# LHC: why proton-proton?

1. Dec 29, 2009

### wpoely

Hi,

I was wondering: why is the LHC a proton-proton collider and not a proton-antiproton collider? Has it a theoretical reasons or is it just for practical reasons?

Thanks,

Ward

2. Dec 29, 2009

### hamster143

At those energies, there's not much difference science-wise, and it's much easier to maintain high luminosity with proton-protons than with proton-antiprotons.

3. Dec 29, 2009

### Bob S

Recall that the CERN SPS (super proton synchrotron) was a relatively low energy p-bar p machine. The CERN anti-proton facility could not produce the high p-bar currents that Fermilab can produce. But in the end, there is not much physics difference between p-p and p-bar p physics, although the cost of building LHC was higher because the magnets all have two beam tubes..
Bob S
[added] It is also useful to note that the planned SSC (Superconducting Super Collider) in Texas was a p-p machine. One reason that the Fermilab Tevatron is a p-bar p machine is because when it was turned on in 1983, it was a rapid (for a superconducting machine) cycling fixed-target machine; 900-GeV pulses 20 seconds long every minute. Converting this to a p - p collider would be very expensive, relative to making it a p-bar p collider.

Last edited: Dec 29, 2009
4. Dec 29, 2009

Staff Emeritus
Hamster has it right. Most processes of interest at the LHC are gluon-gluon initiated, so a proton is as useful as an antiproton. A proton-antiproton collider would have slightly cheaper magnets, but would be starved for antiprotons and would have perhaps 1% of the luminosity of the LHC, so you would end up with a far less capable machine.

I don't believe the Tevatron ever ran at 900 GeV in fixed-target operation. If it did, it certainly didn't run like that way for long. Virtually all the Tevatron fixed-target data was at 800 GeV. Cycling the magnets 1000 times a day to 900 GeV would be extremely unreliable.

5. Dec 30, 2009

### wpoely

So, the low luminosity of proton-antiproton is just because it isn't so easy to produce antiproton's?

Ward

6. Jan 1, 2010

### vitruvianman

It is very hard to find or produce antimatter and much harder to control it especially when it's very near to (nolmal)matter. As we all know it causes nuclear reactions depending on both matter's and antimatter's (in the area) relativistic masses. Then is it impossible? Of course not, but very hard and would cost too much. :)

7. Jan 1, 2010

### blechman

Antiprotons are produced at fixed-target like setup:

$$p+p\rightarrow p+p+p+\overline{p}$$

As you can see, it takes a lot to make one antiproton, and then you have to harvest and "cool" them so they can be turned into a beam. This is difficult, and it is VERY expensive! And, as had already been mentioned, you don't gain anything for it at the LHC energies, since a proton and antiproton have just as much gluons.

8. Jan 1, 2010

### Bob S

As shown above, the total CM energy required to produce an anti-proton is 4M0c2
The transformation to the Lab frame is given in Eq. 38.3:
http://pdg.lbl.gov/2009/reviews/rpp2009-rev-kinematics.pdf
So the threshold laboratory total energy E1 and kinetic energy T1 needed to produce an anti-proton is 7M0c2=6.6 Gev and 6M0c2=5.6 GeV respectively. The Bevatron at Berkeley was built in ~1952 with a kinetic energy of 6.2 GeV specifically to discover anti-protons, which it did, for which Emilio Segre and Owen Chamberlain won the Nobel Prize.
Bob S