R. Horvat

Ruđer Bošković Institute, Zagrabia, Grad Zagreb, Croatia

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Publications (58)174.13 Total impact

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    ABSTRACT: We study one-loop photon (Pi) and neutrino (Sigma) self-energies in a U(1) covariant gauge-theory on d-dimensional noncommutative spaces determined by a antisymmetric-constant tensor theta^{mu nu}. For the general fermion-photon (kappa_f) and photon self-interaction (kappa_g) the closed form results reveal self-energies besetting with all kind of pathological terms: the UV divergence, the quadratic UV/IR mixing terms as well as a logarithmic IR divergent term of the type ln(mu^2(theta p)^2). In addition, the photon-loop produces new tensor structures satisfying transversality condition by themselves. We show that the photon self-energy in four-dimensional Euclidean spacetime can be reduced to two finite terms by imposing a specific full rank of theta^{mu nu} and setting parameters (kappa_f,kappa_g)=(0,3). In this case the neutrino two-point function vanishes. Thus for a specific point (0,3) in the parameter-space (kappa_f,kappa_g), a covariant theta-exact approach is able to produce a divergence-free result for one-loop quantum corrections, having also well-defined both the commutative limit as well as the pointlike limit of an extended object. While in two-dimensional space the photon self-energy is finite for arbitrary (kappa_f,kappa_g) combinations, the neutrino self-energy still contains an superficial IR divergence.
    Journal of High Energy Physics 06/2013; · 5.62 Impact Factor
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    ABSTRACT: In this Letter we report on the results of our search for photons from a U(1) gauge factor in the hidden sector of the full theory. With our experimental setup we observe the single spectrum in a HPGe detector arising as a result of the photoelectric-like absorption of hidden photons emitted from the Sun on germanium atoms inside the detector. The main ingredient of the theory used in our analysis, a severely constrained kinetic mixing from the two U(1) gauge factors and massive hidden photons, entails both photon into hidden state oscillations and a minuscule coupling of hidden photons to visible matter, of which the latter our experimental setup has been designed to observe. On a theoretical side, full account was taken of the effects of refraction and damping of photons while propagating in Sun's interior as well as in the detector. We exclude hidden photons with kinetic couplings chi > (2.2 x 10^{-13}- 3 x 10^{-7}) in the mass region 0.2 eV < m_gamma' < 30 keV. Our constraints on the mixing parameter chi in the mass region from 20 eV up to 15 keV prove even slightly better then those obtained recently by using data from the CAST experiment, albeit still somewhat weaker than those obtained from solar and HB stars lifetime arguments.
    Physics Letters B 10/2012; 721(s 4–5). · 4.57 Impact Factor
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    ABSTRACT: Triple neutral gauge boson and direct photon-neutrino interactions arise quite naturally in noncommutative gauge field theories. Such couplings are absent in ordinary field theory and imply experimental lower bounds on the energy scale Lambda_NC ~ |theta|^{-1/2} of spacetime noncommutativity. Using non-perturbative methods and a Seiberg-Witten (SW) map based covariant approach to noncommutative gauge theory, we obtain theta-exact expressions for the interactions, thereby eliminating previous restrictions to low-energy phenomena. We discuss implications for Z -> gamma gamma and Z -> nu barnu decays, and show that our results behave reasonably throughout all interaction energy scales. Constraining the invisible Z width for this kind of new physics to be under around 1 MeV, one produces Lambda_NC ~ 140 GeV. Although with the current experimental upper limit on the branching ratio BR(Z -> gamma gamma) the obtained bound on Lambda_NC is of the same order of magnitude, we have demonstrated how the expected improvement on the branching ratio from the LHC experiments may significantly strengthen the bound on the spacetime noncommutativity.
    Journal of Physics G Nuclear and Particle Physics 04/2012; · 5.33 Impact Factor
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    R. Horvat, J. Trampetic
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    ABSTRACT: In an approach to noncommutative gauge theories, where the full noncommutative behavior is delimited by the presence of the UV and IR cutoffs, we consider the possibility of describing a system at a temperature T in a box of size L. Employing a specific form of UV/IR relationship inherent in such an approach of restrictive noncommutativity, we derive, for a given temperature T, an upper bound on the parameter of spacetime noncommutativity Lambda_NC ~ |theta|^{-1/2}. Considering such epochs in the very early universe which are expected to reflect spacetime noncommutativity to a quite degree, like the reheating stage after inflation, or believable pre-inflation radiation-dominated epochs, the best limits on Lambda_NC are obtained. We also demonstrate how the nature and size of the thermal system (for instance, the Hubble distance versus the future event horizon) can affect our bounds.
    Physics Letters B 11/2011; 710(1). · 4.57 Impact Factor
  • R.horvat
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    ABSTRACT: The right-handed chiral component of the Dirac neutrino field in a dense medium is derived for two generation mixing. As an application, the modification factor on the luminosity of a supernova core is given for ν+ emission by neutrino-magnetic-moment and neutrino-charge-radius interactions.
    Modern Physics Letters A 11/2011; 07(27). · 1.11 Impact Factor
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    ABSTRACT: One-loop theta-exact quantum corrections to the neutrino propagator are computed in noncommutative U*(1) gauge-theory based on Seiberg-Witten maps. Our closed form results show that the one-loop correction contains a hard 1/epsilon UV divergence, as well as a logarithmic IR-divergent term of the type ln sqrt(theta p)^2, thus considerably softening the usual UV/IR mixing phenomenon. We show that both of these problematic terms vanish for a certain choice of the noncommutative parameter theta which preserves unitarity. We find non-perturbative modifications of the neutrino dispersion relations which are assymptotically independent of the scale of noncommutativity in both the low and high energy limits and may allow superluminal propagation. Finally, we demonstrate how the prodigious freedom in Seiberg-Witten maps may be used to affect neutrino propagation in a profound way.
    Journal of High Energy Physics 11/2011; 2012(4). · 5.62 Impact Factor
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    R.horvat
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    ABSTRACT: We consider the pulsar velocity problem and relate it to some unconventional neutrino oscillation mechanisms based on the violation of the equivalence principle by neutrinos. We show that the observed pulsar velocities may be explained by violations at the level from 10-9 to 10-10 in the case of a non-universal tensor neutrino-gravity coupling, whereas there is no solution in the case of a non-universal scalar neutrino-gravity coupling. Neutrinos may remain massless and the requisite magnetic field strength is similar to that in the conventional mass oscillation mechanism.
    Modern Physics Letters A 11/2011; 13(29). · 1.11 Impact Factor
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    R.horvat
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    ABSTRACT: An extremely light (mϕ≪10-33eV), slowly-varying scalar field ϕ (quintessence) with a potential energy density as large as 60% of the critical density has been proposed as the origin of the accelerated expansion of the universe at present. The interaction of this smoothly distributed component with another predominantly smooth component, the cosmic neutrino background, is studied. The slow-roll approximation for generic ϕ potentials may then be used to obtain a limit on the scalar–neutrino coupling constant, found to be many orders of magnitude more stringent than the limits set by observations of neutrinos from SN 1987A. In addition, if quintessential theory allows for a violation of the equivalence principle in the sector of neutrinos, the current solar neutrino data can probe such a violation at the 10-10 level.
    Modern Physics Letters A 11/2011; 14(32). · 1.11 Impact Factor
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    ABSTRACT: We consider Yukawa couplings in a theta-exact approach to noncommutative gauge field theory and show that both Dirac and singlet Majorana neutrino mass terms can be consistently accommodated. This shows that in fact the whole neutrino-mass extended standard model on noncommutative spacetime can the formulated in the new nonperturbative (in theta) approach which eliminates the previous restriction of Seiberg-Witten map based theories to low-energy phenomena. Spacetime noncommutativity induced couplings between neutrinos and photons as well as Z-bosons appear quite naturally in the model. We derive relevant Feynman rules for the type I seesaw mechanism.
    Physics Letters B 09/2011; · 4.57 Impact Factor
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    ABSTRACT: In formulating gauge field theories on noncommutative (NC) spaces it is suggested that particles carrying gauge invariant quantities should not be viewed as pointlike, but rather as extended objects whose sizes grow linearly with their momenta. This and other generic properties deriving from the nonlocal character of interactions (showing thus unambiguously their quantum-gravity origin) lead to a specific form of UV/IR mixing as well as to a pathological behavior at the quantum level when the noncommutativity parameter theta is set to be arbitrarily small. In spite of previous suggestions that in a NC gauge theory based on the theta-expanded Seiberg-Witten (SW) maps UV/IR mixing effects may be under control, a fairly recent study of photon self-energy within a SW theta-exact approach has shown that UV/IR mixing is still present. We study the self-energy contribution for neutral fermions in the theta-exact approach of NC QED, and show by explicit calculation that all but one divergence can be eliminated for a generic choice of the noncommutativity parameter theta. The remaining divergence is linked to the pointlike limit of an extended object.
    Journal of High Energy Physics 09/2011; · 5.62 Impact Factor
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    ABSTRACT: Photon-neutrino interactions arise quite naturally in noncommutative field theories. Such couplings are absent in ordinary field theory and imply experimental lower bounds on the energy scale Lambda_NC ~ 1/|theta|^2 of noncommutativity. Using non-perturbative methods and a Seiberg-Witten map based covariant approach to noncommutative gauge theory, we obtain theta-exact expressions for the interactions, thereby eliminating previous restrictions to low-energy phenomena. We discuss implications for plasmon decay, neutrino charge radii, big bang nucleosynthesis and ultrahigh energy cosmic rays. Our results behave reasonably throughout all interaction energy scales, thus facilitating further phenomenological applications.
    Physical review D: Particles and fields 03/2011; 84.
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    ABSTRACT: In this experiment we aim to look for keV-mass bosons emitted from the Sun, by looking at a process analogous to the photoelectric/Compton effect inside the HPGe detector. Their coupling to both electrons and nucleons is assumed. For masses above 25 keV, the mass dependence of our limit on the scalar-electron coupling reveals a constraint which proves stronger than that obtained recently and based on the very good agreement between the measured and predicted solar neutrino flux from the 8B reaction. On the other hand, the mass dependence of our limit on the scalar-proton/electron coupling together entails a limit on a possible Yukawa addition to the gravitational inverse square low. Such a constraint on the Yukawa interactions proves much stronger than that derived from the latest AFM Casimir force measurement.
    Physics Letters B 01/2011; · 4.57 Impact Factor
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    R. Horvat
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    ABSTRACT: In a recently proposed scenario for primordial inflation, where the Standard Model (SM) Higgs boson plays a role of the inflation field, an effective field theory (EFT) approach is the most convenient for working out the consequences of breaking of perturbative unitarity, caused by the strong coupling of the Higgs field to the Ricci scalar. The domain of validity of the EFT approach is given by the ultraviolet (UV) cutoff, which, roughly speaking, should always exceed the Hubble parameter in the course of inflation. On the other hand, applying the trusted principles of quantum gravity to a local EFT demands that it should only be used to describe states in a region larger than their corresponding Schwarschild radius, manifesting thus a sort of UV/IR correspondence. We consider both constraints on EFT, to ascertain which models of the SM Higgs inflation are able to simultaneously comply with them. We also show that if the gravitational coupling evolves with the scale factor, the holographic constraint can be alleviated significantly with minimal set of canonical assumptions, by forcing the said coupling to be asymptotically free.
    Physics Letters B 01/2011; 699(3). · 4.57 Impact Factor
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    Raul Horvat, Josip Trampetić
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    ABSTRACT: An important window to quantum gravity phenomena in low energy noncom-mutative (NC) quantum field theories (QFTs) gets represented by a specific form of UV/IR mixing. Yet another important window to quantum gravity, a holography, manifests itself in effective QFTs as a distinct UV/IR connection. In matching these two principles, a useful relationship connecting the UV cutoff ΛUV, the IR cutoff ΛIR and the scale of non-commutativity ΛNC, can be obtained. We show that an effective QFT endowed with both principles may not be capable to fit disparate experimental bounds simultaneously, like the muon g − 2 and the masslessness of the photon. Also, the constraints from the muon g − 2 preclude any possibility to observe the birefringence of the vacuum coming from objects at cosmological distances. On the other hand, in NC theories without the UV completion, where the perturbative aspect of the theory (obtained by truncating a power series in L\textNC - 2 \Lambda_{\text{NC}}^{ - 2} ) becomes important, a heuristic estimate of the region where the perturbative expansion is well-defined E/ΛNC ≲ 1, gets affected when holography is applied by providing the energy of the system E a ΛNC-dependent lower limit. This may affect models which try to infer the scale ΛNC by using data from low-energy experiments. KeywordsNon-Commutative Geometry–Models of Quantum Gravity
    Journal of High Energy Physics 01/2011; 2011(1):1-8. · 5.62 Impact Factor
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    ABSTRACT: If new physics were capable of pushing the neutrino-nucleon inelastic cross section 3 orders of magnitude beyond the standard model prediction, then ultrahigh energy (UHE) neutrinos would have already been observed at neutrino observatories. We use such a constraint to reveal information on the scale of noncommutativity (NC) ΛNC in noncommutative gauge field theories where neutrinos possess a tree-level coupling to photons in a generation-independent manner. In the energy range of interest (1010 to 1011 GeV), the θ expansion (|θ|∼1/ΛNC2) and, therefore, the perturbative expansion, in terms of ΛNC, retains no longer its meaningful character, forcing us to resort to those NC field theoretical frameworks involving the full θ resummation. Our numerical analysis of the contribution to the process coming from the photon exchange impeccably pins down a lower bound on ΛNC to be as high as 900 (450) TeV, depending on the estimates for the cosmogenic neutrino flux. If, on the other hand, one considers a surprising recent result that occurred in Pierre Auger Observatory data, that UHE cosmic rays are mainly composed of highly ionized Fe nuclei, then our bounds get weaker, due to the diminished cosmic neutrino flux. Nevertheless, we show that, even for the very high fraction of heavy nuclei in primary UHE cosmic rays, our method may still yield remarkable bounds on ΛNC, typically always above 200 TeV. Albeit, in this case, one encounters a maximal value for the Fe fraction, from which any useful information on ΛNC can be drawn, delimiting thus the applicability of our method.
    Physical review D: Particles and fields 05/2010; 83(6).
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    R. Horvat
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    ABSTRACT: We study a process of equilibration of holographic dark energy (HDE) with the cosmic horizon around the dark-energy dominated epoch. This process is characterized by a huge amount of information conveyed across the horizon, filling thereby a large gap in entropy between the system on the brink of experiencing a sudden collapse to a black hole and the black hole itself. At the same time, even in the absence of interaction between dark matter and dark energy, such a process marks a strong jump in the entanglement entropy, measuring the quantum-mechanical correlations between the horizon and its interior. Although the effective quantum field theory (QFT) with a peculiar relationship between the UV and IR cutoffs, a framework underlying all HDE models, may formally account for such a huge shift in the number of distinct quantum states, we show that the scope of such a framework becomes tremendously restricted, devoiding it virtually any application in other cosmological epochs or particle-physics phenomena. The problem of negative entropies for the non-phantom stuff is also discussed. Comment: 10 pages, version to appear in PLB
    Physics Letters B 03/2010; · 4.57 Impact Factor
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    R. Horvat
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    ABSTRACT: Effective field theories that manifest UV/IR mode mixing in such a way as to be valid for arbitrarily large volumes, can be used for gravitational, non-black hole events to be accounted for. In formulating such theories with a large number of particle species $N$, we employ constraints from the muon $g-2$, higher-dimensional operator corrections due to the required UV and IR cutoffs as well as the RG evolution in a conventional field-theoretical model in curved space. While in general our bounds on $N$ do reflect $N \simeq 10^{32}$, a bound motivated by the solution to the hierarchy problem in alike theories and obtained by the fact that strong gravity has not been seen in the particle collisions, the bound from the muon $g-2$ turns out to be much stronger, $N \lsim 10^{19}$. For systems on the verge of gravitational collapse, this bound on $N$ is far too restrictive to allow populating a large gap in entropy between those systems and that of black holes of the same size. Comment: 7 pages, version to appear in JHEP
    Journal of High Energy Physics 11/2009; · 5.62 Impact Factor
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    Raul Horvat, Josip Trampetic
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    ABSTRACT: We discuss a constraint on the scale $\Lambda_{\rm NC}$ of noncommutative (NC) gauge field theory arising from consideration of the big bang nucleosynthesis (BBN) of light elements. The propagation of neutrinos in the NC background described by an antisymmetric tensor $\theta^{\mu\nu}$ does result in a tree-level vector-like coupling to photons in a generation-independent manner, raising thus a possibility to have an appreciable contribution of three light right-handed (RH) fields to the energy density of the universe at nucleosynthesis time. Considering elastic scattering processes of the RH neutrinos off charged plasma constituents at a given cosmological epoch, we obtain for a conservative limit on an effective number of additional doublet neutrinos, $\Delta N_\nu =1$, a bound $\Lambda_{\rm NC} \stackrel{>}{\sim}$ 3 TeV. With a more stringent requirement, $\Delta N_\nu \lesssim 0.2$, the bound is considerably improved, $\Lambda_{\rm NC} \stackrel{>}{\sim} 10^3$ TeV. For our bounds the $\theta$-expansion of the NC action stays always meaningful, since the decoupling temperature of the RH species is perseveringly much less than the inferred bound for the scale of noncommutativity. Comment: 4 pages, version to appear in PRD
    Physical review D: Particles and fields 01/2009;
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    ABSTRACT: In this Letter we discuss light neutrino dipole moments, computed in the neutrino-mass extended Standard Model, as a possible source for neutrino condensates which may cause cosmological constant observed today.
    Physics Letters B 01/2009; · 4.57 Impact Factor
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    R. Horvat
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    ABSTRACT: An effective quantum field theory (QFT) with a manifest UV/IR connection, so as to be valid for arbitrarily large volumes, can successfully be applied to the cosmological dark energy problem as well as the cosmological constant (CC) problem. Motivated by recent approaches to the hierarchy problem, we develop such a framework with a large number of particle species. When applying to systems on the brink of experiencing a sudden collapse to a black hole, we find that the entropy, unlike the total energy, now becomes an increasing function of the number of field species. An internal consistency of the theory is then used to infer the upper bound on the number of particle species, showing consistency with the holographic Bekenstein–Hawking bound. This may thus serve to fill in a large gap in entropy of any non-black hole configuration of matter and the black holes. In addition, when the bound is saturated the entanglement entropy matches the black hole entropy, thus solving the multiplicity of species problem. In a cosmological setting, the maximum allowable number of species becomes a function of cosmological time, reaching its minimal value in a low-entropy post-reheating epoch.
    Physics Letters B 01/2009; · 4.57 Impact Factor

Publication Stats

746 Citations
174.13 Total Impact Points

Institutions

  • 1984–2012
    • Ruđer Bošković Institute
      • Division of Experimental Physics
      Zagrabia, Grad Zagreb, Croatia
  • 2009
    • Institute of Physics, Zagreb
      Zagrabia, Grad Zagreb, Croatia
  • 1988
    • University of Ottawa
      • Department of Physics
      Ottawa, Ontario, Canada