140 research outputs found
Spin alignment and violation of the OZI rule in exclusive and production in pp collisions
Exclusive production of the isoscalar vector mesons and is
measured with a 190 GeV proton beam impinging on a liquid hydrogen target.
Cross section ratios are determined in three intervals of the Feynman variable
of the fast proton. A significant violation of the OZI rule is found,
confirming earlier findings. Its kinematic dependence on and on the
invariant mass of the system formed by fast proton
and vector meson is discussed in terms of diffractive
production of resonances in competition with central
production. The measurement of the spin density matrix element of
the vector mesons in different selected reference frames provides another
handle to distinguish the contributions of these two major reaction types.
Again, dependences of the alignment on and on are
found. Most of the observations can be traced back to the existence of several
excited baryon states contributing to production which are absent in
the case of the meson. Removing the low-mass resonant
region, the OZI rule is found to be violated by a factor of eight,
independently of .Comment: 23 pages, 13 figures and 5 table
Kapitel 24. Theorien des Wandels und der Gestaltung von Strukturen
Abschnitt 5 nimmt eine Bestandsaufnahme von Theorien vor, die in einem weiten Sinne Wandel untersuchen. „Theorien des Wandels“ ist ein Überbegriff für all diejenigen Theorien, die helfen, aktuelle Dynamiken der Klimakrise zu verstehen und sowohl die stattfindenden als auch die notwendigen Transformationen zu fassen
Measurement of the charged-pion polarisability
The COMPASS collaboration at CERN has investigated pion Compton scattering,
, at centre-of-mass energy below 3.5 pion
masses. The process is embedded in the reaction
, which is initiated by
190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons
is selected by isolating the sharp Coulomb peak observed at smallest momentum
transfers, \,(GeV/). From a sample of 63\,000 events the
pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\
0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times
10^{-4}\,\mbox{fm}^3\alpha_\pi=-\beta_\pi$, which
relates the electric and magnetic dipole polarisabilities. It is the most
precise measurement of this fundamental low-energy parameter of strong
interaction, that has been addressed since long by various methods with
conflicting outcomes. While this result is in tension with previous dedicated
measurements, it is found in agreement with the expectation from chiral
perturbation theory. An additional measurement replacing pions by muons, for
which the cross-section behavior is unambigiously known, was performed for an
independent estimate of the systematic uncertainty.Comment: Published version: 9 pages, 3 figures, 1 tabl
Interplay among transversity induced asymmetries in hadron leptoproduction
In the fragmentation of a transversely polarized quark several left-right
asymmetries are possible for the hadrons in the jet. When only one unpolarized
hadron is selected, it exhibits an azimuthal modulation known as Collins
effect. When a pair of oppositely charged hadrons is observed, three
asymmetries can be considered, a di-hadron asymmetry and two single hadron
asymmetries. In lepton deep inelastic scattering on transversely polarized
nucleons all these asymmetries are coupled with the transversity distribution.
From the high statistics COMPASS data on oppositely charged hadron-pair
production we have investigated for the first time the dependence of these
three asymmetries on the difference of the azimuthal angles of the two hadrons.
The similarity of transversity induced single and di-hadron asymmetries is
discussed. A new analysis of the data allows to establish quantitative
relationships among them, providing for the first time strong experimental
indication that the underlying fragmentation mechanisms are all driven by a
common physical process.Comment: 6 figure
Interplay among transversity induced asymmetries in hadron leptoproduction
In the fragmentation of a transversely polarized quark several left-right
asymmetries are possible for the hadrons in the jet. When only one unpolarized
hadron is selected, it exhibits an azimuthal modulation known as Collins
effect. When a pair of oppositely charged hadrons is observed, three
asymmetries can be considered, a di-hadron asymmetry and two single hadron
asymmetries. In lepton deep inelastic scattering on transversely polarized
nucleons all these asymmetries are coupled with the transversity distribution.
From the high statistics COMPASS data on oppositely charged hadron-pair
production we have investigated for the first time the dependence of these
three asymmetries on the difference of the azimuthal angles of the two hadrons.
The similarity of transversity induced single and di-hadron asymmetries is
discussed. A new analysis of the data allows to establish quantitative
relationships among them, providing for the first time strong experimental
indication that the underlying fragmentation mechanisms are all driven by a
common physical process.Comment: 6 figure
Leading-order determination of the gluon polarisation from semi-inclusive deep inelastic scattering data
Using a novel analysis technique, the gluon polarisation in the nucleon is
re-evaluated using the longitudinal double-spin asymmetry measured in the cross
section of semi-inclusive single-hadron muoproduction with photon virtuality
. The data were obtained by the COMPASS experiment at
CERN using a 160 GeV/ polarised muon beam impinging on a polarised LiD
target. By analysing the full range in hadron transverse momentum ,
the different -dependences of the underlying processes are separated
using a neural-network approach. In the absence of pQCD calculations at
next-to-leading order in the selected kinematic domain, the gluon polarisation
is evaluated at leading order in pQCD at a hard scale of . It is determined in three intervals
of the nucleon momentum fraction carried by gluons, , covering the
range ~ and does not exhibit a significant
dependence on . The average over the three intervals, at
, suggests that the gluon polarisation
is positive in the measured range.Comment: 14 pages, 6 figure
Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised proton
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.Comment: 15 pages, 13 figures and 1 tabl
Search for exclusive photoproduction of Z(3900) at COMPASS
A search for the exclusive production of the hadron by
virtual photons has been performed in the channel . The data cover the range from 7 GeV to 19 GeV in the
centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS
data set collected with a muon beam between 2002 and 2011 has been used. An
upper limit for the ratio of has been established at the confidence
level of 90%.Comment: 8 pages, 3 figures, 1 tabl
Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering
A semi-inclusive measurement of charged hadron multiplicities in deep
inelastic muon scattering off an isoscalar target was performed using data
collected by the COMPASS Collaboration at CERN. The following kinematic domain
is covered by the data: photon virtuality (GeV/), invariant
mass of the hadronic system GeV/, Bjorken scaling variable in the
range , fraction of the virtual photon energy carried by the
hadron in the range , square of the hadron transverse momentum
with respect to the virtual photon direction in the range 0.02 (GeV/ (GeV/). The multiplicities are presented as a
function of in three-dimensional bins of , , and
compared to previous semi-inclusive measurements. We explore the
small- region, i.e. (GeV/), where
hadron transverse momenta are expected to arise from non-perturbative effects,
and also the domain of larger , where contributions from
higher-order perturbative QCD are expected to dominate. The multiplicities are
fitted using a single-exponential function at small to study
the dependence of the average transverse momentum on , and . The power-law behaviour of the
multiplicities at large is investigated using various
functional forms. The fits describe the data reasonably well over the full
measured range.Comment: 28 pages, 20 figure
The Spin Structure Function of the Proton and a Test of the Bjorken Sum Rule
New results for the double spin asymmetry and the proton
longitudinal spin structure function are presented. They were
obtained by the COMPASS collaboration using polarised 200 GeV muons scattered
off a longitudinally polarised NH target. The data were collected in 2011
and complement those recorded in 2007 at 160\,GeV, in particular at lower
values of . They improve the statistical precision of by
about a factor of two in the region . A next-to-leading order
QCD fit to the world data is performed. It leads to a new determination
of the quark spin contribution to the nucleon spin, ranging
from 0.26 to 0.36, and to a re-evaluation of the first moment of .
The uncertainty of is mostly due to the large uncertainty in
the present determinations of the gluon helicity distribution. A new evaluation
of the Bjorken sum rule based on the COMPASS results for the non-singlet
structure function yields as ratio of the axial and
vector coupling constants , which validates the sum rule to an accuracy of about
9\%.Comment: 19 pages, 8 figures and table
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