CMS-PAS-PPS-17-001

CMS-PAS-PPS-17-001
Evidence for proton-tagged, central semi-exclusive production of high-mass muon pairs at 13 TeV with the CMS-TOTEM Precision Proton Spectrometer
CMS Collaboration
May 2017

Abstract: The process $pp \rightarrow p \mu^+\mu^- p^{()}$ has been observed at the LHC for dimuon masses larger than 110 GeV in $pp$ collisions at $\sqrt{s}= $ 13 TeV. Here $p^{()}$ indicates that the second proton is undetected, and either remains intact or dissociates into a low-mass state $p^{*}$. The scattered proton has been measured in the CMS-TOTEM Precision Proton Spectrometer (CT-PPS), which operated for the first time in 2016. The measurement is based on an integrated luminosity of approximately 10 fb$^{-1}$ collected in regular, high-luminosity fills. A total of 12 candidates with $m(\mu\mu) > $ 110 GeV, and matching forward proton kinematics, is observed. This corresponds to an excess of more than four standard deviations over the background. The spectrometer and its operation are described, along with the data and background estimation. The present results constitute the first evidence of this process at such masses. They also demonstrate that CT-PPS performs as expected.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 07 (2018) 153. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Production of muon pairs by $\gamma \gamma $ fusion. The "elastic" (left), single proton dissociation (center), and double proton dissociation (right) topologies are shown. The left and center diagrams result in at least one intact final state proton, and are considered as signal in this analysis. The rightmost diagram is considered as a background.
png pdf	Figure 1-a: Production of muon pairs by $\gamma \gamma $ fusion in the double proton dissociation topology. This diagram is considered as a background.
png pdf	Figure 1-b: Production of muon pairs by $\gamma \gamma $ fusion. The "elastic" (left), single proton dissociation (center), and double proton dissociation (right) topologies are shown. The left and center diagrams result in at least one intact final state proton, and are considered as signal in this analysis. The rightmost diagram is considered as a background.
png pdf	Figure 1-c: Production of muon pairs by $\gamma \gamma $ fusion. The "elastic" (left), single proton dissociation (center), and double proton dissociation (right) topologies are shown. The left and center diagrams result in at least one intact final state proton, and are considered as signal in this analysis. The rightmost diagram is considered as a background.
png pdf	Figure 2: The layout of the beam line (seen from above) between the interaction point (IP5) and the region where the RPs are located in LHC sector 56. Dipoles (D1, D2), quadrupoles (Q1-Q6), collimators (TCL4-TCL6), as well as absorbers (TAS, TAN) are shown. The 210 near and 210 far units are indicated, along with the timing RPs. 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 belong to TOTEM. The red arrow indicates the outgoing beam, while the blue one the incoming beam. The arm in LHC sector 45 is symmetric with respect to the IP. The drawing is not to scale.
png pdf	Figure 3: Example of track impact point distribution measured in RP 210F, sector 45, at 15$\sigma $ from the beam. The beam center is at $x=y=$ 0.
png pdf	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.
png pdf	Figure 5: The vertical effective length $L_{y}$ as a function of the proton momentum loss $\xi $ at different RPs calculated with MAD-X [6].
png pdf	Figure 6: Distribution of the track impact points measured in RP 210F, in sector 45, in the alignment fill. The point where $L_{y}=0$ and its effect in the impact point distribution are shown. The beam center is at $x=y=0$.
png pdf	Figure 7: Dimuon acoplanarity vs the distance between the closest extra track and the dimuon vertex in signal and background simulation. The points represent the simulated Drell-Yan (red), elastic $\gamma \gamma \to \mu ^+\mu ^-$ (blue), single dissociative $\gamma \gamma \to \mu ^+\mu ^-$ (green), and double dissociative $\gamma \gamma \to \mu ^+\mu ^-$ (yellow) processes. The dashed lines indicate the region selected for the analysis.
png pdf	Figure 8: Dimuon invariant mass (above) and rapidity (below), after all central detector selection cuts. The lower panel in each plot shows the ratio of the data to the sum of all signal and background predictions.
png pdf	Figure 8-a: Dimuon invariant mass, after all central detector selection cuts. The lower panel shows the ratio of the data to the sum of all signal and background predictions.
png pdf	Figure 8-b: Rapidity, after all central detector selection cuts. The lower panel shows the ratio of the data to the sum of all signal and background predictions.
png pdf	Figure 9: Generator-level relative difference $\frac {\xi (\mu \mu ) - \xi (p)}{\xi (\mu \mu )}$ vs. $\xi (\mu \mu )$ for single dissociative $\gamma \gamma \rightarrow \mu ^{+}\mu ^{-}$ production. Of the two possible solutions for $\xi (\mu \mu )$, only the one corresponding to the side with the intact proton is shown.
png pdf	Figure 10: Correlation between $\xi (\mu \mu )$ and $\xi $ measured in the Roman Pots, for both Roman Pots in each arm combined. The 45 (left) and 56 (right) arms are shown. The light shaded region corresponds to the kinematical region outside the acceptance of both the near and far RPs, while the darker shaded 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.
png pdf	Figure 10-a: Correlation between $\xi (\mu \mu )$ and $\xi $ measured in the Roman Pots in sector 45, for both Roman Pots combined. The light shaded region corresponds to the kinematical region outside the acceptance of both the near and far RPs, while the darker shaded 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.
png pdf	Figure 10-b: Correlation between $\xi (\mu \mu )$ and $\xi $ measured in the Roman Pots in sector 56, for both Roman Pots combined. The light shaded region corresponds to the kinematical region outside the acceptance of both the near and far RPs, while the darker shaded 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.
png pdf	Figure 11: Expected coverage in the rapidity vs invariant mass plane overlaid with the observed dimuon signal candidate events. Following the CMS convention, the positive rapidity (negative rapidity) region corresponds to the 45 (56) LHC sector.

Tables
png pdf	Table 1: Estimated backgrounds from Drell-Yan production, with proton kinematics matching within 1$\sigma $, 2$\sigma $, 3$\sigma $, and within the full acceptance range in at least one of the FAR and NEAR Roman Pots of a given arm.
png pdf	Table 2: Estimated backgrounds from double-dissociation $\gamma \gamma \rightarrow \mu \mu $ production, with proton kinematics matching within 1$\sigma $, 2$\sigma $, 3$\sigma $, and within the full acceptance range in at least one of the FAR and NEAR Roman Pots of a given arm.

Summary

We have studied $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ production with forward protons reconstructed in the CMS-TOTEM Precision Proton Spectrometer, using a sample of about $10$ fb$^{-1}$ collected in high luminosity LHC data taking at 13 TeV. The Roman Pot alignment and LHC optics corrections have been derived using a high statistics sample of forward protons. A total of 12 $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ candidates with $m(\mu\mu) > $ 110 GeV, and matching forward proton kinematics, is observed. This corresponds to an excess larger than four standard deviations over the background. The result confirms the reliability of the alignment and optics determination, and represents the first evidence for proton-tagged $\gamma\gamma$ collisions at the electroweak scale. The present data demonstrate the excellent performance of CT-PPS and its potential. With its 2016 operation, CT-PPS has proven for the first time the feasibility of operating a near-beam proton spectrometer at a high luminosity hadron collider on a regular basis.

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