This week I am in Warsaw, where I attend the XI workshop on particle correlations and femtoscopy. I am actually here to give a seminar on statistical methods in particle physics next Thursday, but of course I am also going to try and deepen my understanding of the field of investigations of heavy ion collisions.

Jan Pluta, one of the old-schoolers of the field, gave an introductory talk this morning. It was titled "A brief history of femtoscopy and particle correlations - a personal view". I am reporting below some impressions from his presentation.

What is femtoscopy ? Jan started by warning that he would indeed only give a personal view of the history of the field, and that the view of others may be very different.

We know what is a microscope: an object used to observe the structure of objects with a dimension of microns. Further down the distance scale, nanotechnology works with structures of the order of nanometers. On the other hand, femtosecond lasers produce very short pulses of light, at the level of a femtosecond. Now, femtoscopy deals with the structure of matter at the level of a femtometer: 10^-15 meters; but the time intervals are much shorter than the shortest directly measurable time intervals: 10^-23 seconds. We are thus talking of the smallest space-time scales ever probed.

By colliding heavy nuclei we are confronted with the dynamics of creation of hundreds of particles; it is a challenging task to understand the dynamics of such a complex system.

One may study the vector difference of pairs of particle momenta to investigate the dynamics of the produced bodies, and extract information on their source. The time and space scales of the source are unmeasurable, but the momentum difference of the emitted particles is, and the latter is directly connected to the former.

Jan Pluta mentioned that the measurement of the size of the bright star Sirius by Hanbury-Brown/Twiss correlations in 1959 laid the foundations of correlation femtoscopy, although the analogy with particle physics is rather deep. Indeed, the pioneering measurement of Hanbury and Twiss used an intensity interferometer, measuring the interference in the intensity of the light detected by the star due to the bosonic nature of light. Bose-Einstein correlations are also at the root of the phenomena studied in femtoscopy.

The second milestone in the development of the field was the observation of how quantum statistics played a role in proton-antiproton annihilation in 1960, in a paper by G.Goldhaber and collaborators. Later, in 1967 Siemiarczuk and Zielinski performed the first important study of correlated proton pairs in high-energy nuclear reactions (Phys Lett 24B n.13, p675). Then in 1969 K.Eskreys observed angular correlations between neutral pions produced in interactions of 9 GeV negative pions with xenon atoms. 

The above were seminal studies, but the real beginning of femtoscopy was due to two scientists: Podgorecki and Kopylov. They are the true fathers of correlation femtoscopy, as they introduced correlation functions, mixing techniques, and explained the role of space-time characteristics in these measurements. Yet the word "femtoscopy" did not exist yet back then.

Significant steps forward heppened in 1981, when Lednicky and Lyuboshitz solved the problem of the final state interaction of particles of nearly equal momenta on the two-particle correlation; and in 1986, when a paper by J.Bartke showed the dependence of the emission source on the projectile mass. Then the study of quark-gluon plasma which started in the late eighties and nineties started by applying correlation techniques. Pluta also mentioned in this context the work of Y.Sinyukov, who in 1994 introduced the notion of length of homogeneity.

It was professor G.A.Leksin from ITEP who first used the name "Femtometry". According to Pluta, the field got his name in a seminar in Dubna, when he suggested that the physics that studies the spacetime structure of particle emissions, with all the tools developed, should be called that whay.

Something that played a role in the creation of WPCF conferences was the meeting in Budapest, in 2003. There, Tamas Csorgo gave a very interesting conclusion to his summary talk: "A new world discovered: but is it India or America ? Is it QGP or some other form of matter?" Indeed, we are still discovering the unknown world of QGP and the properties of this state of matter, and the progress are periodically addressed in the WPCF conferences.

Pluta concluded his talk by tracing the history of the WPCF conferences. The one starting today in Warsaw is the eleventh edition. In the room a dozen of participants were identified who participated in the first edition, and three who participated in all editions. The field of particle correlations and femtoscopy appears thriving and I expect to hear lots of interesting talks this week!