DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi• bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer• ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom• mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. MUON COLLIDER DESIGN R. Palmer1'2, A. Sessler3, A. Skrinsky4, A. ToUestrup5, A. Baltz1, S. Caspi3, P. Chen2, W-H. Cheng3, Y. Cho6, D. Cline7, E. Courant1, R. Fernow1, J. Gallardo1, A. Garren3'7, H. Gordon1, M. Green3, R. Gupta1, A. Hershcovitch1, C. Johnstone5, S. Kahn1, H. Kirk1, T. Kycia1, Y. Lee1, D. Lissauer1, A. Luccio1, A. Mclnturfl3, F. Mills5, N. Mokhov5, G. Morgan1, D. Neuffer5'8, K-Y. Ng5, R. Noble5, J. Norem6, B. Norum9, K. Oide 10, Z. Parsa1, V. Polychronakos1, M. Popovic5, P. Rehak1, T. Roser1, R. Rossmanith11, R. Scanlan3, L. Schachinger3, G. Silvestrov4,1. Stumer1, D. Summers12, M. Syphers1, H. Takahashi1, Y. Torun1'13, D. Trbojevic1, W. Turner3, A. Van Ginneken5, T. Vsevolozhskaya4, R. Weggel14, E. Willen1, W. Willis1-15, D. Winn16, J. Wurtele17, Y. Zhao1 1. INTRODUCTION FERMILAB machines would also be possible (see second Ref.[4]). 1.1. Technical Considerations Hadron collider energies are limited by their The possibility of muon colliders was intro• size, and technical constraints on bending mag• duced by Skrinsky et al.[l], Neuffer[2], and others. netic fields. At very high energies it will More recently, several workshops and collabora• also become impractical to obtain the re• tion meetings have greatly increased the level of quired luminosities, which must rise as the en• discussion[3],[4]. In this paper we present scenar• ergy squared. e+e~colliders, because they un• ios for 4 TeV and 0.5 TeV colliders based on an dergo simple, single-particle interactions, can optimally designed proton source, and for a lower reach higher energy Anal states than an equiv• luminosity 0.5 TeV demonstration based on an alent hadron machine. However, extension of upgraded version of the AGS. It is assumed that e+e~ colliders to multi-TeV energies is severely a demonstration version based on upgrades of the performance-constrained by beamstrahlung, and cost-constrained because two full energy linacs 1Brookhaven National Laboratory, Upton, NY 11973- are required[6] to avoid the excessive synchrotron 5000, USA radiation that would occur in rings. Muons 2Stanford Linear Accelerator Center, Stanford, CA 94309, (2j£- = 207) have the same advantage in en• USA ergy reach as electrons, but have negligible beam• 'Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA strahlung, and can be accelerated and stored 4BINP, RU-630090 Novosibirsk, Russia in rings, making the possibility of high energy 5 Fermi National Accelerator Laboratory, Batavia, IL ft+H~ colliders attractive. There are however sev• 60510, USA eral major technical problems with /*'s: 'Argonne National Laboratory, Argonne, IL 60439-4815, USA 7 Center for Advanced Accelerators, UCLA, Los Angeles, 6 CA 90024-1547.USA • they decay with a lifetime of 2.2 x 10" *CEBAF, Newport News, VA 23606, USA s. This problem is partially overcome by 'University of Virginia, Charlottesville, VA 22901, USA rapidly increasing the energy of the muons, 10KEK, Tsukuba-shi, Ibaraki-Ken 305, Japan and thus benefitting from their relativistic UDESY, Hamburg, Germany ,2University of Mississippi, Oxford, MS 38677, USA 7 factor. At 2 TeV, for example, their life• 1SSUNY, Stony Brook, NY 11974, USA time is 0.044 s: sufficient for approximately 14 Francis Bitter National Magnet Laboratory, MIT, Cam• 1000 storage-ring collisions; bridge, MA 02139, USA "Columbia University, New York, NY 10027, USA 16Fairneld University, Fairfield, CT 06430-5195, USA • another consequence of the muon decays is ,TUC Berkeley, Berkeley, CA 94720-7300, USA that the decay products heat tlf|if£g: DISTRIBUTION OF iniS DuAMtNT IS UNLIMITED * S 2 of the collidet ring and cieate backgrounds • It will be relatively hard to obtain both in the detector; high polarization and good luminosity in a ii+fi~ collider, whereas good polariza• • Since the muons axe created through pion tion of one beam can be obtained in an decay into a diffuse phase space, some e+e~ collider without any loss in luminosity. form of cooling is essential. Conventional One notes however that in the muon case, stochastic or synchrotron cooling is too slow moderate polarization could be obtained for to be effective before they decay. Ioniza• both beams. tion cooling, can be used, but the final emittance of the muon beams will remain • because of the decays of the muons, there larger than that possible for electrons in an will be a considerable background of pho• e+e~ collider. tons, muons and neutrons in the detec• tor. This background may be acceptable Despite these problems it appears possible that for some experiments, but it cannot be as + high energy muon colliders might have lumi• clean as in an e e~ collider. nosities comparable to or, at energies of several + 1.3. Discussion TeV, even higher than those in e e~colliders[5]. + We conclude that a muon collider has both And because the fi n~ machines would be much technical advantages and disadvantages when smaller[7], and require much lower precision (the compared with an e+e~ machine. Similarly it has final spots are about three orders of magnitude specific physics advantages and disadvantages. It larger), they may be significantly less expen• thus seems reasonable to consider fi+fi~ colliders sive. It must be remembered, however, that a + + as complementary to e e~ colliders, just as fi p~ collider remains a new and untried concept, e+e~ colliders are complementary to hadron ma• and its study has just begun; it cannot yet be chines. compared with the more mature designs for an + e e~ collider. 1.4. Overview of Components The basic components of the n+p~ collider are 1.2. Physics Considerations shown schematically in Fig.l. Tb.l shows pa• There are at least two physics advantages + rameters for the candidate designs. The normal• of a (t fi~ collider, when compared with an N + ized emittance e is defined as the rms transverse e e~ collider: phase space divided by ir. Notice that more pre• • Because of the lack of beamstrahlung, a cisely a factor of jr must appear in the dimensions ft+ti~ collider can be operated with an en• of emittance (i.e. Tmmmrad). A high intensity ergy spread of as little as 0.01 %. It is thus proton source is bunch compressed and focussed possible to use the fi+fi~ collider for preci• on a heavy metal target. The pions generated sion measurements of masses and widths, are captured by a high field solenoid and trans• that would be hard, if not impossible, with ferred to a solenoidal decay channel within a low an e+e~ collider. frequency linac. The linac serves to reduce, by phase rotation, the momentum spread of the pi• • The direct coupling of a lepton-lepton sys• ons, and of the muons into which they decay. Sub• tem to a Higgs boson has a cross section sequently, the muons are cooled by a sequence of that is proportional to the square of the ionization cooling stages. Each stage consists of mass of the lepton. As a result, the cross energy loss, acceleration, and emittance exchange section for direct Eiggs production from the by energy absorbing wedges in the presence of dis• fi+fi~ system is 40,000 times that from an persion. Once they are cooled the muons must be e+e~ system. rapidly accelerated to avoid decay. This can be done in recirculating accelerators (a la CEBAF) However, there are liabilities: or in fast pulsed synchrotrons. Collisions occur Table 1 Parameters of Collider Rings 4TeV .5TeV Demo. Beam energy TeV 2 .25 .25 Beam f 19,000 2,400 2,400 Repetition rate Hz 15 15 2.5 Muons per bunch 1012 2 4 4 Bunches of each sign 2 1 1 Normalized rms emittance eN nnm mrad 50 90 90 Bending Field T 9 9 8 Circumference Km 7 1.2 1.5 Average ring mag. field B T 6 5 4 • Effective turns before decay 900 800 750 /?* at intersection mm 3 8 8 rms beam size at I.P. fim 2.8 17 17 Luminosity cm~2s-1 1035 51033 6 1032 OVERVIEW intensity (2.5 x 1013 protons per pulse) 30 GeV proton synchrotron. The preferred cycling rate Proton Source would be 15 Hz, but for a demonstration machine using the AGS [8], the repetition rate would be limited to 2.5 Hz and to 24 GeV. For the lower Target Decay Channel energy machines, 2 final bunches are employed (one to make fi~'s and the other to make /*+'s). For the high energy collider, four are used (two ft Cooling bunches of each sign). Earlier studies had suggested that the driver could be a 10 GeV machine with the same charge Acce leration per bunch, but a repetition rate of 30 Hz. This specification was almost identical to that stud• ied^] at ANL for a spallation neutron source. Studies at FNAL[10] have further established that such a specification is not unreasonable.