Evidence for Unequal Lifetimes of theD0andD+

Abstract

In the reaction e*e~ -^"(3770), events containing either one to two electrons have been observed originating in semileptonic decays of D mesons. A comparison of these samples provides a determination of the branching,ratios: bif>^-*Xev) <4.0% (at 95% confidence level) and b(D + -*Xev) = (22.o!f ;f)%. These values imply that the ratio of D° and D + lifetimes is T(D + )/T(D°) >4.3 (at 95% confidence level).PACS numbers: 14.40.Pe, 13.65.+ iThe conventional model of the decay of charmed mesons 1 assumes that the light quarks are merely spectators and therefore predicts the D° and D + lifetimes to be equal. We report here a measurement of unequal lifetimes from a study of DD pairs produced me + e~ annihilations. 2 The analysis is based on a comparison of two data samples: In one, bothD and!) decay semileptonically, leading to events with a detected e + and e~ ("2e events"); in the other, only one semileptonic decay is observed ("le"). The data, recorded by the DELCO detector 3 at SPEAR, are selected from energies at the ty" and below. The ty" provides a pure sample of DD events with known charged and neutral composition, and the lower-energy data are used in the background measurements.The selection criteria for the le sample 4 require that events contain ^ 3 observed charged tracks, of which one is identified as an electron by having in-time Cherenkov-and shower-counter pulses. The backgrounds to the le sample come largely from r decays, accidental coincidences of a hadronic track and a Cherenkov pulse, misidentified photon conversions and Dalitz decays, and two-photon processes. Subtracting contributions from these backgrounds, we obtain 734 ± 44 le events due to charm decays at the */>".Events in the 2e sample must have two electrons and at least one nonelectron, defined as a track having momentum above 200 MeV/c, which passes through the Cherenkov counter but produces no Cherenkov pulse. To obtain unambiguous electron identification, we demand that only one track enter each triggered Cherenkov cell. The candidates are scanned to eliminate the majority of eey final states and misidentified photon conversions in hadronic events. A background due to ^'^7r + 7r" -~ e + e"ir +, n" is reduced by requiring an acollinearity of at least 20° between the e + and e~ in the azimuthal projection. Events involving 7T° or t] Dalitz decays are suppressed by requiring that the electron pair mass exceed m ^ . 21 2e events satisfy these criteria at the ip".The residual backgrounds in the 2e events result from (1) two real electrons, such as from twophoton processes or Dalitz decays; (2) one real electron plus one false electron, such as a D or r decay together with a hadron misidentified as an electron; and (3) two false electrons. We study le events at the ^(3100) to determine the mis identification probability for a track as an electron (1.9xl0" 3 ). Backgrounds (2) and (3) are then measured from the le sample at the ip" by randomly assigning a Cherenkov tag to eligible tracks at a rate equal to this misidentif ication probability....

Relations between inclusive decay rates of heavy baryons

“…The problem of unequal inclusive weak decay rates of hadrons containing a heavy quark (c or b) attracts considerable experimental and theoretical interest ever since the first experimental indication [1] of substantially different lifetimes of the charmed D 0 and D ± mesons. The total weak decay rates of charmed hadrons are presently known to be vastly different, with the ratio of the longest known lifetime to the shortest: τ (D ± )/τ (Ω c ) ∼ 20, while the differences among the b hadrons are much smaller.…”

Absolute Branching Fraction Measurements forD+andD0Inclusive Semileptonic Decays

“…Indeed the early measurements of the ratio of the D and D 0 semileptonic branching fractions gave the first surprising evidence for the lifetime difference between these two charmed mesons [5,6]. Later, the first measurement of the individual charged and neutral D inclusive semileptonic branching fractions was performed by Mark III [7], with an overall relative error of about 12%-16% on the individual branching fractions, and 19% on their ratio.…”

Lignins as Components of Polymeric Composite Materials

“…Kgn' invariant mass spectrum for events which have a lepton This ratio of lifetime was first measured by DELCO [21] at SPEAR and more recently by MARK I1 [22]. The methods used are rather complicated, but the basic idea is to make use of the equality of the semi-leptonic widths of D' and Do:…”