light Wilson fermions, three low energy constants (LECs) appear in terms proportional to a^2 (a being the lattice spacing). We propose methods to separately calculate these LECs, typically called W'_6, W'_7 and W'_8. While only one linear combination of these constants enters into physical quantities, different combinations enter into the description of the spectral density and eigenvalue distributions of the lattice Dirac operator and its Hermitian counterpart. Thus it is useful to be able to determine the LECs separately. Our methods require studying certain correlation functions for either two or three pion scattering, which are accessible only in the partially quenched extension of the theory.

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arXiv2011

Constraint on the Low Energy Constants of Wilson Chiral Perturbation Theory

Wilson chiral perturbation theory (WChPT) is the effective field theory describing the long- distance properties of lattice QCD with Wilson or twisted-mass fermions. We consider here WChPT for the theory with two light flavors of Wilson fermions or a single light twisted-mass fermion. Discretization errors introduce three low energy constants (LECs) into partially quenched WChPT at O(a^2), conventionally called W'_6, W'_7 and W'_8 . The phase structure of the theory at non-zero a depends on the sign of the combination 2W'_6 + W'_8, while the spectrum of the lattice Hermitian Wilson-Dirac operator depends on all three constants. It has been argued, based on the positivity of partition functions of fixed topological charge, and on the convergence of graded group integrals that arise in the epsilon-regime of ChPT, that there is a constraint on the LECs arising from the underlying lattice theory. In particular, for W'_6 = W'_7 = 0, the constraint found is W'_8 \le 0. Here we provide an alternative line of argument, based on mass inequalities for the underlying partially quenched theory. We find that W'_8 \le 0, irrespective of the values of W'_6 and W'_7. Our constraint implies that 2W'_6 > |W'_8| if the phase diagram is to be described by the first-order scenario, as recent simulations suggest is the case for some choices of action.

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