{"id":26906,"date":"2025-07-05T23:16:43","date_gmt":"2025-07-05T17:31:43","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=26906"},"modified":"2025-07-05T23:26:06","modified_gmt":"2025-07-05T17:41:06","slug":"can-the-large-hadron-collider-snap-string-theory","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/can-the-large-hadron-collider-snap-string-theory\/","title":{"rendered":"Can the Large Hadron Collider snap string theory?"},"content":{"rendered":"\n

Penn physicists and collaborators at Arizona State University “test” the fallibility of a framework that seeks to unite physics across the universe.<\/em><\/strong><\/p>\n\n\n

Penn Today<\/p><\/div><\/div>\n\n\n

In physics<\/strong>, two great pillars of thought don\u2019t quite fit together. <\/p>\n\n\n\n

The Standard Model of particle physics describes all known fundamental particles and three forces: electromagnetism, the strong nuclear force, and the weak nuclear force. Meanwhile, Einstein\u2019s general relativity describes gravity and the fabric of spacetime.<\/p>\n\n\n\n

However, these frameworks are fundamentally incompatible in many ways, says Jonathan Heckman<\/a>, a theoretical physicist at the University of Pennsylvania<\/a>. The Standard Model treats forces as dynamic fields of particles, while general relativity treats gravity as the smooth geometry of spacetime, so gravity \u201cdoesn\u2019t fit into physics\u2019 Standard Model,\u201d he explains.<\/p>\n\n\n\n

In a recent paper<\/a>, Heckman, Rebecca Hicks, a Ph.D. student at Penn\u2019s School of Arts & Sciences<\/a>, and their collaborators turn that critique on its head. Instead of asking what string theory predicts, the authors ask what it definitively cannot create. <\/p>\n\n\n\n

Their answer points to a single exotic particle that could show up at the Large Hadron Collider (LHC). If that particle appears, the entire string-theory edifice would be, in Heckman\u2019s words, \u201cin enormous trouble.\u201d<\/p>\n\n\n\n

\"\"
Event display in the signal region from data taken in 2018. The pixel tracklet candidate with pT<\/sub> = 1.2 TeV is shown by the red solid line, and other inner detector tracks by the thin orange lines. Jets are shown by the transparent yellow, blue, and red cones. The missing transverse momentum is shown by the white dotted line. The green and yellow bars indicate energy deposits in the liquid argon and scintillating tile calorimeters, respectively. The event is common to both the electroweak and strong production signal regions. Event and run numbers are shown in the bottom left corner.<\/sup><\/em> Credit: ATLAS Collaboration CERN<\/sup><\/figcaption><\/figure>\n\n\n\n

String theory: the good, the bad, the energy-hungry<\/h3>\n\n\n\n

For decades, physicists have sought a unified theory that can reconcile quantum mechanics and, by extension, the behavior of subatomic particles with gravity, which is described as a dynamic force in general relativity but is not fully understood within quantum contexts, Heckman says.<\/p>\n\n\n\n

A good contender for marrying gravity and quantum phenomena is string theory, which posits that all particles, including a hypothetical one representing gravity, are tiny vibrating strings and which promises a single framework encompassing all forces and matter.<\/p>\n\n\n\n

\u201cBut one of the drawbacks of string theory is that it operates in high-dimensional math and a vast \u2018landscape\u2019 of possible universes, making it fiendishly difficult to test experimentally,\u201d Heckman says, pointing to how string theory necessitates more than the familiar four dimensions\u2014x, y, z, and time\u2014to be mathematically consistent.<\/p>\n\n\n\n

\u201cMost versions of string theory require a total of 10 or 11 spacetime dimensions, with the extra dimensions being sort of \u2018curled up\u2019 or folding in on one another to extremely small scales,\u201d Hicks says.<\/p>\n\n\n\n

To make matters even trickier, string theory\u2019s distinctive behaviors only clearly reveal themselves at enormous energies, \u201cthose far beyond what we typically encounter or even generate in current colliders,\u201d Heckman says.<\/p>\n\n\n\n

Hicks likens it to zooming in on a distant object: at everyday, lower energies, strings look like regular point-like particles, just as a faraway rope might appear to be a single line. <\/p>\n\n\n\n

\u201cBut when you crank the energy way up, you start seeing the interactions as they truly are\u2014strings vibrating and colliding,\u201d she explains. <\/p>\n\n\n\n

\u201cAt lower energies, the details get lost, and we just see the familiar particles again. It\u2019s like how, from far away, you can\u2019t make out the individual fibers in the rope. You just see a single, smooth line.\u201d<\/p>\n\n\n\n

That\u2019s why physicists hunting for signatures of string theory must push their colliders\u2014like the LHC\u2014to ever-higher energies, hoping to catch glimpses of fundamental strings rather than just their lower-energy disguises as ordinary particles.<\/p>\n\n\n\n

Why serve string theory a particle it likely won\u2019t be able to return?<\/h3>\n\n\n\n

Testing a theory often means searching for predictions that confirm its validity. But a more powerful test, Heckman says, is finding exactly where a theory fails. If scientists discover that something a theory forbids actually exists, the theory is fundamentally incomplete or flawed.<\/p>\n\n\n\n

Because string theory\u2019s predictions are vast and varied, the researchers instead asked if there\u2019s a simple particle scenario that string theory just can\u2019t accommodate.<\/p>\n\n\n\n

They zeroed in on how string theory deals with particle \u201cfamilies,\u201d groups of related particles bound together by the rules of the weak nuclear force, responsible for radioactive decay. Typically, particle families are small packages, like the electron and its neutrino sibling, that form a tidy two-member package called a doublet. String theory handles these modest particle families fairly well, without issue.<\/p>\n\n\n\n

However, Heckman and Hicks identified a family that is conspicuously absent from any known string-based calculation: a five-member particle package, or a 5-plet. Heckman likens this to trying to order a Whopper meal from McDonald\u2019s: \u201cNo matter how creatively you search the menu, it never materializes.\u201d<\/p>\n\n\n\n

\u201cWe scoured every toolbox we have, and this five-member package just never shows up,\u201d Heckman says.<\/p>\n\n\n\n

But what exactly is this elusive 5-plet?<\/p>\n\n\n\n

Hicks explains it as an expanded version of the doublet: \u201cThe 5-plet is its supersized cousin, packing five related particles together.\u201d<\/p>\n\n\n\n

Physicists encapsulate this particle family in a concise mathematical formula known as the Lagrangian, essentially the particle-physics cookbook. The particle itself is called a Majorana fermion, meaning it acts as its antiparticle, akin to a coin that has heads on both sides.<\/p>\n\n\n\n

Identifying such a particle would directly contradict what current string theory models predict is possible, making the detection of this specific particle family at the LHC a high-stakes test, one that could potentially snap string theory.<\/p>\n\n\n\n

Why hasn\u2019t a 5-plet been spotted, and the vanishing-Track clue<\/h3>\n\n\n\n

Hicks cites two major hurdles for spotting these 5-plet structures: \u201cproduction and subtlety.\u201d<\/p>\n\n\n\n

In a collider, energy can literally turn into mass; Einstein\u2019s E = mc\u00b2 says that enough kinetic oomph (E) can be converted into the heft (m) of brand-new particles, so the heavier the quarry, the rarer the creation event.<\/p>\n\n\n\n

\u201cThe LHC has to slam protons together hard enough to conjure these hefty particles out of pure energy,\u201d Hicks explains, citing Einstein\u2019s E = mc\u00b2, which directly links energy (E) to mass (m). \u201cAs the masses of these particles climb toward a trillion electron volts, the chance of creating them drops dramatically.\u201d<\/p>\n\n\n\n

Even if produced, detection is challenging. The charged particles in the 5-plet decay very quickly into nearly invisible products. \u201cThe heavier states decay into a soft pion and an invisible neutral particle, zero (X0),\u201d Hicks says. <\/p>\n\n\n\n

\u201cThe pion is so low-energy it\u2019s basically invisible, and X0 passes straight through. The result is a track that vanishes mid-detector, like footprints in snow suddenly stopping.\u201d<\/p>\n\n\n\n

Those signature tracks get picked up by LHC\u2019s ATLAS (short for A Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid), house-sized \u201cdigital cameras\u201d wrapped around the collision center. <\/p>\n\n\n\n

They sit at opposite collision points and operate independently, giving the physics community two sets of eyes on every big discovery. Penn physicists like Hicks are part of the ATLAS Collaboration, helping perform the searches that look for quirky signals like disappearing tracks.<\/p>\n\n\n\n

Why a 5-plet matters for dark matter<\/h3>\n\n\n\n

Hicks says finding the 5-plet isn\u2019t only important for testing string theory, pointing to another exciting possibility: \u201cThe neutral member of the 5-plet could explain dark matter, the mysterious mass shaping up most of our universe\u2019s matter.\u201d<\/p>\n\n\n\n

Dark matter constitutes roughly 85 percent of all matter in the universe, yet scientists still don’t know what exactly it is.<\/p>\n\n\n\n

\u201cIf the 5-plet weighs around 10 TeV\u2014about 10,000 proton masses\u2014it neatly fits theories about dark matter\u2019s formation after the Big Bang,\u201d Hicks says. \u201cEven lighter 5-plets could still play a role as part of a broader dark matter landscape.\u201d<\/p>\n\n\n\n

\u201cIf we detect a 5-plet, it\u2019s a double win,” says Hicks. \u201cWe\u2019d have disproven key predictions of string theory and simultaneously uncovered new clues about dark matter.\u201d<\/p>\n\n\n\n

What the LHC has already ruled out<\/h3>\n\n\n\n

Using existing ATLAS data from collider runs, the team searched specifically for 5-plet signals. <\/p>\n\n\n\n

\u201cWe reinterpreted searches originally designed for \u2018charginos\u2019\u2014hypothetical charged particles predicted by supersymmetry\u2014and looked for 5-plet signatures,\u201d Hicks says of the team\u2019s search through the repurposed ATLAS\u00a0disappearing-track data<\/a>. <\/p>\n\n\n\n

\u201cWe found no evidence yet, which means any 5-plet particle must weigh at least 650\u2013700 GeV, five times heavier than the Higgs boson.\u201d<\/p>\n\n\n\n

For context, Heckman says, \u201cThis early result is already a strong statement; it means lighter 5-plets don\u2019t exist. But heavier ones are still very much on the table.\u201d<\/p>\n\n\n\n

Future searches with upgraded LHC experiments promise even sharper tests. \u201cWe’re not rooting for string theory to fail,\u201d Hicks says. \u201cWe’re stress-testing it, applying more pressure to see if it holds up.”<\/p>\n\n\n\n

\u201cIf string theory survives, fantastic,” Heckman says. “If it snaps, we’ll learn something profound about nature.\u201d<\/p>\n\n\n\n

This work received support from the Department of Energy, the U.S.-Israel Binational Science Foundation, and the National Science Foundation. <\/p>\n\n\n\n

The article title “How to falsify string theory at a collider<\/a>” was published in the journal Physical Review Research on 27 May 2025. <\/p>\n\n\n\n

The story is provided by the University of Pennsylvania.<\/p>\n","protected":false},"excerpt":{"rendered":"

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