1,414 research outputs found
Quantifying jet transport properties via large hadron production
Nuclear modification factor for large single hadron is studied
in a next-to-leading order (NLO) perturbative QCD (pQCD) parton model with
medium-modified fragmentation functions (mFFs) due to jet quenching in
high-energy heavy-ion collisions. The energy loss of the hard partons in the
QGP is incorporated in the mFFs which utilize two most important parameters to
characterize the transport properties of the hard parton jets: the jet
transport parameter and the mean free path , both at
the initial time . A phenomenological study of the experimental data
for is performed to constrain the two parameters with
simultaneous fits to RHIC as well as LHC data. We obtain
for energetic quarks GeV/fm and
fm in central collisions at
GeV, while GeV/fm, and
fm in central collisions at
TeV. Numerical analysis shows that the best fit favors a
multiple scattering picture for the energetic jets propagating through the bulk
medium, with a moderate averaged number of gluon emissions. Based on the best
constraints for and , the estimated value for the
mean-squared transverse momentum broadening is moderate which implies that the
hard jets go through the medium with small reflection.Comment: 8 pages, 6 figures, revised versio
Optimal Distributed Beamforming for MISO Interference Channels
We consider the problem of quantifying the Pareto optimal boundary in the
achievable rate region over multiple-input single-output (MISO) interference
channels, where the problem boils down to solving a sequence of convex
feasibility problems after certain transformations. The feasibility problem is
solved by two new distributed optimal beamforming algorithms, where the first
one is to parallelize the computation based on the method of alternating
projections, and the second one is to localize the computation based on the
method of cyclic projections. Convergence proofs are established for both
algorithms.Comment: 7 Pages, 6 figures, extended version for the one in Proceeding of
Asilomar, CA, 201
Spectrum sensing by cognitive radios at very low SNR
Spectrum sensing is one of the enabling functionalities for cognitive radio
(CR) systems to operate in the spectrum white space. To protect the primary
incumbent users from interference, the CR is required to detect incumbent
signals at very low signal-to-noise ratio (SNR). In this paper, we present a
spectrum sensing technique based on correlating spectra for detection of
television (TV) broadcasting signals. The basic strategy is to correlate the
periodogram of the received signal with the a priori known spectral features of
the primary signal. We show that according to the Neyman-Pearson criterion,
this spectral correlation-based sensing technique is asymptotically optimal at
very low SNR and with a large sensing time. From the system design perspective,
we analyze the effect of the spectral features on the spectrum sensing
performance. Through the optimization analysis, we obtain useful insights on
how to choose effective spectral features to achieve reliable sensing.
Simulation results show that the proposed sensing technique can reliably detect
analog and digital TV signals at SNR as low as -20 dB.Comment: IEEE Global Communications Conference 200
1I/2017 U1 (`Oumuamua) is Hot: Imaging, Spectroscopy and Search of Meteor Activity
1I/2017 U1 (`Oumuamua), a recently discovered asteroid in a hyperbolic orbit,
is likely the first macroscopic object of extrasolar origin identified in the
solar system. Here, we present imaging and spectroscopic observations of
\textquoteleft Oumuamua using the Palomar Hale Telescope as well as a search of
meteor activity potentially linked to this object using the Canadian Meteor
Orbit Radar. We find that \textquoteleft Oumuamua exhibits a moderate spectral
gradient of , a value significantly lower
than that of outer solar system bodies, indicative of a formation and/or
previous residence in a warmer environment. Imaging observation and spectral
line analysis show no evidence that \textquoteleft Oumuamua is presently
active. Negative meteor observation is as expected, since ejection driven by
sublimation of commonly-known cometary species such as CO requires an extreme
ejection speed of m s at au in order to reach the
Earth. No obvious candidate stars are proposed as the point of origin for
\textquoteleft Oumuamua. Given a mean free path of ly in the solar
neighborhood, \textquoteleft Oumuamua has likely spent a very long time in the
interstellar space before encountering the solar system.Comment: ApJL in pres
Depth Sensitivity and Source-Detector Separations for Near Infrared Spectroscopy Based on the Colin27 Brain Template
Understanding the spatial and depth sensitivity of non-invasive near-infrared spectroscopy (NIRS) measurements to brain tissue–i.e., near-infrared neuromonitoring (NIN) – is essential for designing experiments as well as interpreting research findings. However, a thorough characterization of such sensitivity in realistic head models has remained unavailable. In this study, we conducted 3,555 Monte Carlo (MC) simulations to densely cover the scalp of a well-characterized, adult male template brain (Colin27). We sought to evaluate: (i) the spatial sensitivity profile of NIRS to brain tissue as a function of source-detector separation, (ii) the NIRS sensitivity to brain tissue as a function of depth in this realistic and complex head model, and (iii) the effect of NIRS instrument sensitivity on detecting brain activation. We found that increasing the source-detector (SD) separation from 20 to 65 mm provides monotonic increases in sensitivity to brain tissue. For every 10 mm increase in SD separation (up to ∼45 mm), sensitivity to gray matter increased an additional 4%. Our analyses also demonstrate that sensitivity in depth (S) decreases exponentially, with a “rule-of-thumb” formula S = 0.75*0.85depth. Thus, while the depth sensitivity of NIRS is not strictly limited, NIN signals in adult humans are strongly biased towards the outermost 10–15 mm of intracranial space. These general results, along with the detailed quantitation of sensitivity estimates around the head, can provide detailed guidance for interpreting the likely sources of NIRS signals, as well as help NIRS investigators design and plan better NIRS experiments, head probes and instruments
- …