The LHC (Large Hadron Collider), located at CERN, is the world’s largest and most powerful particle accelerator and the largest scientific instrument ever built to explore the new high-energy frontier. It started up operations in September 2008 after about 25 years of design, development of the required technologies and construction of the accelerator and detector components. The LHC has already delivered some sound successes: the discovery of the Higgs boson, an open window into some very rare processes and exotics like pentaquarks. The particle physicists’ Standard Model is now closed: all the standard particles have been observed and the measurements agree with the model predictions depressingly well.  New physics, were it to exist, should manifest itself in heavier particles, or maybe it shows up only rarely and we haven’t yet been able to spot it among the background.

The High-Luminosity Large Hadron Collider project (HL-LHC) aims to crank up the performance of the LHC in order to extend the potential for discoveries after 2025. The goal is to upgrade the LHC luminosity (the number of collisions per second) by a factor close to 10 beyond the original design value. The HL-LHC upgrade is, in fact, a major intervention that will involve more than 1.2 km of the accelerator. It will allow precise studies of the new particles observed at the LHC, such as the Higgs boson, and will open up the observation of very rare processes that are inaccessible at the LHC’s current sensitivity level. For example, the High-Luminosity LHC will produce up to 15 million Higgs bosons per year, compared to the 1.2 million produced in 2011 and 2012.

Increasing the luminosity increases the sensitivity to rare events, but why would it increase the discovery potential? The LHC, as its name suggests, accelerates protons, which are composite particles made up of swarms of quarks and gluons moving at high speeds. When the proton beams meet the particles that actually collide are not the protons, but the quarks and gluons within, and each carries only a fraction of the proton energy. As the energy involved in each collision is the reservoir from which new particles are created, we would like it to be as high as possible. Most of the times, unfortunately, quarks and gluons carry just a small fraction of the total energy, but rarely one of the components of the proton can store most of its energy and yield a high-energy collision. A means of increasing the number of such collisions is simply to augment the collision rate, that is, the luminosity of the accelerator.

The HL-LHC development depends on several technological innovations. The first phase of the project began in 2011 with the “HiLumi LHC” design study. This phase brought together many laboratories from CERN’s member and associate states, and the Instituto de Física Corpuscular (IFIC) also participated. The design study came to a close in October 2015 with the publication of the technical design report, marking the start of the construction phase for the project, both at CERN and at the industry.

The particle detectors, which register what happens during the beam collisions, will have their own upgrade too. They have to operate in very harsh conditions, with extremely high radiation levels that require very specific designs for the sensors and the electronics within them. These detectors have been designed to last for 10 years under LHC conditions. By the time that the accelerator will be upgraded the detectors will also have to be replaced, either because the radiation-induced damage is already critical or because a better performance is needed to cope with the HL-LHC environment.

When the HL-LHC comes online the detectors will encounter ten times more particles during each beam cross ‒and there are 40 million of them every second‒ while performing equally or better than they currently do at the LHC. As a consequence the trackers, the parts of the detector that register the trajectory of the particles, will need to increase their granularity by a factor of 5, the equivalent to augmenting the number of pixels in a digital camera to keep the level of detail in the picture. Other parts of the detector, like the calorimeters, the ones that absorb the particles to measure their energy, will not suffer that much from the increased number of particles, provided that the trackers give the correct information about their trajectory.

While the tracking capabilities of the detector are critical, the “intelligence” of the system, its ability to determine whether the data collected during one collision is worth to be stored for further studies, is also of paramount importance. Clearly we do not want to miss the picture of a new particle either because we are busy storing data or because our built-in intelligence decided that this event was not interesting enough.

As in the case of the accelerator, the development of the new detectors requires large collaborations where physicists and engineers from every branch one can imagine work together to solve the technological challenges. IFIC is playing a very important role in two fundamental components of the future upgrade of ATLAS, one of the general-purpose experiments that operate at the LHC. The first component is the electronics that will extract the data from the calorimeters, which must do its job fast enough to allow the system to decide whether the data from a given collision should be kept or can be dropped. The second is the design and construction of part of the trackers that register the particle trajectories, which must be capable of operating with a spatial resolution of about 20 millionths of a meter. We are designing and characterizing the silicon strip sensors, the local supports for these sensors, the services (cooling, optical fibers for data and control signals and cables for voltages) needed to operate the sensors and  the mechanical structure, three meters long and two meters in diameter, that will allow to maintain the overall spatial resolution as well as to support the sensing elements of the tracker.