LHC with higher luminosity

The large CERN particle accelerator will soon become even more powerful

On 15 June, the European Particle Physics Laboratory (CERN) in Geneva officially celebrates the upcoming upgrade of the Large Hadron Collider (LHC). By the year 2026, the performance of the world's largest particle accelerator will be significantly improved by many technical optimizations in order to empower new insights into the nature of matter.

HL-LHC
Image: CERN, Switzerland

Anyone upgrading its rental car gets more power. More power: this is the aim of the upcoming upgrade of the CERN particle accelerator, the LHC. The technical modification of the accelerator is a huge project, so for the start of the work CERN has established a special celebration: On June 15, the particle physics laboratory will perform a 'High Luminosity LHC kick-off event' in Geneva (link to the press conference).

From the LHC to the HL-LHC

Key innovative technologies will be deployed to render the LHC a 'High-Luminosity-LHC' (HL-LHC) in 2016. In a simplistic way, one could say that the future particle accelerator will allow to illuminate the properties of matter even more clearly than the current particle accelerator, which has been in operation since 2009 and made possible the discovery of the Higgs boson in 2012.

'Luminosity' is, in simple words, a term that refers to the rate of proton-proton collisions in the LHC - and is thus the most important measure of its performance. The background: The LHC is a 27km-long circular tunnel in which protons circulate almost with the speed of light - one half in a clockwise direction, the other half in a counter clockwise direction. At four points, the protons are being brought to collision, their constituents interact and the results of these interactions, an abundance of particles, are then examined with detectors (e.g. ATLAS).

Four times higher luminosity

The protons do not circulate individually in the LHC, but in bunches. Each bunch contains more than 100 billion protons. When two proton bunches collide, not all these protons collide but 'only' about 50 of them (since there is much empty space between the protons, in the scale of their size). Since the proton bunches in the accelerator circulate about 30 million times per second, the ATLAS detector sees 1.5 billion proton-proton collisions per second.

The HL-LHC upgrade will result in a rate of collisions that is about four times higher compared to that current data-taking rate. This means that the ATLAS detector will be seeing 200 simultaneous proton-proton collisions every time the proton bunches are colliding (as shown in the figure attached). This will result to unprecedented amounts of data that particle physicists can use for their research. One of the most exciting things particle physicists search for is a sign for new phenomena. These would be extremely rare and can only be given access to by producing more and more proton collisions. That way the probability of seeing something rare increases and the possibility that CERN physicists see it in their detectors opens up.

The LHC detectors at the HL-LHC upgrade

With the accelerator producing significantly more proton-proton collisions than ever before, at the HL-LHC conditions the current LHC detectors will not be able to exploit the amounts of input data at the maximum. This is the reason why the detectors themselves are undergoing upgrades.

The upcoming upgrade of the detectors is a major project in itself, involving thousands of people over the next eight years. These upgrades can easily be compared to the upgrade of a photographic camera: choosing a large number of pixels and a fast lens allows us to take pictures with better resolution even when the object of the picture is moving. Similarly, upgrading CERN detectors and their “triggering” systems allows CERN physicists to have a more detailed picture of the results of the proton-proton collision and take more such pictures.

As previously written, the accelerator is colliding 30 million proton bunches per second. The detector sees all these collisions, however CERN physicists can only know of a small fraction of them: it would be impossible to store, process and analyse them all. The system that decides which collision events to store is called the “trigger” system. It is composed of super-fast electronics and a very performant large computing farm, which runs complicated algorithms. The higher the rate of events the trigger system has to process (x 4 for the HL-LHC compared to the LHC) and the “busier” the collision events are (200 simultaneous collisions at the HL-LHC rather than 50 at the LHC), the more complicated the trigger system has to become in order to cope with the challenge.

A better ATLAS trigger

And here comes Anna Sfyrla into play. Born in Greece, Sfyrla has been working as assistant professor at the University of Geneva since autumn 2015. Anna Sfyrla’s work focuses mainly on the trigger and its upgrade. Sfyrla takes care of the adjustments needed to get the ATLAS trigger ready for the increased data volumes of the HL-LHC. She is leading a group that studies the requirements for the upgraded system and the performance of the new design. She is also participating in the R&D for a new system that will render the trigger more robust to the challenging conditions. “This is a challenging project that will provide the trigger system the necessary ingredients to face its own challenge" says Anna Sfyrla.

Door to new physics

The HL-LHC project has started design studies and R&D since many years. The next few years will be about development, prototyping, testing and implementation. The new components for the ATLAS trigger will be designed in the next few years and will be in production from 2022 onwards. In 2025 the ATLAS trigger and all other detector components should be ready for commissioning before the HL-LHC embarks its operations in 2026. In the next few years, Anna Sfyrla and her colleagues will have a lot of work. This is a worthwhile investment, says Sfyrla: "Thanks to the increased luminosity that will lead to large datasets, the properties of the Higgs particle will be better understood and - hopefully - very rare processes will be observed, which have the potential to open the door to new physics”.

Author: Benedikt Vogel

This is how 200 simultaneous proton-proton collisions will show up at the detector. Most of these proton-proton collisions do not result to “interesting” interactions between the constituents of the protons. It is in the very challenging job of high energy physicists to look for the needle in the haystack: interactions that probably resulted not from processes we well know about, but something new, leading to a discovery. Graphic: ATLAS
This is how 200 simultaneous proton-proton collisions will show up at the detector. Most of these proton-proton collisions do not result to “interesting” interactions between the constituents of the protons. It is in the very challenging job of high energy physicists to look for the needle in the haystack: interactions that probably resulted not from processes we well know about, but something new, leading to a discovery. Graphic: ATLASImage: ATLAS, CERN, Switzerland
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