Dark matter from the top

Distribution of the significance of the imbalance in the transverse energy flow in the detector: events with dark matter would appear as an excess of black points on the right side of the plot.

Distribution of the significance of the imbalance in the transverse energy flow in the detector: events with dark matter would appear as an excess of black points on the right side of the plot.

Dark matter neither emits nor scatters light. The existence of dark matter was suggested over 70 years ago by Fritz Zwicky to account for the gravitational anomalies of galaxies. Currently, 23 percent of the mass energy of the universe is attributed to dark matter. In the last decade, a few experiments claimed its direct observation, while competing experiments are excluding these results.

The discovery of the existence of dark matter would be a major scientific breakthrough. If dark matter could be produced in the controlled environment of particle colliders, we could study its properties in great detail. The search is challenging since dark matter would fly through the collider detectors without leaving any trace. But if it could be produced together with ordinary matter, as predicted by many theories, then its passage would be revealed by an imbalance in the energy flow in the collision. Therefore most searches for dark matter at collider facilities focus on its production together with ordinary Standard Model particles.

During their last run, CDF physicists searched for dark matter produced simultaneously with top quark pairs. The top quark is believed by many to be a window into new physics. It has a very short lifetime and decays into a b-quark and a W boson. The latter then decays preferentially into two quarks. Therefore, in the CDF detector, the production of top quark pairs and dark matter candidates would leave events with a large imbalance in the energy flow. There would also be many particle jets originating from the six quarks decayed from the top quark pairs.

CDF scientists studied this signature for the first time, developing novel techniques to understand and constrain the backgrounds. Unfortunately, from the perspective of new physics, the data was found to be described very well by the predictions of the Standard Model and the authors set stringent limits on the production of dark matter with top quarks at CDF.

One should not despair though since the LHC data will soon help shed more light on this obscure-but-intriguing topic!

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—Edited by Andy Beretvas

These CDF physicists contributed to this data analysis. Clockwise from top left: Marco Bentivegna and Marco Rescigno, both from Sapienza, University of Rome, Italy; Fabrizio Margaroli, Daniela Bortoletto and Qiuguang Liu, all from Purdue University.

These CDF physicists contributed to this data analysis. Clockwise from top left: Marco Bentivegna and Marco Rescigno, both from Sapienza, University of Rome, Italy; Fabrizio Margaroli, Daniela Bortoletto and Qiuguang Liu, all from Purdue University.