CMS-TOP-15-014

CMS-TOP-15-014 ; CERN-EP-2016-202
Measurement of the mass of the top quark in decays with a $\mathrm{ J } / \psi$ meson in pp collisions at 8 TeV
CMS Collaboration
11 August 2016
JHEP 12 (2016) 123
Abstract: A first measurement is presented of the top quark mass using the decay channel $\mathrm{ t }\to(\mathrm{ W }\to\ell\nu)\,(\mathrm{ b }\to\mathrm{ J } / \psi+\mathrm{X}\to\mu^+\mu^-+\mathrm{X})$ , in events selected in proton-proton collisions and recorded with the CMS detector at the LHC at a center-of-mass energy of 8 TeV. The data correspond to an integrated luminosity of 19.7 fb $^{-1}$ , with 666 $\mathrm{ t \bar{t} }$ and single top quark candidate events containing a reconstructed $\mathrm{ J } / \psi$ candidate decaying into an oppositely-charged muon pair. The mass of the ( $\mathrm{ J } / \psi+\ell$ ) system, where $\ell$ is an electron or a muon from W boson decay, is used to extract a top quark mass of 173.5 $\pm$ 3.0 (stat) $\pm$ 0.9 (syst) GeV.
Links: e-print arXiv:1608.03560 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Pictorial view of the $\mathrm{ J } / \psi$ meson produced in a ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ system.
png pdf	Figure 2: Distributions of the dimuon invariant mass between 2.8 and 3.4 GeV (a) and of the ${p_{\mathrm {T}}}$ of the $\mathrm{ J } / \psi$ meson candidate (b). Processes are normalized to their theoretical cross sections. The simulation assumes a value of $m_\mathrm{ t } =$ 172.5 GeV. The lower panel shows the ratio of the number of events observed in data to the number expected from simulation.
png pdf	Figure 2-a: Distributions of the dimuon invariant mass between 2.8 and 3.4 GeV (a) and of the ${p_{\mathrm {T}}}$ of the $\mathrm{ J } / \psi$ meson candidate (b). Processes are normalized to their theoretical cross sections. The simulation assumes a value of $m_\mathrm{ t } =$ 172.5 GeV. The lower panel shows the ratio of the number of events observed in data to the number expected from simulation.
png pdf	Figure 2-b: Distributions of the dimuon invariant mass between 2.8 and 3.4 GeV (a) and of the ${p_{\mathrm {T}}}$ of the $\mathrm{ J } / \psi$ meson candidate (b). Processes are normalized to their theoretical cross sections. The simulation assumes a value of $m_\mathrm{ t } =$ 172.5 GeV. The lower panel shows the ratio of the number of events observed in data to the number expected from simulation.
png pdf	Figure 3: Distributions of the invariant mass of the $\mathrm{ J } / \psi$ meson candidate and the leading lepton combination, in the leading $\mu$ (a) and leading e (b) combinations. Processes are normalized to their theoretical cross sections. The simulation assumes a value of $m_\mathrm{ t } =$ 172.5 GeV. The lower panel shows the ratio of the number of events observed in data to the number expected from simulation.
png pdf	Figure 3-a: Distributions of the invariant mass of the $\mathrm{ J } / \psi$ meson candidate and the leading lepton combination, in the leading $\mu$ (a) and leading e (b) combinations. Processes are normalized to their theoretical cross sections. The simulation assumes a value of $m_\mathrm{ t } =$ 172.5 GeV. The lower panel shows the ratio of the number of events observed in data to the number expected from simulation.
png pdf	Figure 3-b: Distributions of the invariant mass of the $\mathrm{ J } / \psi$ meson candidate and the leading lepton combination, in the leading $\mu$ (a) and leading e (b) combinations. Processes are normalized to their theoretical cross sections. The simulation assumes a value of $m_\mathrm{ t } =$ 172.5 GeV. The lower panel shows the ratio of the number of events observed in data to the number expected from simulation.
png pdf	Figure 4: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 4-a: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 4-b: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 4-c: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 4-d: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 4-e: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 4-f: Mean (a) and standard deviation (b) of the Gaussian distribution describing the peak of the $m_{\mathrm{ J } / \psi +\ell }$ distributions, relative contribution of the Gaussian distribution to $P_\text {sig+bkg}$ (c), and shape (d), scale(e), and shift (f) parameters of the gamma distribution, as a function of input $m_\mathrm{ t }$ . The solid lines are the result of the simultaneous fit described in Section 3.1, while the dashed lines indicate the 68% confidence level of the fit. The superimposed data points are the result of the alternative fitting method described in Section 3.2.
png pdf	Figure 5: Ratio of the ${p_{\mathrm {T}}}$ of the b hadrons to the ${p_{\mathrm {T}}}$ of the matched generator-level jet for the Z2* LEP $r_\mathrm{ b }$ tune(upper), the ratio to Z2* LEP $r_\mathrm{ b }$ for the Z2, $\text {Z}2^\ast \text {LEP} r_\mathrm{ b } ^{-}$ , and $\text {Z}2^\ast \text {LEP} r_\mathrm{ b } ^{+}$ tunes (middle), and the ratio to Z2 LEP $r_\mathrm{ b }$ for the P12, P12FT, and P12FL tunes (lower). As neutrinos are not clustered within jets, it happens in very rare cases that ${p_{\mathrm {T}}} ^\text {gen}(\text {B})> {p_{\mathrm {T}}} ^\text {gen}(\text {jet})$ . For this effect to be visible, the horizontal axis range is extended beyond 1 unit.
png pdf	Figure 6: Dependence of the extracted $m_\mathrm{ t }$ value on the average fragmentation ratio $< {p_{\mathrm {T}}} ^\text {gen}(\text {B})/ {p_{\mathrm {T}}} ^\text {gen}(\text {jet}) >$ , fitted to a linear function.
png pdf	Figure 7: Distribution in the invariant mass of the $\mathrm{ J } / \psi$ meson candidate and the leading lepton combination, fitted to $P_\text {sig+bkg}$ of Eq.(1) through the maximization of a likelihood function. The inset shows the negative logarithm of the likelihood function $L$ relative to its maximum $L_\text {max}$ as a function of the only free parameter, which is $m_\mathrm{ t }$ .

Tables
png pdf	Table 1: Number of selected events from simulations and observed in data. The uncertainties are statistical.
png pdf	Table 2: Summary of the impact of systematic uncertainties on the top quark mass according to the contributions from each source.

Summary

The first measurement of the mass of the top quark is presented in the decay channel

$\mathrm{ t }\to(\mathrm{ W }\to\ell\nu)\,(\mathrm{ b }\to\mathrm{ J } / \psi+\mathrm{X}\to\mu^+\mu^-+\mathrm{X})$ . An event selection is implemented in proton-proton collisions recorded with the CMS detector at

$\sqrt{s} =$ 8 TeV, to obtain a sample of high purity leptonically-decaying top quarks in

$\mathrm{ t \bar{t} }$ and single top quark production events containing one

$\mathrm{ J } / \psi$ meson candidate that decays into an oppositely-charged muon pair. The data correspond to an integrated luminosity of 19.7 fb

$^{-1}$ . There are 355 events observed with a muon and 311 with an electron as leading isolated lepton, in agreement with expectations from simulation. The top quark mass is extracted from an unbinned maximum-likelihood fit to the invariant mass of the leading lepton and

$\mathrm{ J } / \psi$ meson candidate. The resulting

$m_\mathrm{ t }$ measurement is 173.5 GeV, with a statistical uncertainty of 3.0 GeV and a systematic uncertainty of 0.9 GeV. This is the first time that this method has been applied to a physics analysis and the systematic uncertainty is of the same order of magnitude as that estimated in Ref. [9]. Even though the results are statistically limited, the dominant systematic uncertainties are different from those of the most precise direct reconstruction methods. As the sensitivity to jet-related uncertainties is negligible, this allows the possibility to contribute significantly in combination with other

$m_\mathrm{ t }$ measurements. Furthermore, with the larger data set expected in the next runs of the LHC, the method described in this paper will provide a result which will be more competitive with those obtained from the conventional reconstruction techniques.

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