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Simulating the heart of the CMS detector to study particles that make up our universe  Aug 03, 2017

The CMS experiment is a general purpose detector which is used to study the results of high energy proton-proton collisions at the Large Hadron Collider (LHC) at CERN. Analyzing the collision data gives particle physicists insight into the fundamental particles that make up our universe, and the interactions between them.

The CMS detector consists of several sub-detectors which are optimized to identify specific kinds of particles. Much like the way a camera takes a picture, these sub-detectors prepare a snapshot of what happened after a collision. At the heart of CMS, the pixel detector is used to track charged particles created after the collisions. The charged particles travel through the pixel detector leaving signs of their presence, and allow scientists to recreate the trajectories of each particle.

Shown here are the results of simulating the passage of charged particles through the detector. Each charged particle hits sensors in the different layers, creating an 'image' of the pixel detector itself. On the right (in blue) are the results for the old pixel detector with 3 barrel layers and 2 disk layers. To the left (in green) are the results with the new pixel detector with a total of 4 barrel layers and 3 disk layers. (Image Credit: Sourabh Dube)
New and Old Detector Images
The CMS experiment had been collecting collision data for much of 2016. During a planned shutdown in December 2016, the collisions were stopped for maintenance and upgradation of the CMS detector, and other experiments located at the LHC. The CMS experiment was using a pixel detector first commisioned in 2009 consisting of 64 million pixels (silicon sensors) that were part of multiple layers arranged in an onion-like structure. In March 2017, the experiment upgraded the old pixel detector with a new pixel detector which has more layers and consists of 124 million pixels. In simple words, the new pixel detector can now take a 124 megapixel image compared to the 64 megapixel image of the old detector. The new pixel detector is also faster and lighter compared to the old one.
An engineering design of the barrel layers of the old and the new pixel detector.
An engineering design of the barrel layers of the old and the new pixel detector (Image Credit: Sourabh Dube)

PhD students Anshul Kapoor and Angira Rastogi, working with their supervisor Sourabh Dube, are developing the simulation framework for the new pixel detector. The work requires a critical understanding of the various components of the pixel detector including the precise geometry of the various layers and the supporting electronics used to acquire the data and process the 124 megapixel image. The work involves development of tracking algorithms for the new detector that can help physicists track the trajectories of the charged particles produced after the collisions in simulations.

In the first step, now complete, Anshul and Angira wrote code to describe the geometry of the new pixel detector. The code for the older layers was replaced with code for new layers including their correct dimensions.

Now the team is developing and testing a new algorithm for reconstructing the actual trajectories of the charged particles. Once fully developed, other CMS physicists will then use the new algorithm in their simulations, and in the data collected by the experiment, to study the results of the high energy proton-proton collisions. The LHC has already swung back to action and the CMS experiment is ready to collect more data this year.

- with inputs from Anshul Kapoor, Angira Rastogi, Sourabh Dube