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ESSnuSB: a Project for Leptonic CP Violation Discovery based on the European Spallation Source Linac
Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 265–266 (2015) 195–197 www.elsevier.com/locate/nppp
ESSnuSB: a Project for Leptonic CP Violation Discovery based on the European Spallation Source Linac E. Wildner CERN, 1211 Geneva 23
On behalf of the ESSnuSB Collaboration
Abstract Very intense neutrino beams and large neutrino detectors will be needed in order to discover of CP violation in the leptonic sector. The proton driver of the European Spallation Source (ESS) currently under construction in Lund, Sweden, could provide, in parallel with the spallation neutron production, a very intense, cost effective and high performance neutrino beam. The ESS linac will be fully operational at 5 MW average power by 2022. The proposal presented here is to upgrade the linac to give another 5 MW to produce neutrinos i.e. 10 MW average power, in total. The high current required in the pulsed neutrino horn, requires proton pulses compressed from 2.86 ms to a few μs in an accumulator ring. A long baseline experiment using this Super Beam and a megaton underground Water Cherenkov detector located in existing mines 300-600 km from Lund, will enable precision measurements at the second νμ → νe neutrino oscillation maximum and thereby discover leptonic CP violation at 5 σ significance level in more than 50% of the leptonic Dirac CP-violating phase range. The detector would also be used to measure the proton lifetime, detect cosmological neutrinos and neutrinos from supernova explosions. The presentation will describe the proposed facility and the physics possible with this experiment. Keywords: neutrino, leptonic CP violation, Super Beam, ESSnuSB, ESS, Water Cherenkov detector
1. Introduction In 2012 the last unknown leptonic mixing angle θ13 was measured and found to be non zero and relatively large [1]. This makes it possible to observe possible CP violation (CPV) in the leptonic sector using classical neutrino beams. Moreover, the presently measured values of θ13 enhances the performance of experiments with the detector placed at the second oscillation maximum as compared to those with the detector placed at the first oscillation maximum [2]. TheEuropean Spallation Source Neutrino Super Beam (ESSnuSB) project succeeds the studies made by the FP7 Design Study EUROnu [3], regarding future neutrino facilities and in particular the EUROnu Super Beam (SB) using the MEMPHYS large Water Cherenkov detector in the Fr´ejus tunnel located at the http://dx.doi.org/10.1016/j.nuclphysbps.2015.06.050 2405-6014/© 2015 Published by Elsevier B.V.
first neutrino oscillation maximum (130 km). ESSnuSB [4] proposes to study a SB facility, that uses the high power linac of the European Spallation Source (ESS) in Lund (Sweden) as proton driver with a MEMPHYS type detector located in a deep mine at between 300 to 600 km distance, near the second neutrino oscillation maximum. The primary purpose of ESS is the production and use of spallation neutrons. The ESS proton driver (2.0 GeV protons, 5 MW) will be used for the two applications with no reduction of the neutron production.The requirement to reduce the 2.86 μs long pulses of the ESS linac to a few μs long pulses is imposed by the necessity to have short current pulses in the hadronic collector (horn). The linac pulse will therefore be injected and accumulated in a proton accumulator ring. To be
196
E. Wildner / Nuclear and Particle Physics Proceedings 265–266 (2015) 195–197
able to inject efficiently 5 MW into the accumulator, H− ions will be accelerated in the linac. Detailed studies of the modifications of the ESS proton linac required to allow simultaneous acceleration of H+ and H− ions at an average power of 5+5 MW are ongoing. Necessary upgrades have to be identified early on to enable the smooth construction of the ESSnuSB and to keep costs low. Future spallation neutron users at ESS will also be able to profit of short pulses. The extra costs of the H− beam and the accumulator ring could therefore be shared. The highly synergetic H− beam and the ring will be studied with the aim to design a common facility satisfying the requirements for both the short pulse neutron measurements and the neutrino measurements. Simulations of the optimal distance of the detector from the neutrino source will determine the mine in Sweden that is at a distance closest to the optimum. ESSnuSB will profit from the studies already performed within the FP7 LAGUNA Design Study [5]. 2. The facility The H− pulses for ESSnuSB will be injected between the proton pulses and the H− -beam will be transferred to the accumulator at highest available energy. The accumulator, the target station and the different transfer lines will be implanted on the ESS site, taking into account available space and radiation limits (figure 1). An additional source for H− ions will also be needed.
Table 1: The ESS Linac parameters.
Parameter Average beam power Proton kinetic energy Proton kinetic energy Macropulse length Pulse repetition rate Max. accelerating cavity surface field Max. linac length (no contingency) Annual operating period Reliability Protons on target/year
Value 5 MW 2.0 GeV 62.5 mA 2.86 ms 14 Hz 45 MV/m 352.5 m 5000 h 95% > 2.7 1023
of filling the cavities more often. Alternatively, 4 accumulators could be stacked and the incoming linac beam would then be switched between the four rings. The accumulator is needed to shorten the current pulses in the focusing structures downstream of the targets (horns). The pulse length (related to the circumference of the accumulator) and the delivery sequence from the accumulator to the targets have to satisfy technological constraints of the current generators and the focussing systems of the target station. Simulations of the final design of the accumulator ring, including machine impedances, will give indications of dimensions of the the ring and the beam pipe diameter to guarantee a stable beam. The total mean power of the ESS linac, running the two facilities simultaneously, will be 10 MW.
2.1. ESS ESS will provide slow neutrons for research laboratories and industry. A first proton beam for neutron production will be delivered at reduced energy and power by 2019. Full design power of 5 MW (2 GeV) will be delivered by 2022. There will be 14 pulses of 62.5 mA current and 2.86 ms length containing 1.1 1015 protons, per second. The ESS linac parameters are shown in table 1. 2.2. ESSnuSB accelerators and target The duty factor of the ESS linac is relatively low (4%), therefore extra pulses can be generated for production of neutrinos. 2.86 ms pulses can be inserted between the neutron pulses, which would give a repetition rate of 28 Hz. In case 1.1 1015 charges cannot be comfortably stored in the accumulator, the repetition rate could be increased to 70 Hz to permit 4 additional acceleration cycles of shorter pulses (2.86/4 ms long) . This implies some increase in power loss by the need
Figure 1: Sketch of the ESS site including a possible accumulator ring design, a target station and the different transfer lines.
The accumulator receives the linac beam pulse in a continuous way during several turns. To do this efficiently, H− stripping injection is used, ideally laser stripping (still an R&D topic [6]), however foil stripping is a technology in use today. Simulations show that foil stripping is possible to use as an option for this application [7]. An extra ion source for H− production has to be
E. Wildner / Nuclear and Particle Physics Proceedings 265–266 (2015) 195–197
installed and the H− ions would be injected after the Radio Frequency Quadrupole of the linac. The H− beam will be extracted and guided to the accumulator at the highest available energy of the linac, presently 2 GeV. Feasibility studies and tests of necessary upgrades of the linac for the neutrino facility (linac modulators, power generation and distribution, new buildings, accelerator physics aspects, radiation and Lorenz stripping in H− transfer lines) are ongoing as well as civil engineering and layout studies of the transfer lines and accumulator rings. Four targets will, in turn, be hit by a 1.25 MW proton beam to produce the pions needed. A packed bed of titanium spheres cooled with cold helium gas is the baseline target design for a Super Beam based on a 2.55 GeV proton beam with a power of up to ∼1 MW per target [3]. Figure 2 displays the total number of neutrinos crossing a surface of 100 m2 placed on-axis at 100 km from the target during 200 days. The total contamination for positive (negative) polarity coming from ν¯ μ (νμ ), ν¯ e and νe is 2.3% (5.1%).
197
hierarchy can be determined at more than 3 σ confidence level for baselines in the range 300 km to 500 km.
Figure 3: The second oscillation maximum is well covered by the ESS neutrino spectrum
Figure 4 presents a comparison with other projects of the fraction of the full δCP range covered for which a one standard deviation error in δCP , ΔδCP , could be achieved [4]. The same values for the systematic errors have been assumed for all experiments in the figure. This plot shows that only the Neutrino Factory (IDS-NF [3]) would have a better performance than ESSnuSB.
Figure 2: Neutrino fluence as a function of energy at a distance of 100 km on axis from the target station, for 2.0 GeV protons and positive (left) and negative (right) horn current polarity, respectively.
2.3. Detectors A study of the geological parameters and existing mining infrastructures for Garpenberg mine located at 540 km from the ESS site in Lund. Once a suitable choice of location for the MEMPHIS detector underground detector, of total volume of 6 105 m3 , has been determined, the geometry and construction methods of the required underground halls will be studied in further detail. 3. Physics with ESSnuSB First evaluations indicates that leptonic CP violation could be discovered at 5 σ confidence level within at least 50% of the CP phase range for baselines in the range 300 km to 550 km with an optimum of about 58% of the phase range at a baseline of about 420 km. According to the same first evaluations, the neutrino mass
Figure 4: Fraction of the full δCP range for which 1 σ error of ΔδCP or better could be achieved evaluated for several of the facilities proposed today.
References [1] [2] [3] [4] [5]
F. Anet, et al., Phys. Rev. Lett.108 (2012) 171803. T. Abe, et al., Phys. Rev. Lett. 107 (2011) 041801. R. Edgecock, et al., Phys. Rev. STAB 16 (2013) 021002. E. Baussan, et al., Nucl.Phys.B885(2014)127. LAGUNA - Large Apparatus studying Grand Unification and Neutrino Astrophysics, FP7 INFRA-2007-2.1-01, ref. 212343. [6] W. Bartmann, CERN Thesis, http://cds.cern.ch/record/1209262. [7] H. Schonauer, et al., Private communication.
Nuclear and Particle Physics Proceedings 265–266 (2015) 195–197 www.elsevier.com/locate/nppp
ESSnuSB: a Project for Leptonic CP Violation Discovery based on the European Spallation Source Linac E. Wildner CERN, 1211 Geneva 23
On behalf of the ESSnuSB Collaboration
Abstract Very intense neutrino beams and large neutrino detectors will be needed in order to discover of CP violation in the leptonic sector. The proton driver of the European Spallation Source (ESS) currently under construction in Lund, Sweden, could provide, in parallel with the spallation neutron production, a very intense, cost effective and high performance neutrino beam. The ESS linac will be fully operational at 5 MW average power by 2022. The proposal presented here is to upgrade the linac to give another 5 MW to produce neutrinos i.e. 10 MW average power, in total. The high current required in the pulsed neutrino horn, requires proton pulses compressed from 2.86 ms to a few μs in an accumulator ring. A long baseline experiment using this Super Beam and a megaton underground Water Cherenkov detector located in existing mines 300-600 km from Lund, will enable precision measurements at the second νμ → νe neutrino oscillation maximum and thereby discover leptonic CP violation at 5 σ significance level in more than 50% of the leptonic Dirac CP-violating phase range. The detector would also be used to measure the proton lifetime, detect cosmological neutrinos and neutrinos from supernova explosions. The presentation will describe the proposed facility and the physics possible with this experiment. Keywords: neutrino, leptonic CP violation, Super Beam, ESSnuSB, ESS, Water Cherenkov detector
1. Introduction In 2012 the last unknown leptonic mixing angle θ13 was measured and found to be non zero and relatively large [1]. This makes it possible to observe possible CP violation (CPV) in the leptonic sector using classical neutrino beams. Moreover, the presently measured values of θ13 enhances the performance of experiments with the detector placed at the second oscillation maximum as compared to those with the detector placed at the first oscillation maximum [2]. TheEuropean Spallation Source Neutrino Super Beam (ESSnuSB) project succeeds the studies made by the FP7 Design Study EUROnu [3], regarding future neutrino facilities and in particular the EUROnu Super Beam (SB) using the MEMPHYS large Water Cherenkov detector in the Fr´ejus tunnel located at the http://dx.doi.org/10.1016/j.nuclphysbps.2015.06.050 2405-6014/© 2015 Published by Elsevier B.V.
first neutrino oscillation maximum (130 km). ESSnuSB [4] proposes to study a SB facility, that uses the high power linac of the European Spallation Source (ESS) in Lund (Sweden) as proton driver with a MEMPHYS type detector located in a deep mine at between 300 to 600 km distance, near the second neutrino oscillation maximum. The primary purpose of ESS is the production and use of spallation neutrons. The ESS proton driver (2.0 GeV protons, 5 MW) will be used for the two applications with no reduction of the neutron production.The requirement to reduce the 2.86 μs long pulses of the ESS linac to a few μs long pulses is imposed by the necessity to have short current pulses in the hadronic collector (horn). The linac pulse will therefore be injected and accumulated in a proton accumulator ring. To be
196
E. Wildner / Nuclear and Particle Physics Proceedings 265–266 (2015) 195–197
able to inject efficiently 5 MW into the accumulator, H− ions will be accelerated in the linac. Detailed studies of the modifications of the ESS proton linac required to allow simultaneous acceleration of H+ and H− ions at an average power of 5+5 MW are ongoing. Necessary upgrades have to be identified early on to enable the smooth construction of the ESSnuSB and to keep costs low. Future spallation neutron users at ESS will also be able to profit of short pulses. The extra costs of the H− beam and the accumulator ring could therefore be shared. The highly synergetic H− beam and the ring will be studied with the aim to design a common facility satisfying the requirements for both the short pulse neutron measurements and the neutrino measurements. Simulations of the optimal distance of the detector from the neutrino source will determine the mine in Sweden that is at a distance closest to the optimum. ESSnuSB will profit from the studies already performed within the FP7 LAGUNA Design Study [5]. 2. The facility The H− pulses for ESSnuSB will be injected between the proton pulses and the H− -beam will be transferred to the accumulator at highest available energy. The accumulator, the target station and the different transfer lines will be implanted on the ESS site, taking into account available space and radiation limits (figure 1). An additional source for H− ions will also be needed.
Table 1: The ESS Linac parameters.
Parameter Average beam power Proton kinetic energy Proton kinetic energy Macropulse length Pulse repetition rate Max. accelerating cavity surface field Max. linac length (no contingency) Annual operating period Reliability Protons on target/year
Value 5 MW 2.0 GeV 62.5 mA 2.86 ms 14 Hz 45 MV/m 352.5 m 5000 h 95% > 2.7 1023
of filling the cavities more often. Alternatively, 4 accumulators could be stacked and the incoming linac beam would then be switched between the four rings. The accumulator is needed to shorten the current pulses in the focusing structures downstream of the targets (horns). The pulse length (related to the circumference of the accumulator) and the delivery sequence from the accumulator to the targets have to satisfy technological constraints of the current generators and the focussing systems of the target station. Simulations of the final design of the accumulator ring, including machine impedances, will give indications of dimensions of the the ring and the beam pipe diameter to guarantee a stable beam. The total mean power of the ESS linac, running the two facilities simultaneously, will be 10 MW.
2.1. ESS ESS will provide slow neutrons for research laboratories and industry. A first proton beam for neutron production will be delivered at reduced energy and power by 2019. Full design power of 5 MW (2 GeV) will be delivered by 2022. There will be 14 pulses of 62.5 mA current and 2.86 ms length containing 1.1 1015 protons, per second. The ESS linac parameters are shown in table 1. 2.2. ESSnuSB accelerators and target The duty factor of the ESS linac is relatively low (4%), therefore extra pulses can be generated for production of neutrinos. 2.86 ms pulses can be inserted between the neutron pulses, which would give a repetition rate of 28 Hz. In case 1.1 1015 charges cannot be comfortably stored in the accumulator, the repetition rate could be increased to 70 Hz to permit 4 additional acceleration cycles of shorter pulses (2.86/4 ms long) . This implies some increase in power loss by the need
Figure 1: Sketch of the ESS site including a possible accumulator ring design, a target station and the different transfer lines.
The accumulator receives the linac beam pulse in a continuous way during several turns. To do this efficiently, H− stripping injection is used, ideally laser stripping (still an R&D topic [6]), however foil stripping is a technology in use today. Simulations show that foil stripping is possible to use as an option for this application [7]. An extra ion source for H− production has to be
E. Wildner / Nuclear and Particle Physics Proceedings 265–266 (2015) 195–197
installed and the H− ions would be injected after the Radio Frequency Quadrupole of the linac. The H− beam will be extracted and guided to the accumulator at the highest available energy of the linac, presently 2 GeV. Feasibility studies and tests of necessary upgrades of the linac for the neutrino facility (linac modulators, power generation and distribution, new buildings, accelerator physics aspects, radiation and Lorenz stripping in H− transfer lines) are ongoing as well as civil engineering and layout studies of the transfer lines and accumulator rings. Four targets will, in turn, be hit by a 1.25 MW proton beam to produce the pions needed. A packed bed of titanium spheres cooled with cold helium gas is the baseline target design for a Super Beam based on a 2.55 GeV proton beam with a power of up to ∼1 MW per target [3]. Figure 2 displays the total number of neutrinos crossing a surface of 100 m2 placed on-axis at 100 km from the target during 200 days. The total contamination for positive (negative) polarity coming from ν¯ μ (νμ ), ν¯ e and νe is 2.3% (5.1%).
197
hierarchy can be determined at more than 3 σ confidence level for baselines in the range 300 km to 500 km.
Figure 3: The second oscillation maximum is well covered by the ESS neutrino spectrum
Figure 4 presents a comparison with other projects of the fraction of the full δCP range covered for which a one standard deviation error in δCP , ΔδCP , could be achieved [4]. The same values for the systematic errors have been assumed for all experiments in the figure. This plot shows that only the Neutrino Factory (IDS-NF [3]) would have a better performance than ESSnuSB.
Figure 2: Neutrino fluence as a function of energy at a distance of 100 km on axis from the target station, for 2.0 GeV protons and positive (left) and negative (right) horn current polarity, respectively.
2.3. Detectors A study of the geological parameters and existing mining infrastructures for Garpenberg mine located at 540 km from the ESS site in Lund. Once a suitable choice of location for the MEMPHIS detector underground detector, of total volume of 6 105 m3 , has been determined, the geometry and construction methods of the required underground halls will be studied in further detail. 3. Physics with ESSnuSB First evaluations indicates that leptonic CP violation could be discovered at 5 σ confidence level within at least 50% of the CP phase range for baselines in the range 300 km to 550 km with an optimum of about 58% of the phase range at a baseline of about 420 km. According to the same first evaluations, the neutrino mass
Figure 4: Fraction of the full δCP range for which 1 σ error of ΔδCP or better could be achieved evaluated for several of the facilities proposed today.
References [1] [2] [3] [4] [5]
F. Anet, et al., Phys. Rev. Lett.108 (2012) 171803. T. Abe, et al., Phys. Rev. Lett. 107 (2011) 041801. R. Edgecock, et al., Phys. Rev. STAB 16 (2013) 021002. E. Baussan, et al., Nucl.Phys.B885(2014)127. LAGUNA - Large Apparatus studying Grand Unification and Neutrino Astrophysics, FP7 INFRA-2007-2.1-01, ref. 212343. [6] W. Bartmann, CERN Thesis, http://cds.cern.ch/record/1209262. [7] H. Schonauer, et al., Private communication.