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| CMS-PAS-EXO-18-002 | ||
| Search for heavy long-lived charged particles with large ionization energy loss in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 27 March 2024 | ||
| Abstract: A search for heavy, long-lived, charged particles depositing large ionization within the silicon tracker is presented. This analysis uses proton-proton collision data with center of mass energy at $ \sqrt{s} = $ 13 TeV, collected in 2017 and 2018 by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 101 fb$^{-1}$. Two different approaches for the search are taken. One new method exploits the independence of the silicon pixel and silicon strips measurements, and an improved version of the mass method used in previous iterations of the analysis is further employed. No significant excess of events is observed. The results are interpreted in the context of pair production of supersymmetric particles, namely gluinos, top squarks and tau sleptons, or for Drell-Yan pair production of fourth generation ($ \tau' $) leptons with an electric charge equal to or twice the electron charge ($ e $). This search presents also for the first time an interpretation of a Z' boson decaying to two $ \tau' $ leptons having an electric charge equal to 2$ e $. The 95% confidence upper limits on the production cross section are extracted for each of these hypothetical particles. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
The $ F_{\text{i}}^{\text{Pixels}} $ vs $ G_{\text{i}}^{\text{Strips}} $ distribution for the SM MC (left), and the 1800 GeV mass $ \mathrm{\widetilde{g}} $ R-hadron (right), after passing the selection criteria listed in Table 2. |
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Figure 1-a:
The $ F_{\text{i}}^{\text{Pixels}} $ vs $ G_{\text{i}}^{\text{Strips}} $ distribution for the SM MC (left), and the 1800 GeV mass $ \mathrm{\widetilde{g}} $ R-hadron (right), after passing the selection criteria listed in Table 2. |
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Figure 1-b:
The $ F_{\text{i}}^{\text{Pixels}} $ vs $ G_{\text{i}}^{\text{Strips}} $ distribution for the SM MC (left), and the 1800 GeV mass $ \mathrm{\widetilde{g}} $ R-hadron (right), after passing the selection criteria listed in Table 2. |
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Figure 2:
The $ G_{\text{i}}^{\text{Strips}} $ distribution in the FAIL (left) and PASS (right) regions for events passing the event selection and with 55 $ < p_{\mathrm{T}} < $ 200 GeV. The data recorded in 2017 and 2018 are represented by black markers. The background predicted by the ionization method is shown in yellow, with the shaded area indicating the background uncertainty. The histograms associated to the 557 GeV $ \widetilde{\tau} $ and 1800 GeV $ \mathrm{\widetilde{g}} $ signals are not visible on the plots, their contribution being negligible in this region. The lower panel displays the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
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Figure 2-a:
The $ G_{\text{i}}^{\text{Strips}} $ distribution in the FAIL (left) and PASS (right) regions for events passing the event selection and with 55 $ < p_{\mathrm{T}} < $ 200 GeV. The data recorded in 2017 and 2018 are represented by black markers. The background predicted by the ionization method is shown in yellow, with the shaded area indicating the background uncertainty. The histograms associated to the 557 GeV $ \widetilde{\tau} $ and 1800 GeV $ \mathrm{\widetilde{g}} $ signals are not visible on the plots, their contribution being negligible in this region. The lower panel displays the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
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Figure 2-b:
The $ G_{\text{i}}^{\text{Strips}} $ distribution in the FAIL (left) and PASS (right) regions for events passing the event selection and with 55 $ < p_{\mathrm{T}} < $ 200 GeV. The data recorded in 2017 and 2018 are represented by black markers. The background predicted by the ionization method is shown in yellow, with the shaded area indicating the background uncertainty. The histograms associated to the 557 GeV $ \widetilde{\tau} $ and 1800 GeV $ \mathrm{\widetilde{g}} $ signals are not visible on the plots, their contribution being negligible in this region. The lower panel displays the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
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Figure 3:
The $ G_{\text{i}}^{\text{Strips}} $ distribution in the FAIL (left) and PASS (right) regions for events passing the the event selection and with $ p_{\mathrm{T}} > $ 200 GeV. The data recorded in 2017 and 2018 are represented by black dots. The background predicted by the ionization method is shown in yellow, with the hatched area indicating the background uncertainty. As an example, the blue line shows the 1800 GeV $ \mathrm{\widetilde{g}} $ signal distribution and the red line shows the 557 GeV $ \widetilde{\tau} $ signal distribution. The lower panel displays the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
png pdf |
Figure 3-a:
The $ G_{\text{i}}^{\text{Strips}} $ distribution in the FAIL (left) and PASS (right) regions for events passing the the event selection and with $ p_{\mathrm{T}} > $ 200 GeV. The data recorded in 2017 and 2018 are represented by black dots. The background predicted by the ionization method is shown in yellow, with the hatched area indicating the background uncertainty. As an example, the blue line shows the 1800 GeV $ \mathrm{\widetilde{g}} $ signal distribution and the red line shows the 557 GeV $ \widetilde{\tau} $ signal distribution. The lower panel displays the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
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Figure 3-b:
The $ G_{\text{i}}^{\text{Strips}} $ distribution in the FAIL (left) and PASS (right) regions for events passing the the event selection and with $ p_{\mathrm{T}} > $ 200 GeV. The data recorded in 2017 and 2018 are represented by black dots. The background predicted by the ionization method is shown in yellow, with the hatched area indicating the background uncertainty. As an example, the blue line shows the 1800 GeV $ \mathrm{\widetilde{g}} $ signal distribution and the red line shows the 557 GeV $ \widetilde{\tau} $ signal distribution. The lower panel displays the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
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Figure 4:
Mass prediction in the signal region defined by $ G_{\text{i}}^{\text{Strips}} > $ 0.22 and $ p_{\mathrm{T}} > $ 70 GeV. The data recorded in 2017 and 2018 are represented by black dots. The data driven background estimate is displayed as red markers with the yellow envelope representing the quadratic sum of the statistical uncertainty and the systematic uncertainty. Several signal scenarios are also displayed. The last bin includes the overflow. The lower panel shows the ratio of the observed yields (Data) to those predicted (Bkg). The lower panel shows the pulls, defined as the difference between the observed (Data) and predicted (Bkg) yields divided by the associated uncertainty ($ \sigma $). |
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Figure 5:
Cross section limits for $ \mathrm{\widetilde{g}} $ R-hadrons obtained with the ionization method on the left and with the mass method on the right. The numerical values corresponding to the expected and observed mass limits are written vertically on the plot in light and dark grey, respectively. |
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Figure 5-a:
Cross section limits for $ \mathrm{\widetilde{g}} $ R-hadrons obtained with the ionization method on the left and with the mass method on the right. The numerical values corresponding to the expected and observed mass limits are written vertically on the plot in light and dark grey, respectively. |
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Figure 5-b:
Cross section limits for $ \mathrm{\widetilde{g}} $ R-hadrons obtained with the ionization method on the left and with the mass method on the right. The numerical values corresponding to the expected and observed mass limits are written vertically on the plot in light and dark grey, respectively. |
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Figure 6:
Cross section limits for $ \widetilde{t} $ R-hadrons obtained with the ionization method on the left and with the mass method on the right. |
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Figure 6-a:
Cross section limits for $ \widetilde{t} $ R-hadrons obtained with the ionization method on the left and with the mass method on the right. |
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Figure 6-b:
Cross section limits for $ \widetilde{t} $ R-hadrons obtained with the ionization method on the left and with the mass method on the right. |
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Figure 7:
Cross section limits for the direct pair-production of $ \widetilde{\tau} $, considering both handednesses, obtained with the ionization method (left) and the mass method (right). |
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Figure 7-a:
Cross section limits for the direct pair-production of $ \widetilde{\tau} $, considering both handednesses, obtained with the ionization method (left) and the mass method (right). |
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Figure 7-b:
Cross section limits for the direct pair-production of $ \widetilde{\tau} $, considering both handednesses, obtained with the ionization method (left) and the mass method (right). |
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Figure 8:
Cross section limits for the $ \widetilde{\tau} $ production within the GMSB model, obtained with the ionization method (left) and the mass method (right). |
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Figure 8-a:
Cross section limits for the $ \widetilde{\tau} $ production within the GMSB model, obtained with the ionization method (left) and the mass method (right). |
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Figure 8-b:
Cross section limits for the $ \widetilde{\tau} $ production within the GMSB model, obtained with the ionization method (left) and the mass method (right). |
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Figure 9:
Cross section limits for the DY-produced $ \tau^\prime $ fermions with $ |Q| = $ 1$e $ for the ionization method (left) and the mass method (right). |
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Figure 9-a:
Cross section limits for the DY-produced $ \tau^\prime $ fermions with $ |Q| = $ 1$e $ for the ionization method (left) and the mass method (right). |
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Figure 9-b:
Cross section limits for the DY-produced $ \tau^\prime $ fermions with $ |Q| = $ 1$e $ for the ionization method (left) and the mass method (right). |
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Figure 10:
Cross section limits for the DY-produced $ \tau^\prime $ fermions with $ |Q| = $ 2$e $ for the ionization method (left) and the mass method (right). |
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Figure 10-a:
Cross section limits for the DY-produced $ \tau^\prime $ fermions with $ |Q| = $ 2$e $ for the ionization method (left) and the mass method (right). |
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Figure 10-b:
Cross section limits for the DY-produced $ \tau^\prime $ fermions with $ |Q| = $ 2$e $ for the ionization method (left) and the mass method (right). |
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Figure 11:
2D exclusion showing the observed cross section limit as a function of the masses of the $ \tau^\prime $ (on the $ x $-axis) and of the Z' boson (on the $ y $-axis), for the ionization method on the left and for the mass method on the right. The black shaded region corresponds to the area that is compatible with the ATLAS excess from Ref. [35] and the black star corresponds to the best fit of the ATLAS excess with this model. |
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Figure 11-a:
2D exclusion showing the observed cross section limit as a function of the masses of the $ \tau^\prime $ (on the $ x $-axis) and of the Z' boson (on the $ y $-axis), for the ionization method on the left and for the mass method on the right. The black shaded region corresponds to the area that is compatible with the ATLAS excess from Ref. [35] and the black star corresponds to the best fit of the ATLAS excess with this model. |
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Figure 11-b:
2D exclusion showing the observed cross section limit as a function of the masses of the $ \tau^\prime $ (on the $ x $-axis) and of the Z' boson (on the $ y $-axis), for the ionization method on the left and for the mass method on the right. The black shaded region corresponds to the area that is compatible with the ATLAS excess from Ref. [35] and the black star corresponds to the best fit of the ATLAS excess with this model. |
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Figure 12:
Cross section limits for the Z'$_\psi$ model with a fixed $ \tau^\prime $ mass of 600 GeV, for the ionization method (left) and the mass method (right). |
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Figure 12-a:
Cross section limits for the Z'$_\psi$ model with a fixed $ \tau^\prime $ mass of 600 GeV, for the ionization method (left) and the mass method (right). |
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Figure 12-b:
Cross section limits for the Z'$_\psi$ model with a fixed $ \tau^\prime $ mass of 600 GeV, for the ionization method (left) and the mass method (right). |
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Figure 13:
Cross section limits for the Z'$_{\text{SSM}} $ model with a fixed $ \tau^\prime $ mass of 600 GeV, for the ionization method (left) and the mass method (right). |
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Figure 13-a:
Cross section limits for the Z'$_{\text{SSM}} $ model with a fixed $ \tau^\prime $ mass of 600 GeV, for the ionization method (left) and the mass method (right). |
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Figure 13-b:
Cross section limits for the Z'$_{\text{SSM}} $ model with a fixed $ \tau^\prime $ mass of 600 GeV, for the ionization method (left) and the mass method (right). |
| Tables | |
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Table 1:
Summary of K and C values for 2017 and 2018 data and Monte Carlo. |
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Table 2:
Cumulative selection efficiency for the data and for two signal hypotheses. |
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Table 3:
Expected and observed mass limits obtained using 2017-2018 data for various HSCP candidate models, for the two background estimate methods. |
| Summary |
| A dedicated search for heavy long-lived charged particles produced in proton-proton collisions at $ \sqrt{s} = $ 13 TeV using the CMS detector was presented. Since the experimental signature is a highly ionizing isolated track of high $ p_{\mathrm{T}} $, the analysis is based on anomalous $ \text{d}E/\text{d}x $ deposits in the silicon tracker. The data, corresponding to an integrated luminosity of 101 fb$ ^{-1} $, are compatible with the background expectations, performed by two different estimation methods. The ionization method is based on the newly introduced ionization variable $ F_{\text{i}}^{\text{Pixels}} $ using only the pixel information and on the $ G_{\text{i}}^{\text{Strips}} $ discriminant using the charges collected in the silicon strip detector. The mass method is based on the reconstruction of mass using the ionization variable $ I_{\text{h}} $, providing an estimate of the most probable value of $ \text{d}E/\text{d}x $ of the track, and the momentum of the candidate. The two methods lead to similar sensitivity. This note presents cross section limits interpreted in the context of several models predicting the pair production of gluinos, top squark R-Hadrons, tau sleptons, and $ \tau^\prime $ leptons with an electric charge equal to 1$ e $ or 2$ e $. Gluino R-hadrons are excluded at 95% CL with a mass up to 2.13 TeV, top squarks R-hadrons up to 1.52 TeV, pair produced $ \widetilde{\tau}_{L/R} $ up to 0.69 TeV, and $ \tau^\prime $ fermions with an electric charge equal to 1$ e $ (2$ e $) up to 1.20 TeV (1.47 TeV). The model predicting a pair of doubly-charged $ \tau^\prime $ fermions from the Z' boson decay motivated by an excess reported by the ATLAS Collaboration [34] is directly explored, and no significant deviation from the SM is observed. Cross section limits are extracted as a function of the Z' and $ \tau^\prime $ masses. For a $ \tau^\prime $ mass of 600 GeV, Z'$_\psi$ (Z'$_{\text{SSM}} $) bosons are excluded at 95% CL with a mass up to 4.22 TeV (4.76 TeV). |
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