Searching for a Dark Matter Candidate in Particle Collider Experiments Paul Geffert

1. Searching for a Dark Matter Candidate in Particle Collider Experiments Paul Geffert Cosmologically Favored Region Current Excluded Region In collaborationwithMax Goncharov, Eunsin Lee, David Toback - Texas A&M, Slava Krutelyov – UCSB, Rishi Patel – NYU, Peter Wagner – Penn This research is supported, in part, by funds from the Honors Programs Office Abstract: Objective: Astronomical observations have shown that the amount of visible matter in the universe comprises only a fraction of the total mass of the current universe. This extra mass is described as “dark matter”. Different physical theories have been proposed that account for this dark matter with new predicted particles. I present a search for the neutralino ( ), a predicted particle that decays into a dark matter candidate, the gravitino ( ). Method: We analyze data from particle collisions at the Fermilab Tevatron supercollider in Chicago. We perform a search for the undiscovered neutralino. If produced in a collision, the neutralino might decay into a gravitino and a photon (g) inside the detector. Our search focuses on the distinctive signature such a decay would produce in the particle detector. Results: No evidence of new physics was discovered. However, we use our results to set limits on the mass and lifetime of the neutralino. Our result is currently the best in the world for high mass neutralinos. Conclusions: Our current result probes nearly into the cosmologically favored region of parameter space. Imroved versions of this analysis using larger amounts of data should explore well into the favored region, thus increasing our chances for discovery. Results: We did not find evidence of new physics in our search. However, this is not the end of the story. We use our data to set 95% confidence level limits on the mass and lifetime of the neutralino (we exclude the area shaded in yellow in the figure below). The blue shaded area represents the region that is favored based on astronomical observations. As you can see, we are very close to this region. The previous excluded area based on prior experiments is to the left of the red dashed line. Therefore our result is currently the best in the world for high mass neutralinos. Supersymmetry: Supersymmetry is a theory of particle physics that predicts many new particles. Among these new predicted particles may be the dark matter. Due to conservation principles, the lightest supersymmetric particle might be stable and therefore not decay. This quality is essential for a dark matter candidate. Supersymmetry has been modeled many different ways, and it is very unclear which model is correct. For this analysis, we use a gauge-mediated supersymmetry breaking (GMSB) model. This model predicts a gravitino, as its lightest supersymmetric particle. Another particle predicted by our model is the neutralino. We are interested in neutralinos because they decay to gravitinos and are the supersymmetric particle we are most likely to be able to produce in the Tevatron. Confirming the existence of neutralinos would be a giant step toward solving the mystery of dark matter. Neutralino Mass/Lifetime Sensitivity A Neutralino Decay inside the Detector Energy in the Universe Searching for Neutralinos in particle collisions: A neutralino produced in the Tevatron would do one of three things. (1) Leave the detector without a trace, (2) decay almost immediately, or (3) travel some non-negligible distance and then decay inside the detector. We are concerned with case 3. This case arises if the neutralino has a lifetime on the order of nanoseconds, as is very likely in our model. With such a lifetime, the neutralino would travel a macroscopic distance within the detector but still decay before leaving. A neutralino decays to a photon and a gravitino, our dark matter candidate. The gravitino will not interact with the detector and hence escape undetected as shown in the above figure. This will result in an imbalance of energy measured by the detector, called “missing energy”. The photon, on the other hand, is readily detected with great precision by the detector. As shown in the figure above, the path length from the collision point to the detector for the neutralino decay photon is greater than that of a photon produced directly by the collision. This increased path length causes the neutralino photon to arrive at the detector significantly later than expected, i.e. “delayed”. Our search focuses on finding events with delayed photons and a large missing energy signature. We use our GMSB model to predict how many events in our data set should produce this signature and compare this to what we actually observe. Dark Matter: Various astronomical observations have shown that there is much more energy in the universe than just the visible matter that composes stellar bodies and gases. In fact, such visible matter only accounts for 4% of the energy of the universe. Dark energy, which is very poorly understood and is necessary to explain the rate of expansion of the universe, comprises 74% of the universe’s energy. We however, are interested in dark matter, which we know to reasonable accuracy is 22% of the energy of the universe. The existence of this huge amount of unseen mass was first hinted at when the rotational velocities of galaxies differed wildly from predictions. Dark matter gets its name from the fact that it will not interact electromagnetically and hence emits no light. Conclusions: In the figure above, the two black dotted lines represent the extent of the mass/lifetime region we expect our analysis to have sensitivity to when more data is used. In fact, we already have the dataset that corresponds to the innermost predicted region and plan to use it in the near future. Additionally, we are attempting to improve our analysis technique to extend our sensitivity even further into the cosmologically favored region. The preliminary results for this improvement are very promising. With a larger dataset and improved analysis technique, we may be able to help solve the mystery of dark matter.