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.
The present contribution reviews the principle of RF separation and explains its dependence on different parameters of beam optics and hardware. The first examination of potential showstoppers for the RF-separated beam implementation for Phase-2 of the AMBER experiment in the M2 beam line of the North Area is presented. Different beam optics settings have been examined, providing either focused or parallel beams inside the RF cavities. The separation and transmission capability of the different optics settings for realistic characteristics of RF cavities are discussed and the preliminary results of the potential purity and intensity of the RF-separated beam are presented. These show that a trade-off between the overall beam intensity and the share of the required particle type in the overall beam needs to be established. No showstoppers have been identified for achieving the beam parameters required for AMBER's kaonic Primakoff reactions, kaon spectroscopy, prompt-photon production and kaon charge-radius programs. However, the high beam intensity requirements of the AMBER Drell–Yan programme cannot be satisfied with an RF-separated beam.
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 $^6$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 $u$ and $d$ quark, as well as the tensor charge in the measured $x$-range are extracted. In particular, the accuracy of the $d$ quark results is significantly improved.
The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<\eta<2.4$, $p$-going direction) and backward rapidity ($-2.2<\eta<-1.2$, $A$-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central $p$$+$Al and $p$$+$Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in $p$+$p$ collisions. The magnitude of enhancement at backward rapidity is larger in $p$$+$Au collisions than in $p$$+$Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement.
The fraction of $J/\ensuremath{\psi}$ mesons which come from $B$-meson decay, ${\mathrm{F}}_{B\ensuremath{\rightarrow}J/\ensuremath{\psi}}$, is measured for $\mathrm{J}/\ensuremath{\psi}$ rapidity $1.2<|y|<2.2$ and ${p}_{T}>0$ in $p+p$ and Cu+Au collisions at $\sqrt{{s}_{{}_{NN}}}$ = 200 GeV with the PHENIX detector. The extracted fraction is ${\mathrm{F}}_{B\ensuremath{\rightarrow}J/\ensuremath{\psi}}=0.025\ifmmode\pm\else\textpm\fi{}0.006$ (stat) $\ifmmode\pm\else\textpm\fi{}$ 0.010(syst) for $p+p$ collisions. For Cu+Au collisions, ${\mathrm{F}}_{B\ensuremath{\rightarrow}J/\ensuremath{\psi}}$ is 0.094 $\ifmmode\pm\else\textpm\fi{}$ 0.028(stat) $\ifmmode\pm\else\textpm\fi{}$ 0.037(syst) in the Au-going direction ($\ensuremath{-}2.2<y<\ensuremath{-}1.2$) and 0.089 $\ifmmode\pm\else\textpm\fi{}$ 0.026(stat) $\ifmmode\pm\else\textpm\fi{}$ 0.040(syst) in the Cu-going direction ($1.2<y<2.2$). The nuclear modification factor, ${R}_{\mathrm{CuAu}}$, of $B$ mesons in Cu+Au collisions is consistent with binary scaling of measured yields in $p+p$ at both forward and backward rapidity.
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 ${Q}^{2}>1\text{ }\text{ }(\mathrm{GeV}/c{)}^{2}$, invariant mass of the hadronic system $W>5\text{ }\text{ }\mathrm{GeV}/{c}^{2}$, Bjorken scaling variable in the range $0.003
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 the 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 quantitative relationships to be established among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.
The COMPASS collaboration has collected the currently largest data set on diffractively produced ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ final states using a negative pion beam of $190\text{ }\text{ }\mathrm{GeV}/c$ momentum impinging on a stationary proton target. This data set allows for a systematic partial-wave analysis in 100 bins of three-pion mass, $0.5<{m}_{3\ensuremath{\pi}}<2.5\text{ }\text{ }\mathrm{GeV}/{c}^{2}$, and in 11 bins of the reduced four-momentum transfer squared, $0.1<{t}^{\ensuremath{'}}<1.0\text{ }\text{ }(\mathrm{GeV}/c{)}^{2}$. This two-dimensional analysis offers sensitivity to genuine one-step resonance production, i.e. the production of a state followed by its decay, as well as to more complex dynamical effects in nonresonant $3\ensuremath{\pi}$ production. In this paper, we present detailed studies on selected $3\ensuremath{\pi}$ partial waves with ${J}^{PC}={0}^{\ensuremath{-}+}$, ${1}^{++}$, ${2}^{\ensuremath{-}+}$, ${2}^{++}$, and ${4}^{++}$. In these waves, we observe the well-known ground-state mesons as well as a new narrow axial-vector meson ${a}_{1}(1420)$ decaying into ${f}_{0}(980)\ensuremath{\pi}$. In addition, we present the results of a novel method to extract the amplitude of the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ subsystem with ${I}^{G}{J}^{PC}={0}^{+}{0}^{++}$ in various partial waves from the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ data. Evidence is found for correlation of the ${f}_{0}(980)$ and ${f}_{0}(1500)$ appearing as intermediate ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ isobars in the decay of the known $\ensuremath{\pi}(1800)$ and ${\ensuremath{\pi}}_{2}(1880)$.
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