The search narrows for a mysterious form of matter predicted from Einstein’s theory of special relativity. After more than a decade of looking, scientists at the world’s largest particle collider believe that they are on the verge of finding it.
But the researchers are not searching in the exploded guts of particles smashed together at nearly light speed.
Instead, physicists at the Large Hadron Collider (LHC), a 17-mile (27 kilometers) ring buried underground near the border between France and Switzerland, are looking for the missing matter, called a color glass condensate, by studying what happens when particles don’t collide, but instead zoom past each other in near misses.
In the Standard Model of physics, the theory which describes the zoo of subatomic particles, 98% of the visible matter in the universe is held together by fundamental particles called gluons. These aptly named particles are responsible for the force that glues together quarks to form protons and neutrons. When protons are accelerated to near the speed of light, a strange phenomenon occurs: The concentration of gluons inside them skyrockets.
“In these cases, gluons split into pairs of gluons with lower energies, and such gluons split themselves subsequently, and so forth,” Daniel Tapia Takaki, an associate professor of physics and astronomy at the University of Kansas, said in a statement. “At some point, the splitting of gluons inside the proton reaches a limit at which the multiplication of gluons ceases to increase. Such a state is known as the color glass condensate, a hypothesized phase of matter that is thought to exist in very high-energy protons and as well as in heavy nuclei.”
According to Brookhaven National Laboratory, the condensate could explain many unsolved mysteries of physics, such as how particles are formed in high-energy collisions, or how matter is distributed within particles. However, confirming its existence has eluded scientists for decades. But in 2000, physicists at Brookhaven’s Relativistic Heavy Ion Collider found the first signs that the color glass condensate could exist.
When the lab smashed together gold atoms stripped of their electrons, they found a strange signal in the particles streaming out of the collisions, hinting that the atoms’ protons were jam-packed with gluons and beginning to form the color glass condensate. Further experiments with colliding heavy ions at the LHC have had similar results. However, colliding protons together at relativistic speeds can only give a fleeting glimpse of the protons’ innards before the subatomic particles violently explode. Probing the insides of protons takes a more gentle approach.
When charged particles, such as protons, are accelerated to high speeds, they create strong electromagnetic fields and release energy in the form of photons, or particles of light. (Thanks to the dual nature of light, it is also a wave.) These energy leaks were once dismissed as an unwanted side-effect of particle accelerators, but physicists have learned novel ways to use these high-energy photons to their advantage.
If protons find themselves whizzing past each other in the accelerator, the storm of photons they release can cause proton-on-photon collisions. These so-called ultra-peripheral collisions are the key to understanding the inner workings of high-energy protons.
“When a high-energy light wave hits a proton, it produces particles — all kinds of particles — without breaking the proton,” Tapia Takaki, said in a statement. “These particles are recorded by our detector and allow us to reconstruct an unprecedentedly high-quality picture of what’s inside.”
Tapia Takaki and an international collaboration of scientists are now using this method to track down the elusive color glass condensate. The researchers published early results of their study in the August issue of The European Physical Journal C. For the first time, the team was able to indirectly measure the density of gluons at four different energy levels. At the highest level, they found evidence that a color glass condensate was just beginning to form.
The experimental results “…are very exciting, giving new information about the gluon dynamics in the proton, [b]ut there are many theoretical questions that have not been answered,” Victor Goncalves, a professor of physics at the Federal University of Pelotas in Brazil and a co-author of the study, said in the statement.
For now, the existence of color glass condensate remains an elusive mystery.
September 24, 2019 at 5:32 pm
*Sabine Hossenfelder *is an author and theoretical physicist who researches quantum gravity. She is a Research Fellow at the Frankfurt Institute for Advanced Studies where she leads the Analog Systems for Gravity Duals group. some clips from her book: *Lost in Math.*
I used to attend the international conference series “Supersymmetry and Unification of Fundamental Interactions.” Since 1993, it has taken place annually, and at its peak it gathered more than five hundred participants. Every year the talks laid out the benefits of supersymmetry: naturalness, unification, and dark matter candidates. Every year the searches for superpartners came back with negative results. Every year the models were updated to accommodate the lack of evidence. The failure to date of the LHC to provide evidence of superpartners has taken a toll on theorists’ mood. “It is not time to be desperate yet… but maybe it is time for depression already,” remarked the Italian physicist Guido Altarelli in 2011.10 Ben Allanach, from the University of Cambridge, has described his reaction to a 2015 analysis of LHC data as “a bit depressing for a supersymmetry theorist like me.”11 Jonathan Ellis, a theorist at CERN, has referred to the possibility that the LHC would find nothing but the Higgs boson as “the real five-star disaster.”12 The name that has stuck, however, is “the nightmare scenario.”13 We’re now living this nightmare.
Sabine Hossenfelder, Lost in Math, pg. 70
Cosmic Poker The multiverse has gained in popularity while naturalness has come under stress, and physicists now pitch one as the other’s alternative. If we can’t find a natural explanation for a number, so the argument goes, then there isn’t any. Just choosing a parameter is too ugly. Therefore, if the parameter is not natural, then it can take on any value, and for every possible value there’s a universe. This leads to the bizarre conclusion that if we don’t see supersymmetric particles at the LHC, then we live in a multiverse. *I can’t believe what this once-venerable profession has become. Theoretical physicists used to explain what was observed. Now they try to explain why they can’t explain what was not observed. And they’re not even good at that.* In the multiverse, you can’t explain the values of parameters; at best you can estimate their likelihood. But there are many ways to not explain something. “So,” I say to Weinberg, “in the case that we live in a multiverse, the requirement that the theory is interesting enough, or gives rise to interesting enough physics, isn’t this an empty requirement? You could do the same thing with any theory that doesn’t predict the parameters.” “Well, you have to predict some things,” Weinberg says. “Even if you don’t predict the parameters, you may predict correlation between them. Or you may predict the parameters in terms of some theory, like the standard model but a more powerful theory which actually tells you the mass of the electron and so on, and then if you ask, ‘Why is that theory correct?’ you say, ‘Well, that’s the farthest we can go, we can’t go beyond that.’” He continues: “I wouldn’t be in a hurry to set clear requirements for what a good theory has to be. But I can certainly tell you what a better theory has to be. A theory better than the standard model would be one that makes it inevitable that you have six rather than eight or four quarks and leptons. There are many things in the standard model that seem arbitrary, and a better theory would be one that makes these things less arbitrary, or not arbitrary at all. “But we don’t know how far we can go in that direction,” he continues. “I don’t know how much elementary particle physics can improve over what we have now. I just don’t know. I think it’s important to try and continue to do experiments, to continue to build large facilities…. But where it will wind up I don’t know. I hope it doesn’t just stop where it is now. Because I don’t find this entirely satisfying.”
Sabine Hossenfelder, Lost in Math, pg. 118
In reaction, most string theorists discarded the idea that their theory would uniquely determine the laws of nature and instead embraced the multiverse, in which all the possible laws of nature are real somewhere. They are now trying to construct a probability distribution for the multiverse according to which our universe would at least be likely. Other string theorists left behind the foundations of physics entirely and tried to find applications elsewhere—for example, by using string theoretical techniques to understand collisions of large atomic nuclei (heavy ions). In such collisions (which are also part of the LHC’s program), a plasma of quarks and gluons can be created for a short amount of time. The plasma’s behavior is difficult to explain with the standard model, not because the standard model doesn’t work but because nobody knows how to do the calculations. Nuclear physicists thus welcomed new methods from string theory. Unfortunately, the string-theory-based predictions for the LHC didn’t match the data, and string theorists quietly buried the attempt.3 They now claim their methods are useful for understanding the behavior of certain “strange” metals, but even string theorist Joseph Conlon likened the use of string theory for the description of such materials to the use of a map of the Alps for traveling in the Himalayas.4 String theorists’ continuous adaptation to conflicting evidence has become so entertaining that many departments of physics keep a few string theorists around because the public likes to hear about their heroic attempts to explain everything. Freeman Dyson’s interpretation of the subject’s popularity is that “string theory is attractive because it offers jobs. And why are so many jobs offered in string theory? Because string theory is cheap. If you are the chairperson of a physics department in a remote place without much money, you cannot afford to build a modern laboratory to do experimental physics, but you can afford to hire a couple of string theorists. So you offer a couple of jobs in string theory and you have a modern physics department.”
Sabine Hossenfelder, Lost in Math, pg. 189
IN CASE I left you with the impression that we understand the theories we work with, I am sorry, we don’t. We cannot actually solve the equations of the standard model, so what we do instead is solve them approximately by what is known as “perturbation theory.” For this, we first look at particles that don’t interact at all to learn how they move when undisturbed. Next, we allow the particles to bump into each other, but only softly, so that they don’t knock each other off their paths too much. Then we make successive refinements that take into account an increasing number of soft bumps, until the desired precision of the calculation is reached. It’s like first drawing an outline and then adding more details. However, this method works only when the interaction between the particles isn’t too strong—the bumps aren’t too violent—because otherwise the refinements don’t get smaller (or aren’t refinements, respectively). That’s why, for example, it’s hard to calculate how quarks combine to form atomic nuclei, because at low energies the strong interaction is strong indeed and the refinements don’t get smaller. Luckily, since the strong interaction becomes weaker at higher energies, calculations for LHC collisions are comparably straightforward. Even though the method works in some cases, we know the math will eventually fail because the refinements don’t continue to get smaller forever. For the pragmatic physicist, a method that delivers correct predictions is just fine, regardless of whether mathematicians can agree on why it works. But as Xiao-Gang points out, fundamentally we don’t understand the theory. This might be a case of missing mathematics, or it might hint at a deeper problem.
Sabine Hossenfelder, Lost in Math, pg. 211
During experiments, the LHC creates about a billion proton-proton collisions per second. That’s too much data to store even for CERN’s computing capacity. Hence the events are filtered in real time and discarded unless an algorithm marks them as interesting. From a billion events, this “trigger mechanism” keeps only one hundred to two hundred selected ones.27 We trust the experimentalists to do the right thing. We have to trust them, because not every scientist can scrutinize every detail of everybody else’s work. It’s not possible—we’d never get anything done. Without mutual trust, science cannot work. That CERN has spent the last ten years deleting data that hold the key to new fundamental physics is what I would call the nightmare scenario.
Sabine Hossenfelder, Lost in Math, pg. 236
October 2, 2019 at 5:30 pm
This seems like a quantum version of baseball where the pitchers fast ball just delivered strike three 👍
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