Cameron Christie, 13th October 2003

 

 

 

 

 

 

 

 

 

4C00 Project Outline: Diffractively produced W and Z bosons at the Tevatron

 

 

 

 

Introduction

 

Diffraction of light was described for the first time by the Italian physicist Grimaldi in 1665. However, it was only following the research of Einstein and Stark, demonstrating that light also had particle properties, that physicists began to suspect that all particles may undergo diffraction. A pioneering experiment in high-energy particle physics performend at CERN confirmed that jet-sprays of particles were indeed being diffractively produced in proton/anti-proton collisions. The CDF experiment, being performed using the Tevatron collider at Fermilab, has shown that diffraction can be responsible for the production of very massive W and Z bosons (approximately 80-90 times the mass of a proton). This project will attempt to clarify the explanation(s) surrounding this diffractive production of W and Z bosons.

 

Theory

 

W and Z bosons can only be produced by quark/anti-quark annihilation. It would therefore appear logical to suggest that the quarks and anti-quarks comprising (respectively) the colliding protons and anti-protons collide to produce the W and Z bosons. This, however, cannot be the case. Due to the restrictions of Quantum Chromodynamics, the ‘coloured’ quarks are not permitted to break free and take part in interactions, as the strong nuclear force binds them. It was with this in mind that the Russian physicist Isaak Pomeranchuk (1913-1966) predicted that the colliding hadrons could release a theoretical entity called a Pomeron, which would be neutrally coloured. Both Pomerons could then collide, producing the requisite W or Z bosons. There follows two possibilities of what could compose such a Pomeron. The Pomeron could either be made of a quark/antiquark pair, and these composite quarks would then collide to produce a W or Z boson, or the Pomeron could be made of two gluons, which would have to decay into quark/antiquark pairs before production of W or Z bosons could take place. Both these arrangements are colour neutral, so would thus be allowed. Diagams 1 and 2 show Feynman diagrams corresponding to the interactions necessary to diffractively produce Z and W bosons from these arrangements.

Diagram 1: Feynman Diagram showing the production of a diffractive Z or W boson if the Pomeron were composed of a quark/anti-quark pair (pom denotes a Pomeron).

 

 

Diagram 2: Feynman Diagram showing the production of a diffractive Z or W boson if the Pomeron were composed of a pair of gluons (pom denotes a Pomeron).

 

In Diagram 2, the decay of each gluon into a quark/anti-quark pair has an associated coupling constant Öas which is relatively low. By consequence, the probability of production of a W or Z boson is proportional to (Öas)­2 = as. Hence the interaction shown in Diagram 2 should be less likely than that of Diagram 1 by a factor of as.

 

 

 

Method

 

It now remains for the theory suggested by Pomeranchuk to be thouroughly tested and fully understood. If the measured amount of of W and Z bosons diffractively produced is relatively high, it would suggest that the Pomeron is composed primarily of quark/anti-quark pairs. If, however, the amount of W and Z bosons appears low, it would indicate the Pomeron is mainly made of gluons. Measurements made at HERA, studying Pomeron/photon interactions indicate that the quark/anti-quark component of the Pomerons is low, and therefore the amount of W bosons produced diffractively at the Tevatron should also be low. Measurements made at the Tevatron in 1998 suggest this is not the case, but further measurements are needed.

 

This project aims to clarify the above situtation using data recently acquired from CDF. It is possible to distinguish diffractive particle production from collisional production by the fact that a collision will entail the breaking up of the hadrons, with showers of particles produced in all angles, including low angles, whereas diffraction will leave the hadrons intact, and will probably not produce any particles at low angles. Hence, if a gap in the amount of particles produced is detected at low-angles, chances are that it was a diffractive event that took place. This is illustrated in Diagram 3.

 

Diagram 3: Diagram showing the difference in direction between collisionally-produced particles and diffractively-produced ones.

 

It would thus be possible to write a program that scans through the data counting the number of events at low angles, and use this to determine whether the Pomerons are mostly quark/anti-quark pairs, or gluon pairs.

 

Two major problems are foreseeable with this method: What do we define as a low-angle? Can the detector detect events at very low angle? These problems shall hopefully be dealt with throughout the course of the project.

 

Conclusion

 

Thus, the main focus of the project is on the composition of these theoretical entities, the ‘Pomerons’. Will the model of a Pomeron as a particle still make sense? Will the data from CDF agree with that from HERA?

 

The research undertaken in this project could have important repercussions: Pomerons are of particular interest as they could provide a close insight to the behaviour of the vacuum. Indeed, it is believed that the existence of ‘virtual’ particles in the vacuum could be the consequence of Pomeron-like objects colliding.

 

References