Gluon Radiation and Top Quark Physics

Radiation of gluons gives rise to extra jets in top quark events that can lead to complications in event reconstruction and mass measurement. I review recent results for gluon radiation in top quark production and decay, and discuss their implications for top quark physics. 1 Tevatron The presence of extra jets in t ¯ t events can complicate attempts to reconstruct the mass of the top quark from the momenta of its decay products. Depending on whether the associated gluon was emitted during top production or decay, it may or may not be appropriate to include its momentum in top reconstruction. Unfortunately, it is not easy to tell the difference. Therefore we must consider top production and decay together in our treatment of gluon radiation. We have computed the complete order α 3 s tree-level cross section for t ¯ t production and decay with an extra jet at the Tevatron 1 and the LHC 2 ; for more details see the references. At the Tevatron we find 1 that the contribution (after kinematic cuts) from gluons radiated in the top production process is comparable to that from decay. As we might expect, the decay-stage gluons appear mostly in the central rapidity region. Production-stage radiation occupies a wider rapidity range, but it, too, contributes substantially in the central region. The distribution in the angle between the extra jet and the b quark is dominated at small angles by the decay-stage contribution, but the production-stage piece is non-negligible in that region as well. The bottom line is that there is no clear way experimentally to distinguish the two contributions, and they must be taken into account together in our analyses. 2 LHC The higher energy of the LHC compared to the Tevatron leads to a large increase in radiation in t ¯ t events, most of it associated with production 2. The increase a Presented by L.