Relating D0-anti-D0 Mixing and D0 -> l l- with New Physics Page: 2 of 25
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Following many years of effort, there is now indisputable experimental evidence for D0-D
mixing. The current values (the HFAG 'no CPV-allowed' fit ) of the D0 mixing parameters
XD = AMD = 0.0100+0.026 and YD AFD = .076+.00 (1
These show that (i) charm mixing occurs at about the percent level, (ii) XD, YD are compara-
ble in magnitude and (iii) the signs of XD and YD are positive (although a direct measurement
of the sign of XD is yet to be made).
While it is quite likely that the observed mixing amplitude is dominated by the Standard
Model contributions, the exact predictions are quite difficult.' There are several reasons
for this [2, 3, 4]. For example, in the "short distance" approach  at leading order in the
Operator Product Expansion (OPE) formalism (operators of dimension D = 6), the individ-
ual diagrams are CKM-suppressed to the level O(A2) (A ~ 0.22 is the familiar Wolfenstein
parameter), hinting that the observed charm mixing is a simple consequence of CKM struc-
ture. This is, however, not correct because severe cancellations between diagrams (even
through 0(as)) greatly reduce the D = 6 mixing to 0(10-6) [3, 4]. As for higher (D > 6)
orders in OPE, it is true that certain enhanced contributions have been identified [6, 7], but
a definitive evaluation is lacking due to the large number of D > 6 operators and the inabil-
ity to determine their matrix elements. A promising alternative approach which involves a
hadron-level description  may be able to account for the observed magnitude of YD and
XD, but predicts their relative sign to be opposite. It is fair to say that this is probably not
the final word on the SM analysis.
Given the uncertain status of the SM description, it would be tempting but premature 
to attribute the observed XD to New Physics.2 But clearly, the possibility that NP makes
a significant or even dominant contribution to the observed mixing is open. A recent com-
prehensive treatment of NP models  shows that a large number of such models can
1 Henceforth, we will make frequent use of the abbreviations SM for Standard Model and NP for New
2 We will focus on XD in this paper. Not only does the SM estimate for yD work reasonably well when long
distance effects are included , but it has also been shown that NP effects are too small to have any
significant impact [11, 12, 13].
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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/m1/2/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.