The decision to employ a method that involves hoisting thousands of tonnes of expensive machinery, the result of endless hours of work, down a 100 metre gaping hole was never going to be taken lightly. But CMS’s unique method of slicing up the detector and lowering each piece into the cavern ready-made, has been nothing short of a success.

“It’s a huge amount of responsibility; you have all the physicists waiting for an element to arrive, and if you make a serious mistake, it could be the end of CMS,” says Hubert Gerwig, whose task it was to ensure each sub-detector’s safe arrival.

But lowering CMS by means of heavy lifting was a decision taken at the very beginning, some 16 years ago, inspired by experiences with LEP. “The concept of building large objects on the surface and transferring them completed to the underground area was the clear way to go,” says Alain Hervé, CMS’s original technical coordinator. Though not in LEP’s initial plans, success in lowering two pieces into position in L3, weighing 300 and 350 tonnes, suggested the method could work.

Being able to work in parallel on the civil engineering and the detectors was clearly a huge advantage, and the ‘slicing’ also meant that, through complicated loops of cable chains, water, gas and cooling leads, each piece remained accessible within the cavern, a contrast with experiments built in a ‘Russian dolls’ style, from the inside, out.

But the implications of using this method needed careful consideration too: “As soon as I saw the sketch I had a clear vision of how such an experiment had to be organized,” says Alain. “We were already two years inside LEP running, and lessons from the construction and the first shut-downs were clear to me. The way the experiment had to be sectioned and installed was the first priority, not developing the design,” says Alain.

And so the experimental area, pits and surface hall all had to be designed around the lowering method. Foundations for the gantry and a massive strong plug to hold the pieces over the shaft before the cranes took the strain were incorporated. Anchor points for lifting were integrated in the design of the yoke as well as HCAL (Hadron Calorimeter) and HF (Forward Calorimeter) platforms and cradles.

The first lowering took place in November 2006, and the last on 22 January 2008. “Everything has been calculated and calculated again, but that’s still not the real thing,” says Hubert. “In the end we were successful and nothing was damaged. So there is a sense of relief; it’s a bit like an exam, you feel better once it is over and you can celebrate!”

Trials and Tribulations of Heavy Lowering

Ten companies internationally were capable of taking on such a project as CMS, but VSL Heavy Lifting, a Swiss company, were the eventual winners of the 2 million CHF contract.

Though VSL were used to lifting such heavy loads as the 7500-tonne roof of the Airbus Assembly Hall, and the walkway between the Kuala Lumpur Twin Towers, CMS presented unique challenges. For a start, the equipment needed to lower, not lift, and a long way. Adding to this was the fact that the scientific cargo was far more delicate than its industrial counterparts.

The gantry support system was rented and made from existing towers, but the 3.4m high horizontal beam, that took the strain of the load, had to be custom made. Even such an enormous beam still bent 5cm under the weight of the heaviest element, the 2000 tonne central barrel ring known as YB0.

During each lowering, the element is supported at four points, but by many more cables as each bunch is made up of 55 individual 15.7mm diameter strands of cold-drawn steel, each with a capacity to hold 28.4 tonnes. Within each bundle, half of the strands are twisted one way and half the other to make overall turning force zero and preventing any turning of the load during the descent.

The lowering in fact happens in 200 small steps as the cables are fed through the hydraulic system half a metre at a time. This works on a grip and release basis, similar to your own hands lowering a bucket down a well, and at all times the load is safely supported by one or other of the clamping mechanisms. Once the cargo reaches the ground, it comes to a soft landing on hovercraft-like airpads that take the load off the cables and later on allow easy horizontal movement within the experiment hall.

Each lowering was a big event, requiring a week’s preparation to manoeuvre the plug, element and attachments in place, and each was a success. Though there were also, of course, a few challenges that arose along the way: YB0, for instance, the largest segment to enter the cavern, at one point cleared the sides of the shaft by just 10cm either side, and the team needed surveillance video cameras to carefully monitor its movements and ensure that the element passed the bottleneck safely.

The slight slope in the CMS cavern presented another issue. Whilst unnoticeable underfoot, the 1.23% gradient meant that when one side of any element touched down, the other still had 3cm to go. “Initially we tried to compensate for the slope, so that there was an even landing,” explains Hubert Gerwig, the engineer in charge of heavy lifting, “but soon we realised it wasn’t necessary because under the load, the metal strands actually elongate by 10cm, making them somewhat elastic.” The elasticity meant they could simply carry on lowering the piece until both ends were on the ground, also giving rise to the slight “bouncing” effect visible as each piece finally came to rest on the experiment hall floor.

That some of the elements couldn’t be supported on just four attachment points presented another issue and the endcap disks , for example, were instead carried down on a special crate. But being supported nine metres below their centre of gravity made them naturally unstable. The team solved this dilemma by using stabilisers at the top of the endcap such that if it tilted, it would simply lean against the lowering cables.

In the end each slice of CMS found its way to the experiment hall safely, thanks to the hard work and dedication of all involved, and the group and their equipment can now be disbanded. The VSL team will now take their gantries and hydraulic equipment to Durban in South Africa to build a stadium for the 2010 World Cup. Some parts, such as the cables that have carried down every piece of CMS, are now too fatigued to be used again. Finishing the lowering is a sad event for some, but the VSL mechanics like to travel the world and are eager to move on. “I think some of the workers get the opposite of home sickness,” says Hubert, “they cannot wait to get away again!” And we wish them good luck.

Between 1998 and 2005 a total of almost 250,000 m³ of soil and rock was excavated from the CMS site at “Point 5” of the LHC, in Cessy. The contractors - Seli, an Italian company and Dragados, Spanish - had previously worked on the Madrid metro but here faced some unique surprises.

It started with an unexpected find: “We did trial pits around the site, because the archaeology of an area is always something we have to consider, and what we found was great – a Roman villa, complete with pots, tiles and coins,” explains John Osborne, project manager of CMS civil engineering. “And even better for us was that it wasn’t anywhere near any planned shafts or buildings.”

They were able to leave archaeologists to spend a year thoroughly investigating the site whilst the project got underway. But it wasn’t an easy task. “Funnily enough Point 5 was one of he worst places possible to build the cavern in terms of geology, though best from a physics point of view. And CMS was pretty much the only experiment that could cope with being built here,” remarks John.

The first challenge came in the form of underground water tables approximately ten to twenty metres below the surface. To dig down through them the team had to first freeze the ground around the shafts to act as a barrier to the water. By drilling holes around each shaft’s circumference and pumping down brine, cooled to -25^oC, the water froze into a 3-metre wall of ice. But the water coming from Cessy was moving even faster than predicted, and combined with the channelling effect of water between the two shafts, pressure built up until it penetrated the walls.

Injecting the holes with an even colder substance: liquid nitrogen, at less than at -195^oC, finally solved the problem. With its help engineers formed a wall of ice around the shafts that was solid-enough for teams to keep on digging.

However, the problems were not over yet. Once the shafts were dug out, work had to begin on the caverns, but because the ground materials were so soft, with no intervention any cavern excavated would collapse. “At Point 5 there is only 15 metres of solid rock. For the first 75 metres of digging down it’s just a type of glacial deposit called moraine - basically a mixture of sand and gravel,” explains John. “And the rock we did have was a kind of soft sandstone called Molasse. A large cavern built in this would just collapse.”

The solution was to build a large supporting structure underground that could hold up the caverns and withstand the mass of the soil above it. Engineers envisaged a concrete pillar as the divider between the two caverns that could do just this.

“Knowing we would have to build this structure anyway, we asked radiation teams how thick we’d need it to be to protect people from radiation in the cavern next-door to the experiment,” explains John. “They gave the figure at 7 metres. The width needed to ensure adequate support for the caverns was 7.2 metres, so this worked out very well, and in fact the second cavern can now be safely used when the machine is on.”

After these delays, engineers experimented with using explosives to clear away rock at a faster rate, but in the end it had little benefit for a big disruption. Instead the best trick seemed to be to excavate small sections at a time and immediately install “shotcrete”, a sprayed concrete that sets as soon as it hits the walls, and drilling steel anchors that reached 12 metres into surrounding rock: “If we didn’t do this, the whole thing would collapse whilst we were building it. We constantly monitored any movement with a host of instruments and adjusted the support as necessary.”

For the workers on the site, another exciting yet daunting nature of working on the excavation was descending into the CMS cavern in the lift. During the construction period, access underground was via a lift cage being lowered down the shaft on a system of ropes. “This was good as it meant you could always be lowered to exactly the right level, as opposed to fixed lifts,” explains John. “But being on a rope also meant that it had a tendency to sway as you went down. And 100 metres is a long way!”

As the environment 100 metres below the surface is also full of water, the final stages involved waterproofing, installing drainage systems and painting the cavern, as water could otherwise turn soft rock into mud. “Once everything was in place we could seal off the cavern with waterproofing and put in a permanent concrete wall up to 4 metres thick, reinforced with steel bars.”

Throughout the whole construction, consideration for the environment was at the forefront of people’s minds. In order to keep noise levels to a minimum, sound barriers were built all around the site. And instead of removing the many tonnes of earth and rock excavated from the site, causing noise and road disruption, it was deposited right by the buildings, covered with fresh soil and planted with vegetation, creating a now flourishing artificial hill.

Whilst this was going on, thanks to the “pre-packaged” design of CMS, split up into 15 slices, where each is assembled and as near to complete as possible on the surface, work on building the detector could carry on this whole time. And once the cavern was complete, in 2005, pieces of CMS could be lowered underground and installed.

But though the civil engineering stage of building CMS is long over, the underground nature of the experiment must always be taken into account. Though the detector weighs almost as much as the soil and rock it replaced, the caverns within the spongy earth are like bubbles in water, and they could potentially rise by as much as one millimetre (mm) per year. Geologists predict that CMS, along with the immediately surrounding sections of the LHC machine, will rise 15 mm over 10 years. This may seem small, but when you think that the detector’s tracker and muon systems, for instance, must align to within 0.15 mm, one hundredth of the distance, this small amount becomes significant. In fact the ability to adjust by this amount is built into the detector and movement will be monitored throughout the life of the experiment, so civil engineering continues to play a part in the running of CMS.

The CMS magnet is the central device around which the experiment is built, with a 4 Tesla magnetic field that is 100,000 times stronger than the Earth’s.

Its job is to bend the paths of particles emerging from high-energy collisions in the LHC. The more momentum a particle has the less its path is curved by the magnetic field, so tracing its path gives a measure of momentum. CMS began with the aim of having the strongest magnet possible because a higher strength field bends paths more and, combined with high-precision position measurements in the tracker and muon detectors, this allows accurate measurement of the momentum of even high-energy particles.

The CMS magnet is a “solenoid” - a magnet made of coils of wire that produce a uniform magnetic field when electricity flows through them. The CMS magnet is “superconducting”, allowing electricity to flow without resistance and creating a powerful magnetic field. In fact at ordinary temperatures the strongest possible magnet has only half the strength of the CMS solenoid.

The tracker and calorimeter detectors (ECAL and HCAL) fit snugly inside the magnet coil whilst the muon detectors are interleaved with a 12-sided iron structure that surrounds the magnet coils and contains and guides the field. Made up of three layers this “return yoke” reaches out 14 metres in diameter and also acts as a filter, allowing through only muons and weakly interacting particles such as neutrinos. The enormous magnet also provides most of the experiment’s structural support, and must be very strong itself to withstand the forces of its own magnetic field.

The CMS magnet…

• is the largest superconducting magnet ever built
• weighs 12,000 tonnes,
• is cooled to -268.5ºC, a degree warmer than outer space,
• is 100,000 times stronger than the Earth’s magnetic field,
• stores enough energy to melt 18 tonnes of gold,
• uses almost twice much iron as the Eiffel Tower.

How big can you build a magnet?

More coils give a stronger field, a stronger field gives more precise results, and with more precise results we can spot more physics. But whilst getting the best magnetic field possible was the most important consideration in designing the detector, its size was also limited. For the sake of efficiency the magnet was to be built offsite and transported to CMS by road, which meant it physically could not be more than 7 metres in diameter, else it would not fit through the streets on its way to Cessy.

Visit the link for magnet updates: https://cms.cern/tags/magnet

As the name “Compact Muon Solenoid” suggests, detecting muons is one of the most important tasks of CMS. Muons are charged particles that are just like electrons and positrons, but 200 times heavier. We expect them to be produced in the decay of a number of potential new particles; for instance, one of the clearest "signatures" of the Higgs Boson is its decay into four muons.

Because muons can penetrate several metres of material losing little energy, unlike most particles, they are not stopped by any of CMS calorimeters. Therefore, chambers to detect muons are placed in the outer part of the experiment where they are the only particles likely to produce a clear signal.

A particle is measured by fitting a curve to the hits registered in the four muon stations, which are located outside of the magnet coil, interleaved with iron "return yoke" plates. The particle path is measured by tracking its position through the multiple active layers of each station; for improved precision, this information is combined with the CMS silicon tracker measurements. Measuring the trajectory provides a measurement of particle momentum. Indeed, the strong magnetic field generated by the CMS solenoid bends the particle's trajectory, with a bending radius that depends on its momentum: the more straight the track, the higher the momentum.

In total there are 1400 muon chambers: 250 drift tubes (DTs) and 540 cathode strip chambers (CSCs) track the particle positions and provide a trigger, while 610 resistive plate chambers (RPCs) and 72 gas electron multiplier chambers (GEMs) form a redundant trigger system, which quickly decides to keep or discard the acquired muon data. Because of the many layers of detector and different specialities of each type, the system is naturally robust and able to filter out background noise.

A muon, in the plane perpendicular to the LHC beams, leaves a curved trajectory in four layers of  muon detectors ("stations")

DTs and square-shaped RPCs are arranged in concentric cylinders around the beam line (‘the barrel region’), whilst CSCs, trapezoidal RPCs, and GEMs form the ‘endcap’ disks that “close” the ends of the barrel.

The muon system…

• contains 2 million cathode strip chamber wires each as thin as a human hair;
• is aligned with the central tracker to within one sixth of a millimetre in order for the detectors to work together in reconstructing tracks;
• is made of components built in 15 countries. As of today, the muon system group counts 63 institutions from 22 countries. 

Making Tough Chambers

Most of the muon chambers  were built in different laboratories around the world before being shipped all the way to CERN, so the chambers needed to be robust as they are meant to act as precision detectors. Their mechanical robustness and performance was extensively tested in several phases of prototyping, design, test beams, assembly and commissioning, both at the respective labs and at CERN. 

As a demonstration of their robustness, when physicists overseeing the beam test decided to reposition a chamber, one side of its support structure suddenly fell by more than 30 cm. Such an unprecedented test of robustness had the physicists worry about the fate of that unlucky chamber. Instead, they found it was still "taking data" in spite of its hard drop.

Visit the link for muon system updates: https://cms.cern/index.php/tags/cms-muons-subsystem 

 

In order to build up a picture of events occurring in the LHC, CMS must measure the energies of emerging particles. Of particular interest are electrons and photons, because of their use in discovering the Higgs boson and other new physics.

The energies of electrons and photons are measured using the CMS electromagnetic calorimeter (ECAL). Measuring their energies with the necessary precision in the very strict conditions of the LHC - a high magnetic field, high levels of radiation, and only 25 nanoseconds between collisions - requires dedicated detector materials. 

Lead tungstate crystal is made primarily of metal and is heavier than stainless steel, but with a touch of oxygen in this crystalline form, it is highly transparent and “scintillates” when electrons and photons pass through it. This means it produces light in proportion to the impinging particle’s energy. These high-density crystals produce light in fast, short, well-defined photon bursts that allow for a precise, fast, and fairly compact detector.

Photodetectors, which have been especially designed to work within the high magnetic field, are glued onto the back of each of the crystals to detect the scintillation light and convert it to an electrical signal that is amplified and sent for analysis.

The ECAL, made up of a “barrel” section and two “endcaps”, forms a layer between the tracker and the HCAL. The cylindrical barrel consists of 61,200 crystals formed into 36 “supermodules”, each weighing around three tonnes and containing 1700 crystals. The flat endcaps seal off the barrel at each end and are made up of almost 15,000 more crystals.

For extra spatial precision, the ECAL also contains a preshower detector that sits in front of the endcaps. These allow CMS to distinguish between single high-energy photons (often signs of exciting physics) and the less interesting close pairs of low-energy photons.

The CMS ECAL…

• crystals each weigh 1.5kg but with a volume roughly equal to that of a small coffee cup,
• contains nearly 80,000 such crystals, each of which took two days to grow.

CMS ECAL Technical Design Report

Production Story ....

A Russian factory in a former military complex took on the job of producing most of the crystals, while the remainder were produced in China. It took about ten years to grow all 78,000 crystals to stringent specifications, taking around two days to grow each one. Inside the detector, the crystals face high radiation, so the lead tungstate material had to be carefully chosen and developed.

The measurement of the momentum of particles is crucial in helping us to build up a picture of what happens at the heart of the collision. One method to calculate the momentum of a particle is to track its path through a magnetic field; the more curved the path, the less momentum the particle had. The CMS tracker records the paths taken by charged particles by measuring their positions at a number of points.
The tracker can reconstruct the paths of high-energy muons, electrons and hadrons (particles made up of quarks) as well as see tracks coming from the decay of very short-lived particles such as beauty or “b quarks” that are for example being used to study the differences between matter and antimatter.

The tracker needs to record particle paths accurately yet be lightweight so as to disturb the particle as little as possible. It does this by taking position measurements so accurate that tracks can be reliably reconstructed using just a few measurement points. Each measurement is accurate to 10 µm, a fraction of the width of a human hair. It is also the innermost part of the CMS detector and so receives the highest volume of particles: the construction materials were therefore carefully chosen to resist radiation.
The final design consists of a tracker made entirely of silicon detection elements: the pixels, at the very core of the detector and dealing with the highest intensity of particles, and the silicon microstrip detectors that surround it. As particles travel through the tracker the pixels and microstrips produce tiny electric signals that are amplified and detected. The signals are stored in chips’ memory for several microseconds and then processed before being sent to a laser to be converted into infrared pulses. These are then transmitted over a 100m fibre optic cable for analysis in a radiation-free environment. The tracker uses 40,000 such fibre optic links providing a low power, lightweight way of transporting the signal. Much of the technology behind the tracker electronics came from innovation in collaboration with industry.

The tracker employs sensors covering an area the size of a tennis court, with 135 million separate electronic readout channels: in the pixel detector there are some 6600 connections per square centimetre. Silicon sensors are highly suited to receive many particles in a small space due to their fast response and good spatial resolution.
The CMS strip tracker was installed in CMS in late 2007 and has been in continuous operation since 2008. The original CMS pixel detector was operated during the years 2009 – 2016 and was replaced by the CMS Phase-1 pixel detector in an extended year-end stop 2016/2017.

Visit the link for Pixel Tracker updates: https://cms.cern/tags/tracker

More details about the CMS Tracker can be found in the following selection of documents:
The CMS experiment at the CERN LHC (paper written in ~2008 describing the original CMS experiment as build for LHC Run 1)
The CMS Phase-1 Pixel Detector Upgrade (paper from 2020 describing the Phase-1 pixel detector)
The very original description of the CMS tracking detectors can be found in the Technical Design Report (TDR) from 1998 and 2000, respectively
CMS: The Tracker Project Technical Design Report
The CMS tracker : addendum to the Technical Design Report
While many design aspects have evolved from the original concept, they still provide interesting insights about the challenges for the detectors and how they were thought to be overcome. A few more words on this are in the following paragraph.

Some design history ...

In the very first design sketch of CMS, the tracker section was left blank because it was thought that with the intensity of particles experienced in the LHC it would be impossible to make a tracker that could withstand it. In particular, making readout electronics that could work in this radiation would be a challenge. Hints that the US military, needing such electronics for space travel and nuclear warheads, might have solved some of the potential problems encouraged the team to explore this field.
However, the technology actually came from a most unexpected source; as the military market shrank at the end of the cold war and commercial electronics began to excel, the team took a gamble on a very fine feature manufacturing process, produced for a commercial market, which was not formally in the realm of “radiation hard electronics”. With some modifications, a few simple design tricks and selection of the right technology, it could be as radiation hard as needed. And it was a remarkable success. Eighteen months later the teams had rebuilt the old chips. They had low power consumption, low noise, and high overall performance and on top of this could be easily produced on a large scale, were relatively cheap and thoroughly tested. It was a big breakthrough. The same technology was then also used for the ECAL as well as CMS and ATLAS pixels.

When CMS is performing at its peak, about one billion proton-proton interactions will take place every second inside the detector. There is no way that data from all these events could be read out, and even if they could, most would be less likely to reveal new phenomena; they might be low-energy glancing collisions for instance, rather than energetic, head-on interactions.

We therefore need a “trigger” that can select the potentially interesting events, such as those which will produce the Higgs particle, and reduce the rate to just a few hundred “events” per second, which can be read out and stored on computer disk for subsequent analysis.

However, with groups of protons colliding 40 million times per second there are only ever 25 nanoseconds (25 billionths of a second) before the next lot arrive. New waves of particles are being generated before those from the last event have even left the detector! The solution is to store the data in pipelines that can retain and process information from many interactions at the same time. To not confuse particles from two different events, the detectors must have very good time resolution and the signals from the millions of electronic channels must be synchronised so that they can all be identified as being from the same event.

Triggering

Level 1 of the trigger is an extremely fast and wholly automatic process that looks for simple signs of interesting physics, e.g. particles with a large amount of energy or in unusual combinations. This is like a reader simply scanning the headlines of a newspaper to see if anything catches their eye. This way we select the best 100,000 events or “issues” each second from the billion available. For the next test, the higher level trigger, we assimilate and synchronise information from different parts of the detector to recreate the entire event - like collating the different pages to form the full newspaper - and send it to a farm of more than 1000 standard computers.

Here the PCs are like speed readers, who with more detailed information review the information for longer, less than a tenth of a second. They run very complex physics tests to look for specific signatures, for instance matching tracks to hits in the muon chambers, or spotting photons through their high energy but lack of charge. Overall they select 100 events per second and the remaining 99,900 are thrown out. We are left with only the collision events that might teach us something new about physics. Despite the trigger system, CMS still records and analyses several petabytes of data, that’s millions of gigabytes, around the same amount of information as in 10,000 Encyclopaedia Britannica every second, although not all these data are stored.

For a more detailed account of CMS Triggering see:
CMS The TriDAS Project Technical Design Report

The History of CMS Data Acquisition and Triggering

Early History and Key Decisions

The CMS Trigger Group was originally led by Fritz Szoncsó. Wesley Smith was appointed Trigger Project Manager in 1994. One of the earliest decisions made by the group was not to follow existing three-level trigger-system designs.

Just a few years prior to this, in 1985, the ZEUS experiment was the first to decide to use a three-level trigger system which consisted of: pure hardware (L1), mostly custom hardware (L2) and the computer farm (L3). The Tevatron experiments also adopted this architecture.

At CMS, TriDAS (Trigger and Data Acquisition System) Project Manager Sergio Cittolin, TriDAS Institution Board Chair Paris Sphicas and Smith decided not to have a second level trigger. They would take the output of L1 straight to the computer farm for software processing. The main reason for doing this was that the L2 hardware was too restrictive. It was not fully programmable, and was only used at the time because there was no telecom switch that could convey the full L1 output of 100 kHz of 1 MB events to the farm.

However, Cittolin, studying technology trends and extrapolating world-wide computing network infrastructure, was convinced that a switch with the required bandwidth would be available and affordable by the year 2000. So, when the technical proposal was written in ’94-’95, a plan to go from L1 to the computer farm was laid out.

When the technical proposal was presented to the LHC Experiments Committee (LHCC) in 1994, a different approach was adopted to deal with the bandwidth problem, in order to pass the reviews: The proposal said that 10% of the data would go through the switch for processing and would be used as a basis to knock down the rate by a factor of 10 — sufficient for the switches of 1994 to handle! This 10% — including main calorimeter information, summary of tracking information and so on — would then be used to decide whether to keep the event or not. The group hoped, though, that they would not have to face the problem at all.

Prototyping and testing followed until the Technical Design Report (TDR) was published in 1999/2000 and defended in front of the LHCC. By 2002, network switches with the required bandwidth were available and the problem was solved.

Consequences of Two-Level Trigger

One consequence of the decision was that L1 has to be more efficient than in previous systems in order to perform certain tasks traditionally performed by L2.

Prior to the CMS Trigger, triggers were designed to count objects: the number of electrons/muons over a certain threshold and the like, providing a histogram. In CMS, characteristics of objects, including their energy and co-ordinates would be retained, which required sorting of the objects so that only the prime candidates would be selected.

However, sorting consumes time. Which is why the latency — trigger processing time — is longer than it would have been for the L1 trigger in a three-level trigger system. Custom chips were built for the calorimeter trigger to perform the sorting fast enough.

The upside to this, Smith says, is that instead of asking how many jets or electrons we have, we can ask, “Is there an electron above an ET threshold back-to-back with a jet above that threshold?” and other such questions. This had never been done before and almost allows CMS to perform offline operations at the L1 stage itself.

Wesley Smith adds: “I think it’s been proven that we really built it almost optimally. It was a long extrapolation but I think in retrospect it’s a fairly successful design. The higher level triggers worked out quite well, are well designed, and showed the flexibility of the system. In the end, if you look back at the history, we did make the right decisions. Although at the time, as always, it was not clear.”