Tag: ATLAS

Mass effect: Maybe Higgs, maybe not

By Phil Plait | December 13, 2011 9:29 am

Today, scientists at CERN in Geneva announced their results for their search for the Higgs boson, a subatomic particle that, if it exists, is thought to be responsible for giving other particles mass. It’s no exaggeration to call it a keystone in quantum mechanics, and finding it for sure will be a huge accomplishment for particle physicists.

So, did they find it?

Maybe. Then again, maybe not.

Um, what? OK, this’ll take a wee bit of explaining.


Last things first

I said Higgs, Magnum. HIGGS.

First, the conclusion, so at least you have that in mind as you read the rest. There are two experiments running at CERN looking for the Higgs particle. They don’t smash particles together, look around with magnifying glasses and tweezers, and then yell “AHA!” when they find one. Instead, they build up a picture of it after doing gazillions of particle collisions. After a year of runs, both experiments see something that might be Higgs, but they’re not 100% sure. One sees something at about the 94% confidence level, the other at 98%. That’s pretty good, but it’s not enough to be completely sure. It seems likely they’ve found something, but it’s like a fuzzy picture: it looks like Higgs, but it still might be something else.

So why can’t they be sure one way or another?


CSI: Geneva

Basically, what the Large Hadron Collider at CERN does is whip protons around at nearly the speed of light, then smashes them into each other. At that speed they have huge energies, and when they collide that energy gets converted into matter: other particles. Like shrapnel, these new particles explode away from the collision site. Many of these new particles aren’t stable; they decay into yet lower energy particles after incredibly short time intervals. For example, electron and protons are almost certainly stable over long times (like the lifetime of the Universe), but neutrons decay after only a few minutes, turning into a proton, and electron, and a particle called an antineutrino.

So these daughter particles from the proton collisions in LHC decay, and they have daughter particles, and some of those decay, and so on. At the LHC there are two ginormous detectors called ATLAS and CMS. Both of these, in essence, measure the energy of the particles that hit them; like forensics team, they look at the aftermath of the collision and try to work backwards to figure out what happened.

We know to some extent how much energy is expected from these collisions due to all the particles that are currently known, so those can be accounted for. But if there’s some excess of energy, that could very well indicate a new particle. And we have theories as to how much energy the Higgs particle should have. So the energies are measured, calibrated for known particles, and the excesses are examined.

What both experiments found is an excess of energy — a bump in the graph — indicating a particle that has an energy* about 125 times that of a proton — right in the expected range for the Higgs particle. That’s exciting! But what they’re doing is counting up things statistically, so they can’t be 100% sure. The bump in the graph is still fuzzy.

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CATEGORIZED UNDER: Cool stuff, Science, Top Post
MORE ABOUT: ATLAS, CERN, CMS, Higgs, Higgs boson, LHC

Perspective on four moons

By Phil Plait | September 8, 2010 7:00 am

The Cassini probe orbiting Saturn returned an interesting picture yesterday. It shows four tiny moons and the rings seen nearly edge-on. Take a look:

cassini_4moons_rings

[Click to enjovianate.]

From left to right the moons are Epimetheus (113 km/70 miles across), Janus (179 km/111miles), Prometheus (86 km/53 miles) and Atlas (30 km/19 miles). Like I said, tiny.

When I see images like this I like to amuse myself by fiddling in my mind with their perspective. For example, is Epimetheus closer to us (well, to Cassini when the picture was taken) than Janus was? Even more interestingly, are we looking down on, or up at the rings?

Images like this don’t give us the clues we usually get here on Earth to figure out distance. Look at the picture: the rings make a tight curve across the field. We know we’re seeing a circular ring nearly edge-on… but are we looking down on it, so that the top of the curve is farther away, or are we looking up at it, so that the bottom of the curve is farther away?

For example, take a DVD and hold it so that you’re looking at it almost edge-on. Tilt the near edge down a bit so you’re looking down on the top side. Now tilt the near side up so you’re looking up on the bottom side. See the issue? Without lighting, focus, or other cues, it’s hard to tell which way you’re seeing an object.

So for the Saturn picture, which is it? I’ll tell you below, but see if you can figure it out.
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CATEGORIZED UNDER: Pretty pictures, Science

The Big Picture on LHC

By Phil Plait | November 23, 2009 7:35 am

bigpicture_cmsSpeaking of the LHC, the Boston Globe’s terrific feature The Big Picture has a slew of gorgeous pictures of the Large Hadron Collider up on the site.

These images, as beautiful and hi-res as they are, still cannot convey the awesome size and scale of the LHC. It’s been a year and a half since I stood there, 100 meters below of the surface of the Earth, gawking slack-jawed at ATLAS, CMS, and the other magnificent machinery, and it almost seems like a dream to me. But then I shake out of it and remember: this is what we do, and it’s real.

Secrets of the Universe? We humans figure that stuff out over coffee. What’s next?

CATEGORIZED UNDER: Cool stuff, Pretty pictures, Science
MORE ABOUT: ATLAS, CERN, CMS, LHC, The Big Picture
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