A joint Fermilab/SLAC publication

LHCb event display -- decoded!


One of the four major experiments built around the collision points of CERN’s Large Hadron Collider, LHCb studies B particles - particles composed of beauty quarks. By detecting and analyzing the decays of millions of Bs, it explores the imperfect symmetry between matter and antimatter, the slight imbalance that allowed matter to survive over antimatter and build the universe we inhabit today.

This walk through the event display of the first B particle detected by LHCb shows how the collision data is analyzed.

LHCb event display. <em>Image courtesy of LHCb collaboration.</em>

LHCb event display. Image courtesy of LHCb collaboration.

All four views of the event display show data from the same collision.

1 -> Collision point: Marks the spot where protons from the 2 beams smash into each other.
2 -> Beamline: The arrows show the paths of the proton beams.
3 -> Vertex Locator
4 -> Tracking detectors
5 -> Magnet
6 -> Ring Imaging Cherenkov detectors
7 -> Calorimeters
8 -> Muon detectors

A -> Top view : Looking down onto LHCb (scale 24m x 12m)

This view gives a complete picture of the collision, showing data from all layers of the LHCb detector. The proton beams collide on the left, and B particles fly out close to the path of the proton beams. This property of B particles is reflected in the design of the experiment: The sub-detectors sit side by side along the beam path, like books on a giant bookshelf.

Dominating the image on the left are the large coils of LHCb’s magnet, shown in white [5]. The magnetic field is perpendicular to the page and curves the paths of charged particles in the plane of the page.

On either side of the magnet are tracking detectors [4] that measure the positions of particles as they pass through. The signals the particles leave behind are shown as crosses. There are no sub-detectors inside the magnet, so the paths of the different particles spraying out from the collision are reconstructed in a sophisticated version of “connect the dots” using data from all the LHCb detector’s layers.

Particles from the collision also pass through the Ring Imaging Cherenkov detectors [6], which are used for particle identification. They work by measuring emissions of Cherenkov radiation. This phenomenon, often compared to the sonic boom produced by an aircraft breaking the sound barrier, occurs when a charged particle passes through a certain medium (in this case, a dense gas) faster than light does. As it travels, the particle emits a cone of light, which the Cherenkov detectors reflect onto an array of sensors using mirrors.

Then, particles are stopped by the calorimeters [7], and the energy they deposit is represented in the event display by histograms. The red bars represent the energy of lighter particles (such as photons and electrons), while the blue bars show the energy of the group of mostly heavier particles (such as protons).

The position of the muon detectors [8], furthest from the collision point on the right, is shown by the vertical green lines. The tracks left by the two muons created in this collision are colored in magenta.

B -> Along the beam: Looking straight through the collision point (scale 1m x 0.5m)

This view shows data only from LHCb’s two tracking detectors [4] and the Vertex Locator (VELO) [3]. The curved tracks show how the strong magnetic field affects the direction of charged particles as they spray out from the collision. Some particles are only seen in the very central layer, the VELO, whose main job is to identify the primary interaction point, or vertex, where the collision occurred and also any secondary vertices where short-lived particles decayed. The VELO is very close to the collision point – a mere 8 millimeters from the point of impact.

C -> Where it all happens: The collision region (scale 0.7mm x 10mm)

Here, there are no data points showing actual signals from the sub-detectors. Rather, this is a reconstruction of the innermost region surrounding the collision point, using information gleaned from all of the detector layers. This is where the exciting physics happens.

The image has been stretched in the vertical direction to make it easier for our eyes to decipher – particles spray out from collisions so close to the beamline that without changing the scale, different particle tracks could not be separated by our eyes.

The exact collision point is clearly identified as the point of origin of almost all the lines, labelled “primary vertex.” However the two muons (the magenta tracks labelled μ+ and μ-) originate from a point a few millimeters away, the “secondary vertex.” This indicates that other particles were created in the collision and that they decayed before reaching the first layer of detectors, at the point where the two muons emerge.

The combined mass of the two muons corresponds to the mass of a very short-lived particle called a Jpsi (whose path is represented as a short green dash). The Jpsi was created at the same time as a kaon (red track) at a point close to — but not exactly — the muons’ vertex. The Kaon survived longer than the Jpsi and was detected in several of the layers of LHCb detectors.

Working backwards, each time comparing the direction of the momentum of the different particles, their masses and energies, the full story emerges. A B particle (orange track) was created in the collision and, after a few millimeters decayed into a kaon (red) and a Jpsi (green), which in turn decayed into two muons (magenta).

Other posts in this series include: CMS event display — decoded!, ATLAS event display — decoded!, ALICE event display — decoded! and LHC Page 1 — decoded!

For visitors to CERN, a new photo book telling the story of the LHCb experiment is available from the shop at CERN’s reception and from the CERN library.

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