Model-Independent search for high-mass narrow resonances in the dielectron channel at CDF

We describe a search for high-mass narrow resonances in 0.9 fb^-1 dielectron data taken with the CDF detector. From a scan of the mass spectrum from 150-500 GeV/c², we find the most significant excess of data over background occurs for a dielectron mass of about 240 GeV/c². The probability of observing an excess as significant or more than that observed in data is 1.7%. When choosing mass windows around the excess region, the Standard Model prediction would have to fluctuate between 2.8 and 3.7 standard deviations to account for the observed data.

Authors : Byeong Rok Ko, Seog H. Oh, Chiho Wang
Data : RunII, 902/pb
Documentation : PS PDF

Plots and Tables

Invariant mass distribution
	Dielectron invariant mass spectrum, where red dots with error bars are data and blue shadeds are other Standard Model background and green ones are QCD background. White region is Drell-Yan background. Background contributions are added. Left plots show the mass region from 50 to 500 GeV/c² in a log scale and right plots show high mass region above 150 GeV/c² in a linear scale. EPS or GIF.

	(data-prediction)/uncertainty as function of dielectron mass. EPS or GIF.
Signal Significance
	We search for deviations from the Standard Model prediction in the dielectron invariant mass spectrum by estimating the probability of any excess originating only from fluctuations of standard model backgrounds as follows. We scan the spectrum between 150 and 500 GeV/c² in 5 GeV/c² steps. At each step we fit the distribution to a background-only hypothesis and a background + narrow resonance hypothesis. For the background + narrow resonance hypothesis, we add to the background prediction a gaussian distribution centered on the step whose width is fixed to the known detector resolution. We then compute the difference in &chi² (&Delta&chi²) between the two hypotheses as a figure of merit to quantify the significance of any excess (shown in the plot). Finally, we estimate the probability to observe the maximum value of &Delta&chi² we observe in the data anywhere in the spectrum from 150 to 500 GeV/c² under the background-only hypothesis using background-only pseudo-experiments. The probability of observing an excess as significant or more than that observed in data estimated with this method is 1.7%. EPS or GIF.
Event Display
	Typical dielectron event display. COT View (EPS or GIF). CAL View (EPS or GIF).
Other Studies
	A separate analysis using the dielectron channel and slightly more integrated luminosity did not report a similar excess but the two analyses are statistically consistent with each other at the 7% level. Another study using dimuon pairs is in progress.
Plan
	The data size will be increased by about a factor of 2 before the planned shutdown in the summer of 2007. The data will be analyzed through the same procedure and reported.