Christina Mesropian - 2000


Measurement of the Strong Coupling Constant from Inclusive Jet Production at the Tevatron Collider


The best way to introduce Christina Mesropian is by saying a few words about what she has accomplished. I could, of course, begin with her CV, where one reads that she graduated from Armenia's Yerevan University with honors, and that her Master's thesis received an award at an "All Soviet Student Scientific Conference". But I would like to concentrate on what she has achieved here at Rockefeller that qualifies her to be awarded the Degree of Doctor of Philosophy today.

According to our present understanding, the proton is made up of three quarks, which are held together by a force much stronger than the well known electromagnetic force and thus appropriately called "the strong force". If one tries to knock a quark out of a proton, the force holding it to each of the other two quarks increases, just like the force between two balls held by a spring. The theory predicts precisely how the strength of the force, the so called ``strong coupling constant", depends on the distance between quarks. Christina measured this dependence with great precision, confirming the theoretical predictions.

The measurement was performed by analyzing the results of cllisions between very high energy protons and antiprotons. The scattering of colliding quarks from these nucleons produces collimated jets of particles along the outgoing quark directions. The jet momentum transverse to the beam is a measure of the distance between the quarks at the time of scattering. Close encounters produce large transverse momentum jets. On the other hand, the number of events obtained is proportional to the strength of the interaction. Thus, the measurement is reduced to determining the transverse momentum spectrum of jet production in high energy proton-antiproton collisions.

Christina's measurement represents a major contribution towards understanding the nature of the forces that hold matter together, which is essential to understanding the creation, evolution and eventual fate of our Universe.