PHYSICAL REVIEW D VOLUME 49, NUMBER 5 1 MARCH 1994 New magnetic monopole Hux limits from the IMB proton decay detector R. Becker-Szendy, " C. B. Bratton, J. Breault, D. Casper, '~ S. T. Dye, K. Ganezer, W. Gajewski, M. Goldhaber, T. J. Haines, P. G. Halverson, D. Kielczewska, W. R. Kropp, J. G. Learned, J. LoSecco, S. Matsuno, G. McGrath, C. McGrew, R. S. Miller, L. Price, F. Reines, J. Schultz, H. W. Sobel, J. L. Stone, L. R. Sulak, and R. Svoboda University of Hawaii, Honolulu, Hawaii 968M Cleveland State University, Cleveland, Ohio $/115 University of California, Irvine, California 99717 Boston University, Boston, Massachusetts OM15 California State University, Dominguez Hills, Carson, California 907)7 Brookhaven National Laboratory, Upton, New York 11978 University of Maryland, College Park, Maryland 907/9 S'arsam University, 8'arsau, Poland University of Notre Dame, Notre Dame, Indiana $8558 Louisiana State University, Baton Rouge, Louisiana 70808 (Received 30 June 1993) An improved limit on the Bux of magnetic monopoles in the vicinity of the solar system is obtained, assuming that monopoles strongly catalyze nucleon decay (the Rubakov-Callan e8'ect). Flux limits are presented for monopole velocities from 10 c to 10 c and for monopole-nucleon cross sections between 10 cm and 10 cm . For a representative velocity P 10, and cross section cr 10 cm, we obtain a limit I' g 2.7 x 10 cm sr sec and for cr 10 cm, g~ & 1. 0 x 10 cm sr sec at 90/p C.L. PACS number(s): 14. 80.Hv, 13. 30. Ce I. INTRODUCTION If magnetic monopoles exist and if they catalyze proton (or nucleon) decay as suggested by Rubakov [1, 2], Callan [3], and others, then proton decay detectors, such as the IMB detector, may be used to set limits on the local Hux of monopoles. Monopole-induced nucleon decay is more difBcult to detect than spontaneous nucleon decay be- cause the massive monopole can invisibly carry off much of the momentum. This means that the usual searches for proton decay, where events with a significant unbalanced momentum are usually interpreted as neutrino-induced background, are not directly applicable. The possible modes of a nucleon decay catalyzed by a monopole interaction are not well known. However, there have been predictions [4 — 6] that, if minimal SU(5) were correct, the modes and branching ratios of monopole catalyzed nucleon decay would be similar to those of non- catalyzed decay (e.g. , p m e+vro). It is expected that a similar situation would exist for theories other than minimal SU(5). Despite their high stopping power (dE/dX 100 P GeV g cm2 for P ( 0.1 [7]), massive monopoles (M Now at SLAC, Stanford, California 94309. ~Now at CERN, CH-1211 Geneva, Switzerland. 10~s to 10~ GeV for SU(5) monopoles [8)) with velocities typical of objects in the solar neighborhood (P = 10 10 s) can easily reach deep underground detectors such as IMB. To estimate A, q, the mean distance between catalyzed nucleon decay interactions, we use A«q — — 1/(oN~), where N~ is the nucleon density of the target. Estimates of the catalysis cross section 0 are unfortunately subject to great uncertainty, ranging &om about 10 cm [9] to about 10 ~s cm2 [10] at P = 10 4. Because of this large range of possible cross sections, summary listings of monopole fiux limits such as [11] should be interpreted with care. II. PREVIOUS RESULTS The Parker bound I' 10 cm sr sec, a limit based on the survival of the galactic magnetic field [12], has proven difBcult to improve upon by Earth-based de- tectors. More stringent limits can be obtained &om cat- alyzed nucleon decay in pulsars [13], and &om catalyzed nucleon decay in the sun [14 — 16]. These indirect astro- physical measurements may be complemented by more direct measurements of the monopole Bux incident on the Earth as in the case of large water Cerenkov detectors, if the Rubakov-Callan e8ect exists. Recent results from the underwater Cerenkov detec- tor at Lake Baikal [17] put the fiux limit at I" + 2 x 10 ~ cm sr ~sec ~ for P = 10 4. How'ever, to 0556-2821/94/49(5)/2169(5)/$06. 00 49 2169 1994 The American Physical Society