# CERN Alice Particle spectra behavior

1. Mar 19, 2015

### moso

So i have a question regarding the nature of the particle spectre at alice as a function of momentum. The spectre in question can be seen here. http://cerncourier.com/cws/article/cern/48325. My question is, why is it that the particle in the low momentum range are rising and the reaching a maximum and then beginning to decrease again. From what I have understood it has something to do with the Fermi theory of beta decay or? It would be awesome it anyone knew and you give me a hint on it. thanks in advance.

Morten

2. Mar 19, 2015

### Staff: Mentor

The connection to the energy spectrum of beta decays is very indirect, and weak processes (like for beta decays) are negligible for those spectra.

For very small momenta, the phase space is small. In the same way only a very small fraction of gas molecules in the atmosphere is very slow (see Maxwellâ€“Boltzmann statistics), only a small fraction of the particles has a very low (transverse) momentum.
For high energies, the probability that a single particle gets so much energy drops again.

3. Mar 19, 2015

### moso

So to summarizes, at low momentum (transverse) the possibilities for particles being in different states, eq: phase space is low. Could you elaborate, why the low momentum particles have small phase space? and that for high energies gives it self, because there is limited energy available in a collision.

edit:
is it because the combination of the low moment particles has less options than the medium momentum particles? Because the collision is with pb-pb have the same phase space, they can decay to the same and the same momentum each time,

Last edited: Mar 19, 2015
4. Mar 19, 2015

### Staff: Mentor

Draw a coordinate system on paper, randomly fill it with points (particles). The amount of particles you'll find between 0 and 1 cm from the center will be smaller than the amount of particles between 5 and 6 cm away simply because the first set corresponds to a much smaller area.

If you look at each momentum component separately (like momentum in vertical and horizontal direction), you don't see this effect.

I don't understand that part.

5. Mar 20, 2015

### moso

So has this assumption something to do with the detector and that low momentum particles are close to "ground zero" and high momentum particles travels faster and therefor are father away. Or do you coordinate system just states an idea, that the y and x-axis is momentum and the phase space is the area in-between. Do it have anything to do with Gamow Peak?

Last edited: Mar 20, 2015
6. Mar 20, 2015

### Staff: Mentor

No. Low-energetic particles are harder to detect but that has been taken into account to calculate those spectra.

The coordinate system I suggested is momentum, right.

See the Maxwell-Boltzmann distribution. If you understand this, the particle spectra are a natural consequence.
No.