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News from the LHC

Started by Igor, December 13, 2011, 03:28:04 PM

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Igor

The Large Hadron Collider shut off for the winter a few days ago, and there was a conference announcing their most recent results today.
(This is a link to the liveblog of it that I was following)

The conference was almost entirely to do with the Higg's boson. So here's a quick reminder of why it is so difficult to find:
First, it is only detectable at high energies. This is why we need such a powerful machine to look for it.

Second, the theoretical results that predict the existence of the Higgs do not predict its mass. So we do not know at exactly what energy to start looking. This makes things more difficult when combined with the next point.

Third, it does not exist for long. The LHC produces new particles by smashing old ones together very fast. The new high-mass particles do not exist long enough for the detectors to register them. What they do register are the results of how the high-mass particles decay. The decay modes of the Higgs are very well-understood, so they know what to look for. For example; two high energy photons moving in opposite directions perpendicular to the proton beam. The problem then, is that these photons may also have arisen from other "background" effects from normal Standard Model processes. The scientists deal with this by using a lot of statistical analysis. Fortunately, they have vast amounts of data to work with. The LHC can produce 40,000 collisions per second.

So what have they found this year? The conference presented results from the two main detectors; CMS and ATLAS. They both narrowed down the energy range at which the Higgs may be found.

ATLAS has excluded 112.7-115.5GeV and 131-453GeV. (GeV stands for Giga-electronvolt, which is a unit of energy) This leaves a gap between 115.5 and 131GeV The current theoretical prediction for the Higgs is between 114 and 149GeV, so this is in line with what was expected.
So that's where the Higgs isn't. What's more tantalising is an indication that there may be something at an energy of 126GeV. Because of all the statistical analysis necessary to come to this result, this is very much not an announcement that the Higgs has been found. But it is a definite sign that this energy should be probed much more next year. (The 126GeV measurement has an excess of 3.6 sigma)

CMS has excluded energies from 129-238GeV and has seen a small excess at around 125GeV, with a lower confidence of 2.6 sigma. The fact that the two detectors are seeing the same small bulge in their graphs is obviously very exciting, and it is becoming much more likely with these results that the Higgs exists. It seems very likely that more definite results will confirm these preliminary findings next year.
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LMNO

Thanks for the heads up!

It would be awesome if we found it in our lifetime.  It would also be awesome if we proved it didn't exist in our lifetime.

The Good Reverend Roger

Quote from: LMNO, PhD (life continues) on December 13, 2011, 03:38:18 PM
Thanks for the heads up!

It would be awesome if we found it in our lifetime.  It would also be awesome if we proved it didn't exist in our lifetime.

It would be even more awesome if we could make a death ray out of it.

I mean, if we can't, I'm not seeing a practical application, here.
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PeregrineBF

http://profmattstrassler.com/ is a really good site if you want to learn about this but don't have the desire/time to really learn about all the math needed to understand it.
We will almost certainly find it or prove it doesn't exist within the next few years.
Then the real work starts: finding the Higgs Boson is the first step to studying the Higgs field, which is what we really care about.

Faust

Wait, they have a bulge of 3sigma confidence? That's a lot more then a hunch.
I assume they are going for a 6sigma undeniable proof before making the announcement?
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Rumckle

#5
From what I read, last year they seemed to have 3-4 sigma proof at 145GeV, but after more data taking this was dismissed, so I guess they are just being cautious.

(ETA: Though stats has never been my strong point)
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PeregrineBF

#6
In particle physics, >5 sigma is considered a discovery, though they still want multiple labs to confirm. 2.5 sigma is a vague hint of a signal. 99.5% chance that what you're seeing is real simply isn't good enough for the systems involved. There are too many other reactions that could produce the same detector outputs.
Also, the 2.8 sigma two-photon signal at ATLAS is not accounting for the look-elsewhere effect, it drops to 1.5 sigma when you do. When combine with other signals you regain some confidence.

Remember, you need the probability of an event happening, the probability of a false positive, and the probability of a false negative to make any judgements about what happened. These experiments have shown a high probability that a higgs was produced around 125 GeV, but they also have rather high false positive rates. Remember that we're talking about around 5 events at each detector, so +-1 event from false positives/negatives will cause significant changes.

The following is a nice article on the arguments for and against this being a real detection.
http://profmattstrassler.com/articles-and-posts/the-higgs-particle/holiday-higgs-hints-confidence-inspiring-or-not/

Igor

Bump for this mornings announcement!

The OP (and Peregrine's post) still stand as an explanation of what's happening, the real news is that the confidence in the results are now over 5 sigma!

So the Higgs exists (and has an energy of 125 GeV), what remains is to discover whether it is the expected Standard Model version of the Higgs, or if its something slightly different.

More about that near the end of this article:
http://phys.org/news/2012-07-cern-physicists-strong-evidence-particle.html

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Nephew Twiddleton

Quote from: Igor on July 04, 2012, 11:58:03 AM
Bump for this mornings announcement!

The OP (and Peregrine's post) still stand as an explanation of what's happening, the real news is that the confidence in the results are now over 5 sigma!

So the Higgs exists (and has an energy of 125 GeV), what remains is to discover whether it is the expected Standard Model version of the Higgs, or if its something slightly different.

More about that near the end of this article:
http://phys.org/news/2012-07-cern-physicists-strong-evidence-particle.html

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Kai

#10
Quote from: Igor on July 04, 2012, 11:58:03 AM
Bump for this mornings announcement!

The OP (and Peregrine's post) still stand as an explanation of what's happening, the real news is that the confidence in the results are now over 5 sigma!

So the Higgs exists (and has an energy of 125 GeV), what remains is to discover whether it is the expected Standard Model version of the Higgs, or if its something slightly different.

More about that near the end of this article:
http://phys.org/news/2012-07-cern-physicists-strong-evidence-particle.html

Vegetta, what do the statistics say about our confidence of discovery?

                \




It's over FIVE SIIIIIIIIIIIIIIIIIIIIGGGGGGGGGGGGGGMMMMMAAAAAAAAAAAAAAAA!!!!


                \

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Triple Zero

Quote from: ZL 'Kai' Burington, M.S. on July 04, 2012, 09:15:52 PM
Quote from: Igor on July 04, 2012, 11:58:03 AM
Bump for this mornings announcement!

The OP (and Peregrine's post) still stand as an explanation of what's happening, the real news is that the confidence in the results are now over 5 sigma!

So the Higgs exists (and has an energy of 125 GeV), what remains is to discover whether it is the expected Standard Model version of the Higgs, or if its something slightly different.

More about that near the end of this article:
http://phys.org/news/2012-07-cern-physicists-strong-evidence-particle.html

Vegetta, what do the statistics say about our confidence of discovery?

                \




It's over FIVE SIIIIIIIIIIIIIIIIIIIIGGGGGGGGGGGGGGMMMMMAAAAAAAAAAAAAAAA!!!!


                \





might be the best LHC joke I've heard so far :)
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LMNO

I was kind of hoping this would kick String Theory in the nuts, but because ST is a "God in the Gaps" argument (i.e. the theory itself is currently untestable), they're still chugging along, saying that the HB supports ST.

Still, chalk up another point for the Standard Model.

Kai

Quote from: LMNO, PhD (life continues) on July 05, 2012, 06:08:07 PM
I was kind of hoping this would kick String Theory in the nuts, but because ST is a "God in the Gaps" argument (i.e. the theory itself is currently untestable), they're still chugging along, saying that the HB supports ST.

Still, chalk up another point for the Standard Model.

It sounds like String Theory is a religion.
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