EUROPEAN FACILITIES AND PLANS
John Ellis CERN CH-1211 Geneva 23, Switzerland
Summary provision for excavating two other RF galleries and filling them with superconducting RF which should give The capabilities and time schedules of present 32 2 and planned accelerators in Europe are reviewed. The rs= 180 GeV at a luminosity of 10 cm- sec-l • history of the Large Hadron Collider (LHC) project is The cost of LEP II is estimated to be about 8 recalled, and the results of machine feasibility 10 $, 1 1/2 times the part of the annual CERN budget studies are summarized. It seems possible to build available for LEP construction. The additional RF for in the LEP tunnel a pp collider with ;; = 10 to LEP II could be installed during shutdowns of LEP I, 33 2 18 TeV and a luminosity of 10 cm- sec-l Results which is not expected to run for more than about 2000 hours per year. It is not unreasonable to foresee from the Lausa"ne Workshop on LHC physics are reviewed, and some aspects of Higgs and supersymmetric LEP II starting to operate above the w+w- threshold in 1992, and then running there for at least two years. particle production are discussed. Four detectors are currently under construction for Present and Planned Accelerators LEP. Among these are ALEPH and DELPHI which feature TPC's and other slow drift devices, and OPAL and L3 which do not have such slow drift devices, and could Spps Collider. In the past this machine has perhaps be used in a modified form at the LHC. operated at a centre-of-mass energy rs = 546 GeV at 29 -2 -1 an instantaneous luminosity up to 2 x 10 cm sec , HERA. This ep collider was finally approved in and has accumulated an integrated luminosity of about April 1984, and civil engineering is expected to start 102 nb-l • Plans for this year include a test for an soon. It is planned to collide 30 GeV e± with 820 GeV protons for a total center-of-mass energy of exploratory run in a pulsed mode, with the beam energy 32 2 oscillating between 120 and 450 GeV on a time-scale 314 GeV, with a luminosity around 10 cm- sec-l • measured in tens of seconds. It is hoped to achieve A simpler, cheaper and higher field magnet design is next year an instantaneous luminosity of order now being prepared, which might enable higher proton energies of 0(1) TeV to be achieved. It is expected 10 25 cm -2 at vsr 900 GeV , which should enable = to have electrons in their ring by early 1988, and the searches for Centauro events 01. any other novel protons in mid 1989, with ep collisions in mid phenomena with a threshold below /S = 1 TeV • Also 1990. Intensive e+e- physics is not a priority for planned for this year is a conventional physics run at the machine, and going to 40 GeV per beam in the 118 = 620 GeV for about 12 weeks between September and e-+ ri .. ngs would require superconducting RF: this makes December 1984, during which, with the aid of some it unlikely that HERA will be suitable for a toponium improvement in the instantaneous luminosity, one factory. As for possible subsequent developments of should be able to increase the present integrated HERA, the internal diameter of the tunnel is large luminosity by a factor of 3 to 5. In the future, it enough to contain another ring. There are no prizes If is planned to upgrade the Spps Collider with a new for guessing what that ring might be! The present accumulator ring called ACOL, which should increase cost of the HERA projected is estimated at around the peak luminosity by 0(10) from 1987 onwards. One 8 may hope to accumulate an integrated luminosity of 2.5 x 10 $ • About a third of this is being covered 4 by contributions of labour, materials and components 0(103) nb-l before then, and perhaps 0(10 ) nb-l by countries outside West Germany, including Italy, afterwards. Plans for detector upgrades include a Canada, France, the Netherlands, Great Britain and microvertex detector for UAl during 1984, and Israel. This is a novel model for international subsequently improved calorimetry. UA2 plans include participation in accelerator construction, which Ring Imaging Cerenkov (RICH) in the forward direction might be interesting for future projects such as the during 1984, and improved solid angle coverage for SSC. No experiments for HERA have yet been chosen their calorimetry in 1987. and it is expected that decisions will be made in iate 1985. LEP. Access pits are being dug, as well as the part ~the tunnel under the Jura mountains. The History of the Large. Hadron Collider (LHC) Idea. first e+e- collisions are expected towards the end There were many informal discussions of a possible of 1988. In its first phase (LEP I) the centre-of hadron collider in the LEP tunnel even before LEP was mass energy rs ~ 103 GeV with an instantaneous officially approved. Indeed, the possible subsequent 31 -2 -1 luminosity ~ 10 cm sec . The cost of LEP I is installation of a hadron ring or rings was taken into 8 consideration when choosing the LEP tunnel radius (as about 4.5 x 10 $ • Probably it will be run for at large as possible, since it might be the last piece of least two years at the Z0 peak, gathering up to 107 0 real estate available for accelerator construction in Z decays. Also, the latest data on the t quark Europe, as well as to get to the highest possible make it seem quite likely that toponium is too heavy e+e- energies) and diameter (large enough to leave to be produced at previous lower energy e+e room for hadron magnets). Discussion of an LHC was machines, and that LEP may provide the first detailed greatly stimulated by the recent success of the SppS exploration of the toponium system, something which Collider. Serious work was, however, triggered by the might take a year or more. Beyond LEP I there are initiation of the SSC project in the United States. options for increasing the available rs . called There were prepatory meetings at CERN in December 1983 generically LEP II. One possibility is to fill up the and February 1984, in which about 40 leading European first two planned RF galleries with superconducting physicists participated. Also during the winter RF. This would provide rs up to 140 GeV at 1983/1984 there was an LHC machine study group at CERN 2 luminosities up to 1032 cm- sec-l There is also which produced the feasibility studyl reviewed in the
-799- next section. Then, in March 1984, about 150 physicists met and worked on LHC physics at Lausanne for four days, with a subsequent two day open presen tation at CERN.2 All this activity was summarized in presentations3 at the ICFA meeting at KEK near Tokyo in May 1984.
LHC Machine Studies
Possible Options. Although I am a total ignoramus about accelerators, I have been asked to review here the "feasibility study of possible options" for a Large Hadron Collider in the LEP tunnel 1033 made by the CERN Machine Group, so I will do the best that I can. The three main options considered have •.,, been: N I E (A) pp in a single beam channel, with either u present day or futuristic high-field magnets. Such a device would yield relatively low luminosity and ...J 1032 need a relatively large aperture so as to allow separation of the beams (Fig. la).
(a) (b) (c)
1030 1.-----1.--1--l...... L.J...LL.L.l....----''--.1-J.-'-'..u.J"'-~.__..._..._,__...... , I 10 100 1000 Figure 1 Possible Options for LHC Beam Channels
Figure 2 Luminosity and Bunch Separation
(B) Two beam channels with a common magnetic circuit (Fig. lb). The space available in the LEP a Large Hadron Collider suggest that it may be diffi tunnel can only accommodate one cryostat to contain cult to imagine working with n larger than 1, while both beam channels. The most interesting possibility one can do experiments with TX ~ 25 ns. If these are is to put them side by side, which permits a high strict limits, then the luminosity of a collider in luminosity pp machine with many bunches. If one 32 2 1 uses present-day lower-field magnets one can have the the LEP tunnel is restricted to L ~ 4 x 10 cm- s- • magnetic fields in the two channels either antiparal However, if higher values of n are tolerable, one can 33 2 lel (allowing pp in separated channels or pp in increase L to 1.5 x 10 cm- s-l (corresponding to the same channel) or parallel (allowing pp in n ~ 4 ) before hitting the beam-beam tune-shift limit separated channels). If one goes to high-field mag of 0.0025 reached at the Spps collider. Such a nets only the antiparallel possibility (permitting luminosity is feasible with the pp option, but would pp ) is available. require between 3000 and 4000 bunches. Matching the RF in the SPS and the LHC is only possible for a few (C) Two beam channels with independent magnetic choi ces of bunch number, and the only choice in this circuits (Fig. le). In this case one is restricted 13 range is 3564. With N = 5 x 10 protons the to moderate magnetic fields, but either pp or pp p could be realized. Among these various options, (A) stored energy would be 70 MJ , a reasonable number, has received some attention but most interest has been while there should be no problems with beam-beam focused on (B) which is the most demanding technologi effects as long as n ~ 1 ( 6Q ~ 0.0013). The lumino cally as well as making the most physics available.l sities in pp machines are restricted by the number The attention paid to the different options in this of antiprotons available. Possible choices of TX brief review will reflect these priorities. 12 13 and n for N_ = 10 and 10 are shown in Fig. 2: p Luminosity and Bunch Separation. Most high 31 -2 -1 v s to energy hadron-hadron colliders use bunched beams: let 1 um i nos it ies' in ' th e range L = 1 . 5 ~ 10 cm 32 2 us call the separation between successive beam 1.5 x io cm- s-l would be possible. crossings TX , and the average number of collisions 1 per beam crossing n • It is ~lear that if n is pp Option. General parameters of the LHC PP kept fixed, an increase in the luminosity L can only option are set out in the Table. There would be eight be obtained at the price of decreasing TX , as seen 2 in Fig. 2. Feasibility studies of experimentation at
-800- Table: General Parameters and Performance within this cross-section while having two side by side beam channels is shown in Fig. 3. General Parameters
80 K Shield Collider Type in LEP Proton-Proton
Separation Between Orbits (mm) 165-180 Number of Bunches 3564 Bunch Spacing (ns) 25 Number of Crossing Points 8 Beta Value at Crossing Point (m) 2 Normalized Emittance 411yu /B (µm) 511 Full Bunch Length (m) 0.31 Full Crossing Angle (µrad) 96
Lattice Period Length (m) 79 158 Lattice Phase Advance w/3 w/2 Dipole Magnetic Field (T) 10 10 Operating Beam Energy (TeV) 8.14 8.99
Performance
Figure 4 Perfonnances of Candidate Superconductors 2) To achieve this, one needs a vigorous programme for the development of materials and techniques needed to make such magnets. schedule for such an R and D programme to be under taken, and several European laboratories are interest 3) One would easily reach = 10 to 13 TeV ed in participating in it. rs with six to seven T magnets after a shorter development programme. Other Subject s Stud:!.ed. The purpose of this section is j ust t o i nform you that a serious study 4) All other components and systems are feasible has been made 0£ most machine systems: please see with present technology. the LUC machi ne s tudy group report 1 for details. Practicable designs for quadrupoles and sextupoles What would be the cost of the LHC? The cost of have been made. The scaling of the magnets for the magnets, scaled up from HERA, would be about different apertures between 35 and 50 mm has been 400 M $ , namely less than the total LEP cost and studi ed. A 4.5° K cryqgenlc system has been about 1/6 of the recently projected cost of the SSC. designed-one i nteresting feat ure is t he 1.5° slope A simple-minded theorist's way of understanding this of the tunnel whi ch crea tes as pressut:e difference of ratio is as follows. A factor of 1/3 comes from the 1 . 4 bar across the ring for liquid helium- annoying smaller size of the LEP ring, and is reflected in the but not i mpossible to l ive wi th. The vacuwn s ystem correspondingly lower energy of the LHC. Since has been studied. 1l'or the system it is proposed RF calculations 4,5 suggest that the physics reach of t o use a frequency of 400 .8 MHz , j ust t wice the such hadron-hadron colliders grows roughly as S).'S R!1 fl;equency, which gives flexibility i n t i e 1 4 choice of bunch number. The SPS will be used to s / for fixed L , it becomes relevant to ask how 11 much an extra factor of 1.5 to 2 in physics reach is inject bunches of 10 particles at 450 GeV • Since worth. Another factor of 1/2 in cost comes from the 13 the SPS already provides up to 3 x 10 protons per existence of a tunnel and laboratory infrastructure pulse and has a repetition rate of 10s , it will at CERN. FNAL can provide the latter in the U.S., 13 but not the fonner, so that siting the SSC at or near quickly provide the 5 x 10 protons needed for LHC FNAL would not save such a large factor in cost. in the pp option. Since the ratio of LHC to SPS circumferences is 27/7 , the entire LHC ring can be A final comment concerns a possible ep collider. filled with four SPS pulses. Two possibilities for This would be available for free in the LEP tunnel: the transfer lines have been investigated. One b,Y colliding LEP and the LHC one could get involves two 2 km transfer lines with large bending Is ~ 2 TeV for ep collisions. This possibilit6 radii for which conventional magnets could be used, has been discussed enthusiastically by theorists, while the other involves two 1 km transfer lines but no detailed machine study has yet been undertaken. with smaller radii which would need superconducting magnets. The shorter transfer lines would need a Lausanne Workshop 12% slope to get down to the LEP/LHC tunnel, which is lower than the SPS tunnel, but the problems this Questions. As mentioned earlier, this meeting raises for the transfer line cryogenic system seem involved 150 physicists meeting at Lausanne for four to be soluble. Among other topics studied which seem days, followed by a two day public presentation at to raise no particular problems are the beam dumps and CERN. Needless to say, most of the work was done radiation protection. beforehand by the different working groups. The big questions to be addressed by the physicists at pp Option. In this case the attainable lumi Lausanne were: nosity is limited by the p accumulation rate A • The rate required is ~ N.._ divided by the luminosity p a) Do you want rs 18 TeV or would you be satisfied decay time TL For T = 20 hours and with 10 TeV ? ' 12 L 10 N__ = 10 one needs A ~ 5 x 10 per hour. This is -2 -1 p b) Can you work with L "' 1033 cm s or are you about an order of magnitude larger than that attained -2 -1 ' limited to L ~ 1032 cm s -802- c) Wou l d you prefer n "' 1 and "' 25 ns , or would c) While much physics could be done with n > 1 , n "' 1 is preferred for tracking in particular, while you pref er n "' 6 and TX "' 150 ns nobody objected strongly to TX "' 25 ns . d) How interesting do you find pp at d) No-one saw an overwhelming physics case for pp 32 2 L = 10 cm- s-l ? 32 2 collisions at L 'V 10 cm- s-l . The experimental physicists seeking answers to these Summary of Theoretical Discussions. For reasons questions were divided into 8 working groups, whose subjects and conveners are listed in ref. 6. To aid of time, space and to avoid duplication, I will only summarize here some studies which are of particular (?) them in their deliberations there were the seven interest to the focus of this Workshop, namely electro plenary talks by theorists listed in ref. 7, and a weak symmetry breaking, Higgses etc. This area was continuous th~oretical discussion session held in studied before, during and after the Lausanne meeting parallel to the experimental working groups. by Graciela Gelmini, Heni ek Kowalski and myself. 5 Our most detailed investigations were of Weinberg-Salam Summary of Experimental Conclusions elemenEary Higgs production, signatures and back grounds, and of supersymmetric particles, mainly Jets. A TeV jet is expected to have a gluinos and squarks. multiplicity or order 100, with 1/2 of its energy as carried by particles with energy fractions We founds that the detection of H produced by Z 0.04 , and 1/2 of the jet energy dispersed outside < gg or WW collisions was difficult for a 5° cone about the jet axis. These expectations 2~ ~ ~ 0(400) GeV because of the large pair must be kept in mind when considering jet energy < WW resolution and separation in angle. In order to production background. One reaction which seems to determine transverse momentum balance and hence be give a more favourable signal-to-background ratio, able to look for missing transverse energy one needs though a smaller total cross-section, is to be able to measure muon momenta. Going from n = qq + w* + H + W p~oduction, where there is an event per crossing to 0(5) has little effect on additional W in the final state to tag the event. such physics desiderata as the mass resolution in Another possibly interesting reaction might be dijet combinations. It was not obvious that a gg or qq + t t + H , although the background from magnetic field was needed for jet ph~·sics. The l:t + WW has not yet been evaluated and it is not working group felt that it could operate with bunch clear how efficiently the trigger l:t pair could be separations as low as TX = 10 ns . tagged, We also thought about the detection of Higgses with 100 GeV < ~ < 2 ~ , in which range Electrons and Photons. It was thought that their dominant decay mode would be tt . The back calorimetry was t he onlv feasible technique, and that grounds in gg + WW + H and gg or qq + tt + H a "traditional" resolution ll E/E"' 0.15/El/2 would seem to be overwhelming, but the bac~ground from qq + tt + W to the reaction qq + w" ->- H + W seems to be adequate. No obstacle was seen to working with be no larger than the signal, and might enable it to n > 1 and T = 25 ns. Most of the above remarks be detected. apply to the aetection of isolated e and y . The only conceivable technique for detecting electrons inside a hadronic jet was thought to be transition As concerns supersyrnme try, in addition to computing radiation detec tion, but this was not purs ued in the total cross-sections for gg and i{¥ production, detail. This group also did not feel a strong need we also investigated their missing Pr decay for a magnetic field. signatures using a Monte Carlo programme based on 'V -'V 'V 'V 'V Muons. This group felt able to identify muons g -+ qqy and q -+ qg or qY° decay. We found that g with ~larger than a few GeV , and to measure the or q with masses up to 0(2) TeV could be detected sign up to momenta of 2 TeV A preference was with missing pT of several hundred GeV and several expressed for n = 1 , while TX = 25 ns was found 'V well-separa.ted hadronic jets (four from """gg . g + qq\' • acceptable. ,..;;:; 'V 'V six from qq • q + qg . ~ + qq'1 . two from -803- References 1. CERN Machine Group - Large Hadron Collider in the LEP Tunnel, a feasibility study of possible options - CERN report DIR-TECH/84-01 (1984). 2. For a review, see G. Brianti et al. - Summary Report of Workshop on the feasibility of hadron col liders in the LEP tunnel, Lausanne and CERN, March 1984, CERN report (1984). 3. G. Brianti, J. Ellis and H. Schopper - presenta tions at the ICFA meeting, KEK, May 1984, to appear in the Proceedings (1984). 4. E. Eichten, I. Hinchliffe, K. Lane and C. Quigg - Fermilab preprint Pub-84/17-T (1984). 5. J. Ellis, G. Gelmini and H. Kowalski - DESY pre print (1984). 6. Experimental physics working groups and conveners for the Lausanne meeting were: Jets - P. Jenni, ey - P. Block, Muons - W. Bartel, Tracking Detectors - A. Wagner, Triggering - J. Garvey, Forward Physics - G. Mathiae. 7. Theorists who talked at the Lausanne meeting included: A. Ali - Hard Collisions, G. Altarelli - ep, B. Andersson- Soft Collisions, A. De Rujula - Neutrino Physics, J. Ellis - New Particles, H. Fritzsch, R. Peccei and R. Petronzio - Composite Models. -804-