- Email: [email protected]
Second-Generation B Factories at the ϒ(4S)
Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
Second-Generation B Factories at the B(4S) Jim Alexander* Department of Physics, Cornell University, Newman Laboratory, Ithaca, NY 14853-5001, USA Accepted 16 June 2000
Abstract Physics goals and technical issues related to very high luminosity B Factories are discussed. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 13.25.Hw; 14.40.Nd; 12.15.Hh
1. Introduction Today, the total world's sample of B meson decay data is that consisting of the 10 BBM events in the CLEO II dataset. Three laboratories } KEK, SLAC, and Cornell } are beginning operations of accelerators and experiments designed to generate and study unprecedented numbers of BBM meson pairs. In the next few years each experiment expects to bring in a data sample of about 10 BBM events. This will answer many, but probably not all, of the scienti"c questions that face us. Since we work in a "eld that by its scale demands long-range planning, we are compelled even in this nascent state to begin to think of what tasks may remain incomplete at the end of the period we now enter. If 10 BBM events will be needed to do the physics, we must ask the relevant scienti"c and technical questions now. This talk is intended to give an overview of the issues, both technical and scienti"c, of what we might call the Second-Generation B Factories. It would be folly however to project two orders of * Tel.: #1-607-2555259; fax: #1-607-2544552. E-mail address: [email protected] (J. Alexander).
magnitude and claim any accuracy. Inevitably this is a view rooted in the state of the "eld in late 1999, and any future reader stumbling onto this paper may be best advised to "nd something more current. In the following sections, I will review both where we are with respect to major issues of the day, and what directions we are heading in. Necessarily, this discussion will touch only on highlights and leave details to the references. In particular, basics are assumed and any reader requiring more information is directed to review articles to "ll in the gaps. (For a recent review see Ref. [1].) The task of B meson physics is to elucidate the relationships among quark #avors and between matter and anti-matter. Despite the enormous successes of the Standard Model, the underpinnings are either unstated (the 18 free parameters) or seemingly ad hoc (the Higgs mechanism) and the search for a more fundamental understanding of the underlying issues of electroweak symmetry breaking largely drives the course of high-energy physics as a whole today. Broadly speaking, there are two approaches, direct and indirect. The direct search for particles and phenomena outside this standard model is largely the province of experiments at the
0168-9002/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 6 0 3 - 3
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
energy frontier. The indirect search for manifestations of physics outside the standard model is a task for B Factory experiments, and others, which work at the luminosity frontier. Statistics determines the sensitivity and the reach of these experiments. B mesons o!er unique advantages, particularly as compared against the studies of charm and strange meson decays. The large b quark mass permits access to a large number of diverse "nal states. The relative smallness of < gives the B meson a long lifetime which makes possible a wide range of studies involving mixing and mixing-induced CP violation. < is nonzero, so CP violation is possible in B decay. And "nally, the top quark mass is huge, breaking GIM symmetry and making preeminent, the role of penguin diagrams in rare B decays. This last opens up numerous opportunities for the appearance of new physics and the manifestation of direct CP violation. These four characteristics lead to a rich pattern of decays and a long menu of delicious and sometimes subtle phenomena.
2. The unitarity triangle: sides The Standard Model makes only one statement about the Cabibbo}Kobayashi}Moskawa (CKM) quark mixing matrix: by construction, it is unitary. For B physics the most accessible manifestation of this statement is that the so-called unsquashed unitarity triangle be in fact a triangle. (A discussion of unitarity triangles may be found in Ref. [2].) Current knowledge of the unitarity triangle comes from measurements of branching ratios and BBM mixing which determine the lengths of the sides. We note the following: E bPulm inclusive measurements and BP(o,p)lm exclusive measurements give us "< ". The exclus ive branching ratios are measured with about 10% precision, but theoretical uncertainties in extracting "< " itself result in an overall uncer tainty of 20}25%. This latter uncertainty is due to uncertainty in the theoretical treatment of hadronization of the quark fragments. E B mixing is extremely well measured (*m " 0.477$0.017 ps\) but extraction of < is hampered by uncertainty in B f , currently esti-
33
mated by lattice QCD to be in the range of 160}220 MeV. E B mixing is still a couple years over the horizon. Q Its eventual measurement will pin down the ratio of "< "/"< " with much less theoretical uncertain ty than extracting "< " from B mixing alone. E In the far future BPoc/BPKHc will also help to pin down "< "/"< " provided theoretical un certainties about long distances contributions can be reduced. E Also in the far future, BPqm decays o!er hope. The ratio Br(BPqm)/*m &"< /< " has only mild model dependence but o!ers information similar to measurements of "< ". Thus, we see that advancement in de"ning the sides of the unitarity triangle will depend strongly on advancement in theoretical tools.
3. The unitarity triangle: angles The set of internal angles of the unitarity triangle, a( ), b( ), and c( ) can be checked for consist ency or inconsistency with the measured sides. sin 2b has already been attempted by CDF [3], and will soon be measured by almost every experiment in the world. Precision of around $0.1 or better should be available in a few years. sin 2a is a more challenging problem. Although time-dependent asymmetries in pionic B decays can be well-measured [4], the asymmetry is the result of contributions both from mixing-induced CP violation and direct CP violation. Extraction of sin 2a from the measured asymmetry requires detailed knowledge of the direct component. It is well known that for BPp>p\ this demands an isospin decomposition which in turn depends on measurements of BPpp which is expected to be very small. Thus, despite the fact that we anticipate time-dependent asymmetry measurements from SLAC and KEK in the next few years, an accurate extraction of sin 2a will require signi"cant data samples and a good understanding of the role of penguin modes. There are several approaches to the measurement of the angle c, the phase of
34
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
[5,6] which uses the ratio R ,Br(B!Pp!K)/ H 2Br(B!PpK!). With 10 M BBM events in the current data sample, this method is statistically limited and places as yet no limit on c. It will remain statistically limited for a long time, as 100 M BBM events are required to reach a precision of even $123. A method [7] based on B!P DK! decays will require &10 BBM to realize a comparable precision. A new approach proposed this year [8] trades theoretical cleanliness for higher statistical power. In this method, many rare B decay branching ratio measurements are "t to a factorization model parametrization. The "ve "tted parameters include c, which with the current (CLEO) data sample of 10 M BBM events is found to be (114$25)3. The validity or limitations of this method will be established only by examining a large sweep of branching ratio measurements, which in turn will require a substantially larger dataset than is currently available. It is likely that substantial progress on this last issue can be made with the B Factories currently in progress.
4. New physics in rare B decays As mentioned earlier, the large mass of the top quark makes penguin decays of the bottom quark signi"cant and in many cases dominant contributors to rare decays. The presence of the unseen loop particles in these decays opens up the possibility to see non-standard model physics through its indirect e!ects on either branching ratios or CP violating phases. The most sensitive test so far is the measurement of the inclusive branching ratio Br(bPsc). Stringent limits on non-standard model charged Higgs mass can be set with current measurements [9]. Direct searches at high-energy collider experiments are not competitive with this indirect method. In the next few years we can also expect to see the "rst measurements of Br(bPsl>l\) which o!er additional information through the l>l\ invariant mass, forward}backward asymmetry, and, in principle, polarization. Direct CP violation in rare B decays is also a place where new physics could show up. In rare charmless hadronic modes the Standard Model
predictions [10] for A are generally small, of !. order "A ":(0.01}0.10). Sensitivity to such asym!. metries is wholly statistics dominated [11], and scales as 1/(Luminosity; sensitivity in the $0.01 range for some rare decays will be just reachable with 10 BBM events. Non-standard model contributions can be many tens of percent. Large "nal state interaction phases, however, could amplify the standard model predictions which are based only on short distance contributions to the strong phase di!erence. Signi"cant progress in both experimental measurements and theoretical control of such e!ects will be required to distinguish new physics from old physics.
5. Issues for B-Factory luminosity upgrades The design parameters and status of the three B Factories at the time of this conference can be summarized by Table 1. The design luminosities range (2}10);10 cm\ s\. The translation of luminosity into BBM per year depends on the fraction of time the accelerator runs for highenergy physics and the fraction of that time spent at the peak of the B(4S) resonance. Assuming the product of these is 30% (an optimistic assumption), then 1;10 cm\ s\ translates to 10 M BBM per year. The formula for luminosity in a symmetric energy machine with no crossing angle makes a convenient starting point to discuss issues surrounding luminosity upgrade plans:
p m ¸"(7;10 cm\ s\) 1# W p 0.06 V I 1 cm E ; (1) 1A bH 5.3 GeV We summarize below the issues related to each of the terms in this equation, beginning with the beam current I which historically has been the parameter most amenable to improvement.
E I } Increases in beam current a demand corresponding increase in available RF power. Beam instabilities grow with current, demanding additional feedback. Vacuum quality worsens, and
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36 Table 2 Parameters of possible CESR upgrade options
Table 1 Parameters of the three B Factories
Energy (GeV) Circumference (m) Beam current (A) Number of bunches Bunch spacing (m) Crossing angle (mr) Vertical tune shift (m ) bH (mm) Bunch length (mm) Luminosity (cm\ s\) Status Peak luminosity (cm\ s\) Integrated luminosity
35
KEK-B
PEP-II
CESR-III
3.5;8.0 3016 2.6;1.1 5000 0.59 $11 0.05 10 10 10;10 running 0.4;10
3.1;9.0 2199 2.1;1.0 1658 1.26 0 0.03 15 15 3;10 running 1.4;10
5.3 768 1.0 54!90 1.8!4.2 $2.5 0.05 13 13 2;10 starting (0.8;10)
0.1 fb\
1.4 fb\
(14 fb\)
Items in ( ) refer to CESR Phase II achievements.
experimental backgrounds increase. Synchrotron radiation power increases, demanding additional cooling and additional wall plug power. Increasing the beam current may require altering the bunch structure, the bunch separation, the crossing angle, and/or adding crabbing capability. E bH } Reductions in bH require changes to IR optics and generally encroach on territory claimed by experiment detectors. Smaller bH demands correspondingly shorter bunch length to avoid hourglass degradation, and this in turn imposes demands on the RF cavity design and power load. E m } Increases in the tune shift parameter are known to be di$cult. Improvements in magnetic "eld quality, quadrupole alignment, and careful tuning of machine operating parameters all contribute. Large advances are unlikely. E p /p } Flat beam conditions (p ;p ) are W V W V normal for e>e\ machines. Round beam conditions (p "p ) are possible in principle, W V and o!er a factor of 2 enhancement of luminosity. Additional gains are expected from increased tune shift, which has been demonstrated [12] to at least double. However, constraints on bH and beam current may undermine other advances.
C Superconducting RF cavities Energy (GeV) Beam current (A) Number of bunches Bunch spacing (m) Crossing angle (mr) Tune shift m bH Bunch length (mm) Luminosity (cm\ s\)
Flat beams
Round beams
10 5.3 3.1 420 1.8 $2.3 0.06 7 7 3;10
8 5.3 2.3 45 18.7 0 0.15 30 30 2.5;10
PEP-II upgrade plans are under discussion at SLAC. Likely paths include increasing the beam currents by a factor 1.8, adding addition RF power, IR vacuum pumping, and feedback; reducing bH from 1.5 cm to 1.0 cm, with corresponding shortening of bunch length; and relaxing energy transparency conditions and improve tune shift from 0.03 to 0.05. CESR upgrade plans [13] require a new ring to be built as the bunch density in the current ring is close to maximum. A double ring design which "ts in the existing tunnel is under study. Dual aperture quadrupole magnets are required for positions closes to the IR, and prototypes meeting the necessary criteria have been developed and tested. With 3 A beam current, 7 mm bH, and 0.06 tune shift, 3;10 is possible. Round beam designs are also under study (see Table 2). Experimental backgrounds are an important consideration. Based on experience so far it is known that the dominant background comes from `Bremsstrahlunga events in which inelastic collisions with residual gas in the accelerator vacuum chamber yields o!-energy beam particles. These can strike surfaces near the detector and produce showers which lead to radiation damage of detector components, increased detector occupancy, increased trigger rates, and increased raw event size. Other players of usually lesser importance include elastic scattering o! residual gas, synchrotron radiation, beam gas events, and of course accidents. Di!erent components and sources may 2. FUTURE ACCELERATORS
36
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
be identi"ed by a combination of pressure `bumpa studies, orbit bump studies, beam current dependence, and detailed comparison of all of these with Monte Carlo simulation. Experimental design considerations which may be used to minimize the e!ects of machine backgrounds or simply to deal with their irreducible e!ects include masking and shielding in the IR, extra pumping in the IR region, upstream collimation, front-end electronics design, and trigger design. Experience at CLEO [14], where running conditions have been stable for several years has shown that Monte Carlo simulation is an extremely important tool which can yield results that are in good agreement (factor of two) with the data in radiation dose, occupancy, and trigger rates.
6. Summary and outlook B physics addresses many of the issues related to quark #avor and identity that are raised but not answered by the Standard Model. In addition to exploring CP violation, it o!ers the chance to probe high mass scales and search through indirect e!ects for signs of new physics. Small branching ratios, a rich set of phenomena, and challenging theory all indicate that large data sets will be required to obtain a complete picture. Advances in theory will be essential but these will be driven by advances in the precision and breadth of the experimental results. Both hadron colliders and dedicated B(4S) e>e\ machines will contribute in complementary ways. The former o!er extremely high yield in simple modes, as well as B physics, while
the latter o!er broad spectrum capability. Accelerator physics will be pushed to deliver beams of high intensity and tighter focus than ever before to underwrite the scienti"c endeavours at the frontier of luminosity.
References [1] A.J. Buras, CP Violation and Rare Decays of K and B Mesons, Lectures given at the 14th Lake Louise Winter Institute, February 1999. hep-ph/9905437. [2] I.I. Bigi, A.I. Sanda, On the Other Five KM Triangles, hep-ph/9909479. [3] A Measurement of sin 2b from BPJ/w K with the CDF Detector; http://www-cdf.fnal.gov/physics/new/bottom/ cdf4855/cdf4855.html. [4] `The Babar Physics Booka, SLAC-R-504. [5] M. Neubert, J. Rosner, Phys. Lett. B 441 (1998) 403. [6] M. Neubert, JHEP 9902 (1999) 14. [7] D. Atwood, I. Dunietz, A. Soni, Phys. Rev. Lett. 78 (1997) 3257. [8] W.-S. Hou, J.G. Smith, F.K. Wuerthwein, hep-ph/9910014. [9] J.P. Alexander, Heavy quark decay, Proceedings of the 29th International Conference on High Energy Physics, and references therein. [10] A. Ali, G. Kramer, C.D. LuK , Phys. Rev. D 59 (1999) 014005 (hep}ph/9805403). [11] Measurement of Charge Asymmetries in Charmless Hadronic in B Meson Decays, CLEO Collaboration, hep-ex/0001009. [12] E. Young et al., Collisions of resonantly coupled round beams at the Cornell electron}positron storage ring (CESR) Proceedings of the 1998 Particle Accelerator Conference, Vancouver. [13] D.H. Rubin, Very high luminosity collider at Cornell, Presented at e>e\ Factories 1999, KEK Laboratory. [14] S. Henderson, D. Cinabro, Beam-generated detector backgrounds at CESR, Presented at e>e\ Factories 1999, KEK Laboratory, Cornell CBN 99-30.
Second-Generation B Factories at the B(4S) Jim Alexander* Department of Physics, Cornell University, Newman Laboratory, Ithaca, NY 14853-5001, USA Accepted 16 June 2000
Abstract Physics goals and technical issues related to very high luminosity B Factories are discussed. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 13.25.Hw; 14.40.Nd; 12.15.Hh
1. Introduction Today, the total world's sample of B meson decay data is that consisting of the 10 BBM events in the CLEO II dataset. Three laboratories } KEK, SLAC, and Cornell } are beginning operations of accelerators and experiments designed to generate and study unprecedented numbers of BBM meson pairs. In the next few years each experiment expects to bring in a data sample of about 10 BBM events. This will answer many, but probably not all, of the scienti"c questions that face us. Since we work in a "eld that by its scale demands long-range planning, we are compelled even in this nascent state to begin to think of what tasks may remain incomplete at the end of the period we now enter. If 10 BBM events will be needed to do the physics, we must ask the relevant scienti"c and technical questions now. This talk is intended to give an overview of the issues, both technical and scienti"c, of what we might call the Second-Generation B Factories. It would be folly however to project two orders of * Tel.: #1-607-2555259; fax: #1-607-2544552. E-mail address: [email protected] (J. Alexander).
magnitude and claim any accuracy. Inevitably this is a view rooted in the state of the "eld in late 1999, and any future reader stumbling onto this paper may be best advised to "nd something more current. In the following sections, I will review both where we are with respect to major issues of the day, and what directions we are heading in. Necessarily, this discussion will touch only on highlights and leave details to the references. In particular, basics are assumed and any reader requiring more information is directed to review articles to "ll in the gaps. (For a recent review see Ref. [1].) The task of B meson physics is to elucidate the relationships among quark #avors and between matter and anti-matter. Despite the enormous successes of the Standard Model, the underpinnings are either unstated (the 18 free parameters) or seemingly ad hoc (the Higgs mechanism) and the search for a more fundamental understanding of the underlying issues of electroweak symmetry breaking largely drives the course of high-energy physics as a whole today. Broadly speaking, there are two approaches, direct and indirect. The direct search for particles and phenomena outside this standard model is largely the province of experiments at the
0168-9002/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 6 0 3 - 3
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
energy frontier. The indirect search for manifestations of physics outside the standard model is a task for B Factory experiments, and others, which work at the luminosity frontier. Statistics determines the sensitivity and the reach of these experiments. B mesons o!er unique advantages, particularly as compared against the studies of charm and strange meson decays. The large b quark mass permits access to a large number of diverse "nal states. The relative smallness of < gives the B meson a long lifetime which makes possible a wide range of studies involving mixing and mixing-induced CP violation. < is nonzero, so CP violation is possible in B decay. And "nally, the top quark mass is huge, breaking GIM symmetry and making preeminent, the role of penguin diagrams in rare B decays. This last opens up numerous opportunities for the appearance of new physics and the manifestation of direct CP violation. These four characteristics lead to a rich pattern of decays and a long menu of delicious and sometimes subtle phenomena.
2. The unitarity triangle: sides The Standard Model makes only one statement about the Cabibbo}Kobayashi}Moskawa (CKM) quark mixing matrix: by construction, it is unitary. For B physics the most accessible manifestation of this statement is that the so-called unsquashed unitarity triangle be in fact a triangle. (A discussion of unitarity triangles may be found in Ref. [2].) Current knowledge of the unitarity triangle comes from measurements of branching ratios and BBM mixing which determine the lengths of the sides. We note the following: E bPulm inclusive measurements and BP(o,p)lm exclusive measurements give us "< ". The exclus ive branching ratios are measured with about 10% precision, but theoretical uncertainties in extracting "< " itself result in an overall uncer tainty of 20}25%. This latter uncertainty is due to uncertainty in the theoretical treatment of hadronization of the quark fragments. E B mixing is extremely well measured (*m " 0.477$0.017 ps\) but extraction of < is hampered by uncertainty in B f , currently esti-
33
mated by lattice QCD to be in the range of 160}220 MeV. E B mixing is still a couple years over the horizon. Q Its eventual measurement will pin down the ratio of "< "/"< " with much less theoretical uncertain ty than extracting "< " from B mixing alone. E In the far future BPoc/BPKHc will also help to pin down "< "/"< " provided theoretical un certainties about long distances contributions can be reduced. E Also in the far future, BPqm decays o!er hope. The ratio Br(BPqm)/*m &"< /< " has only mild model dependence but o!ers information similar to measurements of "< ". Thus, we see that advancement in de"ning the sides of the unitarity triangle will depend strongly on advancement in theoretical tools.
3. The unitarity triangle: angles The set of internal angles of the unitarity triangle, a( ), b( ), and c( ) can be checked for consist ency or inconsistency with the measured sides. sin 2b has already been attempted by CDF [3], and will soon be measured by almost every experiment in the world. Precision of around $0.1 or better should be available in a few years. sin 2a is a more challenging problem. Although time-dependent asymmetries in pionic B decays can be well-measured [4], the asymmetry is the result of contributions both from mixing-induced CP violation and direct CP violation. Extraction of sin 2a from the measured asymmetry requires detailed knowledge of the direct component. It is well known that for BPp>p\ this demands an isospin decomposition which in turn depends on measurements of BPpp which is expected to be very small. Thus, despite the fact that we anticipate time-dependent asymmetry measurements from SLAC and KEK in the next few years, an accurate extraction of sin 2a will require signi"cant data samples and a good understanding of the role of penguin modes. There are several approaches to the measurement of the angle c, the phase of
34
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
[5,6] which uses the ratio R ,Br(B!Pp!K)/ H 2Br(B!PpK!). With 10 M BBM events in the current data sample, this method is statistically limited and places as yet no limit on c. It will remain statistically limited for a long time, as 100 M BBM events are required to reach a precision of even $123. A method [7] based on B!P DK! decays will require &10 BBM to realize a comparable precision. A new approach proposed this year [8] trades theoretical cleanliness for higher statistical power. In this method, many rare B decay branching ratio measurements are "t to a factorization model parametrization. The "ve "tted parameters include c, which with the current (CLEO) data sample of 10 M BBM events is found to be (114$25)3. The validity or limitations of this method will be established only by examining a large sweep of branching ratio measurements, which in turn will require a substantially larger dataset than is currently available. It is likely that substantial progress on this last issue can be made with the B Factories currently in progress.
4. New physics in rare B decays As mentioned earlier, the large mass of the top quark makes penguin decays of the bottom quark signi"cant and in many cases dominant contributors to rare decays. The presence of the unseen loop particles in these decays opens up the possibility to see non-standard model physics through its indirect e!ects on either branching ratios or CP violating phases. The most sensitive test so far is the measurement of the inclusive branching ratio Br(bPsc). Stringent limits on non-standard model charged Higgs mass can be set with current measurements [9]. Direct searches at high-energy collider experiments are not competitive with this indirect method. In the next few years we can also expect to see the "rst measurements of Br(bPsl>l\) which o!er additional information through the l>l\ invariant mass, forward}backward asymmetry, and, in principle, polarization. Direct CP violation in rare B decays is also a place where new physics could show up. In rare charmless hadronic modes the Standard Model
predictions [10] for A are generally small, of !. order "A ":(0.01}0.10). Sensitivity to such asym!. metries is wholly statistics dominated [11], and scales as 1/(Luminosity; sensitivity in the $0.01 range for some rare decays will be just reachable with 10 BBM events. Non-standard model contributions can be many tens of percent. Large "nal state interaction phases, however, could amplify the standard model predictions which are based only on short distance contributions to the strong phase di!erence. Signi"cant progress in both experimental measurements and theoretical control of such e!ects will be required to distinguish new physics from old physics.
5. Issues for B-Factory luminosity upgrades The design parameters and status of the three B Factories at the time of this conference can be summarized by Table 1. The design luminosities range (2}10);10 cm\ s\. The translation of luminosity into BBM per year depends on the fraction of time the accelerator runs for highenergy physics and the fraction of that time spent at the peak of the B(4S) resonance. Assuming the product of these is 30% (an optimistic assumption), then 1;10 cm\ s\ translates to 10 M BBM per year. The formula for luminosity in a symmetric energy machine with no crossing angle makes a convenient starting point to discuss issues surrounding luminosity upgrade plans:
p m ¸"(7;10 cm\ s\) 1# W p 0.06 V I 1 cm E ; (1) 1A bH 5.3 GeV We summarize below the issues related to each of the terms in this equation, beginning with the beam current I which historically has been the parameter most amenable to improvement.
E I } Increases in beam current a demand corresponding increase in available RF power. Beam instabilities grow with current, demanding additional feedback. Vacuum quality worsens, and
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36 Table 2 Parameters of possible CESR upgrade options
Table 1 Parameters of the three B Factories
Energy (GeV) Circumference (m) Beam current (A) Number of bunches Bunch spacing (m) Crossing angle (mr) Vertical tune shift (m ) bH (mm) Bunch length (mm) Luminosity (cm\ s\) Status Peak luminosity (cm\ s\) Integrated luminosity
35
KEK-B
PEP-II
CESR-III
3.5;8.0 3016 2.6;1.1 5000 0.59 $11 0.05 10 10 10;10 running 0.4;10
3.1;9.0 2199 2.1;1.0 1658 1.26 0 0.03 15 15 3;10 running 1.4;10
5.3 768 1.0 54!90 1.8!4.2 $2.5 0.05 13 13 2;10 starting (0.8;10)
0.1 fb\
1.4 fb\
(14 fb\)
Items in ( ) refer to CESR Phase II achievements.
experimental backgrounds increase. Synchrotron radiation power increases, demanding additional cooling and additional wall plug power. Increasing the beam current may require altering the bunch structure, the bunch separation, the crossing angle, and/or adding crabbing capability. E bH } Reductions in bH require changes to IR optics and generally encroach on territory claimed by experiment detectors. Smaller bH demands correspondingly shorter bunch length to avoid hourglass degradation, and this in turn imposes demands on the RF cavity design and power load. E m } Increases in the tune shift parameter are known to be di$cult. Improvements in magnetic "eld quality, quadrupole alignment, and careful tuning of machine operating parameters all contribute. Large advances are unlikely. E p /p } Flat beam conditions (p ;p ) are W V W V normal for e>e\ machines. Round beam conditions (p "p ) are possible in principle, W V and o!er a factor of 2 enhancement of luminosity. Additional gains are expected from increased tune shift, which has been demonstrated [12] to at least double. However, constraints on bH and beam current may undermine other advances.
C Superconducting RF cavities Energy (GeV) Beam current (A) Number of bunches Bunch spacing (m) Crossing angle (mr) Tune shift m bH Bunch length (mm) Luminosity (cm\ s\)
Flat beams
Round beams
10 5.3 3.1 420 1.8 $2.3 0.06 7 7 3;10
8 5.3 2.3 45 18.7 0 0.15 30 30 2.5;10
PEP-II upgrade plans are under discussion at SLAC. Likely paths include increasing the beam currents by a factor 1.8, adding addition RF power, IR vacuum pumping, and feedback; reducing bH from 1.5 cm to 1.0 cm, with corresponding shortening of bunch length; and relaxing energy transparency conditions and improve tune shift from 0.03 to 0.05. CESR upgrade plans [13] require a new ring to be built as the bunch density in the current ring is close to maximum. A double ring design which "ts in the existing tunnel is under study. Dual aperture quadrupole magnets are required for positions closes to the IR, and prototypes meeting the necessary criteria have been developed and tested. With 3 A beam current, 7 mm bH, and 0.06 tune shift, 3;10 is possible. Round beam designs are also under study (see Table 2). Experimental backgrounds are an important consideration. Based on experience so far it is known that the dominant background comes from `Bremsstrahlunga events in which inelastic collisions with residual gas in the accelerator vacuum chamber yields o!-energy beam particles. These can strike surfaces near the detector and produce showers which lead to radiation damage of detector components, increased detector occupancy, increased trigger rates, and increased raw event size. Other players of usually lesser importance include elastic scattering o! residual gas, synchrotron radiation, beam gas events, and of course accidents. Di!erent components and sources may 2. FUTURE ACCELERATORS
36
J. Alexander / Nuclear Instruments and Methods in Physics Research A 453 (2000) 32}36
be identi"ed by a combination of pressure `bumpa studies, orbit bump studies, beam current dependence, and detailed comparison of all of these with Monte Carlo simulation. Experimental design considerations which may be used to minimize the e!ects of machine backgrounds or simply to deal with their irreducible e!ects include masking and shielding in the IR, extra pumping in the IR region, upstream collimation, front-end electronics design, and trigger design. Experience at CLEO [14], where running conditions have been stable for several years has shown that Monte Carlo simulation is an extremely important tool which can yield results that are in good agreement (factor of two) with the data in radiation dose, occupancy, and trigger rates.
6. Summary and outlook B physics addresses many of the issues related to quark #avor and identity that are raised but not answered by the Standard Model. In addition to exploring CP violation, it o!ers the chance to probe high mass scales and search through indirect e!ects for signs of new physics. Small branching ratios, a rich set of phenomena, and challenging theory all indicate that large data sets will be required to obtain a complete picture. Advances in theory will be essential but these will be driven by advances in the precision and breadth of the experimental results. Both hadron colliders and dedicated B(4S) e>e\ machines will contribute in complementary ways. The former o!er extremely high yield in simple modes, as well as B physics, while
the latter o!er broad spectrum capability. Accelerator physics will be pushed to deliver beams of high intensity and tighter focus than ever before to underwrite the scienti"c endeavours at the frontier of luminosity.
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