arXiv subjects

Maxim Pospelov

Publications and source records attributed to Maxim Pospelov.

At least 163 records · Page 9Linked to original sources

arXiv2009

Scalar-tensor theories with pseudoscalar couplings

We consider the scalar-tensor theories of gravity extended by the pseudoscalar couplings to matter and gauge fields and derive constraints on the CP-odd combinations of scalar and pseudoscalar couplings from laboratory spin precession experiments and from the evolution of photon polarization over cosmological distances. We show the complimentary character of local and cosmological constraints, and derive novel bounds on the pseudoscalar couplings to photons from the laboratory experiments. It is also shown that the more accurate treatment of the spin content of nuclei used in the spin precession experiments allows to tighten bounds on Lorentz-violating backgrounds coupled to the proton spin.

hep-ph↗

arXiv2008

Astrophysical Signatures of Secluded Dark Matter

We analyze the indirect astrophysical signatures of secluded models of WIMP dark matter, characterized by a weak-scale rate for annihilation into light MeV-scale mediators which are metastable to decay into Standard Model states. Such scenarios allow a significant enhancement of the annihilation cross section in the galactic halo relative to its value at freeze-out, particularly when the mediator is light enough for this process to proceed through radiative capture to a metastable `WIMP-onium' bound state. For MeV-scale vector mediators charged under a hidden U(1)' gauge group, the enhanced annihilation rate leads predominantly to a sizable excess positron flux, even in the absence of astrophysical boost factors.

hep-ph↗

arXiv2008

Secluded U(1) below the weak scale

A secluded U(1) sector with weak admixture to photons, O(10^{-2}-10^{-3}), and the scale of the breaking below 1 GeV represents a natural yet poorly constrained extension of the Standard Model. We analyze g-2 of muons and electrons together with other precision QED data, as well as radiative decays of strange particles to constrain mass--mixing angle (m_V-κ) parameter space. We point out that m_V = 214 MeV and κ^2 > 3\times 10^{-5} can be consistent with the hypothesis of HyperCP collaboration, that seeks to explain the anomalous energy distribution of muon pairs in the Σ^+ \to p μ^+μ^- process by a resonance, without direct contradiction to the existing data on radiative kaon decays. The same parameters lead to O({\rm few} \times 10^{-9}) upward correction to the anomalous magnetic moment of the muon, possibly relaxing some tension between experimental value and theoretical determinations of g-2. The ultra-fine energy resolution scan of e^+e^-\to μ^+μ^- cross section and dedicated analysis of lepton spectra from K^+\to π^+ e^+e^- decays should be able to provide a conclusive test of this hypothesis and improve the constraints on the model.

hep-ph↗

arXiv2008

Bosonic super-WIMPs as keV-scale dark matter

We consider models of light super-weakly interacting cold dark matter, with O(10-100) keV mass, focusing on bosonic candidates such as pseudoscalars and vectors. We analyze the cosmological abundance, the gamma-background created by particle decays, the impact on stellar processes due to cooling, and the direct detection capabilities in order to identify classes of models that pass all the constraints. In certain models, variants of photoelectric (or axioelectric) absorption of dark matter in direct-detection experiments can provide a sensitivity to the superweak couplings to the Standard Model which is superior to all existing indirect constraints. In all models studied, the annual modulation of the direct-detection signal is at the currently unobservable level of O(10^{-5}).

hep-ph↗

arXiv2008

Constraints on Supersymmetric Models from Catalytic Primordial Nucleosynthesis of Beryllium

The catalysis of nuclear reactions by negatively charged relics leads to increased outputs of primordial ^6Li and ^9Be. In combination with observational constraints on the primordial fractions of ^6Li and ^9Be, this imposes strong restrictions on the primordial abundance and the lifetime of charged relics. We analyze the constraints from the catalysis of ^9Be on supersymmetric models in which the gravitino is the lightest supersymmetric particle and a charged slepton--such as the lighter stau--the next-to-lightest supersymmetric particle (NLSP). Barring the special cases in which the primordial fraction of the slepton NLSP is significantly depleted, we find that the ^9Be data require a slepton NLSP lifetime of less than 6x10^3 seconds. We also address the issue of the catalytic destruction of ^6Li and ^9Be by late forming bound states of protons with negatively charged relics finding that it does not lead to any significant modification of the limit on the slepton lifetime.

hep-ph↗

arXiv2008

Electric Dipole Moments as Probes of CPT Invariance

Electric dipole moments (EDMs) of elementary particles and atoms probe violations of T and P symmetries and consequently of CP if CPT is an exact symmetry. We point out that EDMs can also serve as sensitive probes of CPT-odd, CP-even interactions, that are not constrained by any other existing experiments. Analyzing models with spontaneously broken Lorentz invariance, we calculate EDMs in terms of the leading CPT-odd operators to show that experimental sensitivity probes the scale of CPT breaking as high as 10^{12}GeV.

hep-ph↗

arXiv2008

Resonant scattering and recombination of pseudo-degenerate WIMPs

We consider the direct and indirect detection signatures of WIMPs χ^0 in kinematic regimes with a heavier, but nearly degenerate, charged state χ^{+-}. For small splittings of O(10) MeV, the scattering of WIMPs off nuclei may be dominated by inelastic recombination processes mediated by the formation of (χ^- N) bound states, leading to a distinct signature for direct detection. These cross-sections are bound primarily by limits on the abundance of heavy isotopes, and may be considerably larger than the elastic scattering cross section in more conventional models. If the mass splitting is too large for recombination to occur, there may still be a significant resonant enhancement of loop-induced electromagnetic form-factors of the WIMP, which can enhance the elastic scattering cross-section. We also discuss how this regime affects the annihilation cross-section and indirect detection signatures, and note the possibility of a significant mono-energetic γ-signal, mediated by resonant processes near the (χ^+χ^-) bound state threshold.

hep-ph↗

arXiv2008

Pseudoscalar perturbations and polarization of the cosmic microwave background

We show that models of new particle physics containing massless pseudoscalar fields super-weakly coupled to photons can be very efficiently probed with CMB polarization anisotropies. The stochastic pseudoscalar fluctuations generated during inflation provide a mechanism for converting E-mode polarization to B-mode during photon propagation from the surface of last scattering. The efficiency of this conversion process is controlled by the dimensionless ratio H/(2πf_a), where H is the Hubble scale during inflation, and f_a^{-1} is the strength of the pseudoscalar coupling to photons. The current observational limits on the B-mode constrain this ratio to be less than 0.07, which in many models of inflation translates to a sensitivity to values of f_a in excess of 10^{14} GeV, surpassing the sensitivity of other tests.

astro-ph↗

arXiv2008

Low-energy constraints on kappa-Minkowski extension of the Standard Model

We investigate the phenomenological consequences of kappa-Minkowski extension of the Standard Model, working in the linear order in inverse $κ$. At this order the *-deformed Lagrangian can be expanded in the series of dimension five operators that have non-trivial transformation properties under the ordinary Lorentz invariance. Such operators cause the Lorentz-violating signatures at low energies, and in particular lead to the anomalous spin precession linked to the external direction. The experimental bounds on this phenomenon then restrict parameter $κ$ to be above 10^{23} GeV, making it difficult to impose a direct connection between this theory and quantum gravity.

hep-ph↗

arXiv2008

Primordial Lithium Abundance in Catalyzed Big Bang Nucleosynthesis

There exists a well known problem with the Li7+Be7 abundance predicted by standard big bang nucleosynthesis being larger than the value observed in population II stars. The catalysis of big bang nucleosynthesis by metastable, τ_X \ge 10^3 sec, charged particles X^- is capable of suppressing the primordial Li7+Be7, abundance and making it consistent with the observations. We show that to produce the correct abundance, this mechanism of suppression places a requirement on the initial abundance of X^- at temperatures of 4\times 10^8 K to be on the order of or larger than 0.02 per baryon, which is within the natural range of abundances in models with metastable electroweak-scale particles. The suppression of Li7+Be7, is triggered by the formation of (Be7X^-), compound nuclei, with fast depletion of their abundances by catalyzed proton reactions, and in some models by direct capture of X^- on Be7. The combination of Li7+Be7 and Li6 constraints favours the window of lifetimes, 1000s \la tau_X \leq 2000 s.

hep-ph↗

arXiv2008

Environmental Dependence of Masses and Coupling Constants

We construct a class of scalar field models coupled to matter that lead to the dependence of masses and coupling constants on the ambient matter density. Such models predict a deviation of couplings measured on the Earth from values determined in low-density astrophysical environments, but do not necessarily require the evolution of coupling constants with the redshift in the recent cosmological past. Additional laboratory and astrophysical tests of Δαand Δ(m_p/m_e) as functions of the ambient matter density are warranted.

hep-ph↗

arXiv2007

Bridging the primordial A=8 divide with Catalyzed Big Bang Nucleosynthesis

Catalysis of nuclear reactions by metastable charged particles X^- opens the possibility for primordial production of elements with A>7. We calculate the abundance of ^9Be, where synthesis is mediated by the formation of (^8Be X^-) bound states, finding a dramatic enhancement over the standard BBN prediction: ^9Be/^1H = 10^{-13}\times(Y_X/10^{-5}). Thus observations of ^9Be abundances at low metallicity offers a uniquely sensitive probe of many particle physics models that predict X^-, including variants of supersymmetric models. Comparing the catalytically-enhanced abundances of primordial ^6Li and ^9Be, we find the relation ^9Be/^6Li = (2-5)\times 10^{-3} that holds over a wide range of X^- abundances and lifetimes.

hep-ph↗

arXiv2007

Secluded WIMP Dark Matter

We consider a generic mechanism via which thermal relic WIMP dark matter may be decoupled from the Standard Model, namely through a combination of WIMP annihilation to metastable mediators with subsequent delayed decay to Standard Model states. We illustrate this with explicit examples of WIMPs connected to the Standard Model by metastable bosons or fermions. In all models, provided the WIMP mass is greater than that of the mediator, it can be secluded from the Standard Model with an extremely small elastic scattering cross-section on nuclei and rate for direct collider production. In contrast, indirect signatures from WIMP annihilation are consistent with a weak scale cross-section and provide potentially observable γ-ray signals. We also point out that γ-ray constraints and flavor physics impose severe restrictions on MeV-scale variants of secluded models, and identify limited classes that pass all the observational constraints.

hep-ph↗

arXiv2007

Classification of Dimension 5 Lorentz Violating Interactions in the Standard Model

We give a complete classification of mass dimension five Lorentz-non-invariant interactions composed from the Standard Model fields, using the effective field theory approach. We identify different classes of Lorentz violating operators, some of which are protected against transmutation to lower dimensions even at the loop level. Within each class of operators we determine a typical experimental sensitivity to the size of Lorentz violation.

hep-ph↗

arXiv2007

Electric dipole moment constraints on minimal electroweak baryogenesis

We study the simplest generic extension of the Standard Model which allows for conventional electroweak baryogenesis, through the addition of dimension six operators in the Higgs sector. At least one such operator is required to be CP-odd, and we study the constraints on such a minimal setup, and related scenarios with minimal flavor violation, from the null results of searches for electric dipole moments (EDMs), utilizing the full set of two-loop contributions to the EDMs. The results indicate that the current bounds are stringent, particularly that of the recently updated neutron EDM, but fall short of ruling out these scenarios. The next generation of EDM experiments should be sufficiently sensitive to provide a conclusive test.

hep-ph↗

arXiv2007

Particle physics catalysis of thermal Big Bang Nucleosynthesis

We point out that the existence of metastable, tau > 10^3 s, negatively charged electroweak-scale particles (X^-) alters the predictions for lithium and other primordial elemental abundances for A>4 via the formation of bound states with nuclei during BBN. In particular, we show that the bound states of X^- with helium, formed at temperatures of about T=10^8K, lead to the catalytic enhancement of Li6 production, which is eight orders of magnitude more efficient than the standard channel. In particle physics models where subsequent decay of X^- does not lead to large non-thermal BBN effects, this directly translates to the level of sensitivity to the number density of long-lived X^-, particles (τ>10^5 s) relative to entropy of n_{X^-}/s < 3\times 10^{-17}, which is one of the most stringent probes of electroweak scale remnants known to date.

hep-ph↗

arXiv2007

The galactic 511 keV line from electroweak scale WIMPs

We consider possible mechanisms via which electroweak scale WIMPs χ^0 could provide the source of the INTEGRAL/SPI 511 keV photon flux from the galactic centre. We consider scenarios where the WIMP spectrum contains near-degeneracies, with MeV-scale splitting, and focus on three possible production mechanisms for galactic positrons: (i) collisional excitation of the WIMP to a nearby charged state, χ^0 + χ^0 -> χ^+ + χ^-, with the subsequent decay producing positrons; (ii) capture of the WIMP by nuclei in the galactic interstellar medium, χ^0 + N -> e^+ + (χ^- N); and (iii) the decay of a nearby long-lived state surviving from the Big Bang, χ^0_2 -> χ_1^0 + e^+ + e^-. We find that process (i) requires a cross-section which is significantly larger than the unitarity bound, process (ii) is allowed by unitarity, but is impractical due to terrestrial bounds on the χ-N cross-section, while process (iii) is viable and we construct a simple model realization with singlet dark matter fields interacting with the Standard Model via the Higgs sector.

hep-ph↗

arXiv2006

CPT-odd Leptogenesis

We calculate the baryon asymmetry of the Universe resulting from the combination of higher-dimensional Lorentz-noninvariant CPT-odd operators and dimension five operators that induce the majorana mass for neutrinos. The strength of CPT-violating dimension five operators capable of producing the observed value of baryon abundance is directly related to neutrino masses and found to be in the trans-Planckian range 10^{-24}-10^{-22} GeV^{-1}. Confronting it with observational tests of Lorentz symmetry, we find that this range of Lorentz/CPT violation is strongly disfavored by the combination of the low-energy constraints and astrophysical data.

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