## Publications

113 results found

Cable A, Rajantie A, 2021, Free scalar correlators in de Sitter space via the stochastic approach beyond the slow-roll approximation, *PHYSICAL REVIEW D*, Vol: 104, ISSN: 2470-0010

Cable A, Rajantie A, 2021, Free scalar correlators in de Sitter space via the stochastic approach beyond the slow-roll approximation, *Physical Review D*, Vol: 104, ISSN: 2470-0010

Gould O, Ho DL-J, Rajantie A, 2021, Schwinger pair production of magnetic monopoles: Momentum distribution for heavy-ion collisions, *PHYSICAL REVIEW D*, Vol: 104, ISSN: 2470-0010

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- Citations: 3

Karam A, Markkanen T, Marzola L,
et al., 2021, Higgs-like spectator field as the origin of structure, *EUROPEAN PHYSICAL JOURNAL C*, Vol: 81, ISSN: 1434-6044

Ho DL-J, Rajantie A, 2021, Instanton solution for Schwinger production of 't Hooft-Polyakov monopoles, *PHYSICAL REVIEW D*, Vol: 103, ISSN: 2470-0010

Mantziris A, Markkanen T, Rajantie A, 2021, Vacuum decay constraints on the Higgs curvature coupling from inflation, *JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS*, ISSN: 1475-7516

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- Citations: 1

Acharya B, Alexandre J, Benes P,
et al., 2021, First Search for Dyons with the Full MoEDAL Trapping Detector in 13 TeV pp Collisions, *PHYSICAL REVIEW LETTERS*, Vol: 126, ISSN: 0031-9007

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- Citations: 3

Karam A, Markkanen T, Marzola L,
et al., 2020, Novel mechanism for primordial perturbations in minimal extensions of the Standard Model, *JOURNAL OF HIGH ENERGY PHYSICS*, ISSN: 1029-8479

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- Citations: 1

Ho DL-J, Rajantie A, 2020, Electroweak sphaleron in a strong magnetic field, *PHYSICAL REVIEW D*, Vol: 102, ISSN: 1550-7998

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- Citations: 2

Barausse E, Berti E, Hertog T,
et al., 2020, Prospects for fundamental physics with LISA, *General Relativity and Gravitation*, Vol: 52, Pages: 1-33, ISSN: 0001-7701

In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant to LISA,we present here a sample of what we view as particularly promising fundamental physics directions. We organize these directions through a “science-first” approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics.

Bruce R, dEnterria D, de Roeck A,
et al., 2020, New physics searches with heavy-ion collisions at the CERN Large Hadron Collider, *Journal of Physics G: Nuclear and Particle Physics*, Vol: 47, Pages: 1-20, ISSN: 0954-3899

This document summarises proposed searches for new physics accessible in the heavy-ion mode at the CERN Large Hadron Collider (LHC), both through hadronic and ultraperipheral γγ interactions, and that have a competitive or, even, unique discovery potential compared to standard proton–proton collision studies. Illustrative examples include searches for new particles—such as axion-like pseudoscalars, radions, magnetic monopoles, new long-lived particles, dark photons, and sexaquarks as dark matter candidates—as well as new interactions, such as nonlinear or non-commutative QED extensions. We argue that such interesting possibilities constitute a well-justified scientific motivation, complementing standard quark-gluon-plasma physics studies, to continue running with ions at the LHC after the Run-4, i.e. beyond 2030, including light and intermediate-mass ion species, accumulating nucleon–nucleon integrated luminosities in the accessible fb−1 range per month.

Ho DL-J, Rajantie A, 2020, Classical production of 't Hooft-Polyakov monopoles from magnetic fields, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 101, Pages: 055003-1-055003-6, ISSN: 1550-2368

We show that in the SU(2) Georgi-Glashow model, ’t Hooft–Polyakov monopoles are produced by a classical instability in magnetic fields above the Ambjørn-Olesen critical field, which coincides approximately with the field at which Schwinger pair production becomes unsuppressed. Below it, monopoles can be produced thermally, and we show that the rate is higher than for pointlike monopoles by calculating the sphaleron energy as a function of the magnetic field. The results can be applied to production of monopoles in heavy-ion collisions or in the early Universe.

Markkanen T, Rajantie A, 2020, Scalar correlation functions for a double-well potential in de Sitter space, *Journal of Cosmology and Astroparticle Physics*, Vol: 2020, Pages: 1-23, ISSN: 1475-7516

We use {the spectral representation of }the stochastic Starobinsky-Yokoyama approach to compute correlation functions in de Sitter space for a scalar field with a symmetric or asymmetric double-well potential. The terms in the spectral expansion are determined by the eigenvalues and eigenfunctions of the time-independent Fokker-Planck differential operator, and we solve them numerically. The long-distance asymptotic behaviour is given by the lowest state in the spectrum, but we demonstrate that the magnitude of the coeffients of different terms can be very different, and the correlator can be dominated by different terms at different distances. This can give rise to potentially observable cosmological signatures. In many cases the dominant states in the expansion do not correspond to small fluctuations around a minimum of the potential and are therefore not visible in perturbation theory. We discuss the physical interpretation these states, which can be present even when the potential has only one minimum.

Rajantie A, 2019, Monopole-antimonopole pair production by magnetic fields, *PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES*, Vol: 377, ISSN: 1364-503X

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- Citations: 3

Imrith S, Mulryne DJ, Rajantie A, 2019, Primordial curvature perturbation from lattice simulations, *PHYSICAL REVIEW D*, Vol: 100, ISSN: 2470-0010

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- Citations: 4

Markkanen T, Rajantie A, Stopyra S, et al., 2019, Scalar correlation functions in de Sitter space from the stochastic spectral expansion, Publisher: IOP PUBLISHING LTD

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- Citations: 19

Gould O, Ho DL-J, Rajantie A, 2019, Towards Schwinger production of magnetic monopoles in heavy-ion collisions, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 100, Pages: 015041-1-015041-19, ISSN: 1550-2368

Magnetic monopoles may be produced by the Schwinger effect in the strong magnetic fields of peripheral heavy-ion collisions. We review the form of the electromagnetic fields in such collisions and calculate from first principles the cross section for monopole pair production. Using the worldline instanton method, we work to all orders in the magnetic charge, and hence are not hampered by the breakdown of perturbation theory. Our result depends on the spacetime inhomogeneity through a single dimensionless parameter, the Keldysh parameter, which is independent of collision energy for a given monopole mass. For realistic heavy-ion collisions, the computational cost of the calculation becomes prohibitive and the finite size of the monopoles needs to be taken into account, and therefore our current results are not applicable to them—we indicate methods of overcoming these limitations, to be addressed in further work. Nonetheless, our results show that the spacetime dependence enhances the production cross section and would therefore lead to stronger monopole mass bounds than in the constant-field case.

Acharya B, Alexandre J, Baines S,
et al., 2019, Magnetic monopole search with the full MoEDAL trapping detector in 13 TeV pp collisions interpreted in photon-fusion and Drell-Yan production, *Physical Review Letters*, Vol: 123, ISSN: 0031-9007

MoEDAL is designed to identify new physics in the form of stable or pseudostable highly ionizing particles produced in high-energy Large Hadron Collider (LHC) collisions. Here we update our previous search for magnetic monopoles in Run 2 using the full trapping detector with almost four times more material and almost twice more integrated luminosity. For the first time at the LHC, the data were interpreted in terms of photon-fusion monopole direct production in addition to the Drell-Yan-like mechanism. The MoEDAL trapping detector, consisting of 794 kg of aluminum samples installed in the forward and lateral regions, was exposed to 4.0 fb−1 of 13 TeV proton-proton collisions at the LHCb interaction point and analyzed by searching for induced persistent currents after passage through a superconducting magnetometer. Magnetic charges equal to or above the Dirac charge are excluded in all samples. Monopole spins 0, ½, and 1 are considered and both velocity-independent and-dependent couplings are assumed. This search provides the best current laboratory constraints for monopoles with magnetic charges ranging from two to five times the Dirac charge.

Gould O, Mangles S, Rajantie A,
et al., 2019, Observing thermal Schwinger pair production, *Physical Review A*, Vol: 99, ISSN: 1050-2947

We study the possibility of observing Schwinger pair production enhanced by a thermal bath of photons. We consider the full range of temperatures and electric field intensities from pure Schwinger production to pure thermal production, and identify the most promising and interesting regimes. In particular, we identify temperatures of ∼ 20 keV/kB and field intensities of ∼ 10²³ W cm¯² where pair production would be observable. In this case, the thermal enhancement over the Schwinger rate is exponentially large and due to effects which are not visible at any finite order in the loop expansion. Pair production in this regime can thus be described as more nonperturbative than the usual Schwinger process, which appears at one loop. Unfortunately, such high temperatures appear to be out of reach of foreseeable technologies, though nonthermal photon distributions with comparable energy densities are possible. We suggest the possibility that similar nonperturbative enhancements may extend out of equilibrium and propose an experimental scheme to test this.

Markkanen T, Rajantie A, Tenkanen T, 2018, Spectator dark matter, Publisher: AMER PHYSICAL SOC

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- Citations: 27

Markkanen T, Rajantie A, Stopyra S, 2018, Cosmological aspects of Higgs vacuum metastability, *Frontiers in Astronomy and Space Sciences*, Vol: 5, ISSN: 2296-987X

The current central experimental values of the parameters of the Standard Model give rise to a striking conclusion: metastability of the electroweak vacuum is favored over absolute stability. A metastable vacuum for the Higgs boson implies that it is possible, and in fact inevitable, that a vacuum decay takes place with catastrophic consequences for the Universe. The metastability of the Higgs vacuum is especially significant for cosmology, because there are many mechanisms that could have triggered the decay of the electroweak vacuum in the early Universe. We present a comprehensive review of the implications from Higgs vacuum metastability for cosmology along with a pedagogical discussion of the related theoretical topics, including renormalization group improvement, quantum field theory in curved spacetime and vacuum decay in field theory.

Gould O, Rajantie A, Xie C, 2018, Worldline sphaleron for thermal Schwinger pair production, *Physical Review D - Particles, Fields, Gravitation and Cosmology*, Vol: 98, ISSN: 1550-2368

With increasing temperatures, Schwinger pair production changes from aquantum tunnelling to a classical, thermal process, determined by a worldlinesphaleron. We show this and calculate the corresponding rate of pair productionfor both spinor and scalar quantum electrodynamics, including the semiclassicalprefactor. For electron-positron pair production from a thermal bath of photonsand in the presence of an electric field, the rate we derive is faster thanboth perturbative photon fusion and the zero temperature Schwinger process. Wework to all-orders in the coupling and hence our results are also relevant tothe pair production of (strongly coupled) magnetic monopoles in heavy ioncollisions.

Imrith SV, Mulryne DJ, Rajantie A, 2018, Nonperturbative delta N formalism, *Physical Review D - Particles, Fields, Gravitation and Cosmology*, Vol: 98, ISSN: 1550-2368

We revisit the question of how to calculate correlations of the curvature perturbation, ζ, using the δN formalism when one cannot employ a truncated Taylor expansion of N. This problem arises when one uses lattice simulations to probe the effects of isocurvature modes on models of reheating. Working in real space, we use an expansion in the cross-correlation between fields at different positions and present simple expressions for observables such as the power spectrum and the reduced bispectrum, fNL. These take the same form as those of the usual δN expressions, but with the derivatives of N replaced by nonperturbative δN coefficients. We test the validity of this expansion and, when compared to others in the literature, argue that our expressions are particularly well suited for use with simulations.

Figueroa DG, Rajantie A, Torrenti F, 2018, Higgs field-curvature coupling and postinflationary vacuum instability, *Physical Review D - Particles, Fields, Gravitation and Cosmology*, Vol: 98, ISSN: 1550-2368

We study the postinflationary dynamics of the Standard Model (SM) Higgs field in the presence of a nonminimal coupling ξ|Φ|2R to gravity, both with and without the electroweak gauge fields coupled to the Higgs field. We assume a minimal scenario in which inflation and reheating are caused by chaotic inflation with a quadratic potential, and no additional new physics is relevant below the Planck scale. By using classical real-time lattice simulations with a renormalization group improved effective Higgs potential and by demanding the stability of the Higgs vacuum after inflation, we obtain upper bounds for ξ, taking into account the experimental uncertainty of the top-Yukawa coupling. We compare the bounds in the absence and presence of the electroweak gauge bosons and conclude that the addition of gauge interactions has a rather minimal impact. In the unstable cases, we parametrize the time when such instability develops. For a top-quark mass mt≈173.3 GeV, the Higgs vacuum instability is triggered for ξ≳4–5, although a slightly lower mass of mt≈172.1 GeV pushes up this limit to ξ≳11–12. This, together with the estimation ξ≳0.06 for stability during inflation, provides tight constraints to the Higgs field-curvature coupling within the SM.

Collaboration M, Acharya B, Alexandre J,
et al., 2018, Search for magnetic monopoles with the MoEDAL forward trapping detector in 2.11 fb$^{-1}$ of 13 TeV proton-proton collisions at the LHC, *Physics Letters B*, Vol: 782, Pages: 510-516, ISSN: 0370-2693

We update our previous search for trapped magnetic monopoles in LHC Run 2using nearly six times more integrated luminosity and including additionalmodels for the interpretation of the data. The MoEDAL forward trappingdetector, comprising 222~kg of aluminium samples, was exposed to 2.11~fb$^{-1}$of 13 TeV proton-proton collisions near the LHCb interaction point and analysedby searching for induced persistent currents after passage through asuperconducting magnetometer. Magnetic charges equal to the Dirac charge orabove are excluded in all samples. The results are interpreted in Drell-Yanproduction models for monopoles with spins 0, 1/2 and 1: in addition tostandard point-like couplings, we also consider couplings withmomentum-dependent form factors. The search provides the best currentlaboratory constraints for monopoles with magnetic charges ranging from two tofive times the Dirac charge.

Markkanen T, Nurmi S, Rajantie A,
et al., 2018, The 1-loop effective potential for the Standard Model in curved space time, *Journal of High Energy Physics*, Vol: 2018, ISSN: 1029-8479

The renormalisation group improved Standard Model effective potential in an arbitrary curved spacetime is computed to one loop order in perturbation theory. The loop corrections are computed in the ultraviolet limit, which makes them independent of the choice of the vacuum state and allows the derivation of the complete set of β-functions. The potential depends on the spacetime curvature through the direct non-minimal Higgs-curvature coupling, curvature contributions to the loop diagrams, and through the curvature dependence of the renormalisation scale. Together, these lead to significant curvature dependence, which needs to be taken into account in cosmological applications, which is demonstrated with the example of vacuum stability in de Sitter space.

Gillman E, Rajantie A, 2018, Kibble Zurek mechanism of topological defect formation in quantum field theory with matrix product states, *Physical Review D*, Vol: 97, ISSN: 2470-0010

The Kibble Zurek mechanism in a relativistic ϕ4 scalar field theory in D=(1+1) is studied using uniform matrix product states. The equal time two point function in momentum space G2(k) is approximated as the system is driven through a quantum phase transition at a variety of different quench rates τQ. We focus on looking for signatures of topological defect formation in the system and demonstrate the consistency of the picture that the two point function G2(k) displays two characteristic scales, the defect density n and the kink width dK. Consequently, G2(k) provides a clear signature for the formation of defects and a well defined measure of the defect density in the system. These results provide a benchmark for the use of tensor networks as powerful nonperturbative nonequilibrium methods for relativistic quantum field theory, providing a promising technique for the future study of high energy physics and cosmology.

Rajantie A, 2018, Higgs cosmology, *PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES*, Vol: 376, ISSN: 1364-503X

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- Citations: 1

Rajantie A, Stopyra S, 2018, Standard model vacuum decay in a de Sitter background, *Physical Review D*, Vol: 97, ISSN: 2470-0010

We present a calculation of thick-wall Coleman-de Luccia (CdL) bounces in the standard model effective potential in a de Sitter background. The calculation is performed including the effect of the bounce backreaction on the metric, which we compare with the case of a fixed de Sitter background, and with similar full-backreaction calculation in a model polynomial potential. The results show that the standard model potential exhibits nontrivial behavior: rather than a single CdL solution, there are multiple (nonoscillating) bounce solutions which may contribute to the decay rate. All the extra solutions found have higher actions than the largest amplitude solution, and thus would not contribute significantly to the decay rate, but their existence demonstrates that CdL solutions in the standard model potential are not unique, and the existence of additional, lower action, solutions cannot be ruled out. This suggests that a better understanding of the appearance and disappearance of CdL solutions in de Sitter space is needed to fully understand the vacuum instability issue in the standard model.

Gould O, Rajantie A, 2017, Magnetic monopole mass bounds from heavy-ion collisions and neutron stars, *Physical Review Letters*, Vol: 119, ISSN: 0031-9007

Magnetic monopoles, if they exist, would be produced amply in strong magnetic fields and high temperatures via the thermal Schwinger process. Such circumstances arise in heavy-ion collisions and in neutron stars, both of which imply lower bounds on the mass of possible magnetic monopoles. In showing this, we construct the cross section for pair production of magnetic monopoles in heavy-ion collisions, which indicates that they are particularly promising for experimental searches such as MoEDAL.

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