CMS (Compact Muon Solenoid) - Pixel Tracker

• 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.

Anti-kt algorithm

• 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.

1. 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.

• Anti-kt  algorithm.

1. If we let p = -1, this is anti-kt algorithm.

• Why we used anti-kt algorithm?

Discussion Jan 15, 2015.

1. Cross section
1. 1 barn = 10^(-28) m^2
2. 1 mb = 10 ^(-27) cm^2
3. pp cross section at 7 TeV ~ 100 mb
   Luminosity ~ 10^34 cm^2 / s
   Cross-section ~ 100 mb
   than : about 10^9 events / s
4. Bunch spacing : 50 ns at 7 & 8 TeV and will be 25s at 13 TeV
2. Cross section
1. ttbar -> 250 pb at 8TeV , 800 pb (~10^-33 cm^2) at 14TeV
2. Instant Luminosity : 10 ^ 33 cm^-2 s^-1
3. about 1 ttbar events per 1 sec, about 10pi M events per year.
3. Next time
1. quark and gluon property
2. Neutrino cross section -> 1GeV of it 10^-38cm^2 (nu p)

Discussion Jan 9th, 2015.

1. 1. How can we distinguish the elastic scattering and the inelastic scattering?
1. In particle physics point of view, we can say if objects are same before and after the collision then the collision is elastic. This is true only when objects have no internal structure.
2. In macro-scopic world, how can we define the elastic scattering?
       
2. For p-p collision
1. Elastic scattering.
1. What can be the medium particle of p-p elastic scattering?
1. Gamma*
2. Z*
3. Pomoron(gluon-gluon bound state)
2. These interactions can be happened only in t-channel, not in s-channel.
3. How is the order of interactions?
1. Gamma* : square of alpha_em. alpha_em = 1/137. So about 10^-4 scale.
2. Z* : really small. About 10^-3 times of alpha_em. Therefore, we do not consider the weak interaction in elastic case.
3. Pomoron : alpha_s is about 1 in long distance where elastic collision happens. So, if we consider higher order interaction, they do not go to be smaller. However, electromagnetic interaction goes to be smaller 10^-4 times whenever the order goes up 1 step.
4. So, the strong interaction is dominant.
2. Inelastic scattering.
1. You know? hc = 1.24 keV nm and hbar c = 197 MeV fm.
2. If we give 7TeV to each proton, the medium particle gets certain portion of proton's momentum. Let's say the medium particle has 10 GeV momentum. Then the matter wave length of the medim particle is about 0.1fm that is smaller than proton size(1 fm). So, we can see the internal structure of proton. We should consider as parton-parton scattering.
3. What process is dominant?
1. When we consider parton-parton scattering, now the single gluon can be medium particle. When the medium particle is gluon, what parton is most attractive to gluon? That is gluon. Gluon has more color than quark. So, gluon-gluon interaction(diffractive) has the biggest cross-section in parton-parton scattering.
3. Let's do
1. p-p cross-section, p-pbar's difference, p-pbar annihilation's portion.