Text for the blessed web page - CDF note 6273

**The CDF Collaboration**

**March 14, 2003
**

We present a search for the flavor changing neutral current (FCNC)
decay .
With of two-track trigger (TTT) data we set an upper limit on the
branching ratio of

This is nearly a factor of 2 improvement over the previous best
limit of .

For the FCNC decay , the Standard Model predicts . The present experimental limit is from BEATRICE ( from E771), 7 orders of magnitude from the prediction.

New physics can substantially enhance this mode. In charm meson decays we are constraining couplings to up-type quarks not necessarily constrained by B decays. presents and immaculate and unexplored region to search for new physics.

We use the TTT data for a clean analysis (all information can be
determined from the data).
The limit on the branching ratio is computed as:

where:

- is the 90% limit on the number of observed decays,
- is the number of reconstructed in the normalization mode,
- is the ratio of efficiencies between the signal and normalization modes, , and
- is the branching ratio of the normalization mode,

We performed a ``blind'' analysis. The selction cuts were optimized on a kinematically similar but statistically independent set of events. We normalize to the kinimatically similar decay . We select events that satisfy a requirements, with both tracks fiducial to the CMU, and satisfying

- projected decay length ,
- impact parameter of the , and
- angular separation at the CMU .

We divide our background estimate into two parts:

- misidentification, and
- combinatorics (all other sources).

With these values, our sensitivity (expected limit for 1.7 observed events) is .

We now ``unblind'' the search window and find zero events. This gives at 90% confidence level, and at 95% confidence level.

This section summarizes the numbers which have been blessed for this analysis.

**Table 1:** Probability (in percent) to misidentify a or as a muon for the bins listed.

The uncertainties on the number of events in the normalization mode, the ratio of efficiencies, and the branching ratio to the normalization mode are included in the limit following the procedure outlined by Cousins&Highland. All of the mentioned uncertainties are of order 5%, and have a negligible effect on the limit.

This result improves on the existing best limits by almost a factor of 2 ( from BEATRICE, and from E771 at 90% confidence level).

Our 95% CL limit is

The following figures have been blessed for the search. More text can be found in CDF note 6273.

**Figure 1:** The calculation of the normalization.
Both tracks are assigned the pion mass and required to project into a
CMU chamber (CMU fiducial).
The fit is to a Gaussian plus linear function, and we count the events
in the range 1.840 to .

**Figure 2:** The misidentification background is determined from the
number of events that fall into the search window when
reconstructed as , times the square of the average pion misID
probability.

**Figure 3:** The combinatoric background is determined from the number of
dimuon events in a wide high mass sideband.
We assume the background is linear in mass, and scale by the ratio of
the search window width to the sideband width.

**Figure 4:** Using tagged decays we determine the
probability that a pion is misidentified as a muon.
The dependence is shown, and the average probability is 1.3%.

**Figure 5:** We performed the same misidentification analysis with the
kaons.
Note that the misidentification probability is significantly
higher than the owing to the smaller interaction cross
section.

**Figure 6:** The unblinding of the search window (red hatched region)
reveals zero events.

**
Search for the FCNC Decay
Text for the blessed web page - CDF note 6273
**

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