Search for

The decay is a distinctive signature of some popular classes of SUSY models [4]. The current experimental bound on the branching fraction is BR() < , a Run I result from CDF [5]. In the Standard Model, the branching fraction is predicted to be only [6]. Elliott Cheu and postdoc Peter Tamburello are searching for in the present Run II data.

The mass distribution of muon pairs is shown in Figure 13. A peak from would have a standard deviation of about 100 MeV. Selection criteria are next applied in order to decrease the backgrounds.


Figure 13. Muon pair mass distribution for data (solid) and Monte Carlo (dashed). The Monte Carlo normalization represents BR() = .

We require that , the distance from the primary vertex to the muon pair vertex projected onto the muon pair direction, be at least 550m, and that the uncertainty on be less than 150m. This removes prompt tracks while retaining decay products of long lived B hadrons. The component of the displacement from the primary vertex to the muon pair vertex perpendicular to the muon pair direction () is required to be less the two standard deviations. This removes muon pairs that do not point back to the primary vertex. The PT of the muon pair is required to be at least 4 GeV/c. The isolation, defined as the scalar sum of the PT of all tracks except for the two muons within a cone of around the muon pair direction, is required to be less than 0.39 GeV/c. Figure 14 shows the distributions of these quantities for signal and background.


Figure 14. Quantities used to separate signal from background. The background distributions (solid) are from data with a muon pair mass between 4.5 and 7.0 GeV. The signal distributions (dashed) are from Monte Carlo. , PT, and isolation are shown after the cut on .

After cuts, the muon pair masses are distributed as shown in the top left and the bottom of Figure 15. In the signal region, 5.22 GeV 5.51 GeV/c2, there are three candidates with a background of. The background is estimated from a straight line fit to the sidebands around the signal region.


Figure 15. Mass spectra after cuts for (upper left and bottom) and (upper right). In the bottom plot, the signal region is indicated by arrows and the linear sideband fit is shown. The upper right plot shows the fit to with a quadratic background shape

For three events with a background of 3.42, the 90% C.L. upper limit on the number of

signal events is 4.03 using the method of Reference [7]. To set an upper limit on BR() we calculate

(0.1)

where is the number of events in the data, is the efficiency for , and is the efficiency for the signal. The candidates are selected using the same requirements as for the signal, except that the mass is required to be near the and an additional track is required for the kaon. The fit shown in the upper right of Figure 15 is used to determine. The ratios are computed using Pythia and the DØ detector simulation. The result is BR() < . Taking into account systematic uncertainties raises the limit to [7].

W-bosonplus jets studies and searches for

In preparation for a search for, postdoc Peter Tamburello investigated the properties of jets produced in association with a W-boson using data.

To begin, we check our understanding of high PT muon events by plotting the dimuon mass in events with a pair of isolated muons (Figure 16). We first smear the PT resolution in the Monte Carlo to reproduce the width of the Z-boson peak. After normalizing the luminosity using the number of events in the Z-bosonpeak, we find good agreement between the shape and number of events in both the low mass region of Figure 16, and the transverse mass of events with an isolated muon and missing ET (Figure 17).

With reasonable W and Z-boson signals, we turn to the jets in the W–boson events. The number of 20 GeV jets per event is shown in Figures 18 and 19. The transverse momentum of the leading jet in the data (before jet ET cuts) is compared to Pythia and to Alpgen in Figure 20. Finally, we require at least two jets with ET > 20 GeV and compare data to the Alpgen Monte Carlo, in each case normalizing the Monte Carlo so that the area under the QCD plus Monte Carlo distribution is the same as the area under the data distribution. The scalar sum of the ETof the two highest ET jets is shown in Figure 21, and the dijet mass of the two highest jets is shown in Figure 22. These studies are continuing and we expect to be able to have a competitive result in the near future.


Figure 16. The dimuon mass distribution for pairs of good-isolated muons.

The normalization is the same as that used for inclusive W–boson events.


Figure 17. Transverse mass of the W–boson candidates. The points are data. The filled histogram is QCD background estimated from data. The open histogram is the sum of Pythia and QCD.


Figure 18. The inclusive jet multiplicity for 20 GeV jets, shown for data (points), Pythia (solid), Alpgen Wj (dashed), Alpgen Wjj (dot-dashed), and QCD (hatched). The Monte Carlo distributions include QCD.


Figure 19. The inclusive jet multiplicity for 20 GeV jets. Data (points); Pythia with nominal jet scale (solid), one  high jet scale (dashed), and one  low jet scale (dotted); and QCD (hatched). The Monte Carlo distributions include QCD. The Monte Carlo scale error and the data scale error were combined in quadrature.


Figure 20.ET of the leading jet. The points are data. The filled histogram is QCD. The solid histogram is QCD plus Pythia. The dashed histogram is QCD plus Alpgen (single jet).


Figure 21.ETof the first jet plus the ET of the second jet. Data (points), background with nominal (solid), high (dashed), and low (dotted) jet energy scale. For each jet scale, the Monte Carlo is normalized so that the Monte Carlo plus QCD distribution has equal area to the data distribution.


Figure 22. Dijet mass. Data (points), background with nominal (solid), high (dashed), and low (dotted) jet energy scale. For each jet scale, the Monte Carlo is normalized so that the Monte Carlo plus QCD distribution has equal area to the data distribution.

References

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