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We report the results of a search for the flavor-changing neutral-current decays D 1 ! p 1 m 1 m 2 and D 1 ! p 1 e 1 e 2 in data from Fermilab charm hadroproduction experiment E791. No signal above background is found, and we obtain upper limits on branching fractions, B͑D 1 ! p 1 m 1 m 2 ͒ , 1.8 3 10 25 and B͑D 1 ! p 1 e 1 e 2 ͒ , 6.6 3 10 25 , at the 90% confidence level. PACS numbers: 13.20.Fc, 11.30.Hv Flavor-changing neutral-current (FCNC) decays have played a major role in our understanding of weak decays. For example, absence of an s ! d quark transition in neutral kaon decays led to the Glashow-Iliopoulos-Maiani mechanism [1] that predicted the charm quark. This Letter presents limits from Fermilab experiment E791 for FCNC decays of charm, D 1 ! p 1 m 1 m 2 , p 1 e 1 e 2 [2], which would proceed through an effective c ! u quark transition. These decays are forbidden at tree level within the standard model, but can occur through higher-order loop diagrams and are thus sensitive to physics at very high mass scales [3].Within the standard model, the contribution of shortdistance electroweak processes to B͑D 1 ! p 1 ᐉ 1 ᐉ 2 ͒ is expected to be less than 10 28 [4]; in contrast, the best published experimental limits are only ͑223͒ 3 10 23 ͑p 1 e 1 e 2 ͒ [5] and 2 3 10 24 (p 1 m 1 m 2 ) [6]. Threebody FCNC decays such as D 1 ! p 1 ᐉ 1 ᐉ 2 are more sensitive to new physics involving vector or axial vector currents than are two-body decays such as D 0 ! ᐉ 1 ᐉ 2 , because three-body decays do not suffer from helicity suppression imposed by angular momentum conservation.Our search finds no evidence for FCNC D 1 ! p 1 ᐉ 1 ᐉ 2 decays. We set branching-fraction limits that are an order of magnitude below those previously published.Data were recorded from 500 GeV͞c p 2 interactions in five thin foils (one platinum, four diamond) separated by gaps of 1.34 to 1.39 cm. The E791 spectrometer is an upgrade of apparatus used in Fermilab charm 364 0031-9007͞96͞76(3)͞364(4)$06.00

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