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| CMS-FSQ-16-006 ; CERN-EP-2020-005 | ||
| Study of central exclusive $\pi^{+}\pi^{-}$ production in proton-proton collisions at $\sqrt{s} = $ 5.02 and 13 TeV | ||
| CMS Collaboration | ||
| 5 March 2020 | ||
| Eur. Phys. J. C 80 (2020) 718 | ||
| Abstract: Central exclusive production of $\pi^{+}\pi^{-}$ pairs is measured with the CMS detector in proton-proton collisions at the LHC at center-of-mass energies of 5.02 and 13 TeV. The theoretical description of these nonperturbative processes, which have not yet been measured in detail at the LHC, poses a significant challenge to models. The two pions are measured and identified in the CMS silicon tracker based on specific energy loss, whereas the absence of other particles is ensured by calorimeter information. The total and differential cross sections of exclusive central $\pi^{+}\pi^{-}$ production are measured as functions of invariant mass, transverse momentum, and rapidity of the $\pi^{+}\pi^{-}$ system in the fiducial region defined as transverse momentum ${p_{\mathrm{T}}}(\pi) > $ 0.2 GeV and pseudorapidity $|{\eta(\pi)}| < $ 2.4. The production cross sections for the four resonant channels $\mathrm{f}_{0}(500)$, $\rho^{0}(770)$, $\mathrm{f}_{0}(980)$, and $\mathrm{f}_{2}(1270)$ are extracted using a simple model. These results represent the first measurement of this process at the LHC collision energies of 5.02 and 13 TeV. | ||
| Links: e-print arXiv:2003.02811 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Diagrams of the dominant mechanisms for $\pi^{+} \pi^{-} $ production via CEP in proton-proton collisions: (a) continuum; (b) resonant double pomeron exchange; and (c) vector meson photoproduction. |
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Figure 2:
Left: The distribution of the logarithm of the mean energy loss and absolute value of the momentum of tracks from low-multiplicity ($N_\text {track} \leq $ 4) events collected at $\sqrt {s} = $ 13 TeV. The $\pi $-selection region is shown in the 0.3-10 GeV range. All tracks outside this momentum range are identified as pions. Right: The fit energy loss distributions in a given momentum bin with the sum of three Gaussian curves. Plots are similar for the 5.02 TeV data. |
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Figure 2-a:
The distribution of the logarithm of the mean energy loss and absolute value of the momentum of tracks from low-multiplicity ($N_\text {track} \leq $ 4) events collected at $\sqrt {s} = $ 13 TeV. The $\pi $-selection region is shown in the 0.3-10 GeV range. All tracks outside this momentum range are identified as pions. |
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Figure 2-b:
The fit energy loss distributions in a given momentum bin with the sum of three Gaussian curves. |
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Figure 3:
The number of extra calorimeter towers over threshold in events containing an identified pion pair with opposite (left) and same (right) charge. The known contributions, denoted with the red hatched areas, are used to estimate the background in the zero bin of the opposite-sign distribution, which is denoted by the blue hatched area. The error bars correspond to statistical uncertainties, whereas the error rectangle on the background denotes the 14% systematic uncertainty in the background normalization. Plots are similar for 5.02 TeV data. |
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Figure 3-a:
The number of extra calorimeter towers over threshold in events containing an identified pion pair with opposite charge. The error bars correspond to statistical uncertainties, whereas the error rectangle on the background denotes the 14% systematic uncertainty in the background normalization. |
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Figure 3-b:
The number of extra calorimeter towers over threshold in events containing an identified pion pair with same charge. The error bars correspond to statistical uncertainties, whereas the error rectangle on the background denotes the 14% systematic uncertainty in the background normalization. |
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Figure 4:
Background distributions as functions of kinematic variables estimated by data-driven methods. The proton dissociation background is not shown here, since it is included via scaling of the final cross section values. The error bars correspond to statistical uncertainties. The results for the 5.02 TeV data set are similar. |
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Figure 4-a:
Background distribution as a function of $m(\pi^{+}\pi^{-})$ estimated by data-driven methods. The proton dissociation background is not shown here, since it is included via scaling of the final cross section values. The error bars correspond to statistical uncertainties. |
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Figure 4-b:
Background distribution as a function of $p_{\mathrm{T}}(\pi^{+}\pi^{-})$ estimated by data-driven methods. The proton dissociation background is not shown here, since it is included via scaling of the final cross section values. The error bars correspond to statistical uncertainties. |
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Figure 4-c:
Background distribution as a function of $y(\pi^{+}\pi^{-})$ estimated by data-driven methods. The proton dissociation background is not shown here, since it is included via scaling of the final cross section values. The error bars correspond to statistical uncertainties. |
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Figure 5:
Differential cross sections as functions of mass (upper row), transverse momentum (middle row), and rapidity (bottom row), compared with generator-level simulations for the 5.02 (left) and 13 TeV (right) data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 5-a:
Differential cross sections as functions of mass, compared with generator-level simulations for the 5.02 TeV data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 5-b:
Differential cross sections as functions of mass, compared with generator-level simulations for the 13 TeV data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 5-c:
Differential cross sections as functions of transverse momentum, compared with generator-level simulations for the 5.02 TeV data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 5-d:
Differential cross sections as functions of transverse momentum, compared with generator-level simulations for the 13 TeV data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 5-e:
Differential cross sections as functions of rapidity, compared with generator-level simulations for the 5.02 TeV data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 5-f:
Differential cross sections as functions of rapidity, compared with generator-level simulations for the 13 TeV data sets. The error bars correspond to statistical, whereas the open boxes to systematic uncertainties. |
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Figure 6:
Fit to the measured cross section with the sum of four interfering relativistic Breit-Wigner functions convolved with a normal distribution (to account for the the experimental resolution of the detector) for the 5.02 (left) and 13 TeV (right) data sets. The error bars correspond to statistical, whereas the open boxes correspond to systematic uncertainties. |
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Figure 6-a:
Fit to the measured cross section with the sum of four interfering relativistic Breit-Wigner functions convolved with a normal distribution (to account for the the experimental resolution of the detector) for the 5.02 TeV data set. The error bars correspond to statistical, whereas the open boxes correspond to systematic uncertainties. |
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Figure 6-b:
Fit to the measured cross section with the sum of four interfering relativistic Breit-Wigner functions convolved with a normal distribution (to account for the the experimental resolution of the detector) for the 13 TeV data set. The error bars correspond to statistical, whereas the open boxes correspond to systematic uncertainties. |
| Tables | |
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Table 1:
The value of calorimeter thresholds for different calorimeter constituents, used in the selection of exclusive events. |
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Table 2:
Correction factors. |
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Table 3:
Checking the validity of Eq. (9) by comparing the true and predicted number of background events in inclusive MC samples. |
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Table 4:
The semiexclusive/exclusive ratio $R$ calculated from different MC event generators. The average $R$ value is also shown together with its systematic uncertainty. |
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Table 5:
The sources and average values of systematic uncertainties, used as the systematic uncertainty of the total cross section. |
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Table 6:
Cross sections of the resonant processes in the $ {p_{\mathrm {T}}} (\pi) > $ 0.2 GeV, $ {| \eta (\pi) |} < $ 2.4 fiducial region, extracted from the simple model fit using the sum of the continuum distribution obtained from the DIME MC model and four dominant resonances. The STARLIGHT predictions for ${\mathrm{p}} {\mathrm{p}} \to {\mathrm{p}} '{\mathrm{p}} '\rho^{0} \to {\mathrm{p}} '{\mathrm{p}} ' \pi^{+} \pi^{-} $ processes are 2.3 and 3.0 $\mu$b for 5.02 and 13 TeV, respectively, which is compatible with the fit results. |
| Summary |
| The cross sections for central exclusive pion pair production have been measured in ${\mathrm{p}}{\mathrm{p}}$ collisions at 5.02 and 13 TeV center-of-mass energies. Exclusive events are selected by vetoing additional energy deposits in the calorimeters and by requiring two oppositely charged pions identified via their mean energy loss in the tracker detectors. These events are used together with correction factors to obtain invariant mass, transverse momentum, and rapidity distributions of the $\pi^{+}\pi^{-}$ system. Four resonant peaks can be identified in the mass spectrum: $\mathrm{f}_{0}(500)$, $\rho^{0}(770)$, $\mathrm{f}_{0}(980)$, and $\mathrm{f}_{2}(1270)$, which are fitted with a simple model containing four interfering Breit-Wigner functions and a continuum contribution modeled by the DIME MC. There is an indication that the DIME MC model overestimates the high invariant mass region of the exclusive pion pair. The measured total exclusive $\pi^{+}\pi^{-}$ production cross section is 19.6 $\pm$ 0.4 (stat) $\pm$ 3.5 (syst) $\pm$ 0.01 (lumi) and 19.0 $\pm$ 0.6 (stat) $\pm$ 3.4 (syst) $\pm$ 0.01 (lumi) $\mu$b for 5.02 and 13 TeV, respectively. The exclusive production cross sections for the resonances are obtained assuming the most important resonances in the invariant mass spectrum are described by Breit-Wigner resonances interfering with a continuum contribution. The high-mass parts of the spectra are overestimated, which can be attributed to the DIME MC mismodeling of the continuum shape and further resonances, which might influence spectrum shape via interference effects. The obtained cross sections of $\rho^{0}(770)$ production are consistent with the STARLIGHT model prediction. |
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