arXiv subjects

Francis Bursa

Publications and source records attributed to Francis Bursa.

At least 19 recordsLinked to original sources

arXiv2021

The Undulator Radiation Collider: An Energy Efficient Design For A $\sqrt{s}=10^{15}$ GeV Collider

We discuss the main factors affecting the design of accelerators aiming to investigate physics at the GUT scale. The most important constraints turn out to be the energy used and the time taken to accumulate sufficient luminosity. We propose a photon collider design, where the photons are generated by undulator radiation from high energy muon beams. This reduces the energy requirements by a factor of more than $10^7$ compared to a pp collider. Much of the reduction in energy use is achieved by using a periodic magnetic field, and by splitting the muon wavefunctions spatially to reduce the photon beam divergence; these prevent a cascade of secondary reactions at the collision points. The proposed collider would be powered by (part of) a Dyson swarm constructed around the Sun, and efficient use of energy will be important to reduce the time needed to reach the desired number of collisions. We also discuss why a neutrino collider would be much less efficient.

physics.acc-ph↗

arXiv2021

Optimisation of complex integration contours at higher order

We continue our study of contour deformation as a practical tool for dealing with the sign problem using the $d$-dimensional Bose gas with non-zero chemical potential as a toy model. We derive explicit expressions for contours up to the second order with respect to a natural small parameter and generalise these contours to an ansatz for which the evaluation of the Jacobian is fast ($O(1)$). We examine the behaviour of the various proposed contours as a function of space-time dimensionality, the chemical potential, and lattice size and geometry and use the mean phase factor as a measure of the severity of the sign problem. In turns out that this method leads to a substantial reduction of the sign problem and that it becomes more efficient as space-time dimensionality is increased. Correlations among contributions to $\operatorname{Im}\langle S \rangle$ play a key role in determining the mean phase factor and we examine these correlations in detail.

hep-lat↗

arXiv2018

A simple approach towards the sign problem using path optimisation

We suggest an approach for simulating theories with a sign problem that relies on optimisation of complex integration contours that are not restricted to lie along Lefschetz thimbles. To that end we consider the toy model of a one-dimensional Bose gas with chemical potential. We identify the main contribution to the sign problem in this case as coming from a nearest neighbour interaction and approximately cancel it by an explicit deformation of the integration contour. We extend the obtained expressions to more general ones, depending on a small set of parameters. We find the optimal values of these parameters on a small lattice and study their range of validity. We also identify precursors for the onset of the sign problem. A fast method of evaluating the Jacobian related to the contour deformation is proposed and its numerical stability is examined. For a particular choice of lattice parameters, we find that our approach increases the lattice size at which the sign problem becomes serious from $L \approx 32$ to $L \approx 700$. The efficient evaluation of the Jacobian ($O(L)$ for a sweep) results in running times that are of the order of a few minutes on a standard laptop.

hep-lat↗

arXiv2017

Pion electromagnetic form factor from full lattice QCD

We present the first calculation of the pion electromagnetic form factor at physical light quark masses. This form factor parameterises the deviations from the behaviour of a point-like particle when a photon hits the pion. These deviations result from the internal structure of the pion and can thus be calculated in QCD. We use three sets (different lattice spacings) of $n_f = 2+1+1$ lattice configurations generated by the MILC collaboration. The Highly Improved Staggered Quark formalism (HISQ) is used for all of the sea and valence quarks. Using lattice configurations with $u$/$d$ quark masses very close to the physical value is a big advantage, as we avoid the chiral extrapolation. We study the shape of the vector ($f_+$) form factor in the $q^2$ range from $0$ to $-0.15$~GeV$^2$ and extract the mean square radius, $\langle r^2_v\rangle$. The shape of the vector form factor and the resulting radius is compared with experiment. We also discuss the scalar form factor and radius extracted from that, which is not directly accessible to experiment. We have also calculated the contributions from the disconnected diagrams to the scalar form factor at small $q^2$ and discuss their impact on the scalar radius $\langle r^2_s\rangle$.

hep-lat↗

arXiv2014

Lattice String Field Theory: The linear dilaton in one dimension

We propose the use of lattice field theory for the study of string field theory at the non-perturbative quantum level. We identify many potential obstacles and examine possible resolutions thereof. We then experiment with our approach in the particularly simple case of a one-dimensional linear dilaton and analyse the results.

hep-th↗

arXiv2013

Pion electromagnetic form factor from full lattice QCD

We present preliminary results from the first calculation of the pion electromagnetic form factor at physical light quark masses. This form factor parameterises the deviations from the behaviour of a point-like particle when a photon hits the pion. These deviations result from the internal structure of the pion and can thus be calculated in QCD. We use three sets (different lattice spacings) of n_f=2+1+1 lattice configurations generated by the MILC collaboration. The Highly Improved Staggered Quark formalism (HISQ) is used for all of the sea and valence quarks. Using lattice configurations with u/d quark masses very close to the physical value is an advantage, as we avoid the chiral extrapolation. We study the shape of the vector (f_+) form factor in the q^2 range from 0 to -0.12 GeV^2 and extract the mean square radius, . The shape of the vector form factor and the resulting radius is compared with experiment.

hep-lat↗

arXiv2013

Mesons in large-N QCD

We present the results of a systematic, first-principles study of the spectrum and decay constants of mesons for different numbers of color charges N, via lattice computations. We restrict our attention to states in the non-zero isospin sector, evaluating the masses associated with the ground-state and first excitation in the pseudoscalar, vector, scalar, and axial vector channels. Our results are based on a new set of simulations of four dimensional SU(N) Yang-Mills theories with the number of colors ranging from N=2 to N=17; the spectra and the decay constants are computed in the quenched approximation (which becomes exact in the 't Hooft limit) using Wilson fermions. After discussing the extrapolations to the chiral and large-N limits, we present a comparison of our results to some of the numerical computations and analytical predictions available in the literature - including, in particular, those from holographic computations.

hep-lat↗

arXiv2013

The meson spectrum in large-N QCD

We present lattice results on the meson spectrum and decay constants in large-N QCD. The results are obtained in the quenched approximation for N = 2, 3, 4, 5, 6, 7 and 17 and extrapolated to infinite N.

hep-lat↗

arXiv2012

SO(2N) and SU(N) gauge theories in 2+1 dimensions

We perform an exploratory investigation of how rapidly the physics of SO(2N) gauge theories approaches its N=oo limit. This question has recently become topical because SO(2N) gauge theories are orbifold equivalent to SU(N) gauge theories, but do not have a finite chemical potential sign problem. We consider only the pure gauge theory and, because of the inconvenient location of the lattice strong-to-weak coupling 'bulk' transition in 3+1 dimensions, we largely confine our numerical calculations to 2+1 dimensions. We discuss analytic expectations in both D=2+1 and D=3+1, show that the SO(6) and SU(4) spectra do indeed appear to be the same, and show that a number of mass ratios do indeed appear to agree in the large-N limit. In particular SO(6) and SU(3) gauge theories are quite similar except for the values of the string tension and coupling, both of which differences can be readily understood.

hep-lat↗

arXiv2011

Calculation of the One W Loop $H \to γγ$ Decay Amplitude with a Lattice Regulator

There has been a controversial recent claim that the standard result on the Higgs to two photon decay rate is incorrect, with the use of dimensional regularization fingered as the alleged culprit. Given the great importance of the $H\to γγ$ process as a possible Standard Model Higgs discovery channel at the LHC if the Higgs mass is light, it is critical to find a way to check the correctness of the results of dimensional regularization for this process. Here we report the results of a perturbative calculation of the $H\to γγ$ decay amplitude using a spacetime lattice as a UV regulator, which is the only known gauge-invariant regulator for non-Abelian gauge theories other than dimensional regularization. We find that the decay amplitude calculated using lattice-regularized perturbation theory is consistent to very high statistical accuracy with the decay amplitude obtained using dimensional regularization.

hep-ph↗

arXiv2011

Improved Lattice Spectroscopy of Minimal Walking Technicolor

We present a numerical study of spectroscopic observables in the SU(2) gauge theory with two adjoint fermions using improved source and sink operators. We compare in detail our improved results with previous determinations of masses that used point sources and sinks and we investigate possible systematic effects in both cases. Such comparison enables us to clearly assess the impact of a short temporal extent on the physical picture, and to investigate some effects due to the finite spatial box. While confirming the IR-conformal behaviour of the theory, our investigation shows that in order to make firm quantitative predictions, a better handle on finite size effects is needed.

hep-lat↗

arXiv2011

Mass anomalous dimension in SU(2) with six fundamental fermions

We simulate SU(2) gauge theory with six massless fundamental Dirac fermions. We measure the running of the coupling and the mass in the Schroedinger Functional scheme. We observe very slow running of the coupling constant. We measure the mass anomalous dimension gamma, and find it is between 0.135 and 1.03 in the range of couplings consistent with the existence of an IR fixed point.

hep-ph↗

arXiv2010

Improved Spectroscopy of Minimal Walking Technicolor

We present an improved study of spectroscopic observables in the $SU(2)$ Yang-Mills theory with two adjoint fermions. We make an improvement on the precision of previous results which clarify the scale of finite volume effects present. This analysis adds to the evidence for near-conformal dynamics of this theory, while indicating a preference for a low anomalous mass dimension of the massless theory.

hep-lat↗

arXiv2010

Mass anomalous dimension and running of the coupling in SU(2) with six fundamental fermions

We simulate SU(2) gauge theory with six massless fundamental Dirac fermions. By using the Schrödinger Functional method we measure the running of the coupling and the fermion mass over a wide range of length scales. We observe very slow running of the coupling and construct an estimator for the fermion mass anomalous dimension giving $0.135 <γ< 1.03$ in the region compatible with an IR fixed point.

hep-ph↗

arXiv2010

Lattice String Field Theory

String field theory is a candidate for a full non-perturbative definition of string theory. We aim to define string field theory on a space-time lattice to investigate its behaviour at the quantum level. Specifically, we look at string field theory in a one dimensional linear dilaton background. We report the first results of our simulations.

hep-lat↗

arXiv2009

Mass anomalous dimension in SU(2) with two adjoint fermions

We study SU(2) lattice gauge theory with two flavours of Dirac fermions in the adjoint representation. We measure the running of the coupling in the Schroedinger Functional (SF) scheme and find it is consistent with existing results. We discuss how systematic errors affect the evidence for an infrared fixed point (IRFP). We present the first measurement of the running of the mass in the SF scheme. The anomalous dimension of the chiral condensate, which is relevant for phenomenological applications, can be easily extracted from the running of the mass, under the assumption that the theory has an IRFP. At the current level of accuracy, we can estimate 0.05 < gamma < 0.56 at the IRFP.

hep-ph↗

arXiv2009

Running of the coupling and quark mass in SU(2) with two adjoint fermions

We simulate SU(2) gauge theory with two massless Dirac fermions in the adjoint representation. We calculate the running of the Schroedinger Functional coupling and the renormalised quark mass over a wide range of length scales. The running of the coupling is consistent with the existence of an infrared fixed point (IRFP), and we find 0.07 < gamma < 0.56 at the IRFP, depending on the value of the critical coupling.

hep-ph↗

arXiv2008

Mesons at large Nc from lattice QCD

We calculate the pion and rho meson masses in quenched SU(N) gauge theories for N = 2, 3, 4 and 6. Extrapolating these results to the chiral and large-N limits, we find m_rho = (1.670 +/- 0.024) sqrt(sigma) = (741 +/- 11) MeV for the rho meson mass at a fixed lattice spacing a \approx 0.2093 sqrt(sigma) \approx 0.093 fm where we use the (arbitrary) value (444 MeV)^2 for the string tension. We estimate a continuum limit large-N value, m_rho = 1.77 +/- 0.05 sqrt(sigma). We find 1/N^2 corrections to be small and we compare our results to predictions from AdS/QCD.

hep-lat↗