481
LONG BASE-LINE NEUTRINO OSCILLATION BEAMS AND EXPERIMENTS
Stavros Katsanevas University of Athens 104 Solonos, GR-106 80 Athens.
Abstract
Strong interest has recently been shown in very long base-line neutrino beams1 directed at existing or planned massivedetector facilities, in order to extend the search forneutrino oscil lations. There are currently proposed experiments in the U.S, Japan and Europe. Among such possibilities are beams from CERN pointing towards the Gran Sasso Underground Laboratory in Italy and the NESTOR Underwater Laboratory in the Ionian Sea off the west coast of the Peloponnese. After a brief review of the long baseline beam possibilities in the U.S and Japan, the basic parameters of a CERN beam are studied. A number of possible configurations cov ering a range of neutrino energy bands are studied and estimates of the neutrino fluxes, event rates and backgrounds at typical detectors are reported. A neutrino oscillation search down to limits of sin2 0.01 and could be made with currently proposed detectors. 20 � �m2 � O.OOleV2 482
1 Introduction The relatively old idea of a long baselineaccelerator beam pointing to a detector located tens to hundreds of kilometers away hasevolved from the phase of first estimations [l] to that of realistic designs and detailed proposals [2]. Further, results of the KAMIOKANDE, !MB, and SOUDAN [3] collaborations suggest that perhaps as much as 40 % of the atmospheric v" events in the energy range from 0.2-1.5 GeV have oscillated to some other type of neutrino. On then other hand, FREJUS [4] did not record any effect. The recent 'multi-GeV' studies by KAMIOKANDE [5], have confirmed their previous result, and have even shown an azimuthal distribution of the ratio ;;,:- consistent with avµ --+ v, or vµ --+ vr oscillation signal. Clearly one would like to continue with controlled beam experiments because they have the fo llowing advantages over atmospheric data: Initial flavourcom position well known (typically 11, is 1% of 11µ ),
Control of the energy. One can typically obtain beam energy dispersions of as � 5 GeV One can assume the direction cosines and time of arrival of the neutrino, improving efficiency and reducing backgrounds substantially Higher statistics, giving sensitivity to lower mixing angles Control of the beam polarity. One can switch between v and ;;; beams to study matter enhanced oscillation (MSW) effects [6]).
2 Currently proposed beams and experiments The possibilities of long base-line beams are under active study in the US and Japan. In the US the two main proposals are: a beam from the Fe rmilab 120 GeV Main Injector to the Soudan underground laboratory at 732 km distance (MINOS proposal) [7] and the 24 GeV Brookhaven proton beam to sites up to 68 km on Long-Island [9]. There is also a Japanese proposal [10] of a beam from KEK {12 GeV) to Superkamiokande, 250 km away. MINOS The Collaboration [7] proposes to conduct a search for IIµ --+ v, and IIµ --+ llr oscillations detected by the comparison of signals in a "near" detector at Fermilab and a "far" detector at the Soudan site. A new 10 kt {36m long,4m radius) detector consisting of a sandwich of 4 cm thick octagonal steel plates separated by 2 cm gaps of active elements in a toroidal magnetic field of � 1.5 Tesla will be built at Soudan. In a standard year they expect ') 3.7xl020 protons on the target. They propose two focusing scenarii: a wide-band beam horn-reflector system peaking at 10 GeV in Ev and delivering 2100 111, Charged Current (CC) events per kiloton-year at the far detector and a narrow-band beam , using lithium lenses that can be tuned between 10 GeV and 30 GeV in Ev with energy spread of ±15%. The 11, backgrounds are estimated to be in both beams less than 1%. The decay tunnel, they presently favour, is 800m long and has lm radius. The near detector will have the same granularity and detector technology as the far detector. Simulations show that the neutrino energy spectrum in the center of the beam (� 25 cm radius) will be similar to that of the far detector. They will be able to start taking data in 2001. The existing high resolution lkt detector Soudan2 [8] in the same site, can provide an independent test with lower statistics. Among the lower energy proposals the Japanese proposal [10] proposes a setup where the 12 GeV proton beam of KEK after a 90 degrees bend is directed to 3 water cherenkov detectors and a near fine-grained detector. Their fiducial masses and distances are 1.6 ton at 500m, 2kt at 25 km and 22kt at 250 km (the Super-Kamioka detector). A fine grained detector of 10 tons will be positionned immediately in front of the first cherenkov detector. There is a horn focusing and the decay pipe is 200m long and l.5m in radius. For a total 1020 protons on target, which corresponds to 2-3 years of running, more than 500 CC events will be observed in the 22 kt fiducial volume for Super-Kamioka, 1700 CC events at the intermediate detector and 8000 CC events in the near detector. The 11, contamination is estimated to be less than 0.7% of the 111, flux. They wish to start taking data during 1999. Assuming protons on target every seconds for seconds/year l) 4xl013 l.9 l.75xl07 483
BNL The second low energy proposal 889 uses the BNL AGS. The experiment will search for vµ oscillations by means of four identical 4.6 kton {18m high, 9m radius) water Cherenkov neutrino detectors located 1,3, 24 and 68 km from the AGS neutrino source. The experiment will capitalize on the advantages of AGS: a) high 28 GeV proton beam intensity (6xl013 p per pulse every 1.6 sec) and narrow time-structure (8 bunches per pulse, 20-30ns wide) of the fast extracted protom beam to permit the detectors to be loacted on the earths surface. They have requested 102l protons on the target. A key aspect particular to this design, involves placing the detectors 1.5 degrees offthe center line of the neutrino beam, in order to obtain a lower neutrino mean energy (� 1 GeV). They use a double horn focusing sustem and the neutrinos are produced in a decay pipe 180m long having a radius of l.5m at the far end. For the above integrated luminosity they expect 2.2xl03, l.8xl04, ll.5xl05, and 10.3xl06 quasi-elastic muon events in decreasing order of distance in the 4 detectors. They also expect 4.5xl02, 3.6xl03, 23.2xl04, and 20.7xl05 neutral current events respectively. They could be ready to start ir0 taking data by the year 2000. Interest in a long baseline neutrino beam from CERN has been stimulated by the fact that as part of the LHC project at CERN, new transfer lines are required to bring fast extracted protons from the SPS to the LHC. It has been shown [11] that it would be possible to derive a neutrino beam from the Tl 87 line linking SPS-LSS4 to the LHC/LEP ring near point 8. Two substantial neutrino detector facilities exist or are under development which lie in the general direction of such a beam: the Gran Sasso underground laboratory (in particular the ICARUS detector [12]) at a dis tance of 731 km, azimuth 122.502 and declination 3.283 degrees w.r.t CERN. The proposal describes a 5 kt liquid argon TPC detector. An intermediate, fu nded, 600 ton detector will be ready to take data2l in 2 years time. the deep ( 4000m below sea level) underwater neutrino laboratory NESTOR [13] in the Mediterranean at a distance of 1676 km, azimuth 124.1775 and declination 8.526 degrees w.r.t CERN. The NESTOR protype module under construction consists of a 200 kton tower (32m diameter,240m height) having 12 floors distant by 20m with sparse photomultiplier (14/floor) coverage. It will be deployed during 1997. 3) The directional angles from CERN differ by 1.68 degrees in azimuth and 5.24 degrees in declination [14]. The possibility of providing neutrino beams to both of these facilities using common or shared beam equipment is clearly attractive; one cannot avoid though the cost of two decay tunnels. . 3 CERN Beam simulation A. Ball et al. [15] use GEANT [16] to fully simulate the beam line with realistic values for material thicknesses, magnetic field strengths, secondary interaction effects, absorption and multiple scattering in all materials. Incident proton beams of different energy (80, 120, 160 and 450 GeV) have been simulated.
They were assumed to have a normal intensity distribution with a = 0.5 mm at a multiply segmented beryllium target of 3 mm diameter and 80-120 cm overall length. A major systematic uncertainty in estimating neutrino fluxes is the model of hadronic interactions used to obtain the production spectra. Three production models have been com pared, GHEISHA [17], FLUKA [18] and a thermodynamic model using fitted parameters [19]. The thermodynamic model compared to FLUKA/GHEISHA underestimates by 50% the neu trino flux, but it is known that no attempt hasbeen made to fitparameters to experimental data below 25% of the proton energy. FLUKA was chosen as the production model since it appears to be more consistently checked against experimental data. Production spectra from GHEISHA are higher than FLUKA by 25-30 %. However, without experimental particle production data The limits reported in the last section correspond to year of protons on target of this detector 2l l 2xl019 The limits presented at the last section concern a modification of the present prototype which has a) modules 3) of 5m inter-floor distance, b) twice the number of photomultipliers per floor and c} floors twisted in angle for uniform space coverage. A fiducial volume of kt and years of runnjng have beem assumed. 100 2 484
at energies well below that of the incident beam, it is not possible to conclude that any one model simulates correctly the production or the I
E PF Hl5R25 H20R20 J-!20R40 H40R70 GeVp fluxes in 10 / 10l3 p/m2 v 4 v 80 3.70(0.67) 1.10(0.17) 0.96(0.17) 0.80(0.17) 120 6.00(1.10) 1.60(0.29) 1.40(0.28) 1.60(0.31) 160 9.00(1.70) 2.10(0.42) 2.10(0.40) 2.60(0.46) 450 31.00(5.80) 7.0(1.39) 6.50(0.12) 7.60(1.50) 8.40(1.60)
GeV v Events/ 1019 p/kt 80 151(27) 38(7) µ 38(7) 32(8) 120 341(61) 65(12) 55(11) 77(15) 160 578(116) 90(19) 86(16) 133(23) 450 3516(657) 390(72) 338(64) 473(93) 722(134) GeV E:; 2 in GeV < >-' 80 6.8(6.3) 7.0(6.9) 6.5(6.3) 6.5(7.5) 120 8.6(7.7) 7.0(7.6) 7.1(7.2) 8.3(8.1) 160 9.3(9.7) 8.0(8.0) 7.1(7.3) 8.9(9.3) 450 14.6(14.6) 9.4(9.4) 8.8(9.0) 10.5(10.7) 14.2(14.6) GeV v,/vµ (%) 80 0.66(0.66) 0.74(0.14) 0.78(0.83) 0.60(0.74) 120 0.57(0.61) 0.78(0.77) 0.92(1.00) 0.88(1.00) 160 0.50(0.50) 1.20(1.25) 0.93(1.00) 1.00(1.00) 450 0.50(0.50) 1.57(1.57) 1.58(1.71) 1.40(1.50) 1.11(1.20)
GeV iJ Events/10l9 p/kt 80 50(9) 0.5(-) µ 0.5(-) 0.6(-) 120 115(22) 1.5(0.2) 1.75(0.4) 1.9(0.4) 160 210(40) 4(0.7) 4(0.8) 4(0.8) 450 1500(300) 52(10) 51(9) 47(8) 33(6) Table 1: Event rates at Gran Sasso(NESTOR) for different focusinggeometries and beam ener gies.
The PF (perfect focusing) data are included to show the idealistic flux one would obtain
Note that the distance from FNAL to the MINOS/SOUDAN2 detector is also 732 Km <) 485
if all mesons entering the hornaperture could be focused parallel to the axis without absorption or scattering. Operating at the highest available energy is clearly the most efficient and cannot be compensated by even tripling the repetition rate at lower beam energies. Of the different focusing systems considered it would seem that the 20 GeV horn and 40 GeV reflectorsystem (H20R40) gives the best compromise between the flux/event rate obtained and the mean energy of the neutrino spectrum in order to obtain good sensitivity to oscillations with the expected detector efficiencies. Figure 1 shows that it possible to design focusing systems which select different energy "bites" from the available production spectrum.
The combined v and iJ1, fluxes obtained using quadrupole triplet structures designed to focus 20 GeV (40 GeV)µ mesons from the target with an incident proton beam of 450 GeV, are roughly equivalent to the yield from a single horn system without reflector. One obtains 374(76) events/kt/1019 for Gran Sasso(NESTOR) fora 20 GeV focusand 537(110) events/kt/1019p for a 40 GeV focus. One obtains generally higher neutrino event mean energies, (�13(18) GeV for a
20(40)GeV focus), v, backgrounds are slightly higher and the sign selection capability is ofcourse lost. The radial distribution of the beam at the two detector sites with the focusing system (H20R40) is shown in figure 2 for event rates. The distributions are relatively flatwith nominal radii of the order of 1.75 - 2 km at Gran Sasso and 3.5 - 4 km at NESTOR. About 90% of the flux is contained within a radius of l.5km at Gran Sasso so the beam direction should be maintained within 2 milliradians. A decay tunnel of � l.5m radius contains 95% of the decays that generate events within the detector acceptance. The number of detected events is roughly proportional to the tunnel length but it is mostly higher neutrino energy events that are detected by increasing the tunnel length. Another important element that enters into considerations of the decay tunnel length is the v, contamination in the beam. Table 1 shows that the ratio of v, to v is �1%, rising to 1.5% at 450 GeV; there is some evidence that a shorter decay tunnel might" decrease the low energy component of the spectrnm wich comes from secondary decays. v, µ
4 Neutrino oscillation studies The probability for a two flavour neutrino oscillation is: . . . 1.27om2L om Sin 2 2 Slll 2 2 Sin2 ( ) (1) P(• 2 , e) = e Ev 2 2 where om = mi - m� is the diffe rence of the mass eigenvalues in eV , ()is the two flavourmixing angle, L is the v propagation length in kilometres and Ev is the neutrino energy in GeV. The maximal oscillation is achieved for and it is e.g 123 km/GeV for om2 Ev = = O.OleV2/c2• As a rule of thumb the minimum statisticaln;m , sensitivities forthe mass differenceand mixing angle, for a large class of detection methods, are given approximatively by the followingformulae [l]:
(2)
(3)
Where N is the number of detected neutrino (total or CC) events 5l . Most of the experiments propose to measure oscillations using different complementary methods: disappearance. Measuring the CC (or only the quasi-elastic for the "low energy" vµ vµ beams) rate at a far and near detector is a means of measuring the oscillation to either V7 or For a detailed discussion of the exact numerical factors that enter in the above formulae, see e.g (2] s) 486
. The matching of the acceptances can become a serious source of systematic errors forthese v, large distances. From the "low energy" beamlines, BNL 889 estimate the departure from the f; law clue to the extended source to be less than 0.2 3, especially at 1.5° off-axis. Their overall detector and beam uncertainty extrapolation systematics are estimated to be of the order of 13. Among the " high energy" beamlines MINOS has also shown that one can match the energy distributions of the far and near detectors with appropriate fiducial cuts. They initially estimated a 43 remaining uncertainty on the average mean neutrino energy. The newer understanding of the origin of these discrepancies tends to lower this value to 23, out of which 13 is clueto the uncertainty of the hadronization model. The MINOS collaboration further intends to use the event energy differences (distinctive "oscillation dips") as a sign for oscillations. appearance. A clear excess over the expected contamination of the beam (� 13) v, v, v, can be an anambiguous and extremely sensitive probe of neutrino oscillations. In most cases a front detector monitors the presence in the beam, which is the most serious systematic v, uncertainty. The main background events come from misidentified 's from the hadronic system. 7l'o They tend to produce lower energy showers, which are cut using the longitudinal development of the shower or the number of photoelectrons in the water cherenkov detectors. Typical values for misidentified for "low energy" water cherenkov detectors are 3-53 of the CC for above 903 v,, efficiency for 11'0 CC, while one can obtain the same percentage of contamination for a reduced v, efficiency of 25-303 for "high energy" detectors of the type of MINOS or NESTOR. Ratio of Neutral Current NC/CC and direct detection. The ratio of muonless vT "NC" events to muonfull "CC" events is the most sensitive generic probe of oscillation since all and 823 of CC interactions result to muonless events. The "low energy" setups use the v, vT ratio of NC single pion production to the CC µ production. This experimental ratio is more sensitive for than since there is a phase space suppression factor and the branching ratio v, vT T BR(r µX) is � 183, affecting the sensitivity to oscillations. This test remains nevertheless -t vT the most senitive test for oscillations. A smaller part of the parameter space can be probed by vT direct search for by using kinematical cuts (different hadronic energy distribution, planarity r, tests etc) for the "high energy" type of experiments. The MINOS collaboration hopes to take advantage of the narrow band beam kinematical constraints, once a signal has been detected, to unambiguously tag a appearance by tagging a high unbalanced electron or muon. T PT Fig 3 shows the estimated 903 CL limits of the 5 reviewed setups for the 3 methods above6>. If neutrino oscillations exist within the accessible range they should manifest in more than one experimental ratios. The consistency of the signals would be a compelling discovery and would permit full 3 Oavour analyses[20] .
5 Conclusions The long baseline studies have reached their age of maturity where realistic studies con cerning both the possible beams and detectors have been conducted. A series of proposals have been submitted in the US and Japan. The Gran Sasso laboratory has issued a call for proposals and CERN has seriously considered the case of a long baseline beam during the 1995 Cogne meeting[21]. In what concerns the CERN beam, a realistic, compact beam design using horn and reflector focusing, with a 1000 metre long decay tunnel of 1.5 metre radius, can produce a low energy neutrino beam providing typically � 500 events/kiloton at 731 km (G.S) and 100 events/kiloton at 1676 km (NESTOR) for 1019 protons on target at 450 GeV. The energy dispersion of such a beam is �±5 GeV and the mean neutrino energy can be tuned to between 8 and 20 GeV by selecting the focusing elements. The contamination is at the level of 13 v, of the rate. The beam dispersion is such that one can confidently target the detector with a v1, beam alignment accuracy of 1 milliradian. A series of complementary studies can detect oscillations to or to the 3 level for the v, vT mixing angle statistical sensitivity. The 6.m2 sensitivity, determined by the lever arm and the
In the case of disappearance for Super-Kamioka the limits correspond to 3 6l Vµ a. 487
mm1mum detectable neutrino energy, is of the order of 10-3 eV2• All 5 beam-detector setups studied here can cover the I References [l] See e.g S. J. PARKE, FERMILAB Conf-91/251-T September 1991, M. Goodman, ANL-HEP CP- 92-17, Summary talk of the Workshop on Long Baseline Neutrino Oscillations, Fermilab, Novem ber 17-20, 1991, R.Bernstein, FERMILAB-Conf-92/63, Ideas for a Long-Baseline Neutrino Detec tor,February 1992. [2] See e.g the Gran Sasso workshops for a Long Baseline beam {April to December 1994) and D. Crane and M. Goodman in "Proceedings of Particle and Nuclear Astrophysics and Cosmology in the Next Millenium" , June 22-July 14, 1994, Snowmass, Colorado. [3] KS.Hirata et al. Phys. Lett. B280, 146 {1992) D.Casper et al. Phys. Rev. Lett. 66, 2561 {1991) R. Becker-Szendy et al. Phys. Rev. D46, 3720 (1992) Maury Goodman et al. ANL-HEP-CP-92-124 [4] Ch. Berger et al .,Phys. Lett. B227, 489 {1989), B245, 305 (1990) H. Meyer talk at the 1994 Snowmass Summer Study, July 7, 1994. [5] Y. Fukuda et al. Phys. Lett. B335, 205 (1994) [6] L. Wolfenstein Phys. Rev. Dl7 (1978) 2369; S.P.Mikheyev and A.Yu. Smirnov, Yad . Fiz. 42 (1985) 1441. [7] P-875 A Long-Baseline Neutrino oscillation Experiment at Fe rmilab. The MINOS collaboration. Proposal February 1985. Addendum NuMi-L-79 April 1995. [8] P-822 Proposal for a long baseline neutrino oscillation experiment using the Soudan 2 detector March 1991 [9] Long Baseline Neutrino Oscillation Experiment E889 Collaboration. BNL No 52459 April 1995. [10] K. Nishikawa et al. KEK Preprint 93-55. [11] A. Ball et al. SL/Note 92-75(BT) [12] Benetti P. ICARUS: a neutrino observatory at the Gran Sasso Laboratory. Nucl. Phys. B, Proc. Suppl. : 35 {1994) 280-283 [13] Resvanis, L K. NESTOR: a neutrino particle astrophysics underwater laboratory in the Mediter- ranean. Proceedings / Neutrino Telescopes, Venice 1993. Ed. by M. Baldo Ceolin [14] S. Mayoud calculations, unpublished. [15] A. Ball, S.1 n.,...,t.-lno ev•nts/10••l9/l. Figure 1: Comparison of the energy distribution of events at GS and NESTOR for different horn/reflector systems (Hl5Rl5,H20R40,H40R70) and perfect focusing with a 450 GeV proton beam. 0 2000 4000 6000 8000 1 0000 12000 1 4000 1 6000 1 15000 20000 �•) ;!1 rn• 00 ., Figure 2: Event radial distributions for incident beam energies of 80, 120 160 and 450 GeV , in ascending order, with H20R40 focusingat A) Gran Sasso and B) NESTOR 489 .. ..� � 10 .\ NE <] ·2 10 . 10 3 v" 10 4 disappearanc 2 10 .3 10 · 10 ·l 1 sin2 20 1 N .. 60ot ICARUS UPERKAM � 10 .\ NE <] .2 10 . 10 3 v. 10 4 appearance ·2 10 10 10 ·l 1 .3 sin2 29 1 N.. � 10 ·1 NE <] -2 10 10 .3 10 4 Ratio NC/CC. 3 .2 10 · 10 10 ·l 1 2 sin 20 Figure limits limits using different methods the planned experiments 3: 90 % CL 3 for 5 LBL reported here. The KAMioka signal (vµ to v. or is also reported Vr )