A conventional low-energy neutrino beam of great intensity could be produced by the Super Proton Linac at CERN as a first stage of a Neutrino Factory. Water Cherenkov and liquid scintillator detectors are studied as possible candidates for a neutrino oscillation experiment which could improve our current knowledge of the atmospheric parameters Δmatm2, θ23 and measure or severely constrain θ13. It is also shown that a very large water detector could eventually observe leptonic CP violation

Blondel, Alain

Burguet Castell, Jordi

Casper, D.

Donega, M.

Gilardoni, Simone

Gómez Cadenas, Juan José

Hernández Gamazo, Pilar

Mezzetto, Mauro

Repositori d'Objectes Digitals per a l'Ensenyament la Recerca i la Cultura

Superbeam Studies at CERNA. Blondela, J. Burguet-Castellb, D. Casperc, M. Donegaa,S. Gilardonia, J. J. Gomez Cadenasb;d, P. HernandezdM. Mezzettoe;,aDepartement de Physique, Universite de Geneve, SwitzerlandbIFIC, Edicio de Institutos de Paterna, Paterna, Valencia, SpaincUniversity of California at Irvine, USAdCERN, Ch-1211 Geneve 23, SwitzerlandeINFN, Sezione di Padova, ItalyAbstractA conventional low energy neutrino beam of great intensity could be produced bythe Super Proton Linac at CERN as a rst stage of a Neutrino Factory. WaterCerenkov and liquid scintillator detectors are studied as possible candidates for aneutrino oscillation experiment which could improve our current knowledge of theatmospheric parameters m2atm, 23and measure or severely constrain 13. It isalso shown that a very large water detector could eventually observe leptonic CPviolation.1 The SPL neutrino beamThe planned Super Proton Linac (SPL) is a 2.2 GeV proton beam of 4 MWpower [1] which could be used as a rst stage of the Neutrino Factory complexat CERN[2]. It would work with a repetition rate of 75 Hz delivering 1:5  1014protons per pulse (1023protons on target (pot) in a 107s conventional year).Pions are produced by the interactions of the 2.2 GeV proton beam with aliquid mercury target and focused with a magnetic horn [3]. Target and hornassumed in the following are precisely those studied for the Neutrino Factoryand no optimization has been attempted for this Superbeam.Pions next transverse a cylindrical decay tunnel of 1 meter radius and 20Corresponding authorEmail address: Mauro.Mezzetto@pd.infn.it ( M. Mezzetto).Preprint submitted to Elsevier Science 8 October 2001CERN-OPEN-2002-02520/02/2002Table 1The SPL neutrino uxes, for +(left) and  (right) focused in the horn, computedat 50 km from the target.+focused beam  focused beamFlavor Absolute Flux Relative hEi Flavor Absolute Flux Relative hEi(=1023pot=m2) (%) (GeV) (=1023pot=m2) (%) (GeV)1:7  1012100 0.26 1:1  1012100 0.234:1  10102.4 0.24 6:3  10105.7 0.25e6:1  1090.36 0.24 e4:3  1093.9 0.25e1:0  1080.006 0.29 e1:6  1080.15 0.29meters length. These dimensions have been optimized in order to keep theecontribution low and ux as high as possible.We have used the MARS program[4] to generate and track pions. We have addedpolarization eects on  decays and used analytical calculations to follow and  decays and particle trajectories [5].The resulting neutrino spectra are shown in Fig. 1 and Table reftab:uxes.Energy (GeV)ν/m2 /20 meV10 610 710 810 910 1010 110 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1ννµeννµeEnergy (GeV)ν/m2 /20 meV10 610 710 810 910 1010 110 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1ννµeννµeFig. 1. The SPL neutrino spectra, for +(left) and  (right) focused in the horn,computed at 50 km from the target.The average energy of the neutrinos is around 250 MeV with a econtami-nation of  0:4%.ein the beam can be produced only by  decays, since center-of-mass en-ergy is below kaon production. and ecome from the same decay chain+! +! e+eand so ecan be predicted by a direct measure of thethemselves. We estimate that combining the measurement of interactionrate at the far detector with the and erates in a close detector (0.5 ktonat 1 km from the target), the econtamination can be established with 2%systematic and statistical errors.2 Detector scenariosFig. 2 shows the oscillation probability P (! e) as a function of the dis-tance. Notice that the rst maximum of the oscillation at L  130 km matches2L (km)p(ν µν e)LMALMA with -∆m223SMA00.0050.010.0150.020.0250 100 200 300 400 500 600 700 800Fig. 2. P (! e) computed for hEi = 0:26 GeV, m2atm= 2:5  10 3eV2,m2sun= 5  10 5, 6  10 6eV2, m223, sin213= 0:01, Æ = 0.the distance between CERN and the Modane laboratory in the Frejus tunnel,where one could conceivably locate a large neutrino detector[6].Detection of low-energy neutrinos at O(100km) from the source requires amassive target with high eÆciency. Moreover, a search for eappearance de-mands excellent rejection of physics backgrounds, namely  misidenticationand neutral current 0production, which should be controlled to a lower levelthan the irreducible beam-induced background.Backgrounds from atmospheric neutrinos remain negligible if the accumulatorforeseen in the Neutrino Factory complex is used; it provides a duty cycle of4000.In this paper we consider two detector technologies, which have demonstratedexcellent performance in the low energy regime, while being able to pro-vide massive targets. These are, waterCerenkov detectors, as used in Super-Kamiokande[7], and diluted liquid scintillator detectors as used by the LSNDexperiment[8] and planned for the forthcomingMiniBoone experiment[9], wherebothCerenkov and scintillation light are measured.2.1 WaterCerenkov detectorsWe have considered an apparatus of 40 kton ducial mass and sensitivity iden-tical to the Super-Kamiokande experiment. The response of the detector tothe neutrino beam discussed in section 1 was studied using the NUANCE[10]neutrino physics generator and detector simulation and reconstruction algo-rithms developed for the Super-Kamiokande atmospheric neutrino analysis.These algorithms, and their agreement with real neutrino data, have been de-scribed elsewhere[7,11,12].The SuperKamiokande standard algorithms for =e separation are very eec-3tive in the SPL-SuperBeam energy regime.aEvents with a 0are easily rejected in events where the two rings are foundby a standard 0search algorithm, a la Super-Kamiokande. To reject eventswhere only one  has been initially identied, an algorithm[14] forces theidentication of a candidate for a second ring, which, if the primary ring istruly a single electron, is typically either very low energy, or extremely for-ward. By requiring that the invariant mass formed by the primary and thesecondary rings is less than 45MeV/c2, almost all remaining 0contaminationis removed. Being the SPL-SuperBeam more clean and at lower energies thanthe beam of Ref. [15], we have not attempted to introduce more aggressiveÆrejection algorithms.Data reduction is summarized in Table 2. Contamination by the intrinsic beameis dominant.Table 2Summary of simulated data samples a +and  focused neutrino beams. The num-bers in the rightmost column (after all cuts) represent the sample used to estimatethe oscillation sensitivity.Fit in d. vol. TightInitial Visible Single-ring particle No mChannel sample events 100   450MeV=c2ID ! e  45MeV=c2+focused beam3250 887 578.4 5.5 2.5 1.5e18 12. 8.2 8.0 8.0 7.8NC 2887 36.9 8.7 7.7 7.7 1.7! e100% 82.4% 77.2% 76.5% 70.7% 70.5% focused beam 539 186 123 2.3 0.7 0.7 e4 3.3 3 2.7 2.7 2.7NC 687 11.7 3.3 3 3 0.3! e100% 79.3% 74.1% 74.0% 67.1% 67.0%2.2 Liquid scintillator detectorsThe energy range (50 MeV-1 GeV) and the rejections against backgroundneeded by MiniBoone nicely match the requirements of our study. Neutrino-12C cross sections are taken from reference[16].bTable 3 shows data reduction assuming no -eoscillation, for a 200 kton-aWe have only tightened the cut value of the particle identication criterium, whichis based on a maximum likelihood t of both -like and e-like hypotheses (PandPerespectively). We use Pe> P+ 1 instead of the default Pe> P.bThey come from an upgraded version of the continuous random phase approxi-mation method and in average they are lower by about  15% from what quotedby the MiniBoone experiment.4Table 3Summary of data samples in a 40 kton liquid scintillator detector at L = 130 kmfor a 200 kton/year exposure.+focused beam  focused beamChannel Initial sample Final sample Channel Initial sample Final sampleCC2538 2.5 CC451 0.5CCe12 6 eCC2.3 1.0NC (visible) 48 0.5 NC 10 < 0:1! e100% 50% ! e100% 50 %year exposure. As before, intrinsic beam econtamination results to be thedominant background.As one can see comparing Tables 2 and 3 the performances of both devicesare quite similar. The conclusion is that one would prefer for this experimenta water detector, where truly gargantuan sizes can be aorded.3 Physics Potential3.1 Sensitivity to the atmospheric parametersTo illustrate the precision in measuring Æm2atmand 23Fig. 3 shows theresult of a 200 kton-years exposure disappearance experiment on a liquidscintillator detector. The computation is performed dening 4 energy bins inthe 0.1-0.7 GeV energy range and including Fermi motion, that is by far themost limiting factor to energy reconstruction at these energies. See[17] formore details. We nd that m223can be measured with a standard deviationof 1  10 4eV2=c4while sin2223is measured at the  1% precision level.In case of waterCerenkov, the rst energy bin would be lost, being the below threshold. In this case the resolution on the atmospheric parameters isslightly worsened, especially in case of low m2.3.2 Sensitivity to 13The dierent solar solutions don't aect very much P (! e) for L =130 km as can be seen in Fig.2. We will use SMA parameters, a hierarchicalmass model, Æ = 0. Given the low statistic and the poor energy resolution weconsider in this case a counting experiment, not exploiting spectral distortions.Fig. 3(right) shows the expected sensitivity for a 5-year run with a 40 kton(400 kton) water target at a distance of 130 km.The sensitivity of this search results to be about 15 (60) times higher than thepresent experimental limit of sin213coming from the Chooz experiment[18].5sin 2(2 θ 23 )m2 2390% CL, syst=2 %3σ, syst=2 %0.88 0.9 0.92 0.94 0.96 0.98 10.220.240.260.280.30.320.340.360.380.4x 10-2Testing points∆sin 2( θ13 )10-410-310-210-410-310-210-1∆m2  13 (eV2 )400 kton 40 ktonFig. 3. LEFT: Fits of Æm2atm(eV2),sin2(223) after a 200 kton/year expo-sure, L = 130km, 2% systematic errors. The crosses sign the initial points(0:98; 3:8  10 3), (0:98; 3:2  10 3), (0:98; 2:5  10 3) in Æm2atm, sin2(223) coor-dinates. RIGHT: ! eoscillation sensitivity for +focused neutrino beamswith a 40 (400) kton detector.3.3 Sensitivity to CP in the LMA-MSW scenarioIn the remaining of this section we will assume Æm212= 10 4eV2(in the upperpart of LMA) and sin2212= 1.The +16O cross-section is approximately six times less than that for +16Oat E' 0:25 GeV; to compensate that we have considered a 10 year run withfocused  and a 2 year run with focused +.We follow the approach in[19,20] and t simultaneously the number of de-tected electron-like events to the CP phase Æ and 13. Notice that, althoughwe apply a full three family treatment to our calculations, including mattereects, these are not important at the short distances and low energies consid-ered. Fig. 4(left) shows the condence level contours including statistical andbackground subtraction errors. A maximally violating CP phase (Æ = 90Æ)would be just distinguishable from a non CP violating phase (Æ = 0Æ).Fig. 4(right) shows the result of the same t, now assuming a very large waterdetector, such as the proposed UNO[21], with a ducial mass of 400 kton.Clearly, the prospects to observe CP violation are much improved.4 ConclusionsWe have examined the physics potential of a low energy Superbeam whichcould be produced by the CERN Super Proton Linac. WaterCerenkov andliquid oil scintillator detectors have been considered.62      3       4       5       6        7      8       9       10     11    12150100500−150−100−50δθ132      3       4       5       6        7       8       9      10     11     12150100500−150−100−50δθ13Fig. 4. 1, 90 %CL and 99 %CL intervals resulting from a simultaneous t tothe 13and Æ parameters. The generated values were 13= 5Æ; 8Æ; 10Æ, Æ = 90Æ.Computed for a 10 year anti-neutrino and a 2 year neutrino run, L = 130 km witha 40 kton detector (left) or with a 400 kton detector (right).The low energy of the beam studied has several advantages. Beam systematicsare kept low because 2.2 GeV protons are below the kaon production thresh-old. Furthermore, e= and e=0separation in a water (liquid oil) detector isnear optimal at these low energies. The drawback is the small anti neutrinocross section, which is more than a factor ve smaller than neutrino cross sec-tion. These cross-sections are in addition quite uncertain and would certainlyneed to be measured.The peak of the oscillation is at a distance of about 130 km where an ideallocation exists, the Modane laboratory in the Frejus tunnel.A \moderate" size detector (\only" twice as big a Super-Kamiokande) at thisbaseline could, in a ve year run, improve the precision in the determinationof the atmospheric parameters by about one order of magnitude (with respectto the expected precision of next generation neutrino experiments, such asMinos). It could also improve by more than one order of magnitude the sen-sitivity on sin213compared to the present experimental limits.Such a detector could also, if the solution to the solar neutrino problem lies inthe upper part of the LMA, detect a maximally violating CP phase. For CPviolation studies a very large detector, a la UNO (400 kton ducial mass), ismandatory.5 AcknowledgementsWe wish to thank the Super-Kamiokande collaboration for allowing use of itssimulation and analysis software in this study.7References[1] B. Autin et al., \Conceptual design of the SPL, a high-power superconductingH linac at CERN", CERN-2000-012., Dec 2000. 89pp.[2] B. Autin, A. Blondel and J. Ellis editors: \Prospective study of muonstorage rings at CERN", CERN yellow report CERN 99-02. Home pagehttp://muonstoragerings.cern.ch/.[3] A. E. Ball et al. Nucl. Instrum. Meth. A 451 (2000) 359. S.Gilardoni et al.\Updated results for the Horn Study for the Nufact" note 2000/42, available athttp://molat.home.cern.ch/molat/neutrino/nfnotes.html.[4] N.V. Mokhov, Fermilab-FN-628(1995).[5] A. Blondel, M. Donega and S. Gilardoni \Study of (Anti-)Neutrino Fluxes froma Horn Neutrino Beam Using 2.2 GeV Protons" Nufact note 2001/78. Availableat http://mdonega.home.cern.ch/mdonega/lowe/homepage.html[6] Luigi Mosca, talk given in the ECFA Neutrino OscillationWorking Group, May,10th, 2001. Michel Spiro, talk given in the plenary session, muon days, ECFAworking group, May, 12th, 2001. http://muonstoragerings.cern.ch/.[7] Super-Kamiokande Collaboration, Phys. Lett. B 433, 9 (1998).[8] LSND collaboration, Phys. Rev. Lett. 81, 1774 (1998).[9] Boone proposal, http://www.neutrino.lanl.gov/BooNE/boone proposal.ps[10] D. Casper, private communication.[11] M. D. Messier, UMI-99-23965.[12] M. Shiozawa, Nucl. Instrum. Meth. A 433, 240 (1999).[13] S. Kasuga et al., Phys. Lett. B 374, 238 (1996).[14] T. Barszczak, University of California, Irvine Ph.D thesis (to be published).[15] Y. Itow et al., \The JHF-Kamioka neutrino project", hep-ex/0106019[16] E.Kolbe et al. Nucl.Phys A613 (1997) 382, Nucl.Phys A652(1999) 91.[17] M. Mezzetto: \Neutrino Oscillations at the SPL SuperBeam",NuFact internal note 2000/60, December 2000, available athttp://molat.home.cern.ch/molat/neutrino/nfnotes.html.[18] Chooz collaboration, Phys. Lett. B 466, 415 (1999) [hep-ex/9907037].[19] A. Cervera et al., Nucl. Phys. B579, 17 (2000).[20] J. Burguet-Castell et al., Nucl. Phys. B 608 (2001) 301 [hep-ph/0103258].[21] UNO Collaboration: \Physics Potential and Feasibility of UNO", preprintSBHEP01-3, available at http://superk.physics.sunysb.edu/~superk/uno/8

Superbeam studies at CERN

http://dx.doi.org/10.1016/S0168-9002(03)00667-3

Superbeam studies at CERN

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Repositori d'Objectes Digitals per a l'Ensenyament la Recerca i la Cultura