On hadron colliders, dark matter and black holes

[torstan]

@Waldonrate: That's pretty interesting - and way outside my field. Thanks for the link.

@Turgenev: Thanks! It's really exciting to see the LHC finally starting to come together. We're still a long way from new physics data, but given the fact that this was first conceived in '83 - this is a huge milestone. It's just great to have news that things are working. Certainly exciting times ahead.

[Redrobes]

Thanks Torstan for keeping up to date. First collisions - wow I thought that was some weeks away...

Yeah I dont get the entangled thing at all. They were suggesting that you could see into the blast from a bomb drop by not looking through the debris but by some other means. If you have to create both photons together then surely you have to send one set into the blast zone in order to look at the other entangled one. Are they suggesting that you send it in but it never gets back out but you dont care cos you have the other one to look at... hmmm ????

I have to admit I still see the entanglement thing with the "spooky action at a distance" metaphor. Its just too spooky for me...

[torstan]

I don't think they were expecting them either - but they got them. See here:

http://cms.web.cern.ch/cms/Media/Ima...nts/index.html

You're looking at the middle image on the bottom row. That's a set of particles emerging from the center of the detector and being picked up by the layers of the detector. That's the first collision in CMS.

Also, you can see a press conference here (basically just covering the fact that the beam went all the way round):

http://cdsweb.cern.ch/record/1223965

@Redrobes: I think that's what they're saying, but there's subtleties in there that I didn't really look into.

[torstan]

Here are the images!

http://press.web.cern.ch/press/Press.../PR17.09E.html

[Gidde]

Thanks for the response, Torstan, been out of it for a couple weeks myself I too really appreciate the updates in this thread.

[torstan]

@Gidde: You're welcome.

New World Record!

Last night the LHC became the highest energy collider in the world, having accelerated particles up to 1.18TeV, beating the current highest energy machine - Fermilab's tevatron. To put this in perspective, CERN had been saying that the beam commissioning up to these energies was going to take until Christmas, and they did it in a week. This is fantastically fast. I think everyone was expecting that we'd have glitches and hiccups along the way with such a large and complex machine but it's going wonderfully. They are now aiming to increase luminosity (density of the beams effectively) before running some callibration collisions for the detectors.

Link to the press release:
http://press.web.cern.ch/press/Press.../PR18.09E.html

[Turgenev]

So torstan, which one is you in the press release picture?

Congrats on the new world record. *thumbs up*

[torstan]

As a parasitic theorist I get to sit on the other side of the atlantic and, like the kid in the back seat of a car, ask "are we nearly there yet" to the hard working experimentalists. That's why they get to be in the pictures and all the theorists (including the whole theory division of CERN) are noticeably absent. This time is the experimentalists, specifically the beam and accelerator guys. When we start getting results, then you'll see the pics of theorists and detector guys (though still not my own mug unless I come up with something *really* good).

They've done an amazing job. Let's just hope it continues to go this well.

[Redrobes]

Yeah what phenomenal progress. Remind me again. Whats the energy / conditions that the Higgs should show itself and how long after that beam has collided does it take before that data ought to get processed to say just initially that its there or not there. Cos I thought about 1.2TeV was sorta ballpark for this to be the case...

[torstan]

The Higgs boson needs 120GeV or so of energy to be produced. Now this machine is running at 10 times that. However, the tevatron in the US has been running at 1000GeV for years and hasn't found the Higgs. That's not to say it's not there. This comes down to event rates and backgrounds.

Say you have an experiment running at one energy and you expect to see 1 Higgs event per billion events. Now you need many billions of events to be sure that you should have a few Higgs boson events in there. Now you've also got to be clear that out of all of those other events that you've got, you're not going to misdiagnose one of them as a Higgs event. Any event that can mimic the one you're looking for is called background and there are usually many more background events than signal events. So you need *lots* of Higgs events before you can say that they're certainly not part of the background. This is shown by summing together all of the events that you've found that look like a Higgs - both signal and background - and showing that your count rate is significantly above what you'd expect if it were just the background. This means you have to understand the background signals really well before you can categorically say that the excess is down to a Higgs.

Now the rates of signal and background events change depending on how you look for it. So you look in lots of different types of events. It also changes depending on the energy. At higher energies I believe that the Higgs backgrounds are better so it should be easier to spot than at the tevatron. But essentially the name of the game is to get the largest data sample so that you can say that an excess is real rather than a random statistical fluctuation. If you predict that out of 1 billion events you'd epect 5 background events and you see 6 then that's not particularly significant. However if you say that in 1 trillion events you'd expect to see 5239 events and you see 6050 events then you can say you've seen something and people can get excited.

So the name of the game is to finish building up the energy to 3500GeV and then run the concentrated beams of particles through. This will generate the large event rates we need and give us the huge number of events that we need to do these studies. It's the luminosity (number of events per second) as much as the energy that contains the real advantage of the LHC. They've not started with the luminosity challenge yet.

Once it's up and running at full luminosity then we'll be collecting yearly data samples and building up a record of all the events. It's a job of statistical analysis at that point to find signals. The more events you see, the more significant the signals are. So the machine becomes more and more sensitive to new physics the longer it runs.

The LHC is designed to be able to see the Higgs with a year or so of data at a good luminosity. It might take an extra year or so to get there, and the analysis will take a while too.