PoS(ICHEP2012)007 Long Baseline Neutrinos Takashi Kobayashi∗Institute for Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK) E-mail: [email protected] Status and results of accelerator-based long baseline neutrino oscillation experiments are re- viewed. Future prospects are also discussed. 36th International Conference on High Energy Physics 4-11 July 2012 Melbourne, Australia ∗Speaker. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ Long Baseline Neutrinos Takashi Kobayashi 1. Introduction In the last decades, great progress in understanding the nature of neutrinos has been made and it is now established that the neutrino have finite mass and “surprisingly” large mixing unlike quarks. However still there are many unanswered questions such as whether the standard 3x3 mixing matrix picture is correct?, all three flavors participate to the mixing? Why the mixing looks so different from the quarks, CP is violated?, what is the absolute mass?, why so light?, what is the mass spectrum, normal or inverted hierarchy?, etc. The neutrino keeps attracting much attention because PoS(ICHEP2012)007 unraveling these mysteries could provide breakthrough to understand fundamental questions in particle physics, physics at high energy scale beyond the standard model and origin of matter dominated universe. Accelerator based long baseline (LBL) neutrino oscillation experiments produce 1 ∼ 10 GeV nm beam and detect it at several 100 km from the source. The nm beam is produced in the “con- ventional” method where pions produced by hitting proton beam on a target decay in flight into + + nm as p ! nm + m [1]. The physics goals of the LBL experiments have been to understand nature of neutrino, especially mass and flavor mixing structure through the finding and measure- ments of neutrino oscillation. Neutrino oscillation is a phenomena that the type of neutrino (ne, nm , nt ) changes its type during flight if the mass eigenstates of neutrino have different mass eigenval- ues each other and the flavor eigenstates are expressed as a linear combinations of more than one 2 2 mass eigenstates. The typical oscillation length is characterized by L(km) ∼ E(GeV)= Dmi j (eV ), 2 2 2 where Dmi j ≡ mi − m j is the mass squared difference of the neutrino mass eigenvalues mi. The LBL experiments with 1 ∼ 10 GeV and several 100 km flight distance are optimized for measuring oscillation with jDm2j ∼ 10−2∼−3 eV2 at which the first evidence of neutrino oscillation was found in atmospheric neutrino observation by Superk-Kamiokande (SK) [2]. The first accelerator-based LBL experiment was the K2K experiment started in 1999 and finished in 2004 in Japan in which the nm beam is produced at KEK in Tsukuba-city and detected by Super-Kamiokande at 250 km [3]. The experiment confirmed the existence of neutrino oscillation by observing disappearance of nm after 250 km flight. The MINOS experiment in US [4] and the OPERA [5] and ICARUS [6] ex- periments in Europe were planned to further confirm/establish the neutrino oscillation discovered by SK. Soon after the discovery of neutrino oscillation by SK, the importance of the sub-leading electron neutrino appearance channel was pointed out [7, 8]. In the three flavor mixing picture, the probability of electron neutrino appearance gives a measure of the mixing angle q13. The existence of electron neutrino appearance at the atmospheric oscillation length means non-zero q13. Because the CP violating observable, the phase d, appears always in the product with sinq12 sinq23 sinq13 and q23 and q12 are known to be large, the size of q13 decides the feasibility of the future CP violation search. With the goal to discover electron neutrino appearance and determine q13, the T2K experiment [9] in Japan started taking data in 2010 and the NOnA experiment [10, 11, 12] is being constructed to start data taking in 2013. In these proceedings, the present on-going experiments and future prospects are summarized. 2 Long Baseline Neutrinos Takashi Kobayashi 735km Figure 1: MINOS experiment. Left picture is the far detector in the Soudan mine. The right graphs are ex- pected energy spectra of nm CC interactions (=flux × CC cross section). The black histogram is the spectrum PoS(ICHEP2012)007 in the default MINOS running condition. 2. Current experiments and their status 2.1 MINOS MINOS is a long baseline experiment with the precise nm disappearance measurements as a main physics goal, in which muon neutrino beam is produced by the 120 GeV proton beam from Main Injector at FNAL and detected by the MINOS detector located in the Soudan mine at 735 km from FNAL (Fig. 1)[4]. The neutrino beam is a 2-horn focused on-axis wide band beam with the peak energy of around 3 GeV as seen in the expected neutrino spectrum in Fig. 1(right). The beam power of the Main Injector had reached at 350 kW. Magnetized tracking calorimeters are placed at near (980 ton) and far (5,400 ton) site. Physics data taking had been started in May 2005 and 20 20 finished on Apr. 30, 2012 with the total accumulated POTs of 10:7 × 10 and 3:4 × 10 for nm and n m , respectively. 2.2 CNGS experiments CERN neutrino to Gran Sasso (CNGS) is a high energy muon neutrino beam of hEn i ∼ 20 GeV produced at CERN with the SPS 400-GeV proton beam and directed to the detectors placed in Gran Sasso laboratory at 732 km distance (Fig. 2)[13]. Main goal of the CNGS experiments is to detect nt appearance. There are two experiments now taking beam data from CNGS, OPERA [5] and ICARUS [6]. OPERA is an emulsion based detector which identify t decay by its decay topology (kink). The detector consists of emulsion based active target part and electronic tracker part. The target part is made of 150,000 7:5 × 12:5 × 10 cm3 emulsion cloud chamber (ECC) blocks, each of which is a stack of 57 emulsion sheets and 56 1mm-thick lead sheets, with the total weight of 1.25 kt. The OPERA detector started to take neutrino beam data since 2008 and accumulated 1:4 × 1020 POT by 2011 and ∼ 1:8 × 1020 POT in 2012. The other detector is a Liquid Argon TPC detector, ICARUS, which consists of two 3:6 × 3:9 × 19:6 = 275 m3 chambers with 476 ton total Liq. Argon mass. It has been taking data since Oct. 2010 and have accumulated 5:8 × 1019 POT equivalent data. 2.3 T2K The T2K (Tokai to Kamioka) experiment is a long baseline experiment in Japan with the main goal of discovering ne appearance. The muon neutrino beam is produced by the 30 GeV 3 Long Baseline Neutrinos Takashi Kobayashi 732km Figure 2: Cern Neutrino beam to Gran Sasso (CNGS). The expected energy spectrum of nm flux is plotted in the left panel (red histogram) overlaid with (nt appearance probability) × (nt CC cross section) as a PoS(ICHEP2012)007 function of neutrino energy which demonstrate that the nm spectrum is optimized to maximize number of nt appearance events. ×106 ] 2 1.2 1 0.8 POT/50 MeV/cm 0.6 21 0.4 Flux[/10 0.2 0 0 0.5 1 1.5 2 2.5 3 3.5 Eν (GeV) Figure 3: T2K experiment. Expected energy spectrum of nm flux at SK is plotted in the right panel. Main Ring (MR) of J-PARC (Japan Proton Accelerator Research Complex) and detected by the Super-Kamiokande detector at 295 km from J-PARC. In order to maximize the sensitivity, off-axis scheme is applied for the first time to have intense narrow energy spectrum peaked at the expected oscillation maximum of 0.6 GeV. T2K started data taking in Jan. 2010 and achieved 200 kW stable operation. After one year of recovery work from the big east Japan earthquake on March 11, 2011, the experiment resumed data taking from March 2012 and total accumulated POT reached 3:01 × 1020 from the beginning to June 9, 2012. 3. Important issues in LBL experiments The basic method of the analyses in LBL experiments is to compare the observed number of obs events Nf ar(En ) in a far detector to the expected number of events with oscillation which can be symbolically expressed as exp Nf ar (En ) = Posc · F f ar · s f ar · e f ar; (3.1) in a simplified formula omitting to explicitly write integration of, for example, final state particle phase space, and Posc, F f ar, s f ar and e f ar are oscillation probability, neutrino flux, neutrino inter- action cross section and detection efficiency at the far detector. The precision or sensitivity of a exp LBL experiment is determined by how precisely Nf ar can be evaluated (and of course the statistical obs error of Nf ar). 4 Long Baseline Neutrinos Takashi Kobayashi PoS(ICHEP2012)007 Figure 4: Example of results from hadron production measurements in NA61 for T2K for pion (left) and Kaon (right) exp In order to predict Nf ar precisely, usually it is evaluated by extrapolating the measurement by obs a near detector Nnear exp obs Posc · F f ar · s f ar · e f ar Nf ar (En ) = Nnear · (3.2) Fnear · snear · enear The energy spectra of nm flux at near and far site (without oscillation) are not the same due to geometrical difference relative to the neutrino production region and also the cross section and effi- ciencies are usually different for near and far detector, so this extrapolation does not follow simple 1=R2 law.