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Polarized Protons at Rhic POLARIZED PROTONS AT RHIC I G. Bunce, J. Collins, S. Heppelmann, R. Jaffe, S.Y. Lee, Y. Makdisi, R.W. Robinett, J. Soffer. M. Tannenbaum, D. Underwood and A. Yokosawa I Abstract Recent technical developments in the acceleration of polarized protons, using so-called Siberian snakes, now allow for the achievement of polarized proton—proton collisions at collider energies. We review the machine physics of achieving such collisions and describe a comprehensive program of spin physics which can be carried out at the RHIC facility using polarized pp collisions. 1. Introduction produce high [)T jets and weak bosons copiously while at the The hugely successful program of QCD and electroweak tests same time ensuring that the contributing quark processes are in a at the hadron colliders at CERN and FNAL has provided a wealth region of documented high polarization. In these regards. a RHIC of information on the Standard Model of particle physics. Tests spin program will be unique. involving jets (single and multi-jet events), direct photons, Tests using polarized beams and targets at fixed—target Drell—Yan weak boson production, and heavy quark and energies involving direct photons and quarkonium production quarkonium production have probed the structure of the QCD have been put forth as possible probes of the longitudinally hard-scatterings, often beyond leading order, and have yielded polarized gluon distributions. The relatively low energies additional information on the parton structure of the proton itself. involved at presently envisaged polarized fixed—target facilities One aspect of our understanding which has not benefitted put the proposed experiments near the limit of applicability of from such experiments at high—energy colliders, however, is the perturbative QCD. In contrast, polarized pp collisions at RHIC, at area of spin physics, both the spin structure of the proton itself collider energies (\lv 2 50 — 500 GeV) and with high luminosity and the spin-dependence of the fundamental interactions. (reaching L = 2.1032 cm’zs") would provide a huge sample of Polarized pp collisions, which can address both issues, have large transverse momentum events to which perturbative QCD previously been restricted to relatively low-energy (by collider would be just as applicable as at existing unpolarized machines. standards) fixed-target experiments. So, spin, although Furthermore, this can likely be achieved while providing ~ 70% historically central to the development of particle physics, has polarization in each beam. Even with limited running time (say occasionally been relegated to a “soft physics" compartment in one month a year), this could yield well over 50 pbrl of data and our field, despite indisputably large effects seen in many different so would allow for high statistics studies of QCD and experiments. At issue has been the interpretation whether a electroweak phenomena [5] and their spin dependence at a perturbative QCD analysis is possible or whether a complete previously unattained level. understanding of non-perturbative effects is necessary. Besides being of unquestioned importance in its own right, With the successful tests of the Siberian snake concept [1], it such a program of collider spin physics is a crucial step in the now seems clear that a polarization option at a hadron collider, assessment of the possible importance of polarization options at especially the Relativistic Heavy Ion Collider (RHIC) at BNL, is future supercolliders such as SSC or LHC. Given the extent to now a definite possibility [2—5]. A comprehensive program of which the particle-physics community is committed to high— experiments at such a facility would allow one to measure the energy collider physics, it is of utmost importance that the parton spin—dependent parton distributions of the proton (both longitu- content of the proton, including its spin dependence, be dinal and transverse) and would help to resolve any “spin crisis” understood and it seems that a polarized RHIC facility would be raised by some theoretical interpretations of the much discussed the ideal machine for economically pursuing this program. Electromagnetic Calorimeter (EMC) experiments on polarized Motivated by these possibilities, a number of experimen— lepton—nucleus scattering [6]. Furthermore, it would provide talists, particle and nuclear theorists, and machine physicists have fundamental spin tests of QCD and the electroweak interactions formed the RHIC Spin Collaboration (RSC) [7] to develop a not accessible to unpolarized colliders, probing the helicity comprehensive and compelling program of spin physics for structure of the matrix elements of the fundamental hard RHIC. Some aspects of the physics prospects for such a facility scattering processes. Finally, the high energy and luminosity of have been discussed elsewhere [4,8,9] and in this article we will such a machine would guarantee that a perturbative QCD review various components of the proposal to achieve a program approach would be applicable and would also be sufficient to of polarized pp collisions at the RHIC. In sect. 2 we describe © |‘)‘)2 Gordon and BlCtll Science Publishers S./\ Photocopying [)Cl mitted by license only l’alticlc World, Vol 3 No l, p LIZ, [992 (i. Bunce el til some of the technical considerations relevant for producing, facility. We describe a program to map out the helicity—depcndcnt storing and monitoring polarized proton beams of the proposed parton distributions and to test the spin structure of the basic spin RHIC complex (fig. 1), while in sect. 3 we discuss in sotne QCD interactions, as well as that of the electroweak theory using detail the physics questions which can be addressed at such a the unique tools provided by the existence of polarization infor— mation; we will also emphasize the importance of transverse spin asytntnetrics. in sect. 4, we discuss briefly some of the Polarized proton collisions at BNL itnplications that the desired physics program may have for possible detectors and we give some concluding remarks. Polarized Ii H‘ source 2. Technical considerations of high-energy polarized beams Linac 2.1 Polarized beam luminosity in RHIC with Siberian snakes To achieve high—energy polarized proton collisions in RHIC, 200 MeV the AGS polarized proton source may be used. The source | |0| polarimeter produces 30 ~ 40 pA of H’ with 75—80% polarization in 500 its pulses at a repetition rate of 5 Hz. The polarized H’ ions are accelerated to 200 MeV with an RFQ and 200 MHz with the Partial Linae. Twenty pulses of H‘ ions are strip—injected and aceti— Siberian snake mulated in the AGS booster. Each bunch in the booster will contain NB : (1—3) X 10" polarized protons with normalized emittance about 6 N = 10 TC mm-mrad depending on the harmonic Pulsed number of the r.f. system used in the booster. quadrupoles The accumulated proton bunch will be transferred and accelerated to 24 GeV in the AGS. During the acceleration, the polarization may be lost when the spin precession frequency Cy // (G is the Pauli anomalous g factor) is equal to the spin AGS polarimeter depolarizing resonance. These resonances occur at an integer (imperfection resonance) due to vertical closed orbit errors or at kP i vy (intrinsic resonance) due to the vertical betatron motion. Here, P = 12 is the superperiod of the AGS, vy : 8.8 is the vertical betatron tune, and k is an integer. Traditionally, these spin depolarizing resonances are corrected by the tedious harmonic correction method for the imperfection resonances and the tune—jump method for the intrinsic resonances. In an approved experiment [1 1] at the AGS to be performed 4 split Siberian snakes in 1993, the feasibility of polarized proton acceleration by using a (2 per ring,180°apart) 5% solenoid Siberian snake will be tested. Previous experiences with polarized proton acceleration in the AGS indicate that a 5% RHIC snake is indeed sufficient to overcome imperfection resonances without using the harmonic correction method. The remaining six important intrinsic resonances can be corrected by the tune—jump method. At 24 GeV kinetic energy, the polarized protons are transferred to the RHIC. The transfer line between the AGS and the RHIC is spin—transparent at this energy. In the RHIC, spin depolarizing resonances can be overcome with two Siberian snakes [1] located symmetrically in each of the RHIC rings. Using the split snake configuration [12] (fig. 2), RHIC polarimeter proton collisions with longitudinal or transverse polarization can be achieved without using additional spin rotators. The maximum tFIGUILE; spin depolarizing resonance strength is found to be ~ 0.4 for a Pioposcd spin RHIC complex normalized emittance of 10 1t mm-mrad. (l Bunce el al l“; fill lL’l [iii/Er; ~.____. / l<—2d——>l \f 10 N 20 Tm Total field strength Schematic thawing of the split-snake configuration Two snakes can preserve [3] the polarization for an energy up 2.2 Measurement of polarization to 250 GeV provided that the vertical r.m.s. closed orbit is Polarimeters for the polarized beams at the ZGS and AGS corrected to < 0.3 mm and the low—order snake resonances [13] energies up to 25 GeV/c' relied on the analyzing power in pp are avoided. The expected beam polarization is 70%. elastic scattering at small III. For higher energies such polari— The corresponding luminosity for the RHIC at the top energy meters can no longer be used because the analyzing power of 250 GeV, is given by becomes very small at III S 0.3 (GeV/(')2 and the cross section becomes small at III 2 1.5 (GeV/c)2 where the analyzing power is relatively large. (swig-[37) -7 .1 For the FNAL polarized—beam facility up to 200 GeV/c, two L:(5.7)103' 7 CIT] “S at EN ’ kinds of polarimeters were developed and these can be used energies much higher than 200 GeV/c.
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