arXiv subjects

A. Segarra

Publications and source records attributed to A. Segarra.

7 recordsLinked to original sources

arXiv2019

Review and new concepts for neutron-capture measurements of astrophysical interest

The idea of slow-neutron capture nucleosynthesis formulated in 1957 triggered a tremendous experimental effort in different laboratories worldwide to measure the relevant nuclear physics input quantities, namely ($n,\gamma$) cross sections over the stellar temperature range (from few eV up to several hundred keV) for most of the isotopes involved from Fe up to Bi. A brief historical review focused on total energy detectors will be presented to illustrate how, advances in instrumentation have led, over the years, to the assessment and discovery of many new aspects of $s$-process nucleosynthesis and to the progressive refinement of theoretical models of stellar evolution. A summary will be presented on current efforts to develop new detection concepts, such as the Total-Energy Detector with $\gamma$-ray imaging capability (i-TED). The latter is based on the simultaneous combination of Compton imaging with neutron time-of-flight (TOF) techniques, in order to achieve a superior level of sensitivity and selectivity in the measurement of stellar neutron capture rates.

physics.ins-det↗

arXiv2018

Signatures of the genuine and matter-induced components of the CP violation asymmetry in neutrino oscillations

CP asymmetries for neutrino oscillations in matter can be disentangled into the matter-induced CPT-odd (T-invariant) component and the genuine T-odd (CPT-invariant) component. For their understanding in terms of the relevant ingredients, we develop a new perturbative expansion in both $Δm^2_{21},\, |a| \ll |Δm^2_{31}|$ without any assumptions between $Δm^2_{21}$ and $a$, and study the subtleties of the vacuum limit in the two terms of the CP asymmetry, moving from the CPT-invariant vacuum limit $a \to 0$ to the T-invariant limit $Δm^2_{21} \to 0$. In the experimental region of terrestrial accelerator neutrinos, we calculate their approximate expressions from which we prove that, at medium baselines, the CPT-odd component is small and nearly $δ$-independent, so it can be subtracted from the experimental CP asymmetry as a theoretical background, provided the hierarchy is known. At long baselines, on the other hand, we find that (i) a Hierarchy-odd term in the CPT-odd component dominates the CP asymmetry for energies above the first oscillation node, and (ii) the CPT-odd term vanishes, independent of the CP phase $δ$, at $E =0.92~\mathrm{GeV}\,(L/1300~\mathrm{km})$ near the second oscillation maximum, where the T-odd term is almost maximal and proportional to $\sinδ$. A measurement of the CP asymmetry in these energy regions would thus provide separate information on (i) the neutrino mass ordering, and (ii) direct evidence of genuine CP violation in the lepton sector.

hep-ph↗

arXiv2018

Disentangling genuine from matter-induced CP violation in neutrino oscillations

We prove that, in any flavor transition, neutrino oscillation CP violating asymmetries in matter have two disentangled components: i) a CPT-odd T-invariant term, non-vanishing iff there are interactions with matter; ii) a T-odd CPT-invariant term, non-vanishing iff there is genuine CP violation. As function of the baseline, these two terms are distinct L-even and L-odd observables to separately test (i) matter effects sensitive to the neutrino hierarchy and (ii) genuine CP violation in the neutrino sector. For the golden $ν_μ\to ν_e$ channel, the different energy distributions of the two components provide a signature of their separation. At long baselines, they show oscillations in the low energy region, with zeros at different positions and peculiar behavior around the zeros. We discover a magic energy $E = (0.91 \pm 0.01)$ GeV at L = 1300 km with vanishing CPT-odd term and maximal genuine CP asymmetry proportional to $\sinδ$, with $δ$ the weak CP phase. For energies above 1.5 GeV, the sign of the CP asymmetry discriminates the neutrino hierarchy.

hep-ph↗

arXiv2017

The W and Z boson spin observables as messengers of new physics at LHC

The successful LHC operation suggests going beyond the search of excess of events for the quest of new physics. We demonstrate that the eight multipole parameters describing the spin state of the W or Z bosons are able to disentangle their hidden production mechanism. They can be separately extracted from well defined angular asymmetries in the leptonic distribution of boson decays. The discriminating power of this analysis is well illustrated by: (i) polarised top quark decays, (ii) two body decay of heavy resonances, (iii) Drell-Yan production of Z plus jets, (iv) Z boson plus missing transverse energy.

hep-ph↗

arXiv2017

Stimulated X-rays in resonant atom Majorana mixing

Massive neutrinos demand to ask whether they are Dirac or Majorana particles. Majorana neutrinos are an irrefutable proof of physics beyond the Standard Model. Neutrinoless double electron capture is not a process but a virtual $ΔL=2$ mixing between a parent $^AZ$ atom and a daughter $^A(Z-2)$ excited atom with two electron holes. As a mixing between two neutral atoms and the observable signal in terms of emitted two-hole X-rays, the strategy, experimental signature and background are different from neutrinoless double beta decay. The mixing is resonantly enhanced for almost degeneracy and, under these conditions, there is no irreducible background from the standard two-neutrino channel. We reconstruct the natural time history of a nominally stable parent atom since its production either by nature or in the laboratory. After the time periods of atom oscillations and the decay of the short-lived daughter atom, at observable times the relevant "stationary" states are the mixed metastable long-lived state and the short-lived excited state, as well as the ground state of the daughter atom. Their natural population inversion is most appropriate for exploiting the bosonic nature of the observed X-rays by means of stimulating X-ray beams. Among different observables of the atom Majorana mixing, we include the enhanced rate of stimulated X-ray emission from the long-lived metastable state by a high-intensity X-ray beam. A gain factor of 100 can be envisaged in a facility like European XFEL.

hep-ph↗

arXiv2017

The Z boson spin observables as messengers of new physics

We demonstrate that the 8 multipole parameters describing the spin state of the $Z$ boson are able to disentangle known $Z$ production mechanisms and signals from new physics at the LHC. They can be extracted from appropriate asymmetries in the angular distribution of lepton pairs from the $Z$ boson decay. The power of this analysis is illustrated by (i) the production of $Z$ boson plus jets; (ii) $Z$ boson plus missing transverse energy; (iii) $W$ and $Z$ bosons originating from the two-body decay of a heavy resonance.

hep-ph↗

arXiv2016

Neutrino-Pair Exchange Long-Range Force Between Aggregate Matter

We study the long-range force arising between two neutral---of electric charge---aggregates of matter due to a neutrino-pair exchange, in the limit of zero neutrino mass. The conceptual basis for the construction of the effective potential comes from the coherent scattering amplitude at low values of t. This amplitude is obtained using the methodology of an unsubtracted dispersion relation in t at threshold for s, where (s, t) are the Lorentz invariant scattering variables. The ultraviolet behavior is irrelevant for the long-range force. In turn, the absorptive part in the t-dependence is given by the corresponding unitarity relation. We show that the potential describing this force decreases as $r^{-5}$ at large separation distance r. This interaction is described in terms of its own charge, which we call the weak flavor charge of the interacting systems, that depends on the flavor of the neutrino as $Q_W^e = 2Z-N$, $Q_W^μ= Q_W^τ= -N$. The flavor dependence of the potential factorizes in the product of the weak charges of the interacting systems, so that the resulting force is always repulsive. Furthermore, this charge is proportional to the number of constituent particles, which differs from the global mass, so this interaction could be disentangled from gravitation through deviations from the Equivalence Principle.

hep-ph↗