# Nuclear Theory/RIKEN Seminar

## "Universal Transverse Momentum Dependent Fragmentation"

#### Presented by Duff Neill, LANL

Friday, March 17, 2017, 2:00 pm — Small Seminar Room, Bldg. 510

Fragmentation is the earliest and perhaps most interesting QCD jet

observable, since it directly deals with the parton-hadron duality at

the end of the QCD cascade. The most basic fragmentation observables

all enjoy the property of being universal, in the sense that a

sufficiently energetic parton that initiates the cascade factorizes

from the rest of the event, so that the underlying soft structure of

the event to a good approximation does not change the fragmentation

spectrum. With the luminosities and resolution of modern detectors, we

can begin to study the fragmentation process in three dimensions: both

the energy spectrum and the transverse fluctuations to the collinear

direction of initiating hard parton. However, when one wants to study

the transverse fluctuations, one becomes very sensitive to the

underlying jet definition, in particular, how the collinear direction

is defined. Intuitive definitions of the jet direction, like the total

momentum of the jet constituents, are inherently sensitive to soft

processes, and can spoil the universality of the spectrum. I will

discuss how a simple change in the jet definition removes this soft

sensitivity, and allows one to study the intrinsic three dimensional

structure of collinear splittings, which should be process

independent.

Hosted by: Heikki Mantysaari

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