- Email: [email protected]
NuFact00 workshop summary
Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
NuFact00 workshop summary Stanley G. Wojcickia, Jonathan Wurteleb,* b
a Physics Department, Stanford University, Stanford, CA 94305-4060, USA Lawrence Berkeley National Laboratory (LBNL), and Physics Department, Center for Beam Physics, University of California, Berkeley, CA 94720-7300, USA
Abstract A brief summary of the workshop is presented. r 2001 Elsevier Science B.V. All rights reserved. PACS: 13.15.+g; 29.27.a Keywords: Neutrino factories
1. Physics oriented working groups The physics activities at NuFact00 were divided into three working groups. The first one, led by R. Bernstein, P. Hernandez, and O. Yasuda addressed the issue of Neutrino Oscillations at a Nu Factory. The second one, organized by M. Mangano, K. McFarland, and N. Sasao, focused on the Short Baseline n Physics and Rare Processes. The third group, led by J.J. GomezCadenas, S. Geer and Y. Mori, discussed the Interface Between Machine and Physics. Clearly, there was significant overlap between these groups and some overlap with accelerator groups, i.e., working groups 4 and 5. Thus, a number of joint sessions were held during the workshop to discuss issues spanning two or more groups. All the groups built on the results of NuFact99, significantly augmented by the work done during the year between the two workshops. The two broad goals of the first working group were quantifica*Corresponding author. E-mail address: [email protected] (J. Wurtele).
tion of the physics potential of a n factory to explore the neutrino flavor sector of the Standard Model and definition of the optimal machine parameters as defined by the physics considerations. The latter studies were then refined through interactions with working group 3. The work of the first working group is summarized in a paper by Hernandez and Yasuda. The group started out by defining what the most interesting physics issues will be 5–10 years from now, i.e. angle y13 ; sign of Dm223 ; parameter d defining leptonic CP violation, and a need to have model independent experimental confirmation of MSW effect (if that is the explanation of the solar neutrino deficit). y13 determination is probably the golden measurement at the neutrino factory, the transitions of interest being detected by observations of the wrong sign muons. For small angle MSW solution, solar oscillation parameters do not affect measurements at terrestrial distances. Studies of potential backgrounds as well as of rates suggest that an improvement in sin2 y13 of three orders of magnitude over the present limit can be expected (assuming 1021 muon decays). For large values of
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 2 7 2 - 4
324
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
y13 it was shown that the measurement without charge identification is competitive. The ne -nm transition is sensitive to the sign of Dm223 ; or equivalently to the asymmetry due to matter effects between neutrinos and antineutrinos. Thus, to measure the sign, measurements with both neutrinos and antineutrinos have to be performed. It was shown that for baselines > 2000 km, the sign can be measured relatively easily. For the large angle MSW solution, the solar parameters do affect the measurements. The CP parameter in the lepton sector, d; can be measured in this physics scenario. The extraction of the parameters is made complicated by the fact that y13 and d are strongly correlated. Because of this correlation, one cannot determine values of y13 and d at short baselines. At very long baselines, there is a large matter effect which hides the dependence on the CP violating phase. The measurements are performed optimally with baselines in the range of 2000–4000 km using the energy spectrum information for both polarities of the beam. Some work was done comparing the neutrino factory with conventional beams. The three main disadvantages of the conventional beams that have been identified are: beam background (ne ’s in a nm beam), lower intensity, and systematics in understanding the beam spectrum. Some preliminary results were shown on what could be done with low energy intense beams. The four neutrino scenarios were also investigated. The solutions consistent with current data group the four neutrinos into two doublets separated by the LSND-suggested mass difference. The mixing matrix is now significantly more complex but it can be explored in detail with the neutrino factory. Of special interest is the CP violation which is now not suppressed by the solar parameters and can be measured by looking at nm -nt transitions with baselines of the order of 10 km. The wish list for the beam parameters includes: 1021 muon decays, muon energy of at least 20 GeV and optimally 50 GeV, a baseline in the 2000–4000 km range, and capability of running beams with both polarities. Working group 2 started from the premise that the muon and neutrino non-oscillation experiments to be discussed were to be ‘‘parasitic’’, in the
sense that they would not drive the design nor the construction of a neutrino factory. However, if a compelling case can be made for one or more of these experiments, the ability of the neutrino factory to provide a diverse program of physics beyond long-baseline neutrino oscillation experiments could give a significant boost in support for a neutrino factory from the broader community. The potential for high rate experiments at a neutrino factory is clear. Such a facility would be the world’s premier source for low-energy muons, and neutrino interaction rates downstream of a 50 GeV storage ring with 800 m straight sections and 1020 muon decays per year would be 106 / kg/yr. This rate, approximately three orders of magnitude beyond the capabilities of planned conventional neutrino beams, opens up a whole new range of physics that can be done with small fully active, low density or otherwise exotic targets. One of the most promising possibilities for fruitful programmatic investigation with such a beam is the study of nucleon spin structure using neutrino beams on polarized nuclear targets. The parity-violating nature of the neutrino interaction, plus the possibility of identifying s quarks via the charged-current production of final state charm, leads to the possibility of a model-independent separation of the spin content of the nucleon among sea and valence quarks on a flavor-byflavor basis. Targets considered in the workshop were approximately 10 g=cm2 thick, 10 cm in radius, and had dilution factors in the range of 0.2–0.5. It was concluded that a significant experimental effort would be able to produce precise measurements of quark polarization, albeit over a small range of x and Q2 ; limited by the low energy of the neutrino beam. Low Z targets would also allow investigation of nuclear dependence of parity violating structure functions, e.g., to study shadowing effects. Other possibilities for high-rate neutrino experiments include precision electroweak measurements, and charm decay measurements using flavor-tagged charm. Among the former measurements, neutrino–electron scattering seemed the best candidate for a precision measurements because of the relatively clean theoretical prediction for the cross-section; practical proposal for a
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
measurement of sin2 yW at the 0.0001 or 0.0002 level from this process was presented. In the field of charm decay physics, charm continues to be an attractive lab for studying new physics beyond the SM processes. Charm mesons are produced at high rates in charged-current neutrino interactions, and each meson is tagged as a charm or anti-charm meson by the charged lepton in the final state. This leads to a sufficiently large tagged sample to perform competitive measurements of D0 2D0 mixing using semi-leptonic D decays, and in a discussion led by Debbie Harris, the working group concluded that the level of systematic uncertainty would likely be competitive with and complementary to B factory measurements. The high-rate neutrino beams also lead to the possibility of observing non-standard neutrino interactions, such as lepton flavor violating processes or t production, or interactions due to anomalous neutrino magnetic moments. The physics reach of such searches are hard to quantify without concrete models, but certainly the improved discovery potential of such experiments over those at conventional facilities is obvious. A number of interesting possibilities involving stopped or low energy muon physics were discussed, mostly in the context of improving existing experimental proposals. Experiments presented included muon to electron conversion (MECO, PRISM), muonium to anti-muonium conversion and precise measurements of GF and g 2; a number of other possibilities for studying rare muon decays (m-3e; m-eg) were discussed as well. Among the muon experiments, a common concern appeared to be the need for a debunched muon beam to avoid pile-up, which is in conflict with design requirements for the driver for a neutrino factory. In summary, a number of interesting ‘‘parasitic’’ experiments at a neutrino factory were identified, several of which have the merits to support significant experimental efforts should such a facility be built. For the neutrino experiments, a common issue was the need for the highest possible energy available, preferably at least 50 GeV, to improve prospects for these experiments. Understanding the tradeoffs between the physics motivated desires and accelerator realities was the
325
goal of working group 3. The main conclusions are summarized in the paper by Gomez-Cudenas, Geer and Mori. The major parameters converged on were: (a) Muon beam energy of 50 GeV. (b) No of decays in beam-forming straight section times detector mass of the order of 1022 kt decays. (c) Baseline greater than, or equal to, 3000 km. A start was made on understanding possible detectors based on different technologies. Much more work needs to be done here since one has to look in detail not only at the issues of cost for a given mass with a given technology but also the cuts that have to be imposed on the data for different physics experiments and the backgrounds present. There is also the question of how much detector R&D needs to be done before one arrived at a credible design. A software design tool was described which allows one to generate readily a catalogue of potential accelerator–far detector site combinations together with their properties. The work done so far focused on the existing underground sites but possibilities of developing new ones were also discussed. Requirements for near detector sites where a non-oscillation physics detector would be located have also been discussed. The rates are now very high so that a small and compact detector (ro10 cm) and short straight section (about 50 m) would be adequate. The distance from the source to the ring is determined primarily by the requirement of adequate shielding so as to reduce the backgrounds to a manageable level. A specific shielding model for a 50 m source to detector distance was studied and shown to be adequate for 1020 decays of 50 GeV muons per year. The rate (for a 30 m shielding distance and a 50 m straight section) was estimated to be about 5000 CC events/kg/1020 muons. The following requirements on beam parameters were shown to be required for a 50 GeV storage ring: (a) beam divergence o0:1=g; (b) knowledge of polarization at the level of 2% (to give flux knowledge at the 1% level),
326
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
(c) beam divergence knowledge o15%; and (d) pointing accuracy of 10 mm. (e) Polarization issues were studied and as a result the tentative conclusion was reached that polarization is desirable but not mandatory. Some studies were presented of the capabilities of conventional super beams. Such beams might become available in the first phase of the program, once a hot proton source is available. The working group provided a wish list of studies that should be done before the NuFact01. The key topics that should be studied are: (a) (b) (c) (d) (e) (f)
optimization of detector, required depth for the location of the detector, candidate detector sites, examples of non-oscillation capabilities, physics motivation for polarization, and physics capabilities of superbeams.
2. Accelerator working groups The accelerator physics activity at NuFact00 was divided into two working groups. Working Group 4, led by J. Gareyte, S. Kamada, and M. Zisman, reviewed the design, theory and simulation work for the various Neutrino Factory scenarios, while Working Group 5, led by I. Hofmann, Y. Iwashita, and M. Tigner, examined R&D plans, goals, and requirements. The immediate need for both groups was to review recent and ongoing studies from CERN, FNAL, KEK, and INP. This, in itself, was a non-trivial undertaking. The year between NuFact99 and NuFact00 was one of intensive effort. The Summary Report of Working Group 4 gives an overview of the participants’ reports. These were categorized as developments in the mainstream studies, alternate proposals that are not as advanced as the mainstream work, and speculative new ideas that require further investigation. The mainstream studies differ in their proton driver concepts, which are strongly biased by the experience and existing facilities at each labora-
tory. Target designs range from ‘conservative’ extrapolations of existing technology to more ambitious higher performance schemes. The target is inside a high-field solenoid or, possibly, a magnetic horn. Capture and cooling, in the mainstream designs, is based on solenoidal transport and ionization cooling. Two approaches to longitudinal phase rotation, induction linac and low frequency, were presented. Recirculating linac accelerators were envisioned in the US and CERN scenarios, while the KEK group is actively investigating the Fixed Field Alternating Gradient Synchrotron as an alternative. The trade-offs between simplicity of construction and polarization preservation in the racetrack and bow tie storage ring geometries were discussed. Among the presentations were the results of the FNAL Feasibility Study; the developing, benchmarking, and sharing of simulation tools such as ICOOL, DPGEANT and a modified version of PATH; the first integrated front-end simulations, significant progress in hardware design; and greater realism in the simulation effort (much of this brought about through iteration with engineers during the FNAL study). The importance of agreeing on and using a common figure of merit for the performance of the different factory scenarios was discussed. Theoretical advances were reported on beam dynamics in cooling channels. Transverse phase space dynamics in a cooling channel can now be studied with analytical methods and appropriate beam moment equations. Unfortunately, for a muon collider the transverse dynamics is coupled to the the longitudinal dynamics through emittance exchange. Furthermore, present neutrino factory designs incur substantial losses in the cooling and capture region because of a longitudinal beam phase space volume that is too large. Emittance exchange is essential for a muon collider and would likely be of benefit to the neutrino factory as well. Thus, there was a general belief that work is required, with the goal of developing a full six-dimensional theory capable of handling emittance exchange. There were lively discussions on some of the more novel ideas as well. Working Group 5 addressed the R&D program. Details of its activities can be found in its
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
Summary Report. The major R&D efforts are described in the papers of Haseroth, Mori, and Zisman. Overall, it was felt that duplication of effort was not a problem. There are projects underway to investigate a wide range of critical issues, including target survivability, muon production, scattering and energy loss in absorbers, and component development. Experimental plans include a production experiment (HARP), a scattering experiment at TRIUMF, and component development (for example, magnets, RF, and liquid helium absorber cells) at various laboratories. The balance among the R&D programs, which must address a wide range of problems, was felt to be good. An important element of the program, a muon test beam, is missing. Such a beam could be used for the development and testing of instrumentation and a cooling experiment. The discussion of which experimental program comprises a convincing demonstration of ionization cooling was somewhat contentious. Of course, part of the problem stems from the lack of an intense muon beam, and the expense, in time and money, required to develop one. Different proposals for experimental programs were put forward. One line of thinking was that the testing of components and critical physics parameters used in the simulations (such as scattering and energy loss in absorbers) could be used in conjunction with detailed numerical studies. In this (minority) view a full cooling experiment could be postponed. An opposing viewpoint was that we have no experience with muon beams and the associated instrumentation, and a full experimental program involving the cooling of a bunch and the implementation of appropriate instrumentation would be needed as soon as possible. This debate lead to one of the concrete actions taken by the Scientific Organizing Committee at the end of the Workshop. The discussions at NuFact00 made clear that we need muon test beam(s) for development of credible neutrino factory designs. Several beamlines are in use around the world and there is potential for the creation of several more at the various labs in Japan, Europe and North America. Muon beamlines are expensive and at present have
327
a low intensity. Moreover, the requirements we want to impose on such a beamline have yet to be determined. This will naturally occur as we develop a better understanding of what a capture and cooling section, and related instrumentation, will look like. The development of a muon beamline for neutrino factory studies thus has excellent potential for genuine international collaboration at the grass roots level. In order to foster such a development and to facilitate the development and discussion of what is needed for a cooling experiment, the Scientific Program Committee appointed an International Working Group on muon beamlines, led by Norbert Holtkamp. The initial Charge to the International Working Group was: (A) Survey and characterize existing (committed or under construction) muon beam lines around the world. Include also beam lines that are not now operating but could be put into operation for a modest cost. The characterization should contain energy range, energy resolution, emittance, flux, purity, physical size of the area available for use of the beam and commitments for use of the beam over the next five years. (B) Survey and characterize the expressed desires for a muon test beam. Characterization should include the estimated date when needed and estimated duration of use in addition to the characteristics listed in (A). It is, of course, expected that some of the numerical characterizations will be very approximate. The survey should also include the names of members and affiliations of the groups proposing the beam use. (C) If possible, determine whether there are existing beam lines that meet the needs expressed in (B) and suggest whether or not it will be necessary to establish an entirely new line or lines to meet the needs. (D) In the case that a new line or lines are required, for each facility identified in (A) estimate, if possible, what is required in money, manpower and time to establish the line(s).
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S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
(E) Submit the report to the Scientific Program Committee of NuFact00 by November 30, 2000 for distributionFwith or without commentFby the Committee to the entire mailing list for NuFact00. After some discussion, it was agreed that the working group would restrict itself to Task A, namely survey and characterize existing (committed or under construction) muon beam lines around the world. We look forward to the survey results. The other Tasks will wait for NuFact01 and beyond.
Acknowledgements In closing, the Workshop Chairs appreciate the efforts of the Editor, Swapan Chattopadhyay, and of the Workshop Staff, Joy Kono, Sam Vanecek, and Tom Gallager. We thank the Naval Postgraduate School and Professor William B. Colson for hosting NuFact00. We further thank LBNL and the DOE, the NSF, and Hamamatsu Corporation for financial support.
NuFact00 workshop summary Stanley G. Wojcickia, Jonathan Wurteleb,* b
a Physics Department, Stanford University, Stanford, CA 94305-4060, USA Lawrence Berkeley National Laboratory (LBNL), and Physics Department, Center for Beam Physics, University of California, Berkeley, CA 94720-7300, USA
Abstract A brief summary of the workshop is presented. r 2001 Elsevier Science B.V. All rights reserved. PACS: 13.15.+g; 29.27.a Keywords: Neutrino factories
1. Physics oriented working groups The physics activities at NuFact00 were divided into three working groups. The first one, led by R. Bernstein, P. Hernandez, and O. Yasuda addressed the issue of Neutrino Oscillations at a Nu Factory. The second one, organized by M. Mangano, K. McFarland, and N. Sasao, focused on the Short Baseline n Physics and Rare Processes. The third group, led by J.J. GomezCadenas, S. Geer and Y. Mori, discussed the Interface Between Machine and Physics. Clearly, there was significant overlap between these groups and some overlap with accelerator groups, i.e., working groups 4 and 5. Thus, a number of joint sessions were held during the workshop to discuss issues spanning two or more groups. All the groups built on the results of NuFact99, significantly augmented by the work done during the year between the two workshops. The two broad goals of the first working group were quantifica*Corresponding author. E-mail address: [email protected] (J. Wurtele).
tion of the physics potential of a n factory to explore the neutrino flavor sector of the Standard Model and definition of the optimal machine parameters as defined by the physics considerations. The latter studies were then refined through interactions with working group 3. The work of the first working group is summarized in a paper by Hernandez and Yasuda. The group started out by defining what the most interesting physics issues will be 5–10 years from now, i.e. angle y13 ; sign of Dm223 ; parameter d defining leptonic CP violation, and a need to have model independent experimental confirmation of MSW effect (if that is the explanation of the solar neutrino deficit). y13 determination is probably the golden measurement at the neutrino factory, the transitions of interest being detected by observations of the wrong sign muons. For small angle MSW solution, solar oscillation parameters do not affect measurements at terrestrial distances. Studies of potential backgrounds as well as of rates suggest that an improvement in sin2 y13 of three orders of magnitude over the present limit can be expected (assuming 1021 muon decays). For large values of
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 2 7 2 - 4
324
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
y13 it was shown that the measurement without charge identification is competitive. The ne -nm transition is sensitive to the sign of Dm223 ; or equivalently to the asymmetry due to matter effects between neutrinos and antineutrinos. Thus, to measure the sign, measurements with both neutrinos and antineutrinos have to be performed. It was shown that for baselines > 2000 km, the sign can be measured relatively easily. For the large angle MSW solution, the solar parameters do affect the measurements. The CP parameter in the lepton sector, d; can be measured in this physics scenario. The extraction of the parameters is made complicated by the fact that y13 and d are strongly correlated. Because of this correlation, one cannot determine values of y13 and d at short baselines. At very long baselines, there is a large matter effect which hides the dependence on the CP violating phase. The measurements are performed optimally with baselines in the range of 2000–4000 km using the energy spectrum information for both polarities of the beam. Some work was done comparing the neutrino factory with conventional beams. The three main disadvantages of the conventional beams that have been identified are: beam background (ne ’s in a nm beam), lower intensity, and systematics in understanding the beam spectrum. Some preliminary results were shown on what could be done with low energy intense beams. The four neutrino scenarios were also investigated. The solutions consistent with current data group the four neutrinos into two doublets separated by the LSND-suggested mass difference. The mixing matrix is now significantly more complex but it can be explored in detail with the neutrino factory. Of special interest is the CP violation which is now not suppressed by the solar parameters and can be measured by looking at nm -nt transitions with baselines of the order of 10 km. The wish list for the beam parameters includes: 1021 muon decays, muon energy of at least 20 GeV and optimally 50 GeV, a baseline in the 2000–4000 km range, and capability of running beams with both polarities. Working group 2 started from the premise that the muon and neutrino non-oscillation experiments to be discussed were to be ‘‘parasitic’’, in the
sense that they would not drive the design nor the construction of a neutrino factory. However, if a compelling case can be made for one or more of these experiments, the ability of the neutrino factory to provide a diverse program of physics beyond long-baseline neutrino oscillation experiments could give a significant boost in support for a neutrino factory from the broader community. The potential for high rate experiments at a neutrino factory is clear. Such a facility would be the world’s premier source for low-energy muons, and neutrino interaction rates downstream of a 50 GeV storage ring with 800 m straight sections and 1020 muon decays per year would be 106 / kg/yr. This rate, approximately three orders of magnitude beyond the capabilities of planned conventional neutrino beams, opens up a whole new range of physics that can be done with small fully active, low density or otherwise exotic targets. One of the most promising possibilities for fruitful programmatic investigation with such a beam is the study of nucleon spin structure using neutrino beams on polarized nuclear targets. The parity-violating nature of the neutrino interaction, plus the possibility of identifying s quarks via the charged-current production of final state charm, leads to the possibility of a model-independent separation of the spin content of the nucleon among sea and valence quarks on a flavor-byflavor basis. Targets considered in the workshop were approximately 10 g=cm2 thick, 10 cm in radius, and had dilution factors in the range of 0.2–0.5. It was concluded that a significant experimental effort would be able to produce precise measurements of quark polarization, albeit over a small range of x and Q2 ; limited by the low energy of the neutrino beam. Low Z targets would also allow investigation of nuclear dependence of parity violating structure functions, e.g., to study shadowing effects. Other possibilities for high-rate neutrino experiments include precision electroweak measurements, and charm decay measurements using flavor-tagged charm. Among the former measurements, neutrino–electron scattering seemed the best candidate for a precision measurements because of the relatively clean theoretical prediction for the cross-section; practical proposal for a
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
measurement of sin2 yW at the 0.0001 or 0.0002 level from this process was presented. In the field of charm decay physics, charm continues to be an attractive lab for studying new physics beyond the SM processes. Charm mesons are produced at high rates in charged-current neutrino interactions, and each meson is tagged as a charm or anti-charm meson by the charged lepton in the final state. This leads to a sufficiently large tagged sample to perform competitive measurements of D0 2D0 mixing using semi-leptonic D decays, and in a discussion led by Debbie Harris, the working group concluded that the level of systematic uncertainty would likely be competitive with and complementary to B factory measurements. The high-rate neutrino beams also lead to the possibility of observing non-standard neutrino interactions, such as lepton flavor violating processes or t production, or interactions due to anomalous neutrino magnetic moments. The physics reach of such searches are hard to quantify without concrete models, but certainly the improved discovery potential of such experiments over those at conventional facilities is obvious. A number of interesting possibilities involving stopped or low energy muon physics were discussed, mostly in the context of improving existing experimental proposals. Experiments presented included muon to electron conversion (MECO, PRISM), muonium to anti-muonium conversion and precise measurements of GF and g 2; a number of other possibilities for studying rare muon decays (m-3e; m-eg) were discussed as well. Among the muon experiments, a common concern appeared to be the need for a debunched muon beam to avoid pile-up, which is in conflict with design requirements for the driver for a neutrino factory. In summary, a number of interesting ‘‘parasitic’’ experiments at a neutrino factory were identified, several of which have the merits to support significant experimental efforts should such a facility be built. For the neutrino experiments, a common issue was the need for the highest possible energy available, preferably at least 50 GeV, to improve prospects for these experiments. Understanding the tradeoffs between the physics motivated desires and accelerator realities was the
325
goal of working group 3. The main conclusions are summarized in the paper by Gomez-Cudenas, Geer and Mori. The major parameters converged on were: (a) Muon beam energy of 50 GeV. (b) No of decays in beam-forming straight section times detector mass of the order of 1022 kt decays. (c) Baseline greater than, or equal to, 3000 km. A start was made on understanding possible detectors based on different technologies. Much more work needs to be done here since one has to look in detail not only at the issues of cost for a given mass with a given technology but also the cuts that have to be imposed on the data for different physics experiments and the backgrounds present. There is also the question of how much detector R&D needs to be done before one arrived at a credible design. A software design tool was described which allows one to generate readily a catalogue of potential accelerator–far detector site combinations together with their properties. The work done so far focused on the existing underground sites but possibilities of developing new ones were also discussed. Requirements for near detector sites where a non-oscillation physics detector would be located have also been discussed. The rates are now very high so that a small and compact detector (ro10 cm) and short straight section (about 50 m) would be adequate. The distance from the source to the ring is determined primarily by the requirement of adequate shielding so as to reduce the backgrounds to a manageable level. A specific shielding model for a 50 m source to detector distance was studied and shown to be adequate for 1020 decays of 50 GeV muons per year. The rate (for a 30 m shielding distance and a 50 m straight section) was estimated to be about 5000 CC events/kg/1020 muons. The following requirements on beam parameters were shown to be required for a 50 GeV storage ring: (a) beam divergence o0:1=g; (b) knowledge of polarization at the level of 2% (to give flux knowledge at the 1% level),
326
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
(c) beam divergence knowledge o15%; and (d) pointing accuracy of 10 mm. (e) Polarization issues were studied and as a result the tentative conclusion was reached that polarization is desirable but not mandatory. Some studies were presented of the capabilities of conventional super beams. Such beams might become available in the first phase of the program, once a hot proton source is available. The working group provided a wish list of studies that should be done before the NuFact01. The key topics that should be studied are: (a) (b) (c) (d) (e) (f)
optimization of detector, required depth for the location of the detector, candidate detector sites, examples of non-oscillation capabilities, physics motivation for polarization, and physics capabilities of superbeams.
2. Accelerator working groups The accelerator physics activity at NuFact00 was divided into two working groups. Working Group 4, led by J. Gareyte, S. Kamada, and M. Zisman, reviewed the design, theory and simulation work for the various Neutrino Factory scenarios, while Working Group 5, led by I. Hofmann, Y. Iwashita, and M. Tigner, examined R&D plans, goals, and requirements. The immediate need for both groups was to review recent and ongoing studies from CERN, FNAL, KEK, and INP. This, in itself, was a non-trivial undertaking. The year between NuFact99 and NuFact00 was one of intensive effort. The Summary Report of Working Group 4 gives an overview of the participants’ reports. These were categorized as developments in the mainstream studies, alternate proposals that are not as advanced as the mainstream work, and speculative new ideas that require further investigation. The mainstream studies differ in their proton driver concepts, which are strongly biased by the experience and existing facilities at each labora-
tory. Target designs range from ‘conservative’ extrapolations of existing technology to more ambitious higher performance schemes. The target is inside a high-field solenoid or, possibly, a magnetic horn. Capture and cooling, in the mainstream designs, is based on solenoidal transport and ionization cooling. Two approaches to longitudinal phase rotation, induction linac and low frequency, were presented. Recirculating linac accelerators were envisioned in the US and CERN scenarios, while the KEK group is actively investigating the Fixed Field Alternating Gradient Synchrotron as an alternative. The trade-offs between simplicity of construction and polarization preservation in the racetrack and bow tie storage ring geometries were discussed. Among the presentations were the results of the FNAL Feasibility Study; the developing, benchmarking, and sharing of simulation tools such as ICOOL, DPGEANT and a modified version of PATH; the first integrated front-end simulations, significant progress in hardware design; and greater realism in the simulation effort (much of this brought about through iteration with engineers during the FNAL study). The importance of agreeing on and using a common figure of merit for the performance of the different factory scenarios was discussed. Theoretical advances were reported on beam dynamics in cooling channels. Transverse phase space dynamics in a cooling channel can now be studied with analytical methods and appropriate beam moment equations. Unfortunately, for a muon collider the transverse dynamics is coupled to the the longitudinal dynamics through emittance exchange. Furthermore, present neutrino factory designs incur substantial losses in the cooling and capture region because of a longitudinal beam phase space volume that is too large. Emittance exchange is essential for a muon collider and would likely be of benefit to the neutrino factory as well. Thus, there was a general belief that work is required, with the goal of developing a full six-dimensional theory capable of handling emittance exchange. There were lively discussions on some of the more novel ideas as well. Working Group 5 addressed the R&D program. Details of its activities can be found in its
S.G. Wojcicki, J. Wurtele / Nuclear Instruments and Methods in Physics Research A 472 (2001) 323–328
Summary Report. The major R&D efforts are described in the papers of Haseroth, Mori, and Zisman. Overall, it was felt that duplication of effort was not a problem. There are projects underway to investigate a wide range of critical issues, including target survivability, muon production, scattering and energy loss in absorbers, and component development. Experimental plans include a production experiment (HARP), a scattering experiment at TRIUMF, and component development (for example, magnets, RF, and liquid helium absorber cells) at various laboratories. The balance among the R&D programs, which must address a wide range of problems, was felt to be good. An important element of the program, a muon test beam, is missing. Such a beam could be used for the development and testing of instrumentation and a cooling experiment. The discussion of which experimental program comprises a convincing demonstration of ionization cooling was somewhat contentious. Of course, part of the problem stems from the lack of an intense muon beam, and the expense, in time and money, required to develop one. Different proposals for experimental programs were put forward. One line of thinking was that the testing of components and critical physics parameters used in the simulations (such as scattering and energy loss in absorbers) could be used in conjunction with detailed numerical studies. In this (minority) view a full cooling experiment could be postponed. An opposing viewpoint was that we have no experience with muon beams and the associated instrumentation, and a full experimental program involving the cooling of a bunch and the implementation of appropriate instrumentation would be needed as soon as possible. This debate lead to one of the concrete actions taken by the Scientific Organizing Committee at the end of the Workshop. The discussions at NuFact00 made clear that we need muon test beam(s) for development of credible neutrino factory designs. Several beamlines are in use around the world and there is potential for the creation of several more at the various labs in Japan, Europe and North America. Muon beamlines are expensive and at present have
327
a low intensity. Moreover, the requirements we want to impose on such a beamline have yet to be determined. This will naturally occur as we develop a better understanding of what a capture and cooling section, and related instrumentation, will look like. The development of a muon beamline for neutrino factory studies thus has excellent potential for genuine international collaboration at the grass roots level. In order to foster such a development and to facilitate the development and discussion of what is needed for a cooling experiment, the Scientific Program Committee appointed an International Working Group on muon beamlines, led by Norbert Holtkamp. The initial Charge to the International Working Group was: (A) Survey and characterize existing (committed or under construction) muon beam lines around the world. Include also beam lines that are not now operating but could be put into operation for a modest cost. The characterization should contain energy range, energy resolution, emittance, flux, purity, physical size of the area available for use of the beam and commitments for use of the beam over the next five years. (B) Survey and characterize the expressed desires for a muon test beam. Characterization should include the estimated date when needed and estimated duration of use in addition to the characteristics listed in (A). It is, of course, expected that some of the numerical characterizations will be very approximate. The survey should also include the names of members and affiliations of the groups proposing the beam use. (C) If possible, determine whether there are existing beam lines that meet the needs expressed in (B) and suggest whether or not it will be necessary to establish an entirely new line or lines to meet the needs. (D) In the case that a new line or lines are required, for each facility identified in (A) estimate, if possible, what is required in money, manpower and time to establish the line(s).
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(E) Submit the report to the Scientific Program Committee of NuFact00 by November 30, 2000 for distributionFwith or without commentFby the Committee to the entire mailing list for NuFact00. After some discussion, it was agreed that the working group would restrict itself to Task A, namely survey and characterize existing (committed or under construction) muon beam lines around the world. We look forward to the survey results. The other Tasks will wait for NuFact01 and beyond.
Acknowledgements In closing, the Workshop Chairs appreciate the efforts of the Editor, Swapan Chattopadhyay, and of the Workshop Staff, Joy Kono, Sam Vanecek, and Tom Gallager. We thank the Naval Postgraduate School and Professor William B. Colson for hosting NuFact00. We further thank LBNL and the DOE, the NSF, and Hamamatsu Corporation for financial support.