Roadmap for the international, accelerator-based neutrino programme
J. Cao, A. de Gouvea, D. Duchesneau, S. Geer, R. Gomes, S.B. Kim, T. Kobayashi, K.R. Long, M. Maltoni, M. Mezzetto, N. Mondal, M. Shiozawa, J. Sobczyk, H.A. Tanaka, M. Wascko, G. Zeller
AApril 27, 2017 The ICFA Neutrino Panel Final (revision 1)
Roadmap for the international, accelerator-based neutrinoprogramme
The ICFA Neutrino Panel
Overview
The neutrino, with its tiny mass and large mixings, offers a window on physics beyond the StandardModel. Precise measurements made using terrestrial and astrophysical sources are required tounderstand the nature of the neutrino, to elucidate the phenomena that give rise to its uniqueproperties and to determine its impact on the evolution of the Universe. Accelerator-driven sourcesof neutrinos will play a critical role in determining its unique properties since such sources providethe only means by which neutrino and anti-neutrino transitions between all three neutrino flavourscan be studied precisely.In line with its terms of reference [1] the ICFA Neutrino Panel [2] has developed a roadmapfor the international, accelerator-based neutrino programme. A “roadmap discussion document”[3] was presented in May 2016 taking into account the peer-group-consultation described in thePanel’s initial report [4]. The “roadmap discussion document” was used to solicit feedback fromthe neutrino community—and more broadly, the particle- and astroparticle-physics communities—and the various stakeholders in the programme. The roadmap, the conclusions and recommenda-tions presented in this document are consistent with the conclusions drawn in [4] and take intoaccount the comments received following the publication of the roadmap discussion document.With its roadmap the Panel documents the approved objectives and milestones of the experi-ments that are presently in operation or under construction. Approval, construction and exploita-tion milestones are presented for experiments that are being considered for approval. The timetableproposed by the proponents is presented for experiments that are not yet being considered formallyfor approval. Based on this information, the evolution of the precision with which the critical pa-rameters governing the neutrino are known has been evaluated. Branch or decision points havebeen identified based on the anticipated evolution in precision. The branch or decision points havein turn been used to identify desirable timelines for the neutrino-nucleus cross section and hadro-production measurements that are required to maximise the integrated scientific output of the pro-gramme. The branch points have also been used to identify the timeline for the R&D required totake the programme beyond the horizon of the next generation of experiments. The theory andphenomenology programme, including nuclear theory, required to ensure that maximum benefit isderived from the experimental programme is also discussed. a r X i v : . [ h e p - e x ] A p r ontents Introduction
The Standard Model (SM) gives a precise, quantitative description of the fundamental constituents of matterand the forces through which they interact. The study of the properties and interactions of the neutrino hasbeen seminal in the development of the electroweak theory and decisive in the development of the quark-partonmodel and quantum chromodynamics. Recently, the discovery of neutrino oscillations, which implies thatneutrinos have mass and that the flavour eigenstates mix, has led to the realisation that the SM is incomplete.Measurements of the parameters that govern neutrino oscillations will have a profound impact on our un-derstanding of particle physics, astrophysics and cosmology. Such a breadth of impact justifies a far-reachingexperimental programme that exploits terrestrial and astrophysical sources of neutrino. Accelerator-based mea-surements of neutrino oscillations are an essential component of the programme since they are the only meansby which each of the possible appearance channels can be studied with sufficient precision.In 2013 the International Committee for Future Accelerators (ICFA) [5] established a Neutrino Panel [2]with the mandate to “ promote international cooperation in the development of the accelerator-based neutrino-oscillation program and to promote international collaboration in the development of a neutrino factory as afuture intense source of neutrinos for particle physics experiments [6]. In its Initial Report the Panel outlinedan ambitious programme by which the discovery potential of the accelerator-based neutrino programme couldbe optimised [4]. Greater international cooperation was identified as being key to the successful deliveryof this programme. The establishment of the Deep Underground Neutrino Experiment (DUNE) [7, 8], theShort Baseline Programme (SBN) [9] and the Long Baseline Neutrino Facility (LBNF) at the Fermi NationalAccelerator Laboratory (FNAL) [10] and the CERN Neutrino Platform [11] as international facilities for theadvancement of the field represents substantial progress in the necessary internationalisation of the programme.The Panel notes the recent developments in the consideration of the international Hyper-K programme [12–14].Together, the complementary long-baseline experiments DUNE and Hyper-K [15], the SBN programme andthe CERN Neutrino Platform will provide the basis for a robust discovery programme.To complete our understanding of neutrino oscillations it will be necessary to determine [4]:• Whether mixing among the three neutrino flavours violates the matter-antimatter (CP) symmetry. Suchleptonic CP-invariance violation (CPiV) would be something new and might have cosmological conse-quences;• The ordering of the three neutrino mass eigenstates is and what is the absolute neutrino-mass scale;• Whether empirical relationships between neutrino-mixing parameters, or between neutrino- and quark-mixing parameters, can be established and whether the neutrino is its own antiparticle; and• Whether the few measurements of neutrino oscillations that are not readily accommodated within theelegant framework of three-neutrino mixing are statistical fluctuations, systematic effects or indicationsthat there is even more to discover.By addressing these issues it may be possible to develop a theory that can explain why neutrino masses areso tiny, at least a million times smaller than any other known matter particle, and why the strength of mixingamong the neutrino flavours is so much stronger than the mixing among the quarks.The purpose of this document is to present concisely the elements of the global accelerator-based neutrinoprogramme in such a way that branch or decision points can be identified. The accelerator-based experimentsthat are in operation or that are being planned will make substantial contributions to this programme and havethe potential to discover leptonic CP-invariance violation, determine the mass hierarchy and, perhaps, findevidence for sterile neutrinos. Improvements in accelerator and detector techniques will be required to takethe programme forward, for example, to seek to establish empirical relationships between neutrino- and quark-mixing parameters. Therefore, the accelerator and detector R&D programmes are considered alongside theneutrino-experiment programme. Consideration of the accelerator-based programme as a whole will allow5hoices to be made that exploit regional strengths and ambitions to optimise the discovery potential. In thisway the impact of each contribution on the global programme will be maximised.The non-accelerator-based neutrino-physics programme is vibrant and plays a crucial role in determining theproperties of the neutrino. The Neutrino Panel’s terms of reference [1] restrict the scope of its recommendationsto the accelerator-based programme.
A vibrant programme that is able to attract the interest of researchers and the support of funding agencies andlaboratories must:• Have both discovery potential and deliver critical measurements in the short term ( < years) and in themedium term (between 5 and 15 years); and• Develop the capabilities required to build on and go beyond the performance of the near- and medium-term experiments through appropriately-resourced detector and accelerator R&D programmes.By preparing this roadmap, the Panel seeks to identify an accelerator-based programme that satisfies theseimperatives. In the short term (less than five years), experiments such as T2K [16], NO ν A [17], MicroBooNE[18] and MINER ν A [19] will provide a steady stream of results. Over the five- to fifteen-year timescale, themedium-term programme will seek evidence for CPiV by exploiting the Deep Underground Neutrino Experi-ment (DUNE) in the USA and the Hyper-K experiment in Japan. The branch or decision points identified in theanalysis of the roadmap are intended to facilitate the discussions necessary to maximise the scientific output of,and technological benefit from, the global investment in the accelerator-based neutrino programme.The categories into which the neutrino programme has been broken down are:1. The study of neutrino oscillations;2. Searches for sterile neutrinos;3. The supporting programme that includes:(a) The study of hadro-production and neutrino-nucleus scattering necessary to allow the neutrino fluxand interaction rates to be estimated with the requisite precision; and(b) The detector, accelerator and software R&D that supports the current programme and builds capa-bility for the discovery programme in the medium to long term;4. Experiments that use neutrinos produced by nuclear reactors;5. Experiments that exploit non-terrestrial sources and radio-active sources; and6. The non-oscillation programme.Categories 1, 2 and 3 constitute the accelerator-based neutrino discovery and measurement programmes. Cate-gories 4 and 5 are included as the results from these programmes may impact the accelerator-based programme.The needs of the discovery and measurement programmes were considered when analysing the timetable forthe programme of supporting measurements and R&D (category 3). The objectives and approved or proposedtimetable is reported for each of the projects considered together with an indication of the number of scientistsengaged in the execution or development of the activity.The Panel’s terms of reference [1] relate to the accelerator-based neutrino programme. In line with theseterms of reference, the Panel provides conclusions and recommendations on: the accelerator-based long-baseline neutrino-oscillation programme; sterile-neutrino searches with accelerator-based experiments; and thesupporting programme. For completeness, sections 5 to 7 contain brief summaries of the non-accelerator-basedprogramme. The Panel’s conclusions and recommendations are summarised in section 8.6 .2 The international accelerator-based neutrino-physics community
It is of interest to consider the strength of the neutrino-physics community that exploits accelerator-generatedneutrino beams. Surveys of the particle-physics community have been carried out on a national or regionalbasis [20, 21]. No consistent survey of the international accelerator-based neutrino community exists, makingit necessary to draw information from a number of sources to give an indication of the size of the community.The European Committee on Future Accelerators (ECFA) [22] carries out a regular survey of the particle-physics community in Europe. The most recent [20], published in 2010, was based on data collected in 2009.The ECFA survey counted personnel, rather than “full-time equivalents”; only persons spending 20% or moreof their time on particle-physics activities were counted, each such person was counted with a weight of one.The fraction of research time allocated by a particular individual to a particular project was then recorded.Personnel were classified as permanent staff, time-limited researchers, PhD students and engineers with aUniversity degree. With these definitions, ∼ members (approximately 10%) of the European particle-physics community were engaged in neutrino physics. Of these, ∼ were involved in the accelerator-basedprogramme. The 2009 survey is now somewhat outdated and ECFA is considering repeating the survey inpreparation for the European Strategy update that is planned for 2019/20.The number of researchers involved in neutrino experiments based at FNAL is shown in figure 1. Researchersholding a PhD and graduate students are included in the count; all such researchers who are engaged in the pro-gramme are included in the count, regardless of their affiliation. The data has been drawn from the archives heldat FNAL. The raw number of researchers and graduate students has been corrected for the effect of “overlaps”;cases in which an individual is recorded against more than one experiment. The size of this correction is .An estimate of the uncertainty arising from the overlap correction is also shown. The data indicate that theFNAL-based neutrino community grew by ∼ from 2005 to 2007. The community was stable during theperiod when the Tevatron p ¯ p collider was in operation (2007–2012). The change of emphasis of the Laboratoryin recent years is clearly seen in the steady growth in the accelerator-based neutrino community supported byFNAL.In Canada, a survey of particle physics activities was conducted in the context of a long-range planning exer-cise [23] held in 2015. According to this survey, approximately 35 scientists and graduate students participate inaccelerator-based long-baseline neutrino physics through the T2K and Hyper-K collaborations. An additional70 scientists and graduate students are members of other neutrino efforts, such as EXO [24], IceCube [25],HALO [26] and SNO+ [27].For Asia, there had been no survey of the size of community for accelerator-based neutrino experiments. Asa first trial, Asian members of the Panel compiled the number of researchers (staff members, post-docs, andstudents) employed by Asian institutes who are working on accelerator-based neutrino experiments around theworld; experiments such as T2K, Hyper-K, MINOS [28], DUNE, etc. were included in the survey. Two statis-tics were prepared: (a) individuals were counted with weight one if they were working on an accelerator-basedexperiment; the count did not take into account the fraction of time spent by an individual on an accelerator-based neutrino experiment; and (b) the full-time equivalent effort invested in accelerator-based neutrino exper-iments. The results of the survey are ∼ and ∼ for (a) and (b) respectively. The distribution of thenumber of individuals engaged in the accelerator-based programme and the effort invested is shown for theAsian countries taking part in the survey in figure 2. 7igure 1: The number of PhD-holding researchers and graduate students engaged in neutrino experimentsbased at Fermi National Accelerator Laboratory. The data has been corrected for “overlaps”; cases in which anindividual is listed on more than one experiment. The uncertainty on the overlap-correction is indicated by theshaded band. The data were taken from the archives of the Fermi National Accelerator Laboratory.Figure 2: The number of Asian researchers (staff, post-doc and students) engaged in accelerator-based neutrinoexperiments around the world. (a) number of individuals counted with a weight of one if the individual isworking on an accelerator-based experiment with any fraction of their time. (b) full time equivalent (FTE)number of persons working on accelerator-based neutrino experiments.8 .3 Conclusions and recommendations The neutrino has a tiny mass, much smaller than any other fundamental fermion, and its type, or“flavour”, changes as it propagates through space and time. These properties imply the existence ofnew phenomena not described by the Standard Model of particle physics and may have profoundconsequences for our understanding of the Universe. The tiny neutrino mass seems likely to berelated to phenomena that occur at very high energy scales, well beyond the reach of the present orproposed colliding-beam facilities. The study of the neutrino is therefore the study of physics beyondthe Standard Model and a fundamentally important component of the particle-physics programme.
The accelerator-based neutrino programme is global in scope, engagement and intellectual contribu-tion. Continued and enhanced cooperation in a coherent global programme will maximise the impactof each individual contribution and of the programme as a whole.
Recommendation 1.1: The present roadmap document should be revised and updated at ap-propriate intervals.1.3:
By collating data from a number of sources the Panel has gained a partial understanding of the strengthof the global accelerator-based neutrino community. Accurate, up-to-date, consistent and completecensus data for the global accelerator-based neutrino community will be valuable in planning thedevelopment of the programme.
Recommendation 1.2: ICFA should support the Panel in its efforts to work with the stakehold-ers to gather the necessary census data as part of the consultation process that will followthe completion of this roadmap discussion document.1.4:
Experiments that exploit extra-terrestrial sources of neutrinos, neutrinos produced by nuclear reactorsand radio-active decays all have a fundamentally important role to play in the development of a com-plete understanding of neutrino physics. The Panel’s terms of reference restricted its considerationsto the accelerator-based neutrino programme. During the period of consultation that followed thepublication of the roadmap discussion document a compelling case was made that a future consul-tation or roadmapping process should encompass the field of neutrino physics as a whole. This casewas made by members of the neutrino-physics community and by stakeholders in the programme.The Panel agrees that a holistic approach should be developed and welcomes the positive responseit has received in initial discussions of the way forward.90
Accelerator-based long-baseline neutrino-oscillation programme
Neutrino oscillations, in which a neutrino created in an eigenstate of flavour α is detected in flavour state β , mayreadily be described in terms of the mixing of three mass eigenstates, ν i , i = 1 , , [29, 30]. The probabilityfor the transition ν α → ν β in vacuum is given by [31]: P ( ν α → ν β ) = (cid:88) i,j U αi U ∗ βi U ∗ αj U βj exp (cid:34) − i ∆ m ji LE (cid:35) ; (1)where E is the neutrino energy, L is the distance between source and detector and ∆ m ji = m j − m i . Theunitary matrix, U , may be parameterised in terms of three mixing angles, θ ij and one phase parameter δ CP : U = c s − s c c s e − iδ CP − s e iδ CP c c s − s c
00 0 1 (2)where c ij = cos θ ij and s ij = sin θ ij . The values of the various parameters that define the “Standard NeutrinoModel” (S ν M) obtained in fits to the present data obtained using terrestrial and non-terrestrial sources aregiven in table 1 [31]. Possible Majorana phases can not be measured in neutrino-oscillation experiments andare omitted from equation 2. Values for all of the mixing angles and ∆ m have been determined. Themagnitude of ∆ m is also known. The value of the CP-invariance violating phase, δ CP , and the sign of ∆ m are unknown.The goals of the future neutrino-oscillation programme are to:• Complete the S ν M: – Determine the mass hierarchy; and – Search for (and discover?) leptonic CP-invariance violation;• Establish the S ν M as correct description of nature: – Determine precisely θ and the degree to which it differs from π/ ; – Determine θ precisely; and – Determine θ precisely;• Search for deviations from the S ν M: – Provide redundant measurements of sufficient precision to test the unitarity of the neutrino-mixingmatrix; and – Search for sterile neutrinos and non-standard neutrino interactions.• Seek relationships between the parameters of the S ν M or between neutrinos and quarks.The accelerator-based neutrino-oscillation experiments presently in operation are presented in section 2.1and the planned next-generation of experiments is presented in section 2.2. Projects that seek to go beyond theperformance of the present or planned experiments are presented in section 2.3.
Physics goals
The collaboration uses the off-axis J-PARC neutrino beam, for which the neutrino-energy distribution peaks at0.6 GeV, to collect large samples in the near (ND280) and far (Super-K) detectors to [32]:• Search for CP-invariance violation using (cid:44) (cid:45)ν e appearance;11able 1: Summary of neutrino-oscillation parameters [31]. The best-fit values and σ allowed ranges of the 3-neutrino oscillation parameters, derived from a global fit to the current terrestrial and non-terrestrial oscillationdata. The definition of ∆ m used is: ∆ m = m − ( m + m ) / . Thus, ∆ m > , if m < m < m , and ∆ m < for m < m < m .Parameter Value ( ± σ ) ( σ range) sin θ . ± . sin θ , ∆ m > . +0 . − . sin θ , ∆ m < . +0 . − . sin θ , ∆ m > . +0 . − . sin θ , ∆ m < . +0 . − . ∆ m (7 . +0 . − . ) × − eV (6 . − . × − eV | ∆ m | , ∆ m > . ± . × − eV (2 . − . × − eV | ∆ m | , ∆ m < . ± . × − eV (2 . − . × − eV sign of ∆ m unknown δ CP unknown• Measure θ with high precision using (cid:44) (cid:45)ν µ disappearance;• Make a variety of measurements of neutrino-nucleus interactions, to improve neutrino oscillation mea-surements;• Contribute to the neutrino mass-hierarchy determination; and• Search for non-standard interactions and exotic phenomena.The T2K programme was originally approved for an exposure of . × protons on target (POT). A data setcorresponding to . × POT has been accumulated. T2K has published results based on . × POTwhich, in conjunction with the reactor constraint on θ , disfavour values of δ CP around π (see figure 3). Atotal exposure of × POT is projected by the end of Japanese financial year 2016. The projected sensitivityto CPiV is shown in figure 3 [33].
Institutes 61; collaborators 500
TRIUMF, University of British Columbia, University of Regina, University of Toronto, University of Victoria,University of Winnipeg, York University (Canada); Institute of Nuclear Physics of Lyon (IPNL), Institute ofResearch into the Fundamental Laws of the Universe, CEA Saclay, Laboratoire Leprince-Riguet, Ecole Poly-technique (IN2P3), LPNHE, UPMC, Paris, (France); RWTH Aachen University (Germany); INFN Bari andUniversity of Bari, INFN Napoli and Napoli University, INFN Padova and Padova University, INFN Romaand University of Roma “La Sapienza” (Italy); Institute for Cosmic Ray Research (ICRR), Kamioka Observa-tory, University of Tokyo, ICRR, Research Center for Cosmic Neutrino (RCCN), University of Tokyo, KavliInstitute for the Physics and Mathematics of the Universe, University of Tokyo, High Energy Accelerator Re-search Organization (KEK), Kobe University, Kyoto University, Miyagi University of Education, OkayamaUniversity, Osaka City University, Tokyo Metropolitan University, University of Tokyo, Yokohama NationalUniversity (Japan); Institute for Nuclear Research (Cracow), National Centre for Nuclear Research (Warsaw),University of Silesia (Katowice), Warsaw University of Technology, University of Warsaw, Wrocław Univer-sity, (Poland); INR (Russia), IFAE, Barcelona, UAM Madrid, IFIC , Valencia (Spain), ETH Zurich, Universityof Bern, University of Geneva (Switzerland); Imperial College London, Oxford University, Queen Mary, Uni-versity of London, Royal Holloway University of London, STFC Daresbury Laboratory, STFC RutherfordAppleton Laboratory, University of Lancaster, University of Liverpool, University of Sheffield, University of12 π ( CP δ -1 -0.5 0 0.5 1 χ ∆ Normal HierarchyInverted Hierarchy (NH) χ∆ FC 90 % critical (IH) χ∆ FC 90 % critical excluded at 90% CLexcluded at 90% CL
FIG. 37: Profiled as a function of CP with the results of the critical values forthe normal and inverted hierarchies for the joint fit with reactor constraint, with theexcluded regions found overlaid.94 Figure 3: Left panel: Profiled ∆ χ as a function of the CP phase ( δ CP ) for a fit to T2K ν e appearance datacorresponding to . × POT combined with the reactor measurement of θ (taken from [33]). Right panel:Sensitivity to CP violation as a function of POT with a 50% improvement in the effective statistics, assumingthe true the normal mass hierarchy and the true value of δ CP = − π/ and sin θ = 0 . for differentassumptions of the systematic uncertainties [34].Warwick (United Kingdom); Boston University, Colorado State University, Duke University, Louisiana StateUniversity, Michigan State University, Stony Brook University, University of California, Irvine, University ofColorado, University of Pittsburgh, University of Rochester, University of Washington (United States). Future programme
Following the J-PARC Main Ring upgrade, the beam power will be 700 kW in 2018, 800 kW in 2019 and900 kW in 2020. Assuming these beam-power projections and “five-cycle” operation (one “cycle” is 22–23days of operation in a month), T2K will achieve the approved number of POT goal by around 2021. The fardetector (Super-Kamiokande) will be loaded with gadolinium to enhance the neutrino-tagging capability at atime to be determined. Data-taking must be suspended during the detector work needed to prepared for thegadolinium-loading procedure. An upgraded near detector is being studied. A second phase of the experiment,aiming at > σ evidence of CP-invariance violation when CP is maximally violated by accumulating ∼ × POT by around 2026 with the beam power reaching 1.3 MW, has been given Stage 1 Status by the J-PARCDirectorate. ν A Physics goals
The NO ν A detector is located 810 km from the source of the Main Injector neutrino beam [35]. The off-axisangle of 14 mrad results in a neutrino-energy spectrum peaked at ∼ GeV. The collaboration will use the nearand far detectors to:• Maximise sensitivity to the neutrino mass hierarchy;• Constrain the value of δ CP ;• Resolve the octant of θ at better than . σ for of all values of δ CP for either mass hierarchy;• Achieve world-leading precision on ∆ m and sin θ ; and• Search for oscillations associated with sterile neutrinos.Using an exposure of . × POT the NO ν A collaboration has isolated a ν e appearance signal in the fardetector [36]. These events have been used to determine allowed regions in the sin θ , δ CP plane (see figure13 C P δ C P δ θ sin0 2 π π π π
2 2 π π π π LID
Best fit68% C.L.90% C.L.68% C.L.Reactor
NHIH C P δ C P δ θ sin0 2 π π π π
2 2 π π π π LEM
NHIH
FIG. 3: Allowed values of CP vs sin ✓ . Top (bottom)plots show the NH (IH). Left (right) plots show results forthe primary (secondary) selector. Both have sin ✓ fixed at0.5.FIG. 4: Significance of the di↵erence between the selectedand the predicted number of events as a function of CP andthe hierarchy. The primary (secondary) selection technique isshown with solid (dotted) lines. range of oscillation parameters favored in global fits [46],but 13% of pseudo-experiments generated at the NOvAbest fit find at least as many events than that observedin the data. With the secondary selector all values of CP in the IH are disfavored at greater than 90% C.L. Therange of 0 . ⇡<