Low Energy Background Study of the KamLAND Detector Tatjana Miletić advisor: Dr. Charles Lane

Outline Neutrino Discovery Mass and Oscillations KamLAND Experiment First KamLAND Results Low Energy Background Study Summary

Neutrino Discovery Wolfgang Pauli, theoretical prediction Enrico Fermi, named the particle to1959 Reines and Cowan, discovery of particle fitting the expected characteristics of neutrino, detected anti-neutrino via the inverse  decay Danby, muon neutrino discovery tau lepton discovered by Perl, implied the existence of tau neutrino Kodama directly observed tau neutrino

Mass and Oscillations -Standard Model prediction (postulated that neutrinos are mass less, consistent with observation that individual lepton flavors seemed to be conserved and total lepton number as well) -Phenomenon of neutrino oscillations (direct tests for neutrino mass lack at present required sensitivity, the recent hints for neutrino mass are indirect, based on phenomena of neutrino oscillations)

Phenomenon of Neutrino Oscillations - neutrinos are massive particles which behave in analogy to quarks, the states with definite mass, “mass eigenstates”, are not necessarily the partners of the charged leptons The weak eigenstates the mass eigenstates are linear superpositions of where coefficients form the leptonic mixing matrix.,,(1) (2)

Phenomenon of Neutrino Oscillations Consider the time development of the mass eigenstate where L is the flight path and it is assumed that the laboratory momenta and energies are much larger than neutrino rest mass. If we consider propagation of a neutrino which was created at L=0 as a weak eigenstate,at distance L this state will be described by Thus, the neutrino of flavor l acquired components corresponding to the flavors l’. This is the consequence of coherence in the superposition of the states in equation 2. (4) (3)

Phenomenon of neutrino oscillations Probability that “transition” l → l’ happens at L is This is an oscillation function of a distance L. Neutrino oscillation experiments are often analyzed in a simplified way by assuming only two neutrino flavor mix, e.g. e and μ. Mixing matrix is then simplified as well as oscillation probability. (5) (6)

Phenomenon of neutrino oscillations To test for oscillation, one can perform: -appearance search (looking for neutrino flavor – e.i. deviations of from 0) -disappearance search (looking for a change in flux normalization – e.i. deviation from unity) The oscillation length is: (7) Numerous searches for neutrino oscillations were performed in last two decades. Most of them resulted in exclusion plot, based on them certain ranges of parameters and can be excluded from future consideration.

Exclusion plot Two isolated islands on a exclusion plot ~ two solutions, both corresponding to (SMA) (LMA) Various solutions for solar neutrino problem

Phenomenon of neutrino oscillations At present time, there are three groups of measurements that suggest the existence of neutrino oscillations: -”atmospheric neutrino anomaly” (cosmic rays impinging on the N and O nuclei at the top of the earth’s atmosphere produce mostly pions which decay via the chain ) -”solar neutrino puzzle” (the Sun produces an intense flux of electron neutrinos as a byproduct of the fusion reactions, the most popular solution scenario MSW – effect which explains neutrino oscillations in matter) - involving man-made neutrinos (first indication came from LSND experiment and finally – )

KamLAND Experiment Kamioka Liquid scintillator Anti-Neutrino Detector -The largest low-energy anti-neutrino detector built so far -Located at the site of former Kamiokande experiment -High concentration of nuclear reactors at the right distance

1000 ton Liquid Scintillator Balloon made of transparent nylon/EVOH composite film, supported by cargo net structure. Stainless steel tank filled with paraffin oil (0.04% lighter than LS) inch inch PMT’s Photosensitive coverage ~ 34% 3mm thick acrylic wall: Rn barrier 3.2Kton water Cherenkov outer detector 225 – 20inch PMT’s

KamLAND Experiment Designed to detect: - anti-neutrino interactions via inverse beta decay or electron scattering - neutrinos from the Sun - terrestrial anti-neutrinos - anti-neutrinos from the past Supernova

Reactor anti-neutrino detection in liquid scintillator reaction process: inverse-β decay distinctive two step signature - prompt part: - delayed part (2.2MeV) - tagging: correlation of time, energy and position between prompt and delayed signal

KamLAND Experiment - Electronics MACRO ElectronicsTriggerLBL Electronics DAQ PMT’s waveforms trigger command E-sum trigger command Nsum event data run conditions event data

MACRO Electronics - borrowed from MACRO experiment - 4 crates, 44 cards each each card has 4 channels - constantly writing data into circular buffer - trigger issuing Start and Stop commands - depending on a type of interrupt DAQ reads out the buffer for fixed amount of time before the Stop.

First KamLAND Results First results, published in December 2002, revealed the evidence for reactor anti-neutrino disappearance - data obtained (March 4 to October 6, 2002) - total of 370 million events in days of live time - ratio of the number of observed reactor anti-neutrino events to the expected in the absence of neutrino oscillations is - - is estimated number of background effects

Some photos…

Backgrounds Produced by: - cosmic muon induced processes - natural radioactivity Two types of physics signatures interesting for KamLAND: - double events (reactor anti-neutrino signature, supernova neutrino scattering on C) - single events (low energy neutrino scattering on electrons, minimum energy deposition of 1MeV) Cosmic muons: - prompt neutrons from muon capture and muon spallation - radioactive isotopes produced by cosmic ray activation

Backgrounds Backgrounds from natural radioactivity in KamLAND derive from various sources: - decays chains of the long lived naturally present in small amounts in rocks surrounding the detector and materials used in detector construction; - radioactive impurities contained in scintillator (includingand ),and - decay of artificially added to steel during production for quality monitoring purposes - decay of continuously produced in the disintegration of, radon readily diffuses into many materials and has a life time of only few days it effectively act as a carrier that disperses the radioactivity through the entire detector. Accidental coincidence makes another type of background, uncorrelated background.

KamLAND Data ROOT currently used in all High Energy and Nuclear Physics laboratories to monitor, to store and to analyze data. KamLAND data: - converted to ROOT format - processed to remove muon events so that data contain only low energy single events Typical ROOT file contains: - event number information - time - approximate position - PMT hits number (Nsum) Nsum gives the number of photoelectrons at PMTs, therefore Nsum information is proxy for energy deposition. To obtain Nsum we devide recorded waveforms to time bins and count number of pulses from all waveforms in the corresponding time bin.

NSUM KamLAND Data Run numbers : 1335 to 1340 ~ 5 days of data taking Expected Poisson distribution, since with no radiation background only thermionic PMT noise would form the signal coming from uncorrelated events.

KamLAND Data Monte Carlo simulation of Poisson distribution and background with energy deposition of 30 photoelectrons. Actual Nsum distribution.

Nuclear  -decay: Fermi theory The first weak interaction studied was the nuclear beta-decay (decay of a free or bounded neutron) in terms of quark constituents: Initially this reaction was studied: Being a two-bodies decay A  B+C, the electron E should have been completely determined as:  E e  # events m n -m p -m e  17 keV  E e  # events Not this ! But this

Nuclear  -decay: Fermi theory Probability of emission of an electron in an energy interval dE: where is the matrix element for two particle interacting: and wave functions describe the nucleus before and after decay respectively. is an extremely short range potential so it can be replaced with. and are introduced for convenience of measuring energies and momenta. - energy and momentum of an electron - neutrino energy, W – total disintegration energy - Coulomb interaction between nucleus and electrons

KamLAND Data Histogram fitted using probability distribution for, form of beta spectrum or Kurie plot. Z=6 Histogram fitted using probability distribution for and, form of beta spectrum or Kurie plot. Z=36

Summary - Improvement of detector necessary for “Solar Neutrino Phase” - The calibration of detector using low energy sources needed - Understanding of energy scale and origin of background - Development of software, analysis tools - Work in progress…

References 1.Proposal for US Participation in KamLAND 2.Measurement of Electron Anti-Neutrino Oscillations with a Large Liquid Scintillator Detector, KamLAND, Osamu Tajima, Department of Physics, Tohoku University, Sendai, Japan, March Nuclei and Particles, Emilio Serge, 1977, W.A.Benjamin, Inc., Reading, Massachusetts 4.Reactor-based Neutrino Oscillation Experiments, Carlo Bemporad, Giorgio Gratta, Petr Vogel, Reviews of Modern Physics, volume 74, April Readout Issues for the New Waveform Digitizer, Edward Kearns, Department of Physics, Boston University, February 20, First Results from KamLAND: Evidence for Reactor Anti-Neutrino Disappearance, Phys.Rev.Lett. 90, (2003) 7. Discovery of the Neutrino, editors; C.E.Lane and R.I.Steinberg, Franklin Institute, Philadelphia, WorldScientific publishing Co, 1993