Last post I discussed how we reconstruct tracks in LHCb. The next logical step is to talk about how we identify what sort of particle left which track. Continuing with my analogy about animal tracking, animals leave very distinctive tracks related to their paw prints and how they move. You can basically tell what animal left a track by examining it carefully…

Unfortunately is is not possible with particle tracks. Only given the parameters of a reconstructed track, there is no way to determine what type of particle left that track. More information is required and that is where the RICH1 and RICH2 detectors come in.

The identity of a particle can be determined from its mass. The mass of a particle can be determined from its momentum and speed. The momentum of a charged particle is measured by its deflection in a magnetic field. The purpose of the RICH detectors is to match this information with a measurement of the particle’s speed.

RICH detectors work by measuring emissions of Cherenkov radiation. A charged particle traveling faster than the local speed of light in a medium emits Cherenkov radiation in the form of light, in a cone at an angle which depends on the speed of the particle. The RICH detectors focus the cone of Cherenkov light into a ring using mirrors onto an array of detectors. The radius of this ring provides information about the particle’s speed. Here are a few of the rings seen in RICH2 from an early LHC event.

The system of RICH detectors consists of an upstream detector (RICH1) which uses silica aerogel and \(C_{4}F_{10}\) gas as Cherenkov media located just behind the VELO, and a downstream detector (RICH2) using \(CF_{4}\) positioned after the magnet and tracking system. The use of silica aerogel allows the detector to identify low momentum particles (order of a few GeV), the use of \(C_{4}F_{10}\) allows the identification of higher momentum particles (between 10 GeV to around 65 GeV), while the use of \(CF_{4}\) allows the identification of even higher momentum particles (between 15 GeV to around 100 GeV).

Here is a schematic of the RICH1. Particles will enter the detector from the VELO on the left, then travel through the Cherenkov media, producing Cherenkov light which are reflected by the mirrors into the photon detectors. RICH2 is fairly similar.

The two RICH detectors are responsible for identifying a range of different particles that result from the decay of B mesons. Particle identification is crucial to reduce background in selected final states. For example, in the plots below, we are searching for the decay of a \(B_s\) meson into two \(K\) mesons. On the left, you can see that without the RICH it would be very hard to separate the signal, shown in red, from the backgrounds, since we would have no way of accurately differentiating \(K\) mesons from \(\phi\) mesons and \(\rho\) mesons. We would also have problems differentiating between \(B_s\) mesons and \(B_d\) mesons. On the right, using the RICH detectors, you can see that the signal is much much cleaner. They are very nice, useful detectors!

Today’s post is a continuation of my description of the LHCb detector. From my other post on identifying vertices using the VELO, we naturally progress to tracking charged particles. As I mentioned in my first post, the VELO along with the TT, T1, T2, and T3 stations are used to reconstruct particle tracks inside LHCb.

Particle tracking is somewhat akin to animal tracking. The first thing you need is some material where particle tracks will leave a trace. It is very hard to find animal tracks on concrete, but very easy on sand or snow…

This where the VELO along with the TT, T1, T2 and T3 come in. When charged particles pass though these detector components, they leave hits. Two different types of technology are used to measure particle interactions. The VELO, TT and the inner sections of the T stations are made of layers of silicon strips while the outer sections of the T stations consist of straw tubes filled with a mixture of argon and carbon dioxide gas. The layout of the TT and T stations is shown below. The silicon sections are coloured purple, while the drift tube sections are coloured blue.

Depending on which detector components register hits, tracks can be classified into four different groups:

1. Long tracks which pass through all parts of the tracking system, from the VELO, through the TT to the outer T stations;
2. Upstream tracks which only pass through the VELO and TT stations;
3. Downstream tracks which only pass though the TT and T stations;
4. VELO tracks which only pass through the VELO; and
5. T tracks which only pass through the T stations.

Each of these track types is shown in the image below:

All of these types of tracks are useful for reconstructing B meson events. An example of a reconstructed event is displayed below. The average number of successfully reconstructed tracks in fully simulated B meson events is about 72, which are distributed among the track types as follows: 26 long tracks, 11 upstream tracks, 4 downstream tracks, 26 VELO tracks and 5 T tracks.

You may notice that in the images above, the tracks are curved. This is due to the LHCb dipole magnet. The experiment contains what essentially is a very large horseshoe magnet, which produces a field of 4 Tesla between its two large coils. 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.

So that’s how we measure particle tracks in LHCb and the types of tracks we record. Stay tuned for how we figure out what type of particle left which track…

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…

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.