Proton Driver Meeting

10 - May - 2001
FNAL Black Hole

Presentations:-

• Steve Werkema: pbar issues
• Stephen Pordes: Charmonium Studies
• Dan Kaplan: Prospects for Low-Energy Physics with Antiprotons

Steve summarized the pbar experience from the Fixed Target run of 2000. The experimental focus was E835 which collided a pbar beam onto a hydrogen gas jet. (The physics goals of this experiment were the detailed study of charmonium states.)

• integrated luminosity: 113 pb-1
• maximum instantaneous luminosity: 4.0x10**31 cm-2s-1
• pbar stack rate: 2-3 x 10**10 pbar/hr equiv 2-3 mA
• largest single stack: 2x10**11 pbar
• Ecm ranged over 2240 - 4270 MeV
• The transition was arranged to be above the psi' state.
• largest store that went through transition: 25mA

Deceleration was managed by a program called PAUX. This program has been lost when the PC was upgraded. To reproduce this program from scratch would be a major task and would require quite a lot of time.

Typically 5 - 25% of the beam was lost during deceleration, before the target could be turned on. The underlying causes are not understood. The losses were not reproducible, and tended to be very spontaneous, ie, difficult to link to any other properties of the beam or the machine. Each store was very different in this regard. For example, the time for deceleration varied from 1/2 to 5 hrs. Sometimes it became necessary to recool the antiprotons before completing the deceleration.

The time for stacking was much shorter than the store time. The time to begin stacking after a store was ended tended to be very short. They tried re-accelerating the antiprotons, without success. It is felt that an energy feedback mechanism is needed. The biggest stack decelerated was 80x10**10 antiprotons.

They implemented an automatic control of the beam energy, which was very successful. The energy spread was limited to 50keV. They also measured directly the beam energy from the length of the orbit and its period. For example, the absolute beam energy is was known to 224keV on the psi'. The distribution of beam energies was also obtained by this method.

Can the pbar source be operated like this again? Probably, yes. What will have changed by the time the booster is upgraded?

• the stacktail cooling will be upgraded.
• the stack rate will be increased allegedly from 20 to 100mA.
• the max stack size will decrease from 200 to 20x10**10.
• The accumulation of antiprotons will be accomplished by the Recycler, rather than the Accumulator, which otherwise serves a very limited function. In other words, the functions of storing and cooling will be separated in different machines.
• All of this depends crucially on the Recycler.
• One could contemplate installing two sets of pickups for the stochastic cooling, to service two antiproton customers: the TeV/BTeV program, and low-E antiproton experiments, which have different demands as far as stack size and rate are concerned.

Question: Can TeV/BTeV be run concurrently with future FT experiments? That is, what can be done by carrying out the stacking operations in an interleaved fashion?

Suggestion: Use the Recycler as a `pool' of antiproton to feed the TeV/BTeV experiments, freeing up the antiproton source for the FT work?

For the next generation of charmonium experiments, there are a few obviously worthwhile: mass, total width, and two-photon partial width for the eta_c', 1P1, eta_c. Stephen points out that charmonium provides and excellent challenge to QCD theory. "Charmonium is to QCD what the Hydrogen atom was to QED" (paraphrased).

A new experiment would need larger acceptance for better background rejection. J.Rosen is suggesting adding a magnetic to investigate the charged modes.

A very worthwhile program of measurements can be carried out for a modest luminosity: 200 pb-1, equiv 2000 mA, equiv 2000 hours. Stacking requirements are low, what matters is long-term delivery. Main point: one could not use the pbar source for TeV/BTeV during this time.

clarification from S. Pordes:

The main issue for Charmonium is availability of the pbar source. I suggested that one would want continuous periods of 24 hrs (say) for Charmonium running during which time the Accumulator would be unavailable for Tevtron business. (I took 24hrs as a reasonable length of time for one set of data taking). After the 24 hrs of Charmonium data, the pbar source would be available to prepare more antiprotons for the Tevatron. Such a cycle might be acceptable if one could store enough antiprotons for 24hrs of Tevatron running in the recycler, for example.

Questions: A timeline or timeplan is needed. Can this be achieved with slip stacking? Do additional protons from an upgraded proton driver help in any way?

This is a summary of selected results from the pbar2000 workshop, for which proceedings will soon be available. The central questions of the workshop were: What will the pbar capabilities of Fermilab be, and how can this community take advantage?

Dan suggests bringing in segments of the nuclear physics and atomic physics communities to help swell the ranks of users for pbars. Fermilab has the best pbar source in the world, but thus far it is reserved for use by the TeVatron experiments.

Some possible physics topics:

• ppbar --> charmonium (see Pordes' talk above)
• hyperon production
• light quark resonances: Where are the gluonic states?
• physics with trapped antiprotons
• Gerry Jackson has already made a nice presentation of creating and storing antiprotons for interesting commercial uses.

CERN turned off LEAR before the physics was done, and those physicists are still interested in continuing their work. Perhaps they would like to come here. GSI is getting into the act, proposing a 10 GeV storage ring. This would be a very expensive project and would require on the order of 10 years for completion.

One would need luminosities of 10**33 cm-2sec-1. It would be good to build a dedicated 2 GeV ring. Another ring would be more flexible, reaching energies up to 10 GeV.

Physics: We need to confront QCD (which includes both perturbative and non-perturbative elements) with good charmonium data. Measurements of hyperon beta decay give values for Vus.

There is a search for hyperon CP violation: lambda -> p pi. Look at the angular correlations between p and pbar. PS185 reached a statistical error of 0.015 on the angular decay asymmetry. The SM prediction is 10**-5. HyperCP, and ongoing experiment, will reach the 10**-4 level.

Proposal for a more sensitive experiment: 10**33 cm-2sec-1, stack at 30x10**10 pbar/hr, pbar current of 500 mA. Take data for one year and reach 2x10**-5 on the decay asymmetry. Again, one envisions a dedicated 2 GeV ring.

Question: Is this realistic? It would require very good cooling. Does this need the proton driver? Perhaps yes, in order to achieve high stacking rates.

Question: Do you really need pbar collisions with a gas jet, can't you do this with fixed target? Answer: getting rid of the Lorentz boost would be very helpful for the analysis and event reconstruction.

comments from D. Kaplan:

In partial answer to Stephen's excellent question yesterday, I believe the background rate is a few charged particles per interaction, and 10^33 implies 100 MHz interaction rate, or a few 10^8 Hz of charged particles. This yields about 10^5 Hz of Lambda-bar Lambda events, of which (I think) of order 1% are triggered and reconstructed and pass offline cuts. In comparison, HyperCP had a 20 MHz secondary beam rate for about 100 Hz of Lambda-bars, so the yield per interaction is higher by about an order of magnitude. Also, the background rate is spread over a larger solid angle since the Lorentz boost is so much lower. More work is needed to refine these estimates.

Keep in mind that I am proposing a fixed-target (gas-jet) pbar p experiment - the small boost and detectable primary-secondary-vertex separation are believed to be essential for triggering and background rejection.

Question: What energy? sqrt{s} = 2*M_lambda + a little bit.