Relating D0-anti-D0 Mixing and D0 -> l l- with New Physics Page: 3 of 25
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accommodate a value of XD at the per cent level. This encourages us to further explore the
NP option. In particular, New Physics could affect charm-related processes beyond mixing,
such as rare decays . Of special interest are the D0 -> f+f- decays. At present, there
are only the upper limits [16, 17, 18, 19]
BD0-+p- < 1.3 x 10-6, BDO-e+e- < 1.2 x 10-6, and
<D+e+ < 8.1 x 107 , (2)
all at CL=90%. Such branching fractions place bounds on possible NP couplings, which can
be compared with that obtained from D0-D0 mixing. In this paper we study the impact of
NP on the combined system of D0-D0 mixing and the rare decays D0 -> f+f--. It should
be stressed that the SM rate for the decay mode, BDO-,+- ~ 3 x 103 can be estimated
fairly reliably even upon accounting for the effect of long distance enhancement . This
smallness of the SM signal makes it easier for NP contributions to stand out. In this paper,
we point out how, in certain NP models, the same couplings occur in the amplitudes for
both D0-D0 mixing and D0 -> +f- decay. If the NP effects are significant in mixing, then
a correlation will exist between the magnitudes of each. In fact the correlation can be very
simple and striking, with the branching fraction BDO-+f- being proportional to the mixing
For each NP model considered in this paper, we shall make the simplifying assumption
that the NP dominates D0-D0 mixing and then derive the correlated branching fraction
that is then predicted. Obviously, if NP does not dominate, all our results for the branching
fractions become upper bounds. We shall stress the general issue of which conditions allow
for such correlations and give specific examples. Finally, even if the number of parameters
in a given NP model is too large to give a unique prediction for BDO-+f- in terms of XD
(e.g. Z' models, etc), we show in Sects. III B-D how it is possible to estimate the scale of
BDO--+f- by using the value of XD as input.
II. EFFECTIVE LAGRANGIANS
Heavy particles present in NP models are not produced in final states of charm quark
decays. Yet, effects generated by exchanges of these new particles can be accounted for
in effective operators built out of the SM degrees of freedom. That is, by integrating out
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Golowich, Eugene; /Massachusetts U., Amherst; Hewett, JoAnne; /SLAC; Pakvasa, Sandip; U., /Hawaii et al. Relating D0-anti-D0 Mixing and D0 -> l l- with New Physics, article, March 31, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc925940/m1/3/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.