Alterations in white matter microstructure in neurofibromatosis-1.

Neurofibromatosis (NF1) represents the most common single gene cause of learning disabilities. NF1 patients have impairments in frontal lobe based cognitive functions such as attention, working memory, and inhibition. Due to its well-characterized genetic etiology, investigations of NF1 may shed light on neural mechanisms underlying such difficulties in the general population or other patient groups. Prior neuroimaging findings indicate global brain volume increases, consistent with neural over-proliferation. However, little is known about alterations in white matter microstructure in NF1. We performed diffusion tensor imaging (DTI) analyses using tract-based spatial statistics (TBSS) in 14 young adult NF1 patients and 12 healthy controls. We also examined brain volumetric measures in the same subjects. Consistent with prior studies, we found significantly increased overall gray and white matter volume in NF1 patients. Relative to healthy controls, NF1 patients showed widespread reductions in white matter integrity across the entire brain as reflected by decreased fractional anisotropy (FA) and significantly increased absolute diffusion (ADC). When radial and axial diffusion were examined we found pronounced differences in radial diffusion in NF1 patients, indicative of either decreased myelination or increased space between axons. Secondary analyses revealed that FA and radial diffusion effects were of greatest magnitude in the frontal lobe. Such alterations of white matter tracts connecting frontal regions could contribute to the observed cognitive deficits. Furthermore, although the cellular basis of these white matter microstructural alterations remains to be determined, our findings of disproportionately increased radial diffusion against a background of increased white matter volume suggest the novel hypothesis that one potential alteration contributing to increased cortical white matter in NF1 may be looser packing of axons, with or without myelination changes. Further, this indicates that axial and radial diffusivity can uniquely contribute as markers of NF1-associated brain pathology in conjunction with the typically investigated measures

Freeze-Out Time in Ultrarelativistic Heavy Ion Collisions from Coulomb Effects in Transverse Pion Spectra

The influence of the nuclear Coulomb field on transverse spectra of $\pi^+$ and $\pi^-$ measured in $Pb+Pb$ reactions at 158 A GeV has been investigated. Pion trajectories are calculated in the field of an expanding fireball. The observed enhancement of the $\pi^-/\pi^+$ ratio at small momenta depends on the temperature and transverse expansion velocity of the source, the rapidity distribution of the net positive charge, and mainly the time of the freeze-out.Comment: 11 pages including 2 figure

Testing the Flyby Anomaly with the GNSS Constellation

We propose the concept of a space mission to probe the so called flyby anomaly, an unexpected velocity change experienced by some deep-space probes using earth gravity assists. The key feature of this proposal is the use of GNSS systems to obtain an increased accuracy in the tracking of the approaching spacecraft, mainly near the perigee. Two low-cost options are also discussed to further test this anomaly: an add-on to an existing spacecraft and a dedicated mission.Comment: 8 pages, 1 figure, 4 table

The relationship between particle freeze-out distributions and HBT radius parameters

The relationship between pion and kaon space-time freeze-out distributions and the HBT radius parameters in high-energy nucleus-nucleus collisions is investigated. We show that the HBT radius parameters in general do not reflect the R.M.S. deviations of the single particle production points. Instead, the HBT radius parameters are most closely related to the curvature of the two-particle space-time relative position distribution at the origin. We support our arguments by studies with a dynamical model (RQMD 2.4).Comment: RevTex, 10 pages including 3 figures. v2: Discussion of the lambda parameter has been added. PRC, in prin

(Anti)Proton and Pion Source Sizes and Phase Space Densities in Heavy Ion Collisions

NA44 has measured mid-rapidity deuteron spectra from AA collisions at sqrt{s}=18GeV/A at the CERN SPS. Combining these spectra with published proton, antiproton and antideuteron data allows us to calculate, within a coalescence framework, proton and antiproton source sizes and phase space densities. These results are compared to pion source sizes and densities, pA results and to lower energy (AGS) data. The antiproton source is larger than the proton source at sqrt{s}=18GeV/A. The phase space densities of pions and protons are not constant but grow with system size. Both pi+ and proton radii decrease with transverse mass and increase with sqrt{s}. Pions and protons do not freeze-out independently. The nature of their interaction changes as sqrt{s}, and the pion/proton ratio increases.Comment: 4 pages, Latex 2.09, 3 eps figures. Changes for January 2001. The proton source size is now calculated assuming a more realistic Hulthen, rather than Gaussian, wavefunction. A new figure shows the effect of this change which is important for small radii. A second new figure shows the results of RQMD calculations of the proton source size and phase density. Because of correlations between position and momentum coalesence does not show the full proton source size. The paper has been streamlined and readability improve

Nuclear Modification Factor for Charged Pions and Protons at Forward Rapidity in Central Au+Au Collisions at 200 GeV

We present spectra of charged pions and protons in 0-10% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV at mid-rapidity ($y=0$) and forward pseudorapidity ($\eta=2.2$) measured with the BRAHMS experiment at RHIC. The spectra are compared to spectra from p+p collisions at the same energy scaled by the number of binary collisions. The resulting nuclear modification factors for central Au+Au collisions at both $y=0$ and $\eta=2.2$ exhibit suppression for charged pions but not for (anti-)protons at intermediate $p_T$. The $\bar{p}/\pi^-$ ratios have been measured up to $p_T\sim 3$ GeV/$c$ at the two rapidities and the results indicate that a significant fraction of the charged hadrons produced at intermediate $p_T$ range are (anti-)protons at both mid-rapidity and $\eta = 2.2$

Two-kaon correlations in central Pb + Pb collisions at 158 A GeV/c

Two-particle interferometry of positive kaons is studied in Pb + Pb collisions at mean transverse momenta $\approx 0.25$ and 0.91 GeV/c. A three-dimensional analysis was applied to the lower $p_T$ data, while a two-dimensional analysis was used for the higher $p_T$ data. We find that the source size parameters are consistent with the $m_T$ scaling curve observed in pion correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.Comment: 4 pages incl. 1 table and 3 fig's; RevTeX; accepted for publication in PR

Baryon phase-space density in heavy-ion collisions

The baryon phase-space density at mid-rapidity from central heavy-ion collisions is estimated from proton spectra with interferometry and deuteron coalescence measurements. It is found that the mid-rapidity phase-space density of baryons is significantly lower at the SPS than the AGS, while those of total particles (pion + baryon) are comparable. Thermal and chemical equilibrium model calculations tend to over-estimate the phase-space densities at both energies.Comment: 5 pages, 2 tables, no figure. RevTeX style. Accepted for publication in Phys. Rev. C Rapid Communicatio

The New Physics at RHIC. From Transparency to High pt_t Suppression

Heavy ion collisions at RHIC energies (Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV) exhibit significant new features as compared to earlier experiments at lower energies. The reaction is characterized by a high degree of transparency of the collisions partners leading to the formation of a baryon-poor central region. In this zone, particle production occurs mainly from the stretching of the color field. The initial energy density is well above the one considered necessary for the formation of the Quark Gluon Plasma, QGP. The production of charged particles of various masses is consistent with chemical and thermal equilibrium. Recently, a suppression of the high transverse momentum component of hadron spectra has been observed in central Au+Au collisions. This can be explained by the energy loss experienced by leading partons in a medium with a high density of unscreened color charges. In contrast, such high $p_t$ jets are not suppressed in d+Au collisions suggesting that the high $p_t$ suppression is not due to initial state effects in the ultrarelativistic colliding nuclei.Comment: 15 pages, 11 figures. to appear in Nucl. Physics A. Invited talk at 'Nucleus-Nucleus Collisions 2003' conference, Mosco

Squeezed Correlations and Spectra for Mass-Shifted Bosons

Huge back-to-back correlations are shown to arise for thermal ensembles of bosonic states with medium-modified masses. The effect is experimentally observable in high energy heavy ion collisions.Comment: 4 pages (RevTex) including 2 eps figures via psfig, published versio