Abstract
The time-integrated $CP$ asymmetry in the decay $D^0 \to K^0_S K^0_S$ is measured using $3 fb^{-1}$ of proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The flavour of the $D^0$ meson is determined by use of the decay $D^{*+} \to D^0 \pi^+$ and its charge conjugate mode. The result is \[ {\cal A}_{CP} = -0.029 \pm 0.052 \pm 0.022, \] where the first uncertainty is statistical and the second systematic. The result is consistent with Standard Model expectations and improves the uncertainty with respect to the only previous measurement of this quantity by more than a factor of three.
Highlights
LLtrig D*+ LLtrig D*− 45 LHCbLD Data D*+ Total fit Signal Background DD D*+ LD D*− DD D*−to the signal extraction is estimated by comparing the nominal fit with an alternative one, where outside of the signal region of ±1.5 MeV around the known ∆m-value, only the background component is fitted
For each category, where N + (N −) is the yield determined from a fit to the data for a positive charge of the slow pion
For the combined CP asymmetry a difference of 0.019 is found, which is assigned as a systematic uncertainty
Summary
The LHCb detector [9, 10] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The KS0 → π+π− decays are reconstructed in two different categories: the first involves KS0 mesons that decay early enough for the daughter pions to be reconstructed in the vertex detector; the second contains KS0 mesons that decay later such that daughter track segments are only reconstructed in the tracking detectors downstream of the vertex detector. These categories are referred to as long (L) and downstream (D), respectively. The less abundant long category has better momentum and vertex resolution than the downstream category
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