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UCSB physicists publish a null LHC black hole search as an exclusion limit
Tamas Vami and Danyi Zhang looked through CMS data for microscopic black holes, found none, and published the bound. They also describe the search itself as a new way to look for new particles.
The Scientist · Science desk
What happened
- Researchers at UC Santa Barbara working on the CMS experiment extended the LHC search for microscopic black holes, found no signal, and published the resulting bound in Progress in High Energy Physics.
- The objects were proposed as an answer to puzzles including the hierarchy problem, which asks why gravity is so much weaker than the other fundamental forces.
- The instant evaporation of any such object was misunderstood in the popular imagination, which latched onto reports of potentially stable black holes forming at the collider.
- Searches so far have excluded a large space of possibilities, and the absence of any new physics at the LHC is what the group calls a real conundrum for fundamental physics.
- The group also presents the technique it used as a new way of searching for new particles, as well as a test of one black hole scenario.
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Why it matters
- constraint A theorist proposing extra-dimension physics inside the excluded region now has to explain why CMS saw nothing there. The limit narrows where the next model can put its signal.
- capability If the selection was written around a detector signature, the same data can constrain models that come later, without anyone rerunning the analysis.
- precedent Publishing an empty search as its own paper keeps negative parameter-space statements in the citable literature that future collider searches get planned against.
An exclusion limit is a claim about a region of parameter space. Danyi Zhang, a graduate student researcher in Joe Incandela's lab at UC Santa Barbara [6], said of the empty search, "It's not a dead end" [7]. She spelled out what the limit does say: "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," she said [8].
Steven Giddings keeps the recipe short. "So what do you need to make a black hole? Well, you have to compress some energy into a really small volume," said Giddings, a UCSB theorist and one of the few who proposed, when the idea first appeared, that such objects could exist under certain conditions [13][12]. That really small volume is supposed to sit inside two or more extra spatial dimensions, too small for us to register in our 3 + 1 dimensional reality [14]. String theory already requires extra dimensions, and with them, high enough energies could in principle produce quantum black holes among the trillions of proton-proton collisions the LHC delivers [10]. None would last: "if you made one, it would disintegrate immediately," Giddings said [11].
Behind the geometry is a problem of scales. Everything we observe sits far below the Planck scale, the one fundamental energy scale we know of, and theorists have argued that the new physics explaining that gap could appear at energies the LHC reaches [17]. If gravity is in fact much stronger and is leaking into extra dimensions, the Planck scale is much closer to our own energy scale than assumed [16].
Had a signal appeared, the payoff would have been direct. "Had we found evidence, we could have begun to directly study quantum gravity," said Tamas Vami, a researcher on the CMS experiment doing postdoctoral work under Incandela [3][4]. He called it "a step toward unifying all of the known fundamental forces, which has been a goal of physicists for more than a century" [5].
Phys.org's account describes the method as a new way of searching for new particles [2] and does not include the excluded mass range or the size of the collision dataset [21]. Whether the technique carries over to other searches depends on how the selection was defined. A cut built around a signature the detector records can be reinterpreted later against models nobody has written yet; a cut tuned to one predicted particle's kinematics cannot. Vami and Zhang's group puts the empty result in the record that guides the next round of ideas and searches [19].
What to watch
- Whether the Progress in High Energy Physics paper defines its selection by detector signature. That would let later models be tested against the same limit.
- Whether another LHC collaboration reuses the method or reinterprets the bound for a different extra-dimensions scenario.
- Whether the limit gets recomputed on larger collision datasets as they accumulate.