- Today, I want to briefly introduce the pixel tracker of CMS.
The pixel tracker is the most inner detector of CMS, and has key role to reconstruct track of particle, to calculate the interaction point position, and etc.
Therefore, I would say that the pixel tracker is the most shiny device of CMS.
- The pixel of pixel tracker is 100 micron * 150 micron size.
66 million of pixels are on the area about 1 square meter.
- The geometrical setting of pixel tracker is here.
You can see the three barrel layers which is length is 53 cm, and two disk layers each front and the backward of barrel layers.
- As you can see, disk layers look like turbine. Each wing of disk layer is rotated 20 degree from the plane which is exactly perpendicular to the z-axis.
It helps to get better resolution than making the disk layer as flat plane.
- The efficiency when we demand three different layer hit is usable when eta < 2.2.
Demanding two layers is usable when eta < 2.5.
- Everything comes from,
- Kt algorithm : http://arxiv.org/pdf/hep-ph/9305266v1.pdf
- Anti-kt algorithm : http://arxiv.org/pdf/0802.1189v2.pdf
- To grab the jet made by hadronization of parton comes from particle collider (such as electron collider, LHC, Tevatrom), physicists have used many algorithms.
In the CMS, many jet analysis are done with anti-kt algorithm. Let's briefly check what it is.
- As a first step, we tag every hadrons as proto-jet. After that, merging hadrons as components of the same jet should be done.
The anti-kt algorithm is about the way to merge hadrons.
- The kt-algorithm uses this variable.
- Now, find the minimum value among d_ij and d_i.
If d_ij is the minimum value, merge i and j proto-jet and give
Energy : E_k = E_i + E_j
phi = (E_i * phi_i + E_j * phi_j ) / E_k
and same way for eta.
- If we let p = -1, this is anti-kt algorithm.
- Why we used anti-kt algorithm?
