2,876 research outputs found
The G0 experiment at Jefferson Laboratory: The nucleon strangeness form factors
The G0 experiment is dedicated to the determination of the strange quark's contribution to the electric and magnetic nucleon form factors, provided by parity-violating asymmetries of cross-sections measured with longitudinaly polarized electrons in elastic electron-proton scattering and quasi-elastic electron-deuteron scattering. Forward angle measurements, which have been performed in Hall C of Jefferson Laboratory, have provided a linear combination of electric and magnetic vector form factors for momentum transfers in the range 0.1 to 1 (GeV/c)^2. Backward angle measurements, which will be performed starting in 2006, will allow the complete separation of the form factors of Q^2 of 0.23 and 0.63 (GeV/c)^2
CEDFPD board : Board in VXI format which realize the coincidence between the CED and the FPD detectors for the G0 experiment
QGP tomography with photon tagged jets in ALICE
jet events provide a tomographic measurement of the medium formed
in heavy ion collisions at LHC energies. Tagging events with a well identified
high direct photon and measuring the correlation distribution of
hadrons emitted oppositely to the photon, allows us to determine, with a good
approximation, both the jet fragmentation function and the back-to-back
azimuthal misalignement of the direct photon and the jet. Comparing these two
observables measured in collisions with the ones measured in
collisions will reveal the modifications of the jet structure induced by the
medium formed in collisions and consequently will infer the medium
properties.Comment: 3 pages, 3 figures, Proceedings for 5th International Conference on
Quarks and Nuclear Physic
The ALICE EMCal L1 trigger first year of operation experience
The ALICE experiment at the LHC is equipped with an electromagnetic
calorimeter (EMCal) designed to enhance its capabilities for jet, photon and
electron measurement. In addition, the EMCal enables triggering on jets and
photons with a centrality dependent energy threshold. After its commissioning
in 2010, the EMCal Level 1 (L1) trigger was officially approved for physics
data taking in 2011. After describing the L1 hardware and trigger algorithms,
the commissioning and the first year of running experience, both in proton and
heavy ion beams, are reviewed. Additionally, the upgrades to the original L1
trigger design are detailed.Comment: Proceedings of TWEPP-12, Oxford. 10 pages, 9 figure
Etude de la fragmentation des partons par mesure de corrélations photon-hadrons auprès de l'expérience ALICE au LHC
La théorie de l interaction forte, ou Chromodynamique Quantique (QCD), prédit l existence d une nouvelle phase de la matière nucléaire à très haute température et/ou très haute densité. Cet état est composé de quarks et de gluons déconfinés connu sous le nom de plasma de quarks-gluons (PQG).La mesure de sa composition et de ses propriétés est un enjeu important pour la physique nucléaire du XXIème siècle afin de parvenir à une meilleure compréhension des symétries et des mécanismes fondamentaux à l origine du confinement des quarks au sein des hadrons et de l interaction forte dans son ensemble.L accélérateur LHC (Large Hadron Collider) au CERN (Organisation Européenne pour la Recherche Nucléaire) permet d atteindre les conditions thermodynamiques nécessaires à la formationdu plasma de quarks-gluons à l aide de collisions d ions lourds (Pb) ultra relativistes. L expérience ALICE (A Large Ion Collider Experiment) permet d accéder à un grand nombre d observables pour caractériser le PQG à partir de la reconstruction et de l identification des particules produites lors descollisions. Parmi ces observables, la perte d énergie des partons (quarks, gluons) de haute impulsiontransverse permet une étude des caractéristiques du milieu telle que sa densité et sa température.La perte d énergie des partons est mise en évidence par la modification de la distribution en énergiedes hadrons produits par fragmentation.Cette thèse s articule autour de l analyse des corrélations photon-hadron dans le but d étudierla modification de la fragmentation partonique par le plasma de quarks-gluons. La première partiede cette thèse est consacrée à la caractérisation du calorimètre électromagnétique EMCal, détecteur central pour la mesure en énergie et l identification des photons. La seconde partie est dédiéeà la mesure des corrélations photon-hadron, dont l analyse a portée sur les collisions proton-protond énergie ps = 7 TeV, avant d être appliquée aux collisions Plomb-Plomb d énergie psNN = 2.76TeV. Un effort particulier a été fourni pour optimiser l identification des photons prompts, un des points clés de cette analyse.The strong interaction theory, Quantum Chromodynamic (QCD), predicts a new phase of nuclearmatter at very high temperature and/or very high density. This state is composed of deconfinedquarks and gluons known as the quark-gluon plasma (QGP). The measurement of its compositionand properties is a challenge for the nuclear physics of the 21st century and should lead to a betterunderstanding of the fundamental symetries and mechanisms related to the quarks confinement insidehadrons and the strong interaction generally.The Large Hadron Collider (LHC) accelerator at CERN (European Organization for NuclearResearch) allows to reach the thermodynamic conditions required to create the quark-gluon plasmausing ultra-relativistic heavy ion collisions (Pb). The ALICE experiment (A Large Ion ColliderExperiment) allows to access several probes to characterize the QGP through particles reconstructionand. Among these probes, high energy parton energy loss is used to access medium characteristicssuch as density or temperature. Parton energy loss is estimated from the modification of the energydistribution of hadrons produced by fragmentation.This thesis is dedicated to the photon-hadron correlations analysis in order to study the modificationof the parton fragmentation due to the quark-gluon plasma. First part of this thesis is devotedto the characterization of the electromagnetic calorimeter (EMCal), the central detector for energymeasurement and photon identification. The second part is dedicated to the photon-hadron correlationmeasurement, for the 7 TeV proton-proton collisions and 2.76 TeV Lead-Lead collisions. Animportant work has been done to improve the prompt photon identification, one of the key point ofthis analysis.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
G Electronics and Data Acquisition (Forward-Angle Measurements)
The G parity-violation experiment at Jefferson Lab (Newport News, VA) is
designed to determine the contribution of strange/anti-strange quark pairs to
the intrinsic properties of the proton. In the forward-angle part of the
experiment, the asymmetry in the cross section was measured for
elastic scattering by counting the recoil protons corresponding to the two
beam-helicity states. Due to the high accuracy required on the asymmetry, the
G experiment was based on a custom experimental setup with its own
associated electronics and data acquisition (DAQ) system. Highly specialized
time-encoding electronics provided time-of-flight spectra for each detector for
each helicity state. More conventional electronics was used for monitoring
(mainly FastBus). The time-encoding electronics and the DAQ system have been
designed to handle events at a mean rate of 2 MHz per detector with low
deadtime and to minimize helicity-correlated systematic errors. In this paper,
we outline the general architecture and the main features of the electronics
and the DAQ system dedicated to G forward-angle measurements.Comment: 35 pages. 17 figures. This article is to be submitted to NIM section
A. It has been written with Latex using \documentclass{elsart}. Nuclear
Instruments and Methods in Physics Research Section A: Accelerators,
Spectrometers, Detectors and Associated Equipment In Press (2007
Eikonal representation in the momentum-transfer space
By means of empirical fits to the differential cross section data on pp and
p(bar)p elastic scattering, above 10 GeV (center-of-mass energy), we determine
the eikonal in the momentum - transfer space (q^2- space). We make use of a
numerical method and a novel semi-analytical method, through which the
uncertainties from the fit parameters can be propagated up to the eikonal in
the - space. A systematic study of the effect of the experimental
information at large values of the momentum transfer is developed and discussed
in detail. We present statistical evidence that the imaginary part of the
eikonal changes sign in the q^2- space and that the position of the zero
decreases as the energy increases; after the position of the zero, the eikonal
presents a minimum and then goes to zero through negative values. We discuss
the applicability of our results in the phenomenological context, outlining
some connections with nonperturbative QCD. A short review and a critical
discussion on the main results concerning "model-independent" analyses are also
presented.Comment: 18 pages, 17 figures, 4 tables, svjour.cls. Revised discussion on the
proton's electromagnetic form factor and references added. To appear in Eur.
Phys. J.
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Rhizosphere bacteria are more strongly related to plant root traits than fungi in temperate montane forests: insights from closed and open forest patches along an elevational gradient
Heterogeneous canopies in temperate montane forests affect microclimate and soil characteristics, with important effects on soil microbial communities and related processes. Here, we studied the interactions between plant root traits and soil bacterial and fungal communities in closed forest and gaps in a mixed forest along an elevational gradient in the French Alps (1400, 1700 and 2000 m).
Samples were separated into three fractions (plant root endosphere, rhizosphere and bulk soil), to further investigate the influence of plant zones on microbial communities. Bacterial (16S) and fungal (ITS) biodiversity was determined using high throughput sequencing, along with standard measures of soil, litter and root traits.
We found that (i) microbial community diversity was higher in gaps than in closed forest because of increased root trait diversity and density; (ii) open versus closed forest patches affected phylogenetic dispersion despite differences in elevations with phylogenetic clustering in closed forest; (iii) the interaction between root traits and microbial communities was stronger for rhizosphere and endosphere compartments than for bulk soil and (iv) bacterial community composition was better explained by root traits than for fungi.
Our findings highlight the importance of open gaps versus closed forest patches and associated root traits affecting microbial community structure, particularly for bacterial assemblages that exhibited a stronger interaction with root traits than for fungi
Evidence for eikonal zeros in the momentum transfer space
We present the results of fitting elastic differential cross section
data at 23.5 62.5 GeV with a novel analytic
parametrization for the scattering amplitude. Making use of a fitting method,
the errors from the free parameters are propagated to the imaginary part of the
eikonal in the momentum transfer space. A novel systematic study of the effects
coming from data at large momentum transfer is also performed. We find
statistical evidence for the existence of eikonal zeros in the interval of
momentum transfer 5-9 .Comment: Text with 9 pages in Revtex (preprint form), 8 figures in PostScript.
Replaced with small changes. Final version to be published in Physical Review
A precise measurement of the deuteron elastic structure function A(Q^2)
The A(Q^2) structure function in elastic electron-deuteron scattering was
measured at six momentum transfers Q^2 between 0.66 and 1.80 (GeV/c)^2 in Hall
C at Jefferson Laboratory. The scattered electrons and recoil deuterons were
detected in coincidence, at a fixed deuteron angle of 60.5 degrees. These new
precise measurements resolve discrepancies between older sets of data. They put
significant constraints on existing models of the deuteron electromagnetic
structure, and on the strength of isoscalar meson exchange currents.Comment: 3 LaTeX pages plus 2 PS figure
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