253 research outputs found
Recent results from COMPASS
No full textRecent data from the COMPASS experiment at CERN, including inclusive and semi-inclusive double-spin asymmetries are presented. The gluon and the valence quarks spin distributions are determined from the data
Initial-state energy loss in cold QCD matter and the Drell-Yan process
No full textInternational audienceThe effects of parton energy loss in nuclear matter on the Drell-Yan process in pA and πA collisions at fixed-target energies are investigated. Calculations are based on the Baier-Dokshitzer-Mueller-Peigné-Schiff (BDMPS) framework embedded in a next-to-leading order calculation, using the transport coefficient extracted from J/ψ measurements. Model calculations prove in good agreement with preliminary measurements by the E906 experiment, despite a slightly different magnitude, supporting a consistent picture between Drell-Yan and J/ψ data. Predictions for the COMPASS future measurements in πA collisions at GeV are also performed. At higher collision energy ( GeV), Drell-Yan measurements are only slightly affected by energy loss effects. On the contrary, the E906 results turn out in clear disagreement with nuclear PDF effects alone. The comparison of E772, E866, and E906 measurements indicates for the first time a clear violation of QCD factorization in Drell-Yan production in pA collisions
Exposing novel quark and gluon effects in nuclei
No full textThe fundamental theory of the strong interaction-quantum chromodynamics (QCD) - provides the foundational framework with which to describe and understand the key properties of atomic nuclei. A deep understanding of the explicit role of quarks and gluons in nuclei remains elusive however, as these effects have thus far been well-disguised by confinement effects in QCD which are encapsulated by a successful description in terms of effective hadronic degrees of freedom. The observation of the EMC effect has provided an enduring indication for explicit QCD effects in nuclei, and points to the medium modification of the bound protons and neutrons in the nuclear medium. Understanding the EMC effect is a major challenge for modern nuclear physics, and several key questions remain, such as understanding its flavor, spin, and momentum dependence. This manuscript provides a contemporary snapshot of our understanding of the role of QCD in nuclei and outlines possible pathways in experiment and theory that will help deepen our understanding of nuclei in the context of QCD
First Measurement of Chiral Dynamics in pi(-)gamma -> pi(-)pi(-)pi(+)
No full textAdolph C, Alekseev MG, Alexakhin VY, et al. First Measurement of Chiral Dynamics in pi(-)gamma -> pi(-)pi(-)pi(+). Physical Review Letters. 2012;108(19): 192001.The COMPASS Collaboration at CERN has investigated the pi(-)gamma -> pi(-)pi(-)pi(+) reaction at center-of-momentum energy below five pion masses, root s < 5m(pi), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t' < 0.001 GeV2/c(2). Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3 pi-invariant mass m(3 pi) = root s were extracted. The absolute cross section was determined in seven bins of root s with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated
New analysis of tensor resonances measured at the COMPASS experiment
No full textWe present a new amplitude analysis of the ηπ D -wave in the reaction π−p→ηπ−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 a2(1320) and the excited a2′(1700) , and perform a comprehensive analysis of their pole positions. For the mass and width of the a2 we find M=(1307±1±6) MeV and Γ=(112±1±8) MeV, and for the excited state a2′ we obtain M=(1720±10±60) MeV and Γ=(280±10±70) MeV, respectively.We present a new amplitude analysis of the -wave in measured by COMPASS. Employing an analytical model based on the principles of the relativistic -matrix, we find two resonances that can be identified with the and the excited , and perform a comprehensive analysis of their pole positions. For the mass and width of the we find ~MeV and ~MeV, and for the excited state we obtain ~MeV and ~MeV, respectively
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