98
4 Standard Model Measurements
and the choice of the factorisation and renormalisation scales. The normalised mass
distribution from boosted top quarks can be used to extract the top quark mass. The
normalised distribution is used since only the shape can be reliably calculated, and
it has the additional benefit that systematic uncertainties partially cancel. The top
quark mass is measured to be m t = 170.8 ± 6.0 (stat) ± 2.8 (sys) ± 4.6 (model) ±
4.0 (theo) GeV in agreement with top quark mass measurement in resolved tt events
(see e.g. Refs. [558–561]), albeit with a much larger uncertainty. This constitutes a
proof-of-principle, presenting the possibility to extract a fundamental SM parameter
from a jet mass distribution. This is of particular interest, as ambiguities arise in the
interpretation of traditional m t measurements [562, 563], which can be circumvented
by measurements and analytical calculations in the highly-boosted regime [564, 565].
A recent CMS measurement using 13 TeV data [557] has reported a precision
improved by a factor of three relative to the 8 TeV measurement. The measurement is based on XCone jets, obtained by a two-step jet clustering [180]. First,
the exclusive XCone algorithm is applied with a distance parameter of R = 1.2
and the specification of returning two jets, corresponding to the two boosted top
quarks in the event. Using the constituents of these two large-R jets as input, XCone
is run again with the distance parameter R = 0.4 and the parameter of the number of subjets in each jet N = 3. This procedure results in exactly two large-radius
XCone jets with three XCone subjets each. Jet energy corrections [70] derived for
anti-k T jets are applied to the XCone subjets. An additional correction applied to
the XCone-subjet momenta is obtained from simulation to account for differences
between the XCone-subjet momenta and the momenta of anti-k T jets. This correction is parametrised as a function of XCone subjet p T and |η|, and has an average
size of 2%, with an average uncertainty of 0.3%. The four-momenta of the three
XCone subjets are combined to form the final XCone jet, where the jet mass is the
invariant mass of all PF candidates clustered into the three XCone subjets. The
resulting distribution in m jet at the particle level has a width half as large as for CA
jets with R = 1.2, as used in the 8 TeV measurement. The improvement is due to
the two-step XCone jet clustering procedure, which acts as a grooming algorithm,
similar to trimming, on the large jet. The advantage of XCone over other grooming
algorithms in this measurement is its dynamical interpolation between the resolved
and boosted regime, i.e., between three well-separated subjets and three subjets close
in R, which would not be resolved by other reconstruction methods. The XConejet reconstruction results also in a large improvement of the experimental resolution
in m jet , due to the accurate subjet calibration. With XCone a resolution of 6% is
achieved, compared to a resolution of approximately 14% for CA jets with R = 1.2.
Figure 4.4 (left) shows the reconstructed XCone jet mass for jets with p T > 400 GeV.
The distribution shows a pronounced and narrow peak close to the value of m t . The
fraction of fully merged t → W b → qq
b decays in the region of the top quark peak
with 140 < m jet < 200 GeV is approximately 75%. The peak position is stable as a
function of the number of pileup vertices, thanks to the inherent grooming of the
two-step XCone jet clustering and the area-based jet energy corrections. The stability
of the peak position is also verified using m jet from fully merged W decays, as calculated from the two XCone subjets with the smallest pairwise mass. The simulation,
4 Standard Model Measurements
and the choice of the factorisation and renormalisation scales. The normalised mass
distribution from boosted top quarks can be used to extract the top quark mass. The
normalised distribution is used since only the shape can be reliably calculated, and
it has the additional benefit that systematic uncertainties partially cancel. The top
quark mass is measured to be m t = 170.8 ± 6.0 (stat) ± 2.8 (sys) ± 4.6 (model) ±
4.0 (theo) GeV in agreement with top quark mass measurement in resolved tt events
(see e.g. Refs. [558–561]), albeit with a much larger uncertainty. This constitutes a
proof-of-principle, presenting the possibility to extract a fundamental SM parameter
from a jet mass distribution. This is of particular interest, as ambiguities arise in the
interpretation of traditional m t measurements [562, 563], which can be circumvented
by measurements and analytical calculations in the highly-boosted regime [564, 565].
A recent CMS measurement using 13 TeV data [557] has reported a precision
improved by a factor of three relative to the 8 TeV measurement. The measurement is based on XCone jets, obtained by a two-step jet clustering [180]. First,
the exclusive XCone algorithm is applied with a distance parameter of R = 1.2
and the specification of returning two jets, corresponding to the two boosted top
quarks in the event. Using the constituents of these two large-R jets as input, XCone
is run again with the distance parameter R = 0.4 and the parameter of the number of subjets in each jet N = 3. This procedure results in exactly two large-radius
XCone jets with three XCone subjets each. Jet energy corrections [70] derived for
anti-k T jets are applied to the XCone subjets. An additional correction applied to
the XCone-subjet momenta is obtained from simulation to account for differences
between the XCone-subjet momenta and the momenta of anti-k T jets. This correction is parametrised as a function of XCone subjet p T and |η|, and has an average
size of 2%, with an average uncertainty of 0.3%. The four-momenta of the three
XCone subjets are combined to form the final XCone jet, where the jet mass is the
invariant mass of all PF candidates clustered into the three XCone subjets. The
resulting distribution in m jet at the particle level has a width half as large as for CA
jets with R = 1.2, as used in the 8 TeV measurement. The improvement is due to
the two-step XCone jet clustering procedure, which acts as a grooming algorithm,
similar to trimming, on the large jet. The advantage of XCone over other grooming
algorithms in this measurement is its dynamical interpolation between the resolved
and boosted regime, i.e., between three well-separated subjets and three subjets close
in R, which would not be resolved by other reconstruction methods. The XConejet reconstruction results also in a large improvement of the experimental resolution
in m jet , due to the accurate subjet calibration. With XCone a resolution of 6% is
achieved, compared to a resolution of approximately 14% for CA jets with R = 1.2.
Figure 4.4 (left) shows the reconstructed XCone jet mass for jets with p T > 400 GeV.
The distribution shows a pronounced and narrow peak close to the value of m t . The
fraction of fully merged t → W b → qq
b decays in the region of the top quark peak
with 140 < m jet < 200 GeV is approximately 75%. The peak position is stable as a
function of the number of pileup vertices, thanks to the inherent grooming of the
two-step XCone jet clustering and the area-based jet energy corrections. The stability
of the peak position is also verified using m jet from fully merged W decays, as calculated from the two XCone subjets with the smallest pairwise mass. The simulation,