40 research outputs found
The microstructure, texture and mechanical properties of AS-ECAE Interstitial-free steel and copper
A comparison between the microstructure, texture and mechanical properties of bcc interstitial-free (IF) steel and fcc copper (Cu) for up to N = 8 passes Equal Channel Angular Extrusion (ECAE) via route BC processing was undertaken. Transmission Electron Microscopy (TEM) and Electron Back-Scattering Diffraction (EBSD) studies revealed that the deformation microstructures of both metals evolves from low-angled microbands and dislocation cells after N=2 passes towards more equiaxed, homogeneous subgrain/grain structures comprising higher-angles of misorientation after N = 8 passes. In both metals, the percentage rise in Σ3 and random boundaries are attributed to mechanisms that favour low-energy boundary configurations during ECAE. Texture evolution involves gradual changes in individual component strengths during multi-pass ECAE. The bcc and fcc textures are correlated by interchanging the Miller indices of the slip plane and slip direction between the two cubic crystal systems. The uniaxial tensile curves of both materials are representative of significant cold-working and depict higher 0.2% proof stresses, a small period of uniform elongation, necking and lastly, failure via geometrical softening. Constitutive modelling suggests that rather than a change in deformation mechanism, the preservation of ductility up to N = 8 passes is associated with an increase in the mean free path of dislocations; with slip via dislocation glide remaining as the dominant carrier of plastic strain in both metals
Quark helicity distributions from longitudinal spin asymmetries in muon-proton and muon-deuteron scattering
Double-spin asymmetries for production of charged pions and kaons in
semi-inclusive deep-inelastic muon scattering have been measured by the COMPASS
experiment at CERN. The data, obtained by scattering a 160 GeV muon beam off a
longitudinally polarised NH_3 target, cover a range of the Bjorken variable x
between 0.004 and 0.7. A leading order evaluation of the helicity distributions
for the three lightest quarks and antiquark flavours derived from these
asymmetries and from our previous deuteron data is presented. The resulting
values of the sea quark distributions are small and do not show any sizable
dependence on x in the range of the measurements. No significant difference is
observed between the strange and antistrange helicity distributions, both
compatible with zero. The integrated value of the flavour asymmetry of the
helicity distribution of the light-quark sea, \Delta u-bar - \Delta d-bar, is
found to be slightly positive, about 1.5 standard deviations away from zero.Comment: 13 pages, 5 figure
Measurement of the Longitudinal Spin Transfer to Lambda and Anti-Lambda Hyperons in Polarised Muon DIS
The longitudinal polarisation transfer from muons to lambda and anti-lambda
hyperons, D_LL, has been studied in deep inelastic scattering off an
unpolarised isoscalar target at the COMPASS experiment at CERN. The spin
transfers to lambda and anti-lambda produced in the current fragmentation
region exhibit different behaviours as a function of x and xF . The measured x
and xF dependences of D^lambda_LL are compatible with zero, while
D^anti-lambda_LL tends to increase with xF, reaching values of 0.4 - 0.5. The
resulting average values are D^lambda_LL = -0.012 +- 0.047 +- 0.024 and
D^anti-lambda_LL = 0.249 +- 0.056 +- 0.049. These results are discussed in the
frame of recent model calculations.Comment: 13 pages, 7 figure
Leading order determination of the gluon polarisation from DIS events with high-p_T hadron pairs
We present a determination of the gluon polarisation Delta g/g in the
nucleon, based on the longitudinal double-spin asymmetry of DIS events with a
pair of large transverse-momentum hadrons in the final state. The data were
obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon
beam scattering off a polarised ^6LiD target. The gluon polarisation is
evaluated by a Neural Network approach for three intervals of the gluon
momentum fraction x_g covering the range 0.04 < x_g < 0.27. The values obtained
at leading order in QCD do not show any significant dependence on x_g. Their
average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (syst.) at x_g=0.09
and a scale of mu^2 = 3 (GeV/c)^2.Comment: 13 pages, 6 figures and 3 table
High-statistics measurement of Collins and Sivers asymmetries for transversely polarised deuterons
New results are presented on a high-statistics measurement of Collins and
Sivers asymmetries of charged hadrons produced in deep inelastic scattering of
muons on a transversely polarised LiD target. The data were taken in 2022
with the COMPASS spectrometer using the 160 \gevv\ muon beam at CERN, balancing
the existing data on transversely polarised proton targets. The first results
from about two-thirds of the new data have total uncertainties smaller by up to
a factor of three compared to the previous deuteron measurements. Using all the
COMPASS proton and deuteron results, both the transversity and the Sivers
distribution functions of the and quark, as well as the tensor charge
in the measured -range are extracted. In particular, the accuracy of the
quark results is significantly improved
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
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
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
Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised protons
Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise. © 2015