438 research outputs found
Variability in the maximum height of the ionospheric F2-layer over Millstone Hill (September 1998?March 2000); influence from below and above
International audienceThe basic aim of this ?case study' is to investigate the variability in the maximum height of the ionospheric F2-layer, hmF2, with periods of planetary waves (2?30 days), and to make an attempt to determine their origin. The hourly data of hmF2 above Millstone Hill (42.6° N, 71.5° W) during 01 September 1998 - 31 March 2000 were used for analysis. Three types of disturbances are studied in detail: (i) the 27- day oscillations observed in the hmF2 above Millstone Hill are generated by the geomagnetic activity and by the global-scale 27-day wave present in the zonal mesosphere/lower thermosphere (MLT) neutral wind. The time delay between the 27-day oscillation in the zonal wind and that in the hmF2 is found to be 5?6 days, while between the 27-day oscillation in the geomagnetic activity and that in the hmF2 is found to be 0.8?1 day; (ii) the 16-day oscillation in the hmF2 observed during summer 1999 is probably generated by the global scale 16-day modulation of the semidiurnal tide observed in the MLT region during PSMOS campaign in June?August. We found that if the modulated semidiurnal tide mediates the planetary wave signature in the ionosphere, this planetary wave oscillation has to be best expressed in the amplitude and in the phase of the 12-h periodicity of the ionosphere; and (iii) the third type of disturbances studied is the quasi-2- day activity in the hmF2 that increases during geomagnetic disturbances. The strong pseudo diurnal periodicities generated during the geomagnetic storms can interact between each other and produce the quasi-2-day oscillations in the ionosphere
Performance of the LHCb Vertex Detector Alignment Algorithm determined with Beam Test Data
LHCb is the dedicated heavy flavour experiment at the Large Hadron Collider
at CERN. The partially assembled silicon vertex locator (VELO) of the LHCb
experiment has been tested in a beam test. The data from this beam test have
been used to determine the performance of the VELO alignment algorithm. The
relative alignment of the two silicon sensors in a module and the relative
alignment of the modules has been extracted. This alignment is shown to be
accurate at a level of approximately 2 micron and 0.1 mrad for translations and
rotations, respectively in the plane of the sensors. A single hit precision at
normal track incidence of about 10 micron is obtained for the sensors. The
alignment of the system is shown to be stable at better than the 10 micron
level under air to vacuum pressure changes and mechanical movements of the
assembled system.Comment: accepted for publication in NIM
Fractal Inspired Models of Quark and Gluon Distributions and Longitudinal Structure Function FL(x, Q2) at small x
In recent years, Fractal Inspired Models of quark and gluon densities at
small x have been proposed. In this paper, we investigate longitudinal
structure function F-L (x, Q2) within this approach. We make predictions using
the QCD based approximate relation between the longitudinal structure function
and the gluon density. As the Altarelli-Martinelli equation for the
longitudinal structure function cannot be applied to Model I due to the
presence of a singularity in the Bjorken x-space we consider Model II only. The
qualitative feature of the prediction of Model II is found to be compatible
with the QCD expectation.Comment: 11 pages, 4 figures, Accepted for publication on 10-07-2010 in Indian
Journal of Physic
Muscular and neuromuscular control following soccer-specific exercise in male youth: Changes in injury risk mechanisms
Poor neuromuscular control has been proposed as a risk factor for non-contact injuries, thus this study aimed to explore the effects of soccer-specific fatigue on leg muscle activation, reactive strength, leg stiffness, and functional hamstring/quadriceps ratio (H/QFUNC) in elite male youth soccer players. Outcome measures were determined in 18 youth players (age 14.4 ± 0.5 years; stature 169.4 ± 9.9 cm; mass 59.3 ± 8.9 kg; maturity offset 0.86 ± 0.88 years) pre and post simulated soccer match play (SAFT90). There was no fatigue-related change in the H/QFUNC; however, reactive strength and leg stiffness were both compromised (P < 0.001) after soccer-specific fatigue. Muscle activation was also locally compromised (P < 0.001) in the medial hamstring and quadriceps but not in the lateral muscles. Where statistically significant changes were observed, the effect sizes ranged from small to large (0.33–0.97). Compromised stiffness when fatigue is present suggests an increased yielding action, greater ground contact times, greater center of mass displacement, and less efficient movement when the limb comes into contact with the ground. This combined with a reduction in medial quadriceps muscle activation may reflect poor kinetic chain control at the hip and an increase in knee injury risk
Q2-evolution of parton densities at small-x values
In the leading twist approximation of the Wilson operator product expansion
with "frozen" and analytic strong coupling constants we show that
Bessel-inspired behavior of the structure function F2 at small x, obtained for
a flat initial condition in the DGLAP evolution equations, leads to good
agreement with the deep inelastic scattering experimental data from HERA.Comment: 19 pages, 5 figures, published in the Proceedings of Workshop on
Physics of Fundamental Interactions, Institute of High Energy Physics,
Protvino, Russia, 22--25 December 2008 (Phys.Atom.Nucl. in press
Dynamical parton distributions of the nucleon and very small-x physics
Utilizing recent DIS measurements (F_{2,L}) and data on dilepton and
high-E_{T} jet production we determine the dynamical parton distributions of
the nucleon generated radiatively from valence-like positive input
distributions at optimally chosen low resolution scales. These are compared
with `standard' distributions generated from positive input distributions at
some fixed and higher resolution scale. It is shown that up to the next to
leading order NLO(\bar{MS}, DIS) of perturbative QCD considered in this paper,
the uncertainties of the dynamical distributions are, as expected, smaller than
those of their standard counterparts. This holds true in particular in the
presently unexplored extremely small-x region relevant for evaluating ultrahigh
energy cross sections in astrophysical applications. It is noted that our new
dynamical distributions are compatible, within the presently determined
uncertainties, with previously determined dynamical parton distributions.Comment: 21 pages, 2 tables, 16 figures, v2: added Ref.[60], replaced Fig.
Searches at HERA for Squarks in R-Parity Violating Supersymmetry
A search for squarks in R-parity violating supersymmetry is performed in e^+p
collisions at HERA at a centre of mass energy of 300 GeV, using H1 data
corresponding to an integrated luminosity of 37 pb^(-1). The direct production
of single squarks of any generation in positron-quark fusion via a Yukawa
coupling lambda' is considered, taking into account R-parity violating and
conserving decays of the squarks. No significant deviation from the Standard
Model expectation is found. The results are interpreted in terms of constraints
within the Minimal Supersymmetric Standard Model (MSSM), the constrained MSSM
and the minimal Supergravity model, and their sensitivity to the model
parameters is studied in detail. For a Yukawa coupling of electromagnetic
strength, squark masses below 260 GeV are excluded at 95% confidence level in a
large part of the parameter space. For a 100 times smaller coupling strength
masses up to 182 GeV are excluded.Comment: 32 pages, 14 figures, 3 table
Forward pi^0 Production and Associated Transverse Energy Flow in Deep-Inelastic Scattering at HERA
Deep-inelastic positron-proton interactions at low values of Bjorken-x down
to x \approx 4.10^-5 which give rise to high transverse momentum pi^0 mesons
are studied with the H1 experiment at HERA. The inclusive cross section for
pi^0 mesons produced at small angles with respect to the proton remnant (the
forward region) is presented as a function of the transverse momentum and
energy of the pi^0 and of the four-momentum transfer Q^2 and Bjorken-x.
Measurements are also presented of the transverse energy flow in events
containing a forward pi^0 meson. Hadronic final state calculations based on QCD
models implementing different parton evolution schemes are confronted with the
data.Comment: 27 pages, 8 figures and 3 table
Small x behavior of parton distributions. A study of higher twist effects
Higher twist corrections to the structure function F_2 at small x are studied
for the case of a flat initial condition for the twist-two QCD evolution in the
next-to-leading order approximation. We present an analytical parameterization
of the contributions from the twist-two and higher twist operators of the
Wilson operator product expansion. Higher twist terms are evaluated using two
different approaches, one motivated by BFKL and the other motivated by the
renormalon formalism. The results of the latter approach are in very good
agreement with deep inelastic scattering data from HERA.Comment: 46 pages, 9 figures, typeset using REVTeX. Misprints corrected, new
results, discussions, references and acknowledgements added, 2 pictures
replaced. The conclusions remain unchange
Deep-Inelastic Inclusive ep Scattering at Low x and a Determination of alpha_s
A precise measurement of the inclusive deep-inelastic e^+p scattering cross
section is reported in the kinematic range 1.5<= Q^2 <=150 GeV^2 and
3*10^(-5)<= x <=0.2. The data were recorded with the H1 detector at HERA in
1996 and 1997, and correspond to an integrated luminosity of 20 pb^(-1). The
double differential cross section, from which the proton structure function
F_2(x,Q^2) and the longitudinal structure function F_L(x,Q^2) are extracted, is
measured with typically 1% statistical and 3% systematic uncertainties. The
measured partial derivative (dF_2(x,Q^2)/dln Q^2)_x is observed to rise
continuously towards small x for fixed Q^2. The cross section data are combined
with published H1 measurements at high Q^2 for a next-to-leading order DGLAP
QCD analysis.The H1 data determine the gluon momentum distribution in the range
3*10^(-4)<= x <=0.1 to within an experimental accuracy of about 3% for Q^2 =20
GeV^2. A fit of the H1 measurements and the mu p data of the BCDMS
collaboration allows the strong coupling constant alpha_s and the gluon
distribution to be simultaneously determined. A value of alpha
_s(M_Z^2)=0.1150+-0.0017 (exp) +0.0009-0.0005 (model) is obtained in NLO, with
an additional theoretical uncertainty of about +-0.005, mainly due to the
uncertainty of the renormalisation scale.Comment: 68 pages, 24 figures and 18 table
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