Search for Production of Two Photons with Large Imbalance in Transverse Momentum


September 8, 2004

Elementary particle physics strives to identify the building blocks of matter and the interactions that bind them. From atoms to protons and neutrons within nuclei, to quarks and gluons within protons and neutrons, physicists are gaining an ever deeper and more fundamental understanding about our world. The current theory, called the Standard Model (SM), postulates that the universe is made of six types of quarks and six types of leptons, bound by three fundamental forces: strong, weak, and electromagnetic (Gravity is very weak, and is ignored in the SM).


Yet recent cosmological experiments suggest that the matter we know about account for only 5% of the total amount of matter in the universe. About 70% corresponds to a universal gravitational pressure that is very poorly understood at this time, and another 25% consist of weakly-interacting non-luminous matter - the Dark Matter. There is no room for such things in the SM, but certain theoretical extensions offer a possible explanation for the Dark Matter. One of the more popular such theories is Supersymmetry (SUSY), which introduces a new symmetry between fundamental particles and predicts that there should be a supersymmetric partner (“superpartner”) for each of the SM particles. In addition, to make SUSY accommodate current experimental observations requires introduction of some extra particles and forces. At least one of the particles predicted by SUSY is thought to be stable and weakly interacting, making it an attractive candidate for Dark Matter.


Physicists from the ("D-Zero") experiment are looking for signs of production of Dark Matter in proton-antiproton collisions at the world's highest energy accelerator, the Fermilab Tevatron collider. Each proton antiproton collision is called an event.


In a recently submitted paper, DØ reports on a search for SUSY in events with two energetic photons (or g rays). One of the few possible reactions that can contribute to such final states is shown in the sketch below:



In this process, quarks within colliding proton and antiproton produce a pair of charginos c1± (superpartners of W+ bosons), which are unstable and decay into W± bosons and neutralinos c10 (superpartners of photons), which in turn decays into photons g and a gravitinos G (superpartners of gravitons), the latter being a candidate for Dark Matter. A gravitino will traverse DØ detector without interacting in it, and can’t therefore be detected directly, but its production can be inferred from an apparent imbalance in transverse momentum.


These kinds of processes would be very rare, and require very sophisticated data reduction methods. Out of about 20 Trillion proton-antiproton collisions, electronic signatures from about 500 Million of events were deemed to be sufficiently interesting to be recorded on tape for careful study off line. Of these, only 1909 events had two very energetic photons, and only two of those events had a significant amount of “missing” transverse momentum.


Can these two events originate from SUSY? One of them looks particularly interesting. In addition to two photons and imbalance in transverse momentum, it also has an energetic electron, as can be seen in the event display below:




However, such events can also arise from “background” SM processes. For example, the above event can in principle be explained by the production of a W boson accompanied by two photons, where W decayed into an electron and a neutrino. The probability for SM processes to give rise to events that look like SUSY can be calculated, and we expect between one and seven events of this type in our data sample. We therefore conclude that, although intriguing, there are no grounds for claiming that the two observed events provide evidence for SUSY.


Does this mean that SUSY does not exist? No, it does not. We can only say that the superpartners may be too heavy to be produced in quantities that are measurable with the current data sample. Our measurement can therefore be used to restrict possible SUSY parameter space, which provides important feedback to theorists.


DØ is planning to take at least 50 times more data which will significantly extend the reach in mass in the search for any possible SUSY particles. We are just beginning our search program and look forward to a discovery.


The full text of the paper can be found here.


For further information contact Dr. Yuri Gershtein, Brown University, at