Greetings and salutations everybody! I’m gratified and honoured to be the first LHCb US LHC blogger. As well being new to US LHC, I’m also new to the LHCb collaboration, so we can all learn about LHCb and B physics together.
Today I thought I would start by introducing the LHCb detector. Here is a schematic:
If you are familiar with the two largest LHC experiments, ATLAS or CMS, this may look a little different to you. LHCb is not a general purpose detector, with the aim of detecting as many different types of physics events as possible; it is especially designed to measure the decays of (B) mesons (more about why we care specifically about (B) mesons in a future post).
ATLAS and CMS are essentially cylindrical in shape, while LHCb is a cone, taking advantage of the strongly forward peaked probability distribution of (B) meson production. This means that while the detector only covers around 4% solid angle, it is able to measure around 40% of (B) meson production.
The decay products of (B) mesons are identified and measured by different detector components:
- Closest to the proton-proton interaction region is the vertex detector, known as the VELO. Its job is to measure the particle tracks to precisely separate primary and secondary vertices. This is important to identify (B) mesons and their decay products. For example, in the image below, we have been able to identify three vertices: PV, SV and TV, and associate them with particular events: PV – the production of an (B_s) meson, SV – the decay of the (B_s) meson into a muon and a (D_s) meson, and TV – the decay of the (D_s) meson into a (K) and two (pi) mesons. These three vertices could not have been identified without the VELO.
- There are two ring imaging Cherenkov detectors, known as RICH1 and RICH2, which are used for particle identification. For example, they can differentiate between the (K) and the (pi) mesons in the decay chain described above.
- Reconstruction of charged particle tracks and momentum measurement is performed by the tracking system, made up of the TT, T1, T2 and T3 stations in the schematic.
- Following these are the electromagnetic and hadronic calorimeters (ECAL and HCAL), which measure the energy of electrons, photons and hadrons.
- Finally, there are there is the muon system (M1, M2, M3, M4 and M5), which identifies and measures muons.
Along with these detector components, there is a dipole magnet to help measure particle momenta. Particles normally travel in straight lines, but in a magnetic field the paths of charged particles curve, with positive and negative particles moving in opposite directions. By examining the curvature of the path, it is possible to calculate the momentum of a particle.
There you have it, a brief overview of the LHCb detector and its components. This is what it looks like in reality:
Note that I’ve flipped the photo horizontally so it is easier to match the detector components in the schematic to their real counterparts.
Hi Anna, I look forward to learning more about LHCb. It is truly a beautiful detector – no pun intended – with awesome capabilities. Your example event above says it all!
regards,
Michael
Very interesting post. It’s amazing how little information about the LHCb reaches us, outside spectators, compared to Atlas and CMS.
It would be interesting if you could briefly mention the technologies used in each subdetector. Ok, you mentioned that RICH (I always write RHIC and have to correct
) is a cerenkov detector, but could you add information about the others as well?
– Rafael.
Hi Rafael,
Thanks for your interest in the LHCb detector. I’ll be going through the subdetectors in my next few posts, so stay tuned!
Cheers,
Anna
OK, dumb question time.
The proton-proton collisions at the LHC are basically symmetrical. Does that mean that as well as the 40% of B mesons that are detectable by LHCb, there are another 40% emitted in the opposite direction? And if so, why can’t you build it as a double cone (with the VELO at their common point) and try for 80% of the B mesons?
Thanks
Steve
Dear Steve,
That’s a very good question.
The reason that we don’t build the other arm of the spectrometer is somewhat complicated, but it is essentially the result of a cost benefit analysis. Building another detector not only involves the expense of the materials and installation, but also would double the amount of data produced, which we wouldn’t actually be able to record due to bandwidth constraints, so it would involve modifying the data acquisition and requiring more computing power and storage.
I’m sure there are other things that would also need to be changed, but I hope you can see that it’s not as easy as double the detector, improve the results by a factor of two.
Cheers,
Anna
I remember reading up a little bit about the ‘other’ detectors at cern; but, I’ve forgotten what they’re about. Well, I suppose I could easily look that up again.
Still, I don’t think I’ve seen anyone explain at least this detector yet; so, thank you vary much!(sorry to type while I’m thinking about it!)
Ms. Phan,
I am currently an undergraduate student attending the University of Advancing Technology. I was hoping to do an email interview with a scientific researcher who works at the LHC, or at least closely associated with the work there. Can you please email me if you are open to participating in such an interview for my classwork? Thank you for your time!
-Justin