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Anna Phan | USLHC | USA

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What made those tracks?

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…

The above image was taken from this webpage.

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!

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  • Zara

    But my question is, Are the particles determined correctly?? RICH 2 detector is very effective than RICH 1 I think. And the post is very informative because I didn’t know it before.

  • Anna Phan

    The RICH system provides good particle identification over the entire momentum range. RICH1 is more for low momenta particles and RICH2 for higher momenta particles. For example, the average efficiency for kaon identification for momenta between 2 and 100GeV/c is ε(K → K) ∼95%, with a corresponding average pion misidentification rate ε(π → K) ∼5%. Around 30 GeV/c the identification probability is ∼97% and the misidentification probability ∼5%.

  • Amir

    How does knowing the magnetic deflection of a particle tell you its momentum? Doesn’t this also depend on the electric charge?

  • Anna Phan

    My next entry will actually be about the many uses of magnets at the LHC, so stay tuned!

  • michelscofield

    RICH 2 looks more accurate than RICH 1, thank you for this particle detection news and looking forward for your next update.

  • Zara

    RICH 2 is more vulnerable than RICH 1. The materials and factors are very effective in RICH 2. RICH 2 particle gives more strong signal than RICH 1. Very useful and important post.

  • http://agleam3387@mypacks.net Christopher

    Presumably all elements of the Standard Model are produced by the LHC in its range of experiments. Does this mean that there’s a specific detector for each of them along the lines of the two RICH detectors? What are the ket detectors for the Higgs?

  • Anna Phan

    Dear Christopher,

    Yes, all the particles of the Standard Model are produced in the LHC collisions. The way each of the detectors goes about identifying the particles depends on what physics they are targeting. LHCb needs to be able to identify mesons well and hence has the RICH subdetectors. With regards to the Higgs, it is identified from its decay products, electrons, muons, photons and jets. These are things which ATLAS and CMS identify and measure very well.

    Cheers,
    Anna