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Linear Electron-Electron Colliders SLAC{PUB{7436 SCIPP97/08 March 1997 Linear Electron-Electron Colliders Clemens A. Heusch Institute for Particle Physics University of California, Santa Cruz, CA 95064 Stanford Linear Accelerator Center Stanford University, Stanford, CA 94309 Invitedcontribution to the Workshop on Future High-Energy Col liders, Santa Barbara, CA, October 21-25, 1996. Work supp orted in part by Department of Energy contracts DE{AC03{76SF00515 and DE{FG03{ 92ER40689. Linear Electron{Electron Colliders Clemens A. Heusch Institute for Particle Physics University of California, Santa Cruz, CA 95064 I INTRODUCTION In the framework of the discussion on what shap e our future machine ar- senal should take so as to maximize our chances of p enetrating b eyond the realm where our astonishingly successful Standard Mo del of Particle Interac- tions holds undisputed sway, the present contribution is somewhat unusual: I am not here to convince our community to build yet another machine. In- stead, my task is to convince you that in the established choices that we are headed towards, it is of great imp ortance that the Electron Col lider of the next generation, i.e., in the 0.5 to 1.5 TeV energy range, should b e con gured just such, as an Electron collider, NOT dedicated to just one incoming charge + state say, e e . Now that wehave exceeded the energy range that can b e reached with circular/recirculating machines, we are freed from the need to have opp ositely charged electrons as pro jectiles and targets. The colliding linac con guration sets no preferential condition on the chosen net charge; in fact, this is the rst time wehave a machine that maywell serve to collide a variety of initial states + + + at full energy 2E and luminositye e , e e , e e , or at slightly reduced B center-of-mass energy, but still full luminosity {e, { . I will not b elab or the case for initial states including high-energy photons beyond mentioning, in Section IV, the intimate connection that a successful realization of these collisions has with the availability of a high-quality e e facility. Rather, I will attempt to show, brie y, that there is little if any problem in con guring an Electron Collider such that it can b e run in either charge mo de with comparable p erformance characteristics, excepting only the p olarization parameter; and I will pro ceed to showyou the very richphysics p otential of the e e collision mo de|some of it unique, some complementary + to the promise of the more thoroughly discussed e e collision mo de. Before embarking on this enterprise, it is fair to remind you that the rst electron collider was, in fact, built for the explicit purp ose of testing the limits of precision to which the Standard Mo del of the 1950s, Quantum Electro dy- namics, could b e shown to follow its theoretically accepted pattern: Barb er, c 1995 American Institute of Physics 1 2 Gittelman, O'Neill, and Richter built their e e circular collider, with two rings, on the Stanford Campus, and were able to reach center-of-mass ener- gies of 1012 MeV, at which they tested Mller scattering for p ossible cuto or form factor e ects. The rst step toward testing the broader, emerging Standard Mo del that included the strong and weak interactions, showed the + virtues of using e e annihilation, and led to the immensely successful op- eration of a slew of storage rings that would teach us a large fraction of our present state of knowledge, was initiated in Frascati by Bruno Touschek with his ADA ring [1]. Today's running of LEPI I is, b eyond any doubt, the last + hurrah of the circular e e machines|and it would b e disingenuous to sug- gest installation of a second ring in its tunnel for the purp ose of running e e exp eriments. Fortunately, the Linear Electron Colliders, the NLC version of whichis describ ed in detail in these pro ceedings, have no problem worth mentioning + + b eing con gured in the e e or, should that b e of separate interest, the e e initial state. II MACHINE CONSIDERATIONS Linear acceleration of electrons and p ositrons is identical once the phase di erence with regard to the RF eld is taken into account. What is not identical is the emittance of the b eams entering the linac structure, and, as a result, the p otential phase space e ects due to wake elds building up in the accelerating structure. More di erentiation needs to b e considered for the interaction of the accelerated b eams at the interaction p oint: the luminosity that can b e reached with opp ositely charged b eams is enhanced by the elec- trostatic attraction of the two b eams \pinch e ect"; conversely, like-charge b eams rep el each other and \blow up" the interaction area \anti-pinch". Also, there is the need for di erent handling of the \sp ent" b eams b eyond the interaction p oint|particularly in the case of non-zero crossing angle: like-sign b eams need more attention b ecause they do not automatically follow the op- tical path of the opp ositely moving antiparticle, up to ejection into a b eam dump. While all of these p oints are basically amenable to given technical so- lutions, there is one qualitative di erence that cannot b e made up for in any known way: electron guns can easily reach high degrees of p olarization for the emerging e b eams, and we do not b elieve there is any relevant limitation as to the available intensity. 80 p olarized electrons are routinely used at SLAC, and there is no reason to b elieve that this value cannot b e raised to ab ove 90 in the intermediate time frame. It turns out that this capability is of immense value for the enhancementofanumberofBeyond-the-Standard-Mo del e ects, and for the suppression of backgrounds, as we will see b elow. For p ositron b eams, there is strictly no way to reach similarly high p olarization values; a numberofschemes are b eing tested, but there is no hop e of reaching anything 3 beyond ab out 60|which is not sucient for precision work in a number of connotations. Fairly detailed studies of the generalized luminosities L in terms of incident Gaussian bunches were made by J. E. Sp encer [2]. He p oints out that while it would b e nice to put the full currentavailable from the gun, p er RF pulse of the linac, into a single bunch for acceleration, this would b e highly undesirable for reasons of emittance growth, energy spread, and b eamstrahlen intensities. Rather, he plays a numb er of scenarios with multi-bunch op eration n > 1 B and f , the numb er of bunch trains p er second which is the same as the T RF rep rate. With N the numb er of electrons p er bunch and a luminosity B enhancement or disruption factor H , the luminosity can b e expressed as D ! 2 2 2 2 ^ f n N H N f n N H f n N P T B D T B D T B b B b B B L = ! = / : 1 2 2 4 4 4^ ^ N n b z y Here, is an eciency factor whichmaywell approach 1, and the are geometrical transverse sp ot sizes. Multibunch trains of n = 100 will help to B distribute the total charge p er RF pulse more evenly down the linac structure, whichmaywell help to make electron currents easier to raise than p ositron currents that are injected into the linac from co oling rings. Many practical problems have to b e addressed|the multibunch op eration will necessitate a crab-crossing interaction geometry, and overall luminosity optimizationmay well make a plasma lens advisable for the comp ensation of the electrostatic b eam-b eam repulsion [3]|but overall there is an exp ectation that the imple- mentation of a highly stable e e op eration of the Next Linear Collider will have little trouble coming in with luminosities commensurate with what an + e e version of the same machine can do. Given the high demands on instrumentation that will b e needed to pro duce ecient photon b eams from laser photons backscattered within less than 1 cm of the IR o the incident electron b eams, it will b e very unwise to couple e e exp erimentation a priori with e{ and/or op eration. Rather, the urgency of the physics program that can fruitfully b e addressed by the e e mo de argues p owerfully for the implementation of the electron{electron version early on, at turn-on time of the colliding linacs. The physics motivations that will have to decide on the appropriate initial-state choice therefore are our paramountinterest. III PHYSICS PROMISE In discussing the motivations for implementing the electron-electron version of the Next Linear Collider or its equivalent, we will follow roughly the 1996 Snowmass Study organization. In an attempt to highlight b oth the uniqueness of the goals that lend themselves to exp erimental investigation from an e e 4 initial state and the complementarity of the di erent approaches, we will treat a few problems in more detail than others; this will serve to illustrate the strengths of this channel, but do es not imply a lackofinterest in the studies more cursorily advanced b elow.
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