arXiv subjects

Maxim Pospelov

Publications and source records attributed to Maxim Pospelov.

At least 199 records · Page 11Linked to original sources

arXiv2003

Cosmological bounds on large extra dimensions from non-thermal production of Kaluza-Klein modes

The existing cosmological constraints on theories with large extra dimensions rely on the thermal production of the Kaluza-Klein modes of gravitons and radions in the early Universe. Successful inflation and reheating, as well as baryogenesis, typically requires the existence of a TeV-scale field in the bulk, most notably the inflaton. The non-thermal production of KK modes with masses of order 100 GeV accompanying the inflaton decay sets the lower bounds on the fundamental scale M_*. For a 1 TeV inflaton, the late decay of these modes distort the successful predictions of Big Bang Nucleosynthesis unless M_*> 35, 13, 7, 5 and 3 TeV for 2, 3, 4, 5 and 6 extra dimensions, respectively. This improves the existing bounds from cosmology on M_* for 4, 5 and 6 extra dimensions. Even more stringent bounds are derived for a heavier inflaton.

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arXiv2003

Conformal matter in warped backgrounds

Warped backgrounds in five dimensional models can provide solutions to various hierarchy problems in particle physics if the standard model matter is associated with the zero modes of bulk fields with nontrivial profiles along the extra dimension. We investigate the case of a {\sl conformally} coupled massless scalar. This field possesses a zero mode whose action density is exponentially enhanced along the transverse direction in a warped background, {\em i.e.} near the negative tension brane in the RS setting. At the zero mode level effective 5D composites of this scalar and its derivatives, which are suppressed by powers of the fundamental gravitational scale, yield 4D interactions of electroweak strength. This suggests a certain importance of conformally coupled scalar fields for extra-dimensional model building.

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arXiv2002

Electric Dipole Moments in The Limit of Heavy Superpartners

Supersymmetric loop corrections induce potentially large CP-violating couplings of the Higgs bosons to nucleons and electrons that do not vanish in the limit of heavy superpartners. The Higgs-mediated CP-odd four-fermion operators are enhanced by tan^3 beta and induce electric dipole moments of heavy atoms which exceed the current experimental bounds for the electroweak scale Higgs masses and tan beta > 10. If only the first two sfermion generations are heavy, the Higgs-mediated contributions typically dominate over the Barr-Zee type two-loop diagrams at tan beta > 30.

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arXiv2002

Constraints on the Variations of the Fundamental Couplings

We reconsider several current bounds on the variation of the fine-structure constant in models where all gauge and Yukawa couplings vary in an interdependent manner, as would be expected in unified theories. In particular, we re-examine the bounds established by the Oklo reactor from the resonant neutron capture cross-section of 149Sm. By imposing variations in Λ_{QCD} and the quark masses, as dictated by unified theories, the corresponding bound on the variation of the fine-structure constant can be improved by about 2 orders of magnitude in such theories. In addition, we consider possible bounds on variations due to their effect on long lived α- and β-decay isotopes, particularly 147Sm and 187Re. We obtain a strong constraint on Δα/ α, comparable to that of Oklo but extending to a higher redshift corresponding to the age of the solar system, from the radioactive life-time of 187Re derived from meteoritic studies. We also analyze the astrophysical consequences of perturbing the decay Q values on bound state β-decays operating in the s-process.

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arXiv2002

On the Stabilization of the Size of Extra Dimensions

We derive a general formalism for constructing a four-dimensional effective action for the radion field in 5 dimensions, taking into account the possible dependence of the scale factor on the internal coordinate. First and second variations of the action with respect to the size of the extra dimension reveals two types of algebraic constraints on the combination of brane and bulk Lagrangians that extremize the action, and provide an effective mass for the radion field. We derive a compact formula for the radion mass squared in terms of brane and bulk potentials for the case of a bulk scalar field. We further specialize these constraints for several, five-dimensional realizations of brane world models.

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arXiv2002

Indirect limits on the CPT violating background in the neutrino sector

CPT violation in the neutrino sector, suggested as a new way to reconcile different neutrino anomalies, induces at the radiative level observable effects among charged leptons, where high-precision tests of the CPT symmetry are available. We show that, in the models with heavy right-handed Majorana neutrinos, constraints imposed by these experiments require CPT violation in neutrino spectrum be suppressed to a level undetectable for any conceivable neutrino experiment. We find that the CPT violation in the neutrino sector may evade indirect constraints only at the expense of light right-handed neutrinos with small Yukawa couplings to the Standard Model sector or by allowing non-locality well below the electroweak scale.

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arXiv2002

Best Values for the CP-odd Meson-Nucleon Couplings from Supersymmetry

In the supersymmetric models, the dominant sources of the hadronic flavor-diagonal CP violation at low energy are the theta term and the chromoelectric dipole moments of quarks. Using QCD sum rules, we estimate the preferred range and the best values for the CP-odd meson-nucleon coupling constants induced by these operators. When the theta term is removed by the axion mechanism, the size of the most important isospin triplet pion-nucleon coupling is estimated to be \bar g_{πNN}^{(1)} = 2 times 10^{-12}(tilde d_u - tilde d_d), where chromoelectric dipole moments are given in units of 10^{-26} cm.

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arXiv2001

Limits on the Non-commutativity Scale

A non-vanishing vacuum expectation value for an antisymmetric tensor field leads to the violation of Lorentz invariance, controlled by the dimension (-2) parameter, theta_{mu nu}. We assume that the zeroth order term in theta-expansion represents the Standard Model and study the effects induced by linear terms in theta_{mu nu}. If coupling to theta_{mu nu} is realized in strongly interacting sector of the theory, the clock comparison experiments place the limit on the possible size of this background at the level of 1/\sqrt{theta}>~5X10^{14} GeV. If the interaction with theta_{mu nu} is initially present only in the QED sector, this limit can be relaxed to 10^{11}-10^{12} GeV level. The strength of these limits obviates the inferiority of collider physics with regard to experimental checks of Lorentz invariance. Limits of similar strength are expected to hold in the case of mixed non-commutativity between four-dimensional and extra-dimensional coordinates. We also show that in certain models mixed non-commutativity can be interpreted as 4d CPT-violating background.

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arXiv2001

Evolution of the Fine Structure Constant Driven by Dark Matter and the Cosmological Constant

Bekenstein's model of a scalar field, ϕ, that affects the electromagnetic permeability (usually identified with ``changing α'') predicts tiny variations of the effective fine structure constant up to very high red-shifts, |α(z=3.5)/α_0-1| < 10^{-10}, when the constraints from Eötvös-Dicke-Braginsky types of experiments are imposed. We generalize this model by allowing additional couplings of ϕto both a dark matter candidate and to the cosmological constant. We show that in a supersymmetric generalization of Bekenstein's model, the coupling to the LSP, which is assumed to contribute significantly to the dark matter density, can be up to six orders of magnitude stronger than the coupling to the baryon energy density. This allows one to evade the present limits on the non-universality of the gravitational attraction due to ϕ-exchange and at the same time accommodate the effective shift in αat the level of α(z=3.5)/α_0-1 \sim 10^{-5}, reported recently from observations of quasar absorption spectra.

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arXiv2001

Breaking CPT by mixed non-commutativity

The mixed component of the non-commutative parameter θ_{μM}, where μ= 0,1,2,3 and M is an extra dimensional index may violate four-dimensional CPT invariance. We calculate one and two-loop induced couplings of θ_{μ5} with the four-dimensional axial vector current and with the CPT odd dim=6 operators starting from five-dimensional Yukawa and U(1) theories. The resulting bounds from clock comparison experiments place a stringent constraint on θ_{μ5}, |θ_{μ5}|^{-1/2} > 5\times 10^{11} GeV. The orbifold projection and/or localization of fermions on a 3-brane lead to CPT-conserving physics, in which case the constraints on θ{μ5} are softened.

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arXiv2001

How natural is a small \barθin left-right SUSY models ?

In the world without an axion the smallness of $\barθ$ may be achieved due to a spontaneously broken discrete left-right symmetry. We analyze the radiatively induced \barθin the context of generic left-right symmetric SUSY models without assuming flavor degeneracy in the squark sector. Left-right symmetry allows to keep \barθwithin its present bound only if the inter-generational mass splitting in the squark sector at the scale of the left-right symmetry breaking is smaller than 0.5%. We also consider the naturalness of m_u=0 solution to the strong CP problem in the context of horizontal flavor symmetries. A strong bound on the combination of the horizontal charges in the Up quark sector is found in this case.

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arXiv2001

(De)Stabilization of an extra dimension due to a Casimir force

We study the stabilization of one spatial dimension in (p+1+1)-dimensional spacetime in the presence of $p$-dimensional brane(s), a bulk cosmological constant and the Casimir force generated by a conformally coupled scalar field. We find general static solutions to the metric which require the fine-tuning of the inter-brane distance and the bulk cosmological constant (leaving the two brane tensions as free parameters) corresponding to a vanishing effective cosmological constant and a constant radion field. Taking these solutions as a background configuration, we perform a dimensional reduction and study the effective theory in the case of one- and two-brane configurations. We show that the radion field can have a positive mass squared, which corresponds to a stabilization of the extra dimension, only for a repulsive nature of the Casimir force. This type of solution requires the presence of a negative tension brane. The solutions with one or two positive tension branes arising in this theory turn out to have negative radion mass squared, and therefore are not stable.

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arXiv2000

Self-Interacting Dark Matter from the Hidden Heterotic-String Sector

It has been suggested recently that self-interacting dark matter fits better the observational characteristics of galaxy dynamics. We propose that the self-interacting dark matter is composed from the glueballs of the hidden sector non-Abelian gauge group, while the hidden matter states exist in vector-like representation and decouple from the light spectrum. It is shown that these glueballs are semi-stable with the life-time larger than the present age of the Universe, if their mass is 1 GeV or less. The constraint on their abundance today suggests that the energy was stored in the hidden sector soon after inflation. This imposes an upper limit on the reheating temperature. We further study the naturalness of this scenario in the context of the free-fermionic string models and point out a class of such models where the self-interacting dark matter from the hidden sector is indeed plausible.

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arXiv2000

Solving the Hierarchy Problem in Two-Brane Cosmological Models

We analyze cosmological solutions in the class of two-brane models with arbitrary tensions which contain matter with general equations of state. We show that the mass hierarchy between the two branes is determined by the ratio of the lapse functions evaluated on the branes. This ratio can be sufficiently small without fine-tuning the brane separation, once the transverse dimension is stabilized. For suitably large interbrane separations, both brane tensions are positive. We also find that the cosmological evolution obeys the standard four-dimensional Friedman equation up to small corrections.

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arXiv2000

Rho-omega mixing in asymmetric nuclear matter via QCD sum rule approach

We evaluate the operator product expansion (OPE) for a mixed correlator of the isovector and isoscalar vector currents in the background of the nucleon density with intrinsic isospin asymmetry [i.e. excess of neutrons over protons] and match it with its imaginary part, given by resonances and continuum, via the dispersion relation. The leading density-dependent contribution to $ρ-ω$ mixing is due the scattering term, which turns out to be larger than any density dependent piece in the OPE. We estimate that the asymmetric density of $n_n-n_p \sim 2.5 \times 10^{-2} ~{\rm fm^3}$ induces the amplitude of $ρ-ω$ mixing, equal in magnitude to the mixing amplitude in vacuum, with the constructive interference for positive and destructive for negative values of $n_n-n_p$. We revisit sum rules for vector meson masses at finite nucleon density to point out the numerical importance of the screening term in the isoscalar channel, which turns out to be one order of magnitude larger than any density-dependent condensates over the Borel window. This changes the conclusions about the density dependence of $m_ω$, indicating $\sim 40$ MeV increase at nuclear saturation density.

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arXiv2000

Low-energy Limits on the Antisymmetric Tensor Field Background on the Brane and on the Non-commutative Scale

A non-vanishing vacuum expectation value for an antisymmetric tensor field leads to the violation of Lorentz invariance on the brane. This violation is controlled by the $θ_{μν}$ parameter, which has a dimension of inverse mass squared. We assume that the zeroth order term in theta-expansion represents the Standard Model and study the effects induced by linear terms in theta_{μν}. Low-energy precision experiments place the limit on the possible size of this background at the level of $1/\sqrtθ > 5 \times 10^{14}$ GeV. This poses certain difficulty for the TeV-range string scale models, in which the antisymmetric field has to acquire a relatively large mass to ensure that $θ_{μν}=0$ and avoid cosmological moduli problem.

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arXiv2000

Static Solutions for Brane Models with a Bulk Scalar Field

We present static solutions of the 5-dimensional Einstein equations in the brane-world scenario by using two different approaches for the stabilization of the extra dimension. Assuming a ``phenomenological'' stabilization mechanism, that creates a non-vanishing $\hat T^5_5$ in the bulk, we construct a two-brane model, which allows both branes to have positive self-energies. We then consider a candidate theory for the dynamical stabilization, through the introduction of a massless scalar field in the bulk, which interacts with the branes. We find exact static solutions for the metric and scalar field in the bulk and demonstrate that the inter-brane distance is determined by the parameters of scalar field-brane interactions. However, these solutions are always accompanied by a correlation between the bulk cosmological constant, the brane self-energies and the interaction terms of the scalar field with the branes and thus cannot be considered as candidates for the phenomenologically viable stabilized geometry. We find that the aforementioned correlation cannot be avoided even in the case of a single-brane solution with positive self-energy where the fifth dimension ends on a singularity.

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arXiv2000

Direct and indirect limits on the electro-magnetic form factors of WIMPs

We use the results of direct and indirect searches for weakly interacting massive particles (WIMPs) to obtain bounds on various electro-magnetic form factors of WIMPs. The limits on the recoil signal in underground dark matter detection experiments, with standard assumptions about the density and r.m.s. velocity of WIMPs in the halo, can be translated into the following model-independent bounds on the magnetic dipole, electric dipole, quadrupole and anapole moments and the polarizability of WIMPs: mu/mu_n < 1.4 10^{-4}, d < 5 10^{-21}e cm, Q < 6 10^{-7}fm^2, a/mu_n < 3 10^{-2} fm, and chi < 5 10^{-8}fm^3 for a WIMP mass of 100 GeV. The limits on fluxes of highly energetic neutrinos produced in the annihilation of WIMPs in the center of the earth and the sun lead to somewhat stronger, but more model dependent bounds.

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