192
5 Direct Searches for New Physics
μ = 1 GeV [251]. This definition of ρ
is different from the usual dimensionless
scaling variable ρ = ln[m
2
SD / p
2
T ] [237], and has been chosen in order to decorrelate
τ
DDT
21
from the jet mass. The range in ρ
is restricted to 0–4, corresponding to a
soft drop jet mass between 30 and 300 GeV for jets with p T > 500 GeV. For ρ
< 0
non-perturbative effects become important and for ρ
> 4 hard wide-angle radiation
leads to out-of-cone effects, both effects resulting in a deviation of the average of the
τ
DDT
21
distribution from a constant. Signal events are classified by τ
DDT
21
< 0.38. The
dominant background from multijet production is obtained using a pass-fail ratio
technique, where the events failing the substructure selection with τ
DDT
21
> 0.38
are weighted by the pass-fail ratio. This ratio is obtained from a fit of polynomial
functions to data as a function of τ
DDT
21
and ρ
, which corrects for any residual
correlations between τ
DDT
21
and ρ
. In order to avoid a bias due to a potential signal,
a region in the soft drop mass around the probed Z
mass is removed from the fit.
Resonant backgrounds from simulated V +jets production are subtracted from the
data prior to the determination of the pass-fail ratio. By analysing the soft drop jet
mass, the analysis places limits on a Z
in the mass range from 100 to 300 GeV. In the
region 140–300 GeV, these limits are better by a factor of up to four than the previous
limits by the UA2 experiment. These have been the first experimental results for Z
masses in the range 100–140 GeV.
An update of this analysis by CMS, based on 35.9 fb
−1 [489], uses N 2 for the
substructure selection and ρ for the decorrelation. For similar reasons as above, only
jets with −5.5 < ρ < −2.0 are considered, with different numerical values because
of the change to ρ instead of ρ
. This translates into jet masses in the range from
25 to 185 GeV for p T = 500 GeV. The definition of N
DDT
2
is improved in relation
to the previous analysis, where an exact decorrelation is performed on simulated
multijet events. This requires a map in the plane of ρ and p T , defining a constant 5%
background efficiency for all values of ρ and p T considered. Residual differences
between data and simulation, which can affect this decorrelation, are accounted for
by allowing for deviations in the pass-fail from a constant. Since the pass-fail ratio
is determined simultaneously to the signal and background fit to data, and because
it is only slowly varying as function of ρ and p T , it is not necessary any more to
remove a window around the Z
mass from its determination. The soft drop jet mass is
measured in five intervals of jet p T , where the distribution for 500 < p T < 600 GeV
is shown in Fig. 5.24. The multijet background, together with smaller contributions
from V +jets and tt production, describe the data well. The improved background
estimation allows to extend the reach in Z
masses down to 50 GeV, with an upper
reach of 300 GeV. The upper reach of this search can be extended by considering
larger jet distance parameters, as done in a search by CMS using data recorded in the
year 2017 with an integrated luminosity of 41.1 fb
−1 [1142]. The analysis strategy
is unchanged in relation to the search based on 2016 data [489], but the analysis is
extended to use CA R = 1.5 jets in addition to anti-k T R = 0.8 jets. The CA R = 1.5
jets are required to have p T > 575 GeV and −4.7 < ρ < −1.0, which translates into
a jet mass range from 55 to 350 GeV for p T = 575 GeV and from 81 to 500 GeV for
p T = 850 GeV. The analysis based on R = 0.8 jets is used to probe Z
masses in
5 Direct Searches for New Physics
μ = 1 GeV [251]. This definition of ρ
is different from the usual dimensionless
scaling variable ρ = ln[m
2
SD / p
2
T ] [237], and has been chosen in order to decorrelate
τ
DDT
21
from the jet mass. The range in ρ
is restricted to 0–4, corresponding to a
soft drop jet mass between 30 and 300 GeV for jets with p T > 500 GeV. For ρ
< 0
non-perturbative effects become important and for ρ
> 4 hard wide-angle radiation
leads to out-of-cone effects, both effects resulting in a deviation of the average of the
τ
DDT
21
distribution from a constant. Signal events are classified by τ
DDT
21
< 0.38. The
dominant background from multijet production is obtained using a pass-fail ratio
technique, where the events failing the substructure selection with τ
DDT
21
> 0.38
are weighted by the pass-fail ratio. This ratio is obtained from a fit of polynomial
functions to data as a function of τ
DDT
21
and ρ
, which corrects for any residual
correlations between τ
DDT
21
and ρ
. In order to avoid a bias due to a potential signal,
a region in the soft drop mass around the probed Z
mass is removed from the fit.
Resonant backgrounds from simulated V +jets production are subtracted from the
data prior to the determination of the pass-fail ratio. By analysing the soft drop jet
mass, the analysis places limits on a Z
in the mass range from 100 to 300 GeV. In the
region 140–300 GeV, these limits are better by a factor of up to four than the previous
limits by the UA2 experiment. These have been the first experimental results for Z
masses in the range 100–140 GeV.
An update of this analysis by CMS, based on 35.9 fb
−1 [489], uses N 2 for the
substructure selection and ρ for the decorrelation. For similar reasons as above, only
jets with −5.5 < ρ < −2.0 are considered, with different numerical values because
of the change to ρ instead of ρ
. This translates into jet masses in the range from
25 to 185 GeV for p T = 500 GeV. The definition of N
DDT
2
is improved in relation
to the previous analysis, where an exact decorrelation is performed on simulated
multijet events. This requires a map in the plane of ρ and p T , defining a constant 5%
background efficiency for all values of ρ and p T considered. Residual differences
between data and simulation, which can affect this decorrelation, are accounted for
by allowing for deviations in the pass-fail from a constant. Since the pass-fail ratio
is determined simultaneously to the signal and background fit to data, and because
it is only slowly varying as function of ρ and p T , it is not necessary any more to
remove a window around the Z
mass from its determination. The soft drop jet mass is
measured in five intervals of jet p T , where the distribution for 500 < p T < 600 GeV
is shown in Fig. 5.24. The multijet background, together with smaller contributions
from V +jets and tt production, describe the data well. The improved background
estimation allows to extend the reach in Z
masses down to 50 GeV, with an upper
reach of 300 GeV. The upper reach of this search can be extended by considering
larger jet distance parameters, as done in a search by CMS using data recorded in the
year 2017 with an integrated luminosity of 41.1 fb
−1 [1142]. The analysis strategy
is unchanged in relation to the search based on 2016 data [489], but the analysis is
extended to use CA R = 1.5 jets in addition to anti-k T R = 0.8 jets. The CA R = 1.5
jets are required to have p T > 575 GeV and −4.7 < ρ < −1.0, which translates into
a jet mass range from 55 to 350 GeV for p T = 575 GeV and from 81 to 500 GeV for
p T = 850 GeV. The analysis based on R = 0.8 jets is used to probe Z
masses in