| With the increasing of collision energy, the particles in a single event in the final state become large enough to investigate the single event or event-by-event fluctuations of a variable. Transverse momentum fluctuations, Φpt suggested by Gazdzicki and Stanislaw Mrowczynski has been considered as a variable which can distinguish the quark-gluon plasma (QGP) from hadronic matter and reflect the degree of thermalization of a system. If particles emit independently in nucleus-nucleus collisions, Φpt will be zero. If nucleus-nucleus collisions are only the simple superposition of hadron-hadron collisions, Φpt will be the same in these two systems. Also it's declared that a correlation between transverse momentum and multiplicity (n ~ n) should be responsible for non-zero Φpt. However, the result reported in NA49 is larger than it in CERES(NA45) nearly by a magnitude. And many disagreements exist in the origin of the fluctuations even in the theoretical aspects, for example whether it comes from (pt)n ~ n or from Bose-Einstein Correlation (BEG).In this article, the purpose and status of investigating the event-by-event fluctuations of transverse momentum are summarized both in theory and in experiment. The fundamental notions related to the event-by-event fluctuations are described and derived in detail. For the first time, in n+p and K+p collisions at 250 GeV/c of NA22 which covers 4π phase space and has an excellent momentum resolution, the dependence of event-by-event fluctuations of transverse momentum on rapidity region, transverse momentum acceptance, multiplicity, mean transversemomentum per event (pt), and on correlation between transverse momentum and multiplicity are systematically presented. The fluctuations under the same acceptance cuts are used in current heavy-ion experiments are also presented. The results are compared with those from the PYTHIA Monte Carlo generator.It's shown that (1) the event-by-event fluctuations of transverse momentum increase drastically with the broadening of rapidity region and lowering of low-pt particle cut. Only the measurements in the total central plateau region are representative for the behavior in the full rapidity region. The contribution of particles to the fluctuations in the fragmentation region are negligible. The lower the low-pt cut is , the smaller the influence on the resolution of measurement will . be; (2) Φpt dependents much on the event classes chosen. The events with high multiplicity show higher fluctuations than the ones with low multiplicity. And the events with large mean pt per event has large fluctuations; (3) The fluctuations are negative in the subsamples with restricted pt. (4) n ~ n is not so closely related to the fluctuations as expected. The correlation doesn't decide the behavior of fluctuations qualitatively; (5) PYTHIA can reproduce the relation between the Φpt and the measurements related to pt, but underestimate the fluctuations in all rapidity regions and also in the relation with multiplicity. So some dynamical mechanism owned by the real physics are lack in PYTHIA; (6) In PYTHIA, Bose-Einstein Correlation doesn't show much influence on Φpt.Apart from the transverse momentum fluctuations, charge fluctuations have been investigated in experiments. Chemical (or flavor) fluctuations and elliptic flow fluctuations belong to the new fields probing the fluctuations.
|
PDF Full Text Request