The Sciences

Beyond the Higgs

Looking for "the smoking gluon"

By Stephen CassAug 13, 2007 5:00 AM


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Although the ATLAS and CMS experiments are focused on the search for the Higgs boson, they are not the only apparatus taking advantage of the huge energies of the Large Hadron Collidor (LHC). By smashing together nuclei of lead—the largest particles the LHC can handle (pdf)—the ALICE experiment will create a quantum-scale fireball 100,000 times the temperature of the core of the sun. This will echo a moment that took place nearly fourteen billion years ago, when time was just a split second old and the universe barely the size of an orange. Temperatures then were so hot that all matter existed in the form of its most basic building blocks—minuscule subatomic particles called quarks. Quarks normally exist only in pairs or triplets, tightly bound by other particles that are aptly named gluons. But in the first eye-blink of existence, individual quarks and gluons floated about freely in the primordial soup.

ALICE’s researchers calculate that each head-on collision at the LHC will spawn a speck of this quark-gluon plasma, which will hang around for less than a trillionth of a trillionth of a second. And then, in accordance with Einstein’s famous equation, E=mc^2, the fireball’s energy will be converted into new matter, spraying out more than 10,000 particles into the waiting arms of the detectors. Analyze these particles, and you will get a handle on how the quark-gluon plasma behaves.

Physicists believe that many of the conditions in the universe today were frozen in during that first split second. For instance, one theory holds that when the quark-gluon soup turned into more ordinary matter, it did so in lumps that eventually gave rise to galaxies and clusters of galaxies. Over lunch in the staff cafeteria, theoretician John Ellis explains that this idea has already fallen out of fashion, mainly because that theory supposed to be a quark-gluon plasma smooth, disconnected gas, but earlier this year, physicists at Brookhaven National Laboratory caught a glimpse of the quark-gluon plasma and discovered that it looks much more like a thick, viscous liquid. If so, how did galaxies get started? ALICE may help find the answer. “If it’s more like treacle,” Ellis says, “surely the gobs of treacle would freeze differently from if it was gas. The Brookhaven results look convincing, but you’d always like better evidence. We really want to see a smoking gun, or in this case a smoking gluon.”

The LHCb experiment seeks to find out why the Big Bang didn’t just create a universe containing nothing but energy. According to standard physics theory, the Big Bang should have created equal amounts of matter and its nemesis, antimatter. Put these two together and they explode in mutual assured destruction, leaving nothing but energy. So why are we here? The LHCb experiment aims to uncover a previously undetected kink in the laws of physics that would explain how enough matter survived to build galaxies, stars, and planets. The idea is to make and study a flood of particles known as B mesons.

B mesons are important because, as they decay into other, more ordinary particles, they display a slight asymmetry: The antimatter versions tend to decay more readily into matter than the reverse. Existing experiments have already spotted a similar feature in the decay of another exotic particle called the kaon, and in certain kinds of B meson. The problem is that the kinds of imbalance seen to date can account for only a tenth of a billionth of the matter that we know is out there. With its super-high energies, LHCb will be able to make many more B mesons, including versions that have not yet been studied. When these particles decay, they could show enough of a matter-antimatter difference to start to explain one of the most basic question of physics, if not philosophy: Why is there something rather than nothing?

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