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| ESPERIMENTO TO31, RESPONSABILE: Mauro Anselmino |
| OBIETTIVI DELL'ESPERIMENTO TO31 |
SPIN PHYSICS AND THE 3-DIMENSIONAL STRUCTURE OF THE PROTON
Spin is a natural and fundamental degree of freedom in any quantum field theory and a complete understanding of QCD and the Standard Model must include the spin sector. The interest in high-energy spin physics has grown enormously in recent years: new data, ideas and phenomenological models are now contributing to the understanding of the nucleon spin structure and to the description of several intriguing spin effects observed in hadronic and electromagnetic inclusive interactions. A detailed 3-dimensional partonic mapping of nucleons is being performed by considering effects related to the intrinsic motion of partons, to their spin, and to possible coupling between spin and intrinsic motion. A coherent picture of the orbital motion of quarks inside the proton is now emerging.
The main issues which have been and are being studied by the groups participating in this scientific network are:
- quark transversity distribution h_1(x,Q^2);
- phenomenology of single (transverse) spin asymmetries (SSA);
- hard scattering processes with unintegrated transverse-momentum dependent parton distribution and fragmentation functions (TMDs);
- extraction of TMDs from data and creation of a database;
- QCD properties of TMD's: TMD factorization and TMD evolution;
- study of Semi Inclusive Deep Inelastic Scattering (SIDIS) processes in the Target Fragmentation Region (TFR).
The transversity distribution h_1 describes the transverse polarisation of quarks inside a transversely polarised proton; it is as fundamental and important a quantity as the unpolarised and helicity distributions. Owing to its chirally odd nature, h_1 cannot be accessed in the usual fully inclusive deeply inelastic scattering (DIS) experiments, which have so far supplied the bulk of information on nucleon partonic structure. Instead, it requires the study of specific processes, in which two chirally odd functions combine in the single physical observable to be measured. This strategy was and is actively pursued by the HERMES, COMPASS and JLab collaborations in polarised semi-inclusive deeply inelastic scattering (SIDIS) processes, which couple the chirally odd h_1 function to a new chirally odd fragmentation function, the so-called Collins function. The latter has been measured independently and found to be sizeable by the Belle and BaBar collaborations, studying the correlated azimuthal dependence of hadron (pion) pairs produced in two back-to-back jets in unpolarised e+e- processes; such correlations depend on the product of two Collins functions.
Using the HERMES, COMPASS and Belle data, the first ever extraction of the transversity distribution for u and d quarks has been performed by the Cagliari and Torino groups participating in this network. More data are now available from COMPASS, Belle and BaBar Collaborations and further, more precise, studies are being performed in order to reduce the present uncertainties. The pieces of information emerging from the transversity distributions derived so-far appear very interesting, but they are not yet so precise as to allow, for example, a precise determination of the first moment of h_1, which is related to the important tensor charge, a fundamental quantity in QCD.
Optimal access to transversity would be offered by a measurement in polarised Drell-Yan (DY) processes of the double transverse-spin asymmetry A_TT, which depends on the direct product of two h_1 functions. Estimates of A_TT at RHIC and at the planned GSI antiproton facility, together with the relevant QCD corrections, have been produced by participants of this program. The crucial issue of polarising antiprotons, proposed by the PAX collaboration, is under intense debate, both from the experimental and theoretical viewpoints; most participants of this network are theoretical members of PAX. Information on h_1, coupled to a new chirally odd distribution function, the Boer-Mulders function, could be obtained by studying SSAs in D-Y processes, which are much easier to achieve. The Boer-Mulders function itself, which describes the distribution of polarised quarks in an unpolarised proton, could be measured in either unpolarised D-Y or SIDIS processes; some results are already available. Again, a combined analysis of these functions, transversity and Boer-Mulders, in different processes, would bring new and valuable information. We also plan to pursue this line of investigation.
The Collins and Boer-Mulders functions are examples of so-called unintegrated or transverse-momentum dependent partonic distribution and fragmentation functions (TMD's); they take into account intrinsic parton motion and, in polarised cases, spin dependence. They not only offer a way of accessing transversity, but appear in several observables, measurable both in SIDIS and D-Y processes; the very rich and interesting full azimuthal dependence of these processes is under active experimental investigation in ongoing experiments (COMPASS, JLab and RHIC). QCD factorisation theorems should hold in these cases, with two clearly defined scales (Q^2 and P_T or q_T); while the full expression for SIDIS processes with TMD's is well known and established, the analogous situation for D-Y processes has been explored only more recently.
Among the chirally even TMD's, which do not couple to h_1, the Sivers function is especially interesting: it describes the distribution of unpolarised quarks inside a transversely polarised proton. It has been studied via SSA in SIDIS processes, and will receive further attention and interpretation; in fact, from a good knowledge of the Sivers function, one can compute the total amount of intrinsic momentum carried by the partons in a polarised nucleon. It is also related to the quark and gluon orbital angular momenta and to the quark anomalous magnetic moments. The phenomenological analysis is presently at such a stage that collection of all available data and existing extractions of the Sivers functions and the other TMD's would greatly help people working in the field. The creation of such a database has been discussed on many occasions recently within the international spin community; we fully intend to participate and contribute.
Several single-spin asymmetries have also been observed in inclusive hadronic interactions, such as p p → π X, with one of the initial protons transversely polarised. These asymmetries persist even at the very high energies attained by RHIC, where the usual leading-twist collinear QCD factorisation approach is perfectly corroborated by unpolarised data, but fails in explaining the SSA. A consistent phenomenological description of these asymmetries, generalising the factorisation approach to TMD's, has been developed by our group with good phenomenological success. In contrast to the situation in SIDIS and D-Y, factorisation is not proven for single inclusive processes such as A B → h X. The issue of factorisation has been studied intensely in recent months and remarkable theoretical progress has been made and is being discussed. The key is the introduction of multi-partonic correlators and, in practice, the use of slightly modified elementary interactions. The relation between the two schemes, TMD's and multi-partonic correlators, is under study by several groups, including participants of this network, and we intend to investigate the matter. Other related fundamental QCD issues, which are important and are part of our program, are the QCD evolution of TMD's and their universality. Also in this case, very recent theoretical progress and new experimental data, allow a first test of the TMD-evolution, which has been studied and is further considered.
A related line of research concerns the inclusive production of hadrons, in the Target Fragmentation Region (TFR) of SIDIS processes. The full formalism of fracture functions, necessary to describle such events in QCD, has been developed by the Alessandria and Torino groups, with a complete classification of these new functions and their expected azimuthal dependence. The related phenomenology is in progress.
The Alessandria, Cagliari, Como and Torino groups have well-known traditional activity in all the above-mentioned fields, documented by the list of publications and long-standing experience of common research. They also have a history of close and active involvement with experimental collaborations, both in the study and interpretation of data and in the proposal of new experiments. |
Istituto Nazionale di Fisica Nucleare - Piazza dei Caprettari, 70 - 00186 Roma
tel. +39 066840031 - fax +39 0668307924 - email: presidenza@presid.infn.it
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