Date: Tue, 27 Nov 2001 15:14:11 -0500 (EST) From: Physical Review Letters To: doug@triumf.ca Cc: prl@ridge.aps.org Subject: ll8059 Dr. D.A. Bryman TRIUMF 4004 Wesbrook Mall Vancouver B.C., CANADA V6T 2A3 doug@triumf.ca Re: LL8059 Further evidence for the decay K^+\rightarrow\pi^+\nu\bar\nu By: S. Adler, A.O. Bazarko, P.C. Bergbusch, E.W. Blackmore, et al. Dear Doug, The above manuscript has been reviewed by our referee(s). Acceptance of your paper for publication is likely, but we first ask you to consider carefully the enclosed comments. Please accompany your resubmittal by a summary of the changes made, and a brief response to any recommendations and criticisms. Sincerely yours, /Robert/ Robert Garisto Senior Assistant Editor Physical Review Letters Email: prl@aps.org Fax: 631-591-4141 -------------------------------------------------------------------- Report of Referee A (LL8059 Adler,S) -------------------------------------------------------------------- I read the paper and it certainly looks ok, worthy of publication in PRL. The authors should give the confidence level for their limit on Im(Lambda_t) on page 6. -------------------------------------------------------------------- Report of Referee B (LL8059 Adler,S) -------------------------------------------------------------------- This paper provides the long-awaited final results from Brookhaven AGS Experiment 787 on its principal rare K decay: K^+ -> pi^+ nu nubar. This rare K decay has been the focus of many decades of effort, almost two decades of effort from this collaboration alone. With their two observed events, above effectively negligible backgrounds, they have indeed made a significant advance. This result is one of the most significant in high-energy physics this year. It fully deserves to be published in Physical Review Letters in a timely way. The decay in question represents one of the few decay modes that provides truely incisive information on the outstanding issues on the CKM matrix. While not directly sensitive to CP violation, a measurement of this branching ratio constrains the magnitude of the top-to-down quark coupling. This information is an important contribution to demonstrating both the internal consistency and the overall adequacy of the CKM picture of quark mixing, including CP violation. This mode, while not completely free of theoretical complications, is relatively free of them, making the measurement easy to interpret. The only problem is practical: the branching ratio is so small, and the signature so kinematically unconstrained, that it requires an experimental tour de force merely to observe the decay. While long in coming (I believe this experiment took its first data in about 1987), E787 is truely such a tour de force. The present article describes the final result. I strongly hope there is a long paper coming (a Phys Rev D style article). Frankly, the limited format of a PRL makes it impossible to describe much of what has been done here. I know enough about the detector, the technique, the analysis, and the group, to have confidence in the validity of this work. However, no one could arrive at that from this paper alone. One curiosity I have is why the analysis reported here relies on the particular signal box they have used. They briefly describe the estimate of backgrounds within the box, and report the acceptance of the box for the signal mode. Nonetheless, from Figure 2 it seems apparant that a rotated box (i.e., a criterion coupling both R and E) would result in larger acceptance with equally small background. Having used 7500 bins in their analysis, it would seem they should have the flexiblity to use such a rotated box. Of course, it is too late to change this now, having already inspected the signal region. But it does seem unfortunate not to make such a simple improvement in the analysis. The preceeding comment should not be interpreted as criticism of this paper. It is well-written. It is an important result of broad interest. I urge immediate publication.