9,253 research outputs found
Majorana vs Pseudo-Dirac Neutrinos at the ILC
Neutrino masses could originate in seesaw models testable at colliders, with
light mediators and an approximate lepton number symmetry. The minimal model of
this type contains two quasi-degenerate Majorana fermions forming a
pseudo-Dirac pair. An important question is to what extent future colliders
will have sensitivity to the splitting between the Majorana components, since
this quantity signals the breaking of lepton number and is connected to the
light neutrino masses. We consider the production of these neutral heavy
leptons at the ILC, where their displaced decays provide a golden signal: a
forward-backward charge asymmetry, which depends crucially on the mass
splitting between the two Majorana components. We show that this observable can
constrain the mass splitting to values much lower than current bounds from
neutrinoless double beta decay and natural loop corrections.Comment: 16 pages, 5 figures; v2: Minor changes, version accepted for
publication in EPJ
Ultrafast relaxation rates and reversal time in disordered ferrimagnets
In response to ultrafast laser pulses, single-phase metals have been classified as “fast” (with magnetization quenching on the time scale of the order of 100 fs and recovery in the time scale of several picoseconds and below) and “slow” (with longer characteristic time scales). Disordered ferrimagnetic alloys consisting of a combination of “fast” transition (TM) and “slow” rare-earth (RE) metals have been shown to exhibit an ultrafast all-optical switching mediated by the heat mechanism. The behavior of the characteristic time scales of coupled alloys is more complicated and is influenced by many parameters such as the intersublattice exchange, doping (RE) concentration, and the temperature. Here, the longitudinal relaxation times of each sublattice are analyzed within the Landau-Lifshitz-Bloch framework. We show that for moderate intersublattice coupling strength both materials slow down as a function of slow (RE) material concentration. For larger coupling, the fast (TM) material may become faster, while the slow (RE) one is still slower. These conclusions may have important implications in the switching time of disordered ferrimagnets such as GdFeCo with partial clustering. Using atomistic modeling, we show that in the moderately coupled case, the reversal would start in the Gd-rich region, while the situation may be reversed if the coupling strength is larger
Can SUSY relax LNV constraints coming from loop corrections to light neutrino masses on the low-scale Seesaw?
Heavy neutrinos from the Type-I Seesaw model can have a large mixing with
active states, motivating their search at collider experiments. However, loop
corrections to light neutrino masses constrain the heavy neutrinos to appear in
pseudo-Dirac pairs, leading to a potential suppression of lepton number
violating parameters.
In this work we perform a detailed review of a proposal to relax constraints
on lepton number violation by adding supersymmetry. We define the conditions
necessary to maximise the SUSY screening effect, with the objective of allowing
a larger mass splitting between low-scale heavy neutrino masses. We find that
the sole addition of SUSY does not guarantee a screening, and that favourable
cases have some degree of fine-tuning.Comment: 10 figure
A search for water maser emission toward obscured post-AGB star and planetary nebula candidates
Water maser emission at 22 GHz is a useful probe to study the transition
between the nearly spherical mass-loss in the AGB to a collimated one in the
post-AGB phase. In their turn, collimated jets in the post-AGB phase could
determine the shape of planetary nebulae (PNe) once photoionization starts. We
intend to find new cases of post-AGB stars and PNe with water maser emission,
including water fountains or water-maser-emitting PNe. We observed water maser
emission in a sample of 133 objects, with a significant fraction being post-AGB
and young PN candidate sources with strong obscuration. We detected this
emission in 15 of them, of which seven are reported here for the first time. We
identified three water fountain candidates: IRAS 17291-2147, with a total
velocity spread of ~96 km/s in its water maser components and two sources (IRAS
17021-3109 and IRAS 17348-2906) that show water maser emission outside the
velocity range covered by OH masers. We have also identified IRAS 17393-2727 as
a possible new water-maser-emitting PN. The detection rate is higher in
obscured objects (14%) than in those with optical counterparts (7%), consistent
with previous results. Water maser emission seems to be common in objects that
are bipolar in the near-IR (43% detection rate). The water maser spectra of
water fountain candidates like IRAS 17291-2147 show significantly less maser
components than others (e.g., IRAS 18113-2503). We speculate that most
post-AGBs may show water maser emission with wide enough velocity spread (> 100
km/s) when observed with enough sensitivity and/or for long enough periods of
time. Therefore, it may be necessary to single out a special group of "water
fountains", probably defined by their high maser luminosities. We also suggest
that the presence of both water and OH masers in a PN is a better tracer of its
youth, rather than the presence of just one of these species.Comment: To be published in Astronomy & Astrophysics. 16 pages, 1 figure
(spanning 5 pages). This version includes some minor language corrections and
fixes some errors in Table
Magnetic Structure of Hydrogen Induced Defects on Graphene
Using density functional theory (DFT), Hartree-Fock, exact diagonalization,
and numerical renormalization group methods we study the electronic structure
of diluted hydrogen atoms chemisorbed on graphene. A comparison between DFT and
Hartree-Fock calculations allows us to identify the main characteristics of the
magnetic structure of the defect. We use this information to formulate an
Anderson-Hubbard model that captures the main physical ingredients of the
system, while still allowing a rigorous treatment of the electronic
correlations. We find that the large hydrogen-carbon hybridization puts the
structure of the defect half-way between the one corresponding to an adatom
weakly coupled to pristine graphene and a carbon vacancy. The impurity's
magnetic moment leaks into the graphene layer where the electronic correlations
on the C atoms play an important role in stabilizing the magnetic solution.
Finally, we discuss the implications for the Kondo effect.Comment: 10 pages, 10 fig
Anticipating the Successor to Mexico's Largest Historical Earthquake
Note in proof: On October 9, as this article was being prepared for publication, a magnitude 7.6 earthquake occurred beneath the Jalisco region and caused significant loss of life and property. This earthquake highlights the societal need for accurate measurements of crustal strain rates in earthquake-prone zones. In the coming months, we plan to measure the amount of displacement that occurred within the GPS network during and after this earthquake
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