Nuclear Instruments and Methods in Physics Research A 503 (2003) 418–420

Conventional neutrino beams at K2K and JHF

Takashi Kobayashi

Institute for Practicle and Nuclear Studies, High Energy Accelarator Research Organization, 1-1 Oho, Tsukuba 305-0801, Japan

Abstract

The neutrino beam line for the K2K, the first accelerator-based long baseline experiment, has been operated very

successfully since the beginning of the experiment. The nm beam is produced by a ‘‘conventional’’ method in which the

decay of horn-focused pions is used. The facility and its performance are introduced. In the JHF-Kamioka neutrino

project, narrow-band neutrino beam will be produced by the conventional method. With an B1MW, 50-GeV protonsynchrotron, a 50 times higher neutrino flux than K2K is expected. The current design and the expected performance of

the facility are introduced.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 14.60.Pq; 13.15.+g; 23.40.Bw; 93.55.Vj

Keywords: Neutrino; Horn; Oscillation; Beam

1. Introduction

All accelerator-based high-energy (\1GeV)neutrino experiments in the past and at presenthave used nm (or %nm) beam from pions and Kaons,which are produced by hitting a target with high-energy protons. A neutrino beam produced in thisway is recently called a ‘‘conventional’’ beam whenit is contrasted with a neutrino beam from muondecay in a neutrino factory.A facility for a conventional beam consists of

proton accelerator, a proton transport line, aproduction target, a focussing device(s), a decaypipe and a beam dump. The pions (Kaons)produced in the target are focussed by focussingdevice(s) in order to increase the neutrino flux inthe forward direction. In many cases, electromag-netic horns have been used for focussing purpose

[1]. Focused pions enter into the decay pipe inwhich they decay.Here, a neutrino beam for the long-baseline experi-

ments K2K and JHF-SK projects are described.

2. Neutrino beam at K2K

The K2K, KEK-to-Kamioka, experiment is thefirst accelerator-based long-baseline experiment[2]. Its primary goal is to give a definite answerconcerning the existence of neutrino oscillationfound in atmospheric neutrino observations. Aconventional beam is produced at KEK, and thefar detector is Super-Kamiokande (SK) located250 km away from KEK. The experiment startedin 1999 and is on going.The proton beam is extracted from the 12-GeV

proton synchrotron (PS) in a single turn with a1.1 ms width every 2.2 s. The design intensity is6� 1012 protons on target/pulse. Every beam spillE-mail address: takashi.kobayashi@kek.jp (T. Kobayashi).

0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0168-9002(03)00728-9

is stamped with time measured by the globalpositioning system (GPS) with an accuracy ofo200 ns [3]. The production target is a 66 cm longAl rod. Its diameter is 2 cm for runs in June 1999,and 3 cm since November 1999. Positive pions arefocused by two electromagnetic horns [4]. Bothhorns are operated by a pulsed current of B1mswidth and 200 kA peak for the June 1999 run and250 kA peak for runs since November 1999. Thelength of the decay pipe is 200m. A beam dump atthe end of the decay pipe is 3-m thick from thetarget, there is a hole in which front neutrinodetector (FD) is installed. The beam-line compo-nents are aligned based on a survey with GPS [5].The precision of the GPS survey is t0.01mrad,while that of the civil construction is t0.1mrad.The expected neutrino spectra and the radialdependence of the flux at SK are plotted inFig. 1. The average neutrino energy is 1.3GeV.The neutrino flux is almost constant within 3mrad(B750m) at SK. The purity of nm in the beam isestimated to be 98.2% and ne contamination isestimated to be 1.3%.After an engineering run, a physics run started

in June, 1999. Until the end of a 2001 run in July,5.6� 1019 protons on target (POT) had beenaccumulated.A muon range detector (MRD), one of the

components of the FD, is a sandwich of iron platesand drift chambers. The size of the iron plates isabout 8� 8m2 and the total iron thickness is 2m.The total mass of the iron is about 1 kt. Because ofits large area coverage and large mass, neutrinointeractions in the iron plates provide the goodmonitoring of the neutrino beam profile and

intensity. Fig. 2 shows the neutrino event rate inMRD. The event rate has been stable throughoutwhole experimental period. The stability of theneutrino beam direction is demonstrated in Fig. 3.The figures plot the fitted center of the vertexdistribution of the MRD Fe events as a function oftime. Since MRD is located 300 cm from theproduction target, a 30 cm displacement at MRDcorresponds to 1mrad. The beam direction hasbeen well controlled within the physics require-ment of 3mrad throughout the experiment.

Fig. 1. (a) Expected nm spectrum and (b) radial dependence of

the flux at the far site.

Fig. 2. Neutrino event rate in MRD Fe plates.

Fig. 3. Fitted center of the vertex distribution of MRD Fe

events as a function of time: (a) is horizontal and (b) is vertical.

The horizontal solid and dashed lines are the SK direction and

1mrad from the SK direction.

T. Kobayashi / Nuclear Instruments and Methods in Physics Research A 503 (2003) 418–420 419

3. Neutrino beam at JHF

The JHF project is a high-intensity protonaccelerator complex which is being constructed atJapan Atomic Energy Research Institute (JAERI),about 60km N.E. of KEK [6]. It will be completedin March 2007. Using a 50-GeV PS in the complex, along-baseline neutrino experiment toward SK isbeing planned [7]. The primary goals of theexperiment are precision measurements of theoscillation parameters in nm disappearance and thediscovery of ne appearance. One of the essentialprinciples of the experiment is the use of the neutrinospectrum with a narrow energy width, whose peak istuned at the oscillation maximum. This greatlyenhances the physics potential of the experiment [7].The proton beam is extracted from the 50-GeV

PS in a single turn with an B5 ms width, and istransported to the production target. The designintensity of the PS is 3.3� 1014 ppp, and therepetition rate is 0.292Hz, resulting in beam powerof 0.77MW. This is about 2 orders of magnitudehigher than the current KEK-PS. We define thetypical 1 yr operation as 1021 POT. At present, wehave three options of beam configurations: wideband beam (WBB), narrow band beam (NBB) andoff axis beam (OAB) [7,8]. NBB and OAB canproduce a tunable monochromatic beam. Thecurrent strategy of the experiment is that withWBB in the first year, pin down Dm2 to 10% level,and then switch to NBB or OAB, whose peak istuned at the oscillation maximum, run B5 yr toexplore ne appearance and measure the oscillationparameters precisely.

Fig. 4 shows the expected neutrino energyspectra at SK for each beam configuration. Here-after, NBB with a selected pion momentum of#GeV/c is called LE#p and OAB with the beamaxis # degree declined is called OA#�. Theexpected numbers of interactions are summarizedin Table 1.

4. Summary

The neutrino beam for the first long baselineneutrino experiment K2K has been working verysuccessfully. The event rate and beam directionwere confirmed to be stable during whole experi-mental period. The design work of the neutrinobeam line for the JHF-Kamioka neutrino projectis now in progress. A neutrino beam with a narrowspectrum will be prepared. The expected neutrinoflux is B50 times higher than that of K2K. Weplan to start experiments form April, 2007.

References

[1] S. Van der Meer, CERN 62-16.

[2] K. Nishikawa, et al., KEK-PS E362 proposal, March, 1995;

Nucl. Phys. B (Proc. Suppl.) 59 (1997) 289;

S.H. Ahn, S. An, S. Aoki, et al., Phys. Lett. B 511 (2001)

178.

[3] H.G. Berns, R.J. Wilkes, IEEE Trans. Nucl. Sci. 47 (2000)

340.

[4] Y. Yamanoi, et al., KEK Preprint 99-178, Feb. 2000.

[5] H. Noumi, et al., Nucl. Instru. Meth. A 398 (1997) 399.

[6] http://jkj.tokai.jaeri.go.jp

[7] http://neutrino.kek.jp/jhfnu; Y. Itow, et al., hep-ex/

0106019, KEK Report 2001-4.

[8] D. Beavis, A. Carroll, I. Chiang, et al., Proposal of BNL

AGS E-889 (1995).

Fig. 4. Neutrino energy spectra of charged–current interac-

tions. The thick solid, dashed and dotted histograms in: (a) are

LE1.5p, LE2p and LE3p, and those in (b) are OA1�, OA2� andOA3�, respectively. WBB is drawn by a thin solid histogram in

both (a) and (b).

Table 1

Expected neutrino interactions at SK

Beam Epeak nm NCC nm=ne

LE1.5p 0.7 360 0.39

LE2p 0.95 620 0.15

OA2� 0.7 2200 0.21

OA3� 0.55 800 0.20

The second column is the peak energy in GeV. The third

column is the number of CC interactions (/22.5 kt/yr). The last

column is the flux ratio of nm to ne in % at the peak of nmspectrum.

T. Kobayashi / Nuclear Instruments and Methods in Physics Research A 503 (2003) 418–420420