Search for Hot Nuclear Matter Continues

A jet—in particle physics—is essentially a shower of collimated particles, produced by an extremely energetic gluon or quark.

In his dissertation, Tomas Snellman studied, whether there are differences in the characteristics of jets between proton-proton and proton-lead collisions. (Image credit: University of Jyväskylä)

Jets in a lead-lead collision have to traverse via quark-gluon plasma, changing their track, energy, and consistency. Experiments have already established this fact.

In his thesis, Tomas Snellman explored whether differences exist in the properties of jets between proton-lead collision and proton-proton collision. The aim of this study was to find out if quark-gluon plasma can be produced in proton-lead collisions, because jets would then begin to look like the observations made in lead-lead collisions.

The term “hot nuclear matter” in particle physics often means quark-gluon plasma, or QGP. This matter is so hot that gluons and quarks are not confined to nucleons anymore, that is, neutrons and protons, but rather shift freely inside the plasma.

In order to convert normal matter into quark-gluon plasma, temperatures of approximately 2000 billion kelvin are generally needed. High temperatures like these can be achieved in high-energy collisions between atomic nuclei in labs, for instance, at the large hadron collider (LHC).

Tomas Snellman investigates particle jets in collisions between lead nuclei and protons, which have been determined at CERN in the ALICE experiment of the LHC.

A significant goal in the measurements carried out in the ALICE experiment was to determine whether the properties of a proton-lead collision can be elucidated using just the characteristics of cold nuclear matter. This cold nuclear matter is merely utilized to refer to the normal state of atomic nuclei, which happens to be cold by the principles of particle physics.

In the field it has been established that quark-gluon plasma is created in lead-lead collisions at LHC. The interesting question is whether this can happen also in proton-lead collisions.

Tomas Snellman, Doctoral Student, Department of Physics¸ University of Jyväskylä

Atomic nuclei are “large” according to the scales in particle physics research. Therefore, the ball of colliding matter in a collision that occurs between two heavy nuclei is sufficiently large to convert into quark-gluon plasma. On the contrary, one proton is so small that it led to an assumption that QGP is unlikely to be produced.

However some proton-lead collisions have shown indications of the creation of QGP. It remains unknown what actually happens in proton-lead collisions. In my research I studied whether jets from either the average proton-lead collision or from an exceptionally active collision differ from jets observed in proton-proton collisions. Especially changes in the active collisions could provide clear proof of the creation of QGP. However, within the current experimental capabilities, no proof could be found.

Tomas Snellman, Doctoral Student, Department of Physics¸ University of Jyväskylä

Thus the question of QGP in proton-lead collision remains an open one. Certain measurements support the creation of QGP, but especially measurements based on particle jets, like this thesis, see no signs. As the potential QGP droplet would be small in proton-lead collisions, the signals would be weak. This explains a part of the discrepancy, but not all of it. A solution would require a better theoretical understanding of the underlying phenomena, but also on the experimental side we need better control of the biases affecting our measurements so that even a weak signal could be detected.

Tomas Snellman, Doctoral Student, Department of Physics¸ University of Jyväskylä


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