Constraints on the tension and the abundance of cosmic strings depend crucially on the rate at which they decay into particles and gravitational radiation. We study the decay of cosmic string loops in the Abelian-Higgs model by performing field theory simulations of loop formation and evolution. We find that our set of string loops emit particle radiation primarily due to kink collisions, and that their decay time due to these losses is proportional to L p with p ≈ 2 where L is the loop length. In contrast, the decay time to gravitational radiation scales in proportion to L, and we conclude that particle emission is the primary energy loss mechanism for loops smaller than a critical length scale, while gravitational losses dominate for larger loops.Cosmic strings play an important role in building theories of the early universe [1] and provide a rare observational probe of String Theory [2]. The search for their signatures has mostly focused on their gravitational effects, and they are among the main science goals of LIGO [3]. The tightest bound on the string tension µ, coming from millisecond pulsar timing measurements [4], is based on the gravitational wave (GW) background produced by decaying cosmic string loops. This bound, Gµ 10 −10 [3, 5], where G is Newton's gravitational constant, assumes that string loops decay primarily into GW with the quantitative predictions obtained from simulations using the Nambu-Goto (NG) approximation that ignores the field composition of the strings [6-10]. While it is widely accepted that the NG description works well for loops much larger than the string width, the exact loop size above which the particle composition of the string cores can be ignored is not firmly established. The few existing field theory simulations of string networks suggest that loops primarily decay into particle radiation [11], with cosmological size loops not surviving beyond one oscillation, potentially leading to a new paradigm for cosmic string evolution in which the GW bounds do not apply. Thus it is critical to examine particle emission by cosmic string loops and to determine their primary decay mode. Previous studies of the particle radiation from cosmic strings included analytical estimates [12], some based on effective couplings of NG strings to other fields [13,14], field theory simulations of standing waves, kinks and cusps on long strings [15,16] and simulations of strings with small oscillations [11,17]. In this Letter, for the first time, we directly examine the decay of a cosmic string loop to particle radiation in the Abelian-Higgs model by simulating loop formation followed by evolution in full field theory. The focus on a single loop is to be contrasted with the very large field theory simulations of an entire network of strings in an expanding spacetime [11,18].We find that string loops emit particle radiation mainly due to features on the strings known as kinks and cusps [1]. The half-life of a loop due to particle radiation is proportional to L p , where L is the length of the loop...