arXiv subjects

Dafne Guetta

Publications and source records attributed to Dafne Guetta.

80 records · Page 5Linked to original sources

arXiv2002

Gamma Ray Bursts from the First Stars: Neutrino Signals

If the first (PopIII) stars were very massive, their final fate is to collapse into very massive black holes. Once a proto-black hole has formed into the stellar core, accretion continues through a disk. It is widely accepted, although not confirmed, that magnetic fields drive an energetic jet which produces a burst of TeV neutrinos by photon-meson interaction, and eventually breaks out of the stellar envelope appearing as a Gamma Ray Burst (GRB). Based on recent numerical simulations and neutrino emission models, we predict the expected neutrino diffuse flux from these PopIII GRBs and compare it with the capabilities of present and planned detectors as AMANDA and IceCube. If beamed into 1% of the sky, we find that the rate of PopIII GRBs is $\le 4 \times 10^6$ yr$^{-1}$. High energy neutrinos from PopIII GRBs could dominate the overall flux in two energy bands [$10^4 - 10^5$] GeV and [$10^5 - 10^6$] GeV of neutrino telescopes. The enhanced sensitivities of forthcoming detectors in the high-energy band (AMANDA-II, IceCube) will provide a fundamental insight on the characteristic explosion energies of PopIII GRBs and will constitute a unique probe of the the Initial Mass Function (IMF) of the first stars and of the redshift $z_f$ marking the metallicity-driven transition from a top-heavy to a normal IMF. The current upper limit set by AMANDA-B10 implies that such transition must have occurred not later than $z_f =9.2$ for the most plausible jet energies. Based on such results, we speculate that PopIII GRBs, if not chocked, could be associated with a new class of events detected by BeppoSax, the Fast X-ray Transient (FXTs), which are bright X-ray sources, with peak energies in the 2-10 keV band and durations between 10-200 s.

astro-ph↗

arXiv2001

On the neutrino flux from Gamma Ray Bursts: an overview

Observations imply that gamma-ray bursts (GRBs) are produced by the dissipation of the kinetic energy of a highly relativistic fireball. Photo-meson interactions of protons with gamma-rays within the fireball dissipation region are expected to convert a significant fraction of the fireball energy into high energy neutrinos. In this talk we summarize the results of an analysis of the internal shock model of GRBs, where production of synchrotron photons and photo-meson neutrinos with energies about 10^14 eV are self-consistently calculated. These neutrinos will be coincident with the GRB. We show that the fraction of fireball energy converted into high energy neutrinos is not sensitive to uncertainties in the fireball model parameters, such as the expansion Lorentz factor and characteristic variability time. Other processes of neutrinos emission from GRBs are also reviewed. Photomeson interactions within the plasma region shocked by the reverse shock, may produce a burst of 10^18 eV neutrinos following the GRB on the time scale of 10 s. Inelastic p-n nuclear collisions result in the production of a burst of around 10 GeV neutrinos in coincidence with the GRB. Planned 1 km^3 neutrino telescopes are expected to detect ten 100 TeV neutrino events and several 10^18 eV events, correlated with GRBs, per year. A suitably densely spaced detector may allow the detection of several 10 GeV events per year.

astro-ph↗

arXiv2000

Determination of the strong g-coupling and of the $μ_{B^*Bγ}-$ strength from two-photon decays of heavy vector mesons

We describe a calculation of the two-photon decays of heavy vector-mesons, $B^*\to Bγγ$ and $D^*\to Dγγ$, by using the ``heavy meson chiral Lagrangian''. The decay amplitudes are expressed in terms of the strong coupling $g$ of the Lagrangian at various powers and the strength of the anomalous magnetic dipole $μ_{B^*Bγ}$ (respectively $μ_{D^*D}γ$). In the charm case we are able to express the branching ratio $Γ(D^{*0}\to D^0γγ)/Γ(D^{*0})$ as a function of $g$ only and we expect it to be in the $10^{-6}-10^{-5}$ range, depending on the value of $g$. The determination of $μ_{B^{*0}B^{0}γ},μ_{B^{*+}B^+}γ$ requires a more involved analysis, including the consideration of bremsstrahlung radiation for the $B^{*+}\to B^+γγ$ case.

hep-ph↗

arXiv2000

$ω-ρ$ Mixing and the $ω\toππγ$ Decay

We reexamine the $ω\to π^{0} π^{0} γ$ decay, adding the effect of $ω-ρ$ mixing to the amplitude calculated with the aid of chiral perturbation theory and vector meson dominance. We predict the neutral decay to occur with a width of $Γ($ω\to π^{0} π^{0} γ$) =(390\pm96) {\rm eV}$ and also analyze the effect of the $ω-ρ$ mixing on the $Γ($ω\to π^{0} π^{0} γ$)/ Γ($ω\to π^{+} π^{-} γ$)$ ratio. Several remarks on the effect of $ω-ρ$ mixing on certain radiative decays of vector mesons are presented.

hep-ph↗

arXiv1999

Two photon decays of heavy vector mesons, $B^{}\to Bγγ$,$D^{}\to Dγγ$, and the possible determination of the $g_{B^{}(D^{})B(D)π}$ and $g_{B^{*}^0B^0γ}$ couplings

We study the novel decays $B^{*}\to Bγγ$ and $D^{*}\to Dγγ$ using the framework of the Heavy Meson Chiral Lagrangian (HM$χ$L) to leading order in chiral perturbation theory. The branching ratios of these decays are expressed in terms of the strong $g_{B^{*}(D^{*})B(D)π}$ and the electromagnetic $g_{B^{*}(D^*)B(D)γ}$ couplings, thus providing a possible tool for their determination. In the charm case, using the experimentally determined ratios $({D^*}^{0,+}\to Dπ)/({D^*}^{0,+}\to Dγ),$ we are able to express the branching ratio as a function of the strong coupling only. We thus find $1.6\times 10^{-6}<{\rm Br} ({D^{*}}^{0}\to D^0γγ)<3.3\times 10^{-5}$ for $0.25 0.6.$ The expected two-photon differential distributions are presented for both ${B^{*}}^{0} \to B^0 γγ$ and ${D^{*}}^{0}\to D^0γγ,$ for different values of the couplings involved.

hep-ph↗

arXiv1998

$B\toτμ(X)$ decays in SUSY models without R-parity

Being strictly forbidden in the standard model, experimental detection of the lepton flavor violating decays $B(\bar B)\toτ^+μ^-$ and $b(\bar b)\to Xτ^+μ^-$ would constitute an unmistakable indication of new physics. We study these decays in supersymmetric models without R-parity and without lepton number. In order to derive order of magnitude predictions for the branching ratios, we assume a horizontal U(1) symmetry with horizontal charges chosen to explain the magnitude of fermion masses and quark mixing angles. We find that the branching ratios for decays with a $τμ$ pair in the final state are not particularly suppressed with respect to the lepton flavor conserving channels. In general in these models ${\rm B}[b\toμ^+μ^-(X)]\lsim {\rm B}[b(\bar b)\toτ^+μ^-(X)] \lsim {\rm B}[b\toτ^+τ^-(X)]$. While in some cases the rates for final states $τ^+τ^-$ can be up to one order of magnitude larger than the lepton flavor violating channel, due to better efficiencies for muon detection and to the absence of standard model contributions, decays into $τμ$ final states appear to be better suited to reveal this kind of new physics.

hep-ph↗

arXiv1998

Effects of R-parity Violating Couplings on CP Asymmetries in Neutral B Decays

A detailed analysis of the effects of supersymmetric models without R-parity on various CP asymmetries in neutral $B$ decays is given. We concentrate on models with Abelian horizontal symmetries that allow us to estimate the order of magnitude of the new effects. We focus on channels where the Standard Model gives clean predictions: $ B_{d}\toψK_{S} $ and $B_{d}\toϕK_{S}$. The two asymmetries can have a value different from $\sin2β.$ Moreover, they can be different from each other.

hep-ph↗

arXiv1998

Searching for New Physics in Rare $B \to τ$ Decays

The rare decays {$B^- \to τ\barν$}, \mbox{$B \to τ^+ τ^-$}, {$b \to X ν\barν$} and \mbox{$b \to X τ^+ τ^-$} all contain third generation leptons in the final state, and hence are sensitive to new physics that couples more strongly to the third family. We present model independent expressions for these decays that can be useful to study several types of new physics effects. We concentrate on supersymmetric models without R-parity and without lepton number. We also assume a horizontal U(1) symmetry with fermion horizontal charges chosen to explain the magnitude of fermion masses and quark mixing angles. This allows us to estimate the order of magnitude of the new effects, and to derive numerical predictions for the various decay rates and for the forward-backward asymmetry and the $τ$ polarization components measurable in \mbox{$b \to X τ^+ τ^-$}. In some cases the branching ratios are enhanced by more than one order of magnitude, rendering foreseeable their detection at upcoming B-factories. We also discuss how a measurement of asymmetries in \mbox{$b \to X τ^+ τ^-$} can be crucial in distinguishing between different sources of new physics.

hep-ph↗