CMS-FSQ-12-004 ; CERN-EP-2016-261 | ||
Exclusive and semi-exclusive $\pi^{+}\pi^{-}$ production in proton-proton collisions at $\sqrt{s} = $ 7 TeV | ||
CMS Collaboration | ||
25 June 2017 | ||
Superseded by FSQ-16-006 | ||
Abstract: A measurement is presented of the exclusive and semi-exclusive production of charged pion pairs in proton-proton collisions, $\mathrm{ p p }\to\mathrm{ p }({\mathrm{p}^*})+ \pi^{+}\pi^{-}+ \mathrm{ p } ({\mathrm{p}^*})$, where the $\pi^{+} \pi^{-}$ pair is emitted at central rapidities, and the scattered protons stay intact ($\mathrm{p}$) or diffractively dissociate (${\mathrm{p}^*} $) without detection. The measurement is performed with the CMS detector at the LHC, using a data sample corresponding to an integrated luminosity of 450 $\mu$b$^{-1}$ collected at a center-of-mass energy of 7 TeV. The dipion cross section, measured for single-pion transverse momentum $p_{\mathrm{T}} > $ 0.2 GeV/$c$ and rapidity $|{y}| < $ 2, is 26.5 $\pm$ 0.3 (stat) $\pm$ 5.0 (syst) $\pm$ 1.1 (lumi) $\mu$b. The differential cross sections measured as a function of the invariant mass, $p_{\mathrm{T}}$, and $y$ of the pion pair, and as a function of single-pion $p_{\mathrm{T}}$, are compared to phenomenological predictions. | ||
Links: e-print arXiv:1706.08310 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Representative diagrams for (semi)exclusive central $\pi^{+} \pi^{-} $ production in proton-proton collisions. (left) Double pomeron exchange continuum, and (right) photon-pomeron interaction with production of a $\rho^+(770)$ meson that subsequently decays into a pair of pions. |
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Figure 1-a:
Representative diagram for (semi)exclusive central $\pi^{+} \pi^{-} $ production in proton-proton collisions: double pomeron exchange continuum. |
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Figure 1-b:
Representative diagram for (semi)exclusive central $\pi^{+} \pi^{-} $ production in proton-proton collisions: photon-pomeron interaction with production of a $\rho^+(770)$ meson that subsequently decays into a pair of pions. |
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Figure 2:
Distribution of the multiplicities of ECAL+HCAL+HF towers above noise thresholds, $N_\text {extra}$, in events with two opposite-sign (solid circles) and same-sign (open circles) tracks. The negative binomial distributions used to reproduce the backgrounds in OS (solid curve) and SS (dashed curve) events are shown in their fitting range (thick lines), as well as their extrapolation below that range (thinner lines). |
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Figure 3:
Detector-level distributions for the 5402 signal events with $N_\text {extra} = $ 0 (filled circles), compared with the background estimated from control regions in data (open circles), as explained in the text. The pion pair a) invariant mass, b) ${p_{\mathrm {T}}} $, c) rapidity, and d) single-pion ${p_{\mathrm {T}}}$ distributions are shown. The vertical error bars indicate the statistical uncertainty and the shaded band indicates the uncertainty in the estimate of the background when the size of the control region is changed. |
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Figure 3-a:
The pion pair invariant mass distribution at detector level for the 5402 signal events with $N_\text {extra} = $ 0 (filled circles), compared with the background estimated from control regions in data (open circles), as explained in the text. The vertical error bars indicate the statistical uncertainty and the shaded band indicates the uncertainty in the estimate of the background when the size of the control region is changed. |
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Figure 3-b:
The pion pair ${p_{\mathrm {T}}} $ distribution at detector level for the 5402 signal events with $N_\text {extra} = $ 0 (filled circles), compared with the background estimated from control regions in data (open circles), as explained in the text. The vertical error bars indicate the statistical uncertainty and the shaded band indicates the uncertainty in the estimate of the background when the size of the control region is changed. |
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Figure 3-c:
The pion pair rapidity distribution at detector level for the 5402 signal events with $N_\text {extra} = $ 0 (filled circles), compared with the background estimated from control regions in data (open circles), as explained in the text. The vertical error bars indicate the statistical uncertainty and the shaded band indicates the uncertainty in the estimate of the background when the size of the control region is changed. |
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Figure 3-d:
The single-pion ${p_{\mathrm {T}}}$ distribution at detector level for the 5402 signal events with $N_\text {extra} = $ 0 (filled circles), compared with the background estimated from control regions in data (open circles), as explained in the text. The vertical error bars indicate the statistical uncertainty and the shaded band indicates the uncertainty in the estimate of the background when the size of the control region is changed. |
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Figure 4:
Shape comparison of the detector-level mass distribution for opposite-sign pairs with $N_\text {extra}= $ 1 for the estimated signal (hatched histogram) and background (shaded histogram) contributions. The background is estimated from events with 2 $ \leq N_\text {extra} \leq $ 10, as explained in the text. The signal shape is extracted from the distribution measured in events with $N_\text {extra}= $ 0. Both distributions are scaled to their predicted normalization according to Fig. 2. |
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Figure 5:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) } + (\pi^{+} \pi^{-}) + {\mathrm{ p } }({ {\mathrm {p}^*} })$ as a function of the pion pair invariant mass, compared to the predictions from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve). The results are also compared to PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the vertical error bars indicate the statistical uncertainty. The results are plotted on (a) linear and (b) logarithmic scales. |
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Figure 5-a:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) } + (\pi^{+} \pi^{-}) + {\mathrm{ p } }({ {\mathrm {p}^*} })$ as a function of the pion pair invariant mass, compared to the predictions from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve). The results are also compared to PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the vertical error bars indicate the statistical uncertainty. The results are plotted on linear scale. |
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Figure 5-b:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) } + (\pi^{+} \pi^{-}) + {\mathrm{ p } }({ {\mathrm {p}^*} })$ as a function of the pion pair invariant mass, compared to the predictions from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve). The results are also compared to PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the vertical error bars indicate the statistical uncertainty. The results are plotted on logarithmic scale. |
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Figure 6:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown are the pion pair ${p_{\mathrm {T}}}$ (a, b) and rapidity (c, d), and single-pion ${p_{\mathrm {T}}}$ (e, f). The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a linear (left) and a logarithmic (right) scale. |
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Figure 6-a:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown is the pion pair ${p_{\mathrm {T}}}$. The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a linear scale. |
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Figure 6-b:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown is the pion pair ${p_{\mathrm {T}}}$. The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a logarithmic scale. |
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Figure 6-c:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown is the pion pair rapidity. The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a linear scale. |
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Figure 6-d:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown is the pion pair rapidity. The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a logarithmic scale. |
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Figure 6-e:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown is the single-pion ${p_{\mathrm {T}}}$. The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a linear scale. |
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Figure 6-f:
Differential cross sections for ${\mathrm{ p } \mathrm{ p } \to {\mathrm{ p } }({ {\mathrm {p}^*} }) }+ \pi^{+} \pi^{-} + {\mathrm{ p } }({ {\mathrm {p}^*} })$, compared to the predictions of DPE production from dime (solid and dashed curves), added to $\rho $ photoproduction from STARlight (long dashed curve), and of PYTHIA 8 MBR (open squares). Shown is the single-pion ${p_{\mathrm {T}}}$. The data are also compared to the PYTHIA 8 MBR (open squares). The shaded band shows the overall systematic uncertainty, and the error bar indicates the statistical uncertainties. The results are plotted on a logarithmic scale. |
Tables | |
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Table 1:
Number of events remaining after each step of the analysis. |
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Table 2:
Predictions for the DPE and $\rho $-photoproduction cross sections from Monte Carlo simulations. Results are given for the full cross section and for the fiducial cross section defined by exactly two oppositely charged pions with $ {p_{\mathrm {T}}} > $ 0.2 GeV/$c$ and $ {| y | }< $ 2. |
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Table 3:
Summary of systematic uncertainties in the exclusive dipion cross sections (for a single-pion phase space defined by $ {p_{\mathrm {T}}} > $ 0.2 GeV/$c$ and $ {| y | } < $ 2). |
Summary |
Cross sections for pion pair production in the reaction ${\mathrm{ p }\mathrm{ p } \to {\mathrm{ p }}({{\mathrm{p}^*} }) }+ \pi^{+}\pi^{-}+ {\mathrm{ p }}({{\mathrm{p}^*} })$, where the undetected protons stay intact (exclusive production) or dissociate into low-mass states (semi-exclusive production), have been measured in proton-proton collisions at $\sqrt{s} = $ 7 TeV with the CMS detector using data corresponding to an integrated luminosity of 450$\mu$b$^{-1}$. By selecting events with exactly two oppositely-charged central particles, the multiplicity distribution of additional calorimeter towers $N_\text{extra}$ shows an excess for 0 or 1 towers relative to a negative binomial distribution that reproduces the inclusive dipion production with $N_\text{extra} > $ 1. This excess is attributed to exclusive and semi-exclusive production of $\pi^{+}\pi^{-}$. The results are compared to phenomenological predictions for (semi)exclusive dipion cross sections from double pomeron exchange (as modeled in PYTHIA8 and DIME) and from $ \rho $-meson photoproduction (as modeled in STARlight). The exclusive and semi-exclusive dipion cross section, for individual pions with $p_{\mathrm{T}}> $ 0.2 GeV/$c$ and $|{y} | < $ 2 and no additional particles produced within $|{\eta} < $ 4.9, is 26.5 $\pm$ 0.3 (stat) $\pm$ 5.0 (syst) $\pm$ 1.1 (lumi) $\mu$b, which is 50% larger than that predicted by the PYTHIA8 (MBR and 4C tune) and DIME models. Such a result is expected as none of the models include the contributions from low-mass proton dissociation, nor the production of specific dipion resonances, which would increase the visible cross section. The $\pi^{+}\pi^{-}$ differential cross sections as a function of the pion pair invariant mass, $p_{\mathrm{T}}$, and $y$ have also been compared to model predictions. The measured $p_{\mathrm{T}}(\pi\pi)$ distribution shows a larger average $p_{\mathrm{T}}$ and a higher tail above $p_{\mathrm{T}}> $ 0.5 GeV/$c$ than predicted by the models, suggesting the presence in the data of a significant contribution from semi-exclusive $\pi^{+}\pi^{-}$ production with proton dissociation. The invariant mass spectrum for dipions shows various resonant peaks (including a possible contribution from $ \rho $ mesons produced in $\gamma$p photoproduction processes) and dips, similar to those observed in lower-energy ${\mathrm{ p }}\mathrm{ \bar{p} }$ and $\mathrm{pp}$ collisions. This is the first measurement at the LHC of exclusive and semi-exclusive production of pion pairs from the nonresonant continuum, and from possible decays of various low-mass meson resonances. The understanding of the data requires the improvement of phenomenological double pomeron exchange models to consistently include continuum and resonant processes, and their interference, as well as similar contributions with proton dissociation. |
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Compact Muon Solenoid LHC, CERN |