Colliding different particle species: The LHC's proton-lead run

Jan 21, 2013 by Achintya Rao
A proton-lead collision at a centre-of-mass energy of 5 TeV per nucleon. In this side-on view, the proton beam enters from the right side of the image and leaves on the left; the lead beam travels in the opposite direction. The event was selected requiring a muon trigger, and the muon (red line) was reconstructed in the CSC detectors.

The new year brings a new type of collision at the LHC: the accelerator will smash protons and lead nuclei together, allowing CMS and the other LHC experiments to study the cold nuclear matter we expect these collisions to produce. Although we caught a glimpse of these asymmetric proton-lead (pPb) collisions during a pilot run last September, the next four weeks will bring the first sustained pPb run and provide valuable data. Indeed the small data sample from 2012 already revealed interesting phenomena, and raised interest in this study.

Preparing the detector

CMS preparations for collisions have been progressing steadily. "We have been commissioning the detector for operations as we recover from the three weeks of downtime during the Christmas shutdown," points out Christof Roland, co-convener of the CMS Heavy- (HI) group. Commissioning involves making sure all of the subdetectors of CMS are operating within their expected parameters.

"Work on recommissioning the detector started on 4 January, during the Christmas break," says Maria Chamizo-Llatas, CMS Run Coordinator. "On 7 January, when everyone returned to , we had cooling and power up and running. By that night, we were already taking with all subdetectors except the RPCs."

"Compared to lead-lead collisions," adds Christof, "you're looking at different physics at different beam intensities. So you need dedicated trigger menus. This was where the biggest effort went in from the HI group: to prepare studies of all the triggers for the -lead run." When colliding lead , there are around 4000 interactions each second, of which around 200 are recorded to tape. The proton-lead collisions, on the other hand, will have around 2,000,000 interactions each second, and CMS will record around 1000 of these. "For all these trigger studies," explains Christof, "the pilot run in September was extremely useful to get an idea of what the rates would be so we could optimise everything."

Since the rate in the pPb run is lower than that in the pp one, two forward subdetectors that couldn't withstand the radiation under the harsher conditions of proton collisions have been reinstalled in CMS: CASTOR and ZDC. These subdetectors will help measure the collision remnants that travel very close to the beampipe, crucial to the study of pPb data.

A proton-lead collision event at a centre-of-mass energy of 5 TeV per nucleon, in which two particle jets are produced. The view shown is along the beam axis. The event was selected requiring a high-transverse-momentum jet trigger, and the jets (yellow cones) were reconstructed in the CMS calorimeters.

Joining hands with TOTEM

For this run, CMS will combine forces with TOTEM so as to cover a greater range of collision data. The two are essentially separate entities—independent experiments that use different analysis software—and they are fully complementary. CMS measures in the central region and TOTEM exclusively measures in the very forward region. "Combining information from both allows us to do a lot of physics studies that previously were impossible to do," says Christof. "You can correlate proton remnants seen in TOTEM with objects such as jets and Upsilon (Υ) particles observed in the central part of CMS."

"We are going to use the TOTEM triggers," says Maria. "We made some tests previously, including in the pilot pPb run last year. We had the TOTEM triggers going to CMS and vice versa."

"The only point where we can synchronise between CMS and TOTEM is at the first level of the trigger system," states Christof. "Then we know we have common triggers and each side stores the information of each trigger in the data stream. Later, you have to synchronise and pick the events that are common in both data sets. This is not a trivial operation," he laughs, "but based on the pilot proton-lead run where we combined triggers, we have proved that it is possible. The first hours of collisions will be spent commissioning this operation."

A proton-lead collision event at a centre-of-mass energy of 5 TeV per nucleon, in which two particle jets are produced. In this view, the proton beam enters from the right side of the image and leaves on the left; the lead beam travels in the opposite direction. The event was selected requiring a high-transverse-momentum jet trigger, and the jets (yellow cones) were reconstructed in the CMS calorimeters.

Physics aims of the pPb run

Julia Velkovska, also a co-convener of the HI group, explains the motivations behind colliding these different particle species together: "Not only does it act as a straight reference for lead collisions, but it is an interesting physics system in its own right. In addition, there are a lot of things that we think we can address now that weren't on the cards a few months ago: the ridge we observed in the pilot run raised a lot of new questions and new approaches on how to analyse the data have been proposed."

Now, if this ridge in pPb collisions is caused by the same effect as in PbPb collisions, you might also hope to see other phenomena only observed so far in PbPb data, such as quenching of jets.

What about other theoretical predictions, though? "That's the fun of heavy-ion physics!" exclaims Christof. "These many-body systems and extended systems are very hard to calculate on a theoretical basis, so the experimental side is usually ahead of what the theory guys can calculate. That's what makes this field also so exciting, because it's really the experimental side to a large extent that drives the progress and understanding."

The LHC is expected to commence the run over the weekend. "On the CMS side, we are ready for collisions!" says Maria with a smile.

Explore further: IHEP in China has ambitions for Higgs factory

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omerbashich
1.3 / 5 (13) Jan 21, 2013
What a hilarious site, look at the "rating system" for comments. You can give little stars to posters. (S)he loves me, (s)he loves me not...

Then if someone rates my post high or low, should I return the favor? As in the "peer review" system. Seriously, what a sicko you have to be to make a rating system even for comments...

It's lab Nazi control-freaks thinking science is about democracy,
when it isn't.

In the meantime, on planet Earth: "How Wineland & Haroche Stole My Discovery (and got 2012 PHYSICS NOBEL PRIZE for it...)" http://sites.goog...ci#Nobel
antialias_physorg
4.1 / 5 (9) Jan 21, 2013
For someone who finds the rating system 'hilarious' you sure are the poster who's most hung up about it/miffed at his ratings.

Especially since it's now certainly half a dozen threads where you copy and pasted your rant.

Why don't you go and DO something about it - like writing to physorg staff (the email can be gotten from a link at the bottom of your screen) - instead of crying like a little baby about it to other posters?

What do you want us to do? Hand you a lolly and tell you "There, there - it's gonna be all right"?
Chromodynamix
not rated yet Jan 22, 2013
Colliding the heavy stuff!
Gawd, I hope we don't get another rash of Black Hole hyperbole.

But as you see, you can have fun with the stars. Mwahahaha!
omerbashich
1.7 / 5 (6) Jan 22, 2013
For someone who finds the rating system 'hilarious' you sure are the poster who's most hung up about it/miffed at his ratings.

Especially since it's now certainly half a dozen threads where you copy and pasted your rant.

Why don't you go and DO something about it - like writing to physorg staff (the email can be gotten from a link at the bottom of your screen) - instead of crying like a little baby about it to other posters?

What do you want us to do? Hand you a lolly and tell you "There, there - it's gonna be all right"?

Oh look, it's a little Nazi again, telling us what to do.

Jawohl!

Now use all your fake accounts to "rate" this. As already noted -- what a hilarious site. ha ha
antialias_physorg
5 / 5 (1) Jan 23, 2013
Now use all your fake accounts to "rate" this.

Sorry to disappoint you. Fake accounts are your game - not mine.
omerbashich
1.8 / 5 (5) Jan 23, 2013
Now use all your fake accounts to "rate" this.

Sorry to disappoint you. Fake accounts are your game - not mine.

Can't believe it, he actually replied to another rhetorical...

Time for therapy?
omerbashich
1.8 / 5 (5) Jan 24, 2013
Now use all your fake accounts to "rate" this.

Sorry to disappoint you. Fake accounts are your game - not mine.

So you're still claiming it's me who is anonymous in here, not you?

Power of self-conviction is infinite indeed...