4,345 research outputs found
Rapidity distribution of particle multiplicity in DIS at small x
Analytical study of the rapidity distribution of the final state particles in
deep inelastic scattering at small x is presented. We separate and analyse
three sources of particle production: fragmentation of the quark-antiquark
pair, accompanying coherent soft gluon radiation due to octet color exchange in
the t-channel, and fragmentation of gluons that form parton distribution
functions. Connection to Catani-Ciafaloni-Fiorani-Marchesini (CCFM) equations
and the role of gluon reggezation are also discussed.Comment: 14 pages, 6 Figures. Abstract extended, minor misprints corrected,
derivations improved and explanations are streamlined. Published in Physics
Letters, B
The four jet production at LHC and Tevatron in QCD
We demonstrate that in the back-to-back kinematics the production of four
jets in the collision of two partons is suppressed in the leading log
approximation of pQCD, compared to the hard processes involving the collision
of four partons. We derive the basic equation for four-jet production in QCD in
terms of the convolution of generalized two-parton distributions of colliding
hadrons in the momentum space representation. Our derivation leads to
geometrical approach in the impact parameter space close to that suggested
within the parton model and used before to describe the four-jet production. We
develop the independent parton approximation to the light-cone wave function of
the proton. Comparison with the CDF and D0 data shows that the independent
parton approximation to the light-cone wave function of the proton is
insufficient to explain the data. We argue that the data indicate the presence
of significant multiparton correlations in the light-cone wave functions of
colliding protons.Comment: 5 pages, 2 figures, final version published in Phys.Rev.D, rapid
communication
Origins of parton correlations in nucleon and multi-parton collisions
We demonstrate that perturbative QCD leads to positive 3D parton--parton
correlations inside nucleon explaining a factor two enhancement of the cross
section of multi-parton interactions observed at Tevatron at as
compared to the predictions of the independent parton approximation. We also
find that though perturbative correlations decrease with decreasing, the
nonperturbative mechanism kicks in and should generate correlation which, at
below , is comparable in magnitude with the perturbative one for
.Comment: 4 pages, 5 figure
Perturbative QCD correlations in multi-parton collisions
We examine the role played in double parton interactions (DPI) by the
parton--parton correlations originating from perturbative QCD parton
splittings. Also presented are the results of the numerical analysis of the
integrated DPI cross sections at Tevatron and LHC energies. To obtain the
numerical results the knowledge of the single-parton GPDs gained by the HERA
experiments was used to construct the non-perturbative input for generalized
double parton distributions. The perturbative two-parton correlations induced
by three-parton interactions contribute significantly to resolution of the
longstanding puzzle of an excess of multi-jet production events in the
back-to-back kinematics observed at the Tevatron.Comment: 16 pages, 13 figure
Star Formation and Tidal Encounters with the Low Surface Brightness Galaxy UGC 12695 and Companions
We present VLA H I observations of the low surface brightness galaxy UGC
12695 and its two companions, UGC 12687 and a newly discovered dwarf galaxy
2333+1234. UGC 12695 shows solid body rotation but has a very lopsided
morphology of the H I disk, with the majority of the H I lying in the southern
arm of the galaxy. The H I column density distribution of this very blue, LSB
galaxy coincides in detail with its light distribution. Comparing the H I
column density of UGC 12695 with the empirical (but not well understood) value
of Sigma_c = 10E21 atoms/cm^2 found in, i.e., Skillman's 1986 paper shows the
star formation to be a local affair, occurring only in those regions where the
column density is above this star formation threshold. The low surface
brightness nature of this galaxy could thus be attributed to an insufficient
gas surface density, inhibiting star formation on a more global scale.
Significantly, though, the Toomre criterion places a much lower critical
density on the galaxy (+/-10E20 atoms/cm^2), which is shown by the galaxy's low
SFR to not be applicable.
Within a projected distance of 300kpc/30kms of UGC 12695 lie two companion
galaxies - UGC 12687, a high surface brightness barred spiral galaxy, and
2333+1234, a dwarf galaxy discovered during this investigation. The close
proximity of the three galaxies, combined with UGC 12695's extremely blue color
and regions of localized starburst and UGC 12687's UV excess bring to mind
mutually induced star formation through tidal activity.Comment: 14 pages, 8 figures (2 color), To be published in A.J., May 2000
Aspects Of Heavy Quark Theory
Recent achievements in the heavy quark theory are critically reviewed. The
emphasis is put on those aspects which either did not attract enough attention
or cause heated debates in the current literature. Among other topics we
discuss (i) basic parameters of the heavy quark theory; (ii) a class of exact
QCD inequalities; (iii) new heavy quark sum rules; (iv) virial theorem; (v)
applications (|V_cb| from the total semileptonic width and from the B->D*
transition at zero recoil). In some instances new derivations of the previously
known results are given, or new aspects addressed. In particular, we dwell on
the exact QCD inequalities. Furthermore, a toy model is considered that may
shed light on the controversy regarding the value of the kinetic energy of
heavy quarks obtained by different methods.Comment: 67 pages, 6 Figures; plain LaTeX. Changes: Some equations in Sect.4
related to spin-nonsinglet sum rules are corrected. The references are
updated
Impact of a mass vaccination campaign against a meningitis epidemic in a refugee camp.
Serogroup A meningococcus epidemics occurred in refugee populations in Zaire in August 1994. The paper analyses the public health impact of a mass vaccination campaign implemented in a large refugee camp. We compared meningitis incidence rates from 2 similar camps. In Kibumba camp, vaccination was implemented early in the course of the epidemic whilst in the control camp (Katale), vaccination was delayed. At a threshold of 15 cases per 100 000 population per week an immunization campaign was implemented. Attack rates were 94 and 134 per 100,000 in Kibumba and Katale respectively over 2 months. In Kibumba, one week after crossing the threshold, 121,588 doses of vaccine were administered covering 76% of all refugees. Vaccination may have prevented 68 cases (30% of the expected cases). Despite its rapid institution and the high coverage achieved, the vaccination campaign had a limited impact on morbidity due to meningitis. In the early phase in refugee camps, the relative priorities of meningitis vaccination and case management need to be better defined
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