The 1996 data taking of the SMC experiment used polarized protons to measure the spin-dependent structure function
g
1 of the proton. Three liters of solid granular ammonia were irradiated at the ...Bonn electron linac in order to create the paramagnetic radicals which are needed for polarizing the protons. Proton polarizations of ±(90±2.5)% were routinely reached. An analysis based on a theoretical line shape for spin-1 systems with large quadrupolar broadening was developed which allowed the nitrogen polarization in the ammonia to be determined with a 10% relative error. The measured quadrupolar coupling constant of
14N agrees well with earlier extrapolated values. The polarization of the nitrogen nuclei was measured as a function of the proton polarization in order to provide a test of the equal spin temperature (EST) hypothesis. It was found to be closely valid under the dynamic nuclear polarization conditions with which the protons are polarized. Large deviations from EST could be induced by cross relaxing the proton and nitrogen spin systems at low fields. Nitrogen polarizations up to 40% were reached by these means.
Dielectric measurements of an organic-rich permafrost soil over the range from 1.0 to 16 GHz and from -30 °C to +25 °C are presented. The measured shrub soil contains up to 90% organic matter and is ...the first soil of this composition for which the soil dielectric has been characterized. Using the dielectric data thus obtained, the process of freezing has been analyzed of unfrozen water contained in the shrub tundra sample.
Evidence for a positive longitudinal double-spin asymmetry 〈
A
1
ρ
〉=0.24±0.11
stat±0.02
syst in the cross section for exclusive diffractive
ρ
0(770) vector–meson production in polarised ...lepton–proton scattering was observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA positron beam was scattered off a longitudinally polarised pure hydrogen gas target. The average invariant mass of the photon–proton system has a value of 〈
W〉=4.9 GeV, while the average negative squared four-momentum of the virtual photon is 〈
Q
2〉=1.7 GeV
2. The ratio of the present result to the corresponding spin asymmetry in inclusive deep-inelastic scattering is in agreement with an early theoretical prediction based on the generalised vector–meson dominance model.
We present a precise measurement of the proton longitudinal double-spin
asymmetry $A_1^{\rm p}$ and the proton spin-dependent structure function
$g_1^{\rm p}$ at photon virtualities $0.006~({\rm ...GeV}/c)^2<Q^2 < 1~({\rm
GeV}/c)^2$ in the Bjorken $x$ range of $4 \times 10^{-5} < x < 4 \times
10^{-2}$. The results are based on data collected by the COMPASS Collaboration
at CERN using muon beam energies of $160~{\rm GeV}$ and $200~{\rm GeV}$. The
statistical precision is more than tenfold better than that of the previous
measurement in this region. In the whole range of $x$, the measured values of
$A_1^{\rm p}$ and $g_1^{\rm p}$ are found to be positive. It is for the first
time that spin effects are found at such low values of $x$.
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 \(Q^{2}>1\) (GeV/\(c\))\(^2\), invariant mass of the hadronic system \(W > 5\) GeV/\(c^2\), Bjorken scaling variable in the range \(0.003 < x < 0.4\), fraction of the virtual photon energy carried by the hadron in the range \(0.2 < z < 0.8\), square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 (GeV/\(c)^2 < P_{\rm{hT}}^{2} < 3\) (GeV/\(c\))\(^2\). The multiplicities are presented as a function of \(P_{\rm{hT}}^{2}\) in three-dimensional bins of \(x\), \(Q^2\), \(z\) and compared to previous semi-inclusive measurements. We explore the small-\(P_{\rm{hT}}^{2}\) region, i.e. \(P_{\rm{hT}}^{2} < 1\) (GeV/\(c\))\(^2\), where hadron transverse momenta are expected to arise from non-perturbative effects, and also the domain of larger \(P_{\rm{hT}}^{2}\), where contributions from higher-order perturbative QCD are expected to dominate. The multiplicities are fitted using a single-exponential function at small \(P_{\rm{hT}}^{2}\) to study the dependence of the average transverse momentum \(\langle P_{\rm{hT}}^{2}\rangle\) on \(x\), \(Q^2\) and \(z\). The power-law behaviour of the multiplicities at large \(P_{\rm{hT}}^{2}\) is investigated using various functional forms. The fits describe the data reasonably well over the full measured range.
Proton transverse-spin azimuthal asymmetries are extracted from the COMPASS 2010 semi-inclusive hadron measurements in deep inelastic muon-nucleon scattering in those four regions of the photon ...virtuality \(Q^2\), which correspond to the four regions of the di-muon mass \(\sqrt{Q^2}\) used in the ongoing analysis of the COMPASS Drell-Yan measurements. This allows for a future direct comparison of the nucleon transverse-momentum-dependent parton distribution functions extracted from these two alternative measurements. Various two-dimensional kinematic dependences are presented for the azimuthal asymmetries induced by the Sivers transverse-momentum-dependent parton distribution function. The integrated Sivers asymmetries are found to be positive with an accuracy that appears to be sufficient to test the sign change of the Sivers function predicted by Quantum Chromodynamics.
We present a new amplitude analysis of the $\eta\pi$ $D$-wave in $\pi^- p\to
\eta\pi^- p$ measured by COMPASS. Employing an analytical model based on the
principles of the relativistic $S$-matrix, we ...find two resonances that can be
identified with the $a_2(1320)$ and the excited $a_2^\prime(1700)$, and perform
a comprehensive analysis of their pole positions. For the mass and width of the
$a_2$ we find $M=(1307 \pm 1 \pm 6)$~MeV and $\Gamma=(112 \pm 1 \pm 8)$~MeV,
and for the excited state $a_2^\prime$ we obtain $M=(1720 \pm 10 \pm 60)$~MeV
and $\Gamma=(280\pm 10 \pm 70)$~MeV, respectively.
We have searched for exclusive production of exotic charmonia in the reaction
$\mu^+~N \rightarrow \mu^+ (J\!/\!\psi\pi^+\pi^-)\pi^{\pm}~N'$ using COMPASS
data collected with incoming muons of 160 ...GeV/$c$ and 200 GeV/$c$ momentum. In
the $J\!/\!\psi\pi^+\pi^-$ mass distribution we observe a signal with a
statistical significance of 4.1 $\sigma$. Its mass and width are consistent
with those of the $X(3872)$. The shape of the $\pi^+\pi^-$ mass distribution
from the observed decay into $J\!/\!\psi\pi^+\pi^-$ shows disagreement with
previous observations for $X(3872)$. The observed signal may be interpreted as
a possible evidence of a new charmonium state. It could be associated with a
neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model. The
product of cross section and branching fraction of the decay of the observed
state into $J\!/\!\psi\pi^+\pi^-$ is determined to be
71$\pm$28(stat)$\pm$39(syst) pb.
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