Lambda Polarization in Lead-Lead Collisions

Abstract

In this thesis the production and polarization of Lambda particles produced in Pb-Pb collisions at a beam energy of 158 GeV/c per nucleon are studied. In these collisions nuclear matter is compressed and heated and under these conditions it is possible that a quark-gluon plasma (QGP) is created. In a QGP the boundaries between individual particles disappear and the quarks and gluons can move around freely. One of the probes to study the creation of the QGP is the so-called Strangeness Enhancement, which is the enhanced production of strange particles, with respect to the production in proton-induced collisions. Another proposed signature of the QGP is the disappearance of transverse Lambda polarization around mid-rapidity. Transverse Lambda polarization can occur when the spin of the produced Lambda has its preferred direction perpendicular to the production plane. In proton-proton and proton-nucleus collisions a negative transverse polarization has been observed and its magnitude was largest for the Lambdas produced in the direction of the beam (i.e. at high xF). This is an indication that the projectile plays an important role and in semi-classical models the following picture is assumed: in the interaction region a spin-zero ud-diquark from the projectile recombines with a negatively polarized s-quark from the sea. This s-quark also gives transverse momentum (pT) to the Lambda and as a result the polarization is dependent on pT. Recently it has been shown that also in nucleus-nucleus collisions a negative transverse polarization occurs, so even in large interaction regions the information on the nature of the projectile is kept. In a QGP however this information is lost and no production plane can be defined. This will cause the transverse polarization of the Lambdas produced in this region to disappear.

The transverse polarization of the produced Lambdas has been measured in the NA57 experiment at CERN around two values of xF: for xF = 0.09 a value of +0.6 +7.4-6.6 % was found and at xF = 0.14 a value of +5.5 +8.9 -7.4 %, where the errors are the combined statistical and systematic error. The latter result is compatible with zero polarization and it seems to favor the Berlin Model. Unfortunately the error is too large to exclude other semi-classical models (which predict a polarization of -5 to –7 %) or to make a statement about the creation of a QGP. However this is the first time that a measurement was done in this region and with this collision system. Keeping these results in mind it would be interesting to see what is the value of the transverse polarization of the Lambdas measured by NA57 in Pb-Pb collisions at a lower beam energy of 40 AGeV/c. This would not only give results in the same region as measured in this thesis, but it would enable a measurement up to xF ~ 0.35, where polarizations of the order of 10-15 % are expected.