|The di-electron mass distribution for the CDF data. The expected Standard Model backgrounds (Drell Yan, QCD, diboson and top-antitop) are summed and compared with the data.|
One question that continues to puzzle physicists is why gravity is so much weaker than the other fundamental forces (strong, electromagnetic and weak). The strength of gravity is only one hundredth of a billionth of a billionth of a billionth of a billionth of the strong interaction, or 10-38 times less.
In 1999, Lisa Randall and Raman Sundrum, physics theorists, proposed a remarkable model of extra dimensions that provides an answer to this question. In their model, they postulated the existence of an undetected spatial dimension, in addition to the four dimensions of ordinary space and time.
The strength of gravity is weakened by the presence of this extra dimension, in which only gravity can enter. The graviton, a particle that transmits the force of gravity, can have momentum in the extra dimension, which will look like mass. The interaction of gravity and the extra dimension will appear to us as a new heavy particle that physicists call a RS graviton. This graviton would transmit gravity the same way that the photon transmits electricity and magnetism or the W and Z particles transmit the weak force or the gluon transmits the strong force.
At the Tevatron, the RS gravitons would appear as heavy, unstable particles decaying into particle-antiparticle pairs. CDF physicists looked for new particles decaying into pairs of charged leptons. The advantage of studying this channel is that scientists know where to expect the background data, so any deviations from the Standard Model would be clearly evident.
Recently CDF physicist performed a generic search for di-electron and di-muon resonances indicating the presence of a graviton. A resonance would look like a spike in an otherwise smooth curve. Physicists used a data sample corresponding to 5.7 inverse femtobarns for this search. There was not an excess over the background prediction, meaning scientists did not find any measurements suggesting the presence of a graviton.
This result, combined with a previous published search in the photon-photon channel, excludes the existence of Randall-Sundrum gravitons below a mass of 1.1 TeV/c2, for an extra dimension of approximately 10-30 meters.
—Edited by Andy Beretvas
|These physicists were responsible for this analysis. From left: Michael Gold (inset), Marcelo Vogel, John Strologas, University of New Mexico; Ray Culbertson and Tingjun Yang, Fermilab.|