Measurement of the Z boson cross-section in - Harvard University ...

Chapter 4: Data Collection and Event Reconstruction 116
Sequential recombination jet finder
Sequential recombination schemes are inherently infrared- and collinear-safe. The
default implementation of this scheme in ATLAS is the anti-kT algorithm. This
algorithm starts by analyzing all pairs of input clusters with respect to their relative
inverse transverse momentum squared, defined as:
dij = min(P −2
T,i ,P −2
T,j ) ∆R2 ij
−2
= min(P 2 T,i ,P −2
T,j ) ∆η2 ij +∆φ2 ij
R
R 2
(4.1)
and the inverse squared pT of cluster i: di = p −2
T,i [17]. ∆η and ∆φ are the η, φ
differences between the two clusters. R, the only free parameter in this definition, is
the distance parameter and allows control of the size of the jets formed. The algorithm
finds the minimum dmin of the dij and the di. If dmin is a dij, the algorithm combines
the clusters i and j into one object by adding their four-momenta. Both clusters are
removed from the input list, and the new object k added to it. If, by contrast, dmin
is a di, the cluster itself is considered to be a jet, and removed from the input list.
The procedure is repeated for updated sets of dij and di until the input list is empty.
Consequently, all clusters in the original list become either a jet by itself or part of a
jet.
Since no clusters are shared between jets, this method is infrared-safe. Since no
seeds are used, it is also collinear-safe. ATLAS uses distance parameters R =0.4 for
narrow jets and R =0.7 for wide jets.
One important difference between using sliding window (tower) clusters versus
topological clusters for jet finding is in the number of cells used in the jet. The
former uses all cells in the calorimeter, while the latter leaves noisy cells out. Hence,
the noise contribution per cell is significantly smaller for topological cluster jets than