A new CMS subdetector – the Precision Proton Spectrometer (PPS) – allows the electroweak sector of the Standard Model to be probed in regions so far unexplored, explain Michele Arneodo, Michael Pitt, Enrico Robutti and Ksenia Shchelina.
Colliding particles at high energies is a tried and tested route to uncover the secrets of the universe. In a collider, charged particles are packed in bunches, accelerated and smashed into each other to create new forms of matter. Whether accelerating elementary electrons or composite hadrons, past and existing colliders all deal with matter constituents. Colliding force-carrying particles such as photons is more ambitious, but can be done, even at the Large Hadron Collider (LHC).
The LHC, as its name implies, collides hadrons (protons or ions) into one another. In most cases of interest, projectile protons break up in the collision and a large number of energetic particles are produced. Occasionally, however, protons interact through a different mechanism, whereby they remain intact and exchange photons that fuse to create new particles (see “Photon fusion” figure). Photon–photon fusion has a unique signature: the particles originating from this kind of interaction are produced exclusively, i.e. they are the only ones in the final state along with the protons, which often do not disintegrate. Despite this clear imprint, when the LHC operates at nominal instantaneous luminosities, with a few dozen proton–proton interactions in a single bunch crossing, the exclusive fingerprint is contaminated by extra particles from different interactions. This makes the identification of photon–photon fusion challenging.
The sensitivity in many channels is expected to increase by a factor of four or five compared to that in Run 2
Protons that survive the collision, having lost a small fraction of their momentum, leave the interaction point still packed within the proton bunch, but gradually drift away as they travel further along the beamline. During LHC Run 2, the CMS collaboration installed a set of forward proton detectors, the Precision Proton Spectrometer (PPS), at a distance of about 200 m from the interaction point on both sides of the CMS apparatus. The PPS detectors can get as close to the beam as a few millimetres and detect protons that have lost between 2% and 15% of their initial kinetic energy (see “Precision Proton Spectrometer up close” panel). They are the CMS detectors located the farthest from the interaction point and the closest to the beam pipe, opening the door to a new physics domain, represented by central-exclusive-production processes in standard LHC running conditions.
Testing the Standard Model
Central exclusive production (CEP) processes at the LHC allow novel tests of the Standard Model (SM) and searches for new phenomena by potentially granting access to some of the rarest SM reactions so far unexplored. The identification of such exclusive processes relies on the correlation between the proton momentum loss measured by PPS and the kinematics of the central system, allowing the mass and rapidity of the central system in the interaction to be inferred very accurately (see “Tagging exclusive events” and “Exclusive identification” figures). Furthermore, the rules for exclusive photon–photon interactions only allow states with certain quantum numbers (in particular, spin and parity) to be produced.
Precision Proton Spectrometer up close
PPS was born in 2014 as a joint project between the CMS and TOTEM collaborations (CERN Courier April 2017 p23), and in 2018 became a subsystem of CMS following an MoU between CERN, CMS and TOTEM. For the specialised PPS setup to work as designed, its detectors must be located within a few millimetres of the LHC proton beam. The Roman Pots technique – moveable steel “pockets” enclosing the detectors under moderate vacuum conditions with a thin wall facing the beam – is perfectly suited for this task. This technique has been successfully exploited by the TOTEM and ATLAS collaborations at the LHC and was used in the past by experiments at the ISR, the SPS, the Tevatron and HERA. The challenge for PPS is the requirement that the detectors operate continuously during standard LHC running conditions, as opposed to dedicated special runs with a very low interaction rate.
The PPS design for LHC Run 2 incorporated tracking and timing detectors on both sides of CMS. The tracking detector comprises two stations located 10 m apart, capable of reconstructing the position and angle of the incoming proton. Precise timing is needed to associate the production vertex of two protons to the primary interaction vertex reconstructed by the CMS tracker. The first tracking stations of the proton spectrometer were equipped with silicon-strip trackers from TOTEM – a precise and reliable system used since the start of the LHC. In parallel, a suitable detector technology for efficient operation during standard LHC runs was developed, and in 2017 half of the tracking stations (one per side) were replaced by new silicon pixel trackers designed to cope with the higher hit rate. The x, y coordinates provided by the pixels resolve multiple proton tracks in the same bunch crossing, while the “3D” technology used for sensor fabrication greatly enhances resistance against radiation damage. The transition from strips was completed in 2018, when the fully pixel-based tracker was employed.
In parallel, the timing system was set up. It is based on diamond pad sensors initially developed for a new TOTEM detector. The signal collection is segmented in relatively large pads, read out individually by custom, high-speed electronics. Each plane contributes to the time measurement of the proton hit with a resolution of about 100 ps. The design of the detector evolved during Run 2 with different geometries and set-ups, improving the performance in terms of efficiency and overall time resolution.
The most common and cleanest process in photon–photon collisions is the exclusive production of a pair of leptons. Theoretical calculations of such processes date back almost a century to the well-known Breit–Wheeler process. The first result obtained by PPS after commissioning in 2016 was the measurement of (semi-)exclusive production of e+e– and μ+μ– pairs using about 10 fb–1 of CMS data: 20 candidate events were identified with a di-lepton mass greater than 110 GeV. This process is now used as a “standard candle” to calibrate PPS and validate its performance. The cross section of this process has been measured by the ATLAS collaboration with their forward proton spectrometer, AFP (CERN Courier September/October 2020 p15).
An interesting process to study is the exclusive production of W-boson pairs. In the SM, electroweak gauge bosons are allowed to interact with each other through point-like triple and quartic couplings. Most extensions of the SM modify the strength of these couplings. At the LHC, electroweak self-couplings are probed via gauge-boson scattering, and specifically photon–photon scattering. A notable advantage of exclusive processes is the excellent mass resolution obtained from PPS, allowing the study of self-couplings at different scales with very high precision.
During Run 2, PPS reconstructed intact protons that lost down to 2% of their kinetic energy, which for proton–proton collisions at 13 TeV translates to sensitivity for
central mass values above 260 GeV. In the production of electroweak boson pairs, WW or ZZ, the quartic self-coupling mainly contributes to the high invariant-mass tail of the di-boson system. The analysis searched for anomalously large values of the quartic gauge coupling and the results provide the first constraint on γγZZ in an exclusive channel and a competitive constraint on γγWW compared to other vector-boson-scattering searches.
Many SM processes proceeding via photon fusion have a relatively low cross section. For example, the predicted cross section for CEP of top quark–antiquark pairs is of the order of 0.1 fb. A search for this process was performed early this year using about 30 fb–1 of CMS data recorded in 2017, with protons tagged by PPS. While the sensitivity of the analysis is not sufficient to test the SM prediction, it can probe possible enhancements due to additional contributions from new physics. Also, the analysis established tools with which to search for exclusive production processes in a multi-jet environment using machine-learning techniques.
The SM provides very accurate predictions for processes occurring at the LHC. Yet, it cannot explain the origin of several observations such as the existence of dark matter, the matter–antimatter asymmetry in the universe and neutrino masses. So far, the LHC experiments have been unable to provide answers to those questions, but the search is ongoing. Since physics with PPS mostly targets photon collisions, the only assumption is that the new physics is coupled to the electroweak sector, opening a plethora of opportunities for new searches.
Photon–photon scattering has already been observed in heavy-ion collisions by the LHC experiments, for example by ATLAS (CERN Courier December 2016 p9). But new physics would be expected to enter at higher di-photon masses, which is where PPS comes into play. Recently, a search for di-photon exclusive events was performed using about 100 fb–1 of CMS data at a di-photon mass greater than 350 GeV, where SM contributions are negligible. In the absence of an unexpected signal, a new best limit was set on anomalous four-photon coupling parameters. In addition, a limit on the coupling of axion-like particles to photon was set in the mass region 500–2000 GeV. These are the most restrictive limits to date.
A new, interesting possibility to look for unknown particles is represented by the “missing mass” technique. The exclusivity of CEP makes it possible, in two-particle final states, to infer the four-momentum of one particle if the other is measured. This is done by exploiting the fact that, if the protons are measured and the beam energy is known, the kinematics of the centrally produced final state can be determined: no direct measurements of the second particle are required, allowing us to “see the unseen”. This technique was demonstrated for the first time at the LHC this year, using around 40 and 2 fb–1 of Run 2 data in a search for pp → pZXp and pp → pγXp, respectively, where X represents a neutral, integer-spin particle with an unspecified decay mode. In the absence of an observed signal, the analysis sets the first upper limits for the production of an unspecified particle in the mass range 600–1600 GeV.
Looking forward with PPS
For LHC Run 3, which began in earnest on 5 July, the PPS team has implemented several upgrades to maximise the physics output from the expected increase in integrated luminosity. The mechanics and readout electronics of the pixel tracker have been redesigned to allow remote shifting of the sensors in several small steps, which better distributes the radiation damage caused by the highly non-uniform irradiation. All timing stations are now equipped with “double diamond” sensors, and from 2023 an additional, second station will be added to each PPS arm. This will improve the resolution of the measured arrival time of protons, which is crucial for reconstructing the z coordinate of a possible common vertex, by at least a factor of two. Finally, a new software trigger has been developed that requires the presence of tagged protons in both PPS arms, thus allowing the use of lower energy thresholds for the selection of events with two particle jets in CMS.
The sensitivity in many channels is expected to increase by a factor of four or five compared to that in Run 2, despite only a doubling of the integrated luminosity. This significant increase is due to the upgrade of the detectors, especially of the timing stations, thus placing PPS in the spotlight of the Run 3 research programme. Timing detectors also play a crucial role in the planning for the high-luminosity LHC (HL-LHC) phase. The CMS collaboration has released an expression of interest to pursue studies of CEP at the HL-LHC with the ambitious plan of installing near-beam proton spectrometers at 196, 220, 234, and 420 m from the interaction point. This would extend the accessible mass range to the region between 50 GeV and 2.7 TeV. The main challenge here is to mitigate high “pileup” effects using the timing information, for which new detector technologies, including synergies with the future CMS timing detectors, are being considered.
PPS significantly extends the LHC physics programme, and is a tribute to the ingenuity of the CMS collaboration in the ongoing search for new physics.
CMS and TOTEM Collab. 2022 CMS-PAS-EXO-19-009.
CMS and TOTEM Collab. 2022 CMS-PAS-EXO-21-007.
CMS and TOTEM Collab. 2022 CMS-PAS-SMP-21-014.
CMS and TOTEM Collab. 2022 CMS-PAS-TOP-21-007.
CMS Collab. 2021 CMS-NOTE-2020-008.