Science1 publisher2 min readPublished
An empty LHC search narrows where quantum black holes could still be hiding
A search of LHC collision data for microscopic quantum black holes found none. The exclusion limit that follows is the publishable result. Where that limit sits decides whether theorists have to move.
The Scientist · Science desk
What happened
- Physicists at UC Santa Barbara working on the CMS experiment searched Large Hadron Collider data for microscopic quantum black holes, found no evidence of them, and published the null result in Progress in High Energy Physics.
- The team states the outcome as an exclusion limit: if the objects existed with the properties tested, the search would have registered them, so that stretch of the theory can be ruled out.
- Steven Giddings, among the theorists who proposed the objects, said one would disintegrate immediately; the public discussion at the time centred on the fear of a stable black hole instead.
- The analysis doubled as a trial of a new technique for picking rare and previously unknown particles out of collision data.
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Why it matters
- constraint A limit is only usable if its numbers travel with it. The mass reach and the assumed production model are what a theorist pruning extra-dimension models needs before this result can displace an older bound.
- decision The group's argument for publishing nothing-found is operational: each exclusion removes options from the list and feeds the choice of where the next search points.
- capability If the rare-particle technique generalises past this one signature, it is the part of the work other analyses can reuse whether or not quantum black holes ever appear.
An exclusion limit is a conditional statement with numbers attached. For an assumed production model, a given mass and a given count of extra spatial dimensions, a signal of a stated size would have appeared in the collisions. None did. Danyi Zhang is a graduate student researcher in Joe Incandela's lab at UC Santa Barbara. "The result is an exclusion limit, which is a real, publishable statement: 'If this thing existed with these properties, we'd have seen it. We didn't, so we can rule it out here.' That's genuine knowledge about how the universe works," Zhang said. [7]
Those numbers are what a theorist uses to prune a model. The UC Santa Barbara announcement of September 22 does not give a mass reach, an assumed number of extra dimensions or the size of the collision dataset. It also skips the new technique for spotting rare particles that the team says the search let it try. [15][16][14] Without them, the statement that the limits are tighter cannot be set beside the many possibilities earlier experiments have already removed. [1][12]
The idea under test is about twenty years old, which places it near 2006. [8][17] Concentrate enough energy into a small enough region, and if extra spatial dimensions exist (string theory already requires them), a quantum black hole could form in one of the trillions of proton-proton collisions the machine produces. [8] The motivation is the distance between everyday energies and the Planck scale, the energy at which quantum gravity is supposed to matter. [11] Only some proposals put their effects within the LHC's reach, and those are the only ones a search like this constrains. [11]
These objects are microscopic and short-lived, a different sort of thing from the black holes astronomers study. "They wouldn't stick around very long -- if you made one, it would disintegrate immediately," said Steven Giddings. The UC Santa Barbara theorist was among the few to propose that such tiny voids in spacetime might exist under certain conditions. [9] When the idea reached a wider audience, the concern was that the LHC would make a stable one. [10]
Tamas Vami, the CMS researcher who carried out the work as a postdoc in Incandela's group, said what a detection would have opened up. "Had we found evidence, we could have begun to directly study quantum gravity." [3][4] The search produced no such evidence. The release describes the continued absence of clear signs of new physics at the LHC as a major challenge for the researchers working on it. [1][12]
What to watch
- The PHEP paper's limit figures: with a mass reach and an assumed production model, a reader can tell whether the excluded region is new ground or a reconfirmation.
- Whether the new technique for finding rare particles is described in detail and picked up by other CMS analyses.