- Email: [email protected]
Neutrino physics
Physics Reports 403–404 (2004) 57 – 67 www.elsevier.com/locate/physrep
Neutrino physics F. Dydak CERN, PH Department, EP Division, CH-1211 Geneva 23, Switzerland editor: R. Petronzio
Abstract Neutrino physics has a glorious past at CERN. It entered the scene with the groundbreaking discovery of neutral currents in 1973, and made essential contributions, until 1984, toward establishing the rule of the Standard Model. Nature’s choice of neutrino oscillation parameters was not favourable to CERN experiments carried out in the subsequent phase, until 1998. However, the new neutrino beam to Gran Sasso permits CERN to play a major role in forthcoming long-baseline studies of neutrino oscillations. © 2004 Elsevier B.V. All rights reserved. PACS: 13.15. + g Keywords: CERN; Neutrino physics; Standard Model
1. Introduction In the area of neutrino physics, CERN’s first 25 years were dominated by the construction, and operation in a wide-band neutrino beam at the CERN Proton Synchrotron (PS), of the heavy-liquid bubble chamber GARGAMELLE. It led in 1973 to the epic discovery of neutral currents, only rivalled by the W and Z discovery one decade later. The second 25 years of neutrino physics at CERN comprised three distinct phases. Nobody doubted that neutrino experiments would play a major role at the newly constructed SuperProton Synchrotron (SPS). A two-pronged approach foresaw GARGAMELLE and the Big European Bubble Chamber (BEBC), and the two large-mass electronic detectors CDHS and CHARM exploiting high-quality, high-intensity narrow-band and wide-band neutrino beams. Unfortunately, in 1978 E-mail address: [email protected] (F. Dydak). 0370-1573/$ - see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physrep.2004.08.011
58
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
GARGAMELLE dropped out from the race toward exciting new results, because of a leak due to material fatigue which could not be repaired. From 1977 onwards, results were pouring in, commensurate with the large investment which CERN and the neutrino community had made into beams and detectors. What appear in retrospect as landmark results of these golden years of neutrino physics at CERN, are reviewed in Section 2. Toward the end of this first phase in 1984, the Standard Model was already firmly established. The interest to look for effects beyond the Standard Model was growing and fuelled proposals to search for neutrino oscillations. Three experiments reported absence of disappearance, and absence of e appearance in a beam at the CERN PS. Rather than being discouraged, a major experimental effort to look for → oscillations was launched, with the CHORUS and NOMAD detectors as work-horses, which lasted until 1998. The highlights of this second phase are reviewed in Section 3. The decisive change of paradigm occurred in 1998 when oscillations of atmospheric neutrinos were first claimed by the SuperKamiokande collaboration. The new results explained in retrospect why all searches at CERN had had negative results: the baseline had been way too short to let oscillations develop. Naturally, the proposal to construct a new neutrino beam from CERN to Gran Sasso, and to exploit this beam with detectors in the Gran Sasso Underground Laboratory, found considerable support. The leitmotiv of this third phase which is still ongoing, is described in Section 4, including a glimpse on ideas on the future of accelerator neutrino physics.
2. 1979–1984: harvesting after the investment The SPS came at the end of 1976 into operation, and neutrino beams were an all-important part of its physics potential. The SPS delivered two neutrino beams: a high-quality narrow-band beam with a flat energy spectrum up to ∼200 GeV, and a wide-band beam with an intensity higher by two orders of magnitude, but concentrated at low energy and steeply falling toward a maximum energy of ∼300 GeV. The novel feature of the narrow-band beam was that from the radial position of the event in the detector, the energy of the incoming neutrino was known. Besides the giant bubble chamber BEBC, two massive electronic detectors took data: CDHS and CHARM. BEBC took pictures of neutrino interactions, with superb high-resolution optics but only 1 t of liquid hydrogen, or 10 t of a liquid neon–hydrogen mixture, as target mass. The CDHS detector, conceived by Steinberger and collaborators, represented a major step forward in neutrino detection technology. Thousand tons of iron, instrumented with scintillator sheets and driftchambers, constituted the first neutrino detector which integrated the functions of target and spectrometer. The resulting acceptance of nearly unity helped greatly to obtain fast and reliable results. The CHARM detector, conceived by Winter and collaborators and distinguished by its noble target material, Carrara marble, was the first integrated detector in which hadronic and electromagnetic showers developed over the same physical length. This design feature made for the first time possible the high-statistics measurement of the direction of the struck quark in neutrino–nucleon interactions (important for the reconstruction of neutral-current events), and of the direction of final-state electrons (important for the reconstruction of scattering events off electrons). Fig. 1 shows a photograph of the CDHS and CHARM detectors.
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
59
Fig. 1. The CDHS (front) and CHARM (back) detectors lined up in CERN’s West Area neutrino beam.
CERN was poised to live up to the challenge of the rich physics potential of neutrino interactions. Controversial results from earlier experiments at Fermilab further contributed to high expectations from CERN experiments. A distinctive change with respect to earlier work was that event numbers would no longer be counted in thousands, but in hundred thousands. 2.1. From the quark–parton model to QCD Electron–nucleon scattering experiments at SLAC had established the quark–parton model of the nucleon, and thus ‘scaling’ of the nucleon structure function F2 . It was generally accepted that the a priori complicated scattering of high-energy neutrinos off nucleons was de facto the simple scattering of point-like neutrinos off point-like quarks, for which the centre-of-mass scattering angles depend only on the relative spin orientation of the scattering partners. Minor controversies (of which the ‘high-y anomaly’ [1] was the most popular) were quickly sorted out, and quark–parton model predictions were convincingly confirmed [2] in 1977 already. The stunning confirmation of the quark–parton model of the nucleon structure had also its dark side: with the exception of BEBC physicists around Perkins, the majority of the neutrino community ignored the theoretical advances of QCD which predicted that the quark–parton model was an approximation only and that ‘scaling’ was violated with a specific dependence on Q2 . More strongly supported by their confidence in QCD predictions than by evidence from their small data sample in the perturbative Q2 region, BEBC physicists were the first to claim scaling violation [3]. This was daring at the time but history proved them right! Soon thereafter, scaling violations as predicted by QCD were confirmed beyond doubt, with precise measurements from large data samples, first by CDHS [4,5] and then by CHARM [6]. Historically, this was the first in a series of triumphal QCD predictions, and contributed in a major way to establish QCD as the correct theory of the strong interaction at high Q2 .
60
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
More and more refined measurements of scaling violation in the nucleon structure functions permitted the first measurement of the strong coupling constant s , first in leading order [4] but soon with quite some sophistication from the inclusion of higher-order corrections [5,6]. As a necessary byproduct of the analysis of the structure functions of the nucleon in terms of QCD, the structure function of the nucleon’s gluon content was determined. In conjunction with analogous measurements of the nucleon structure functions with electrons and muons as probes, the fractional electric charges of the u and d quarks were confirmed through the verification of ∼ 5 Fem 2 = 18 F2 , 5 where the factor 18 is the average of the squared charges of the u and d quarks. The spin of the quarks was confirmed to be 21 , in agreement with results from electron and muon scattering experiments. By virtue of parity violation in their interactions, neutrinos distinguish between quarks and antiquarks, by contrast to electrons and muons. This made them a unique tool to measure q, ¯ the structure function of the nucleon’s ‘sea’ of antiquarks, as well as x F3 , the nucleon structure function of valence quarks. This, in turn, enabled the measurement of the number of valence quarks in the nucleon, which was found compatible with three. So-called ‘opposite-sign’ dimuon events were soon recognized as normal charged-current events where the second muon with opposite sign resulted from the semileptonic decay of a charm quark [7]. While the leading reaction was + d → − + u, the sub-leading reaction was + s → − + c, where the charm quark was easily identified through its decay c → s + + . Therefore, dimuon events were successfully exploited to determine the structure function of the nucleon’s ‘sea’ of strange quarks.
2.2. From neutral currents to electroweak radiative corrections The discovery of neutral currents in neutrino–nucleon scattering marked the birth of the electroweak Standard Model. Its broad and rapid acceptance had been prepared by the proof of its renormalizability by t’Hooft. Attention soon focused on the value of the free parameter of the model, the electroweak mixing angle sin2 w . Guided by advice of Grand Unified Model builders, many were convinced that sin2 w had the value 38 = 0.375. Early experiments which measured sin2 w from the ratio of neutralto charged-current neutrino scattering off nucleons in isoscalar targets, seemed to support strongly this notion. Still in 1977, the average from several experiments was sin2 w = 0.31 ± 0.03 [8]. The CERN experiments which for the first time disposed of a large statistics sample and had better control of systematic errors, found a considerably lower value. The first was sin2 w =0.24±0.02, reported by the CDHS experiment [9] (radiative corrections which were then not yet calculated, would change the published result to 0.23 ± 0.02, perfectly compatible with today’s world average for the leptonic effective mixing angle, sin2 w = 0.23148 ± 0.00017 [10]). BEBC [11] reported 0.22 ± 0.05 and CHARM [12] 0.220 ± 0.014 and thus confirmed the new lower value of sin2 w . An important question was the Lorentz-structure of neutral currents. Establishing the V–A structure of charged currents had taken 25 years. How much time would be needed now? The Standard Model predicted neutral currents as, in general, unequal mixture of vector and axialvector currents constructed from lepton and quark fields. This expectation which was with the known value of sin2 w unambiguous and quantitative, was rapidly confirmed by CDHS [13], BEBC [14], and CHARM [15]. Five years after the
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
61
discovery of neutral currents their Lorentz-structure was settled in favour of the Standard Model prediction. In retrospect, one might say that this result marked the beginning of the Standard Model’s ‘tyranny’: the next 25 years would see all experiments confirming with ever greater precision the predictions of the Standard Model. Rapidly, in the early 1980s, the physics essence of the Standard Model shifted from Born approximations to the level of loop calculations. No prediction in Born approximation was in conflict with confirmed experimental results. Would the same be true at loop level? The first test at loop level was made possible with the Z boson newly discovered in the UA1 and UA2 experiments. Wheater and Llewellyn Smith [16] pointed out that the predictions of its mass in terms of sin2 w differed by ∼ 5 GeV in Born approximation and at one-loop level. With (in the ‘on-shell’ renormalization scheme) = e2 /4 and sin2 w = 1 − m2W /m2Z , electroweak (one-loop) radiative corrections were concentrated in the then popular parameter r introduced by Sirlin [17]. With this parameter, the relation between the Z mass, the Fermi coupling constant GF and the electroweak mixing angle became 1 1 1 GF = √ . 2 2 2 2 mZ sin w (1 − sin w ) 1 − r The r was sensitive to the squared mass of the top quark. That suggestion was followed up, and in a dedicated high-statistics experiment the electroweak mixing angle was determined with much better precision: 0.225 ± 0.006 by CDHS [18], and 0.236 ± 0.006 by CHARM [19]. The existence of the radiative correction was supported at the 3 level. Furthermore, because of the sensitivity of the radiative correction on the top-quark mass, for the first time an indication of an upper limit on the mass of the top quark around 200 GeV had shown up. In parallel to this development, the measurement of elastic scattering off electrons was developed into an art by the successor of CHARM, CHARM II: their final result for the electroweak mixing angle was sin2 w = 0.232 ± 0.008 [20], well consistent with measurements from other processes. 2.3. Charm–quark pair production in hadronic collisions Parallel to the rise of the Standard Model, the quest for physics beyond the Standard Model rose. One of the areas where physicists hoped to break new ground, was a beam dump experiment: as many protons of the highest energy would be dumped into dense matter, in the hope that new particles (e.g. heavy leptons or axions) would be produced. They or their decay products could be penetrating like neutrinos and would be recorded in downstream neutrino detectors. One source of background was known: neutrinos from the decay of normal ± and K ± in the hadronic cascade initiated by the incoming protons. This problem could easily be handled by employing dumps with different density and extrapolating to infinite density. Also, this background should predominantly consist of . The result of the first beam dump experiment was a surprise. All three CERN experiments took part: BEBC, CDHS and CHARM. BEBC, the experiment with the lowest statistics but with the best electron identification capacity, took the lead. Boosted by a favourable upward statistical fluctuation, they reported an anomalous e signal [21]. This early result was confirmed, and measured more precisely in two more beam dump campaigns, by the CDHS [22] and CHARM [23] experiments.
62
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
What had been seen? It was the production of charm quarks in hadronic collisions, with subsequent semileptonic decay which gave rise to a ‘prompt’ flux of , , e and e . Thus, open charm production in hadronic collisions was seen for the first time, with a cross-section one order of magnitude higher than was then expected from the negative results of earlier emulsion experiments. 2.4. Getting used to ‘appearance’ and ‘disappearance’ Although there was no accepted evidence for neutrino oscillation the existence of which was steadfastly advocated by Pontecorvo [24] and others—the significant evidence from solar neutrinos measured by Davis and collaborators [25] was not understood and therefore ignored; the evidence from atmospheric neutrinos measured by NUSEX [26], the Fréjus experiment [27], and IMB [28], was seemingly conflicting and also ignored—, physicists proposed and carried out dedicated searches for neutrino oscillations in low-energy beams at the CERN PS. CDHS [29] and CHARM [30] did not observe the ‘disappearance’ of . BEBC [31] did not observe the ‘appearance’ of e . The experimental results were correct, but as known today, they were carried out in a region of oscillation parameter space where no oscillations could be observed. The CDHS result is still today the best for the generic → x oscillation, where x stands for any type of penetrating neutrino-like particle other than the . 2.5. Handing over the torch In 1984, the neutrino programme at CERN came to a halt. BEBC was closed down to save money and liberate manpower for LEP construction. The CDHS and CHARM detectors stopped data taking and were dismantled. CERN’s West Area Neutrino Facility was preparing for a new, quite different physics challenge: the search for → oscillations. The programme of analysing millions of neutrino–nucleon scattering events was after 1984 solely carried on by the CCFR experiment at Fermilab, which resembled in its design the CDHS experiment at CERN. Their experimental programme lasted for many more years and finally exceeded in precision significantly the earlier results of the CERN experiments. Most noteworthy are their final results on the nucleon structure functions and on the electroweak mixing angle. The studies of the nucleon structure function were of course dominated by H1 and ZEUS results at HERA, however the comparison with precise—and consistent—results from neutrino scattering still provided essential information such as the valence and sea structure functions. The precision attained by CCFR is perhaps best reflected by their measurement of the Q2 dependence of the various nucleon structure functions [32]. The error on the electroweak mixing angle which had been reduced to 0.005 already in the CERN experiments, was further reduced by CCFR to well below 0.002 [33]. The slightly worrying feature of their result sin2 w =0.2277±0.0016 is that it is at the 2.4 level above the combined result from LEP and the SLC [10]. Some see in this a promising deviation from the Standard Model, others are more sceptical and rather suspect problems with the nucleon structure functions that are needed in this analysis.
3. 1985–1998: not reaching the oasis After the first period of the exploitation of neutrino beams at the CERN SPS was completed in 1984, a significant change of paradigm took place: the search for neutrino oscillations became the prime challenge.
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
63
On the one hand, no good theoretical justification for a zero neutrino mass had ever been put forward; on the other hand, speculations abounded on the contribution of finite neutrino masses to hot dark matter, and thus to the mass density of the universe. To close the universe, the sum of masses of all neutrino families was conjectured not to exceed, roughly, 30 eV [34]. Naturally, with the oscillatory term 2 2 2 1.267m [eV ]L[km] sin , E [GeV] neutrino masses in that range would lead to observable oscillations over baselines of order 1 km with neutrino energies of order 10 GeV. The CERN wide-band neutrino beam appeared to be the right vehicle for discovery! In the first proposal, put forward by Vannucci and collaborators [35], would be intercepted on the Jura mountain range west of CERN, at a distance of about 20 km, with a view to searching for disappearance. This then bold proposal marked the beginning of what is called today ‘long-baseline’ oscillation experiment. However, this proposal was politically inconvenient, as negative effects on the construction of LEP were feared. Insofar, this proposal also marked the growing importance of political considerations in scientific decision making. In retrospect, the non-acceptance of the proposal can hardly be criticized because with today’s knowledge of oscillation parameters, the experiment would not have observed oscillations. 3.1. The emulsion approach The second proposal was daring only from the technological point of view. Inside the CERN site, Winter and collaborators [36] proposed to search for the appearance of in a wide-band beam. The ’s produced in their charged-current reactions would be identified by their finite decay path of a few 100 m in a 770 kg target of emulsion sheets. This proposal was accepted and promoted the emulsion technology to a prominent place in the search for the interaction of with nucleons. Today, the technique is referred to as ‘emulsion approach’. So far, it was most successfully exploited in the DONUT experiment at Fermilab which reported in 2001 evidence for the production of four ’s produced by the component in the neutrino flux behind a dump for 800 GeV protons [37]. In first results, CHORUS reported [38] the absence of → oscillations; their final results are still to come. Their experimental method and their goal were perfectly correct, but unfortunately for them and for CERN, oscillations were out of their experiment’s reach. 3.2. The kinematic approach Vannucci and collaborators had not given up. They came back with a novel alternative to the emulsion approach, which became known as the ‘kinematic approach’. The production of a few ’s amidst an overwhelming background of charged-current and neutral-current interactions, would be signalled by sophisticated cuts on the distribution of secondary hadrons in the transverse plane of the event, with a view to distinguishing final-state electrons and muons from the primary neutrino interaction vertex, from electrons and muons from decays. Their proposal [39] was accepted and led to the NOMAD experiment. There was also an element of nostalgia: NOMAD re-used the giant magnet which had been constructed for the UA1 experiment.
64
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
NOMAD took data in the West Area wide-band neutrino beam until 1998, concurrently with CHORUS. Their ‘kinematic approach’ proved successful. In 2001 they published their final result [40]: sin2 2 < 3.3×10−4 for large m2 . Like for CHORUS, their method as well as their goal were perfectly correct but neutrino oscillations were too small in their sensitivity domain.
4. 1999 and beyond: responding to the new challenge 4.1. The irresistible rise of non-accelerator experiments By 1998, the depletion of the solar neutrino flux which had been claimed by Davis and collaborators for 25 years already [41], had been confirmed by GALLEX [42], SAGE [43], Kamiokande [44] and SuperKamiokande [45]. In 1998, the observation of atmospheric neutrino oscillations in the SuperKamiokande detector was announced [46], based on a significant depletion of with respect to e in the atmospheric neutrino flux, with a telling dependence on zenith angle. After 1 year of shock and reluctance, the reaction of the particle physics community was decisive. In the USA, the MINOS experiment which was to search for disappearance between Fermilab and the Soudan mine in 732 km distance, was re-optimized to cope with the m223 ∼ 3 × 10−3 eV2 claimed by SuperKamiokande. In Europe, earlier plans of a search for oscillations in a beam from CERN to the Gran Sasso Laboratory in Italy found strong support, also and particularly at the upper echelons, and condensed into a major European experimental neutrino programme. The third phase had begun. 4.2. Neutrinos from CERN to Gran Sasso The only realistic possibility for a neutrino beam to Gran Sasso was to build on the successful experience with a wide-band neutrino beam at the CERN SPS. As for the experimental programme, the obvious decision was taken to build on the tradition of the emulsion and kinematic approaches that had been successfully developed by CHORUS and NOMAD at CERN. The new ‘CERN-Neutrinos-to-Gran Sasso’ (CNGS) beamline [47] has as new features the direction to Gran Sasso (which implies a downward inclination of 5.6◦ ), a much longer decay tunnel than was usual before (to maximize the neutrino flux), and a horn/reflector focussing which favours the flux around 15 GeV energy (in order to maximize the number of charged-current interactions with nucleons). The decision to launch this neutrino programme was taken in 2000. The construction is well under way. According to the planning, CNGS will be operational in 2006. 4.3. Tradition versus new ideas The experiment which exploits the emulsion approach, is OPERA [48]. Its prominent feature is a 1.8 kt lead target, organized in two sections, finely segmented and instrumented with emulsion sheets. Each target section is followed by a spectrometer which measures particle tracks (to narrow the search for the event’s interaction vertex in emulsion tracks) and calorimetric energy. OPERA is under construction in the Gran Sasso Laboratory and is expected to take first data soon after the commissioning of the CNGS. A second experiment which exploits the kinematic approach, is ICARUS [49]. This experiment has a 3000 t fully sensitive liquid argon target which is read out like a Time Projection Chamber. Although it
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
65
permits ‘pictures’ of events with amazing detail known only from bubble-chamber photographs before, its spatial resolution cannot compete with emulsions when it comes to the identification of the finite flight path. Hence, ICARUS relies on kinematic cuts to isolate interactions. This, however, can be done with unparalleled perfection, given the detailed measurement of each event. Interestingly, despite of the very different methods, OPERA and ICARUS claim to observe the same number of ↔ oscillations: at the presently favoured value m223 ∼ 2.0 × 10−3 eV2 , 11 events on top of a background of 1 event. The anticipated running time is 5 years. The amazingly fast experimental progress in pinning down the numerical values of oscillation parameters in the past five years, has already shifted attention to details of the neutrino mixing matrix. The prime target of attention is the mixing angle 13 , which ‘connects’, if non-zero, solar-neutrino oscillation phenomena with those of atmospheric neutrinos. Most importantly, 13 must be non-zero for CP-violation in the neutrino mixing matrix to occur, with possibly important consequences for leptogenesis and the matter–antimatter asymmetry of the universe. The CNGS with a horn/reflector system optimized to focus 4 GeV/c pions, and exploited in off-axis geometry with a moveable underwater Cherenkov light detector in the Gulf of Taranto south of Italy, would permit not only precise measurements of the amplitude and phase of the leading oscillation ↔ , but also of the amplitude of the sub-leading oscillation ↔ e , and thus of the angle 13 . 4.4. Outlook CERN would not be CERN if it were not thinking, together with its community of physicists, of future opportunities and options for what might come after the LHC. No doubt that neutrino oscillations moved neutrino physics again into the limelight. The discovery of finite neutrino masses constitutes a ‘mild’ extension only of the Standard Model, but there is strong hope of further ramifications which may eventually help understanding important aspects of physics beyond the Standard Model. Experimentally, the challenge neutrino physics at accelerators is to measure as precisely as possible the neutrino masses, the three neutrino mixing angles 12 , 13 and 23 , and the CP-violating phase of the mixing matrix. The accelerator of choice has been identified and both the machine aspects and prospects for experiments have been studied in quite some detail already. It is a storage ring for muons, first proposed by Geer [50], at a momentum of ∼ 50 GeV/c, with long straight sections along which decaying muons lead to and e beams of unparalleled intensity and quality. The machine has become known as ‘Neutrino Factory’ and several studies confirmed that it offers by a large margin the best opportunities for significant advances in measurement accuracy.
5. Summary Neutrino physics has undoubtedly a glorious past at CERN. It entered the scene with the groundbreaking discovery of neutral currents in 1973, and made essential contributions toward establishing firmly the rule of the Standard Model until 1984. Nature’s choice of oscillation parameters was not favourable to past CERN experiments in the search for neutrino oscillations, yet permits CERN neutrino beams to play a major role in forthcoming long-baseline studies of neutrino oscillations.
66
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
References [1] A. Benvenuti, et al., Phys. Rev. Lett. 36 (1976) 1478; A. Benvenuti, et al., Phys. Rev. Lett. 37 (1976) 189. [2] M. Holder, et al., Phys. Rev. Lett. 39 (1977) 433. [3] P.C. Bosetti, et al., Nucl. Phys. B 142 (1978) 1. [4] J.G.H. de Groot, et al., Phys. Lett. B 82 (1979) 292; J.G.H. de Groot, et al., Phys. Lett. B 82 (1979) 456. [5] J.G.H. de Groot, et al., Z. Phys. C 1 (1979) 143; H. Abramowicz, et al., Z. Phys. C 12 (1982) 289; H. Abramowicz, et al., Z. Phys. C 13 (1982) 199; H. Abramowicz, et al., Z. Phys. C 17 (1983) 283; P. Berge, et al., Z. Phys. C 49 (1991) 187. [6] M. Jonker, et al., Phys. Lett. B 109 (1982) 133; F. Bergsma, et al., Phys. Lett. B 123 (1983) 269; F. Bergsma, et al., Phys. Lett. B 153 (1985) 111. [7] M. Holder, et al., Phys. Lett. B 69 (1977) 377. [8] P. Musset, in: F. Gutbrod (Ed.), Proceedings of the International Symposium on Lepton and Photon Interactions, Hamburg, 1977, DESY, Hamburg, 1977, p. 785. [9] M. Holder, et al., Phys. Lett. B 71 (1977) 222; H. Abramowicz, et al., Z. Phys. C 28 (1985) 51. [10] Summary of electroweak results from LEP and SLC, Report CERN-EP/2002-091 (hep-ex/0212036). [11] P.C. Bosetti, et al., Phys. Lett. B 76 (1978) 505; P.C. Bosetti, et al., Nucl. Phys. B 217 (1983) 1. [12] M. Jonker, et al., Phys. Lett. B 99 (1981) 265. [13] M. Holder, et al., Phys. Lett. B 72 (1977) 254. [14] H. Deden, et al., Nucl. Phys. B 149 (1979) 1. [15] M. Jonker, et al., Phys. Lett. B 102 (1981) 67; J.V. Allaby, et al., Phys. Lett. B 231 (1989) 317. [16] J.F. Wheater, C.H. Llewellyn Smith, Nucl. Phys. B 208 (1982) 27; J.F. Wheater, C.H. Llewellyn Smith, Nucl. Phys. B 226 (1983) 547 (Errata). [17] A. Sirlin, Phys. Rev. D 22 (1980) 185. [18] H. Abramowicz, et al., Phys. Rev. Lett. 57 (1986) 298. [19] J.V. Allaby, et al., Phys. Lett. B 177 (1986) 446; J.V. Allaby, et al., Z. Phys. C 36 (1987) 611. [20] P. Vilain, et al., Phys. Lett. B 335 (1994) 246. [21] P.C. Bosetti, et al., Phys. Lett. B 74 (1978) 143; P. Fritze, et al., Phys. Lett. B 96 (1980) 427; H. Grässler, et al., Nucl. Phys. B 273 (1986) 253. [22] T. Hansl-Kozanecka, et al., Phys. Lett. B 74 (1978) 139; H. Abramowicz, et al., Z. Phys. C 13 (1982) 179; P. Berge, et al., Z. Phys. C 56 (1992) 175. [23] M. Jonker, et al., Phys. Lett. B 96 (1980) 435; F. Bergsma, et al., Phys. Lett. B 157 (1985) 458; J.V. Allaby, et al., Z. Phys. C 40 (1988) 497. [24] B. Pontecorvo, Sov. Phys. JETP 26 (1968) 984. [25] R. Davis, et al., Phys. Rev. Lett. 20 (1968) 1205. [26] M. Aglietta, et al., Europhys. Lett. 8 (1989) 611. [27] Ch. Berger, et al., Phys. Lett. B 227 (1989) 489. [28] R. Becker Szendy, et al., Phys. Rev. D 46 (1992) 3720. [29] F. Dydak, et al., Phys. Lett. B 134 (1984) 281. [30] F. Bergsma, et al., Phys. Lett. B 142 (1984) 103.
F. Dydak / Physics Reports 403–404 (2004) 57 – 67 [31] C. Angelini, et al., Phys. Lett. B 179 (1986) 307. [32] W.G. Seligmann, et al., Phys. Rev. Lett. 79 (1997) 1213; U.K. Yang, et al., Phys. Rev. Lett. 86 (2001) 2742; U.K. Yang, et al., Phys. Rev. Lett. 87 (2001) 251802. [33] G.P. Zeller, et al., Phys. Rev. Lett. 88 (2002) 091802; G.P. Zeller, et al., Phys. Rev. Lett. 90 (2003) 239902 (Erratum). [34] See, for example, H. Harari, Phys. Lett. B 216 (1989) 413. [35] T.C. Bacon, et al, Proposal SPSC/82-20/P178 (CERN). [36] N. Armenise, et al., Proposal SPSC/90-42/P254 (CERN). [37] K. Kodama, et al., Phys. Lett. 504 (2001) 218. [38] E. Eskut, et al., Phys. Lett. B 497 (2001) 8. [39] P. Astier, et al., Proposal SPSLC/91-21/P261 (CERN). [40] P. Astier, et al., Nucl. Phys. B 611 (2001) 3. [41] B.T. Cleveland, et al., Astrophys. J. 496 (1998) 505. [42] W. Hampel, et al., Phys. Lett. B 447 (1999) 127. [43] J.N. Abdurashitov, et al., Phys. Rev. Lett. 83 (1999) 4686; J.N. Abdurashitov, et al., J. Exp. Theor. Phys. 95 (2002) 181. [44] Y. Fukuda, et al., Phys. Rev. Lett. 77 (1996) 1683. [45] Y. Fukuda, et al., Phys. Rev. Lett. 81 (1998) 1158; Y. Fukuda, et al., Phys. Rev. Lett. 86 (2001) 5651. [46] Y. Fukuda, et al., Phys. Rev Lett. 81 (1998) 1562; Y. Fukuda, et al., Phys. Lett. B 433 (1998) 9; Y. Fukuda, et al., Phys. Lett. B 436 (1998) 25. [47] G. Acquistapace, et al., Proposal CERN 98-02 (INFN/AE-98/05). [48] M. Guler, et al., Proposal CERN-SPSC/2000-028 (CERN). [49] P. Aprili, et al., Proposal CERN-SPSC/2002-027 (CERN). [50] S. Geer, Phys. Rev. D 57 (1998) 6989; S. Geer, Phys. Rev. D 59 (1999) 039903 (Erratum).
67
Neutrino physics F. Dydak CERN, PH Department, EP Division, CH-1211 Geneva 23, Switzerland editor: R. Petronzio
Abstract Neutrino physics has a glorious past at CERN. It entered the scene with the groundbreaking discovery of neutral currents in 1973, and made essential contributions, until 1984, toward establishing the rule of the Standard Model. Nature’s choice of neutrino oscillation parameters was not favourable to CERN experiments carried out in the subsequent phase, until 1998. However, the new neutrino beam to Gran Sasso permits CERN to play a major role in forthcoming long-baseline studies of neutrino oscillations. © 2004 Elsevier B.V. All rights reserved. PACS: 13.15. + g Keywords: CERN; Neutrino physics; Standard Model
1. Introduction In the area of neutrino physics, CERN’s first 25 years were dominated by the construction, and operation in a wide-band neutrino beam at the CERN Proton Synchrotron (PS), of the heavy-liquid bubble chamber GARGAMELLE. It led in 1973 to the epic discovery of neutral currents, only rivalled by the W and Z discovery one decade later. The second 25 years of neutrino physics at CERN comprised three distinct phases. Nobody doubted that neutrino experiments would play a major role at the newly constructed SuperProton Synchrotron (SPS). A two-pronged approach foresaw GARGAMELLE and the Big European Bubble Chamber (BEBC), and the two large-mass electronic detectors CDHS and CHARM exploiting high-quality, high-intensity narrow-band and wide-band neutrino beams. Unfortunately, in 1978 E-mail address: [email protected] (F. Dydak). 0370-1573/$ - see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physrep.2004.08.011
58
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
GARGAMELLE dropped out from the race toward exciting new results, because of a leak due to material fatigue which could not be repaired. From 1977 onwards, results were pouring in, commensurate with the large investment which CERN and the neutrino community had made into beams and detectors. What appear in retrospect as landmark results of these golden years of neutrino physics at CERN, are reviewed in Section 2. Toward the end of this first phase in 1984, the Standard Model was already firmly established. The interest to look for effects beyond the Standard Model was growing and fuelled proposals to search for neutrino oscillations. Three experiments reported absence of disappearance, and absence of e appearance in a beam at the CERN PS. Rather than being discouraged, a major experimental effort to look for → oscillations was launched, with the CHORUS and NOMAD detectors as work-horses, which lasted until 1998. The highlights of this second phase are reviewed in Section 3. The decisive change of paradigm occurred in 1998 when oscillations of atmospheric neutrinos were first claimed by the SuperKamiokande collaboration. The new results explained in retrospect why all searches at CERN had had negative results: the baseline had been way too short to let oscillations develop. Naturally, the proposal to construct a new neutrino beam from CERN to Gran Sasso, and to exploit this beam with detectors in the Gran Sasso Underground Laboratory, found considerable support. The leitmotiv of this third phase which is still ongoing, is described in Section 4, including a glimpse on ideas on the future of accelerator neutrino physics.
2. 1979–1984: harvesting after the investment The SPS came at the end of 1976 into operation, and neutrino beams were an all-important part of its physics potential. The SPS delivered two neutrino beams: a high-quality narrow-band beam with a flat energy spectrum up to ∼200 GeV, and a wide-band beam with an intensity higher by two orders of magnitude, but concentrated at low energy and steeply falling toward a maximum energy of ∼300 GeV. The novel feature of the narrow-band beam was that from the radial position of the event in the detector, the energy of the incoming neutrino was known. Besides the giant bubble chamber BEBC, two massive electronic detectors took data: CDHS and CHARM. BEBC took pictures of neutrino interactions, with superb high-resolution optics but only 1 t of liquid hydrogen, or 10 t of a liquid neon–hydrogen mixture, as target mass. The CDHS detector, conceived by Steinberger and collaborators, represented a major step forward in neutrino detection technology. Thousand tons of iron, instrumented with scintillator sheets and driftchambers, constituted the first neutrino detector which integrated the functions of target and spectrometer. The resulting acceptance of nearly unity helped greatly to obtain fast and reliable results. The CHARM detector, conceived by Winter and collaborators and distinguished by its noble target material, Carrara marble, was the first integrated detector in which hadronic and electromagnetic showers developed over the same physical length. This design feature made for the first time possible the high-statistics measurement of the direction of the struck quark in neutrino–nucleon interactions (important for the reconstruction of neutral-current events), and of the direction of final-state electrons (important for the reconstruction of scattering events off electrons). Fig. 1 shows a photograph of the CDHS and CHARM detectors.
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
59
Fig. 1. The CDHS (front) and CHARM (back) detectors lined up in CERN’s West Area neutrino beam.
CERN was poised to live up to the challenge of the rich physics potential of neutrino interactions. Controversial results from earlier experiments at Fermilab further contributed to high expectations from CERN experiments. A distinctive change with respect to earlier work was that event numbers would no longer be counted in thousands, but in hundred thousands. 2.1. From the quark–parton model to QCD Electron–nucleon scattering experiments at SLAC had established the quark–parton model of the nucleon, and thus ‘scaling’ of the nucleon structure function F2 . It was generally accepted that the a priori complicated scattering of high-energy neutrinos off nucleons was de facto the simple scattering of point-like neutrinos off point-like quarks, for which the centre-of-mass scattering angles depend only on the relative spin orientation of the scattering partners. Minor controversies (of which the ‘high-y anomaly’ [1] was the most popular) were quickly sorted out, and quark–parton model predictions were convincingly confirmed [2] in 1977 already. The stunning confirmation of the quark–parton model of the nucleon structure had also its dark side: with the exception of BEBC physicists around Perkins, the majority of the neutrino community ignored the theoretical advances of QCD which predicted that the quark–parton model was an approximation only and that ‘scaling’ was violated with a specific dependence on Q2 . More strongly supported by their confidence in QCD predictions than by evidence from their small data sample in the perturbative Q2 region, BEBC physicists were the first to claim scaling violation [3]. This was daring at the time but history proved them right! Soon thereafter, scaling violations as predicted by QCD were confirmed beyond doubt, with precise measurements from large data samples, first by CDHS [4,5] and then by CHARM [6]. Historically, this was the first in a series of triumphal QCD predictions, and contributed in a major way to establish QCD as the correct theory of the strong interaction at high Q2 .
60
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
More and more refined measurements of scaling violation in the nucleon structure functions permitted the first measurement of the strong coupling constant s , first in leading order [4] but soon with quite some sophistication from the inclusion of higher-order corrections [5,6]. As a necessary byproduct of the analysis of the structure functions of the nucleon in terms of QCD, the structure function of the nucleon’s gluon content was determined. In conjunction with analogous measurements of the nucleon structure functions with electrons and muons as probes, the fractional electric charges of the u and d quarks were confirmed through the verification of ∼ 5 Fem 2 = 18 F2 , 5 where the factor 18 is the average of the squared charges of the u and d quarks. The spin of the quarks was confirmed to be 21 , in agreement with results from electron and muon scattering experiments. By virtue of parity violation in their interactions, neutrinos distinguish between quarks and antiquarks, by contrast to electrons and muons. This made them a unique tool to measure q, ¯ the structure function of the nucleon’s ‘sea’ of antiquarks, as well as x F3 , the nucleon structure function of valence quarks. This, in turn, enabled the measurement of the number of valence quarks in the nucleon, which was found compatible with three. So-called ‘opposite-sign’ dimuon events were soon recognized as normal charged-current events where the second muon with opposite sign resulted from the semileptonic decay of a charm quark [7]. While the leading reaction was + d → − + u, the sub-leading reaction was + s → − + c, where the charm quark was easily identified through its decay c → s + + . Therefore, dimuon events were successfully exploited to determine the structure function of the nucleon’s ‘sea’ of strange quarks.
2.2. From neutral currents to electroweak radiative corrections The discovery of neutral currents in neutrino–nucleon scattering marked the birth of the electroweak Standard Model. Its broad and rapid acceptance had been prepared by the proof of its renormalizability by t’Hooft. Attention soon focused on the value of the free parameter of the model, the electroweak mixing angle sin2 w . Guided by advice of Grand Unified Model builders, many were convinced that sin2 w had the value 38 = 0.375. Early experiments which measured sin2 w from the ratio of neutralto charged-current neutrino scattering off nucleons in isoscalar targets, seemed to support strongly this notion. Still in 1977, the average from several experiments was sin2 w = 0.31 ± 0.03 [8]. The CERN experiments which for the first time disposed of a large statistics sample and had better control of systematic errors, found a considerably lower value. The first was sin2 w =0.24±0.02, reported by the CDHS experiment [9] (radiative corrections which were then not yet calculated, would change the published result to 0.23 ± 0.02, perfectly compatible with today’s world average for the leptonic effective mixing angle, sin2 w = 0.23148 ± 0.00017 [10]). BEBC [11] reported 0.22 ± 0.05 and CHARM [12] 0.220 ± 0.014 and thus confirmed the new lower value of sin2 w . An important question was the Lorentz-structure of neutral currents. Establishing the V–A structure of charged currents had taken 25 years. How much time would be needed now? The Standard Model predicted neutral currents as, in general, unequal mixture of vector and axialvector currents constructed from lepton and quark fields. This expectation which was with the known value of sin2 w unambiguous and quantitative, was rapidly confirmed by CDHS [13], BEBC [14], and CHARM [15]. Five years after the
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
61
discovery of neutral currents their Lorentz-structure was settled in favour of the Standard Model prediction. In retrospect, one might say that this result marked the beginning of the Standard Model’s ‘tyranny’: the next 25 years would see all experiments confirming with ever greater precision the predictions of the Standard Model. Rapidly, in the early 1980s, the physics essence of the Standard Model shifted from Born approximations to the level of loop calculations. No prediction in Born approximation was in conflict with confirmed experimental results. Would the same be true at loop level? The first test at loop level was made possible with the Z boson newly discovered in the UA1 and UA2 experiments. Wheater and Llewellyn Smith [16] pointed out that the predictions of its mass in terms of sin2 w differed by ∼ 5 GeV in Born approximation and at one-loop level. With (in the ‘on-shell’ renormalization scheme) = e2 /4 and sin2 w = 1 − m2W /m2Z , electroweak (one-loop) radiative corrections were concentrated in the then popular parameter r introduced by Sirlin [17]. With this parameter, the relation between the Z mass, the Fermi coupling constant GF and the electroweak mixing angle became 1 1 1 GF = √ . 2 2 2 2 mZ sin w (1 − sin w ) 1 − r The r was sensitive to the squared mass of the top quark. That suggestion was followed up, and in a dedicated high-statistics experiment the electroweak mixing angle was determined with much better precision: 0.225 ± 0.006 by CDHS [18], and 0.236 ± 0.006 by CHARM [19]. The existence of the radiative correction was supported at the 3 level. Furthermore, because of the sensitivity of the radiative correction on the top-quark mass, for the first time an indication of an upper limit on the mass of the top quark around 200 GeV had shown up. In parallel to this development, the measurement of elastic scattering off electrons was developed into an art by the successor of CHARM, CHARM II: their final result for the electroweak mixing angle was sin2 w = 0.232 ± 0.008 [20], well consistent with measurements from other processes. 2.3. Charm–quark pair production in hadronic collisions Parallel to the rise of the Standard Model, the quest for physics beyond the Standard Model rose. One of the areas where physicists hoped to break new ground, was a beam dump experiment: as many protons of the highest energy would be dumped into dense matter, in the hope that new particles (e.g. heavy leptons or axions) would be produced. They or their decay products could be penetrating like neutrinos and would be recorded in downstream neutrino detectors. One source of background was known: neutrinos from the decay of normal ± and K ± in the hadronic cascade initiated by the incoming protons. This problem could easily be handled by employing dumps with different density and extrapolating to infinite density. Also, this background should predominantly consist of . The result of the first beam dump experiment was a surprise. All three CERN experiments took part: BEBC, CDHS and CHARM. BEBC, the experiment with the lowest statistics but with the best electron identification capacity, took the lead. Boosted by a favourable upward statistical fluctuation, they reported an anomalous e signal [21]. This early result was confirmed, and measured more precisely in two more beam dump campaigns, by the CDHS [22] and CHARM [23] experiments.
62
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
What had been seen? It was the production of charm quarks in hadronic collisions, with subsequent semileptonic decay which gave rise to a ‘prompt’ flux of , , e and e . Thus, open charm production in hadronic collisions was seen for the first time, with a cross-section one order of magnitude higher than was then expected from the negative results of earlier emulsion experiments. 2.4. Getting used to ‘appearance’ and ‘disappearance’ Although there was no accepted evidence for neutrino oscillation the existence of which was steadfastly advocated by Pontecorvo [24] and others—the significant evidence from solar neutrinos measured by Davis and collaborators [25] was not understood and therefore ignored; the evidence from atmospheric neutrinos measured by NUSEX [26], the Fréjus experiment [27], and IMB [28], was seemingly conflicting and also ignored—, physicists proposed and carried out dedicated searches for neutrino oscillations in low-energy beams at the CERN PS. CDHS [29] and CHARM [30] did not observe the ‘disappearance’ of . BEBC [31] did not observe the ‘appearance’ of e . The experimental results were correct, but as known today, they were carried out in a region of oscillation parameter space where no oscillations could be observed. The CDHS result is still today the best for the generic → x oscillation, where x stands for any type of penetrating neutrino-like particle other than the . 2.5. Handing over the torch In 1984, the neutrino programme at CERN came to a halt. BEBC was closed down to save money and liberate manpower for LEP construction. The CDHS and CHARM detectors stopped data taking and were dismantled. CERN’s West Area Neutrino Facility was preparing for a new, quite different physics challenge: the search for → oscillations. The programme of analysing millions of neutrino–nucleon scattering events was after 1984 solely carried on by the CCFR experiment at Fermilab, which resembled in its design the CDHS experiment at CERN. Their experimental programme lasted for many more years and finally exceeded in precision significantly the earlier results of the CERN experiments. Most noteworthy are their final results on the nucleon structure functions and on the electroweak mixing angle. The studies of the nucleon structure function were of course dominated by H1 and ZEUS results at HERA, however the comparison with precise—and consistent—results from neutrino scattering still provided essential information such as the valence and sea structure functions. The precision attained by CCFR is perhaps best reflected by their measurement of the Q2 dependence of the various nucleon structure functions [32]. The error on the electroweak mixing angle which had been reduced to 0.005 already in the CERN experiments, was further reduced by CCFR to well below 0.002 [33]. The slightly worrying feature of their result sin2 w =0.2277±0.0016 is that it is at the 2.4 level above the combined result from LEP and the SLC [10]. Some see in this a promising deviation from the Standard Model, others are more sceptical and rather suspect problems with the nucleon structure functions that are needed in this analysis.
3. 1985–1998: not reaching the oasis After the first period of the exploitation of neutrino beams at the CERN SPS was completed in 1984, a significant change of paradigm took place: the search for neutrino oscillations became the prime challenge.
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
63
On the one hand, no good theoretical justification for a zero neutrino mass had ever been put forward; on the other hand, speculations abounded on the contribution of finite neutrino masses to hot dark matter, and thus to the mass density of the universe. To close the universe, the sum of masses of all neutrino families was conjectured not to exceed, roughly, 30 eV [34]. Naturally, with the oscillatory term 2 2 2 1.267m [eV ]L[km] sin , E [GeV] neutrino masses in that range would lead to observable oscillations over baselines of order 1 km with neutrino energies of order 10 GeV. The CERN wide-band neutrino beam appeared to be the right vehicle for discovery! In the first proposal, put forward by Vannucci and collaborators [35], would be intercepted on the Jura mountain range west of CERN, at a distance of about 20 km, with a view to searching for disappearance. This then bold proposal marked the beginning of what is called today ‘long-baseline’ oscillation experiment. However, this proposal was politically inconvenient, as negative effects on the construction of LEP were feared. Insofar, this proposal also marked the growing importance of political considerations in scientific decision making. In retrospect, the non-acceptance of the proposal can hardly be criticized because with today’s knowledge of oscillation parameters, the experiment would not have observed oscillations. 3.1. The emulsion approach The second proposal was daring only from the technological point of view. Inside the CERN site, Winter and collaborators [36] proposed to search for the appearance of in a wide-band beam. The ’s produced in their charged-current reactions would be identified by their finite decay path of a few 100 m in a 770 kg target of emulsion sheets. This proposal was accepted and promoted the emulsion technology to a prominent place in the search for the interaction of with nucleons. Today, the technique is referred to as ‘emulsion approach’. So far, it was most successfully exploited in the DONUT experiment at Fermilab which reported in 2001 evidence for the production of four ’s produced by the component in the neutrino flux behind a dump for 800 GeV protons [37]. In first results, CHORUS reported [38] the absence of → oscillations; their final results are still to come. Their experimental method and their goal were perfectly correct, but unfortunately for them and for CERN, oscillations were out of their experiment’s reach. 3.2. The kinematic approach Vannucci and collaborators had not given up. They came back with a novel alternative to the emulsion approach, which became known as the ‘kinematic approach’. The production of a few ’s amidst an overwhelming background of charged-current and neutral-current interactions, would be signalled by sophisticated cuts on the distribution of secondary hadrons in the transverse plane of the event, with a view to distinguishing final-state electrons and muons from the primary neutrino interaction vertex, from electrons and muons from decays. Their proposal [39] was accepted and led to the NOMAD experiment. There was also an element of nostalgia: NOMAD re-used the giant magnet which had been constructed for the UA1 experiment.
64
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
NOMAD took data in the West Area wide-band neutrino beam until 1998, concurrently with CHORUS. Their ‘kinematic approach’ proved successful. In 2001 they published their final result [40]: sin2 2 < 3.3×10−4 for large m2 . Like for CHORUS, their method as well as their goal were perfectly correct but neutrino oscillations were too small in their sensitivity domain.
4. 1999 and beyond: responding to the new challenge 4.1. The irresistible rise of non-accelerator experiments By 1998, the depletion of the solar neutrino flux which had been claimed by Davis and collaborators for 25 years already [41], had been confirmed by GALLEX [42], SAGE [43], Kamiokande [44] and SuperKamiokande [45]. In 1998, the observation of atmospheric neutrino oscillations in the SuperKamiokande detector was announced [46], based on a significant depletion of with respect to e in the atmospheric neutrino flux, with a telling dependence on zenith angle. After 1 year of shock and reluctance, the reaction of the particle physics community was decisive. In the USA, the MINOS experiment which was to search for disappearance between Fermilab and the Soudan mine in 732 km distance, was re-optimized to cope with the m223 ∼ 3 × 10−3 eV2 claimed by SuperKamiokande. In Europe, earlier plans of a search for oscillations in a beam from CERN to the Gran Sasso Laboratory in Italy found strong support, also and particularly at the upper echelons, and condensed into a major European experimental neutrino programme. The third phase had begun. 4.2. Neutrinos from CERN to Gran Sasso The only realistic possibility for a neutrino beam to Gran Sasso was to build on the successful experience with a wide-band neutrino beam at the CERN SPS. As for the experimental programme, the obvious decision was taken to build on the tradition of the emulsion and kinematic approaches that had been successfully developed by CHORUS and NOMAD at CERN. The new ‘CERN-Neutrinos-to-Gran Sasso’ (CNGS) beamline [47] has as new features the direction to Gran Sasso (which implies a downward inclination of 5.6◦ ), a much longer decay tunnel than was usual before (to maximize the neutrino flux), and a horn/reflector focussing which favours the flux around 15 GeV energy (in order to maximize the number of charged-current interactions with nucleons). The decision to launch this neutrino programme was taken in 2000. The construction is well under way. According to the planning, CNGS will be operational in 2006. 4.3. Tradition versus new ideas The experiment which exploits the emulsion approach, is OPERA [48]. Its prominent feature is a 1.8 kt lead target, organized in two sections, finely segmented and instrumented with emulsion sheets. Each target section is followed by a spectrometer which measures particle tracks (to narrow the search for the event’s interaction vertex in emulsion tracks) and calorimetric energy. OPERA is under construction in the Gran Sasso Laboratory and is expected to take first data soon after the commissioning of the CNGS. A second experiment which exploits the kinematic approach, is ICARUS [49]. This experiment has a 3000 t fully sensitive liquid argon target which is read out like a Time Projection Chamber. Although it
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
65
permits ‘pictures’ of events with amazing detail known only from bubble-chamber photographs before, its spatial resolution cannot compete with emulsions when it comes to the identification of the finite flight path. Hence, ICARUS relies on kinematic cuts to isolate interactions. This, however, can be done with unparalleled perfection, given the detailed measurement of each event. Interestingly, despite of the very different methods, OPERA and ICARUS claim to observe the same number of ↔ oscillations: at the presently favoured value m223 ∼ 2.0 × 10−3 eV2 , 11 events on top of a background of 1 event. The anticipated running time is 5 years. The amazingly fast experimental progress in pinning down the numerical values of oscillation parameters in the past five years, has already shifted attention to details of the neutrino mixing matrix. The prime target of attention is the mixing angle 13 , which ‘connects’, if non-zero, solar-neutrino oscillation phenomena with those of atmospheric neutrinos. Most importantly, 13 must be non-zero for CP-violation in the neutrino mixing matrix to occur, with possibly important consequences for leptogenesis and the matter–antimatter asymmetry of the universe. The CNGS with a horn/reflector system optimized to focus 4 GeV/c pions, and exploited in off-axis geometry with a moveable underwater Cherenkov light detector in the Gulf of Taranto south of Italy, would permit not only precise measurements of the amplitude and phase of the leading oscillation ↔ , but also of the amplitude of the sub-leading oscillation ↔ e , and thus of the angle 13 . 4.4. Outlook CERN would not be CERN if it were not thinking, together with its community of physicists, of future opportunities and options for what might come after the LHC. No doubt that neutrino oscillations moved neutrino physics again into the limelight. The discovery of finite neutrino masses constitutes a ‘mild’ extension only of the Standard Model, but there is strong hope of further ramifications which may eventually help understanding important aspects of physics beyond the Standard Model. Experimentally, the challenge neutrino physics at accelerators is to measure as precisely as possible the neutrino masses, the three neutrino mixing angles 12 , 13 and 23 , and the CP-violating phase of the mixing matrix. The accelerator of choice has been identified and both the machine aspects and prospects for experiments have been studied in quite some detail already. It is a storage ring for muons, first proposed by Geer [50], at a momentum of ∼ 50 GeV/c, with long straight sections along which decaying muons lead to and e beams of unparalleled intensity and quality. The machine has become known as ‘Neutrino Factory’ and several studies confirmed that it offers by a large margin the best opportunities for significant advances in measurement accuracy.
5. Summary Neutrino physics has undoubtedly a glorious past at CERN. It entered the scene with the groundbreaking discovery of neutral currents in 1973, and made essential contributions toward establishing firmly the rule of the Standard Model until 1984. Nature’s choice of oscillation parameters was not favourable to past CERN experiments in the search for neutrino oscillations, yet permits CERN neutrino beams to play a major role in forthcoming long-baseline studies of neutrino oscillations.
66
F. Dydak / Physics Reports 403–404 (2004) 57 – 67
References [1] A. Benvenuti, et al., Phys. Rev. Lett. 36 (1976) 1478; A. Benvenuti, et al., Phys. Rev. Lett. 37 (1976) 189. [2] M. Holder, et al., Phys. Rev. Lett. 39 (1977) 433. [3] P.C. Bosetti, et al., Nucl. Phys. B 142 (1978) 1. [4] J.G.H. de Groot, et al., Phys. Lett. B 82 (1979) 292; J.G.H. de Groot, et al., Phys. Lett. B 82 (1979) 456. [5] J.G.H. de Groot, et al., Z. Phys. C 1 (1979) 143; H. Abramowicz, et al., Z. Phys. C 12 (1982) 289; H. Abramowicz, et al., Z. Phys. C 13 (1982) 199; H. Abramowicz, et al., Z. Phys. C 17 (1983) 283; P. Berge, et al., Z. Phys. C 49 (1991) 187. [6] M. Jonker, et al., Phys. Lett. B 109 (1982) 133; F. Bergsma, et al., Phys. Lett. B 123 (1983) 269; F. Bergsma, et al., Phys. Lett. B 153 (1985) 111. [7] M. Holder, et al., Phys. Lett. B 69 (1977) 377. [8] P. Musset, in: F. Gutbrod (Ed.), Proceedings of the International Symposium on Lepton and Photon Interactions, Hamburg, 1977, DESY, Hamburg, 1977, p. 785. [9] M. Holder, et al., Phys. Lett. B 71 (1977) 222; H. Abramowicz, et al., Z. Phys. C 28 (1985) 51. [10] Summary of electroweak results from LEP and SLC, Report CERN-EP/2002-091 (hep-ex/0212036). [11] P.C. Bosetti, et al., Phys. Lett. B 76 (1978) 505; P.C. Bosetti, et al., Nucl. Phys. B 217 (1983) 1. [12] M. Jonker, et al., Phys. Lett. B 99 (1981) 265. [13] M. Holder, et al., Phys. Lett. B 72 (1977) 254. [14] H. Deden, et al., Nucl. Phys. B 149 (1979) 1. [15] M. Jonker, et al., Phys. Lett. B 102 (1981) 67; J.V. Allaby, et al., Phys. Lett. B 231 (1989) 317. [16] J.F. Wheater, C.H. Llewellyn Smith, Nucl. Phys. B 208 (1982) 27; J.F. Wheater, C.H. Llewellyn Smith, Nucl. Phys. B 226 (1983) 547 (Errata). [17] A. Sirlin, Phys. Rev. D 22 (1980) 185. [18] H. Abramowicz, et al., Phys. Rev. Lett. 57 (1986) 298. [19] J.V. Allaby, et al., Phys. Lett. B 177 (1986) 446; J.V. Allaby, et al., Z. Phys. C 36 (1987) 611. [20] P. Vilain, et al., Phys. Lett. B 335 (1994) 246. [21] P.C. Bosetti, et al., Phys. Lett. B 74 (1978) 143; P. Fritze, et al., Phys. Lett. B 96 (1980) 427; H. Grässler, et al., Nucl. Phys. B 273 (1986) 253. [22] T. Hansl-Kozanecka, et al., Phys. Lett. B 74 (1978) 139; H. Abramowicz, et al., Z. Phys. C 13 (1982) 179; P. Berge, et al., Z. Phys. C 56 (1992) 175. [23] M. Jonker, et al., Phys. Lett. B 96 (1980) 435; F. Bergsma, et al., Phys. Lett. B 157 (1985) 458; J.V. Allaby, et al., Z. Phys. C 40 (1988) 497. [24] B. Pontecorvo, Sov. Phys. JETP 26 (1968) 984. [25] R. Davis, et al., Phys. Rev. Lett. 20 (1968) 1205. [26] M. Aglietta, et al., Europhys. Lett. 8 (1989) 611. [27] Ch. Berger, et al., Phys. Lett. B 227 (1989) 489. [28] R. Becker Szendy, et al., Phys. Rev. D 46 (1992) 3720. [29] F. Dydak, et al., Phys. Lett. B 134 (1984) 281. [30] F. Bergsma, et al., Phys. Lett. B 142 (1984) 103.
F. Dydak / Physics Reports 403–404 (2004) 57 – 67 [31] C. Angelini, et al., Phys. Lett. B 179 (1986) 307. [32] W.G. Seligmann, et al., Phys. Rev. Lett. 79 (1997) 1213; U.K. Yang, et al., Phys. Rev. Lett. 86 (2001) 2742; U.K. Yang, et al., Phys. Rev. Lett. 87 (2001) 251802. [33] G.P. Zeller, et al., Phys. Rev. Lett. 88 (2002) 091802; G.P. Zeller, et al., Phys. Rev. Lett. 90 (2003) 239902 (Erratum). [34] See, for example, H. Harari, Phys. Lett. B 216 (1989) 413. [35] T.C. Bacon, et al, Proposal SPSC/82-20/P178 (CERN). [36] N. Armenise, et al., Proposal SPSC/90-42/P254 (CERN). [37] K. Kodama, et al., Phys. Lett. 504 (2001) 218. [38] E. Eskut, et al., Phys. Lett. B 497 (2001) 8. [39] P. Astier, et al., Proposal SPSLC/91-21/P261 (CERN). [40] P. Astier, et al., Nucl. Phys. B 611 (2001) 3. [41] B.T. Cleveland, et al., Astrophys. J. 496 (1998) 505. [42] W. Hampel, et al., Phys. Lett. B 447 (1999) 127. [43] J.N. Abdurashitov, et al., Phys. Rev. Lett. 83 (1999) 4686; J.N. Abdurashitov, et al., J. Exp. Theor. Phys. 95 (2002) 181. [44] Y. Fukuda, et al., Phys. Rev. Lett. 77 (1996) 1683. [45] Y. Fukuda, et al., Phys. Rev. Lett. 81 (1998) 1158; Y. Fukuda, et al., Phys. Rev. Lett. 86 (2001) 5651. [46] Y. Fukuda, et al., Phys. Rev Lett. 81 (1998) 1562; Y. Fukuda, et al., Phys. Lett. B 433 (1998) 9; Y. Fukuda, et al., Phys. Lett. B 436 (1998) 25. [47] G. Acquistapace, et al., Proposal CERN 98-02 (INFN/AE-98/05). [48] M. Guler, et al., Proposal CERN-SPSC/2000-028 (CERN). [49] P. Aprili, et al., Proposal CERN-SPSC/2002-027 (CERN). [50] S. Geer, Phys. Rev. D 57 (1998) 6989; S. Geer, Phys. Rev. D 59 (1999) 039903 (Erratum).
67