COMPASS Research Project

CERN - Geneva, Switzerland

COMPASS (COmmon Muon Proton Apparatus for Structure and Spectroscopy) is a fixed-target experiment at CERN. Since the year 2002, it has been using the muon or hadron beam of the Super Proton Synchrotron (SPS) to scatter off a target of unpolarized or spin-polarized protons or deuterons. The SPS is also used to inject beams into the Large Hadron Collider (LHC).

One of the important missions of COMPASS is the unraveling of the nucleon spin structure - how does the spin of one half (in units of the reduced Planck constant) come along? What is the dynamic inner structure of the proton in terms of longitudinal and transverse momenta, orbital angular momentum, and spin of its constituents, the quarks and gluons? Our group is in particular interested in the quark's transverse spin and transverse momentum, and their correlations. The underlying theory is based on a rather recent extension of Quantum Chromo Dynamics (QCD), the theory of the strong interaction, and introduces new structure functions of the nucleon descriptive of the non-perturbative part of the particle interactions: Transverse-Momentum Dependent Parton Distribution Functions, in short TMDs.

In 2014 and 2015, our group built a large-area drift chamber (DC5) to replace an aging detector in the COMPASS spectrometer. The drift chamber was constructed in the nuclear physics lab (NPL) at UIUC, at ODU, and in a clean area at CERN. DC5 was installed into the COMPASS spectrometer in spring 2015, where it has been collecting valuable data until the end of COMPASS data taking in late 2022. The worldwide first polarization-dependent Drell-Yan measurements using the negatively charged pion beam from the CERN SPS and the COMPASS transversely polarized proton target took place in 2015 and 2018.  Our group contributed to these measurements by the delivery of the drift chamber, the technical coordination of the changeover from the GPD to the Drell-Yan (2017/18), the run coordination (2018), the analysis coordination (2020/21), and by securing petabyte resources on the NSF-funded supercomputers Blue Waters and Frontera  (2016-2021)  for the processing of  the raw experimental data and the simulation of various processes appearing in the pion-proton collisions [1,2]. 

Our analysis of transverse spin asymmetries in the proton data [3] provides one of the first preliminary answers to the outstanding question whether the Sivers TMD changes sign when measured in Drell Yan as compared to semi-inclusive deep inelastic scattering (SIDIS), as is predicted by the QCD-TMD framework. The COMPASS data indicate that the sign change is preferred. We also analyze transverse spin asymmetries in  the production of a J/Psi meson, which can provide sensitivity to gluon TMDs, while the Drell-Yan process is selective on quark TMDs. We also analyzed cross sections in the simultaneously collected data from heavier nuclear targets without polarization and also the proton data. An advanced interpretation of these results promises access to the partonic distribution of the pion and effects in cold nuclear matter.

COMPASS data taking will be completed by the end of 2022. Data analysis and paper writing will continue well beyond that date. 



[3]  First Measurement of Transverse-Spin-Dependent Azimuthal Asymmetries in the Drell-Yan Process, Phys. Rev. Lett. 119, 112002
(2015 data; the publication of the full 2015 + 2018 data set is in preparation as of 2022)

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