Publications
123 results found
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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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.
Markkanen TT, Rajantie AK, Nurmi ST, 2017, Do metric fluctuations affect the Higgs dynamics during inflation?, Journal of Cosmology and Astroparticle Physics, Vol: 2017, ISSN: 1475-7516
We show that the dynamics of the Higgs field during inflation is not affected by metric fluctuations if the Higgs is an energetically subdominant light spectator. For Standard Model parameters we find that couplings between Higgs and metric fluctuations are suppressed by Script O(10−7). They are negligible compared to both pure Higgs terms in the effective potential and the unavoidable non-minimal Higgs coupling to background scalar curvature. The question of the electroweak vacuum instability during high energy scale inflation can therefore be studied consistently using the Jordan frame action in a Friedmann-Lemaître-Robertson-Walker metric, where the Higgs-curvature coupling enters as an effective mass contribution. Similar results apply for other light spectator scalar fields during inflation.
Gillman E, Rajantie A, 2017, Topological defects in quantum field theory with matrix product states, Physical Review D - Particles, Fields, Gravitation and Cosmology, Vol: 96, ISSN: 1550-2368
Topological defects (kinks) in a relativistic $\phi^{4}$ scalar field theoryin $D=(1+1)$ are studied using the matrix product state tensor network. The onekink state is approximated as a matrix product state and the kink mass iscalculated. The approach used is quite general and can be applied to a varietyof theories and tensor networks. Additionally, the contribution ofkink-antikink excitations to the ground state is examined and a general methodto estimate the scalar mass from equal time ground state observables isprovided. The scalar and kink mass are compared at strong coupling and behaveas expected from universality arguments. This suggests that the matrix productstate can adequately capture the physics of defect-antidefect excitations andthus provides a promising technique to study challenging non-equilibriumphysics such as the Kibble-Zurek mechanism of defect formation.
Gould O, Rajantie A, 2017, Thermal Schwinger pair production at arbitrary coupling, Physical Review D - Particles, Fields, Gravitation and Cosmology, Vol: 96, ISSN: 1550-2368
We calculate the rate of thermal Schwinger pair production at arbitrary coupling in weak external fields. Our calculations are valid independently of many properties of the charged particles produced, in particular their spin and whether they are electric or magnetic. Using the worldline formalism, we calculate the logarithm of the rate to leading order in the weak external field and to all orders in virtual photon exchange, taking us beyond the perturbative expansion about the leading order, weak coupling result.
Rajantie A, 2017, Magnetic monopole search, past and present, PHYSICS TODAY, Vol: 70, Pages: 14-14, ISSN: 0031-9228
Acharya B, Alexandre J, Baines S, et al., 2017, Search for magnetic monopoles with the MoEDAL forward trapping detector in 13 TeV proton-proton collisions at the LHC, Physical Review Letters, Vol: 118, ISSN: 1079-7114
MoEDAL is designed to identify new physics in the form of long-lived highly ionizing particles produced in high-energy LHC collisions. Its arrays of plastic nuclear-track detectors and aluminium trapping volumes provide two independent passive detection techniques. We present here the results of a first search for magnetic monopole production in 13 TeV proton-proton collisions using the trapping technique, extending a previous publication with 8 TeV data during LHC Run 1. A total of 222 kg of MoEDAL trapping detector samples was exposed in the forward region and analyzed by searching for induced persistent currents after passage through a superconducting magnetometer. Magnetic charges exceeding half the Dirac charge are excluded in all samples and limits are placed for the first time on the production of magnetic monopoles in 13 TeV pp collisions. The search probes mass ranges previously inaccessible to collider experiments for up to five times the Dirac charge.
Markkanen T, Rajantie A, 2017, Massive scalar field evolution in de Sitter, Journal of High Energy Physics, Vol: 2017, ISSN: 1029-8479
The behaviour of a massive, non-interacting and non-minimally coupledquantised scalar field in an expanding de Sitter background is investigated bysolving the field evolution for an arbitrary initial state. In this approachthere is no need to choose a vacuum in order to provide a definition forparticle states. We conclude that the expanding de Sitter space is a stableequilibrium configuration under small perturbations of the initial conditions.Depending on the initial state, the energy density can approach its asymptoticvalue from above or below, the latter of which implies a violation of the weakenergy condition. The backreaction of the quantum corrections can thereforelead to a phase of super-acceleration also in the non-interacting massive case.
Rajantie A, Stopyra S, 2017, Standard model vacuum decay with gravity, Physical Review D, Vol: 95, ISSN: 1550-7998
We present a calculation of the decay rate of the electroweak vacuum, fully including all gravitational effects and a possible nonminimal Higgs-curvature coupling ξ, and using the three-loop Standard Model effective potential. Without a nonminimal coupling, we find that the effect of the gravitational backreaction is small and less significant than previous calculations suggested. The gravitational effects are smallest, and almost completely suppressed, near the conformal value ξ=1/6 of the nonminimal coupling. Moving ξ away from this value in either direction universally suppresses the decay rate.
Rajantie A, 2016, The search for magnetic monopoles, Physics Today, Vol: 69, Pages: 40-46, ISSN: 0031-9228
Electricity and magnetism appear everywhere in the modern world and form the basis of most of our technology. Therefore, it would be natural to assume that they are already fully understood and no longer pose unanswered fundamental physics questions. Indeed, for most practical purposes they are perfectly well described by classical electrodynamics, as formulated by James Clerk Maxwell in 1864. At a deeper level, a consistent quantum mechanical account is given by quantum electrodynamics, part of the standard model of particle physics. The theory works so well that it predicts the magnetic dipole moment of the electron accurately to 10 significant figures. Nevertheless, there is still an elementary aspect of electromagnetism that we do not understand: the question of magnetic monopoles.1That magnets always have two poles—north and south—seems like an obvious empirical fact. Yet we do not know any theoretical reason why magnetic monopoles, magnets with a single north or south pole, could not exist. Are we still missing some crucial fundamental aspect of the theory? Or do magnetic monopoles exist and we simply have not managed to find them yet?
The MC, Acharya B, Alexandre J, et al., 2016, Search for magnetic monopoles with the MoEDAL prototype trapping detector in 8 TeV proton-proton collisions at the LHC, Journal of High Energy Physics, Vol: 2016, ISSN: 1126-6708
The MoEDAL experiment is designed to search for magnetic monopoles and other highly-ionising particles produced in high-energy collisions at the LHC. The largely passive MoEDAL detector, deployed at Interaction Point 8 on the LHC ring, relies on two dedicated direct detection techniques. The first technique is based on stacks of nucleartrack detectors with surface area ~18m2, sensitive to particle ionisation exceeding a high threshold. These detectors are analysed offline by optical scanning microscopes. The second technique is based on the trapping of charged particles in an array of roughly 800 kg of aluminium samples. These samples are monitored offline for the presence of trapped magnetic charge at a remote superconducting magnetometer facility. We present here the results of a search for magnetic monopoles using a 160 kg prototype MoEDAL trapping detector exposed to 8TeV proton-proton collisions at the LHC, for an integrated luminosity of 0.75 fb–1. No magnetic charge exceeding 0:5gD (where gD is the Dirac magnetic charge) is measured in any of the exposed samples, allowing limits to be placed on monopole production in the mass range 100 GeV≤ m ≤ 3500 GeV. Model-independent cross-section limits are presented in fiducial regions of monopole energy and direction for 1gD ≤ |g| ≤ 6gD, and model-dependent cross-section limits are obtained for Drell-Yan pair production of spin-1/2 and spin-0 monopoles for 1gD ≤ |g| ≤ 4gD. Under the assumption of Drell-Yan cross sections, mass limits are derived for |g| = 2gD and |g| = 3gD for the first time at the LHC, surpassing the results from previous collider experiments.
Herranen M, Markkanen T, Nurmi S, et al., 2015, Spacetime curvature and Higgs stability after inflation, Physical Review Letters, Vol: 115, ISSN: 1079-7114
We investigate the dynamics of the Higgs field at the end of inflation in theminimal scenario consisting of an inflaton field coupled to the Standard Modelonly through the non-minimal gravitational coupling $\xi$ of the Higgs field.Such a coupling is required by renormalisation of the Standard Model in curvedspace, and in the current scenario also by vacuum stability during high-scaleinflation. We find that for $\xi\gtrsim 1$, rapidly changing spacetimecurvature at the end of inflation leads to significant production of Higgsparticles, potentially triggering a transition to a negative-energy Planckscale vacuum state and causing an immediate collapse of the Universe.
Rajantie A, 2015, Exploring the Early Universe with Gravitational Waves Concluding Remarks, EXPLORING THE EARLY UNIVERSE WITH GRAVITATIONAL WAVES, Publisher: SPRINGER-VERLAG BERLIN, Pages: 133-134, ISBN: 978-3-319-17448-8
Rajantie A, 2015, Exploring the Early Universe with Gravitational Waves Foreword, EXPLORING THE EARLY UNIVERSE WITH GRAVITATIONAL WAVES, Publisher: SPRINGER-VERLAG BERLIN, Pages: V-VI, ISBN: 978-3-319-17448-8
Rajantie A, 2015, Exploring the Early Universe with Gravitational Waves Introduction, EXPLORING THE EARLY UNIVERSE WITH GRAVITATIONAL WAVES, Publisher: SPRINGER-VERLAG BERLIN, Pages: 1-51, ISBN: 978-3-319-17448-8
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Rajantie A, 2015, Anisotropic Gravitational Wave Background from Massless Preheating, EXPLORING THE EARLY UNIVERSE WITH GRAVITATIONAL WAVES, Publisher: SPRINGER-VERLAG BERLIN, Pages: 91-131, ISBN: 978-3-319-17448-8
Rajantie A, 2015, Exploring the Early Universe with Gravitational Waves Appendix, EXPLORING THE EARLY UNIVERSE WITH GRAVITATIONAL WAVES, Publisher: SPRINGER-VERLAG BERLIN, Pages: 135-139, ISBN: 978-3-319-17448-8
Rajantie A, 2015, Chiral Tensor Power Spectrum from Quantum Gravity, EXPLORING THE EARLY UNIVERSE WITH GRAVITATIONAL WAVES, Publisher: SPRINGER-VERLAG BERLIN, Pages: 53-90, ISBN: 978-3-319-17448-8
Herranen M, Markkanen T, Nurmi S, et al., 2014, Spacetime curvature and the Higgs stability during inflation, Physical Review Letters, Vol: 113, ISSN: 0031-9007
It has been claimed that the electroweak vacuum may be unstable during inflation due to large fluctuations of the order H in the case of a high inflationary scale as suggested by BICEP2. We compute the standard model Higgs effective potential including UV-induced curvature corrections at one-loop level. We find that for a high inflationary scale a large curvature mass is generated due to renormalization group running of nonminimal coupling ξ, which either stabilizes the potential against fluctuations for ξEW≳6×10−2, or destabilizes it for ξEW≲2×10−2 when the generated curvature mass is negative. Only in the narrow intermediate region may the effect of the curvature mass be significantly smaller.
Acharya B, Alexandre J, Bernabeu J, et al., 2014, The physics programme of the MoEDAL experiment at the LHC, INTERNATIONAL JOURNAL OF MODERN PHYSICS A, Vol: 29, ISSN: 0217-751X
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- Citations: 88
Bethke L, Figueroa DG, Rajantie A, 2014, On the anisotropy of the gravitational wave background from massless preheating, JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS, ISSN: 1475-7516
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- Citations: 32
Orani S, Rajantie A, 2013, Supersymmetric hybrid inflation with a light scalar, PHYSICAL REVIEW D, Vol: 88, ISSN: 1550-7998
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- Citations: 2
Bethke L, Figueroa DG, Rajantie A, 2013, Anisotropies in the Gravitational Wave Background from Preheating, PHYSICAL REVIEW LETTERS, Vol: 111, ISSN: 0031-9007
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- Citations: 48
Weir DJ, Rajantie A, Rummukainen K, 2013, Form factor and width of a quantum string
We show how the form factor for a quantum string can be obtained from field correlation functions calculated in lattice Monte Carlo simulations. As an example, we apply this technique for simulations of the Ising model. We demonstrate that the form factor shows the same logarithmic broadening as observed by other quantities. Various difficulties in finding the intrinsic width of a string are discussed.
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