33,169 research outputs found
Random Numbers Certified by Bell's Theorem
Randomness is a fundamental feature in nature and a valuable resource for
applications ranging from cryptography and gambling to numerical simulation of
physical and biological systems. Random numbers, however, are difficult to
characterize mathematically, and their generation must rely on an unpredictable
physical process. Inaccuracies in the theoretical modelling of such processes
or failures of the devices, possibly due to adversarial attacks, limit the
reliability of random number generators in ways that are difficult to control
and detect. Here, inspired by earlier work on nonlocality based and device
independent quantum information processing, we show that the nonlocal
correlations of entangled quantum particles can be used to certify the presence
of genuine randomness. It is thereby possible to design of a new type of
cryptographically secure random number generator which does not require any
assumption on the internal working of the devices. This strong form of
randomness generation is impossible classically and possible in quantum systems
only if certified by a Bell inequality violation. We carry out a
proof-of-concept demonstration of this proposal in a system of two entangled
atoms separated by approximately 1 meter. The observed Bell inequality
violation, featuring near-perfect detection efficiency, guarantees that 42 new
random numbers are generated with 99% confidence. Our results lay the
groundwork for future device-independent quantum information experiments and
for addressing fundamental issues raised by the intrinsic randomness of quantum
theory.Comment: 10 pages, 3 figures, 16 page appendix. Version as close as possible
to the published version following the terms of the journa
Explaining LSND by a decaying sterile neutrino
We propose an explanation of the LSND evidence for electron antineutrino
appearance based on neutrino decay. We introduce a heavy neutrino, which is
produced in pion and muon decays because of a small mixing with muon neutrinos,
and then decays into a scalar particle and a light neutrino, predominantly of
the electron type. We require values of few eV, being the
neutrino--scalar coupling and the heavy neutrino mass, e.g. in the
range from 1 keV to 1 MeV and . Performing a fit to
the LSND data as well as all relevant null-result experiments, we show that all
data can be explained within this decay scenario. In the minimal version of the
decay model, we predict a signal in the upcoming MiniBooNE experiment
corresponding to a transition probability of the same order as seen in LSND. In
addition, we show that extending our model to two nearly degenerate heavy
neutrinos it is possible to introduce CP violation in the decay, which can lead
to a suppression of the signal in MiniBooNE running in the neutrino mode. We
briefly discuss signals in future neutrino oscillation experiments, we show
that our scenario is compatible with bounds from laboratory experiments, and we
comment on implications in astrophysics and cosmology.Comment: 23 pages, 5 figures, minor improvements, matches published versio
On X-ray Optical Depth in the Coronae of Active Stars
We have investigated the optical thickness of the coronal plasma through the
analysis of high-resolution X-ray spectra of a large sample of active stars
observed with the High Energy Transmission Grating Spectrometer on Chandra. In
particular, we probed for the presence of significant resonant scattering in
the strong Lyman series lines arising from hydrogen-like oxygen and neon ions.
The active RS CVn-type binaries II Peg and IM Peg and the single M dwarf EV Lac
show significant optical depth. For these active coronae, the Lya/Lyb ratios
are significantly depleted as compared with theoretical predictions and with
the same ratios observed in similar active stars. Interpreting these decrements
in terms of resonance scattering of line photons out of the line-of-sight, we
are able to derive an estimate for the typical size of coronal structures, and
from these we also derive estimates of coronal filling factors. For all three
sources we find that the both the photon path length as a fraction of the
stellar radius, and the implied surface filling factors are very small and
amount to a few percent at most. The measured Lya/Lyb ratios are in good
agreement with APED theoretical predictions, thus indicating negligible optical
depth, for the other sources in our sample. We discuss the implications for
coronal structuring and heating flux requirements. For the stellar sample as a
whole, the data suggest increasing quenching of Lya relative to Lyb as function
of both L_x/L_bol and the density-sensitive MgXI forbidden to intercombination
line ratio, as might generally be expected.Comment: Accepted for publication on the Astrophysical Journa
The VLT-FLAMES Tarantula Survey V. The peculiar B[e]-like supergiant, VFTS698, in 30 Doradus
We present an analysis of a peculiar supergiant B-type star (VFTS698/Melnick
2/Parker 1797) in the 30 Doradus region of the Large Magellanic Cloud which
exhibits characteristics similar to the broad class of B[e] stars. We analyse
optical spectra from the VLT-FLAMES survey, together with archival optical and
infrared photometry and X-ray imaging to characterise the system. We find
radial velocity variations of around 400 km/s in the high excitation Si IV, N
III and He II spectra, and photometric variability of ~0.6 mag with a period of
12.7 days. In addition, we detect long-term photometric variations of ~0.25
mag, which may be due to a longer-term variability with a period of ~400 days.
We conclude that VFTS698 is likely an interacting binary comprising an early
B-type star secondary orbiting a veiled, more massive companion. Spectral
evidence suggests a mid-to-late B-type primary, but this may originate from an
optically-thick accretion disc directly surrounding the primary.Comment: 18 pages, 14 figures and 8 tables. Table 8 to be published onlin
Dark matter and spin-1 milli-charged particles
New physics scenarios beyond the Standard Model predict the existence of
milli-charged particles. So far, only spin-1/2 and spin-0 milli-charged
particles have been considered in literature, leaving out the interesting case
of spin-1. We propose a minimal unitary and renormalizable model of massive
milli-charged vector particles. Unitarity requires that these particles are
gauge bosons of a non-abelian spontaneously broken gauge symmetry. The minimal
scenario then consists of an extended Standard Model gauge group together with a dark Higgs boson
responsible for the symmetry breaking in the dark sector. By imposing that the
dark Higgs multiplet has a non-vanishing milli-hypercharge, stable
milli-charged spin-1 fields arise thereby providing a potential dark matter
candidate. We analyse the phenomenological constraints on this scenario and
discuss their implications.Comment: Matches the version that is to appear on JHE
The new surprising behaviour of the two "prototype" blazars PKS 2155-304 and 3C 279
Recent VHE observations have unveiled a surprising behaviour in two
well-known blazars at opposite sides of the blazar sequence. PKS 2155-304 have
shown for the first time in an HBL a large Compton dominance, high gamma-ray
luminosities and a cubic relation between X-ray and VHE fluxes. 3C 279 is the
first FSRQ detected at VHE. The high luminosity required to overcome the
significant absorption caused by the BLR emission cannot be easily reconciled
with the historical and quasi-simultaneous SED properties. Both cases shed a
new light on the structure and ambient fields of blazars. Contrary to previous
claims, it is also shown that 3C 279 --as any FSRQ-- cannot in general provide
robust constraints on the EBL.Comment: Proceedings of "4th Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy 2008" (Gamma 2008), July 7-11, 2008. Slightly refined
text with updated reference
The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation
We present the first large-scale study of E+A (post-starburst) galaxies that
incorporates photometry in the ultraviolet (UV) wavelengths. We find that the
starburst that creates the E+A galaxy typically takes place within the last Gyr
and creates a high fraction (20-60 percent) of the stellar mass in the remnant
over a short timescale (< 0.1 Gyrs). We find a tight correlation between the
luminosity of our E+A galaxies and the implied star formation rate (SFR) during
the starburst. While low-luminosity E+As (M(z) > -20) exhibit implied SFRs of
less than 50 solar masses per year, their luminous counterparts (M(z) < -22)
shows SFRs greater than 300 and as high as 2000 solar masses per year,
suggesting that luminous and ultra-luminous infrared galaxies in the
low-redshift Universe could be the progenitors of massive nearby E+A galaxies.
We perform a comprehensive study of the characteristics of the quenching that
truncates the starburst in E+A systems.We find that, for galaxies less massive
than 10^10 MSun, the quenching efficiency decreases as the galaxy mass
increases. However, for galaxies more massive than 10^10 MSun, this trend is
reversed and the quenching efficiency increases with galaxy mass. Noting that
the mass threshold at which this reversal occurs is in excellent agreement with
the mass above which AGN become significantly more abundant in nearby galaxies,
we use simple energetic arguments to show that the bimodal behaviour of the
quenching efficiency is consistent with AGN and supernovae (SN) being the
principal sources of negative feedback above and below M ~ 10^10 MSun
respectively. (abridged)Comment: MNRAS in press (accepted September 2007
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