How Can Lightning Be Hotter Than the Surface of the Sun? For roughly twenty to thirty microseconds, the channel of air a lightning bolt has just punched through the atmosphere sits at something like 30,000 degrees Celsius. The visible surface of the sun the layer astronomers call the photosphere runs at about 5,500 degrees Celsius. Do the arithmetic and a bolt over an open field briefly outruns a star by a factor of roughly five. That fact gets repeated often enough to have become trivia-night reflex, and repetition has flattened the strangeness out of it. The stranger question sits underneath: what, precisely, is being measured when someone assigns a temperature to a flash of electricity that exists for less time than it takes to blink? Lightning itself is not a thing with a temperature, not the way a stove burner or a cup of coffee has one. It's a current, a sudden and enormous movement of electric charge between cloud and ground, or between two regions of the ...
CERN’s LHC Shutdown: Inside the High Luminosity LHC Upgrade
Just before six in the morning on 27 June 2026, engineers at CERN watched a final beam of protons complete its circuit of the 27-kilometre ring beneath the French–Swiss border, then steered it safely into a graphite beam-dump absorber. The control-room display that operators call Page 1 recorded its last collision of the era. Two days later, on 29 June, the Large Hadron Collider was formally declared closed for physics. It will not see another proton beam until June 2030.
A four-year silence sounds, on the surface, like retirement or failure. Neither is accurate. What began on 29 June is Long Shutdown 3, or LS3 a planned, engineered transformation of the LHC into a substantially different machine, the High-Luminosity LHC, or HL-LHC. Nothing broke. Nobody is walking away. Underground, in a tunnel that has just spent years running at close to absolute zero, thousands of engineers are about to dismantle more than a kilometre of what made the LHC the LHC.
Superconducting magnets only behave as superconductors when kept extraordinarily cold in the LHC's case, at 1.9 kelvin, or roughly minus 271.3 degrees Celsius, colder than deep space. At that temperature, current flows with no electrical resistance, which is how the machine bends and focuses beams of protons travelling at nearly the speed of light without losing them to the tunnel wall. It is also a temperature at which no human being can safely work on anything nearby. Before a single component gets touched, the ring's roughly 36,000 tonnes of cryogenic equipment has to be warmed, gradually and carefully, back to room temperature a process CERN's cryogenics team has been running since the shutdown began, and one that alone takes weeks to complete.
What follows the warm-up is not a pause. Over four years, teams will remove and replace about 1.2 kilometres of magnets and beamline components, install new cryogenic distribution lines, rebuild large sections of the ATLAS and CMS detectors, and carry out consolidation work across the wider accelerator complex, from the Super Proton Synchrotron's North Area to the ISOLDE radioactive-beam facility. Calling LS3 "switching the LHC off" undersells the project about as much as calling a heart transplant "surgery."
None of that, though, is really about making the LHC more powerful in the way people usually mean the phrase. During its third operating run, the LHC collided protons at 13.6 trillion electron-volts energy that determines what kinds of particles a collision can, in principle, produce. HL-LHC will not push that number dramatically higher. The upgrade targets something else: luminosity, a measure of how many collisions the machine delivers, not how forcefully.
The distinction matters because so much of particle physics comes down to odds. A process can be fully permitted by the laws of physics and still be extraordinarily rare the way a quality-control inspector checking parts on an assembly line might need to examine a million units before finding the one flawed part that turns up only once in every few hundred thousand. Running the line faster doesn't help; running it longer, with more units passing through, does. Several Higgs boson decay modes, and most hypothetical new particles or deviations from the Standard Model, behave exactly like that flawed part: real, potentially detectable, buried in a rate too low for a modest sample size to reveal.
CERN tracks the total data collected over a run in inverse femtobarns, a unit that, combined with how often a given process occurs, tells physicists roughly how many examples of it they should expect to find. Run 3 delivered nearly 550 inverse femtobarns to ATLAS and CMS, almost double the LHC's original 300 inverse-femtobarn design target. HL-LHC is built to reach roughly 3,000 inverse femtobarns over about a decade of operation starting in 2030 a tenfold increase over the original design goal, achieved not by running for a hundred years but by making each year of running far more productive.
That productivity comes at a cost accelerator physicists call pileup. Protons circulate in tightly packed bunches rather than as individual particles, and each time two bunches cross at the centre of ATLAS or CMS, several proton-proton collisions happen at once rather than one at a time. Under HL-LHC conditions, an average bunch crossing is expected to contain around 140 overlapping interactions, with instantaneous peaks reaching as many as 200. A detector's single recorded "event" is therefore not one collision but a stack of well over a hundred, superimposed within a sliver of a nanosecond and working out which particle trajectory belongs to which of those simultaneous interactions turns out to be one of the harder computational problems the upgrade creates.
Producing that pileup requires squeezing the beams into a far smaller cross-section at the exact point where ATLAS and CMS sit, and that job belongs to quadrupole magnets, not the dipole magnets most people picture when they think of the LHC's 1,232 ring-bending magnets. Dipoles steer; quadrupoles focus, the way a lens focuses light. The quadrupoles nearest the collision points, known as the inner triplet, do more focusing work than any other magnets in the machine, and HL-LHC needs them to cut beta-star the parameter that sets how tightly the beam is focused at the collision point by roughly a factor of four compared with today's design.
Reaching that requires abandoning the niobium-titanium superconductor used in every LHC magnet built so far, which tops out around 8 to 10 tesla, in favour of niobium-tin, or Nb3Sn, a compound capable of sustaining fields near 12 tesla. Nb3Sn is notoriously brittle once formed, which makes it far harder to wind into cable and shape into a working coil than niobium-titanium engineers at CERN, Fermilab, Brookhaven and Berkeley Lab have spent years solving exactly that fabrication problem. Twenty-four of these large-aperture Nb3Sn quadrupoles, each reaching a field gradient of roughly 132 tesla per metre, will flank the ATLAS and CMS interaction points, marking the first time niobium-tin technology has been installed inside a working particle accelerator rather than a test facility.
Squeezing the beam that tightly creates a second problem the original LHC never had to solve. To keep the two counter-rotating beams from colliding parasitically at points where they shouldn't, HL-LHC's beams must cross at a wider angle than before, and a wider crossing angle means the bunches overlap less cleanly at the moment they actually meet throwing away luminosity that all that magnet work just bought. The fix is a genuinely strange piece of engineering called a crab cavity: a superconducting radiofrequency structure, cooled to about 2 kelvin, that gives each bunch a sideways electromagnetic kick timed so precisely that the bunch appears to rotate around its own centre as it approaches the interaction point. Two bunches meeting at an angle, each tilted by exactly the right amount, present nearly flat faces to each other at the instant of collision, recovering an overlap that geometry alone would have denied them. It is the first time crab cavities have ever been used on a hadron beam anywhere, tested at CERN's Super Proton Synchrotron since 2018, and CERN projects the technique alone could recover something like 65 percent more peak luminosity than a crossing-angle collision managed without it.
More collisions delivered is only half the challenge; someone still has to record them. ATLAS and CMS are each undergoing what CERN calls a Phase-2 upgrade, effectively a rebuild of their innermost systems. ATLAS is replacing its entire inner tracking detector with an all-silicon Inner Tracker, built to survive radiation doses far beyond what its current tracker can tolerate. CMS is building a High-Granularity Calorimeter using hexagonal silicon sensors to measure particle showers with enough spatial precision to separate genuine physics from the pileup crowding around it. Both experiments are adding detectors that measure not just where a particle passed through, but exactly when, with timing resolved to a matter of picoseconds, because with up to 200 interactions layered into a single crossing, a particle's arrival time can be the only way to tell which of those interactions it actually belongs to. None of this is as visually dramatic as a new magnet or a crab cavity, but without it, the extra collisions HL-LHC delivers would amount to unreadable noise.
What the additional data buys, above all, is Higgs bosons. CERN's own estimate is that HL-LHC will produce at least 15 million Higgs bosons a year, compared with roughly 3 million in 2017. That is not a bigger number for its own sake. Several of the Higgs boson's most physically interesting behaviours its decay into a pair of muons, its decay into a photon and a Z boson, and above all its coupling to itself, the quantity that shapes the potential energy underlying the mechanism of electroweak symmetry breaking occur rarely enough that even fifteen years of LHC data have left them only partially measured. A tenfold increase in statistics is what turns a hint into a measurement for processes like these.
Beyond the Higgs sector, the larger dataset sharpens every search HL-LHC runs: for particles predicted by supersymmetry, for candidates that might explain dark matter, for extra spatial dimensions, for any small deviation from Standard Model predictions that might betray physics nobody has written down yet. None of this is guaranteed. What HL-LHC guarantees is sensitivity the statistical power to notice something rare if it exists, and just as importantly, to rule out entire classes of theoretical models if it doesn't. An upgrade that finds no new particle at all would not be a wasted decade; excluding possibilities and tightening the Standard Model's predictions well past their current limits is how the field has always progressed, discovery and elimination working as two sides of the same method.
Some of that work is already visible, years before beams return. At CERN's SM18 test hall, engineers assembled a 95-metre above-ground replica of an entire HL-LHC interaction region, called the Inner Triplet String, bringing together the new quadrupole magnets, superconducting powering links, cryogenics, and protection systems exactly as they will sit underground. It was cooled to 1.9 kelvin in February 2026 and began electrical powering, circuit by circuit, that April a full rehearsal of hardware that has never operated together before, run above ground so that failures happen there rather than in the tunnel. Elsewhere across the complex, the same warm-up and dismantling work that began with LS3 in late June is now proceeding, section by section, magnet by magnet.
When beams do return in 2030, the machine circulating them will occupy the same tunnel, largely the same footprint, and quite possibly some of the same original dipole magnets. Calling it "the LHC starting again" would still undersell what will have changed. Roughly 1.2 kilometres of it will be new hardware. Its two largest detectors will be substantially rebuilt. Its capacity to collect usable data will sit an order of magnitude beyond anything the machine that shut down in June 2026 could manage.
Even with all of that, CERN's own physics projections show the combined precision ATLAS and CMS expect on the Higgs boson's self-coupling after the full HL-LHC run, after ten times the data, after every magnet and cavity and detector described here sitting at only around 50 percent. That is good enough to confirm the coupling exists and is roughly the size the Standard Model predicts. It is nowhere near good enough to settle whether the shape of the Higgs potential leaves the vacuum we occupy stable over cosmological timescales, or metastable, quietly waiting out a decay that could take far longer than the current age of the universe to occur. Ten times more data was never going to be enough to close that question. It was only ever going to be enough to ask it properly.
YOU MAY ALSO LIKE
Comments
Post a Comment