1. Department of Physics, National Taiwan University, Taipei, 106, Taiwan.
2. Hanyang University, Seoul, 133-791, Korea.
3. High Energy Accelerator Research Organization (KEK), Tsukuba, 305-0801, Japan.
4. Department of Physics, University of Tokyo, Tokyo, 113-0033, Japan.
5. Budker Institute of Nuclear Physics, Novosibirsk, 630090, Russian Federation, Russia.
6. Swiss Federal Institute of Technology of Lausanne, EPFL, Lausanne, CH-1015, Switzerland.
7. Institute for Theoretical and Experimental Physics, Moscow, 117218, Russia.
8. Tata Institute of Fundamental Research, Mumbai, 400005, India.
9. University of Sydney, Sydney, New South Wales, 2006, Australia.
10. University of Melbourne, School of Physics, Victoria 3010, Australia.
11. J. Stefan Institute, Ljubljana, SI-1001, Slovenia.
12. H. Niewodniczanski Institute of Nuclear Physics, Krakow, 31-342, Poland.
13. University of Maribor, Maribor, SI-2000, Slovenia.
14. University of Hawaii, Honolulu, Hawaii 96822, USA.
15. Department of Physics, Fu Jen Catholic University, Taipei, 242, Taiwan.
16. National Central University, Chung-li, 32054, Taiwan.
17. Yonsei University, Seoul, 120-749, Korea.
18. Gyeongsang National University, Jinju, 660-701, South Korea.
19. Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Korea.
20. Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
21. University of Cincinnati, Cincinnati, Ohio 45221, USA.
22. Nara Women's University, Nara, 630-8506, Japan.
23. Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, SI-1001, Slovenia.
24. Korea University, Seoul, 136-701, Korea.
25. Osaka University, Osaka, 560-0043, Japan.
26. Nagoya University, Nagoya, 464-8602, Japan.
27. Tohoku Gakuin University, Tagajo, 985-8537, Japan.
28. Kyungpook National University, Daegu, 702-701, South Korea.
29. Saga University, Saga, 840-8502, Japan.
30. Tokyo Institute of Technology, Tokyo, 152-8551, Japan.
31. The Graduate University for Advanced Studies, Hayama, 240-0193, Japan.
32. Chiba University, Chiba, 263-8522, Japan.
33. Niigata University, Niigata, 950-2181, Japan.
34. Seoul National University, Seoul, 151-742, Korea.
35. Panjab University, Chandigarh, 160014, India.
36. Institute of High Energy Physics, Vienna, A-1050, Austria.
37. Princeton University, Princeton, New Jersey 08544, USA.
38. Osaka City University, Osaka, 558-8585, Japan.
39. Tokyo University of Agriculture and Technology, Tokyo, 184-8588, Japan.
40. Toho University, Funabashi, 274-8510, Japan.
41. Kanagawa University, Yokohama, 221-8686, Japan.
42. Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China.
43. University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
44. RIKEN BNL Research Center, Upton, New York 11973, USA.
45. Institute of High Energy Physics, Protvino, 142281, Russia.
46. University of Nova Gorica, Nova Gorica, 301 5000, Slovenia.
47. Tokyo Metropolitan University, Tokyo, 192-0397, Japan.
48. National United University, Miao Li, 36003, Taiwan.
49. Tohoku University, Sendai, 980-8578, Japan.
50. Nippon Dental University, Niigata, 951-8580, Japan.
51. University of Science and Technology of China, Hefei, 230026, China.

Correspondence to: Correspondence and requests for materials should be addressed to P.C. (Email: pchang@phys.ntu.edu.tw).

Equal amounts of matter and antimatter are predicted to have been produced in the Big Bang, but our observable Universe is clearly matter-dominated. One of the prerequisites¹ for understanding this elimination of antimatter is the nonconservation of charge-parity (CP) symmetry. So far, two types of CP violation have been observed in the neutral K meson (K ⁰) and B meson (B ⁰) systems: CP violation involving the mixing² between K ⁰ and its antiparticle (and likewise^{3, 4} for B ⁰ and ), and direct CP violation in the decay of each meson^{5, 6, 7, 8}. The observed effects for both types of CP violation are substantially larger for the B ⁰ meson system. However, they are still consistent with the standard model of particle physics, which has a unique source⁹ of CP violation that is known to be too small¹⁰ to account for the matter-dominated Universe. Here we report that the direct CP violation in charged B K ⁰ decay is different from that in the neutral B ⁰ counterpart. The direct CP-violating decay rate asymmetry, (that is, the difference between the number of observed B ^-K ^{- 0} event versus B ⁺K ^{+ 0} events, normalized to the sum of these events) is measured to be about +7%, with an uncertainty that is reduced by a factor of 1.7 from a previous measurement⁷. However, the asymmetry for versus B ⁰K ^{+ -} is at the -10% level^{7, 8}. Although it is susceptible to strong interaction effects that need further clarification, this large deviation in direct CP violation between charged and neutral B meson decays could be an indication of new sources of CP violation—which would help to explain the dominance of matter in the Universe.