K2K ： KEK-to-Kamioka Long Baseline Neutrino Oscillation Experiment Kenzo NAKAMURA KEK For the K2K Collaboration Les Houches EuroConference on Neutrino Masses and Mixings June 21, 2001 Les Houches Physics Center, France

K2K Collaboration Chonnum National Univ. Donshin Univ. Seoul National Univ. ICRR, Univ. of Tokyo KEK Kobe Univ. Kyoto Univ. Niigata Univ. Okayama Univ. Osaka Univ. Science Univ. of Tokyo Tohoku Univ. Boston Univ. Univ. of California at Irvine Univ. of Hawaii at Manoa SUNY at Stony Brook Univ. of Washington at Seattle Warsaw Univ.

Accelerator ： 12 GeV proton synchrotron Beam intensity ： 6  protons / pulse Repetition ： 1 pulse / 2.2 sec Pulse width ： 1.1  s (9 bunches) Horn-focused wide-band beam Average neutrino energy ： 1.3 GeV Near detector ： 300 m from the target Far detector (Super-Kamiokande) ： 250 km from the target Goal ： protons on target Water Cherenkov detector Total mass ： 50 kton Inner mass ： 32 kton Fiducial mass: 22.5 kton K2K (KEK-to-Kamioka)

200m Near Detector Pion Monitor Muon Monitor Horn current: 250 kA Production target: Al 66cm×3cmφ Beam Line and Magnetic Horn

K2K Near Detector Measure  flux, spectrum, profile Measure e contamination Study interactions at  1 GeV 1 kton water Cherenkov detector  Same structure and systematics as SuperK  Common water target Scintillating-Fiber tracker  Active area: 2.4 m  2.4 m  20 (x,y) layer Water target  19 layers of 60-mm thick water Lead-glass counter Muon/Fe  12 layers of iron plates (total mass  1,000 tons interleaved with muon drift chambers

Reconstructed Neutrino Energy Spectra with POT l + n  l + p l-l- p ( E l, p l ) The neutrino energy is reconstructed by assuming quasi-elastic (QE) scattering.

Oscillated/Non-oscillated with POT non QE Bkg

Expected Sensitivity protons on target in 4 ～ 5 years

Profile and Spectrum at the Near Detector R  100cm R  200cm R  300cm E (GeV) R (cm) MRD (Muon Range Detector) MRD fiducial vol. for monitor- ing the rate. Neutrino profile at 300 m Neutrino spectrum averaged within a given R Number of neutrinos/cm 2 /10 20 POT

Profile and Spectrum at the Far Detector Number of neutrinos/cm 2 /10 20 POT

Far/Near(R  300 cm) Flux Ratio It is vitally important to predict the far spectrum model-independently from the spectrum measured at the near site.  Measure momentum and angular distribution of pions, N(p ,   ).  Neutrino flux  (E ) at any distance using only decay kinematics. E (GeV)

Pion Monitor Pion monitor is a gas-Cherenkov counter which measures the pion p  -   distribution just after the magnetic horn. Sensitive to p  >2 GeV (E > 1 GeV) to avoid background from 12 GeV protons. Predict far/near flux ratio above 1 GeV Normally retracted from the beam line Special low-intensity runs for measurement

Pion Monitor Results and Fitting pp  w1w1 w2w2 w3w3 w4w4 ….. : : : :

Pion Monitor: Unfolded Result beam axis ++  Relative Pion Population in p  -   plane

Spectra from Pion Monitor Measurement

Flux Ratio from Pion Monitor Measurement Good agreement between the pion monitor results and the beam Monte Carlo above 1 GeV. Thus, for the extrapolation of the event rate, MC calculation is used. Systematic error estimated from pion monitor: R(E )=  SK (E )/  FD (E )

Muon Monitor Sensitive to initially high energy  (>5.5 GeV) Spill-by-spill monitoring of  Beam intensity  Targetting  Horn stability  Profile  Beam direction Segmented ionization chamber

K2K Experiment: Schematic 1kt Water Cherenkov MRD Lead Glass SciFi Super-Kamiokande N expected SK N observed SK Far/near flux ratio N observed 1kt N observed Muon/Fe N observed SciFi Consistency check Comparison This talk ： Number of events Future ： Spectrum Near Detector Beam Monitors Muon monitor Primary beam stability Targetting Pion monitor Pion kinematics P  －  Current Transformer Primary beam intensity Beam Monte Carlo Uncertainties in the absolute flux normalization and cross section cancel out between 1kton and SK Confirm above 1 GeV MRD Rate stability Directional stability Spectrum stability

1 kton Event H 2 O target (same as SK) Same detection principle as SK  Rate normalization in calculating the epected number of nm events at the far site. Fid. Mass: 25 ton Event selection: Qtot>1000p.e. ～ 2events/100 spill

MRD Event Neutrino int. in MRD iron plates (CC inclusive) Large area coverage (8m)  profile (vertex dist.) Large mass (fid. Mass = 329 ton)  high rate ( ～ 5/100spill) Monitor  neutrino direction  intensity  spectrum 8m8m Beam

SciFi Event Neutrino int. in SciFi H 2 O target (+Al 20%) Pos. resolution ～ 1mm  well defined fid. vol.  multitrack resolution Fiducial mass = 5.9 ton Event selection: matching SCIFI & MRD track 1 event/1000 spill Study neutrino interaction, e.g.  inelastic /  elastic Beam

Protons Delivered onto the Target Accumulated POT Protons / pulse

Beam Direction Vertex distribution of MRD events (Nov99) MRD Centered within sys. err. of 20cm (0.7mrad) Fitted centerx: 1±5cm y: － 10±4cm from SK dir. (stat) SK

Stability of the Profile Center Measured with the Muon Range Detector Stable to within  1 mrad Well within the required 3 mrad 1mrad MRD

Stability of the Muon Profile Center 1mrad Jun ‘99 Jun ‘00 Nov ‘99 Jan ‘00 Feb ‘00 Mar ‘00 May ‘00 Spill-by-spill measurement with the muon monitor Stable within  1 mrad

Stability of the MRD Event Rate Measured with the MRD Good stability Fe events/5x10 12 POT ～ 5.5events/100spill MRD Horn 250kA Target 3cm  Horn 200kA Target 2cm 

MRD Stability of Muon Energy and Angle E  (GeV)   (deg) Arbitrary Units Measured with MRD Stable within statistical error

Expected Number of SK Events from 1 kton events These results are consistent with each other.

Systematic Error 1kt (mainly  V/V) Near/Far Ratio Total SK(mainly  V/V)

SK Event K2K event selection at SK 1.No pre-activity in 30  sec 2.p.e. in 300ns window > OD N hit in largetst cluster<10 4.Deposite Energy > 30MeV 5.Fiducial cut (distance from wall>2m) Detection efficiency  =79% for CC:93%, for NC inel :68%

SK Event Category FC (Fully Contained; Light in ID only) Vertex inside the 22.5 kton fiducial volume Vertex outside the fiducial volume OD contained （ Light in OD only) Crossing (Light in both ID and OD ）

Events vs POT FC events in 22.5 kton fiducial volume KS prob. = 7% KS prob. = 12 % All events (FC + OD)

Arrival Time Distribution  (  s) No pre. act > 200 p.e. Exp’ed Atm BG <10 -3 in 1.5  s win. 28 observed. T spill T SK T Spill, T SK : Abs. time of spill start, SK event measured with GPS TOF: 0.83ms(Time of flight from KEK to Kamioka)

Observed vs Expected at SK

cos  KEK Distribution Preliminary Data MC w/o osc. Need to estimate syst. err. in MC expect. 1-ring  -like

Reconstructed  Momentum Preliminary Data MC w/o osc. Need to estimate syst. err. in MC expect.

Reconstructed E Fully contained 1-ring  -like (22.5kt) Preliminary Data MC w/o osc. Need to estimate syst. err. in MC expect.

Visible Energy Data MC w/o osc. Preliminary need to estimate syst. err. in MC expect.

K2K Run in 2001 Until July 12

Conclusions Methodology of a long-baseline neutrino experiment established.  Beam steering (direction)  Time synchronization between near site and far site  Monitoring of the neutrino beam at near site  Prediction of neutrino beam properties at far site from near site measurements 2.29 x POT accumulated from Jun ’99 to Jun ’00. # of fully contained events in fiducial SK  Observed : 28  Expected : (w/o osc.) Deficit of ~1GeV  after 250km flight at 90% significance Statistics doubled this summer

Future Spectrum analysis e appearance Study of  interactions at 1GeV region Upgrade of the near detector in summer, 2003

Upgrade of the near detector

Upgrade of the Near Detector (Segmented Liquid Scintillator with WLS Fiber Readout) 4m4m 2m2m 4m4m Large Volume: 4×4×2m 3 ~30tons Fine segment: 20×20×20mm 3 One cell is read-out to both X and Y directions by X,Y fibers. Large Light Yield: 20~30 photo-electrons/cell for MIP Particle ID: p/  : dE/dx  /  : range #channels : ～ 40,000 cell structure

Upgraded Near Detector Liquid Scintillat or

Event Simulation

Neutrino Energy spectrum and the ratio w/ and w/o oscillation Uncertainty (now) Uncertainty (upgrade) Spectrum ratio 2.6  

JHF-to-SK Neutrino Project Letter of Intent: hep-ex/

Overview ~1GeV beam Kamioka JAERI (Tokaimura)  → x  → x disappearance  （  m 23 2 ）～ eV 2,  （ sin 2 2  23 ）～ 0.01  → e  → e appearance sin 2 2  13 ）～ 0.01 NC measurement  →     →  s ～ 1 MW 50 GeV PS ( conventional beam) Super-K: 50 kton Water Cherenkov

JHK Accelerator Complex (60km N.E. of KEK) (Approved in Dec.2000) Power(MW ) Rate(Hz) Int.(10 12 ppp ) E(GeV) K2KMINOSJHF Construction Construction of neutrino beam lines is not yet approved.

Comparison of Three Beams WBB LE2  OA2° Linear Log OAB/WBB long HE tail 800MeV~1GeV Sharp peak for NBB/OAB OAB produce very intense “NBB” Target: Cu. 1cm , 30cm rod SK size: ±500m

Number of CC Events with Various Beams WBB LE1.5  LE2  LE3  OA3° OA2° OA1° WBB:5200 CC int./22.5kt/yr NBB: 620 CC int./22.5kt/yr (2GeV/c  tune) OAB: 2200 CC int./22.5kt/yr (2degree)

MNS Matrix and Parameters MNS mixing matrix three mixing parameters a cp violating phase mass-squared differences  12    

 Disappearance  Disappearance 1ring FC  -like Reconstructed E (MeV) Fit with 1-sin 2 2  ・ sin 2 (1.27  m 2 L/E) Oscillation with  m 2 =3×10 -3 sin 2 2  =1.0 No oscillation Non-QE (linear) (log)  m 2 =3×10 -3 sin 2 2  =1.0 ~3% Ratio after BG subtraction

Precision of the Parameters  (sin 2 2  ) ～ 0.01 in 5 years  (  m 2 ) ～ <1×10 -4 in 5 years  (sin 2 2  )  (  m 2 ) NBB-3GeV , OAB-2degree, NBB-1.5GeV  0.011×10 -4

e Appearance: Expected Signal and Background OAB2 o 5year Reconstruct E as Quasi-Elastic interaction Apply Energy Cut 0.4GeV< E <1.2GeV (OAB2 o )

e Appearance: Sensitivities In the SK allowed region 1.5  <  m 2 <5  eV 2 : sin 2 2   e ～ or sin 2 2  13 ～ 0.01

NC Measurement NC/CC sensitive to s NC Enriched Sample # of NC events log  m 2 ) #  0 + #e-like 3.2×10 -3

JHF-to-SK Second Phase Hyper-Kamiokande: 1 Mton water Cherenkov detector JHF beam power upgrade: 4MW   200 statistics Goal of the 2nd phase  sin 2 2  13 sensitivity below (if  13 not discovered in the 1 st phase)  CP phase  to 10 – 20 degrees (if the solution to the solar neutrino problem is MSW-LMA)  Proton decay (p  K, e +  0 ) 5  JHF  20  SK 5 year

Hyper-Kamiokande: A Next-Generation Nucleon Decay Detector at Kamioka 0.5 Mton 1 Mton: Total Length 400m (8 Compartments)

CP Violation

CP Sensitivity