Exclusive production in CMS

We discuss the CMS capabilities of measuring vector mesons in pp and heavyion collisions based on the recent results of the exclusive dilepton and diboson production. Studies evaluating the detection of exclusive J/ψ and Υ vector meson photoproduction in the dimuon decay channel are presented. As a result, we report the measurements performed with the CMS detector that are in the invariant mass range relevant for future studies of exclusive vector meson photoproduction in CMS.


Introduction
The CMS detector has been studying different production channels of exclusive and diffractive processes at high energies, in some cases widening the previous results obtained by other experiments. Recent studies involving photon interactions have presented outstanding results, e.g., the measurement of W pairs in the electron-muon decay channel [1]. Moreover, the diffractive production of diphotons by means of pomeron exchange in the t-channel has reported an upper limit to the production cross-section and is in agreement with theoretical predictions [2]. In addition, the production of vector mesons are also possible in collisions involving heavy ions beams, which provides a signal enhancement due to the large number of protons in the nucleus, increasing the photon flux [3]. The recent results reported by CMS are relevant for future measurements regarding the photoproduction of vector mesons.

Capabilities for meson photo-production in CMS
Presenting an excellent efficiency in detecting muons [4], the CMS detector takes advantage of its capability to measure different production processes in the dimuon decay channel. The CMS detector can explore the region of small + − invariant masses where the measurement of exclusive J/ψ and Υ photoproduction in pp collisions are possible. This topics have been recently investigated in theoretical works [5][6][7].
A study has been performed to evaluate the capability of CMS in detecting such processes in pp collisions [8]. For the exclusive photoproduction, a double muon trigger is used to collect dimuon events, requiring a vertex with exactly two muons and no additional tracks. As a exclusivity criteria, the pairs are required to have Δφ(μ + μ − ) > 0.9 and Δp T (μ + μ − ) < 1.5. Figure 1  and electromagnetic calorimeters greater than 4.0 and 2.5 GeV, respectively. As a result, the CMS detector allows the search for vector meson produced in exclusive events, which is corroborated by recent results in photon physics.
In the case of pPb and PbPb collisions, a parallel data-taking can be performed with the TOTEM experiment in order to increase the CMS coverage. The e + e − production has been studied in pPb collisions at 5.02 TeV to evaluate the performance of the detector [9], with the p T (e + e − ) distribution shown in Fig. 1 (right). Since the dilepton final state is similar to the decay of a vector meson, the CMS detector has the features to detect exclusive vector meson photoproduction in heavy ions collisions at high energies also in the very forward phase space.
The measurement of dimuons with invariant mass greater than 20 GeV has been used to validate the selection criteria applied in the study of γγ → W + W − . As shown in Fig. 2, the p T (μ + μ − ) distribution, obtained with no additional tracks in the (μ + μ − ) vertex has good agreement between the data and the theoretical predictions. The study of the exclusive two-photon production of W + W − has found two events passing the selection requirements in the region p T (e ± μ ∓ ) > 30 GeV, M(e ± μ ∓ ) > 20 GeV and |η(e ± μ ∓ )| < 2.4. Figure 2 present the acoplanarity distribution with the two events, being in agreement with the standard model expectation of 2.2 ± 0.4 signal events. As for the exclusivity selection, the lepton pairs are required to be within 1 − |Δφ(eμ)/π| < 0.1 and Δp T (μ ± e ∓ ) < 1.0 GeV and with no additional tracks from the μ ± e ∓ vertex. A recent work has discussed sub-leading processes that contribute to this process [11]. As an extension of this study, anomalous couplings for the γγ → W + W − vertex are also considered. No additional events are observed in comparison to the standard model expectation. Then, new limits for the anomalous coupling parameters are obtained and are shown in Fig 2. These results are about two orders of magnitude more stringent than those obtained in LEP [12][13][14][15] and 20 times more stringent obtained by the Tevatron [16].
Although the measurement of the two-photon production of muon pairs at high masses provides the necessary understanding to report the first evidence of a γγ → W + W − events, the results of the exclusive dilepton production at low masses are especially important as they are a possible background events for the study of vector meson photoproduction in CMS.
The measurement of the two-photon production of e + e − at low masses has been performed taking into account the kinematic regions of E T (e) > 5.5 GeV and |η(e)| < 2.5. Including the contribution of proton dissociation, 17 events have been found, which is in agreement with the standard model expectation. Figure 3 shows the p T (e + e − ) distribution in comparison to the predictions for the three contributions for the γγ → e + e − process. In the case of μ + μ − , cuts are applied on single muons, p T (μ) > 4 GeV and |η(μ)| < 2.1, and muon pairs, m(μ + μ − ) > 11.5 GeV, 1 − |Δφ(μ + μ − )/π| < 0.1 and |Δp T (μ + μ − )| < 1.0. In Fig. 3 the p T (μ + μ − ) distribution is presented including the predictions for the γγ → μ + μ − process and the background events from the Drell-Yan process. As expected, a good agreement is observed between the data and theory.

Conclusion
Measurements performed with the CMS detector during the data-taking periods of 2010 and 2011 have shown promising results for the physics of two-photon processes, with remarkable agreement with the standard model expectations. These results present the capability of the CMS detector in measuring lepton pairs in both low-and high-mass regions, being the former a region for the photoproduction of vector mesons. Moreover, the results present a promising scenario for future investigations to detect vector mesons in CMS due to the same final state with lepton pairs. A combined measurement with the TOTEM experiment can provide valuable data to increase the kinematic coverage of CMS and to allow further analyses of exclusive processes.