James P. Kneller

Effective number of neutrinos and baryon asymmetry from BBN and WMAP

Abstract We place constraints on the number of relativistic degrees of freedom and on the baryon asymmetry at the epoch of Big Bang Nucleosynthesis (BBN) and at recombination, using cosmic background radiation (CBR) data from the Wilkinson Microwave Anisotropy Probe (WMAP), complemented by the Hubble Space Telescope (HST) Key Project measurement of the Hubble constant, along with the latest compilation of deuterium abundances and Hii region measurements of the primordial helium abundance. The agreement between the derived values of these key cosmological and particle physics parameters at these widely separated (in time or redshift) epochs is remarkable. From the combination of CBR and BBN data, we find the 2 σ ranges for the effective number of neutrinos N ν and for the baryon asymmetry (baryon to photon number ratio η ) to be 1.7–3.0 and 5.53–6.76×10 −10 , respectively.

Hiding relativistic degrees of freedom in the early universe

Abstract We quantify the extent to which extra relativistic energy density can be concealed by a neutrino asymmetry without conflicting with the baryon asymmetry measured by the Wilkinson Microwave Anisotropy Probe (WMAP). In the presence of a large electron neutrino asymmetry, slightly more than seven effective neutrinos are allowed by Big Bang Nucleosynthesis (BBN) and WMAP at 2 σ . The same electron neutrino degeneracy that reconciles the BBN prediction for the primordial helium abundance with the observationally inferred value also reconciles the LSND neutrino with BBN by suppressing its thermalization prior to BBN.

BBN for pedestrians

The simplest, standard model of Big Bang nucleosynthesis (SBBN) assumes three light neutrinos (Nν = 3) and no significant electron neutrino asymmetry ( asymmetry parameter ξe ≡ μe/kT, where μe is the chemical potential of νe) leaving only one adjustable parameter: the baryon to photon ratio η ≡ nB/nγ. The primordial abundance of any one nuclide can, therefore, be used to measure η and the value derived from the observationally inferred primordial abundance of deuterium closely matches that from current non-BBN data, primarily from the WMAP survey. However, using this same estimate for η, there is a tension between the SBBN-predicted abundances of 4He and 7Li and their current, observationally inferred primordial abundances, suggesting that Nν may differ from the standard model value of three and/or that ξe may differ from zero (or, that systematic errors in the abundance determinations have been underestimated or overlooked). The differences are not large and the allowed ranges of the BBN parameters (η, Nν and ξe) permitted by the data are quite small. Within these ranges, the BBN-predicted abundances of D, 3He, 4He, and 7Li are very smooth, monotonic functions of η10, ΔNν ≡ Nν − 3 and ξe. As a result, it is possible to describe the dependence of these abundances (or powers of them) upon the three parameters by simple, linear fits which, over their ranges of applicability, are accurate to a few per cent or even better. The fits presented here have not been maximized for their accuracy but, rather, for their simplicity. To identify the ranges of applicability and relative accuracies, they are compared with detailed BBN calculations; their utility is illustrated with several examples. Given the tension within BBN, these fits should prove useful in facilitating studies of the viability of various options for non-standard physics and cosmology, prior to undertaking detailed BBN calculations.

The neutrino-neutrino interaction effects in supernovae: the point of view from the matter basis

We consider the Hamiltonian for neutrino oscillations in matter in the case of arbitrary potentials including off-diagonal complex terms. We derive the expressions for the corresponding Hamiltonian in the basis of the instantaneous eigenstates in matter, in terms of quantities one can derive from the flavor-basis Hamiltonian and its derivative, for an arbitrary number of neutrino flavors. We make our expressions explicit for the two-neutrino flavor case and apply our results to the neutrino propagation in core-collapse supernovae where the Hamiltonian includes both coupling to matter and to neutrinos. We show that the neutrino flavor evolution depends on the mixing matrix derivatives involving not only the derivative of the matter mixing angles but also of the phases. In particular, we point out the important role of the phase derivatives, that appear due to the neutrino?neutrino interaction, and show how it can cause an oscillating degeneracy between the diagonal elements of the Hamiltonian in the basis of the eigenstates in matter. Our results also reveal that the end of the synchronization regime is due to a rapid increase of the phase derivative and identify the condition to be fulfilled for the onset of bipolar oscillations involving both the off-diagonal neutrino?neutrino interaction contributions and the vacuum terms.

What can be learned from a future supernova neutrino detection

This year marks the thirtieth anniversary of the only supernova from which we have detected neutrinos - SN 1987A. The twenty or so neutrinos that were detected were mined to great depth in order to determine the events that occurred in the explosion and to place limits upon all manner of neutrino properties. Since 1987 the scale and sensitivity of the detectors capable of identifying neutrinos from a Galactic supernova have grown considerably so that current generation detectors are capable of detecting of order ten thousand neutrinos for a supernova at the Galactic Center. Next generation detectors will increase that yield by another order of magnitude. Simultaneous with the growth of neutrino detection capability, our understanding of how massive stars explode and how the neutrino interacts with hot and dense matter has also increased by a tremendous degree. The neutrino signal will contain much information on all manner of physics of interest to a wide community. In this review we describe the expected features of the neutrino signal, the detectors which will detect it, and the signatures one might try to look for in order to get at these physics.

Neutrino scattering, absorption and annihilation above the accretion discs of gamma ray bursts

The central engine that drives gamma ray burst (GRB) explosions may derive from the ability of electrons/positrons and nucleons to tap into the momentum and energy from the large neutrino luminosity emitted by an accretion disc surrounding a black hole. This transfer of momentum and energy occurs due to neutrino absorption, scattering and annihilation, and the non-spherical geometry of the source both increases the annihilation efficiency and, close to the black hole, directs the momentum transfer toward the disc axis. We focus on the micro-physical aspects of this system and present annihilation efficiencies and the momentum/energy transfers for a number of accretion disc models. Models in which the neutrinos and antineutrinos become trapped within the disc have noticeably different momentum and energy deposition structure compared to thin disc models that may lead to significant differences in the explosion dynamics. Using these results we make estimates for the critical densities of infalling material below which the transfer of neutrino momentum/energy will lead to an explosion.

Consequences of large θ 13 for the turbulence signatures in supernova neutrinos

Here, the transition probabilities for a single neutrino emitted from a point proto-neutron source after passage through a turbulent supernova density profile have been found to be random variates drawn from parent distributions whose properties depend upon the stage of the explosion, the neutrino energy and mixing parameters, the observed channel, and the properties of the turbulence such as the amplitude C*. In this paper we examine the consequences of the recently measured mixing angle θ13 upon the neutrino flavor transformation in supernova when passing through turbulence, in order to provide some clarity as to what one should expect in the way of turbulence effects in the next supernova neutrino burst signal. We find that the measurements of a relatively large value of θ13 means the neutrinos are relatively immune to small, C*≲1%, amplitude turbulence but as C* increases the turbulence effects grow rapidly and spread to all mixing channels. For C*≳10% the turbulence effects in the high density resonance mixing channels are independent of θ13 but nonresonant mixing channels are more sensitive to turbulence when θ13 is large.

Monte Carlo neutrino oscillations

We demonstrate that the effects of matter upon neutrino propagation may be recast as the scattering of the initial neutrino wave function. Exchanging the differential, Schrodinger equation for an integral equation for the scattering matrix S permits a Monte Carlo method for the computation of S that removes many of the numerical difficulties associated with direct integration techniques.

Flavor evolution of supernova neutrinos in turbulent matter

The neutrino signal from the next galactic supernova carries with it an enormous amount of information on the explosion mechanism of a core-collapse supernova, as well as on the stellar progenitor and on the neutrinos themselves. In order to extract this information we need to know how the neutrino flavor evolves over time due to the interplay of neutrino self-interactions and matter effects. Additional turbulence in the supernova matter may impart its own signatures on the neutrino spectrum, and could partly obscure the imprints of collective and matter effects. We investigate the neutrino flavor evolution due to neutrino self-interactions, matter effects due to the shock wave propagation, and turbulence in three progenitors with masses of 8.8 M⊙, 10.8 M⊙ and 18.0 M⊙. In the lightest progenitor we find that the impact of moderate turbulence of the order 10% is limited and occurs only briefly early on. This makes the signatures of collective and matter interactions relatively straightforward to interpret....

ν propagation in turbulent supernova matter

The flavor evolution of neutrinos propagating through a turbulent medium is a highly interesting and complicated problem. Depending upon the hierarchy and the properties of the turbulence, the neutrino spectral signatures of collective effects and/or shock waves in the supernova may be smothered to the point where they are unobservable in the golden channels (νe → μ transitions) of the next Galactic Supernova Neutrino Burst. Additionally, a more comprehensive understanding of the neutrino flavor evolution in supernova matter is necessary if we wish to use neutrinos to learn about the explosion mechanism. We investigate the effects of neutrino self-interactions, MSW conversions as well as the impact of turbulence on the neutrino flavor evolution along single radial directions in turbulent dense matter, paying special attention to the combined impact of these three effects. We find that adding up to 10% turbulence leads to only minor differences in the emerging neutrino spectra, while overall features of ...