The world’s most highly effective particle accelerator, the Massive Hadron Collider on the CERN laboratory close to Geneva, has failed to seek out any of the hoped-for particles that will lead physicists past the Commonplace Mannequin of particle physics. However it’s potential that the LHC has been producing such pivotal new particles all alongside, and that we’re simply not seeing them.
“The core of the story,” stated David Curtin, a physicist on the College of Maryland, “is that the LHC could possibly be making particles that are completely invisible, which decay a ways away from the manufacturing level, whether or not it’s millimeters or many kilometers, and that are related on the most elementary degree to a number of the most vital theoretical mysteries that we have now.”
Such hypothetical particles are referred to as “long-lived,” as a result of their lifetimes would far exceed those who the LHC was designed to detect. If the LHC is certainly producing these particles, then it’s doubtless that some are fleeing the accelerator’s underground tunnel, capturing up by way of the earth, and doubtlessly exploding like fireworks within the sky above the close by farm fields as they decay again into peculiar matter.
To catch the flash of these fireworks, assuming they exist, Curtin and collaborators Henry Lubatti of the College of Washington and John Paul Chou of Rutgers College have proposed constructing an unlimited new detector that will stand in these fields, wanting reasonably like a very large barn. The three lately revealed their proposal in Physics Letters B, christening their detector Mathusla (which, within the grand custom of tortured physics acronyms, stands for MAssive Timing Hodoscope for Extremely Secure neutraL pArticles). The identify is a nod to the legendary determine who lived for over 900 years.
“I feel an experiment like that’s actually an expression of our instances,” stated Gian Giudice, who heads the speculation division at CERN. For a few years there was “a way of conviction about the proper route for exploration,” he stated, with the sphere led by efforts to seek out short-lived particles predicted by a idea referred to as supersymmetry. But these particles haven’t proven up as deliberate. “Now we’re on the lookout for new instructions and new motivations, and that’s what is admittedly altering.”
Sadly, long-lived particles are troublesome to detect. The excellent news is that our greatest hope for seeing them may stem from the one new particle the LHC has found and continues to supply: the Higgs boson.
The invention of the Higgs in 2012 gave physicists two issues directly: the triumph of discovering the final lacking piece of the Commonplace Mannequin, but in addition compelling proof that that very same mannequin is lacking one thing important.
The issue within the Commonplace Mannequin lies in the truth that the measured mass of the Higgs is about 100 million billion instances smaller than what quantum mechanics suggests it needs to be. From the standpoint of the Commonplace Mannequin, this may be true solely on account of a particularly unlikely coincidence involving the values of a number of the universe’s elementary constructing blocks. (The coincidence can also be exceptionally lucky, as a result of with out it, atoms and every part they’re fabricated from couldn’t exist.) Physicists name this example the “hierarchy drawback” and see it as proof that the Commonplace Mannequin is just an approximation of a extra complete idea that will clarify the Higgs mass “naturally”—as the results of some mechanism apart from an obvious miracle.
Supersymmetry has lengthy been the main candidate for the extra complete idea. It solves the hierarchy drawback by way of new particles—one associate particle for every particle within the Commonplace Mannequin. However with no signal of superpartners rising from the LHC, some physicists are critically contemplating the chance that particles addressing the hierarchy drawback belong to what’s referred to as a hidden sector.
A hidden sector is a household of particles that will work together with one another however which don’t really feel the consequences of the Commonplace Mannequin’s three forces—sturdy, electromagnetic and weak. They don’t straight work together with peculiar matter, which makes them very troublesome to detect. However a hidden sector of particles may assist clear up the hierarchy drawback, stated Zackaria Chacko, a physicist on the College of Maryland who was one of many first to suggest this concept within the early 2000s. “Naively, the way in which you may give it some thought is, the Commonplace Mannequin particles wish to pull their Higgs up and make the Higgs heavy,” he stated. “And you’ve got this hidden sector that’s pulling it again down.”
Chacko’s mannequin is named “Twin Higgs,” for the reason that Higgs, like all the opposite Commonplace Mannequin particles, would have a twin residing within the hidden sector. The Higgs would play a particular position by advantage of its potential to go away the Commonplace Mannequin sector—in different phrases, the world wherein we reside—and go into the hidden one by turning into its twin.
Is that this actually occurring? The LHC’s proton-proton collisions create tons of of Higgs bosons per hour, however they’re a tiny minority of the riot of particles constantly spraying out from these crashes and storming the accelerator’s detectors. Solely a fraction of them could be counted. Based mostly on the information collected thus far, as many as 1 / 4 of the LHC’s Higgs bosons could possibly be slipping into the shadows of a hidden sector.
Monitoring down such fugitives could be virtually hopeless if not for a discovery made in 2014 by a staff together with Raman Sundrum, Chacko’s colleague at Maryland. Having reached what Sundrum describes as a “sure degree of desperation” concerning the lack of recent discoveries on the LHC, he and his collaborators gave Chacko’s mannequin a re-assessment and realized that it was only one of an entire class of theories that would restore naturalness by advantage of hidden particles. Additionally they discovered that within the normal case, a number of the hidden particles that the Higgs decays into don’t keep hidden. As an alternative, they themselves decay—after a fraction of a second, which remains to be a Methuselah-like lifetime by subatomic requirements—again into Commonplace Mannequin particles. Such particles could possibly be noticed on the LHC, or close to it.
“There could possibly be numerous these items flying out on a regular basis, and we simply don’t understand it,” stated Christopher Hill, a physicist at Ohio State College and a senior member of CMS, one of many two groups that discovered the Higgs (ATLAS was the opposite). Hill, in collaboration with Andy Haas, a physicist at New York College who works on ATLAS, is behind a special proposal to seek out hidden-sector particles, referred to as milliQan. The experiment’s identify alludes to Robert Millikan, who was the primary to measure the electron’s cost, but in addition highlights the truth that it’s searching for particles with prices as small as one-thousandth of Millikan’s measurement, making them “millicharged.”
“We had been going after a special portal with milliQan,” Hill stated. By means of this portal, an peculiar photon may remodel into “darkish” photons in a hidden sector. In that case, an electron-like particle within the hidden sector would seem as a millicharged one within the Commonplace Mannequin sector.
Hidden sectors and portals could sound suspiciously like science fiction, devised by physicists pushed to the form of desperation Sundrum confessed to, however we already know of not less than one occasion the place nature has performed this recreation. Darkish matter, regardless of how little we find out about it, presents sturdy proof that one other sector exists past the Commonplace Mannequin.
In truth, physicists have invoked long-lived particles and hidden sectors to handle just about all the basic issues presently plaguing physics, together with points comparable to darkish matter and the matter-antimatter asymmetry of the universe. “The concept of on the lookout for hidden sectors is so pure, and so well-motivated, and so fascinating, that it is best to simply begin with that,” stated Nima Arkani-Hamed, a physicist on the Institute for Superior Research in Princeton, New Jersey.
However on the lookout for hidden sectors means smoking out these elusive long-lived particles.
Fireworks within the Barn
“Lengthy lifetimes—you guys, that is insane!” Curtin recollects saying at a workshop final 12 months. “There’s nothing you are able to do; you may’t deflect it, you may’t lure it, you may’t make it scatter. It’s going to simply go off, and it’ll decay at any time when the hell it needs. And you must be proper there when it decays.”
Curtin requested his viewers for assist.
Lubatti and Chou, experimentalists who had been already engaged in searches for long-lived particles at ATLAS and CMS respectively—an endeavor Lubatti compares to “on the lookout for a needle in a haystack, and we’re not even certain which haystack”—had been each current on the workshop. The following dialogue in the end resulted within the Mathusla collaboration. After months of conferences, Skype calls and simulations, the three labored out a preliminary design: a 20-meter-tall constructing that will cowl an space bigger than seven soccer fields. 5 layers of particle trackers would cling from the ceiling, permitting researchers to reconstruct the subatomic particles of particle decays that occur within the huge house. The construction could be constructed on the bottom above both the ATLAS or the CMS detector, the place roughly 100 meters of dust and rock would operate to protect it from the continual chaos of particle collisions down beneath, which may all too simply conceal a long-lived particle’s fireworks.
MilliQan is a smaller experiment that may require extra finesse. Its detector could be wedged into the spare house of a cavelike tunnel left over from the LHC’s building. Its problem is particle having one-thousandth of an electron’s cost kicks out a mere millionth of the photons in a particle detector that an electron would. MilliQan subsequently configures its detectors in three stacked tubes, every of them 1 meter lengthy and all pointed on the spot the place protons collide. The lengthy tubes needs to be sufficient, Hill stated, for a millicharged particle to kick one photon into every of the three layers, creating an unambiguous sign that ought to let physicists distinguish an precise millicharged particle from the stormy sea of background noise.
Though neither experiment has been permitted by CERN, prototypes are already being constructed on the LHC’s grounds. Mathusla’s is a field 2.5 meters sq. and about 5.5 meters tall, cobbled collectively partly from supplies scrounged from a cosmic-ray experiment in Tibet. MilliQan’s prototype is likewise a miniature model of the true factor, additionally constructed from scavenged elements, and benefiting from the kindness of the CMS staff, who personal the house it’s taking over.
“As with most long-lived particle searches … we’re like parasites residing off the true physics that’s occurring,” stated Haas.
In comparison with ATLAS and CMS, every of which employs 1000’s of individuals and makes use of tremendous refined detectors costing over half a billion every, milliQan and Mathusla are decidedly humble proposals, the previous possible for perhaps $1 million, the latter for just a few tens of thousands and thousands, and each primarily based largely on particle detection expertise that dates again to the Manhattan Venture. It appears ridiculous to counsel that they might sniff out one thing the massive canines have missed.
And can they even get the prospect?
“Scientifically, I feel the case will probably be straightforward to make,” stated Giudice, who sits on the committee at CERN that may determine each experiments’ fates. “Finally the difficulty will probably be decided by sensible points like value, house and the time for the experiments to be prepared.” If all goes properly, the experiments will begin gathering information after a deliberate main improve of the LHC within the mid 2020s.
The proposals profit from a newfound sense of humility amongst particle physicists. As Arkani-Hamed put it, many “grand theoretical causes” put ahead to inspire searches for brand new physics over the previous 40 years have “amounted to diddly squat thus far.”
Now, Giudice stated, “we have to look in all potential instructions.”
It’s “simply loopy,” Arkani-Hamed added, that the portals Mathusla and milliQan may probe are so “astonishingly large” and have but to be explored. “It simply tells you the way little we truly find out about what’s on the market within the universe.”
Authentic story reprinted with permission from Quanta Journal, an editorially unbiased publication of the Simons Basis whose mission is to reinforce public understanding of science by protecting analysis developments and traits in arithmetic and the bodily and life sciences.