New approach for jet-shape identification of TeV-scale particles at the LHC

S. V. Chekanov, C. Levy, J. Proudfoot, and R. Yoshida

Phys. Rev. D 82, 094029 – Published 24 November 2010

Abstract

A new approach to jet-shape identification based on linear regression is discussed. It is designed for searches for new particles at the TeV scale decaying hadronically with strongly collimated jets. We illustrate the method using a Monte Carlo simulation for $p p$ collisions at the LHC with the goal to reduce the contribution of QCD-induced events. We focus on a rather generic example $X \to t \bar{t} \to hadrons$ , with $X$ being a heavy particle, but the approach is well suited for reconstruction of other decay channels characterized by a cascade decay of known states.

Received 20 September 2010

DOI:https://doi.org/10.1103/PhysRevD.82.094029

Authors & Affiliations

S. V. Chekanov, C. Levy^*, J. Proudfoot, and R. Yoshida

HEP Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, USA

^*Also with the Physics Department, Northeastern University 110 Forsyth St., 111 Dana Research Center, Boston, MA 02115, USA.

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Issue

Vol. 82, Iss. 9 — 1 November 2010

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Images

Figure 1
Sketch of two approaches for analyzing an approximately elliptical composite object. Each point represents a jet constituent, with the size representing its weight. The geometric major axis is defined by an unweighted linear regression, the geometric minor is by definition perpendicular to the major axis, through the geometric mean. In the non-quadrant method (left figure), weighted centers P[N] ( $N = 1, \dots, 4$ ) are defined for the regions above or below the major and minor axes. In the quadrant method (right figure), each weighted center P[N] is uniquely associated with one of four quadrants shown with dashed lines and denoted as (1)–(4). In both methods, the weighted centers do not need to be located on the axes.Reuse & Permissions
Figure 2
Several examples illustrating the concept of the shape variables in the linear-regression approach. 400 points in $η$ and $ϕ$ phase space are distributed randomly using two overlapping Gaussian distributions: (a) the mean positions of the Gaussian distributions are the same; (b) two Gaussian distributions are shifted by 3 (arbitrary) in the $η$ direction; (c) the same as before, but a new point was added with the weight 50 (open circle); (d) the heavy-weight point was moved closer to the major axis. The thick (red) line shows the linear regression which defines the major axis, while the thin (black) line is the minor axis. The eccentricities are calculated using the NQM and QM as described in the text.Reuse & Permissions
Figure 3
Jet mass and jet-shape variables for the leading in $p_{T}$ jet in inclusive $p p$ collisions (filled histograms) simulated with the PYTHIA model. The jet shapes are shown after using the jet-mass cut $M (jet) > 140 GeV$ . Also shown are the shape variables for $X \to t \bar{t} \to W^{+} b_{1} W^{-} b_{2}$ , with $W$ bosons decaying hadronically into two jets. The state $X$ was simulated using a $Z^{'}$ particle with a mass of 2, 3, and 4 TeV (solid and dashed lines, respectively). Events were selected with at least one jet with $p_{T} > 500 GeV$ using the anti- $k_{T}$ jet algorithm. The vertical lines show the cuts applied to reject inclusive QCD events.Reuse & Permissions
Figure 4
Same as Fig. 3, but for the second leading in $p_{T}$ jet.Reuse & Permissions
Figure 5
The differential cross section for the jet-jet invariant mass after the mass cut $M (jet) > 140 GeV$ before (open histograms and filled symbols) and after (filled histogram and open symbols) the jet-shape cuts. We used the PYTHIA model for the simulation of $Z^{'}$ particles with 2, 3, and 4 TeV masses. The relative size of the signal compared to the QCD background level increases after applying the jet-shape cuts. See the text for more detailed numerical estimates.Reuse & Permissions

Physical Review D

covering particles, fields, gravitation, and cosmology