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| CMS-EXO-18-001 ; CERN-EP-2018-255 | ||
| Search for new particles decaying to a jet and an emerging jet | ||
| CMS Collaboration | ||
| 23 October 2018 | ||
| JHEP 02 (2019) 179 | ||
| Abstract: A search is performed for events consistent with the pair production of a new heavy particle that acts as a mediator between a dark sector and normal matter, and that decays to a light quark and a new fermion called a dark quark. The search is based on data corresponding to an integrated luminosity of 16.1 fb$^{-1}$ from proton-proton collisions at $\sqrt{s} = $ 13 TeV collected by the CMS experiment at the LHC in 2016. The dark quark is charged only under a new quantum-chromodynamics-like force, and forms an "emerging jet'' via a parton shower, containing long-lived dark hadrons that give rise to displaced vertices when decaying to standard model hadrons. The data are consistent with the expectation from standard model processes. Limits are set at 95% confidence level excluding dark pion decay lengths between 5 and 225 mm for dark mediators with masses between 400 and 1250 GeV. Decay lengths smaller than 5 and greater than 225 mm are also excluded in the lower part of this mass range. The dependence of the limit on the dark pion mass is weak for masses between 1 and 10 GeV. This analysis is the first dedicated search for the pair production of a new particle that decays to a jet and an emerging jet. | ||
| Links: e-print arXiv:1810.10069 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Feynman diagrams in the BSSW model for the pair production of mediator particles, with each mediator decaying to a quark and a dark quark $ {\mathrm {Q}_{\mathrm {DK}}} $, via gluon-gluon fusion (left) and quark-antiquark annihilation (right). |
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Figure 1-a:
Feynman diagram in the BSSW model for the pair production of mediator particles, with each mediator decaying to a quark and a dark quark $ {\mathrm {Q}_{\mathrm {DK}}} $, via gluon-gluon fusion. |
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Figure 1-b:
Feynman diagram in the BSSW model for the pair production of mediator particles, with each mediator decaying to a quark and a dark quark $ {\mathrm {Q}_{\mathrm {DK}}} $, via quark-antiquark annihilation. |
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Figure 2:
Distributions of ${< IP_{\mathrm {2D}}>}$ for background (black) and for signals with a mediator mass of 1 TeV and a dark pion proper decay length of 25 mm, for various dark pion masses. |
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Figure 3:
Distributions of ${\alpha _{\mathrm {3D}}}$ for background (black) and for signals with a mediator mass of 1 TeV and a dark pion mass of 5 GeV for dark pion proper decay lengths ranging from 1 to 300 mm. |
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Figure 4:
The signal acceptance A, defined as the fraction of simulated signal events passing the selection criteria, for models with a dark pion mass ${m_{\pi _\mathrm {DK}}}$ of 5 GeV as a function of the mediator mass ${m_{\mathrm {X_{DK}}}}$ and the dark pion proper decay length ${c\tau _{\pi _\mathrm {DK}}}$. The corresponding selection set number for each model is indicated as text on the plot. |
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Figure 5:
Measured misidentification probability distribution as a function of track multiplicity for the EMJ-1 criteria group defined in Table 2. The red up-pointing triangles are for b jets while the blue down-pointing triangles are for light-flavor jets. The horizontal lines on the data points indicate the variable bin width. The uncertainty bars represent the statistical uncertainties of $ {\epsilon _{\mathrm {f1}}} $ , $ {\epsilon _{\mathrm {f2}}} $, $ {f_{\mathrm {b1}}} $, and ${f_{\mathrm {b2}}}$ in Eq. (7), where the uncertainties in $ {\epsilon _{\mathrm {f1}}} $ and $ {\epsilon _{\mathrm {f2}}} $ correspond to Clopper-Pearson intervals [38]. |
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Figure 6:
Determination of the b jet fraction by fitting the CSVv2 discriminator distribution. The red and blue distributions are the CSVv2 discriminator templates of b jets and light-flavor jets, respectively. The black points with uncertainty bars show the data distribution. The uncertainties in the upper panel include statistical uncertainties of the b jet and light-flavor jet templates, and the fit uncertainties, summed in quadrature. The goodness of fit is given by the $\chi ^2$ divided by the number of degrees of freedom (ndof). The bottom panel shows the difference between data and the fit result, divided by the combination of the statistical uncertainty of data and the uncertainty from the upper panel. The distributions are derived from kinematic samples resulting from selection set 1 in Table 3. |
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Figure 7:
The $ {H_{\mathrm {T}}} $ (left) and number of associated tracks (right) distributions for the observed data events (black points) and the predicted background estimation (blue) for selection set 8 (SM QCD-enhanced), requiring at least two jets tagged by loose emerging jet criteria. The bottom panel shows the difference between observed data and predicted background, divided by the sum in quadrature of the statistical uncertainty in data and the predicted uncertainties from misidentification probability estimation. |
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Figure 7-a:
The $ {H_{\mathrm {T}}} $ distribution for the observed data events (black points) and the predicted background estimation (blue) for selection set 8 (SM QCD-enhanced), requiring at least two jets tagged by loose emerging jet criteria. The bottom panel shows the difference between observed data and predicted background, divided by the sum in quadrature of the statistical uncertainty in data and the predicted uncertainties from misidentification probability estimation. |
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Figure 7-b:
The number of associated tracks distribution for the observed data events (black points) and the predicted background estimation (blue) for selection set 8 (SM QCD-enhanced), requiring at least two jets tagged by loose emerging jet criteria. The bottom panel shows the difference between observed data and predicted background, divided by the sum in quadrature of the statistical uncertainty in data and the predicted uncertainties from misidentification probability estimation. |
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Figure 8:
The $ {H_{\mathrm {T}}} $ (left) and number of associated tracks (right) distributions of the observed data events (black points) and the predicted background estimation (blue) for selection set 9 (SM QCD-enhanced), requiring at least one jet tagged by loose emerging jet criteria and large $ {{p_{\mathrm {T}}} ^\text {miss}} $. The bottom panel shows the difference between observed data and predicted background, divided by the sum in quadrature of the statistical uncertainty in data and the predicted uncertainties from misidentification probability estimation. |
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Figure 8-a:
The $ {H_{\mathrm {T}}} $ distribution of the observed data events (black points) and the predicted background estimation (blue) for selection set 9 (SM QCD-enhanced), requiring at least one jet tagged by loose emerging jet criteria and large $ {{p_{\mathrm {T}}} ^\text {miss}} $. The bottom panel shows the difference between observed data and predicted background, divided by the sum in quadrature of the statistical uncertainty in data and the predicted uncertainties from misidentification probability estimation. |
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Figure 8-b:
The number of associated tracks distribution of the observed data events (black points) and the predicted background estimation (blue) for selection set 9 (SM QCD-enhanced), requiring at least one jet tagged by loose emerging jet criteria and large $ {{p_{\mathrm {T}}} ^\text {miss}} $. The bottom panel shows the difference between observed data and predicted background, divided by the sum in quadrature of the statistical uncertainty in data and the predicted uncertainties from misidentification probability estimation. |
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Figure 9:
Event display of an event passing both selection set 1 and selection set 5. The event contains four jets (jets 1 and 4 pass the emerging jet criteria), consistent with the decay of two massive mediator particles, each decaying to an SM quark and a dark QCD quark. In such a scenario, the dark mesons produced in the fragmentation of the dark quark would decay back to SM particles via the mediator, resulting in displaced vertices with decay distances on the mm scale. (Left) 3D display: the green lines represent reconstructed tracks, the red (blue) truncated pyramids represent energy in the ECAL (HCAL) detectors, respectively. (Right) Reconstructed tracks in $\rho $-$\phi $ view. The filled blue circles represent reconstructed secondary vertices, while the filled red circle is the PV. The solid grey lines represent the innermost layer of the silicon pixel detector. |
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Figure 9-a:
Event display of an event passing both selection set 1 and selection set 5. The event contains four jets (jets 1 and 4 pass the emerging jet criteria), consistent with the decay of two massive mediator particles, each decaying to an SM quark and a dark QCD quark. In such a scenario, the dark mesons produced in the fragmentation of the dark quark would decay back to SM particles via the mediator, resulting in displaced vertices with decay distances on the mm scale. 3D display: the green lines represent reconstructed tracks, the red (blue) truncated pyramids represent energy in the ECAL (HCAL) detectors, respectively. |
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Figure 9-b:
Event display of an event passing both selection set 1 and selection set 5. The event contains four jets (jets 1 and 4 pass the emerging jet criteria), consistent with the decay of two massive mediator particles, each decaying to an SM quark and a dark QCD quark. In such a scenario, the dark mesons produced in the fragmentation of the dark quark would decay back to SM particles via the mediator, resulting in displaced vertices with decay distances on the mm scale. Reconstructed tracks in $\rho $-$\phi $ view. The filled blue circles represent reconstructed secondary vertices, while the filled red circle is the PV. The solid grey lines represent the innermost layer of the silicon pixel detector. |
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Figure 10:
Upper limits at 95% CL on the signal cross section and signal exclusion contours derived from theoretical cross sections for models with dark pion mass $ {m_{\pi _\mathrm {DK}}} $ of 5 GeV in the $ {m_{\mathrm {X_{DK}}}} $-$ {c\tau _{\pi _\mathrm {DK}}} $ plane. The solid red contour is the expected upper limit, with its one standard-deviation region enclosed in red dashed lines. The solid black contour is the observed upper limit. The region to the left of the observed contour is excluded. |
| Tables | |
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Table 1:
Parameters used in generating the 336 simulated signal event samples. A sample corresponding to a single model was created for each possible set of parameter values. |
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Table 2:
Groups of requirements (associated operator indicated in parentheses) on the variables used in the identification of emerging jets. The groups EMJ-1 to -6 are used for the selection sets that define the signal regions, while the groups EMJ-7 and -8 are used to define SM QCD-enhanced samples for the tests of the background estimation methods. |
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Table 3:
The seven optimized selection sets used for this search, and the two SM QCD-enhanced selections (sets 8 and 9) used in tests of the background estimation methods. The headers of the columns are: the scalar $ {p_{\mathrm {T}}} $ sum of the four leading jets ($ {H_{\mathrm {T}}} $) [GeV], the requirements on the $ {p_{\mathrm {T}}} $ of the jets ($p_\mathrm {T,i}$) [GeV], the requirement on $ {{p_{\mathrm {T}}} ^\text {miss}} $ [GeV], the minimum number of the four leading jets that pass the emerging jet selection ($ {n_\mathrm {EMJ}} $), and the EMJ criteria group described in Table 2. The last column is the total number of models defined in Table 1 for which the associated selection set gives the best expected sensitivity. |
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Table 4:
Systematic uncertainties affecting the background estimate from control samples in data. For the definition of the selection sets, see Table 3. |
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Table 5:
Ranges of systematic uncertainties over all models given in Table 1 for which a 95% CL exclusion is expected, for the uncertainties from different sources. |
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Table 6:
Expected (mean $\pm$ syst$_1$ $\pm$ syst$_2$) and observed event yields for each selection set. Uncertainties due to the limited number of events in the control sample and statistical uncertainties in the misidentification probabilities are denoted by "syst$_1$'', while "syst$_2$'' combines the systematic uncertainty sources discussed in Table 4. The "Signal'' column shows the expected event yield for the heaviest mediator mass that can be excluded for each set, with the systematic uncertainties from sources discussed in Table 5 summed in quadrature. The associated model parameters are specified in the last three columns. |
| Summary |
| A search is presented for events consistent with the pair production of a heavy mediator particle that decays to a light quark and a new fermion called a dark quark, using data from proton-proton collisions at $\sqrt{s} = $ 13 TeV corresponding to an integrated luminosity of 16.1 fb$^{-1}$. The dark quark is assumed to be charged only under a new quantum-chromodynamics-like dark force, and to form an emerging jet via a parton shower, containing long-lived dark hadrons that give rise to displaced vertices when decaying to standard model hadrons. The data are consistent with the expected contributions from standard model processes. Limits are set at 95% confidence level excluding dark pion decay lengths between 5 and 225 mm for dark mediators with masses between 400 and 1250 GeV. Decay lengths smaller than 5 and greater than 225 mm are also excluded in the lower part of this mass range. The dependence of the limit on the dark pion mass is weak for masses between 1 and 10 GeV. This analysis is the first dedicated search for the pair production of a new particle that decays to a jet and an emerging jet. |
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