CMS PROGRAMME – TOTEM

Project leader: Kenneth Österberg   Personnel

The project is responsible for coordinating the Finnish participation to the TOTEM experiment and the CMS Proton Precision Spectrometer (PPS). The TOTEM experiment focuses on elastic scattering, total cross-section and diffraction. The PPS studies high-mass central exclusive processes in standard high luminosity running.

TOTEM has published the total pp cross-section, σtot, and the ρ parameter, the real to imaginary ratio of the elastic hadronic amplitude at t = 0, measurements at 13 TeV as well as the elastic differential cross-section (dσel ⁄dt) at 2.76 and 13 TeV. Next are the  σtot and measurements at 0.9 and 14 TeV as well as a common analysis with D0 collaboration to quantify the difference between pp and proton-antiproton (pp) dσel ⁄dt with the aim to confirm and characterize t-channel exchange of a C-odd colorless three gluon compound state (“the Odderon”) in elastic scattering. In addition, for glueball candidate and low mass resonance studies in central exclusive processes, TOTEM has twice (2015 and 2018) taken data with CMS in β*= 90 m runs at √s = 13 TeV.

In 2020-21, the TOTEM project focuses on completing and publishing analyses of data taken during Run 2 and on preparations for Run 3. Regarding physics, the emphasis is on Odderon related analyses, studies of glueball candidates and searches for low mass supersymmetry with forward protons. The Run 3 preparations are related to the new T2 (“nT2”), the upgrade of the PPS diamond timing detector and the development of a dedicated PPS supersymmetry trigger to increase the CMS Run 3 sensitivity to low-mass supersymmetric scenarios not yet excluded by standard LHC searches. The nT2 is a scintillator detector to be installed for special runs at √s = 14 TeV to measure the inelastic rate to enable the determination of the σtot and the ρ parameter during Run 3.

In addition, also activities supporting the completion of the PhD theses of the students will be pursued. Funding for the HIP contributions to the nT2 and the PPS upgrade has been granted by the Academy of Finland. To partially substitute the lack of test beam possibilities at CERN during LS2 (2019-20), a GEM-based lab telescope is under construction in order to be able to test prototype detectors at least at low rate using cosmic muons.

Österberg is the physics coordinator of TOTEM and F. Garcia the coordinator of test beam activities in PPS. Currently the group consists of a professor (K. Österberg), an emeritus professor (H. Saarikko), a senior scientist (F. Garcia), one post-doc (L. Forthomme), one PhD student (F. Oljemark) and one MSc student (A. Molander).

Research plan 2020-21

nT2

TOTEM has measured the σtot and the ρ parameter at 13 TeV center-of-mass energy with high precision [Eur. Phys. J. C 79 (2019) 785]. The combination of the two measurements is not compatible with the predictions without adding  t-channel exchange of an Odderon in elastic scattering. Further evidence for Odderon exchange has been the observation by TOTEM of a diffractive “dip” in the pp dσel ⁄dt  at TeV-energy scale [CERN-EP-2018-341, ArXiv: 1812.08610]. The Odderon interpretation should be confirmed as well as characterized by σtot and ρ measurements at √s = 0.9 TeV (data taken in 2018) and at  = 14 TeV (data taking planned in 2021).

To perform the 14 TeV measurements, a new scintillator-based T2 (“nT2”) telescope needs to be constructed, since the previous T2 was not compatible with the new beam pipe to be installed during LS2 (2019-20). HIP is responsible for the detector part of nT2. The tasks during 2020-21 involves prototyping of detector components (fibers, wave length shifters and scintillators), characterization of the  prototype detectors and the procurement of the components for the final detectors as well as the organization of their assembly. The nT2 activities of HIP are led by F. Garcia and done in close collaboration with INFN Pisa-Siena, CERN, Ankara and Protvino.

Diamond TOF detectors for PPS

High mass central exclusive processes, pp → p + X + p, are very sensitive to new physics either via loops through e.g. anomalous quartic couplings, or via direct production of new particle(s) e.g. supersymmetric particles. The PPS [https://cds.cern.ch/record/1753795] has during its exploratory phase 2016-18 proven the feasibility of operating a near-beam detector at high luminosity on a regular basis to measure such processes [JHEP07 (2018) 153]. To fully exploit the PPS potential and to obtain the design timing resolution of about  ps, an additional pair of diamond Time-Of-Flight (TOF) detector modules using the double-diamond technique [JINST 12 (2017) P03026] will be installed for Run 3. As for the current detector, HIP is responsible for providing the additional diamonds, metallize them and making their quality assurance as well as participate in the assembly and testing of the detector modules at CERN. The currently installed diamonds will also be refurbished to double-diamonds.

In 2020, the focus is on metallizing and making the quality assurance of the diamonds procured from Element Six [https://www.e6.com] before assembly into detector modules at CERN. The new hybrids will be tested in secondary vacuum and the high voltage stability of the assembled detector modules will be investigated. The detector modules will also be characterized with minimum ionizing particles (MIPs) in test beams at DESY. L. Forthomme participates in the development of timing detector offline code. In 2020, this work will concentrate on the integration of the online estimation algorithm for the operational parameters of PPS timing detectors (such as per-channel time precision and timing offset) into the CMS prompt calibration loop, allowing the usage of the forward protons timing information already at the prompt reconstruction stage. The PPS activities are led by K. Österberg, involve F. Garcia and L. Forthomme and done in close collaboration with the HIP CMS upgrade project and the PPS groups of INFN Pisa-Siena, CERN and Tomsk.

Evidence of t-channel exchange of a colourless C-odd three-gluon compound state

In 2020, the focus will be on finalizing the joint work with D0 to confirm the evidence of t-channel Odderon exchange through a quantification of the pp and the pp dσel ⁄dt difference by an extrapolation of the TOTEM pp measurements to the D0 pp measurement at 1.96 TeV as well as the elastic analysis at √s = 0.9 TeV for a σtot and ρ determination. During first part of 2020, also the on-going Run 1 analysis (TOTEM only single diffractive 7 TeV analysis) will be completed and published as part of the PhD thesis of F. Oljemark.

Studies of glueball candidates

The common CMS-TOTEM 2015 and 2018 high β* data show good potential to study central exclusive production of 0++ and 2++ glueball candidates. The aim is first to observe them in this ultrapure gluon-gluon environment and then further to measure or put upper limits on their branching fractions to two and four-charged particle final states. The first publication using the 2015 data is under review and expected to be published during 2020. K. Österberg will continue his participation in these analyses with the aim to fully exploit the potential of the high statistics 2018 data during 2020-21.

New physics searches with forward protons

The group is also actively involved in new physics searches in central exclusive processes with forward protons. Searches of low-mass supersymmetric particles in compressed mass scenarios with forward proton tags show good potential to be able to cover scenarios with small mass difference between the next to lightest and lightest supersymmetric particle [e.g. L. Beresford and J. Liu, arXiv:1811.06465] not covered by standard LHC searches. The goal during 2020 is to develop a dedicated high-level trigger for such scenarios using coincidences between central charged leptons and forward protons to significantly increase the CMS sensitivity to such searches in Run 3. In parallel during 2020, a preparatory analysis using the full PPS Run 2 data set (>100 fb-1) will be performed to be able to make more realistic estimates of the sensitivity of a Run 3 analysis when a significantly improved PPS timing resolution needed for reaching a favorable signal-to-background ratio will become available. In addition, a publication based on an exclusive  production analysis using ~ 10  fb-1 PPS data from 2016 with the participation of L. Forthomme is in preparation and expected to be published in 2020.  A similar analysis using the full PPS Run 2 data set is on-going and expected to be finished and published in 2021. Furthermore, phenomenology studies for central exclusive processes (in particular, photon-induced reactions), and their integration into the CepGen [arXiv:1808.06059] event generator will be pursued in collaboration with IFJ PAN Cracow. The high-mass central exclusive production activity with forward proton tags is led by L. Forthomme with participation of A. Molander and K. Österberg and in collaboration with University of Kansas.