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Rolf Hagedorn (20 July 1919 – 9 March 2003) was a German theoretical physicist who worked at CERN. He is known for the idea that QCD matter has a "melting point". The Hagedorn temperature is named in his honor.
After having completed his studies with the usual diploma (1950) and doctorate (1952), with a thesis under Prof. Richhard Becker on thermal solid-state theory, he was accepted as a postdoc at the Max Planck Institute for Physics (MPI), still at Göttingen at the time. While he was there, he was among a group of physicists including Bruno Zumino, Harry Lehmann, Wolfhart Zimmermann, Kurt Symanzik, Gerhard Lüders, Reinhard Oehme, Vladimir Glaser, and Carl Friedrich von Weizsäcker.
When the CERN theory group came to Geneva from Copenhagen in 1957, where it had been located at first, Hagedorn joined the group. Hagedorn brought to the Theory Division an unusual interdisciplinary background which included particle physics and nuclear physics as well as thermal physics, solid state and accelerator physics. Once member of the Theory Division, he exclusively focused on the statistical models of particle production.
Many key ingredients brought soon afterward by experiment helped refine the approach. Among them is the limited transverse momentum with which the overwhelming majority of the secondary particles happen to be produced. They show an exponential drop with respect to the transverse mass. There is also the exponential drop of elastic scattering at wide angles as a function of incident energy. Such exponential behaviors strongly suggested a thermal distribution for whatever eventually comes out of the reaction. Based on this, Hagedorn put forth his thermal interpretation and used it to build production models which turned out to be remarkably accurate at predicting yields for the many different types of secondary particles. Many objections were raised at the time, particularly as to what could actually be 'thermalized' in the collisions, applying straightforward statistical mechanics to the produced gave the wrong results, and the temperature of the system was apparently constant when it should have risen with the incident energy or with the mass of the excited fireball (according to Boltzmann's Law).
For collision energies above approximately 10 GeV, the naive statistical model needed improvement.
The SBM model of strong interactions is based on the observation that hadrons are made of hadrons in an infinite chain. This leads to the concept of a sequence of heavier and heavier particles, each being a possible constituent of a still heavier one, while at the same time being itself composed of lighter particles. In this SBM framework there would be ever increasing particle production at the Hagedorn temperature. Hagedorn gave this extensive summary of the historical path across 50 years of research in particle physics at his last 2-hours public lecture in Divonne 1994, which was recorded and later made available online. Hagedorn interpreted this limiting temperature, visible at that time also in the transverse mass distribution of the secondary particles, in terms of the slope of an exponential spectrum of all strongly interacting particles appearing in the SBM; the value is of the order of ~150-160 MeV. Later work allowed the interpretation of the Hagedorn temperature as the temperature at which hadrons melt into a new phase of matter, the quark-gluon plasma.
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