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| CMS-PAS-EXO-19-009 | ||
| A search for new physics in central exclusive production using the missing mass technique with the CMS-TOTEM precision proton spectrometer | ||
| CMS Collaboration | ||
| March 2022 | ||
| Abstract: A generic search is presented for associated production of a Z boson or a photon with an additional unspecified massive particle X in proton-tagged events from pp collisions at $\sqrt{s} = $ 13 TeV, recorded in 2017 by the CMS detector and the CMS-TOTEM precision proton spectrometer. The so-called missing mass spectrum is analysed in the 600-1600 GeV range and a fit is performed to search for possible deviations from the background expectation. Model-independent upper limits on the visible production cross section of pp $\rightarrow$ pp $+$ Z/$\gamma$ $+$ X are set, in the absence of significant resonant deviations in data with respect to the background predictions. | ||
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These preliminary results are superseded in this paper, Submitted to EPJC. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Schematic diagram of two-photon production of a Z boson or photon with an additional, unknown particle $\chi $ giving rise to a missing mass $m_{\rm miss} = m_\chi $. The production mechanism does not have to proceed through photon exchange. Other colourless exchange mechanisms (e.g. double pomeron) are also allowed. However, at such a high energy scale, electroweak processes become dominant as QCD-based colourless exchanges are expected to be suppressed. |
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Figure 2:
Schematic drawing of the CT-PPS detectors used in this analysis. The left (right) arm in this plot is located on the positive (negative) side of the $z$ axis in the CMS coordinate system. |
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Figure 3:
Comparison of the missing mass shapes for the simulated signal events within the fiducial region and those outside it, after the mixing of pileup protons and selection of the protons, for a generated $m_X$ mass of 1000 GeV. From left to right the distributions are shown for the different proton reconstruction categories: multi(+)-multi(-), multi(+)-single(-), single(+)-multi(-) and single(+)-single(-). |
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Figure 3-a:
Comparison of the missing mass shapes for the simulated signal events within the fiducial region and those outside it, after the mixing of pileup protons and selection of the protons, for a generated $m_X$ mass of 1000 GeV. From left to right the distributions are shown for the different proton reconstruction categories: multi(+)-multi(-), multi(+)-single(-), single(+)-multi(-) and single(+)-single(-). |
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Figure 3-b:
Comparison of the missing mass shapes for the simulated signal events within the fiducial region and those outside it, after the mixing of pileup protons and selection of the protons, for a generated $m_X$ mass of 1000 GeV. From left to right the distributions are shown for the different proton reconstruction categories: multi(+)-multi(-), multi(+)-single(-), single(+)-multi(-) and single(+)-single(-). |
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Figure 3-c:
Comparison of the missing mass shapes for the simulated signal events within the fiducial region and those outside it, after the mixing of pileup protons and selection of the protons, for a generated $m_X$ mass of 1000 GeV. From left to right the distributions are shown for the different proton reconstruction categories: multi(+)-multi(-), multi(+)-single(-), single(+)-multi(-) and single(+)-single(-). |
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Figure 3-d:
Comparison of the missing mass shapes for the simulated signal events within the fiducial region and those outside it, after the mixing of pileup protons and selection of the protons, for a generated $m_X$ mass of 1000 GeV. From left to right the distributions are shown for the different proton reconstruction categories: multi(+)-multi(-), multi(+)-single(-), single(+)-multi(-) and single(+)-single(-). |
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Figure 4:
Product of the acceptance times the combined reconstruction and identification efficiency, as a function of $m_X$, for events generated inside the fiducial volume defined in Tab.1. The curve shown on the left displays the different final states, while the one on the right shows the contributions from the different proton-reconstruction categories to the Z $ \rightarrow \mu \mu $ analysis. The definition of the fiducial region and signal model used to estimate the acceptance is provided in the text. |
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Figure 4-a:
Product of the acceptance times the combined reconstruction and identification efficiency, as a function of $m_X$, for events generated inside the fiducial volume defined in Tab.1. The curve shown on the left displays the different final states, while the one on the right shows the contributions from the different proton-reconstruction categories to the Z $ \rightarrow \mu \mu $ analysis. The definition of the fiducial region and signal model used to estimate the acceptance is provided in the text. |
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Figure 4-b:
Product of the acceptance times the combined reconstruction and identification efficiency, as a function of $m_X$, for events generated inside the fiducial volume defined in Tab.1. The curve shown on the left displays the different final states, while the one on the right shows the contributions from the different proton-reconstruction categories to the Z $ \rightarrow \mu \mu $ analysis. The definition of the fiducial region and signal model used to estimate the acceptance is provided in the text. |
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Figure 5:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-a:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-b:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-c:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-d:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-e:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-f:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-g:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-h:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 5-i:
Modeling of the reconstructed proton $\xi $ in the negative arm (left), positive arm (middle), and the corresponding di-proton rapidity (right) from the proton mixing procedure, using events selected in MC simulation, compared to data. From top to bottom the ee, $\mu \mu $, and photon final states are shown. The ee and $\mu \mu $ events are displayed without Z boson ${p_{\mathrm {T}}}$ requirement. For illustration the simulated signal distributions are superimposed for various choices of $m_X$, normalised to a generated fiducial cross section of 100 pb. |
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Figure 6:
Validation of the background modeling method, using the e$\mu $ control sample. Selected e$\mu $ events are mixed with protons from Z $ \rightarrow \mu \mu $ events with $ {p_{\mathrm {T}}} (\mathrm{Z}) < $ 10 GeV to simulate the combinatorial background shape, while the data points are unaltered e$\mu $ events. The proton $\xi $ distributions for both CT-PPS arms (top), those of the di-proton invariant mass (bottom left), and of ${m_{\rm miss}}$ (bottom right) are shown. The uncertainty band on the background prediction represents the contribution from the limited statistics and the alternative mixing approaches described in the text. |
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Figure 6-a:
Validation of the background modeling method, using the e$\mu $ control sample. Selected e$\mu $ events are mixed with protons from Z $ \rightarrow \mu \mu $ events with $ {p_{\mathrm {T}}} (\mathrm{Z}) < $ 10 GeV to simulate the combinatorial background shape, while the data points are unaltered e$\mu $ events. The proton $\xi $ distributions for both CT-PPS arms (top), those of the di-proton invariant mass (bottom left), and of ${m_{\rm miss}}$ (bottom right) are shown. The uncertainty band on the background prediction represents the contribution from the limited statistics and the alternative mixing approaches described in the text. |
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Figure 6-b:
Validation of the background modeling method, using the e$\mu $ control sample. Selected e$\mu $ events are mixed with protons from Z $ \rightarrow \mu \mu $ events with $ {p_{\mathrm {T}}} (\mathrm{Z}) < $ 10 GeV to simulate the combinatorial background shape, while the data points are unaltered e$\mu $ events. The proton $\xi $ distributions for both CT-PPS arms (top), those of the di-proton invariant mass (bottom left), and of ${m_{\rm miss}}$ (bottom right) are shown. The uncertainty band on the background prediction represents the contribution from the limited statistics and the alternative mixing approaches described in the text. |
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Figure 6-c:
Validation of the background modeling method, using the e$\mu $ control sample. Selected e$\mu $ events are mixed with protons from Z $ \rightarrow \mu \mu $ events with $ {p_{\mathrm {T}}} (\mathrm{Z}) < $ 10 GeV to simulate the combinatorial background shape, while the data points are unaltered e$\mu $ events. The proton $\xi $ distributions for both CT-PPS arms (top), those of the di-proton invariant mass (bottom left), and of ${m_{\rm miss}}$ (bottom right) are shown. The uncertainty band on the background prediction represents the contribution from the limited statistics and the alternative mixing approaches described in the text. |
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Figure 6-d:
Validation of the background modeling method, using the e$\mu $ control sample. Selected e$\mu $ events are mixed with protons from Z $ \rightarrow \mu \mu $ events with $ {p_{\mathrm {T}}} (\mathrm{Z}) < $ 10 GeV to simulate the combinatorial background shape, while the data points are unaltered e$\mu $ events. The proton $\xi $ distributions for both CT-PPS arms (top), those of the di-proton invariant mass (bottom left), and of ${m_{\rm miss}}$ (bottom right) are shown. The uncertainty band on the background prediction represents the contribution from the limited statistics and the alternative mixing approaches described in the text. |
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Figure 7:
${m_{\rm miss}}$ distributions in the Z $ \rightarrow \mu \mu $ (top), Z $ \rightarrow $ ee (middle), and $\gamma $ (bottom) final states. From the left to the right the distributions are shown in categories making use of protons reconstructed with the multi-multi, multi-single, single-multi and single-single methods, respectively. The background distributions are shown after the fit. The expectations for a signal with $m_X = $ 1000 GeV are superimposed, where in each channel the fiducial production cross section is normalised to 1 pb. |
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Figure 7-a:
${m_{\rm miss}}$ distributions in the Z $ \rightarrow \mu \mu $ (top), Z $ \rightarrow $ ee (middle), and $\gamma $ (bottom) final states. From the left to the right the distributions are shown in categories making use of protons reconstructed with the multi-multi, multi-single, single-multi and single-single methods, respectively. The background distributions are shown after the fit. The expectations for a signal with $m_X = $ 1000 GeV are superimposed, where in each channel the fiducial production cross section is normalised to 1 pb. |
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Figure 7-b:
${m_{\rm miss}}$ distributions in the Z $ \rightarrow \mu \mu $ (top), Z $ \rightarrow $ ee (middle), and $\gamma $ (bottom) final states. From the left to the right the distributions are shown in categories making use of protons reconstructed with the multi-multi, multi-single, single-multi and single-single methods, respectively. The background distributions are shown after the fit. The expectations for a signal with $m_X = $ 1000 GeV are superimposed, where in each channel the fiducial production cross section is normalised to 1 pb. |
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Figure 7-c:
${m_{\rm miss}}$ distributions in the Z $ \rightarrow \mu \mu $ (top), Z $ \rightarrow $ ee (middle), and $\gamma $ (bottom) final states. From the left to the right the distributions are shown in categories making use of protons reconstructed with the multi-multi, multi-single, single-multi and single-single methods, respectively. The background distributions are shown after the fit. The expectations for a signal with $m_X = $ 1000 GeV are superimposed, where in each channel the fiducial production cross section is normalised to 1 pb. |
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Figure 8:
Upper limits on the pp $\rightarrow$ pp Z/$\gamma$ X production cross section at the 95%, as function of $m_{X}$. The 95% and 68% CL quantiles of the expected limits are represented by the bands, while the observed limit is superimposed as a curve. The top plots correspond to the Z $ \rightarrow $ ee and Z $ \rightarrow \mu \mu $ final states, while the bottom plots correspond to the combined Z and $\gamma $ analyses. |
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Figure 8-a:
Upper limits on the pp $\rightarrow$ pp Z/$\gamma$ X production cross section at the 95%, as function of $m_{X}$. The 95% and 68% CL quantiles of the expected limits are represented by the bands, while the observed limit is superimposed as a curve. The top plots correspond to the Z $ \rightarrow $ ee and Z $ \rightarrow \mu \mu $ final states, while the bottom plots correspond to the combined Z and $\gamma $ analyses. |
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Figure 8-b:
Upper limits on the pp $\rightarrow$ pp Z/$\gamma$ X production cross section at the 95%, as function of $m_{X}$. The 95% and 68% CL quantiles of the expected limits are represented by the bands, while the observed limit is superimposed as a curve. The top plots correspond to the Z $ \rightarrow $ ee and Z $ \rightarrow \mu \mu $ final states, while the bottom plots correspond to the combined Z and $\gamma $ analyses. |
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Figure 8-c:
Upper limits on the pp $\rightarrow$ pp Z/$\gamma$ X production cross section at the 95%, as function of $m_{X}$. The 95% and 68% CL quantiles of the expected limits are represented by the bands, while the observed limit is superimposed as a curve. The top plots correspond to the Z $ \rightarrow $ ee and Z $ \rightarrow \mu \mu $ final states, while the bottom plots correspond to the combined Z and $\gamma $ analyses. |
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Figure 8-d:
Upper limits on the pp $\rightarrow$ pp Z/$\gamma$ X production cross section at the 95%, as function of $m_{X}$. The 95% and 68% CL quantiles of the expected limits are represented by the bands, while the observed limit is superimposed as a curve. The top plots correspond to the Z $ \rightarrow $ ee and Z $ \rightarrow \mu \mu $ final states, while the bottom plots correspond to the combined Z and $\gamma $ analyses. |
| Tables | |
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Table 1:
Combined CMS+Totem fiducial volume selection criteria in the Z and $\gamma $ analysis. |
| Summary |
| A search is presented for anomalous central exclusive Z/$\gamma$+X production using up to 37.2 fb$^{-1}$ of data recorded in 2017 by the CMS detector and the CMS-TOTEM precision proton spectrometer. A hypothetical X resonance is searched for in the mass region between 0.6 and 1.6 TeV, with cuts optimised for the best expected significance. Benefiting from the excellent mass resolution of 2% offered by the combination of the CMS central detector and the CT-PPS, for the first time at the LHC, the missing mass distribution is used to perform a model-independent search. Upper limits on the visible cross section of the pp $\rightarrow$ pp $+$ Z/$\gamma$ $+$ X process are set in a fiducial volume, using a generic model, in the absence of significant deviations in data with respect to the background predictions. With these results we demonstrate the feasibility of missing mass-based searches at the LHC. |
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