The CESR-CLEO program: Present and future

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Nuclear Physics B (Proc. Suppl.) 66 (1998) 537-540

The C E S R - C L E O Program: Present and Future Karl Berkelman a aLaboratory of Nuclear Studies, Cornell University, Ithaca, NY 14853 The research program of the CorneU Electron-positron Storage Ring and the CLEO experiment concentrates on heavy quark and lepton physics near the e+e - -4 bb threshold. Since progress in b weak interaction physics is paced by the size of the data samples, the storage ring is now being upgraded to provide higher luminosity and the detector is being upgraded for improved sensitivity. A possible future upgrade now under consideration would bring CESR to a peak luminosity of 3 x 1034/cm2sec.

1. P R E S E N T CLEO P R O G R A M The Cornell Electron-positron Storage Ring (CESR) runs most of the time at the T(4S) resonance at a center-of-mass energy of 10.58 GeV, just above the threshold for e+e - -4 B B . CESR has the highest luminosity of any collider, Z:pk -4 x 1032/cm2sec, and produces over 3 million B B pairs per year. The CLEO experiment exploits these collisions to study the physics of the heavy quarks, b and c, and the heavy lepton T. The direct chargedcurrent decays of the b quark to c or u are being investigated through hadronic, semileptonic, and leptonic decays: tests of Heavy Quark Effective Theory, weak form factors, measurements of the CKM elements Vcb and Vub, and tests of our understanding of the Standard Model through the measurement of many decay branching ratios. The much rarer effective neutral-current, loop decays of the b quark to s~/, sg, d',/, and dg (so called 'penguin' decays) are studied to extract information on Vts and Vtd as well as to search for evidence of new physics beyond the Standard Model. In charm physics CLEO studies D and Ds to determine masses of excited states, make H Q E T tests, measure meson decay constants fD and fD,, and so on. CLEO is particularly active in charmed baryon spectroscopy and has measured masses and decay branching ratios for all of the known states of Ac, ~c, and Ec. With the largest sample of r decays, CLEO specializes in rare de0920-5632/98/$19.00© 1998 ElsevierScienceB.V. All fights reserved. PII S0920-5632(98)00103-0

cay modes and sensitive tests of the Standard Model. At CESR the B mesons are produced in a simple two-body B + B - or B°/3 ° final state which accounts for about a quarter of the total cross section for e+e - annihilation into hadrons. The limitation on the physics reach, however, comes from the smallness of the B B cross section, about 1 nb. CLEO's few dozen events of the penguin decay b - r st' can already limit the mass of a charged Higgs to > 0.5 TeV. Larger event samples could give even more incisive tests of the Standard Model, and would enable CLEO to observe b --4 d7 and thus measure [Vtd/Vts[, as well as measure the interesting b ~ si+g - mode. As illustration of the effect of rate limitations I show some of the rare B decay branching ratios recently obtained by CLEO. Table 7 Rare B decay data [1] Mode

CLEO Br x 106

K-~r+ ~-Tr÷

15+ ± 1 ± 1

i - r r ° or 7r-zr° /~071-K - w or 7r-w K - ~ / o r lr-~

K-y~nlr, n > 0

< 15 164-5±2±1 23±114-24-2 28+10±5 <8 78 +27 -22 -i-'- 10 < 44 670 ± 150 ± 110

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K. Berkelman/Nuclear Physics B (Proc. Suppl.) 66 (1998) 537-540

The fact that measured branching ratios bottom out at about 10 -5 is a consequence of the number of produced B B in the data sample. One would like to know more about these rates and especially some of the ones for which we now have only upper limits. Some of them, like ~r+Tr-, K±Tr~:, and K ± r °, are prime modes for C P violation once we have enough events to measure a charge asymmetry. Direct C P violation in a mode like K ± r °, which doesn't need tagging, results from the interference of two amplitudes, tree and penguin in this case. The asymmetry, A =

2 sin7 sin 5 R + 1 / R + 2cos7cosS'

• We have to replace the present copper rf cavities with superconducting cavities. The first of four such cavities is being prepared for installation in September, 1997, and the rest will be installed in the following year. • We have to replace the focusing quadrupoles nearest the collision point with stronger, superconducting magnets. • We have to refurbish the linac injector to provide higher positron currents.

(1)

depends on the C P odd phase 7 (the phase of Vub in the SM), but also on the ratio R of the two amplitudes and 5, the C P even strong phase difference. The recent successes of CLEO in opening up the field of rare B decays is providing a strong incentive for an increase in CESR luminosity. Also, the weakness of the CLEO detector in distinguishing K's from r's at the momenta (around 2 GeV/c) occurring in these two-body decays is a motivation for an upgrade of the detector. 2. P R E S E N T

goal, we have to complete the following work.

UPGRADE

2.1. C E S R The goal of the CESR upgrade is to increase the peak luminosity from what was about 2 x 1032 /cm2sec when the upgrade began to at least 1.7 x 1033. This is being done be increasing the number of circulating bunches in each beam, keeping the charge per bunch about the same. By separating the electron and positron beams into two 'pretzel' orbits that snake around each other, we had been able to store seven bunches per beam prior to the present upgrade. Now by colliding the beams at a +2 mr angle we can accommodate up to 45 bunches per beam and still have collisions only at the center of the CLEO detector. With 18 bunches per beam we now achieve 4 x 1032 luminosity, but the circulating current is limited by the RF accelerating cavities that have to make up the radiation losses. In order to achieve the 1.7 x 1033 luminosity

• We have to upgrade the vacuum in the interaction region. • We have to upgrade the feedback systems for beam stabilization. If all goes well, this work will be complete in 1999. 2.2. CLEO There are two main motives for a concurrent upgrade of the CLEO detector: (a) the study of decay modes like B ~ zrlr and B --+ K~r demands good K - lr separation at momenta between 2 and 3 GeV/c, and (b) the new CESR superconducting quadrupoles intrude into the volume occupied by the existing CLEO main drift chamber. The solution is a replacement of the inner part of the CLEO detector. In terms of their radii from the beam line, the new components are as follows:

r--80-100 c m a ring imaging Cherenkov chamber, with LiF radiator and TEA photon converter; r=12-80 c m - - a new main drift chamber, with stepped end plates; r----2.5-10 c m - - a four-layer double-sided silicon strip tracker. The trigger and readout electronics are also being upgraded to handle the higher data rates. Since the new tracking devices involve less mass in the path of the particles, the reduction in multiple scattering will compensate for the fact that the radial distance covered is reduced to make

K. Berkelman/Nuclear Physics B (Proc. Suppl.) 66 (1998) 537-540

space for the Cherenkov system. The momentum resolution is essentially unchanged. Installation is scheduled for late 1998. 2.3. A F T E R T H E P R E S E N T U P G R A D E Starting in 1999 or soon after, we expect to have data rates that will be at least four times the present rates, and a particle identification system that will have at least 3 standard deviation separation of K ' s and 7r's at all momenta of interest. This will give CLEO improved physics potential: • better accuracy in all past CLEO measurements - c, b, T, T, 77; • more sensitive searches for nonstandard physics - virtual Higgs, SUSY, and other hypothetical particles that can affect rates for loop processes; • measured branching ratios for rarer B decays, at the 10-6 level; • observed C P asymmetries, in untagged time-independent cases. There are several ways to observe C P violation in B decay. If the Standard Model were correct and if the phase in the CKM matrix were sufficient to explain all C P violating phenomena, all the measurements of C P asymmetries would be redundant. None of them would be needed to fix the parameters of the matrix, which are in principle fixed already by the known sides of the unitarity triangle. In practice, two of the CKM parameters, p and z/are still rather poorly known, because of theoretical uncertainties in the interpretation of the B decay and mixing data, but since the theory is steadily improving, this is not really the motive for the measurement of C P asymmetries. The motive is to test the Standard Model and the Kobayashi-Maskawa mechanism for C P violation. Many theorists maintain that we do not have a satisfactory explanation of the matterantimatter asymmetry in the universe. So it is not fair to assume the Standard Model in deciding on a 'minimum set' of C P asymmetries to be measured. If there were, for example, more than six quarks, there would be more than one

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independent phase in the flavor mixing matrix. Or if there were more interactions contributing in penguin loops or in the mixing box diagram, involving SUSY partners for instance, then the C P asymmetries would have little or no relation to the sides of the unitarity triangle. We have to be prepared for anything. And we have to measure as many asymmetries as possible, and by different methods. The first measurement of the angle ~ in the assumed unitarity triangle, by observing the time-dependent asymmetry in tagged B --+ C K s at an asymmetric B Factory will certainly not settle the issue, regardless of the result. Nor will the measurement of one additional angle. We will have to measure direct C P violation in modes like B ~ K r or B --+ D K . Fortunately, these are modes that do not require tagging and the effect is a total rate asymmetry, independent of time. Recent theoretical studies [?] have shown us how to extract the weak phase 7 without prior knowledge of R and 5 (see eqn. 1). Consider a mode with rate ]a±[ 2 in which one observes a C P asymmetry arising from an interference of amplitudes al and a2 (tree and penguin, for instance). One looks for related modes that are pure al and pure a2. The three amplitudes a ±, al, a2 form a triangle, with one angle 3~ that flips sign between the a + case and the a - case. Measurement of all of the four branching ratios [a+[ 2, [a-[ 2, [al[ 2, [a2[ 2, allows one to extract 7 independently of R and 5. In practice, this can get a little more complicated when there are more than two contributing amplitudes; one then needs to measure more modes. Table 2 lists some B decay modes that may be relevant for C P violation. The branching ratios with errors are preliminary CLEO results [1]. Otherwise the branching ratios and asymmetries are Standard Model estimates. The last column contains the sensitivity parameter ~ -13A2/(1 - A2). The number of standard deviations in an asymmetry measurement is given by

.A/~,.4

= ~

× v~,

(2)

For example, if the number of produced B B is NB~ = l0 s and e = 1/3, then one can expect more than four standard deviation significance

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K. Berkelman/Nuclear Physics B (Proc. Suppl.) 66 (1998) 537-540

for any mode for which Y, > 60 x 10-8. But is N e ~ = 108 achievable? Table 2 Some interesting modes mode

Y 10 - 6

K-~r ° K-~ K-~ I K-w /~°lrK*-r ° ~r-~r° lr-~ ~'- r/' ~r-w p-w K-~r + K*-~r + ~r-~r+

16 4- 5 0.1-5 78 + 28 1.5-4 24 + 10 3-11 1.3-11 5 6-17 4-11 13-25 15 + 5 6-22 2-12

IAI % 1-18 6-65 1-3 3 0-1 2 0.4-70 33 16-22 4 4 1-22 2 1-6

:E 10 -8 0.1-40 0.1-70 0.5-10 0.1-0.4 0-0.3 0.1-0.4 0-130 60 15-80 0.6-2 4 0.4-60 1 0.2-3

3. L O O K I N G F A R T H E R A H E A D Early in the next decade CLEO, BaBar, and Belle will be seeing 10-30 million B B events per year. The time-dependent C P asymmetry in B ~ C K s (maybe 7r+Tr- too) will have been measured by BaBar, Belle, HERA-B, CDF, or DO. Direct C P violation may be seen by CLEO, BaBar, or Belle. But we will be crying for more B B events, hundreds of millions, in order to understand C P violation and test the Standard Model. Experiments at hadron colliders will be producing more than enough B's. But they will have to cope with a difficult background environment, they will have to trigger on displaced vertices, and they are not generally sensitive to final states containing % ~r°, or ~?. So what can we do to get more e+e - -~ T(4S)-~ B B ? At Cornell we have begun a design exercise to see whether CESR can be upgraded further to achieve a peak luminosity above 3 x 1034. We need to keep the two beams in separate channels over most of the ring, in order to have enough physical aperture and to avoid

long-range beam-beam interaction. The circulating currents have to be increased by storing more bunches. In going for the highest possible luminosity, equal energies is a distinct advantage. The interaction region focusing is less constrained, the beam-beam interaction limitations are less serious, and the lower Ema= means less synchrotron radiation, few RF cavities, and hence higher stable beam currents. CESR has other advantages for luminosity upgrading: the crossing angle collisions allow for more beam bunches than do head-on collisions; superconducting RF can handle much higher currents; and there are no competing high energy physics priorities at CESR. Our studies indicate that such an upgrade could be accomplished for less than $40 million. The most challenging commponents would be the superconducting RF cavities and superconducting dual-aperture quadrupole focusing magnets. The cavities are the same as developed for the present upgrade, and a prototype of the dual quadrupole has already been built and tested. There is space to mount the new dual aperture ring above the present synchrotron injector in the CESR tunnel without disturbing the present CESR magnets, so that the installation could proceed in stages during routine maintenance down periods without disrupting the ongoing CESRCLEO experimental program. There would probably have to be some upgrading of the CLEO detector to handle the larger beams, but the effort would be modest on the scale of the CESR upgrade. CLEO is inviting new collaborators to help exploit these opportunities.

REFERENCES 1. CLEO Collaboration, submitted to the Lepton-Photon Symposium, Hamburg, July, 1997. 2. See the summary by Robert Fleischer, hepph/9612446.