Henrik Melbeus

KTH Royal Institute of Technology, Tukholma, Stockholm, Sweden

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Publications (12)45.38 Total impact

  • Henrik Melbéus, Tommy Ohlsson
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    ABSTRACT: Henrik Melbéus and Tommy Ohlsson describe three different theories of extra dimensions – universal, large and warped – and how these unseen dimensions could be observed, if they exist at all.
    Physics World 09/2012; 25(9):27-30. · 0.45 Impact Factor
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    Henrik Melbeus, Alexander Merle, Tommy Ohlsson
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    ABSTRACT: We study the first Kaluza-Klein excitation of the Higgs boson in universal extra dimensions as a dark matter candidate. The first-level Higgs boson could be the lightest Kaluza-Klein particle, which is stable due to the conservation of Kaluza-Klein parity, in non-minimal models where boundary localized terms modify the mass spectrum. We calculate the relic abundance and find that it agrees with the observed dark matter density if the mass of the first-level Higgs boson is slightly above 2 TeV, not considering coannihilations and assuming no relative mass splitting among the first-level Kaluza-Klein modes. In the case of coannihilations and a non-zero mass splitting, the mass of the first-level Higgs boson can range from 1 TeV to 4 TeV. We study also the prospects for detection of this dark matter candidate in direct as well as indirect detection experiments. Although the first-level Higgs boson is a typical weakly interacting massive particle, an observation in any of the conventional experiments is very challenging.
    Physics Letters B 07/2012; 715(1-3):164-169. · 4.57 Impact Factor
  • Article: Erratum to
    Henrik Melbéus, Alexander Merle, Tommy Ohlsson
    Physics Letters B 07/2012; 713(3):350-350. · 4.57 Impact Factor
  • Physical review D: Particles and fields 05/2012; 85(10).
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    ABSTRACT: We study how the recent ATLAS and CMS Higgs mass bounds affect the renormalization group running of the physical parameters in universal extra dimensions. Using the running of the Higgs self-coupling constant, we derive bounds on the cutoff scale of the extra-dimensional theory itself. We show that the running of physical parameters, such as the fermion masses and the CKM mixing matrix, is significantly restricted by these bounds. In particular, we find that the running of the gauge couplings cannot be sufficient to allow gauge unification at the cutoff scale.
    Physics Letters B 05/2012; 712(4-5):419-424. · 4.57 Impact Factor
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    ABSTRACT: We investigate monoenergetic gamma-ray signatures from annihilations of dark matter comprised of Z1, the first Kaluza-Klein (KK) excitation of the Z boson in a nonminimal universal extra dimensions (UED) model. The self interactions of the non-Abelian Z1 gauge boson give rise to a large number of contributing Feynman diagrams that do not exist for annihilations of the Abelian gauge boson B1, which is the standard Kaluza-Klein dark matter (KKDM) candidate. We find that the annihilation rate is indeed considerably larger for the Z1 than for the B1. Even though relic density calculations indicate that the mass of the Z1 should be larger than the mass of the B1, the predicted monoenergetic gamma fluxes are of the same order of magnitude. We compare our results to existing experimental limits, as well as to future sensitivities, for image air Cherenkov telescopes, and we find that the limits are reached already with a moderately large boost factor. The realistic prospects for detection depend on the experimental energy resolution.
    Physical Review D 02/2012; 85(4):043524. · 4.69 Impact Factor
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    Henrik Melbeus, Alexander Merle, Tommy Ohlsson
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    ABSTRACT: We calculate the continuum photon spectrum from the pair annihilation of a Z^1 LKP in non-minimal universal extra dimensions. We find that, due to the preferred annihilation into W^+ W^- pairs, the continuum flux of collinear photons is relatively small compared to the standard case of the B1 as the LKP. This conclusion applies in particular to the spectral endpoint, where also the additional fermionic contributions are not large enough to increase the flux significantly. When searching for the line signal originating from Z^1 Z^1 annihilations, this is actually a perfect situation, since the continuum signal can be regarded as background to the smoking gun signature of a peak in the photon flux at an energy that is nearly equal to the mass of the dark matter particle. This signal, in combination with (probably) a non-observation of the continuum signal at lower photon energies, constitutes a perfect handle to probe the hypothesis of the Z1 LKP being the dominant component of the dark matter observed in the Universe.
    Physics Letters B 11/2011; 706(4-5):329-332. · 4.57 Impact Factor
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    ABSTRACT: We study the renormalization group (RG) running of the neutrino masses and the leptonic mixing parameters in two different extra-dimensional models, namely, the Universal Extra Dimensions (UED) model and a model, where the Standard Model (SM) bosons probe an extra dimension and the SM fermions are confined to a four-dimensional brane. In particular, we derive the beta function for the neutrino mass operator in the UED model. We also rederive the beta function for the charged-lepton Yukawa coupling, and confirm some of the existing results in the literature. The generic features of the RG running of the neutrino parameters within the two models are analyzed and, in particular, we observe a power-law behavior for the running. We note that the running of the leptonic mixing angle \theta_{12} can be sizable, while the running of \theta_{23} and \theta_{13} is always negligible. In addition, we show that the tri-bimaximal and the bimaximal mixing patterns at a high-energy scale are compatible with low-energy experimental data, while a tri-small mixing pattern is not. Finally, we perform a numerical scan over the low-energy parameter space to infer the high-energy distribution of the parameters. Using this scan, we also demonstrate how the high-energy \theta_{12} is correlated with the smallest neutrino mass and the Majorana phases.
    Journal of High Energy Physics 04/2011; 2011(4):052. · 5.62 Impact Factor
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    ABSTRACT: We study the generation of small neutrino masses in an extra-dimensional model, where right-handed neutrinos are allowed to propagate in the extra dimension, while the Standard Model particles are confined to a brane. Motivated by the fact that extra-dimensional models are non-renormalizable, we truncate the Kaluza-Klein towers at a maximal extra-dimensional momentum. The structure of the bulk Majorana mass term, motivated by the Sherk-Schwarz mechanism, implies that the right-handed Kaluza-Klein neutrinos pair to form Dirac neutrinos, except for a number of unpaired Majorana neutrinos at the top of each tower. These heavy Majorana neutrinos are the only sources of lepton number breaking in the model, and similarly to the type-I seesaw mechanism, they naturally generate small masses for the left-handed neutrinos. The lower Kaluza-Klein modes mix with the light neutrinos, and the mixing effects are not suppressed with respect to the light-neutrino masses. Compared to conventional fermionic seesaw models, such mixing can be more significant. We study the signals of this model at the Large Hadron Collider, and find that the current low-energy bounds on the non-unitarity of the leptonic mixing matrix are strong enough to exclude an observation.
    Physical Review D 08/2010; 82(4):045023. · 4.69 Impact Factor
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    Mattias Blennow, Henrik Melbeus, Tommy Ohlsson
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    ABSTRACT: We investigate indirect neutrino signals from annihilations of Kaluza-Klein dark matter in the Sun. Especially, we examine a five- as well as a six-dimensional model, and allow for the possibility that boundary localized terms could affect the spectrum to give different lightest Kaluza-Klein particles, which could constitute the dark matter. The dark matter candidates that are interesting for the purpose of indirect detection of neutrinos are the first Kaluza-Klein mode of the U(1) gauge boson and the neutral component of the SU(2) gauge bosons. Using the DarkSUSY and WimpSim packages, we calculate muon fluxes at an Earth-based neutrino telescope, such as IceCube. For the five-dimensional model, the results that we obtained agree reasonably well with the results that have previously been presented in the literature, whereas for the six-dimensional model, we find that, at tree-level, the results are the same as for the five-dimensional model. Finally, if the first Kaluza-Klein mode of the U(1) gauge boson constitutes the dark matter, IceCube can constrain the parameter space. However, in the case that the neutral component of the SU(2) gauge bosons is the LKP, the signal is too weak to be observed.
    Journal of Cosmology and Astroparticle Physics 01/2010; 2010(1):018. · 6.04 Impact Factor
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    Henrik Melbeus, Tommy Ohlsson
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    ABSTRACT: We investigate a model of large extra dimensions where the internal space has the geometry of a hyperbolic disc. Compared with the ADD model, this model provides a more satisfactory solution to the hierarchy problem between the electroweak scale and the Planck scale, and it also avoids constraints from astrophysics. In general, a novel feature of this model is that the physical results depend on the position of the brane in the internal space, and in particular, the signal almost disappears completely if the brane is positioned at the center of the disc. Since there is no known analytic form of the Kaluza-Klein spectrum for our choice of geometry, we obtain a spectrum based on a combination of approximations and numerical computations. We study the possible signatures of our model for hadron colliders, especially the LHC, where the most important processes are the production of a graviton together with a hadronic jet or a photon. We find that the signals are similar to those of the ADD model, regarding both qualitative behavior and strength. For the case of hadronic jet production, it is possible to obtain relatively strong signals, while for the case of photon production, this is much more difficult.
    Journal of High Energy Physics 08/2008; 2008(8):077. · 5.62 Impact Factor
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    Henrik Melbeus