Conclusions from CDF Results on CP Violation in D^0 \to \pi^+\pi^-, K^+K^- and Future Tasks

TL;DR

This paper reviews CDF results on CP violation in D^0 decays, compares them with Standard Model predictions, and discusses future prospects for detecting CP violation and New Physics effects in charm decays.

Contribution

It provides a detailed analysis of CDF measurements on CP violation in D^0 decays and evaluates their implications for New Physics models like Little Higgs with T-Parity.

Findings

CDF's sensitivity to direct CP violation is comparable to B factories.

CDF's indirect CP violation measurement constrains New Physics models.

Standard Model predicts very small CP violation, not exceeding 10^{-4}.

Abstract

Within the Standard Model (SM) one predicts both direct and indirect CP violation in D^0 \to \pi^+\pi^-, K^+K^- transitions, although the effects are tiny: Indirect CP asymmetry cannot exceed O(10^{-4}), probably even O(10^{-5}); direct effects are estimated at not larger than 10^{-4}. At B factories direct and indirect asymmetries have been studied with <t>/\tau_{D^0} ~ 1; no CP asymmetry was found with an upper bound of about 1%. CDF has shown intriguing data on CP violation in D^0 \to \pi^+\pi^- [K^+K^-] with <t>/\tau_{D^0} ~ 2.4 [2.65]. Also, CDF has not seen any CP violation. For direct CP asymmetry, CDF has a sensitivity similar to the combination of the B factories, yet for indirect CP violation it yields a significantly smaller sensitivity of a_{cp}^{ind}=(-0.01 +- 0.06_{stat} +- 0.05_{syst})% due to it being based on longer decay times. New Physics models (NP) like Little Higgs Models with T-Parity (LHT) can produce an indirect CP asymmetry up to 1%; CDF's findings thus cover the upper range of realistic NP predictions ~ 0.1 - 1%. One hopes that LHCb and a Super-Flavour Factory will probe the lower range down to ~0.01%. Such non-ad-hoc NP like LHT cannot enhance direct CP violation significantly over the SM level in D^0 \to \pi^+\pi^-, K^+K^- and D^{\pm} \to \pi^{\pm}K^+K^- transitions, but others might well do so.