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| CMS-PPS-17-001 ; CERN-EP-2018-014 | ||
| Observation of proton-tagged, central (semi)exclusive production of high-mass lepton pairs in pp collisions at 13 TeV with the CMS-TOTEM precision proton spectrometer | ||
| CMS and TOTEM Collaborations | ||
| 13 March 2018 | ||
| JHEP 07 (2018) 153 | ||
| Abstract: The process ${\mathrm{p}}{\mathrm{p}} \to {\mathrm{p}} \ell^{+}\ell^{-} {\mathrm{p}}^{(*)}$, with $\ell^{+}\ell^{-}$ a muon or an electron pair produced at midrapidity with mass larger than 110 GeV, has been observed for the first time at the LHC in pp collisions at $\sqrt{s} = $ 13 TeV. One of the two scattered protons is measured in the CMS-TOTEM precision proton spectrometer (CT-PPS), which operated for the first time in 2016. The second proton either remains intact or is excited and then dissociates into a low-mass state ${\mathrm{p}}^{*}$, which is undetected. The measurement is based on an integrated luminosity of 9.4 fb$^{-1}$ collected during standard, high-luminosity LHC operation. A total of 12 $ \mu^{+} \mu^{-} $ and 8 $ \mathrm{e}^{+} \mathrm{e}^{-}$ pairs with $m(\ell^{+}\ell^{-}) > $ 110 GeV, and matching forward proton kinematics, are observed, with expected backgrounds of 1.49 $\pm$ 0.07 (stat) $\pm$ 0.53 (syst) and 2.36 $\pm$ 0.09 (stat) $\pm$ 0.47 (syst), respectively. This corresponds to an excess of more than five standard deviations over the expected background. The present result constitutes the first observation of proton-tagged $\gamma\gamma$ collisions at the electroweak scale. This measurement also demonstrates that CT-PPS performs according to the design specifications. | ||
| Links: e-print arXiv:1803.04496 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Production of lepton pairs by $ {\gamma} {\gamma} $ fusion. The exclusive (left), single proton dissociation or semiexclusive (middle), and double proton dissociation (right) topologies are shown. The left and middle processes result in at least one intact final-state proton, and are considered signal in this analysis. The rightmost diagram is considered to be a background process. |
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Figure 2:
Schematic layout (not to scale) of the beam line as seen from above between the interaction point (IP5) and the region where the RPs are located in LHC sector 56. Dipole magnets (D1, D2) of single- (MBXW) and twin-aperture, quadrupoles (Q1-Q6), collimators (TCL4-TCL6), absorbers (TAS, TAN), and quadrupole feedboxes (DFBX) are shown. The 210 near and 210 far units are indicated, along with the timing RPs not used here. The 220 near and 220 far units (not used here) are also shown. The RPs indicated in red are the horizontal CT-PPS ones; those in blue are part of the TOTEM experiment. The red (blue) arrow indicates the outgoing (incoming) beam. In the CMS coordinate system, the $z$ axis points to the left. The arm in the opposite LHC sector 45 (not shown) is symmetric with respect to the IP. |
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Figure 3:
Schematic layout of the silicon strip detectors in one RP station. Both the horizontal RP and the vertical RPs, which are used only for special low-luminosity calibration fills, are shown. In the top RP, the silicon strips oriented at $+45^{\circ}$ and $-45^{\circ}$ angles are indicated by the diagonal lines. Tracks in the overlap region, indicated by the shaded area, are used to perform a relative alignment of the RPs in the calibration fills. |
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Figure 4:
Distribution of the track impact points as a function of the horizontal coordinate for the alignment fill (black points), a physics fill before alignment (blue points), and after alignment (red points). The beam center is at $x = $ 0 for the black and red points; the $x$ axis origin is undefined for the blue points. In the alignment procedure the overall normalization of the histogram is irrelevant; the histograms from different fills are therefore rescaled to compare their shapes. |
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Figure 5:
Vertical effective length $L_{y}$ (in meters) as a function of the proton relative momentum loss $\xi $ at two (near and far) RPs calculated with the beam line optics simulation program MAD-X [22]. |
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Figure 6:
Distribution of the track impact points measured in RP 210F, in sector 45, for the alignment fill. The point where $L_{y} = $ 0 is shown with a cross. The beam center is at $x = y = $ 0. The edge of the distribution is slanted because the RP shown has a rotation of 8$^{\circ}$ with respect to the vertical. |
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Figure 7:
Example of track impact point distribution (in the $x, y$ plane) measured in RP 210F, sector 45, at 15$\sigma $ from the beam in the $x$ direction. The beam center is at $x = y = $ 0. The track selection includes a matching requirement with RP 210N, which suppresses noise and beam backgrounds, but slightly reduces the acceptance for low values of the position $x$, given the different acceptance of the near and far RPs. |
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Figure 8:
Dimuon (left) and dielectron (right) acoplanarity versus the distance between the closest extra track and the dilepton vertex for simulated signal and backgrounds. The points represent the Drell-Yan (red), exclusive $ {\gamma} {\gamma} \to \ell ^{+}\ell ^{-}$ (blue), single-dissociative $ {\gamma} {\gamma} \to \ell ^{+}\ell ^{-}$ (green), and double-dissociative $ {\gamma} {\gamma} \to \ell ^{+}\ell ^{-}$ (yellow) processes. The dashed lines indicate the region selected for the analysis. The number of points for each physics process does not reflect its cross section. |
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Figure 9:
Dimuon (left) and dielectron (right) invariant mass (top) and rapidity (bottom), after all central-detector criteria are applied, in pp collisions at 13 TeV. Points with error bars indicate the measured data (with statistical uncertainties only), and the stacked histograms show the different simulated contributions for signal and backgrounds (with statistical uncertainty of similar size as the data). The lower panel in each plot shows the ratio of the data to the sum of all signal and background predictions. |
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Figure 9-a:
Dimuon invariant mass, after all central-detector criteria are applied, in pp collisions at 13 TeV. Points with error bars indicate the measured data (with statistical uncertainties only), and the stacked histograms show the different simulated contributions for signal and backgrounds (with statistical uncertainty of similar size as the data). The lower panel shows the ratio of the data to the sum of all signal and background predictions. |
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Figure 9-b:
Dielectron invariant mass, after all central-detector criteria are applied, in pp collisions at 13 TeV. Points with error bars indicate the measured data (with statistical uncertainties only), and the stacked histograms show the different simulated contributions for signal and backgrounds (with statistical uncertainty of similar size as the data). The lower panel shows the ratio of the data to the sum of all signal and background predictions. |
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Figure 9-c:
Dimuon rapidity, after all central-detector criteria are applied, in pp collisions at 13 TeV. Points with error bars indicate the measured data (with statistical uncertainties only), and the stacked histograms show the different simulated contributions for signal and backgrounds (with statistical uncertainty of similar size as the data). The lower panel shows the ratio of the data to the sum of all signal and background predictions. |
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Figure 9-d:
Dielectron rapidity, after all central-detector criteria are applied, in pp collisions at 13 TeV. Points with error bars indicate the measured data (with statistical uncertainties only), and the stacked histograms show the different simulated contributions for signal and backgrounds (with statistical uncertainty of similar size as the data). The lower panel shows the ratio of the data to the sum of all signal and background predictions. |
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Figure 10:
Generator-level relative difference $(\xi (\ell ^{+}\ell ^{-}) - \xi ({\mathrm {p}}))/(\xi (\ell ^{+}\ell ^{-}))$ vs. $\xi (\ell ^{+}\ell ^{-})$ for simulated single dissociative $ {\gamma} {\gamma} \to \ell ^{+}\ell ^{-}$ events. Of the two possible solutions for $\xi (\ell ^{+}\ell ^{-})$, only the one corresponding to the side with the intact proton is shown. |
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Figure 11:
Correlation between the fractional values of the proton momentum loss measured in the central dilepton system, $\xi (\ell ^{+}\ell ^{-})$, and in the RPs, $\xi $(RP), for both RPs in each arm combined. The 45 (left) and 56 (right) arms are shown. The hatched region corresponds to the kinematical region outside the acceptance of both the near and far RPs, while the shaded (pale blue) region corresponds to the region outside the acceptance of the near RP. For the events in which a track is detected in both, the $\xi $ value measured at the near RP is plotted. The horizontal error bars indicate the uncertainty of $\xi $ (RP), and the vertical bars the uncertainty of $\xi (\ell ^{+}\ell ^{-})$. The events labeled "out of acceptance'' are those in which $\xi ({{{\mu ^{+}}} {{\mu ^{-}}}})$ corresponds to a signal proton outside the RP acceptance; in these events a background proton is detected with nonmatching kinematics. |
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Figure 11-a:
Correlation between the fractional values of the proton momentum loss measured in the central dilepton system, $\xi (\ell ^{+}\ell ^{-})$, and in the RPs, $\xi $(RP), for both RPs in each arm combined. The 45 arms are shown. The hatched region corresponds to the kinematical region outside the acceptance of both the near and far RPs, while the shaded (pale blue) region corresponds to the region outside the acceptance of the near RP. For the events in which a track is detected in both, the $\xi $ value measured at the near RP is plotted. The horizontal error bars indicate the uncertainty of $\xi $ (RP), and the vertical bars the uncertainty of $\xi (\ell ^{+}\ell ^{-})$. The events labeled "out of acceptance'' are those in which $\xi ({{{\mu ^{+}}} {{\mu ^{-}}}})$ corresponds to a signal proton outside the RP acceptance; in these events a background proton is detected with nonmatching kinematics. |
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Figure 11-b:
Correlation between the fractional values of the proton momentum loss measured in the central dilepton system, $\xi (\ell ^{+}\ell ^{-})$, and in the RPs, $\xi $(RP), for both RPs in each arm combined. The 56 arms are shown. The hatched region corresponds to the kinematical region outside the acceptance of both the near and far RPs, while the shaded (pale blue) region corresponds to the region outside the acceptance of the near RP. For the events in which a track is detected in both, the $\xi $ value measured at the near RP is plotted. The horizontal error bars indicate the uncertainty of $\xi $ (RP), and the vertical bars the uncertainty of $\xi (\ell ^{+}\ell ^{-})$. The events labeled "out of acceptance'' are those in which $\xi ({{{\mu ^{+}}} {{\mu ^{-}}}})$ corresponds to a signal proton outside the RP acceptance; in these events a background proton is detected with nonmatching kinematics. |
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Figure 12:
Expected acceptance regions in the rapidity vs. invariant mass plane overlaid with the observed dimuon (closed circles) and dielectron (open circles) signal candidate events. The "double-arm acceptance" refers to exclusive events, $ {\mathrm {p}} {\mathrm {p}}\to {\mathrm {p}}\ell ^{+}\ell ^{-} {\mathrm {p}}$. Following the CMS convention, the positive (negative) rapidity region corresponds to the 45 (56) LHC sector. |
| Tables | |
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Table 1:
Estimated backgrounds from Drell-Yan and double-dissociation ${{{\mu ^{+}}} {{\mu ^{-}}}}$ production, within the acceptance of at least one of the RPs of a given arm, and in the subsample with proton kinematics matching within 2$\sigma $. The bottom row indicates the total background from the sum of Drell-Yan and double dissociation events. |
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Table 2:
Estimated backgrounds from Drell-Yan and double-dissociation ${{\mathrm {e}^{+}} {\mathrm {e}^{-}}}$ production, within the acceptance of at least one of the RPs of a given arm, and in the subsample with proton kinematics matching within 2$\sigma $. The bottom row indicates the total background from the sum of Drell-Yan and double dissociation events. |
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Table 3:
Sources of systematic uncertainties in the estimates of Drell-Yan and double-dissociation backgrounds in the dimuon and dielectron channels. |
| Summary |
| We have studied $\gamma\gamma \to \mu^{+} \mu^{-}$ and $\gamma\gamma \to \mathrm{e}^{+} \mathrm{e}^{-}$ production together with forward protons reconstructed in the CMS-TOTEM precision proton spectrometer (CT-PPS), using a sample of 9.4 fb$^{-1}$ collected in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The Roman Pot alignment and LHC optics corrections have been determined using a high statistics sample of forward protons. A total of 12 $\gamma\gamma \to \mu^{+} \mu^{-}$ and 8 $\gamma\gamma \to \mathrm{e}^{+} \mathrm{e}^{-}$ events are observed with dilepton invariant mass larger than 110 GeV, and a forward proton with consistent kinematics. This corresponds to an excess larger than five standard deviations over the expected background from double-dissociative and Drell-Yan dilepton processes. The result represents the first observation of proton-tagged $\gamma\gamma$ collisions at the electroweak scale. The present data demonstrate the excellent performance of CT-PPS and its potential for high-mass exclusive (proton-tagged) measurements. With its 2016 operation, CT-PPS has proven for the first time the feasibility of continuously operating a near-beam proton spectrometer at a high-luminosity hadron collider. |
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