Eur. Phys. J. C (2019) 79:562
https://doi.org/10.1140/epjc/s10052-019-6951-9
Special Article - Tools for Experiment and Theory
Identification of jet-like events using a multiplicity detector
Ranbir Singh
1,a
, Bedangadas Mohanty
1,2
1
School of Physical Sciences, National Institute of Science Education and Research, HBNI, Jatni 752050, India
2
Experimental Physics Department, CERN, 1211 Geneva 23, Switzerland
Received: 8 December 2016 / Accepted: 14 May 2019 / Published online: 3 July 2019
© The Author(s) 2019
Abstract We present a method for studying the detection
of jets in high energy hadronic collisions using multiplic-
ity detectors at forward rapidity. Such a study enhances the
physics scope of multiplicity detectors at forward rapidities
in LHC. At LHC energies the jets may be produced with sig-
nificant cross section at forward rapidities. A multi-resolution
wavelet analysis technique can locate the spatial position of
jets due to its feature of space-scale locality. The discrete
wavelet proves to be very effective in probing physics simul-
taneously at different locations in phase space and at different
scales to identify jet-like events. The key feature this analy-
sis exploits is the difference in particle density in localized
regions of the detector due to jet-like and underlying events.
We find that this method has a significant sensitivity towards
detecting jet position and its size. The jets can be found with
the efficiency and purity of the order of 46% with a cut of
three times the root mean square of the typical wavelet coef-
ficients distribution in minimum bias events.
1 Introduction
At the center-of-mass (CM) energies of the Large Hadron
Collider (LHC), multi-jet events may be produced with mea-
surable cross-section in forward rapidities [1]. Typical 3-jets
events arising from qg → qgg : gg → ggg should appear
in the ratio of 0.3:1 as discussed in [2]. The partonic interac-
tion of a low “x” gluon and the high “x” quark will lead to
a jet in forward direction, where x is the fraction of momen-
tum carried by the partons. One of the first measurement of
inclusive jet production cross section at forward rapidities
was performed in p ¯p collision at
√
s = 1.8 TeV with the D0
detector at the Fermilab Tevatron [1]. The differential cross-
section d
2
σ/(dE
T
dη) was measured up to |η| < 3, where
E
T
is the transverse energy of the jet, σ is the cross-section
for jet production and η is the jet pseudorapidity. The results
a
e-mail: ransinghep@gmail.com
are found to be in good agreement with next-to-leading order
predictions from QCD [3] and indicate a preference for cer-
tain parton distribution functions.
In pp collisions, di-jet events will appear with jets lying
back-to-back in azimuthal angle. In high energy experiments
(e.g. ALICE) these may be easily studied using the central
barrel detectors. However one may encounter events where
the barrel detectors see two of more than two jets in an event
where the topology may suggest a missing jet which may be
in other part of the phase space (forward rapidity). If even the
direction of such a missing jet can be found, more physical
information can be extracted from such an event. The multi-
jet cross section is sensitive to the strong coupling constant
(α
s
) and parton distribution functions (PDFs) [4,5]. The 3-jet
cross section is directly related to the pQCD matrix element
as α
3
s
and hence poses higher sensitivity towards α
s
than
di-jet cross section [6,7]. The multi-jet azimuthal angular
correlation can also be studied and is a useful tool to test the
theoretical prediction of multi-jet production processes [8–
10].
Jets in general produce particles which are confined to
a cone and hence the spatial particle density within the jet
region is expected to be very different compared to a generic
(minimum bias) event in pp collisions.
The aim of this study is to explore if this distinction of
localized high particle multiplicity density can be exploited
successfully to predict the jet direction or identify jet-like
events. This study focusses on present high energy experi-
ments as ALICE at the LHC [12], with possible applications
also to STAR (at RHIC) [11]. In the forward region of such
experiments there are a set of charged particle detectors and
a photon multiplicity detector. We have conducted this study
using charged particles in the forward rapidity covering 2.3
<η<3.9.
In this paper, we have used a multi-resolution analy-
sis by discrete wavelet transformation (DWT) which has
been successfully used in engineering, mathematics, com-
puter science, astrophysics, multiparticle productions and
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