Publications (62)279.07 Total impact
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ABSTRACT: A novel nonlocal fourphoton interaction on the deformed spacetime is derived and studied in the three selected models (I, II, III). The first two models (I, II) are obtained via two distinct second order thetaexact SeibergWitten maps of the noncommutative U(1) gauge field strength on Moyal space. The third one (III), inspired by the manifestly gauge invariant structures emerging in the first two, due to the model generality has been constructed with a different set of freedom parameters. All model's physical relevancy is analyzed by evaluating the fourphoton tadpole diagram, which, when combined with the bubble graph, enables to fully consider all contributions to the oneloop photon polarization tensor. For an arbitrary noncommutative matrix theta^mu nu, the full quadratic IR divergence cancelation in the oneloop photon twopoint function is obtained with particular combinations of SeibergWitten map/gaugesymmetry freedom parameters in the models (I, II). Finally, our model (III) enables complete elimination of all pathological terms in the photon polarization tensor at oneloop order if a special value for the thetamatrix is chosen.  [Show abstract] [Hide abstract]
ABSTRACT: We present our results for the fourphotontadpole diagram in a noncommutative U(1) gauge field theory deformed by the SeibergWitten (SW) maps and evaluated via a fullfledged thetaexact approach. Such a contribution, vanishing in the fourdimensional commutative theory under dimensional regularization, now reveals a quadratic IR divergence arising from the UV/IR connection in the Feynman loop integrals. We show that the fourphotontadpole diagram in our model has the same tensorial structure as the threephotonbubble diagram studied previously by us. Employing two distinct thetaexact gauge field strength SW maps and including various gaugeinvariance inspired deformation parameters, we perform a sum over both the relevant diagrams to obtain the full result for the oneloop photon selfenergy. We observe that contrary to the bubble diagram alone, where a selection of parameters that makes it free of UV/IR mixing anomalies does exists, one cannot do away with quadratic IR divergences in the sum, thus misses out to achieve a troublefree oneloop result. Certain possibilities of regaining control over these anomalies in the photon polarization tensor are also discussed.  [Show abstract] [Hide abstract]
ABSTRACT: In a spatially infinite and eternal universe approaching ultimately a de Sitter (or quaside Sitter) regime, structure can form by thermal fluctuations as such a space is thermal. The models of Dark Energy invoking holographic principle fit naturally into such a category, and spontaneous formation of isolated brains in otherwise empty space seems the most perplexing, creating the paradox of Boltzmann Brains (BB). It is thus appropriate to ask if such models can be made free from domination by Boltzmann Brains. Here we consider only the simplest model, but adopt both the local and the global viewpoint in the description of the Universe. In the former case, we find that if a parameter $c$, which modulates the Dark Energy density, lies outside the exponentially narrow strip around the most natural $c = 1$ line, the theory is rendered BBsafe. In the later case, the bound on $c$ is exponentially stronger, and seemingly at odds with those bounds on $c$ obtained from various observational tests. 
Article: Selfenergies on deformed spacetimes
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ABSTRACT: We study oneloop photon (Pi) and neutrino (Sigma) selfenergies in a U(1) covariant gaugetheory on ddimensional noncommutative spaces determined by a antisymmetricconstant tensor theta^{mu nu}. For the general fermionphoton (kappa_f) and photon selfinteraction (kappa_g) the closed form results reveal selfenergies 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 photonloop produces new tensor structures satisfying transversality condition by themselves. We show that the photon selfenergy in fourdimensional 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 twopoint function vanishes. Thus for a specific point (0,3) in the parameterspace (kappa_f,kappa_g), a covariant thetaexact approach is able to produce a divergencefree result for oneloop quantum corrections, having also welldefined both the commutative limit as well as the pointlike limit of an extended object. While in twodimensional space the photon selfenergy is finite for arbitrary (kappa_f,kappa_g) combinations, the neutrino selfenergy still contains an superficial IR divergence.  [Show abstract] [Hide abstract]
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 photoelectriclike 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. 
Article: Forbidden and invisible Z boson decays in a covariant θexact noncommutative standard model
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ABSTRACT: Triple neutral gauge boson and direct photonneutrino 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 nonperturbative methods and a SeibergWitten (SW) map based covariant approach to noncommutative gauge theory, we obtain thetaexact expressions for the interactions, thereby eliminating previous restrictions to lowenergy 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. 
Article: A bound on the scale of spacetime noncommutativity from the reheating phase after inflation
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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 preinflation radiationdominated 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.  [Show abstract] [Hide abstract]
ABSTRACT: Oneloop thetaexact quantum corrections to the neutrino propagator are computed in noncommutative U*(1) gaugetheory based on SeibergWitten maps. Our closed form results show that the oneloop correction contains a hard 1/epsilon UV divergence, as well as a logarithmic IRdivergent 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 nonperturbative 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 SeibergWitten maps may be used to affect neutrino propagation in a profound way. 
Article: STRINGENT CONSTRAINT ON THE SCALAR–NEUTRINO COUPLING CONSTANT FROM QUINTESSENTIAL COSMOLOGY
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ABSTRACT: An extremely light (mϕ≪1033eV), slowlyvarying 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 slowroll 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 1010 level.  [Show abstract] [Hide abstract]
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 109 to 1010 in the case of a nonuniversal tensor neutrinogravity coupling, whereas there is no solution in the case of a nonuniversal scalar neutrinogravity coupling. Neutrinos may remain massless and the requisite magnetic field strength is similar to that in the conventional mass oscillation mechanism.  [Show abstract] [Hide abstract]
ABSTRACT: The righthanded 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 neutrinomagneticmoment and neutrinochargeradius interactions.  [Show abstract] [Hide abstract]
ABSTRACT: We consider Yukawa couplings in a thetaexact 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 neutrinomass extended standard model on noncommutative spacetime can the formulated in the new nonperturbative (in theta) approach which eliminates the previous restriction of SeibergWitten map based theories to lowenergy phenomena. Spacetime noncommutativity induced couplings between neutrinos and photons as well as Zbosons appear quite naturally in the model. We derive relevant Feynman rules for the type I seesaw mechanism.  [Show abstract] [Hide abstract]
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 quantumgravity 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 thetaexpanded SeibergWitten (SW) maps UV/IR mixing effects may be under control, a fairly recent study of photon selfenergy within a SW thetaexact approach has shown that UV/IR mixing is still present. We study the selfenergy contribution for neutral fermions in the thetaexact 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.  [Show abstract] [Hide abstract]
ABSTRACT: Photonneutrino 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 nonperturbative methods and a SeibergWitten map based covariant approach to noncommutative gauge theory, we obtain thetaexact expressions for the interactions, thereby eliminating previous restrictions to lowenergy 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.  [Show abstract] [Hide abstract]
ABSTRACT: In this experiment we aim to look for keVmass 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 scalarelectron 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 scalarproton/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.  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: An important window to quantum gravity phenomena in low energy noncommutative (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 noncommutativity 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 welldefined E/ΛNC ≲ 1, gets affected when holography is applied by providing the energy of the system E a ΛNCdependent lower limit. This may affect models which try to infer the scale ΛNC by using data from lowenergy experiments. KeywordsNonCommutative Geometry–Models of Quantum Gravity  [Show abstract] [Hide abstract]
ABSTRACT: If new physics were capable of pushing the neutrinonucleon 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 treelevel coupling to photons in a generationindependent 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.  [Show abstract] [Hide abstract]
ABSTRACT: We study a process of equilibration of holographic dark energy (HDE) with the cosmic horizon around the darkenergy 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 quantummechanical 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 particlephysics phenomena. The problem of negative entropies for the nonphantom stuff is also discussed. Comment: 10 pages, version to appear in PLB  [Show abstract] [Hide abstract]
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, nonblack hole events to be accounted for. In formulating such theories with a large number of particle species $N$, we employ constraints from the muon $g2$, higherdimensional operator corrections due to the required UV and IR cutoffs as well as the RG evolution in a conventional fieldtheoretical 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 $g2$ 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
Publication Stats
1k  Citations  
279.07  Total Impact Points  
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Institutions

19842015

Ruđer Bošković Institute
 Division of Experimental Physics
Zagrabia, Grad Zagreb, Croatia


1988

Ruder Boskovic Institute
Zagrabia, Grad Zagreb, Croatia
