Large Hadron Collider: First Blood Part II

[Fraz]

The LHC is up and running again, and the first beam to be injected into the LHC from the pre-accelerators occurred about an hour ago. If you are interested, there is an e-commentary of the LHC beam commissioning here:

http://cmsdoc.cern.ch/cms/performanc...mmentary09.htm

This commentary is from the perspective of the CMS detector experiment, one of the two general-purpose detectors on the LHC. At time of writing, we are seeing "beam splash" at the CMS detector. Beam splash occurs when they send a single beam around the LHC and then stop it at a collimator just before the detector; since you can't really "stop" a beam very easily, a whole bunch of particles leak through and hit one side of the detector:



So... when they reconstruct the events, they tend to look a bit funny, as the normal mode of operation of the detector is to have the beams smashing together at the centre. In the top-right section of the pic above, you can see the particles have hit the right-hand side of the detector first.

At least, that's my limited understanding of it

[Fraz]

And another beam-splash piccy:

[Funkstar]

oooh! pretty pictures!



(forwarding this onto a friend of mine that will like it)

[Jay]

fantastic but it reminds me of a faulty X1900XT I once had

[Zhaoman]

Brilliant news, I hope this'll work without a hitch this time round. I really hope this'll give the data that the dedicated physicists have worked so hard for. I look forward to what implications the results will have in due time

Also: wonder why is there no 'end of the world' headlines from the media this time round? Were you just too fast with the news Fraz?

[Fraz]

Originally Posted by Zhaoman

Also: wonder why is there no 'end of the world' headlines from the media this time round? Were you just too fast with the news Fraz?

Good question - I don't know why it's been kept relatively quiet. I guess most of the media fuss last time was based around the black hole nonsense. And maybe it was a bit embarrassing when it broke after 9 days after all the high-profile media attention? Who knows.

[unreal]

It's because John & Edward captivate far too much of the general public's minds!

[Blastuk]

Really hard to make sense of those pictures without being a physicist

[malfunction]

Originally Posted by Jay

fantastic but it reminds me of a faulty X1900XT I once had

Reminds me of the intro to Another World:

http://www.youtube.com/watch?v=LZifBl9ZZUQ

(which I'm sure we all said last time around too)

[Fraz]

Originally Posted by Blastuk

Really hard to make sense of those pictures without being a physicist

I shall endeavour to explain, although to be honest, there isn't much sense to be had from these pictures, as what is occurring is not the type of event the software is designed to reconstruct. In these events, a big mess of particles is hitting one side of the detector, which is really not what is usually supposed to occur. In a normal event - which we'll see in a week or so hopefully - protons collide with each other at the centre of the detector, and produce much more sensible looking events.



Anyway, that problem aside, the easiest picture to explain is the top right-hand-side part of the first picture I posted (the "rho z" bit - edit: now shown above). This is a side-on cross-section of the detector, so the LHC beam-pipe - if it were shown - runs left/right through the centre of the image. The "beam splash", i.e. the remnants of a beam that has been stopped at a collimator upstream from the detector, has hit the right hand side of the detector. At this point, I guess I should explain that the detector a bit:

The detector is barrel-shaped, with an endcap at each end; it's about the size of a four-storey building, and weighs about 12,000 tonnes. In design terms, it's basically an onion, with many layers of different detectors. In the centre is the silicon tracker, which is designed to be as transparent as possible to radiation whilst still being able to register the passage of charged particles going through it. This is surrounded by the "ECAL" which is the electro-magnetic calorimeter, and this absorbs and measures the energies of any electrons or photons entering it. This is further surrounded by the "HCAL", which is the hadronic calorimeter. The HCAL absorbs and measures the energies of any particles made up from quarks - so, things like neutrons, protons or mesons. (A meson is a particle made from two quarks, whereas protons and neutrons have 3 quarks). Next is a big solenoidal magnet, which surrounds everything in the detector mentioned so far. You need a magnetic field so you can distinguish positively charged particles from negatively charged ones, simply by seeing which way their tracks curve whilst passing through the detector. Finally - enclosing everything - is the muon tracker/calorimeter, which samples the passage of muons as they escape the detector. Muons (a heavier version of electrons) are very good at passing through material, so in essence, most of the detector is in fact the muon calorimeter.

Soooo, in that "rho z" picture, the muon calorimeters are the dark blue bits (muon endcaps) and the dark red bits (muon barrel). The beam is hitting the right hand muon endcap, and the light blue bits withing the dark blue areas are the energy deposits that have been registered. The red lines that go all over the place are particle tracks that the software is totally failing to reconstruct properly, simple because it's not designed to reconstruct tracks starting outside the detector. Then, in the centre of the picture are a whole bunch of light-blue and yellow bars that seem to be exploding from the centre of the detector. These are representations of the amount of energy deposited in the various bits of the electromagnetic and hadronic calorimeters - yellow for electromagnetic and light blue for hadronic. Again, these seem to be exploding from the centre of the detector, as this is where we'd normally expect the particles to be coming from.

Um, that's about it really. The main part of that first picture (the "rho phi" part) is just a different viewpoint of the detector, looking down into the barrel - so the LHC beam pipe would be going into the centre of the picture. Again, the light blue or yellow bars in the represent energy deposits, and the red lines are bits of badly reconstructed track.

Hope that makes some sense!

[malfunction]

I'll say thanks because it makes much more sense than it did but I can't pretend I've got the complete context - i.e. I now know what it's showing but I'm still not sure *why* - or rather I've no idea what the implications are of looking at the interactions of all these particles (and the elusive higgs bosons if / when they are 'found'). (before anyone starts talking about expanding 'our' understanding of particle physics / they way everything works - I get that - I just have no idea what that in turn means)

[Fraz]

Originally Posted by Zhaoman

Also: wonder why is there no 'end of the world' headlines from the media this time round? Were you just too fast with the news Fraz?

So, I found out the answer to this. Apparently there will be no press releases or media shenanigans at all from CERN until they start proper proton-proton collisions. They never got quite that far last year, and until they get to the point where they are colliding protons, everything will essentially be a repeat of what happened last year - and thus semi-boring to the media.

So... I guess you won't hear anything official from CERN for another couple of weeks. From what I've heard, every effort is going to be made so that the LHC has a chance of taking the crown as the highest energy particle accelerator in the world before the year is out. This aim is really just something to keep the politicians happy, but it'd be quite nice to make the USA's Tevatron second best before 2010

[Dareos]

Oh, i remember this, the 2nd picture brought it all back to me.


This reaction was the predecessor to the initial proton bonding collision which excited the CERN scientists so much, mainly because of the possibilities of renewable energy which resulted from this test. Of course it did all go horribly wrong when they tried to recreate it on a grand scale, resulting in an explosion which wiped out switzerland and knocked the earth off its axis, culminating in a new ice age, which of course i was sent back in time to prevent.


Of course, i might just have been tripping out on acid and looking at a spool of thread at the time...

[Fraz]

First LHC beam collision occurred about 40 minutes ago:



This was a very low-energy collision, but a collision nonetheless! If you compare against pictures I posted a while back in this thread, the above picture looks a lot more normal - i.e. the beam isn't hitting the side of the detector, but stuff is colliding at the centre instead. The outer part of the silicon tracker is now also switched on, which is why there are some properly reconstructed tracks (yellow lines) now seen that weren't in previous images. We couldn't include the tracker before from fear of damaging it with a poorly controlled beam, which is the reason why the inner tracker is still currently off.

[peackshelb]

Great to hear - and see!

Thanks for keeping us updated.

[Dareos]

and as we are all still here, i must have succeeded..


its hard being this good