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CMS event display -- decoded!

The Large Hadron Collider is once again moving into world-record-setting territory as it gears up to smash protons at the unprecedented energy of seven trillion electron volts. As the accelerator gets ready to speed up and smash particles, the LHC experiments—which record and analyze the debris from high-energy collisions—are running through their final checks and preparing to take center stage.

Whenever the first 7 TeV collisions happen, particle physicists from the experiments will proudly show off snapshots of the very first collisions in their detectors. These snapshots, known as event displays, show the information recorded by the massively complex detectors. However, much like a baby’s first ultrasound image, it can be hard to make heads or tails of the physicists’ pictured pride and joy.

Over the next few days, we’ll help you decode ATLAS and CMS event displays, tell pixel detectors from particles, and follow the action in the accelerator with tips on reading LHC Page 1.

What is an event display?

Experiments performed at an accelerator like the LHC use collision event displays to trace the paths of particles produced in a collision. Below is an event display from the Compact Muon Solenoid (CMS) experiment at the LHC. Event displays are very helpful in visualizing specific physical processes and for checking that the detector and software are functioning properly.

CMS Event Display Decoded

Looking at the title and text on this image, we can see that this event occurred on December 14, 2009 at 4:46 a.m. Central European Time. This was the 5,686,693rd event to be recorded in Run 124120. A run is a period of continuous operation in a given part of the detector.

The fourth line tells you that the energy level of the collision was 2.36 trillion electron volts (TeV). This event is significant in that it produced two muons, making this a possible dimuon event. The path of muons can be very clearly reconstructed in the CMS detector and can be produced in multiple kinds of collision processes.

The two muon paths seen in the detector could be the result of a single heavy particle, such as the J/Psi, decaying, or from two separate particles that each decay into a muon. The paths might also be caused by particles that are not muons but appear identical to muons within the detector. For these reasons, it is impossible to define what a single event is with total certainty. Instead, extensive analyses of multiple events can give physicists the probability of what a given event could be. This is why the event is referred to as a candidate.

Split screens

The event display is divided into different screens that give you different views of the split second when collision occurred and the produced particles travelled through the detector. In this display, there are three screens:

View A: On this screen, labeled Rho Phi, we have a beam’s eye view, looking straight at the central collision point (1). The particle beam is running straight through that central point.

View B: In this view, labeled Rho Z, the beam line (2) is running horizontally through the center of the screen and we can see the three main regions of the CMS detector.

View C: This three-dimensional perspective view allows physicists to rotate the collision event display around an axis. Here the beam line is on a diagonal, running from the upper left to the lower right portion of the screen.

These screens trace the paths of particles from the collision point through the detector. The detector records when and where it makes contact with a particle so that computers can reconstruct the particle’s path after the protons collide. Based on their final destination and movements, physicists can determine what kinds of particles were produced in a collision.

Collision and detector components

Surrounding the central collision point, the detector has three main components that record information about particle travel.

1: Collision point
The collision point, what particle physicists call the interaction point, is where the protons collide. You can orient yourself when looking at each screen by spotting the collision point and imagining where the beam line would pass.

2: Beam line
The beam line is the path that protons travel in opposite directions and into collision. On screen B, arrows represent the movement of protons along the beam line and towards the center of the detector for collision.

3: Silicon tracker
The innermost portion of the detector is the silicon tracker, outlined by a thin green line on screen A and screen B, and within the mesh cylinder in 3D view.

The tracker, which includes the pixel and silicon strip detectors, reconstructs the movement of particles point by point. These points are represented by yellow dots (you can also look at event displays of magnified tracker images). When we connect the dots we can see the particle tracks, represented here by red and green lines, tracing a particle’s trajectory.

The tracker detects charged particles, so the tracks you see in this section come only from particles with a charge, namely muons, electrons, and charged hadrons.

Because of the magnetic field in which the inner detector resides, these particle tracks are curved. From the degree of curvature, physicists learn about the particle’s momentum. Physicists can discern whether a particle’s charge is positive or negative from the direction, clockwise or counterclockwise, of the curve.

The magnetic field can bend the path of particles in some but not all directions. This is one reason why seeing the event from multiple angles is important. In screen A, we can clearly see curved tracks but in screen B they are not visible. The program used to create this display can rotate the angle and axis of the 3D image in view C. Doing so gives physicists a better sense of how the particles travel in space.

4: Calorimeters
The next main components of the detector are the electromagnetic and hadronic calorimeters, referred to by physicists as the ECal and HCal, respectively. When particles strike one or both, they leave an energy deposit. These deposits are represented by the bars (red for ECal and blue for HCal) just outside of the tracker. The height of the bar corresponds to the amount of energy deposited.

Particles that stop in the ECal are generally either electrons or photons. The two can be distinguished by the fact that electrons are charged and leave tracks in the tracker while photons are neutral and generally do not appear in the tracker.

Hadrons pass through the ECal but are stopped in the HCal. Charged hadrons leave tracks in the tracker while neutral hadrons do not.

It is also possible for the tracker or calorimeters to record signals that do not get reconstructed as particle trajectories or energies. Further analysis can help physicists decide whether these signals come from particles or other processes in the detector.

There is another particle that sometimes deposits energy in one or both calorimeters. This particle is the muon.

5: Muon chambers
The third and outermost components of the detector are the muon system's muon chambers, so named because they are designed to study muons. A muon can pass through the tracker, calorimeters, and solenoid magnet (not visible in the displays but lying just beyond the calorimeters) to reach the muon chambers.

Muon chambers are visible in screen B as red and blue blocks and the chambers through which a muon has passed have been highlighted. On screen C, only those chambers through which a muon has passed are visible.

6: Muons
This event display illustrates a dimuon event or the production of two muons in a collision. The paths of the muons are shown by the thin red lines on each screen. The muons left signals that were reconstructed into tracks in the silicon tracker, deposited a little energy in the calorimeters, and passed through the muon chambers.

For a good summary of the parts of the CMS detector and the particles that pass through them, take a look at the CMS Detector slice.

Interested in learning more about event displays? Check back tomorrow to learn how to decode an event display from CMS’s sister experiment, ATLAS.

by Daisy Yuhas