Siam Physics Congress 2015
20-22 May 2015
Development of Charm quark Tagger at the CMS Detector
K. Kovitanggoon1*, B. Asavapibhop1, N. Suwonjandee1 and G. P. Van Onsem2
1
Particle Physics Research Laboratory, Department of Physics , Faculty of Science, Chulalongkorn
University, Bangkok 10330, Thailand
2
Interuniversity Institute for High Energies, Vrije Universiteit Brussel, Ixelles, Belgium
*Corresponding author. E-mail: kovitang.cern@gmail.com
Abstract
Various physics processes studied with the Compact muon Solenoid (CMS) detector have final states
with charm quark jets. Charm quarks can hadronize to D mesons which could travel some distance in
the tracker before decaying, resulting in displaced tracks and vertices. The CMS silicon tracker allows
the precise reconstruction of such secondary vertices and tracks that are displaced with respect to the
primary interaction point. The identification of charm jets or "c-tagging" algorithm is constructed based
on the b-tagging algorithms. Particle Physics Research Laboratory at Chulalongkorn University, as a
part of the c-tagging team, is currently studying the c-tagging algorithm in order to maximize its
performance. This study will be vital for both supersymmetry (SUSY) searches for new particles such
as stop (𝑡̃), the SUSY partner of standard model (SM) top, that may subsequently decay to a charm
quark and the lightest supersymmetric particle (LSP), and for SM precision measurements in the new
data taking at the Large Hadron Collider (LHC) in 2015.
Keywords: CMS, LHC, Jet, C-tagging
Introduction
The Standard model (SM) is the underlying
quantum field theory that deals with the fundamental
building blocks of matter and their interactions. SM is
an elegant framework and successfully describes three
of the four known fundamental forces, the weak, the
electromagnetic and the strong force. Although the
SM has been established in 60s and 70s, it still holds
up to many experiments since then.
Particles
predicted by the SM were discovered over the years at
various experiments. However, it suffers from such
shortcoming as the hierarchy problem and cannot
include the fourth fundamental force, the gravity.
Moreover some questions still cannot be explained
with the SM, such as the unification of the four forces
at high energy and dark matter. Supersymmetry
(SUSY) as the extension of the SM is a popular
candidate that postulates the existence of
supersymmetric partners for every SM particle. SUSY
provides the solution to the hierarchy problem and
may provide a framework to integrate gravity into the
forces unification. In some SUSY scenarios, it
suggests the lightest supersymmetric particle (LSP or
𝜒̃10 ) which is neutral and stable. The LSP is a
candidate of dark matter. The decay mode of a stop
particle is dominated by a charm quark and the LSP
(𝑡̃ → 𝑐 + 𝜒
̃ 01 ).
The study involves the final state that is
characterized by the jet from the hadronization
process of the charm quark and the missing transverse
energy (𝐸𝑇𝑚𝑖𝑠𝑠 ) from the undetected LSP. The aim of
this paper is to present the status of the charm quark
tagger in the CMS software framework (CMSSW).
Charm Quark Tagger in the CMS
In the SM, there are six flavours of quarks. They
are grouped into three generations i.e. up and down
quarks (u and d), charm and strange quarks (c and s),
and also top and bottom quarks (t and b). All the
quarks, except top quarks which decay in a time scale
shorter than the hadronization scale, form hadrons and
thus appear as jets after showering inside the detector.
The charm quark is the third heaviest quark, mc ≈ 1.3
GeV. Considerably heavy and long lifetime, jets that
decay from c quarks have the unique properties from
those of jets that decay from lighter quarks and
gluons.
Following the high energy collisions, free quarks
and gluons are created. The color confinement, which
only allows for colorless particles to exist, states that
free quarks and gluons, containing color charge
fragments, cannot exist in the free space, and thus
they cannot be measured by particle detectors. They
combine with quarks and anti-quarks, which are
created in the vacuum to form the hadrons (the bound
states of quarks and gluons). The decay of these
hadronization processes can be detected as the tight
cone of energy deposited in the calorimeter in term of
jets.
The bound states of charm quarks exist in both
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20-22 May 2015
forms of meson and baryons such as J/Ψ meson
consisting of 𝑐𝑐̅, D meson consisting of c and another
lighter quark or 𝛬+
𝑐 consisting of udc quarks. Due to
the high energy of the pp collisions at the LHC and
low branching ratios of other bound states, D mesons
are dominating at the CMS detector. The lifetime of D
mesons is about 1.04 ps for D± and 0.41 ps for D0.
The corresponding relatively long flight paths lead to
characteristic signatures of one displaced vertex,
called secondary vertex and displaced tracks. The
CMS silicon tracker allows the precise reconstruction
of vertices and tracks which can be used to distinguish
c jets from other jets.
and η, A dedicate module (called the
TagVarExtractor) takes the BTagAnalyzer output as
an input to make the flat (pT and η independent)
output and also store needed variables per jet. Using
the TMVA framework, a multivariate analysis
training can be preformed to discriminate c jets from
non-c jets. In this way a discriminator can be
calculated, given the jet-dependent input variables,
both in simulated and real data recorded with the
CMS detector. In the end, the c-tag discriminator
performance is tested by studying the efficiency of
The compact muon solenoid (CMS)
The CMS detector is a large, high field
superconducting magnet detector. The CMS main
design priorities are a redundant muon tracking
system, a good electromagnetic calorimeter and a
high quality inner tracking system. The CMS
structure consists of many cylindrical detecting layers,
coaxial with the beam direction (barrel region), closed
at both end with disks (endcap region), and large
pseudorapidity calorimeter close to beam line
(forward region). A schematic view of the CMS
detector, which is 28.7 m long, 15 m in diameter, and
14,000 tons of the total weight is shown in Figure 1.
tagging c jets as function of the efficiency of non-c
jets.
Figure 2: A schematic overview of charm quark
tagger in the CMS software.
Analysis Status
Figure 1: Illustration with details of the CMS
detector.
Overview of the charm quark tagger in the CMS
software (CMSSW)
The framework is being developed by using the
newly developed bottom quark tagger framework that
is called BTagAnalyzer with a toolkit for multivariate
data analysis (TMVA). A schematic overview of the
workflow is shown in Figure 2. Starting with well
defined QCD and 𝑡𝑡̅ simulations, the BTagAnalyzer
code is modified in order to produce a set of variables
needed for c-tag purposes such as jet pT, jet η, and
secondary vertex information. The output of this
BTagAnalyzer code contains all needed variables per
event. Since this step is done on simulations, jets can
be matched with their mother partons. Since the
training step must not learn anything from the jet p T
The modifications of BTagAnalyzer and
TagVarExtractor codes are done for the c-tag purpose
which includes adding the new variables, optimizing
the new selections, and organizing the variables into
the format that can be used by other c-tag frameworks
that are being developed in parallel. The input
variables in the training trees are defined using the
impact parameter and secondary vertex information.
The example plots of the number of tracks associated
with the secondary vertex (vertexNtracks) and the
signed transverse impact parameter significance
(trackSip2dSig) show the distinctions between signal
(c jets) and background (light quark and gluon jets) as
shown in Figure 3.
Figure 3: The plots on 𝑡𝑡̅ simulation shows that the
number of track associated with the secondary vertex
(left) and the signed transverse impact parameter
significance (right) can be used to differentiate the c
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20-22 May 2015
jets (blue line) and light quark and gluon jets (red
line).
Ongoing efforts
7.
The developments of the c-tagger are currently
being done in three frameworks. Our colleagues are
working on the old frameworks while Chulalongkorn
University term is working on the newly developed
method as discussed above. Since each framework is
using different definitions and algorithms to calculate
the related variables, the collaboration with other
teams to find the sources of discrepancies of each
framework and how to optimize the selections for the
c-tag purpose is ongoing. Also, by working closely
with experts the TMVA training framework will be
implemented to this framework in order to finally
obtain the discriminator training data base that can be
used for Run-II in the CMS experiment in the late
2015.
8.
charm quarks or in compressed supersymmetric
scenarios in pp collisions at s√=8 TeV with the
ATLAS detector”, Phys. Rev. D 90, 052008
(2014).
Nakamura et al., “Review of Particle Physics”, J.
Phys. G 37 (2010) 075021.
CMS Collaboration, “The CMS experiment at
the CERN LHC”,JINST 3 S08004(2008).
Conclusions
For the first time the c-tagger is studied in the
CMS experiment. A c-tagging algorithm will be one
of the crucial elements for new physics searches, for
example for supersymmetric top partners. Also, it can
be beneficial for Standard Model precision
measurements.
Acknowledgments
The author would like to thank to Department of
Physics, Faculty of Science, Chulalongkorn
University for financial support. I also would like to
thank to CMS collaboration, in particular, our ctagger colleagues and All SPC 2015 staffs for
organizing this event. This research is supported by
Rachadapisek Sompote Fund for Postdoctoral
Fellowship, Chulalongkorn University.
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