Large Hadron Collider’s Shortest Awakening Ever. And Its Last.

Year 2026 will be a transition year, marking the end of the LHC and the beginning of the HL-LHC era

The super particle accelerator “LHC” will soon be replaced by the super-super particle accelerator “HL-LHC”; and this is what happens when two bunches of protons accelerated by the HL-LHC smash into each other, giving birth to a very large number of (potentially new and unknown!) particles. [Credits: 2023, CERN / ATLAS Experiment] (Source)

The Large Hadron Collider is currently sleeping in its underground den. It will soon be awakened from its nap, and it will come back to work, colliding protons and lead ions for a community of several thousand physicists around the world. But only for a short time. This year, the largest of the CERN colliders will resume operations for a brief period, marking its final task before retiring to make way for its successor.

A Deserved Winter Nap

The Large Hadron Collider, also known by its short acronym LHC, is the most powerful particle accelerator ever built. It runs in a 27 km-long tunnel at 100 meters deep under the French and Swiss countryside, just outside the city of Geneva, and it’s used to collide subatomic particles, at the highest energy ever reached by humans, to explore and unveil the most hidden secrets of our universe.

At the moment, the Large Hadron Collider is sleeping the sleep of the just, after all the hard work it went through last year to collide particles almost continuously for many months (when it’s running, it’s a 24/7 service!), giving wondrous data to physicists to study.

At this time of year, in fact, the LHC is usually shut down. We’re currently in what we call the “EYETS”, that is, the “Extended Year-End Technical Stop”. Between December and March, the particle accelerator is shut down so physicists, engineers, and technicians can replace parts, calibrate sensors, and upgrade the accelerator and its experiments with new particle detectors.

While the Large Hadron Collider sleeps in its tunnel, engineers and technicians replace parts and test components. In the image, two technicians walk in the LHC tunnel beside the “dipole" magnets close to the cavern of one of the major experiments, ATLAS. Source: CERN

Then, around March, the accelerator is slowly awakened from its winter nap and carefully switched on, and it would collide particles 24/7 until around the end of November, when it stops again for the next winter break.

But this year is different. The CERN engineers will wake up the LHC earlier than usual, and that simple fact will mark the end of an era.

A Packed Short Year for the LHC

The LHC will be awakened and switched back on as early as the third week of February. It will then run until June, first colliding and smashing protons, then lead atoms (actually, we should say lead “ions”, as we’ll see below in more detail). In between, the accelerator will also go through a series of technical and stress tests.

Every year, the CERN team that operates the large particle accelerator publishes a new “LHC Schedule” calendar. It lists, day by day, all the accelerator’s tasks and duties for the upcoming year, as well as all tests, pauses, restarts, and stops. Let’s explore that calendar together!

CERN published the latest version of the LHC calendar at the end of last year. The public announcement came after the accelerator team and the representatives of all experiments using its particle collisions agreed on the new schedule, and the CERN Research Board approved it. The calendar is a public document, and you can access it freely on the CERN documents website. I attach a partial screenshot of it, below, for your eyes’ pleasure.

**☀️ Ric’s Note:** Yes, I know, the graphic of the calendar is not the most visually appealing thing you encountered in your life, nor the choice of colors. If Apple had to publish anything like this for a hypothetical “MacProton” or “iSmash” particle accelerator, it would have been much fancier and more stylish, with an accurate color palette that conveyed the right message and mood to the reader. But, you know, we’re physicists and engineers, so…

Starting in January, the “LHC Schedule” calendar shows the entire year divided into weeks, with columns for each week (Week 1 of the year, Week 2, etc.) and rows for each day of the week (Monday, Tuesday, etc.). The first column is colored in yellow, as CERN was in the “Annual Closure”. After that, from week 2 through week 6, the green color indicates that the LHC is in the aforementioned “EYETS” period (or, simply, “YETS”, as written in the table, where we omit the “Extended” at the beginning and we keep “Year-End Technical Stop”), during which all maintenance and upgrade work is carried out.

Then, in the second half of February, the awakening phase will start. First, a patrol carefully checks the accelerator’s tunnel for potential sources of issues — such as lost screwdrivers or bolts, which the powerful magnetic field in the accelerator could send flying around and smashing into the precious LHC components! —, closes the 27-km tunnel, and gives the “green light”: the “re-commissioning” phase then starts, where the accelerator is awakened and put back to work.

At first, we have the “Hardware re-commissioning” phase (highlighted in the calendar in a lovely pink!), when the accelerator’s team verifies all the components; then, the “beam experts” start injecting particles into the LHC and test the whole acceleration machinery: that is the “Re-commissioning with beams” phase, which, this year, will take place at the beginning of March (you can see it highlighted in the calendar in a shiny acid green!).

An image of the “beamspots” of the two beams of particles circulating in the LHC. At the four collision points along the 27-km tunnel, the beams are steered to meet, and the accelerated particles in them smash into each other! Source: CERN

This year’s recommissioning phase of the particle beams will be the shortest the LHC has ever seen: usually, the CERN engineers take about three weeks to test everything; however, this year everything is faster and condensed, and so the engineers shortened the acceleration tests to just nine days!

And after that, the “fireworks” will begin! 💥

When the teo “stable beams” of protons meet, the collision energy generates many different new particles. Those can be if different type and can recombine in very different ways. Sometimes, they produce huge sprays of particles, which we call what we call “hadronic jets”, and that’s really where “subatomic fireworks” begin!!! — Source: ATLAS Experiment/CERN

At the beginning of Week 10, in fact, the LHC will provide the “First stable beams” to the experiments. That is an important step in restarting a giant particle accelerator such as the LHC: it means that everything has been tested and is under control, and that the machine can operate beams of accelerated particles in a carefully controlled and safe way. The word “safe” does not refer to people or the environment—the LHC runs about 100 meters underground, and all particles it produces are easily stopped by the earth and rocks around—but to the detectors and sensors that help the experiments catch the particles produced by the LHC in the collisions. When in “stable beams”, the collisions occur exactly where they are supposed to happen, at the center of the experiments; that means that all the delicate sensors that will record and measure the particles created in the collisions can be safely switched ON, and the computers can start recording and save the data.

In a few words, we could translate “stable beams” as “collisions good for physics”! And those are exactly what all the experiments on the LHC crave for!

☀️Ric’s Note: On the day when we expect the first “stable beams” from the LHC, the control room of the ATLAS experiment, the largest of the experiments on the Large Hadron Collider, will be packed! All coordinators for the various data-taking activities will be there. I’ll be there, too, coordinating activities to scan the earliest collisions, pick the most interesting ones, and help produce stunning data visualizations. Stay tuned for this year’s images! I’ll share them in one of my next stories 😉

Then, once everything is under control, the LHC will proceed to the next step, which includes the “Intensity Ramp-Up”. The LHC engineers will gradually increase the number of particles injected into the accelerator, the electric field that “pushes” them, and the power of the superconducting magnets that keep them on track to reach the largest number of collisions and the highest energy.

Where the green/pink striped area ends, a dull pink area begins, which is the one that the physicists working on the LHC experiments always await the most: the “data taking”!

1200 Bunches To Start This Year’s Data Taking

In the “data taking” phase, all collisions are well controlled, all the detectors and sensors are ON, all data are good and ready to be saved, and flow into the data centers to fill disks and tapes. And physicists are happy! 😁

This year, data taking will start in week 12, around mid-March, and will begin with an awesome “1200 bunches”. Let’s see together what that value means and why it is important.

Above, I wrote that the accelerator teams inject particles into the accelerator. But the operation is not that simple. Actually, particles are injected and accelerated in “bunches”: each bunch is a packet containing several million particles. Bunches are injected into the accelerator gradually, one after another, with a specific time interval, until the entire pipe is filled. The larger the number of bunches in the accelerator, the higher the number of collisions we can get, and the higher the data we can record.

However, as the number of bunches running in the accelerator increases, it becomes technically more challenging to keep them all on track, aligned, and in sync with the electric field that pushes them. So, there’s an optimal “working point” that is the best compromise between the maximum number of colliding bunches in the accelerator and the efficient, reliable acceleration and collimation of those bunches.

This year, the accelerator team announced that it will start data-taking with 1200 bunches circulating in the machine, as you can see highlighted in the calendar above, in Week 12. After the start, the number of bunches is usually increased to maximize the data-taking and collect more collisions.

🕶️ Geeky Note: You could be wondering how many particles actually circulate in the accelerator… 😉 The number of protons per bunch is another parameter of the accelerator’s setup. The LHC has already successfully handled about 1.4 × 10¹¹ protons per bunch, which means an incredible 140,000,000,000 protons in each of the 1200 bunches! 💥 And the LHC team aims for an even larger 2.3 × 10¹¹ protons per bunch this year!

☀️**Ric’s note:** A fun note, here! 😁As you can see from the calendar, this year, Easter will come when the data-taking phase has already started. This is somewhat new, because in the past “standard” years, the start of data-taking has practically always coincided with Easter or one of the European holidays around it (such as Holy Friday or Whit Monday)! And note that the Easter date changes every year. But yet the LHC team always managed to get the first “stable beams” always on Easter or the days before or after!! 😄 (Which of course always forced all coordinators across the different activity areas in the experiment to be present in the control room or actively on call on that holiday!! — including me! 😅)

Testing the Future

Scattered throughout the data-taking phase, some “Machine Development” tests are planned. You can see them in this year’s calendar, labeled “MD1”, “MD2", and “MD3", and highlighted in sober, classy hues of ancient blue.

Those are periods when the accelerator team tests new features and settings with real particle beams for future data-taking.

In those tests, for example, the LHC engineers test new strategies to accelerate the particles, such as new combinations of electric and magnetic field strengths, or new focusing techniques to better squeeze the particle bunches and obtain more collisions.

**☀️Ric’s Note:** This year, the LHC team will also perform some tests to experiment and monitor the new working conditions of the next CERN particle accelerator at CERN, which I’ll introduce to you at the bottom of this little article. I’ll share more about those important tests in the next article; so, subscribe to get my updates and stay tuned!

Smashing Heavy Lead Atoms

The LHC accelerates protons most of the time, about 85–90% of the time. The rest of the time, it accelerates other particles, especially what we call “heavy ions”.

As we might recall from school or earlier readings, an atom consists of protons and neutrons packed into a nucleus, with a cloud of electrons “orbiting” it. — (Actually, electrons do not really orbit the nucleus, but that’s a good enough metaphor here, and we can save a deeper explanation of electronic “orbits” for another article)

Atoms in the natural world are “neutral”; that is, they have no charge: the number of positive charges in the nucleus (the number of protons, which are positively charged, “+”) and the number of negative charges in the cloud of electrons (which are negatively charged, “-”) are equal, so they balance, and the overall atom has no charge: it’s “neutral”.

However, if we strip off some of the electrons or if we force some electrons to join the atom, we disrupt the balance, and the atom gets a surplus of positive charges (that is, protons, if we strip off some of the electrons) or an excess of negative charges (i.e., electrons, which are negatively charged). Hence, the atom got a charge, and we call it an “ion”.

And those “ions”, specifically “lead ions", are what the LHC will accelerate at the end of this year, 2026, to let physicists study collisions between particles at a very high energy density and to study the evolution of matter under the same conditions that existed in our universe in the very first instants of its life.

But, wait! Why do we need to accelerate “lead ions”?! Could not take the standard lead atoms and accelerate them?

The answer is no, unfortunately. A classic particle accelerator like the LHC uses an electric field to push the particles and a magnetic field to steer them and keep them on track in the accelerator’s pipe. But electric and magnetic fields can only act on charged particles, that is, particles that have an electric charge; they have no effect on neutral particles. So we cannot accelerate standard atoms in a particle accelerator like the LHC: they would not accelerate at all because they would “feel” neither the electric field nor the magnetic field.

Therefore, we need to strip off the electrons from a lead atom to make a lead ion, and then accelerate that.

And this year’s data-taking will include some days of collisions between accelerated lead ions, which will make my colleagues working on the physics of the quark-gluon plasma very happy!

🕶️ **Geeky Note:** The quark-gluon plasma is a very special state of matter, where all base constituents are not bound and free to move around. This is only possible at very high temperatures and density, the conditions there were in our baby universe just a few microseconds after the Big Bang! So, studying the quark-gluon plasma gives insights into the evolution of our universe itself!

The End of an Era

And then, after the collisions between lead ions, the CERN engineers will stop the Large Hadron Collider.

Forever.

At the end of June, in fact, the current data-taking period, called Run3, will end, and with it the entire LHC project as we know it today.

The LHC began colliding particles for scientific research in 2008. Over almost twenty years, it produced new particles at the highest energy ever achieved, allowing particle physicists like me to study the deepest mysteries of our universe.

Over the years, the LHC has enabled many new measurements and discoveries. Among them, the most well-known is possibly the discovery we made of the particle called the “Higgs boson” in 2012, which, the year after, brought the Nobel Prize to two of the three theorists who built a theory to explain how particles get their mass, and where the particle known as the Higgs boson is the intermediary that makes that possible.

But the Higgs Boson is only one example. Using data from LHC collisions, physicists compiled a notable list of important discoveries and new measurements. For the sake of brevity, I mention only the world’s most precise measurement of the mass of the “W” particle (which is the carrier of one of the four fundamental forces, the “weak force”), and finding evidence of very rare physics processes where the Higgs boson plays a major role, which helps physicist further understand how that Higgs boson interact with the other particles.

Data visualizations of two rare decays of a Higgs boson produced in the LHC collisions, as seen in the ATLAS experiment. In the event visualized on the left, the Higgs boson decays into two muons, whose tracks are visualized as red lines. On the right, a Higgs boson decaying into a photon (visualized as a pink cone) and a Z boson, further decaying into two electrons (the green lines). Source: ATLAS/CERN

☀️ Ric’s Note: In 2025, an international committee awarded the LHC four major experiments on the LHC the Breakthrough Prize in Fundamental Physics! The prestigious $ 3M prize was awarded to all the physicists working on the ATLAS, CMS, LHCb, and ALICE experiments during the period 2015–2024, for precise measurements of the Higgs properties, exploring the relationship between matter and antimatter, probing rare interactions between particles, studying the early Universe, and the discovery of new particles. The prize money will be used to fund PhD programs and support doctoral students. — BTW, I’m among the awarded scientists, as well! 🙃

I wrote above that the LHC will stop at the end of Run3. The life of a particle accelerator is usually divided into major “Runs”: years of data-taking with similar configurations, for example, the same energy and a similar number of collisions. Between those Runs, there are periods where physicists and technicians carry out upgrade work; we call those periods without beams and collisions “long shutdowns”.

CERN published a long-term calendar outlining its plans. As you can see in the image below, the current major Run, Run3, will indeed end at the end of June this year, and, with it, the LHC project.

The long-term schedule CERN published in November 2024, outlining plans for the period 2021–2041. Source: CERN.

But, wait! You told me that the LHC will end in June. But here I see other major Runs planned: Run4 and Run5! So, what does that mean?

Yes, that’s true: other Runs are scheduled because even if the LHC ends, collisions between particles at CERN will not stop any time soon…

A New Super-Super Particle Collider!

The next data-taking period, also known as Run4, will begin only in a few years, and in the meantime, the current “super” particle collider, the LHC, will undergo a complete restyling, transforming it into a “super-super” particle collider known as the HL-LHC!

The super-super particle collider, HL-LHC, will occupy the same tunnel as the LHC; it won’t be any longer. And it won’t be much more powerful either, pushing particles to almost the same energy as the LHC, just slightly above. However, its superpower resides in its “luminosity”.

Luminosity is a very important parameter of a particle collider: in a few words, it measures the number of particles the collider is able to collide. More technically, it measures the number of collisions between particles that the collider delivers each time two bunches of accelerated particles bump into each other. Having a high number of collisions increases the probability of creating unknown particles, especially the ones thought to be the rarest, and yields more data, which translates into greater statistical power when analyzing the experimental data.

At the end of June 2026, the new phase, Long Shutdown 3 (LS3), will begin, with a lengthy renovation schedule. CERN engineers, for example, will replace superconducting magnets with new versions to better steer and focus the accelerated particle bunches before they crash into each other, thereby maximizing the interaction area and increasing the number of collisions. At the same time, the experiments along the tunnel, especially those surrounding the points where the accelerated particles collide, such as ATLAS or CMS, will replace several particle detectors to cope with the higher number of collisions per second to record, the higher number of particles to detect, identify, and measure, and the overall larger data volume to analyze.

The Long Shutdown 3 will end in 2030, and the new HL-LHC will be able to start colliding its first beams of particles.

The HL-LHC will empower new physics searches with analytical superpowers to try to answer the many open questions we still have about the most fundamental aspects of our own universe, such as:

• Why do we now observe almost exclusively matter if matter and antimatter were initially created in equal amounts at the beginning of the universe?
• How did the Quark-Gluon Plasma, the hot mixture of quarks and gluons that led to the formation of everything in the universe, initially form?
• How did the Higgs field, which gives mass to particles through its interactions with them, originate?
• And many others…

So, 2026 will be the shortest data-taking period of the LHC and will mark its end. However, the quest for answers will continue beyond that year with the LHC successor, the HL-LHC. Many years of exploring the unknown lie ahead.

☀️**Ric’s note:** Hey, please don’t forget: if you have questions on the subject of this story, feel free to put them in the comments — I love questions! And follow me to be notified as soon as I publish a new story.

About the author

**Riccardo Maria BIANCHI, Ph.D., *is a scientist working at CERN, where he works on the ATLAS experiment at the Large Hadron Collider, the most powerful particle accelerator ever built. In his daily life as a particle physicist, he searches for new particles to shed light on the universe’s deepest mysteries and leads research teams. Here, he writes about CERN, physics, and the history of science. Always in simple words and in an engaging way, of course! — Nice to meet you! 🤝🙋‍♂️*

📚 Resources — If you want to dive deeper

This is a good list of resources if you want to dive deeper into the subject of this story. I took the time to curate this list to be informative without becoming overly technical. I hope you enjoy it!

🕶️ Geeky Note: Of course, if you want more technical information and links to research papers, just let me know that in the comments, and I’ll add another, “even deeper”, section 😉

The Large Hadron Collider at CERN

The awesome Symmetry Magazine has published many good stories about the Large Hadron Collider; you can find them all listed on this page.

LHC Winter Nap

If you want to know more about why we stop the LHC in Winter and how we get the experiments back to life in Spring, I have an article about that, here on Medium:

[embed]The Large Hadron Collider gets reset and refreshed each year — a CERN physicist explains how the… How to wake a giant physics experiment from its winter nap!medium.com

The High-Luminosity LHC

Explain it in 60 seconds: High-Luminosity LHC — A very short piece by Symmetry Magazine about the HL-LHC

Getting Ready for the super-super collider, the HL-LHC

ATLAS prepares for High-Luminosity LHC — A story in which Andreas Hoecker, the former spokesperson of the ATLAS experiment at CERN, the largest of the four main LHC experiments, recounts all the experiment's major upgrades to get ready for the HL-LHC.