# How to determine if 2 particles come from the same process

Hi,
I am studying the Higgs discovery and i've got a doubt. One of the process used to discover Higgs is the decay H -> γγ in 2 photons. At LHC there are detectors of photons that can measure energy and momentum of them. So if you measure the energy and the momentum of both, you can calculate the mass of the particle from which they are produced.
This is easy if you only receive 2 photons, but if you get many of them, how can you decide which of them come from the same process?
For example, if 2 Higgs decay in 4 γ, how can i decide the couples?

Orodruin
Staff Emeritus
Homework Helper
Gold Member
You get a lot of photons in the detectors, from Higgs decays and background processes. The point is that when you plot the distribution of the invariant masses of all photon pairs, the pairs coming from Higgs decays all results in the same invariant mass - giving a peak at that value in the plot.

ChrisVer
Gold Member
One way could be obtained once you ask yourself how ATLAS sees photons.
Roughly speaking, photons will be the energy deposits in the calorimeter system that have no track pointing at them....
Once you get those "photons" you can check their "signals" in the detector by eg checking how the energy is being deposited within the calorimeter components; in that case you can get an idea of what the photon's momentum direction was... once you know the photon's momentum direction, you can track it back and see from where it comes from.
A pair of photons will roughly be seen to come from the same point. (from the paper I ref. to below, you can see that this case, with their selections, corresponds to roughly 8+28=36% of the photon pairs).

There is also the case that the photons convert (giving charged particles) before reaching the detector. In that case what you can obtain by looking at the charged particles' tracks is the conversion point.

For more you can also check Sec4.3 here :
http://arxiv.org/pdf/1108.5895v2.pdf

Last edited:
mfb
You get a lot of photons in the detectors, from Higgs decays and background processes. The point is that when you plot the distribution of the invariant masses of all photon pairs, the pairs coming from Higgs decays all results in the same invariant mass - giving a peak at that value in the plot.
Ok, that point was clear to me, i've got a doubt regarding how you make those pairs given that you receive a lot of photons.

One way could be obtained once you ask yourself how ATLAS sees photons.
Roughly speaking, photons will be the energy deposits in the calorimeter system that have no track pointing at them....
Once you get those "photons" you can check their "signals" in the detector by eg checking how the energy is being deposited within the calorimeter components; in that case you can get an idea of what the photon's momentum direction was... once you know the photon's momentum direction, you can track it back and see from where it comes from.
A pair of photons will roughly be seen to come from the same point. (from the paper I ref. to below, you can see that this case, with their selections, corresponds to roughly 8+28=36% of the photon pairs).

There is also the case that the photons convert (giving charged particles) before reaching the detector. In that case what you can obtain by looking at the charged particles' tracks is the conversion point.

For more you can also check Sec4.3 here :
http://arxiv.org/pdf/1108.5895v2.pdf

Ok, so basically there is a system to pair the photons based on the direction of their momentum and the time they are detected?

ChrisVer
Gold Member
Ok, so basically there is a system to pair the photons based on the direction of their momentum and the time they are detected?

more or less yes.

I don't think that time makes any difference... I may be missing it though.

Luca_Mantani
mfb
Mentor
Ok, so basically there is a system to pair the photons based on the direction of their momentum and the time they are detected?
It helps, but it doesn't guarantee that the photons really come from the same collision. Two high-energetic photons from different collisions are rare, so the remaining background from that effect is small. The collision that produced the photons is important for a different reason: the invariant mass depends on the photon energies and their directions. The calorimeters give some rough estimate of the direction, but the tracking system is much more precise, so you try to find the collision point based on other tracks produced in the same collision.

If you look for two Higgs from the same collision, tracking them doesn't help to find the right combination, as they come from the same spot. You can combine photon 1 with photons 2, 3 and 4, and see which combination best fits to the Higgs mass. Then check if the other two also form a pair that fits to the Higgs mass. If we would not know the Higgs mass yet, you would look for pairings where both pairs lead to approximately the same mass.

Fervent Freyja and Luca_Mantani