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- Mystery Meteorite: The case for (and against) a rock from Mercury
- Hard Times for Theorists in a Post-Higgs World: The Large Hadron Collider’s big success leaves no clear avenue for new physics
- In the Eye of the Tiger: Global spread of Asian tiger mosquito could fuel outbreaks of tropical disease in temperate regions
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Web edition: December 5, 2012
Print edition: January 12, 2013; Vol.183 #1 (p. 12)
Strangely behaving subatomic particles at the world’s most powerful particle accelerator could lead to fresh insight into how matter behaves at the smallest scales and highest energies.
Ordinarily, the Large Hadron Collider near Geneva sends protons hurtling into each other at velocities approaching the speed of light. But for several hours in September, the machine collided protons into lead nuclei — tightly packed bundles of 82 protons and 126 neutrons. It was merely a test run, designed to calibrate instruments for future experiments.
But when physicists with the Compact Muon Solenoid collaboration analyzed the data, they quickly noticed that something was amiss. When a proton and lead nucleus collide, they shatter into smaller particles that jet out in all directions. The movement of each piece of shrapnel should be almost completely random; the direction of one particle should provide no clue to that of any other. Yet during these collisions, the particles’ directions tended to correspond to one other. Even particles located far from each other seemed to be coordinating their paths of travel.
“This is one of the most interesting unexpected effects we’ve observed at the LHC,” says Gunther Roland, a physicist at MIT and member of the CMS team. He and his team describe the odd behavior in an upcoming Physics Letters B. The team does not try to explain why the particles behaved as they did, but Roland notes that other physicists have come up with a few ideas.
One possibility is that the enormous heat and density in the collision zone actually liquefied the tiny bits of matter. Physicists first observed a liquidlike exotic state of matter, known as quark-gluon plasma, seven years ago when they slammed gold nuclei into each other at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York. Because quark-gluon plasma behaves like a liquid, it flows; the observed effect at the LHC could be the result of particles emerging from the outer edge of the plasma as it cools.
Physicists Raju Venugopalan and Kevin Dusling from Brookhaven present an intriguing alternative: The particles are actually influencing the movement of each other. If true, their theory would be yet another quirky manifestation of quantum mechanics, the strange laws of physics that reign at the smallest of scales. The Brookhaven physicists argue that gluons, the tiny particles that hold protons together, can form fields when moving near the speed of light. These fields allow gluons within multiple protons to interact and influence each other. In essence, Venugopalan and Dusling claim that under the right conditions, certain particles can stop behaving like individual pieces of matter and act more like a coordinated posse.
Berndt Mueller, a physicist at Duke University who is not on the CMS team, says he favors the quark-gluon plasma theory because it is the simplest explanation. But either explanation, if proven correct, would help physicists better understand the strange rules that dictate how matter works under the most extreme conditions. Physicists want to learn more about quark-gluon plasma because they believe all the matter in the universe was in this state a few millionths of a second after the Big Bang.
If Venugopalan and Dusling are right, then physicists would learn more about the structure of the proton and how it can influence the behavior of surrounding matter.
Both explanations do have holes. Roland notes that proton-lead collisions should not generate the extreme conditions necessary to produce quark-gluon plasma. And Venugopalan and Dusling’s prediction, while theoretically possible, has never been experimentally verified.
Fortunately, this mystery may find a quick resolution. Next month the LHC will again slam protons into lead nuclei, this time as part of a planned science-gathering run of several weeks. That should provide the team with up to 100,000 times more data to determine which explanation holds up.
Citations
CMS Collaboration. Observation of long-range near-side angular correlations in proton-lead collisions at the LHC. arXiv:1210.5482v2. Posted October 19, 2012. [Go to]
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Prior to the RHIC results in 2005, theoretical physicists predicted that the quark-gluon plasma would behave like a weakly-interacting gas, i.e., a plasma.
When the 2005 RHIC experiments indicated that their predictions were wrong, and that the plasma acted like a strongly-interacting fluid, then the theoretical physicists "adjusted" their expectations and now they only talk about their new "predictions".
Typical model-building and adjusting theory to fit data, as has been the case with so many things in particle physics over the last 40 years.
Please correct me if I am wrong about original weakly-interacting gas predictions for the plasma.
Robert L. Oldershaw
Since you make the same arguments I make all the time on web blogs and journal sites, I am confused by your comment.
In science, theories are supposed to make definitive predictions and then testing of those predictions tell us reasonably unambiguously whether they are right or misguided.
In Ptolemaic model-building, theories are adjusted as needed to fit new experimental data. For example, if you search high and low for decades and find no free "quarks", then the theory gets an ad hoc amendment saying that nature hides "quarks" inside hadrons. We are supposed to have faith that they are there, even though they cannot be observed.
I hope you do not think I am advocating Ptolemaic model-building. That would truly be getting things backwards.
Robert L. Oldershaw
Discrete Scale Relativity
No, not suggesting you advocate Ptolemaic model building. I was mainly addressing your assessment of the past few decades of particle physics research.
But to address some of your misconceptions directly, deep inelastic scattering experiments have shown the existence of quarks. Also, definitive predictions of the quark model were indeed made and verified by accelerator experiments. For example, Fermilab's claim to fame is the discovery of the predicted top quark in 1992.
Also, refreshing my memory on QGP, the theory accommodates both thermodynamic states. My understanding is that it still isn't clear what happens near the transition temperature, but my impression of the LHC experiment is that if a QGP was formed then it was likely below the transition temperature. Thus the QGP would behave like a viscous liquid. So I don't think the QGP model has changed all that significantly. I think it's primarily just the region around the transition point that is messy due primarily to the intense computational power required to do simulations there.
However, I think the misconceptions are definitely yours. Consider the following.
No human in the entire history of this planet has ever observed a "quark". The so-called evidence for "quarks" is based entirely on secondary or tertiary normal decay products and it is INFERRED that they decayed from "quarks".
Likewise the scattering experiments that are interpreted as scattering by unobservable "quarks" have other interpretations, but theoretical physicists desperately wanted "quarks" so they ignored other models.
After Gellman-Mann introduced the "quark" model as a fictional accounting device for particle family structures, physicists looked everywhere from the deep ocean, to the Moon, to outer space and everywhere in between for free "quarks" with fractional charges. They found not a single one. That was a big problem. So they INVENTED confinement, which is a completely ad hoc way of hiding "quarks" inside hadrons where we can never observe them.
Regarding the "quark-gluon" plasma "evidence", they predicted that it would behave like a weakly interacting gas. The observational evidence says this prediction was WRONG. The plasma, much to the surprise of theoretical physicists behaved like a strongly interacting fluid, which is much more like what Discrete Scale Relativity predicts. Of course, given time the theoretical physicists "adjusted" their model to fit the new data, and now they see it as more confirmation of the "quark" fiction. Another epicycle in their Ptolemaic models.
That's particle physics for you: they do not study nature; they tell nature how it should be according to their Platonic fictions.
Non-players in the theoretical physics game are treated like mushrooms: kept in the dark and fed bullsh*t.
And that's the truth, for once.
Robert L. Oldershaw
Discrete Scale Relativity
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