OSCILLATIONS AT ACCELERATORS. Jacques Bouchez. DAPNIA, CEA Saclay. Les Arcs, France march Abstract

Size: px

Start display at page:

Download "OSCILLATIONS AT ACCELERATORS. Jacques Bouchez. DAPNIA, CEA Saclay. Les Arcs, France march Abstract"

Randolf Allison
5 years ago
Views:

1 THE FUTURE OF NEUTRINO OSCILLATIONS AT ACCELERATORS Jacques Bouchez DAPNIA, CEA Saclay Invited talk at the 32 nd rencontre de Moriond on electroweak interactions and unied theories Les Arcs, France march 1997 Abstract The future neutrino oscillation experiments at accelerators are reviewed. Long baseline experiments will address the atmospheric neutrino anomaly. Intermediate baseline experiments will be sensitive to the LSND eect. Short baseline experiments will increase our sensitivity on mixing parameters by an order of magnitude in the domain of cosmologically relevant neutrino masses. 1

2 2

3 1 The present situation and strategies for the future Presently, there exist several indications of neutrino oscillations, that is the possibility for neutrinos born in a given avor to develop components in the other avors, the probability of such an occurence showing an oscillatory pattern with time. Such a phenomenon implies neutrino masses and a mismatch between mass eigenstates and avor eigenstates, leading to lepton number violation. More specically, in the 3-family case, the conversion probability for a of energy E to interact at a distance L as a is given by (assuming CP conservation, which implies that a real rotation matrix U links the avor eigenstates to the mass eigenstates): with and P!(L) =, a 12 sin 2 12, a 13 sin 2 13, a 23 sin 2 23 a ij =4U i U j U i U j ij = m2 j, m2 i 4 L E In case of strong mass hierarchy (m 1 m 2 m 3 ), each avor transition is a sum of two oscillation terms (as 13 ' 23 ) with dierent amplitudes and 2 dierent frequencies respectively governed by m 2 2 (low frequency, that is long oscillation length) and m 2 3 (high frequency, short oscillation length). The sensitivity in neutrino mass of a given experiment isthus determined by the typical value of the ratio L=E. The higher this ratio, the smaller the lower bound of neutrino masses giving rise to an observable avor transition (for comparable luminosities and signal/background ratios). [In particular, it is more ecient, to gain sensitivity to smaller masses, to either lower the energy of the beam or to put a bigger detector at a larger distance from the source rather than to simply increase the detector size]. The 3 present indications of oscillations come from solar neutrino experiments [1], from atmospheric neutrinos [2], and from the Los Alamos experiment [3]. The explanations in terms of oscillations correspond to 3 very distinct mass scales, respectively 10,5, 10,2 and 1 ev 2. It is clear that in a 3-family scheme, at least one of these experiments has to be either wrong or uncorrectly interpreted in terms of oscillations, since only two dierent 3

4 mass scales should be present. To clarify the situation, it is crucial to check these results with other experiments, using if possible dierent techniques to decrease the chance of unidentied common systematic eects. In the case of solar neutrinos, the mass scale of 10,5 ev 2 implies huge oscillation lengths ( km for 1 GeV neutrinos) and small (below 1%) vacuum transitions 1. It is clear that such oscillations are out of reach of accelerator experiments. Fortunately, SNO [5] and Borexino [6] should give precious informations on solar neutrinos within the next few years. The atmospheric neutrino anomaly consists of a decit by about a factor of 2 in the = e ratio, seen by several (but not all) underground experiments. Kamioka has interpreted this as an oscillation with quasi maximal amplitude of 's towards either e 's or 's. They have determined the mass scale to be around 10,2 ev 2 based on the zenithal dependance of the eect [7]. This corresponds to an L/E ratio between 30 and 200 km/gev. Accelerator experiments using neutrinos above a few GeV ying several hundreds of kilometers could check this eect. Several so-called long baseline experiments are foreseen to start around the year In the meantime, Superkamioka should give in a few months more precise updates on the eect. And the CHOOZ experiment [8], 1 km away from a nuclear reactor, should very soon be able to check this oscillation provided that it occurs between and e. In the case of Los Alamos, which has seen an excess of events possibly coming from to e oscillation, the typical L/E value is around 1 km/gev. Within 2 or 3 years, KARMEN [9] which uses the same kind of low energy neutrino beam, should be able to conrm or disprove the Los Alamos result. Accelerator experiments can also check this eect, and furthermore, can search also for appearance at the same mass scale provided that the beam energy is above 5 GeV, allowing the production of tau leptons by charged currents. Intermediate baseline experiments, 20 km away from the source of 20 GeV neutrinos, would be adequate. Finally, in front of the somewhat confused present situation, another totally legitimate strategy is to focuss on unexplored domains of oscillation parameters. In particular the domain of high neutrino masses (above a few ev) is of cosmological relevance as it could explain at least part of the dark matter in the Universe. Two accelerator experiments (CHORUS 1 Very specic matter eects described by the MSW formalism [4] and due to a very high and varying electron density in the sun are believed to strongly suppress solar neutrinos by driving them adiabatically to heavy mass eigenstates. Alternative explanations based on vacuum oscillations give 10 5 bigger oscillation lengths. 4

5 CHORUS TOSCA COSMOS proton on target / year running time CC / Ton CC / Ton target mass eciency gure of merit Table 1: Comparison of the short baseline projects with CHORUS and NOMAD) are presently searching for appearance with a sensitivity of a few 10,4 on the oscillation amplitude. These are short baseline experiments (L/E ' 0:05 km/gev). And projects are underway to gain an order of magnitude on the oscillation probability at high masses. 2 Short baseline projects There are presently 2 projects, one at FNAL (COSMOS [10]) and one at CERN (TOSCA [11]) which aim at reaching a sensitivity of a few 10,5 on the probability of oscillation between and. Both experiments will use emulsions as the active target to have a positive signature of the production and muonic decay of a tau lepton by its direct observation. The background in this case is mainly due to production and decay of a hadron containing a c antiquark, or to the diusion of a onanucleus with no visible recoil (white stars). Kinematic cuts can keep these backgrounds very low. The table 2 shows how these experiments will gain in sensitivity with respect to CHORUS. The gain is due to: a higher number of neutrino interactions (higher beam intensity and bigger target mass, overcompensating the smaller cross-section, as COS- MOS uses 120 GeV protons from the main injector and TOSCA considers using 350 GeV protons from the SPS rather than 450 GeV protons like CHORUS, in order to maintain the interaction rate of 's produced at the target to a negligible level). a better eciency on the signal, obtained by using a spectrometer following the emulsion with higher acceptance and better resolution. 5

6 CHORUS TOSCA reconstructed muons kink nding scanning eciency trigger, dead time total eciency Table 2: Comparison of the eciencies of CHORUS and TOSCA for the muonic decay of the lepton COSMOS will use a ber tracker and hopes to improve the resolution on the momentum of the outgoing muon by a factor of more than 5, giving a sharper jacobian peak in the p T distribution of the! decay. TOSCA will use silicon trackers followed by honeycomb chambers to reach a high spatial resolution. With a 20m precision on the extrapolation of the tracks on the emulsion sheet, they will gain a factor of 100 on the surface of the area to be scanned and improve the matching eciency. Both experiments will benet from the remarkable improvements achieved by CHORUS in the automated scanning and processing of emulsions [12] and will therefore be able to handle the higher statistics in a reasonable time. The table 2 illustrates the improvement in eciency of TOSCA compared to CHORUS on the muonic decay of the tau. These experiments should run somewhere between 2000 and COS- MOS is approved, whereas TOSCA has sent in march a letter of intent. Meanwhile a silicon tracker prototype (STAR) is being tested inside the NOMAD detector during the 1997 run. 3 Long baseline projects Three labs in the world are planning to send a neutrino beam in underground detectors several hundred kilometers away, in order to check whether or not the atmospheric decit is due to oscillations. The rst one is KEK [13], which should send 1-2 GeV neutrinos in 1998 towards SuperKamioka, 250 km away. The neutrino interactions will be separated between e CC and CC using the same ring technique used 6

7 for atmospheric neutrinos. The high mass of the target (22 kt ducial) should give 160 CC per year. A smaller water Cerenkov detector has been installed 500 meters away from the neutrino source to measure the initial e contamination in the beam. A nearly maximal mixing should be seen already after 1 year of data taking, so that rst results are expected before 2000! The only drawback is that the neutrino energy is too low to produce CC, but a to oscillation will be signed by a decit in with no appearance of e. This project is approved and funded, and is ahead by a few years on the other projects. KEK is considering the possibility to build a 5 GeV neutrino beam from 50 GeV protons which could be ready in The second project aims at sending the main injector neutrino beam (used by COSMOS) to the SOUDAN underground laboratory, 730 km away, where the MINOS detector would detect them [14]. This detector exists only on paper as of today, R&D being still underway. It would consist of 3 modules, 3.3 kt each, made of sandwiches of 4cm thick magnetized iron plates interspaced with 200 XY detection planes. This project is approved and if funding is provided, MINOS could start taking data with one module in year CC interactions would be recorded in a 9 month period, enough to see the disappearance and spectrum distorsion predicted by Kamioka. Studies are underway to assess the possibility of identifying interactions, either à la NOMAD or by adding emulsion sheets. The third project is to send a neutrino beam from CERN to the Gran Sasso laboratory, 732 km away. The ICARUS detector [15], consisting of several modules of liquid argon operated as TPC's, could be used to detect neutrino interactions. Owing to its excellent granularity, it could fully reconstruct events and identify electrons, except for muons which have to be measured by a spectrometer downstream (MACRO could be a possibility). The identication à la NOMAD of CC should be feasible. The construction of the beam would take 5years once it is approved, which is not presently the case. The rst ICARUS module is presently built in Italy. 3 such modules would give a 1.2 kt target, which ismuch less than the other detectors. With the presently foreseen luminosity, 3 ICARUS modules at Gran Sasso would record 2000 CC events per year, starting after

8 JAPAN USA EUROPE Accelerator KEK FNAL Main Injector SPS proton energy 12 GeV 120 GeV 450 GeV neutrino energy 1-2 GeV 10 GeV 25 GeV completion > 2003 far detector SuperKamioka MINOS ICARUS ducial mass 22 kt 5kT 1.2 kt CC /(kt year) status running R&D construction front detector yes, built yes,? JURA start of data taking (1/3 of MINOS) after 2003 Table 3: characteristics and status of the 3 long baseline projects The table 3 gives a comparison of the 3 long baseline projects. 4 Intermediate baseline projects In order to check the Los Alamos indication, one needs an L=E ratio of 1 km/gev. Two projects satisfying this condition are presently proposed: The LSND collaboration has sent to FNAL a letter of intent [16] to use the booster to make a low energy (500 MeV) antineutrino beam, with very high avor purity ( e = '10,3 ) and try to detect e interactions with an LSND-like detector placed at 500 m or 1 km from the source. The rate of CC interactions would be around 10 6 per year. Avery unique possibility exists at CERN since the SPS neutrino beam, after entering the Jura moutain, gets out 17 km away from the source near a secondary road at a place where a detector could be installed. A possibility would be to install one ICARUS module at this location [17]. The neutrino ux would be sucient to detect 6000 quasi elastic events for protons on target. In this simple event topology, it is easy to measure accurately the rate of and e induced events and check in 2 or 3 years the LSND result. Furthermore, it is also possible to sign with high eciency and very small background induced quasielastic events giving a high missing transverse momentum. This experiment would thus be the only one able to check both types of oscillations ( 8

9 to e and to ) in the LSND mass range. This project has been proposed recently at CERN together with the long baseline project by C.Rubbia. The so-called JURA project would benet from the good knowledge of the beam attained by NOMAD, and could be used as the near-detector (with identical technique, although not in the same beam) for ICARUS at Gran Sasso. 5 Conclusion Although the present situation on neutrino oscillations is somewhat confused, the experimental eorts, present and future, will hopefully help in clarifying the situation. The rst results to come, within a few months, are those of CHOOZ and SuperKamioka; they will have a big inuence on the interest of long baseline experiments. On the other hand, the LSND result has to wait 2 or 3 years before being conrmed or disproved by KARMEN; a conrmation would give a considerable importance to the JURA experiment, the only one able to test appearance. Of course, positive indications of oscillations in CHORUS and NOMAD would add to the confusion, and make the next generation of short baseline experiments absolutely mandatory. Note added (july 1997) After this report was written, new informations have become available: SuperKamioka has given rst preliminary results on atmospheric neutrinos, conrming the global decit of 's relative to e 's. But the azimuthal dependance for multi-gev events, although not incompatible with Kamioka, looks much less pronounced and is compatible with a at distribution. More precise results are eagerly awaited. The CERN committee SPSLC has recommended the construction of a neutrino beam aiming at Gran Sasso. It seems now possible to complete this beam in Furthermore, a new scheme for the SPS supercycle (after LEP is stopped) would give a factor 3 improvement in neutrino ux. A sooner start with higher intensity makes ICARUS at Gran Sasso more competitive with respect to the KEK and FNAL projects. As a consequence of this scenario, the JURA project is compromised and TOSCA (also recommended by the SPSLC) would have tomove 9

10 to an underground hall in the new beam, 1 km away from the source. It is conceivable however that a totally at azimuthal distribution in SuperKamioka would lead to reconsider this scenario. References [1] GALLEX: W.Hampel et al, Phys.Lett. B388 (1996) 384; SAGE: J.Abdurashitov et al, Nucl.Phys.B (Proc.Suppl.) 48 (1996) 299; Homestake: R.Davis Jr, Nucl.Phys.B (Proc.Suppl.) 48 (1996) 284; SuperKamioka: Y.Suzuki, Nucl.Phys.B (Proc.Suppl.) 38 (1995) 54 and Y.Takeuchi, these proceedings. [2] K.S.Hirata et al, Phys. Lett. B280 (1992) 237; Ch.Berger et al, Phys. Lett. B 245 (1990) 305; D.Casper et al, Phys.Rev.Lett. 66 (1991) [3] H.White, these proceedings. [4] see for example T.K.Kuo and J.Pantaleone, Phys.Rev.D 35 (1987) 3432 [5] G.Ewan et al, Sudbury Neutrino Observatory proposal, SNO [6] C.Arpesella et al, Borexino proposal, University of Milan (1991) [7] Y.Fukuda et al, Phys. Lett. B335 (1994) 237 [8] H. de Kerret et al, LAPP report (1993) [9] K.Eitel, these proceedings. [10] Fermilab experiment E803 [11] A.S.Ayan et al, letter of intent, march 1997, CERN-SPSC/97-5 [12] M.Vanderdonckt, these proceedings. [13] K2K, KEK experiment E362. [14] Fermilab experiment E875 [15] P.Cennini et al, ICARUS II experiment proposal, LNGS-94/99-I& II [16] H.White, private communication. [17] ICARUS collaboration, december 1996, CERN/SPSLC

Neutrino Experiments: Lecture 2 M. Shaevitz Columbia University

Neutrino Experiments: Lecture 2 M. Shaevitz Columbia University 1 Outline 2 Lecture 1: Experimental Neutrino Physics Neutrino Physics and Interactions Neutrino Mass Experiments Neutrino Sources/Beams and