51 research outputs found
High Gradients In Linear Collider Superconducting Accelerator cavities By high Pulsed Power To Suppress Field Emission
The Superconducting TESLA Cavities
The conceptional design of the proposed linear electron-positron collider
TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an
accelerating gradient of Eacc >= 25 MV/m at a quality factor Q0 > 5E+9. The
design goal for the cavities of the TESLA Test Facility (TTF) linac was set to
the more moderate value of Eacc >= 15 MV/m. In a first series of 27
industrially produced TTF cavities the average gradient at Q0 = 5E+9 was
measured to be 20.1 +- 6.2 MV/m, excluding a few cavities suffering from
serious fabrication or material defects. In the second production of 24 TTF
cavities additional quality control measures were introduced, in particular an
eddy-current scan to eliminate niobium sheets with foreign material inclusions
and stringent prescriptions for carrying out the electron-beam welds. The
average gradient of these cavities at Q0 = 5E+9 amounts to 25.0 +- 3.2 MV/m
with the exception of one cavity suffering from a weld defect. Hence only a
moderate improvement in production and preparation techniques will be needed to
meet the ambitious TESLA goal with an adequate safety margin. In this paper we
present a detailed description of the design, fabrication and preparation of
the TESLA Test Facility cavities and their associated components and report on
cavity performance in test cryostats and with electron beam in the TTF linac.
The ongoing R&D towards higher gradients is briefly addressed.Comment: 45 pages (Latex), 39 figures (Encapsulated Postscript), 53 Author
Spin Density Matrix Elements in Exclusive Meson Muoproduction
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard
exclusive meson muoproduction at COMPASS using 160~GeV/ polarised
and beams impinging on a liquid hydrogen target. The
measurement covers the kinematic range 5.0~GeV/ 17.0~GeV/,
1.0 (GeV/) 10.0 (GeV/) and 0.01 (GeV/) 0.5 (GeV/). Here, denotes the mass of the final
hadronic system, the virtuality of the exchanged photon, and
the transverse momentum of the meson with respect to the
virtual-photon direction. The measured non-zero SDMEs for the transitions of
transversely polarised virtual photons to longitudinally polarised vector
mesons () indicate a violation of -channel helicity
conservation. Additionally, we observe a dominant contribution of
natural-parity-exchange transitions and a very small contribution of
unnatural-parity-exchange transitions, which is compatible with zero within
experimental uncertainties. The results provide important input for modelling
Generalised Parton Distributions (GPDs). In particular, they may allow one to
evaluate in a model-dependent way the role of parton helicity-flip GPDs in
exclusive production
Collins and Sivers transverse-spin asymmetries in inclusive muoproduction of mesons
The production of vector mesons in deep inelastic scattering is an
interesting yet scarsely explored channel to study the transverse spin
structure of the nucleon and the related phenomena. The COMPASS collaboration
has performed the first measurement of the Collins and Sivers asymmetries for
inclusively produced mesons. The analysis is based on the data set
collected in deep inelastic scattering in using a
beam impinging on a transversely polarized target. The
mesons are selected from oppositely charged hadron pairs, and the
asymmetries are extracted as a function of the Bjorken- variable, the
transverse momentum of the pair and the fraction of the energy carried by
the pair. Indications for positive Collins and Sivers asymmetries are observed
Double production in pion-nucleon scattering at COMPASS
We present the study of the production of double mesons using
COMPASS data collected with a 190 GeV/ beam scattering off NH,
Al and W targets. Kinematic distributions of the collected double
events are analysed, and the double production cross section is
estimated for each of the COMPASS targets. The results are compared to
predictions from single- and double-parton scattering models as well as the
pion intrinsic charm and the tetraquark exotic resonance hypotheses. It is
demonstrated that the single parton scattering production mechanism gives the
dominant contribution that is sufficient to describe the data. An upper limit
on the double intrinsic charm content of pion is evaluated. No significant
signatures that could be associated with exotic tetraquarks are found in the
double mass spectrum.Comment: 12 pages, 4 figure
Triangle Singularity as the Origin of the a1(1420)
The COMPASS Collaboration experiment recently discovered a new isovector resonancelike signal with axial-vector quantum numbers, the a(1)(1420), decaying to f(0)(980)(pi). With a mass too close to and a width smaller than the axial-vector ground state a(1)(1260), it was immediately interpreted as a new light exotic meson, similar to the X, Y, Z states in the hidden-charm sector. We show that a resonancelike signal fully matching the experimental data is produced by the decay of the a(1) (1260) resonance into K* (-> K pi) (K) over bar and subsequent rescattering through a triangle singularity into the coupled f(0)(980)p channel. The amplitude for this process is calculated using a new approach based on dispersion relations. The triangle-singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having fewer parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time in the lightmeson sector that a resonancelike structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect
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