## Publications

122 results found

Camargo-Molina JE, Rajantie A, 2023, Phase transitions in de Sitter spacetimes: The stochastic formalism, *PHYSICAL REVIEW D*, Vol: 107, ISSN: 2470-0010

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- Citations: 1

Camargo-Molina JE, Gonzalez MC, Rajantie A, 2023, Phase transitions in de Sitter spacetimes: Quantum Corrections, *PHYSICAL REVIEW D*, Vol: 107, ISSN: 2470-0010

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- Citations: 1

Cable A, Rajantie A, 2022, Second-order stochastic theory for self-interacting scalar fields in de Sitter spacetime, *PHYSICAL REVIEW D*, Vol: 106, ISSN: 2470-0010

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- Citations: 2

Mantziris A, Markkanen T, Rajantie A, 2022, The effective Higgs potential and vacuum decay in Starobinsky inflation, *JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS*, ISSN: 1475-7516

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- Citations: 1

Acharya B, Alexandre J, Benes P,
et al., 2022, Search for highly-ionizing particles in <i>pp</i> collisions at the LHC's Run-1 using the prototype MoEDAL detector, *EUROPEAN PHYSICAL JOURNAL C*, Vol: 82, ISSN: 1434-6044

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- Citations: 1

Acharya B, Alexandre J, Benes P,
et al., 2022, Search for magnetic monopoles produced via the Schwinger mechanism, *NATURE*, Vol: 602, Pages: 63-+, ISSN: 0028-0836

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- Citations: 15

Acharya B, Alexandre J, Benes P,
et al., 2022, Search for magnetic monopoles produced via the Schwinger mechanism., *Nature*, Vol: 602, Pages: 63-67

Electrically charged particles can be created by the decay of strong enough electric fields, a phenomenon known as the Schwinger mechanism1. By electromagnetic duality, a sufficiently strong magnetic field would similarly produce magnetic monopoles, if they exist2. Magnetic monopoles are hypothetical fundamental particles that are predicted by several theories beyond the standard model3-7 but have never been experimentally detected. Searching for the existence of magnetic monopoles via the Schwinger mechanism has not yet been attempted, but it is advantageous, owing to the possibility of calculating its rate through semi-classical techniques without perturbation theory, as well as that the production of the magnetic monopoles should be enhanced by their finite size8,9 and strong coupling to photons2,10. Here we present a search for magnetic monopole production by the Schwinger mechanism in Pb-Pb heavy ion collisions at the Large Hadron Collider, producing the strongest known magnetic fields in the current Universe11. It was conducted by the MoEDAL experiment, whose trapping detectors were exposed to 0.235 per nanobarn, or approximately 1.8 × 109, of Pb-Pb collisions with 5.02-teraelectronvolt center-of-mass energy per collision in November 2018. A superconducting quantum interference device (SQUID) magnetometer scanned the trapping detectors of MoEDAL for the presence of magnetic charge, which would induce a persistent current in the SQUID. Magnetic monopoles with integer Dirac charges of 1, 2 and 3 and masses up to 75 gigaelectronvolts per speed of light squared were excluded by the analysis at the 95% confidence level. This provides a lower mass limit for finite-size magnetic monopoles from a collider search and greatly extends previous mass bounds.

Cable A, Rajantie A, 2021, Free scalar correlators in de Sitter space via the stochastic approach beyond the slow-roll approximation, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 104, ISSN: 1550-2368

The stochastic approach to calculating scalar correlation functions in de Sitter spacetime is extended beyond the overdamped “slow-roll” approximation. We show that with the correct noise term, it reproduces the exact asymptotic long-distance behavior of field correlators in free field theory, thereby demonstrating the viability of the technique. However, we also show that the naïve way of calculating the noise term by introducing a cutoff at the horizon does not give the correct answer unless the cutoff is chosen specifically to give the required result. We discuss the implications of this for interacting theories.

Gould O, Ho DL-J, Rajantie A, 2021, Schwinger pair production of magnetic monopoles: momentum distribution for heavy-ion collisions, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 104, ISSN: 1550-2368

Magnetic monopoles may be produced by the dual Schwinger effect in strong magnetic fields. Today, thestrongest known magnetic fields in the Universe are produced fleetingly in heavy-ion collisions. We use thecomplex worldline instanton method to calculate the momentum distribution of magnetic monopolesproduced in heavy-ion collisions, in an approximation that includes the effect of the magnetic field to allorders but neglects monopole self-interactions. The result saturates the preparation time-energy uncertaintyprinciple, and yields a necessary ingredient for experimental monopole searches in heavy-ion collisions.

Karam A, Markkanen T, Marzola L,
et al., 2021, Higgs-like spectator field as the origin of structure, *European Physical Journal C: Particles and Fields*, Vol: 81, ISSN: 1124-1861

We show that the observed primordial perturbations can be entirely sourced by a light spectator scalar field with a quartic potential, akin to the Higgs boson, provided that the field is sufficiently displaced from vacuum during inflation. The framework relies on the indirect modulation of reheating, which is implemented without any direct coupling between the spectator field and the inflaton and does not require non-renormalisable interactions. The scenario gives rise to local non-Gaussianity with fNL≃5 as the typical signal. As an example model where the indirect modulation mechanism is realised for the Higgs boson, we study the Standard Model extended with right-handed neutrinos. For the Standard Model running we find, however, that the scenario analysed does not seem to produce the observed perturbation.

Ho DL-J, Rajantie A, 2021, Instanton solution for Schwinger production of 't Hooft-Polyakov monopoles, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 103, Pages: 1-9, ISSN: 1550-2368

We present the results of an explicit numerical computation of a novel instanton in Georgi-Glashow SU(2) theory. The instanton is physically relevant as a mediator of Schwinger production of ’t Hooft–Polyakov magnetic monopoles from strong magnetic fields. In weak fields, the pair production rate has previously been computed using the worldline approximation, which breaks down in strong fields due to the effects of finite monopole size. Using lattice field theory we have overcome this limit, including finite monopole size effects to all orders. We demonstrate that a full consideration of the internal monopole structure results in an enhancement to the pair production rate, and confirm earlier results that monopole production becomes classical at the Ambjørn-Olesen critical field strength.

Acharya B, Alexandre J, Beneš P, et al., 2021, Timepix3 as a solid-state time-projection chamber in particle and nuclear physics

Timepix3 devices are hybrid pixel detectors developed within the Medipix3 collaboration at CERN providing a simultaneous measurement of energy (ToT) and time of arrival (ToA) in each of its 256 × 256 pixels (pixel pitch: 55 µm). The timestamp resolution below 2 ns allows a measurement of charge carrier drift times, so that particle trajectories can be reconstructed in 3D on a microscopic level (z-resolution: 30-60 µm). The 3D trajectory reconstruction methodology developed elsewhere is validated against simulated data providing ground truth information of the incident angles. The detector response functions and the achievable track angular resolutions are determined. For the first time, data taken with Timepix3 in the MoEDAL experiment are presented. After extracting singly charged minimum ionizing particle (MIP) tracks from the mixed radiation field using characteristic track features, their impact angles are evaluated. The directionality of the MIP radiation field is shown in elevation angle (?) versus azimuthal angle (?) maps, “unfolded” using the simulated detector responses to an omnidirectional radiation field.

Mantziris A, Markkanen T, Rajantie A, 2021, Vacuum decay constraints on the Higgs curvature coupling from inflation, *Journal of Cosmology and Astroparticle Physics*, Vol: 2021, Pages: 1-22, ISSN: 1475-7516

We derive lower bounds for the Higgs-curvature coupling from vacuum stability during inflation in three inflationary models: quadratic and quartic chaotic inflation, and Starobinsky-like power-law inflation. In contrast to most previous studies we take the time-dependence of the Hubble rate into account both in the geometry of our past light-cone and in the Higgs effective potential, which is approximated with three-loop renormalisation group improvement supplemented with one-loop curvature corrections. We find that in all three models, the lower bound is ξ≳ 0.051... 0.066 depending on the top quark mass. We also demonstrate that vacuum decay is most likely to happen a few e-foldings before the end of inflation.

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

The MoEDAL trapping detector consists of approximately 800 kg of aluminum volumes. It was exposed during run 2 of the LHC program to 6.46 fb−1 of 13 TeV proton-proton collisions at the LHCb interaction point. Evidence for dyons (particles with electric and magnetic charge) captured in the trapping detector was sought by passing the aluminum volumes comprising the detector through a superconducting quantum interference device (SQUID) magnetometer. The presence of a trapped dyon would be signaled by a persistent current induced in the SQUID magnetometer. On the basis of a Drell-Yan production model, we exclude dyons with a magnetic charge ranging up to five Dirac charges (5gD) and an electric charge up to 200 times the fundamental electric charge for mass limits in the range 870–3120 GeV and also monopoles with magnetic charge up to and including 5gD with mass limits in the range 870–2040 GeV.

Karam A, Markkanen T, Marzola L,
et al., 2020, Novel mechanism for primordial perturbations in minimal extensions of the Standard Model, *The Journal of High Energy Physics*, Vol: 2020, ISSN: 1029-8479

We demonstrate that light spectator fields in their equilibrium can source sizeable CMB anisotropies through modulated reheating even in the absence of direct couplings to the inflaton. The effect arises when the phase space of the inflaton decay is modulated by the spectator which generates masses for the decay products. We call the mechanism indirect modulation and using the stochastic eigenvalue expansion show that it can source perturbations even four orders of magnitude larger than the observed amplitude. Importantly, the indirect mechanism is present in the Standard Model extended with right- handed neutrinos. For a minimally coupled Higgs boson this leads to a novel lower bound on the quartic coupling and constrains the neutrino Yukawas below unity.

Ho DL-J, Rajantie A, 2020, Electroweak sphaleron in a strong magnetic field, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 102, ISSN: 1550-2368

In an external magnetic field, the energy of the electroweak sphaleron—representing the energy barrier to baryon and lepton number violation—decreases but remains nonzero until the upper Ambjørn–Olesen critical field strength set by the Higgs mass and the electric charge. At this point the sphaleron energy vanishes. We demonstrate this by numerically computing the sphaleron configuration in the presence of an external magnetic field over the full range of field strengths until the energy barrier vanishes. We discuss the implications for baryogenesis in the early universe and the possibility of observing of baryon and lepton number violation in heavy-ion collisions.

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

Quantum electrodynamics predicts that in a strong electric field, electron–positron pairs are produced by the Schwinger process, which can be interpreted as quantum tunnelling through the Coulomb potential barrier. If magnetic monopoles exist, monopole–antimonopole pairs would be similarly produced in strong magnetic fields by the electromagnetic dual of this process. The production rate can be computed using semiclassical techniques without relying on perturbation theory, and therefore it can be done reliably in spite of the monopoles' strong coupling to the electromagnetic field. This article explains this phenomenon and discusses the bounds on monopole masses arising from the strongest magnetic fields in the universe, which are in neutron stars known as magnetars and in heavy ion collision experiments such as lead–lead collisions carried out in November 2018 in the large Hadron collider at CERN. It will also discuss open theoretical questions affecting the calculation.This article is part of a discussion meeting issue ‘Topological avatars of new physics’.

Imrith S, Mulryne DJ, Rajantie A, 2019, Primordial curvature perturbation from lattice simulations, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 100, ISSN: 1550-2368

We study the contribution to the primordial curvature perturbation on observational scales generated by the reheating field in massless preheating. To do so, we use lattice simulations and a recent extension to the δN formalism. The work demonstrates the functionality of these techniques for calculating the observational signatures of models in which nonperturbative reheating involves a light scalar field.

Markkanen T, Rajantie A, Stopyra S,
et al., 2019, Scalar correlation functions in de Sitter space from the stochastic spectral expansion, *Journal of Cosmology and Astroparticle Physics*, Vol: 2019, Pages: 1-23, ISSN: 1475-7516

We consider light scalar fields during inflation and show how the stochastic spectral expansion method can be used to calculate two-point correlation functions of an arbitrary local function of the field in de Sitter space. In particular, we use this approach for a massive scalar field with quartic self-interactions to calculate the fluctuation spectrum of the density contrast and compare it to other approximations. We find that neither Gaussian nor linear approximations accurately reproduce the power spectrum, and in fact always overestimate it. For example, for a scalar field with only a quartic term in the potential, V=λϕ4/4, we find a blue spectrum with spectral index n−1=0.579√λ.

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, *Physical Review D: Particles, Fields, Gravitation and Cosmology*, Vol: 98, Pages: 1-9, ISSN: 1550-2368

The observed dark matter abundance in the Universe can be fully accounted for by a minimally coupled spectator scalar field that was light during inflation and has sufficiently strong self-coupling. In this scenario, dark matter was produced during inflation by amplification of quantum fluctuations of the spectator field. The self-interaction of the field suppresses its fluctuations on large scales and, therefore, avoids isocurvature constraints. The scenario does not require any fine-tuning of parameters. In the simplest case of a single real scalar field, the mass of the dark matter particle is in the range 1 GeV≲m≲108 GeV, depending on the scale of inflation, and the lower bound for the quartic self-coupling is λ≳0.45.

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.

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