COMPASS turns to new round of QCD studies

The COMPASS collaboration has recently submitted the COMPASS-II proposal to the CERN SPS committee. The aim is to explore several new and interesting aspects of QCD through a whole series of measurements with the versatile COMPASS apparatus.

As one part of the proposal, COMPASS plans to study generalized parton distributions (GPDs) by scattering the highly polarized M2 muon beam off a liquid-hydrogen target surrounded by a recoil detector. The experiment will access as yet uncharted territory between the gluon-dominated region of small Bjorken-x measured by H1 and ZEUS at HERA and the valence-quark-dominated region at large Bjorken-x, which was accessed by HERMES at HERA and at Jefferson Lab. The focus of the GPD programme in the COMPASS-II proposal is the study of the GPD H. This distribution describes interactions in which the helicities of both the struck parton and the nucleon are conserved. Access to H at COMPASS will be provided by the deeply virtual Compton scattering (DVCS) process off unpolarized protons. The same data will make possible the mapping of the transverse position of partons with respect to the fraction of the nucleon longitudinal momentum that they carry. Such a mapping is sometimes referred to as "nucleon tomography".

As another part of the proposal, COMPASS plans to scatter pions from the M2 beam line off the existing polarized ammonia target, using an absorber. COMPASS will thereby become the first "polarized" Drell-Yan (DY) experiment to access transverse-momentum-dependent parton distributions (TMDs). Among the distributions to be studied are the Sivers, Boer-Mulders and "pretzelosity" TMDs as well as transversely polarized quark distributions. Examination of the Wilson-line structure of final-state interactions in QCD predicted that the Sivers function should have opposite signs in DY and semi-inclusive DIS (SIDIS) processes. Groups from several laboratories intend to be first to confirm this "restricted universality", which is rooted in fundamental aspects of QCD.

In addition, the proposal shows how new insights into QCD at low energies will be gained by measurements of exclusive final states produced by incoming high-energy pions at very small momentum-transfer to the recoiling nucleus – that is, in the Primakoff region.

To strengthen the first two of these physics opportunities, the COMPASS collaboration organized two international workshops at CERN in spring 2010. The workshops considered both the theoretical and experimental scenes, including plans for studies at other laboratories around the world.

The first workshop, "GPDs@COMPASS", concentrated mainly on theoretical aspects after starting with experimental reviews, including a look at existing DVCS measurements. Etienne Burtin of Saclay presented the outlook for COMPASS-II and underlined that no measurements exist of the specific kinematic domain that COMPASS will access. Matthias Burkardt from New Mexico State University set the theoretical scene by presenting GPDs as an indispensable tool for studying the structure of the nucleon. He also stressed the importance of a (later) continuation of the GPD programme using a transversely polarized proton target to access the GPD E at COMPASS kinematics. Extractions of H and E at similar kinematics are the only known way to determine the total angular momentum of quarks in the nucleon via Ji's sum rule – a long-term goal of the GPD community. The spatial structure of the nucleon transverse to the direction of motion of particles in the beam will be measured at COMPASS for the region 0.01–0.1 in Bjorken-x, and this will be of special interest for the Monte Carlo simulation of backgrounds at the LHC.

Other topics that were discussed include: nucleon structure from a phenomenological viewpoint; the first promising attempts on global GPD fits; experience building a phenomenological GPD model to describe exclusive vector-meson production data; and, as a complementary approach to determine moments of GPDs, the state of the art in QCD calculations on a Euclidean lattice. The one-day workshop was summarized by Andreas Schäfer of Regensburg, who concluded that for understanding non-collinear QCD – a major task that requires progress on different research fronts – GPDs are the only known rigorous and undisputed approach. "They do not answer all questions," he said, "but most questions cannot be answered without them."

The second workshop, "Studying the hadron structure in Drell-Yan reactions", covered two days. It opened with reviews of theoretical and experimental progress in the field. In his talk, Daniel Boer of Groningen underlined that measurements of TMDs in DY processes are well suited for the study of 3D aspects of nucleon structure. Paul Reimer of Argonne covered four decades of fixed target and collider DY experiments, including the first observation of TMD-induced effects in the violation of the Lam-Tung relation.

The first day then continued with a focus on theoretical topics, including recent calculations of higher-order QCD corrections for the DY process. Relevant aspects of the formalism of TMDs and TMD phenomenology in DY were discussed and the complementary nature of DY and SIDIS measurements in accessing the underlying physical distributions was highlighted. The day concluded with a round-table covering TMD models and GPD studies in DY processes. The second day saw reviews on all DY measurements planned at world-leading laboratories such as Fermilab, Brookhaven, J-PARC, GSI, JINR and CERN. The huge variety of proposals reflects the strong interest of the community in DY studies, a main focus being the study of spin-dependent effects in DY reactions using polarized beams and/or targets. With COMPASS-II, CERN would have a great opportunity to play a leading role.

In summary, these two workshops successfully strengthened the physics goals of the COMPASS-II proposal, in particular the necessity of new measurements in the kinematic region that is accessible only to COMPASS at CERN.

• The programme details and all of the presentations at the two workshops are available on the web. For GPDs@COMPASS see http://indico.cern.ch/conferenceDisplay.py?confId=77527; for Studying the hadron structure in Drell-Yan reactions see http://indico.cern.ch/conferenceDisplay.py?confId=85721.

Further reading

COMPASS II Proposal 2010 CERN-SPSC-2010-014/SPSC-P-340.


Visits

On 13 July Nickolay Mladenov, Bulgarian minister of foreign affairs, left, visited CERN. He toured the CMS control centre with Guido Tonelli, right, CMS spokesperson, after receiving a general introduction to CERN's activities from Rolf Heuer, CERN's director general.