COLLIDING BEAMS IN THE FUTURE

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COLLIDING BEAMS IN THE FUTURE

C. Bernardini

To cite this version:

C. Bernardini. COLLIDING BEAMS IN THE FUTURE. Journal de Physique Colloques, 1976, 37 (C2), pp.C2-67-C2-71. �10.1051/jphyscol:1976208�. �jpa-00216481�

JOURNAL DE PHYSIQUE Colloque C2, supplément au n° 2, Tome 37, février 1976, page C2-67

COLLIDING BEAMS IN THE FUTURE

C. BERNARDINI

1st. di Fisica, Univ. di Roma et INFN, Sez. di Roma, Italia

Résumé. — On donne une revue des projets des anneaux de collision dans le monde, avec emphase sur leur exploitation pour les recherches de physique des hautes énergies.

Abstract. — A survey is given of the proposed colliding-beam facilities in the world, with emphasis on their exploitation for high energy physics research.

1. Short historical and introductory remark. — The first high energy e⁺ e~ annihilation experiment took place here in France, 13 years ago.

The ring, AdA, was very small, as usual for pro- totypes : 2 x 200 MeV energy, 60 cm radius. To those people who did not participate to this work, it might be of some interest to read the thesis (pour obtenir le grade de Docteur es Sciences) of Jacques Haissinsky [1], an admirable booklet showing the multitude of new problems we had to face.

To illustrate the world-wide appreciation of the importance of the program initiated by Bruno Tous- chek in Frascati [2], we give here (Table I) the list of existing machines of this type, before discussing what we hope to be the third generation of ee rings.

As for scientific results, they are by now so generally known that we do not attempt to comment on them.

The colliding-beam principle has been applied to other cases than ee : the ee double-ring devices and the pp intersecting rings. These are shown in table II.

Also, studies have been performed on the possi- bility of operating large e-p colliding-beam machines.

These studies have been promoted by the interest in e-p inclusive reactions and the problem of scaling (both for electromagnetic and weak interactions).

Eventually, ideas circulate from time to time on the feasibility of special rings for pp collisions and even fip or nn. We do not insist on these more or less specula- tive problems which however should not be neglected.

To be realistic, we must conclude on the basis of the efforts made up to now that technical machine problems have been adequately solved for the ee and pp case with maximum luminosities of 1032 and 1030 c m ~2s_ 1 respectively. Energy is not a major problem apart from the cost.

Also, we assist today to the already known pheno- menon of the transition from national to international size in the case of the 3rd generation of ee colliding beam facilities. The wish to have a large ring at home favours rich nations in promoting plans. At the same

TABLE I

Rings AdA VEPP2 ACO

ADONE

CEA

SPEAR DORIS VEPP3 DCI

Site Frascati-Orsay Novosibirsk Orsay Frascati

Cambridge (Mass.) Stanford (Cal.) Hamburg Novosibirsk Orsay

E per beam (GeV)

0.22 0.7 0.5 1.5

2.5

3.5 3.5 3.5 1.8

Max L (cm"2s_ 1)

1024

10^{2 8} 5 x 10²⁸

3 x 1029

2 x 1028

5 x 103' 5 x 1029

Beginning of operation

1961 1964 1965

1969

1971

1972 1974 Now ? (construction)

Main scientific results Storage, Touschek effect, ee -> eey (Dismantled 1964)

ee -» p, 03, cp; Q E D ; ee -» eeee ee -> p, co, <p; p-w interf. ; vacuum pol.

at <p ; ny, t\y, can ; QED

Total cross section and multiplicities above 1.2 GeV ( = 2 E); ee -» p', p" ; n form factor ee -» ee + X ; Q E D ; ee -»iji

Total cross section and multiplicities at 4 and 5 G e V ( = 2 E )

(Dismantled 1973)

Total cross section and multiplicities. Inclusive and exclusive cross sections ee -+ iji, <//', ...

e e - • >j/

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976208

Rings particles

-

PR1NC.-PEN.

e-e VEP 1 e-e ISR P-P

DORIS e-e DCI e-e

E per beam Max L

Site (GeV) (cm-'s-')

- - -

Stanford 0.5 1 028

Novosibirsk 0.13 ?

CERN 30 1030

Hamburg 3.5

Orsay 1.8

time, however, the plans are perhaps a bit less ambi- tious than they might be in the frame of an interna- tional collaboration. It would therefore be of some interest to consider what would be adequate for physics independently of national budgets. In this respect, it is appreciable that E.C.F. A ., the European Committee for Future Activities, submitted to the C.E.R.N. Member States the following statement (dated 6 June 1975) :

The European Committee for Future Activities, E.C.F.A., considers that

i) electron-positron storage rings with center of mass above 20 GeV would be an extremely valuable addition t o the European high energy physics faci- lities, complementing existing proton accelerators and national electron-positron facilities at lower energies,

ii) it is of primary importance that such a project is realized with a minimum of delay,

iii) the exploitation of the storage rings should be open to the European scientific community,

iv) there should be no duplication of similar accelerators within Europe.

In view of these considerations E.C.F.A. will, if the Laboratories concerned agree, set up a working group to study and make recommendations about the international exploitation of an electron-positron storage ring facility.

2. d plans above 20 GeV c.m energy. ^- There are 3 well known projects whose names are currently acknowledged by the physics community : P.E.P. [3], in the United States ; E.P.I.C. [4], in the United Kingdom ; P.E.T.R.A. [5], in Germany. To them, we add the more recent (smaller energy) ring proposed by Cornell University [6] and, for completeness, the Italian project Super-Adone [7] (cut out of the approved 5 years plan by government decision).

We have no precise information about USSR enter- prises for larger rings.

Beginning of

operation Main scientific results

- -

1962 MBller scattering 1963 MBller scattering

(very short beam life) 1971 Total cross section

Inclusive cross sections Rapidity correlations Fragmentation-pionization completion

construction

Each of these projects evaluate possible perfor- mances of the proposed machine after reasonable extrapolation of known beam behaviour. One should remark, however, that the most important para- meters, energy and luminosity, are largely constrained a priori by physics considerations. Therefore, the study mainly consists in finding intelligent solutions to these goals and, in less explicit form, in defining what an intelligent solution is. Actually, the questions faced by the groups involved are of a very different nature : which is the most economic project ? which is the project we can realize in the shorter time ? which is the most flexible machine ? etc.

In order to understand the true meaning of the parameters, we need say a few words on their defi- nition. The nominal energy E of the ring is defined by the value at which the luminosity L reaches its maxi- mum for the given R F power. The magnetic field is generally rather low as compared to conventional machines and E can be raised by adding more R F power. L drops down very rapidly at energies larger than E, as shown in the examples figure la, lh, l c ; also, different operation modes are possible (multi- bunch operation at low energy) giving some freedom in optimizing the luminosity at intermediate energies where the R F power is not the major constraint.

A necessary goal for L at nominal energy is believed to be lo3' cm-2 s-' on the basis of general physics considerations as shown by the many examples in the report of professor Renard at this meeting. However, at energies < E, L is smaller than this reference figure and the question of defining the useful energy range for operation is left somewhat open. As a rule, it is conceivable that lo3' < L < lo3' ~ m S - I - ~in the range (E/2, E). Remember in this connection that the cross section for eE + jip (Q.E.D. only !) is [8]

(1 nbarn effective cross section gives 6 eventslmin. at L = lo3' cm-' s-I).

C O L L I D I N G BEAMS IN THE FUTURE C2-69

I . 1. - Luminosity versus encrgy per beam : Ill) Cornell propo-

%II. I h ) FI'I(' pl'olcct. I ( ⁾ I'ETIIA prc?ject.

8 6

4

2

A second relevant question for experimenters is : how much room we dispose of for experiments ? This is usually answered by quoting the free length for experiments corresponding to the part of a long straight section left free in between the low$ optics.

To ask for a longer available space implies to have a -

- - -

-

BEAM ENERGY -GeV

ENERGY PER BEAM (GeV)

h )

lo3: ₅ ^{I I} , ^{I a I}

6 7 8 9 I0

smaller luminosity since the beam size grows up rapidly (luminosity (free length)' = constant). There- fore, space for experiments is generally somewhat unconfortable being restricted to about 10 m in the larger machines. This calls for peculiar design of 4 n detectors, especially for magnetic analysis at angles below

-

After this short summary of problems relevant to the ring exploitation for physics, we give here the proposed parameters of the 5 projects mentioned (Table 111).

3. Studies for very high energy pp intersecting rings

- So far, the C.E.R.N. I.S.R. is the only existing pp colliding beam facility. It is a very sophisticated technical achievement whose performances are with- nessed by the copious scientific results : raising total pp cross section, multiple production and inelastic cross sections, very high transverse momentum pheno- mena, etc.

Here the order of magnitude of the relevant cross sections ( < 50 mbarn) relaxes somewhat the demand for luminosity L : a value in the lo3' cm-2 s-' is already acceptable.

Studies have been initiated at different sites for pp storage rings in the 400 GeV range. We review here as an example some ideas developed at C.E.R.N.

showing the size and the open questions of such colossal devices, following the presentation by K. Johnsen [9].

C . BERNARDINI

Parameter

-

E (GeV) Perimeter (m) R F Power (MW) R F freq. (MHz) Current/bearn (mA) Magn. field (Gs)

# Exp. Sect.

Free length of exp. sect. (m) Total cost (MFr)

CORNELL SUPERADONE

-

10 857 2.6 103

50 5 200 2

PEP

-

15 2 167 7.2 359 loo 2 940 6

EPIC

-

14 2 190 4 403 40 2 715 4

PETRA

-

16 2 304 4 500 95 2 705 4

As for the G case, luminosity and length of free ^&OO sections pose conflicting problems. Low$ insertions

might allow L = cm-2 S-' according to calcu- 300

lations on beam-beam dynamics; but this would -,

.

restrict the field-free space to 10 m, a severe limitation.

However, very high transverse momentum events

--

could be analyzed at 900 with confortable counting ⁰ rates and this is an appealing goal.

On the other hand, different insertions can be devised allowing much more free space at lower

luminosities in the lo3' cm-2 s-' range. This satisfies v (GCV) x 10

the requirements for very high momentum analysis in the forward direction and for measurements in the Coulomb interference region at very small angles.

A race-track scheme with 2 x (3 types) insertions

(2 low-b, 2 general purpose, 2 high-a) has been O _ < L

1-4 GeVlc 00 GeVlc

considered. To these, two more must be added for ⁴⁰⁰⁰

injection and beam-dumping.

Also, two different technical solutions have been

studied, a conventional and a superconducting one. _{( c )}

t

The proposed parameters are summarized in table 1V. Q ~ ( G ~ v ) ~

Conventional

pmomentum (GeV/c) 400

Max Bending field (kGs) 18 Circumference (m) 8 300

Stored current (A) 7

Vacuum ch. aperture(mm) 30

Quad length (m) 3.3

Bending magn. length (m) 7.2

Superconducting

4. e-p colliding beam proposals. - A revolution in the understanding of the structure of elementary particles was initiated around 1968 by the measure- ment of inclusive ep cross sections at S.L.A.C. Some- thing point-like was discovered inside the proton thus repeating the preferite view by physicists : to find constituents to build compounds [lo].

A simple kinematical plot shows the analyzing power of an e-p facility for inclusive reactions. This is indicated by the boundary in the qZ, v plane corres- ponding to a given machine (Figure 2 ; q2 is the

FIG. 2. - The ^q2 ( = - Q2), v plane. (a) Existing data and acces- sible triangle with leptonic beams from 400 GeV p-synchrotrons.

(b) The triangle for e-p in the extension of the Epic proposal.

(c) Event rates, per day, for deep inelastic e-p (electromagnetic only) assuming scaling.

4-momentum transfer squared, v is the energy transfer in suitable units).

The leading physical ideas in pushing for e-p colliding beams are : 1) to have deeper inelastic

COLLIDING BEAMS IN THE FUTURE C2-7 1

electromagnetic scattering, 2) to reach the region where weak and electromagnetic processes have comparable rates.

A less ambitious goal is to study quasi-photopro- duction in (weakly-virtual) y-p collisions; that is, to perform experiments near q2 = 0 at large v values in the q2, v plane. The photons have also a very good linear polarization monitored by the electron energy E and by the electron scattering plane :

The equivalent luminosity for such events is given as a function of L, the luminosity for e-p and k, the photon energy, by

dLequi, = 0.2 L dk -

.

k

This shows that, at L = lo3' ~ m sK1 and with - ~ a,(yp)

-

rate x 200 dk - eventsls k

a very high value ! therefore, luminosity is not a problem for quasi-photoproduction (a second front candidate to retire for unsuccessful machines). On the contrary, the leading goals (deep and weak inelastic processes) call for not less than L = ~ ms-' to - ~ explore the - q2 > 1 000 GeV2 range where counting rates might be as low as 10 eventslday.

A plausible scheme to work with in considering ep machines is to add an e-ring to the 400 GeV p-rings or (perhaps) to existing machines like FNAL or SPS.

Beam behaviour is not experimentally known and also theoretical studies are not yet fully understood.

Therefore, any definite program is quite premature.

Nevertheless, one can say that the range to be possibly considered is 400 GeV protons against 20 GeV eleo trons, corresponding to s x 4 E, E, = 3.2 x lo4 GeV2 or Js = 180 GeV in the c.m.s. (to be compared to 7 GeV at S.L.A.C.).

5. Where is the horizon ? - How will continue the present activity in high energy physics ? This question is not currently considered because of two related reasons : 1) There are many satisfactory plans to be converted into reality. 2) The actual plans will keep physicists occupied for at least 10 years.

Also, following a remark by professor Wideroe in a seminar given at the Accademia dei Lincei, we need

perhaps a new principle for accelerators to find the courage to pass the 1 000 GeV barrier at reasonable cost. The alternating gradient (1953), the particle storage for many hours (1960), the low$ (1965) had this virtue in the past.

There are some indications that the future is still open : the superconductors technique might well be the key for a next step. This is not, in my opinion, a true matter of new principles but rather a question concerning the mise a point of a very difficult techno- logy.

Meantime. one could ask : what would be sensible to achieve on the basis of scientific considerations ? The problem of pi5 rings is certainly of importance to hadronic physics because of the quantum numbers of the initial state. This can be shown in simple words by the consideration that pp could convert the full c.m. energy into a quark-antiquark pair (if any).

ep is at the horizon and has a stimulating low-energy history ; we already mentioned it in paragraph 4.

Concerning eE (the line I must admit to prefer) it is in my opinion still far from a true qualitative step.

In fact, if you want to attack weak interactions by this very clean leptonic device (e.g. by eE ⁺ p) you need go up to

the s value at which weak and e.m. processes give comparable effects.

This means : 2 x 30 GeV eE rings and 10 eventslhour experiments at L = cm-' S - I for entering a new realm.

Even if at first 2 x 30 GeV does not look so much higher than the energies of P.E.P., E.P.I.C., P.E.T.R.A., the required RF power is a formidable task. Also, the size becomes of the same order as for 400 GeV proton machines. Will it be possible to dis- pose of 30 or 40 MW RF power systems in some future ? Many experts are skeptical today, neverthe- less an explorative study is worth doing before aban- doning the field.

Last, but not least, I want to mention a problem which is not usually given due importance : is the physics community well enough trained to manage with very large devices ? I doubt, in the sense that I believe that this is obtained now at the cost of frus- trations by keeping too many people far from the leading ideas.

But this view might well originate in the conserva- torism of a man who lived at the good old AdA times.

Referpnces

[I] HAISSINSKY, J., Thesis, Orsay, Serie A, No 81 (5 fevrier 1965). [6] CORNELL Univ. report (McDaniel, B. D.), May 1975.

[2] BERNARDINI, C., CORAZZA, G. F., GHIGO, G., TOUSCHEK, B., [7] SuperAdone Design Study INFN-LNF, March 1975.

Nuovo Cimenlo 18 (1960) 1293. 181 CABIBBO, N., GATTO, R., Pl~ys. Rev. 124 (1961) 1577.

[3] P.E.P., IXth Int. Conf. on High Energy Accel., SLAC, May [9] JOHNSEN, K. (Reported in C.E.R.N.1E.C.F.A. 7514).

1974. [lo] BLOOM, E. D., Proc. of the 6th Int. Symp. on electron and

[4] E.P.I.C. (Report+ in C.E.R.N.1E.C.F.A. 7514). photon interactions at high energies, Bonn, Aug. 1973, [5] P.E.T.R.A., D.E.S.Y. report Hamburg, Nov. 1974. p. 227.