I’m not ashamed to admit that I spend a fair bit of time monitoring what the LHC is doing. Luckily there is a very good set of web pages that allow me to do this, the first of which Brian discusses in this post.
Below I’ve got stills of Page 1 and Page 3 when the beam is in Adjust and Stable modes.
Adjust Mode
Stable Mode
It’s a little hard to see, but I’ve highlighted particular sections of each page. First on Page 1 there’s a small entry titled Movable Devices Allowed In, while on Page 3 there’s a special entry for the LHCb VELO Position.
These two small entries in the LHC status pages are probably all that people outside LHCb know about the VELO, and even then the first entry isn’t an obvious reference.
So what is the VELO? And why does it move?
The VErtex LOcator is the part of the LHCb detector closest to the collisions at the LHC. As I mentioned in my previous post, its job is to measure particle tracks to precisely separate primary and secondary vertices and that this is important to identify B mesons and their decay products.
This is possible through the use of 42 silicon modules which during physics data taking are positioned as close as 5mm from the interaction point.
While it is important for the sensors to be close to the beam during physics data taking for optimal precision; at these small distances, there is a high probability that the modules will sustain damage due to radiation from the high energy proton beam.
To prevent damage, the silicon modules are mounted on custom made bellows, which allow the VELO to retract to the safer distance of 30mm from the beam during periods of beam instability.
Below is a schematic of the silicon modules from the side and from the front, in both open and closed positions, followed by photo of the actual modules during the assembly process.
The VELO is a very impressive feat of physics and engineering. It is not an easy task to design a set of silicon chips and associated services to operate so close to a high energy proton beam and its associated electromagnetic effects. And it works! Beautifully, as can be seen from this view of one of the first reconstructed B mesons…
I like to think it is because of Syracuse’s involvement in the subdetector, but Syracuse is just a spoke in the collaboration…
The adjust mode picture shows that the two beams’ intensity has been constant for at least 15 minutes and this is true for the Energy as well. Naively, then, it would seem that the beams are stable; however, clearly they aren’t considered so. Could you elaborate why (are the magnets being tuned?)
Dear Nick,
Nice to see a theorist being so interested in the details of the LHC beam.
To answer your question, while the beams’ intensities and energies are stable, as you can see from the message on Page 1, the LHC machine operators are preparing collisions. This means they are squeezing the beams (reducing the size of the proton bunches at the interaction points) to increase the cross sections and also bringing the beams into collisions, as the beams are injected and ramped with a separation at the interaction points.
Cheers,
Anna
Hi Anna—I have an incredibly naive question: why can’t the general purpose detectors (ATLAS, CMS) also use something like VELO to improve their b-tagging? (Or do they do this?) Whats the efficiency for b-tagging at LHCb vs the general purpose detectors?
Thanks!
Flip
Dear Flip,
ATLAS and CMS both have many layers of silicon pixels, followed by silicon strip detectors, which they use for tracking and vertex reconstruction. However, neither detector has as many silicon layers as LHCb and their silicon modules aren’t as close to their interaction points. There is no need for either ATLAS or CMS to have a similar type of vertex locator as LHCb for B physics as neither detector covers the high pseudorapidity region that LHCb does, which is where the B production rate peaks. For this reason, I can’t really give you a proper comparison of the efficiency of B meson identification between ATLAS, CMS and LHCb, since their coverage and geometries are so different.
I hope that answers your questions,
Anna
Thanks, Anna!