The secret lives of long-lived particles

October 3, 2016 by Sarah Charley, Symmetry Magazine, Symmetry Magazine
Credit: ATLAS collaboration

The universe is unbalanced. Gravity is tremendously weak. But the weak force, which allows particles to interact and transform, is enormously strong. The mass of the Higgs boson is suspiciously petite. And the catalog of the makeup of the cosmos? Ninety-six percent incomplete.

Almost every observation of the subatomic universe can be explained by the Standard Model of particle physics—a robust theoretical framework bursting with verifiable predictions. But because of these unsolved puzzles, the math is awkward, incomplete and filled with restrictions.

A few more particles would solve almost all of these frustrations. Supersymmetry (nicknamed SUSY for short) is a colossal model that introduces new particles into the Standard Model's equations. It rounds out the math and ties up loose ends. The only problem is that after decades of searching, physicists have found none of these new friends.

But maybe the reason physicists haven't found SUSY (or other physics beyond the Standard Model) is because they've been looking through the wrong lens.

"Beautiful sets of models keep getting ruled out," says Jessie Shelton, a theorist at the University of Illinois, "so we've had to take a step back and consider a whole new dimension in our searches, which is the lifetime of these particles."

In the past, physicists assumed that new particles produced in particle collisions would decay immediately, almost precisely at their points of origin. Scientists can catch particles that behave this way—for example, Higgs bosons—in particle detectors built around particle collision points. But what if new particles had long lifetimes and traveled centimeters—even kilometers—before transforming into something physicists could detect?

This is not unprecedented. Bottom quarks, for instance, can travel a few tenths of a millimeter before decaying into more stable particles. And muons can travel several kilometers (with the help of special relativity) before transforming into electrons and neutrinos. Many theorists are now predicting that there may be clandestine species of particles that behave in a similar fashion. The only catch is that these long-lived particles must rarely interact with ordinary matter, thus explaining why they've escaped detection for so long. One possible explanation for this aloof behavior is that long-lived particles dwell in a hidden sector of physics.

"Hidden-sector particles are separated from ordinary matter by a quantum mechanical energy barrier—like two villages separated by a mountain range," says Henry Lubatti from the University of Washington. "They can be right next to each other, but without a huge boost in energy to get over the peak, they'll never be able to interact with each other."

High-energy collisions generated by the Large Hadron Collider could kick these hidden-sector particles over this energy barrier into our own regime. And if the LHC can produce them, scientists should be able to see the fingerprints of long-lived particles imprinted in their data.

Long-lived particles jolted into our world by the LHC would most likely fly at close to the speed of light for between a few micrometers and a few hundred thousand kilometers before transforming into ordinary and measurable matter. This incredibly generous range makes it difficult for scientists to pin down where and how to look for them.

But the lifetime of a subatomic particle is much like that of any living creature. Each type of particle has an average lifespan, but the exact lifetime of an individual particle varies. If these long-lived particles can travel thousands of kilometers before decaying, scientists are hoping that they'll still be able to catch a few of the unlucky early-transformers before they leave the detector. Lubatti and his collaborators have also proposed a new LHC surface detector, which would extend their search range by many orders of magnitude.

Because these long-lived particles themselves don't interact with the detector, their signal would look like a stream of spontaneously appearing out of nowhere.

"For instance, if a long lived particle decayed into quarks while inside the muon detector, it would mimic the appearance of several muons closely clustered together," Lubatti says. "We are triggering on events like this in the ATLAS experiment." After recording the events, scientists use custom algorithms to reconstruct the origins of these clustered particles to see if they could be the offspring of an invisible long-lived parent.

If discovered, this new breed of matter could help answer several lingering questions in physics.

"Long-lived particles are not a prediction of a single new theory, but rather a phenomenon that could fit into almost all of our frameworks for beyond-the-Standard-Model physics," Shelton says.

In addition to rounding out the Standard Model's mathematics, inert long-lived particles could be cousins of dark matter—an invisible form of matter that only interacts with the visible cosmos through gravity. They could also help explain the origin of matter after the Big Bang.

"So many of us have spent a lifetime studying such a tiny fraction of the universe," Lubatti says. "We've understood a lot, but there's still a lot we don't understand—an enormous amount we don't understand. This gives me and my colleagues pause."

Explore further: 3 knowns and 3 unknowns about dark matter

More information: New Detectors to Explore the Lifetime Frontier. arxiv.org/abs/1606.06298

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Mimath224
not rated yet Oct 03, 2016
I was under the impression SUSY predicted a lot more particles than is implied in the article...like each known particle has a super-partner?
Hyperfuzzy
1 / 5 (3) Oct 04, 2016
No, the Standard Model is absurd. There exist only the diametrical spherical fields, throughout the infinite universe in an infinite quantity, never created or destroyed, therefore the fields are everywhere. If the universe allows all the charge to gather within a single ball it will take an infinite amount of time. Stability can only be defined with a set of attributes in space-time. Space-Time is only conceptual, the only thing real are these mysterious spherical fields. They have no source, therefore the field must be a part of this entity. Create all the crazy ideas you wish, but this is it! It does not matter what name or temporal conditions we define, this is all there is!
Hyperfuzzy
1 / 5 (3) Oct 04, 2016
Go ahead, simulate stable sets of these centers, from one to infinity. Well at least define the periodic table. Then molecules and compounds. Note: If each charge is an object, the field relative to itself, how can it change, it's not like drag. But you can define the motion by adding a few of its friends. Each set of objects define a continuum. Don't start thinking about time travel, even without catching Einstein's error, the idea of time travel should have ruled him out. Yes with the right instrumentation it may be possible to affect an object. But it would only be oscillation, see it is still happening in our time, 1 4D frame to define an mathematical space isomorphic to reality! Don't try this with the standard model without the fire department! Don't forget, the relative field from any object's center is updated at the speed of light. Must I show proof and disproof of any of this article? I first prove a fundamental particle does not exist. Maybe the composite, n
Hyperfuzzy
1 / 5 (2) Oct 04, 2016
All we see is motion through these fields, whether they overtake us, or we them, and their fronts may have velocities from -infinity to +infinity. So when astro physicist are peering into distant space, how do they know if they are traveling into a wave, or the wave into them. What sort of interference pattern would you expect if the source was behind us. A plane wave may travel through the galaxy, and be always history, i.e. the galaxy travels through it. Please help the astro guys out by giving them a correct physics. If these guys finally get infinite resolution and see something moving backwards in time, OMG. I don't even want to think about it. We are probably fine for while but we are so myopic.
optical
Oct 04, 2016
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optical
Oct 04, 2016
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snoosebaum
5 / 5 (2) Oct 05, 2016
Hyper fuzzy and optical should get together , you guys are either genius or crazy , in any case no one can make any sense of it.
Mimath224
not rated yet Oct 05, 2016
@snoosebaum...or one is the 'SUSY' partner of the other eh? Ha!
optical
Oct 05, 2016
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optical
Oct 05, 2016
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optical
Oct 05, 2016
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optical
Oct 06, 2016
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Seeker2
not rated yet Oct 17, 2016
If these guys finally get infinite resolution and see something moving backwards in time, OMG. I don't even want to think about it. We are probably fine for while but we are so myopic.
If we can see antimatter there should be no problem except recognizing the obvious truth.

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