High Multiplicity Searches at the LHC Using Jet Masses Page: 2 of 9
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II. JET MASS AS AN OBSERVABLE
Jet masses have historically been difficult observables
at hadron colliders because pile-up and underlying event
contribute to the jet mass as R3 or R4. However, using
jet-grooming techniques such as filtering [29], pruning [30],
or trimming [31], the underlying event and pile up
contributions can be removed. The resulting jet is
an accurate measurement of the underlying partonic
event [32, 33]. Of these three methods, filtering is the least
optimal for high multiplicity signals because it requires
a fixed number of subjets to be identified in advance,
whereas the signals studied in this article do not have a
definite number of subjets per jet.
The jet-grooming techniques listed above are designed
to look for boosted hadronic resonances appearing under
a continuum background. The kinematics considered in
this article typically result from particles decaying at
rest and hence, the reconstructed jets do not group the
underlying partons together in any manner that represents
the underlying decay kinematics. As a result, the jet
masses do not correspond to a parent particle's mass.
While jet-grooming with a variable number of subjets
may be useful or beneficial, it is not as necessary and the
details are not as important. For the remaining portion
of the article, no jet-grooming is used, but it should be
understood that jet-grooming can be applied so long as
the algorithm allows the number of subjets per fat jet to
vary on a jet-by-jet basis. In addition, it may be possible
to combine Qjets with jet pruning to even better improve
sensitivity over background [? ].
When a jet is formed via a parton shower, its mean
squared invariant mass is (m ) cx aspN>R2, where as
is the strong coupling constant, pT,2 is the transverse
momentum of the jet, and R is its radius [34, 35]. When
a jet is formed from independent partons through multi-
body decays of heavy particles, however, the typical jet
mass is larger. In high-multiplicity signal events, there is
not enough solid angle for the partons to be well-separated
and therefore multiple partons are clustered together. As
a result, partons will lie close to each other and may be
clustered together into the same jet. For these jets, the
mean squared invariant mass is (min) ac pR2, where
one does not pay the factor of a8.
The visible energy in the event, HT, can be related to
the total jet mass M . In particular,
2=11
cT (,)((nR)2 +1) ~M 1 (
whr = a f s m g d
where = y for jets whose mass is generated by the600
600 800 1000
HT (GeV)1200 1400
FIG. 1: A plot of M1 versus HT after requiring N > 4 "fat"
jets with pT > 120 GeV and pT > 50 GeV on the leading and
sub-leading jets, respectively. QCD (orange) and top (green)
events are shown where the median value for a given HT is
shown in a solid line and the 68% and 95% inclusion bands
are shown in the dotted and dashed lines, respectively. The
higher values of M1 for top events arise from the top mass.
Signals with heavier parent particles than the top give even
larger M1.
parton shower and 1 for jets whose mass arises from
multiple partons being grouped together. Eq. 2 is the
main reason why MJ is a more effective discriminator than
HT for high-multiplicity signals. For high-multiplicity
signals, the jet masses do not usually result from parton
showering (t 1), while for the QCD and V + jets
backgrounds (when V decays into missing energy) they
do (n a8). For signal and background events with
similar HT, the value of M for the background will
always be lower than that for the signal. As a result,
the signal distribution always has a longer tail of high-jet
mass than the background, even if its HT distributions
are similar. The correlation between MJ and HT is shown
in Fig. 1 for QCD and top events. Top events typically
have higher values of MJ for a fixed HT, with a total jet
mass that asymptotes to 2mt. Signal events have even
larger values of M than top events and asymptote to
higher values.
The argument that M is preferable to HT relies on
two assumptions. The first is that the signal has a larger
M than top events, which requires that the signal is
at least as jet-rich as top events and has higher typical
visible energies than top events. This first assumption
is true in many signals of beyond the Standard Model
physics.
The second assumption implicit in Eq. 2 is that jet2
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Hook, Anson; /SLAC /Stanford U., Appl. Phys. Dept.; Izaguirre, Eder; /SLAC /Stanford U., Phys. Dept.; Lisanti, Mariangela; U., /Princeton et al. High Multiplicity Searches at the LHC Using Jet Masses, article, April 24, 2012; United States. (https://digital.library.unt.edu/ark:/67531/metadc836340/m1/2/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.