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| 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 | |
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Figure 1:
Pictorial view of the $\mathrm{ J } / \psi$ meson produced in a ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ system. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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 | |
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Table 1:
Number of selected events from simulations and observed in data. The uncertainties are statistical. |
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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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Compact Muon Solenoid LHC, CERN |
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