# LEAR(low energy anti-proton ring)

1. Nov 2, 2008

### mrausum

1. The problem statement, all variables and given/known data

We've been set an essay question in our first year modern physics course to explain (very basically) how the LEAR at CERN measures the effects of gravity on anti-protons. (I've only just started my first year course so please try and keep things as simple as possible!)

2. Relevant equations

The following hints were given in order to answer the question:

-Gravitational potential difference - p=mgh
-Thermal Kinetic energy of particle at temperature t ~ kt
-Mass of proton (and antiproton) = 1.673 * 10 ^ -27 kg
- if a particle and its antiparticle collide, they annihilate one another with the production of two gamma rays.

3. The attempt at a solution

Another thread on here gave quite a good starting point:

https://www.physicsforums.com/archive/index.php/t-87881.html

Especially the following:

"EFFECT OF GRAVITY on antiprotons will be measured at the Low Energy
Antiproton Ring (LEAR) of the European laboratory for particle physics
(CERN). Antiprotons are extracted from LEAR at an energy of two million
electron volts, decelerated to between 10,000 and 20,000 electron volts and
captured in the catching trap and storage trap, where they are cooled to 10
degrees Kelvin (-263 degrees Celsius). They are then launched, 100 at a
time, up a one-meter-high drift tube. The antiprotons most useful to the
experiment will have a starting velocity averaging four meters per second.
As they drift upward the tug of gravity will slow them down. Hence the more
energetic particles will reach the detector first and the less energetic
ones will reach it later. There eventually will be a cutoff time after
which no more particles will reach the detector because the slowest
particles will not have enough speed to reach the region of the accelerating
grid before their upward motion is overcome by gravity. The experiment will
separately measure and compare the cutoff time both for antiprotons and for
negative hydrogen ions (black curve), which have the same charge and almost
the same mass as antiprotons. If antimatter were subject to a larger
gravitational force downward than ordinary matter, the antiprotons would
have a shorter cutoff time (colored curve) than the hydrogen."

However, I'm struggling to see how some of the hints given in the question tie into the above (vague) explanation.

I'm guessing the cooling down of the positrons to 10 kelvins has something to do with the initial velocity up the drift tube?

The gravitational potential has something to do with the potential energy of the positrons at the trop of the drift tube? I'm assuming this is why the mass of a positron was also given?

I don't know what place the production of gamma rays has in the explanation?

A more detailed explanation of how the drift chamber works would also help.