30TH INTERNATIONAL COSMIC RAY CONFERENCE

Low Energy Event Reconstruction and Selection in Super–Kamiokande–III

M ICHAEL SMY1 FOR THESUPER–KAMIOKANDE COLLABORATION1Department of Physics & Astronomy, 3117 Frederick Reines Hall, University of California, Irvine, Cali-fornia 92697msmy@uci.edu

Abstract: Super–Kamiokande–I studied low energy neutrino interactions above 4.5 MeV. Photo-cathodecoverage has been restored to 40% in Super-Kamiokande-III in order to observe Cherenkov events withan energy even below 4.5 MeV. This is motivated by the transition of solar neutrino oscillations betweenvacuum and matter-dominated oscillations near 3 MeV and delayed neutron detection from inverse-betainteractions. I will discuss the progress in event reconstruction and background suppression as well asfuture measurements which a lower analysis and hardware energy threshold will make possible.

Introduction

Super–Kamiokande is a cylindrical 50,000 ton wa-ter Cherenkov detector described in detail in [1]. Itis optically divided into an inner 32,000 tons (innerdetector, or ID) viewed by about 11,000 inward-facing 20” diameter photomultiplier tubes (PMTs)and an outer 18,000 tons (OD) viewed by outward-facing 8” diameter PMTs. The fraction of surfacearea covered by the PMT photo cathode is 40%.We see about six photo-electrons from Cherenkovlight per MeV. Below about 100 MeV most PMTsignals are due to single photon hits. The resolu-tion of the hit arrival time is about 3ns for singlephoton hits. The dark noise rate is about 5kHz perPMT. Due to the large size of Super–Kamiokande,it takes Cherenkov photons up to 220ns to traversethe detector, and the relative PMT hit times canbe used to reconstruct the point of origin (eventvertex) of the light. Once the vertex is recon-structed the time-of-flight subtracted hit times sep-arate the dark noise (random) from the Cherenkovphoton hits (coincident within≈5ns). The elec-tron’s or positron’s direction is reconstructed fromthe direction of the characteristic 420 opening an-gle Cherenkov cone, the energy is determined us-ing the observed Cherenkov light intensity.

Above about 10 MeV, this event reconstructionworks very well since there are many moreCherenkov light hits (about 60) than hits due to

dark noise (about 12 within 220ns). Near andbelow 5 MeV sophisticated vertex reconstructionprograms are needed, since there are only about25 to 30 photo-electrons from Cherenkov light.Also, below 5 MeV events from radioactive back-ground increase exponentially with decreasing en-ergy. Most of this background comes from thePMTs themselves: the PMT glass and the fiber-glass backing of the PMT enclosures (which pro-tect against PMT implosion chain reactions). Dueto vertex resolution and mis-reconstruction, thesebackgrounds reach far inside the detector, and theirintensity depends strongly on the quality of the ver-tex reconstruction program.

Vertex Reconstruction Improvements

To be able to reconstruct low energy electronsin Super–Kamiokande–II which had only about46% of the 20” diameter PMTs compared toSuper–Kamiokande-I and Super–Kamiokande–III,we improved the vertex fit program. In Super–Kamiokande–I we maximized an ad-hoc “good-ness” of the Cherenkov signal. Our new recon-struction, BONSAI (branch optimization navigat-ing successive annealing iterations), performs in-stead a maximum likelihood fit to the timing resid-uals of the Cherenkov signal as well as the darknoise background for each vertex hypothesis. The

Proceedings of the 30th International Cosmic Ray ConferenceRogelio Caballero, Juan Carlos D’Olivo, Gustavo Medina-Tanco,Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.)Universidad Nacional Autónoma de México,Mexico City, Mexico, 2008

Vol. 5 (HE part 2), pages 1279–1282

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hypothesis with the largest likelihood is chosen asthe reconstructed vertex. Technically, this is ratherdifficult, since the accidental coincidence of darknoise hits after time-of-flight subtraction can pro-duce local maxima of the likelihood at several po-sitions far away from the true vertex (or globalmaximum). The large size of Super–Kamiokanderenders the search for the true global maximumtricky and time consuming. Also, most standardmaximization strategies are often “caught” by lo-cal maxima, if they are close to the starting posi-tion. To improve both the speed performance aswell as reduce the number of mis-reconstructions,the systematic search for a good starting positionwith a regular grid (spaced about 4m) was replacedby a search of a list of points. Combinations of fourhits define these points by requiring that all four hittime residuals of a combination are zero at its asso-ciated point. With BONSAI Super–Kamiokande–II was able to analyze 7MeV electrons recoilingfrom elastic solar neutrino-electron scattering.

For Super–Kamiokande–III, we improved BON-SAI’s maximization strategy. BONSAI is now fastenough to be run on-line and has a slightly im-proved mis-reconstruction ratio. The on-line re-construction is needed, since the BONSAI ver-tex provides an “intelligent trigger”1 for Super–Kamiokande’s lowest energy events: all eventscloser than 2m to any PMT are deleted from theraw data file thereby reducing the number of rawevents by more than an order of magnitude.

Event Selection and Background

Low energy events compete with three differenttypes of background: decays from nuclear spalla-tion products induced by cosmic ray muons, ex-ternal radioactive background and internal radioac-tive background. External radioactive backgroundoriginates mostly in the PMT glass and the fiber-glass backings of the PMT enclosures. Internalradioactive background is either injected by thewater system during recirculation, or instable el-ements are produced by the detector material andthen dissolved in the water (a typical example isradon gas from the U/Th decay chain). Most of thespallation background is reduced by reconstruct-ing muon tracks preceding the low energy event

and removing events with good spatial and tempo-ral correlation. The spallation cut produces about20% of dead time in the detector. Remaining spal-lation background could come from muon show-ers (which are hard to reconstruct completely),mis-fit muons, and very long-lived spallation prod-ucts. Most of the external radioactive backgroundis removed by a vertex cut requiring events to bemore than two meters away from any PMT. Sincethis background is huge, a high rate of externalbackground events survives this cut due to mis-reconstruction of the vertex. Typically, such eventshave a poor hit time residual coincidence comparedto Cherenkov events. For Super–Kamiokande-IIwe developed a powerful vertex goodness criteriontesting the time residual coincidence:

g(~v) =N∑

i=1

wie−0.5(ti−| ~xi−~v|/c)/σ)

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whereti are the measured PMT hit times,~xi thePMT locations,~v is the reconstructed event ver-tex, σ is the effective timing resolution expectedfor Cherenkov events (5ns) andwi are Gaussianhit weights also based on the hit time residuals,but with a much wider effective time resolution(60ns). The weights reduce the dark noise con-tamination of the Cherenkov light. We combinedthis timing goodness with a circular KS test check-ing the azimuthal symmetry around the Cherenkovcone, which is also sensitive to mis-reconstructionof the vertex. As a consequence, the external ra-dioactive background reconstructing deep insidethe detector was reduced by more than an order ofmagnitude. We are left with the internal radioac-tive background (radon gas). Since radon producesCherenkov light from realβ decays of its daughter214Bi it is hard to distinguish such events from trueelectrons near five MeV. The only difference is themultiple scattering of the electron since the214Bidecay electrons are lower in energy. Most of this

1. as in Super–Kamiokande–I and II, the intelligenttrigger is actually a sucession of two vertex reconstruc-tion programs: a fast, first reconstruction (which wascompletely redesigned for Super–Kamiokande–III) fitsall events while the precise, second fitter (BONSAI) pro-cesses only events further than 2m from any PMT ac-cording to the first fitter. This procedure reduces thespeed performance by a factor of about three. It alsoimproves the background reduction.

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Figure 1: Reconstructed Positions of Single 4.4MeV Electrons Injected at Six Positions by a Lin-ear Accelerator.

background has to be reduced by changes to thedetector hardware (e.g. change of the water flow).

Background Studies for Super–Kamiokande–III

To understand the backgrounds in Super–Kamiokande-III we developed several newsources. We used a linear accelerator to injectsingle low energy electrons at six different posi-tions in the detector. While this linear acceleratorwas also used in Super–Kamiokande–I and –IIas well, this time we successfully reduced thelowest electron energy to 4.4 MeV. In addition wetook data for 4.8MeV and 8.6MeV electrons. Theinjection positions were(−3.9,−0.7, +16.0)m,(−3.9,−0.7,−12.1)m, (−12.4,−0.7, +16.0)m,(−12.4,−0.7, +12.0)m, (−12.4,−0.7,−0.1)m,and (−12.4,−0.7,−12.1)m. Figure 1 shows thereconstructed vertex positions of the 4.4 MeV data.Although there are a few mis-reconstructions, al-most all the events cluster within a few metersof the injection position. All events travel in thedownward direction. We are currently using thisdata to design the cuts of the event selection.

We also made a 60kBq Thorium source combin-ing many camping lantern mantles to understand2.6MeV gamma ray contamination from208Tl

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Peak 1 30.49Center 1 735.4Width 1 83.11Peak 2 50.59Center 2 1128.Width 2 77.22Bkgd 25.01

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Figure 2: Reconstructed Low Energy Events Af-ter Radon Injection. Radon was injected at(0.4,−0.7, 9.5)m and (11.0,−0.7, 5.5)m. Theleft, top panel show the reconstructed height, theleft, bottom panel the mean height near each in-jection point as a function of time. The z vertexresolution is 70-80cm. After injection, the radonappears to follow a circular path in the xy-planearound the detector center as shown in the rightfour panels. The red data points are the mean val-ues of x and y as a function of time.

originating in the fiberglass backing of the PMTenclosures. We are currently studying the trans-port and decays of radon gas by artificially inject-ing radon rich water at two points into the detec-tor. Figure 2 shows the path of the resulting “radonbubbles”. As expected, the timing goodness andazimuthal symmetry around the Cherenkov cone ofthe decays is the same as for low energy electrons.

Trigger Energy Threshold of Super–Kamiokande–III

Next year, we plan to upgrade and modernize theSuper–Kamiokande electronics. We expect to beable to record every PMT hit using a clock to readout time “chunks” of data. A new intelligent trig-ger system will then search this data stream forgood low energy Cherenkov events and simulta-neously reconstruct them. If it reconstructs thevertex further than two meters from any PMT, the

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Vertex Deviation vs. Energy MeV

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Figure 3: Deviation of the Reconstructed Vertexfrom the True Position of Monte-Carlo ReactorNeutrino Inverse-β Reactions. The red error barsare the mean deviations as a function of recon-structed energy. Low energy electrons show a sim-ilar behavior as the positrons from inverse-β reac-tions.

event will be written to disk, otherwise it will bediscarded. We therefore expect the trigger energythreshold to be only limited by the abilities of thevertex reconstruction.

Capabilities of Super–Kamiokande–III

The motivation for a lower energy threshold inSuper–Kamiokande–III is the study of solar (viaelastic electron-neutrino scattering) and reactorneutrino (via inverse-β reactions) oscillations. Thedirection of recoiling electrons from solar neu-trino interactions provide a good signature, so asignal to background ratio (after cuts) as low as5% is possible. A measurement of reactor neu-trinos requires tagging the produced neutron in(delayed) coincidence with the prompt positron.Such tagging might be done with captures on dis-solved Gd ions [2]. Figure 3 shows the vertexresolution of simulated reactor neutrino events asa function of the reconstructed total positron en-ergy. Super–Kamiokande is able to reconstructsuch positrons, if the energy reconstructs abovethree MeV. Low energy electrons have the same

vertex resolution, since the annihilation photons donot produce Cherenkov light.

Since solar neutrinos are single events, the analy-sis threshold is defined by the background rate af-ter all cuts are applied. Our goal is to analyze solarneutrino events above 4 MeV of total reconstructedrecoil electron energy to explore the transition be-tween MSW [3] and vacuum solar neutrino oscil-lations. Achieving this goal will require further re-duction of backgrounds, in particular internal ra-dioactive backgrounds.

The energy threshold of reactor neutrinos (if a neu-tron capture tag is available) are only limited bythe event reconstruction, so a 3 MeV thresholdshould be possible. Due to its large size, Super–Kamiokande would be able to collect several thou-sands of reactor neutrino interactions per year anddetermine the solar neutrino oscillation parametersby itself with unprecedented precision [4] within afew years.

Summary

Many improvements and studies were performedto reduce the Super–Kamiokande energy thresh-old. These new tools should enable us to doexciting new measurements: distortions of therecoil electron spectrum from solar neutrino–electron elastic scattering from the transition be-tween MSW [3] and vacuum solar neutrino oscil-lations and a precision determination of the solarneutrino oscillation parameters with a high statis-tics reactor neutrino spectrum.

References

[1] S.Fukuda et al., Nucl. Instrum. Methods A501, 418 (2003).

[2] J.F.Beacom and M.R.Vagins, Phys. Rev. Lett.93, 171101 (2004.)

[3] S.P.Mikheyev and A.Y.Smirnov, Sov. Jour.Nucl. Phys. 42, 913 (1985); L.Wolfenstein,Phys. Rev. D 17, 2369 (1978).

[4] S.Choubey and S.T.Petcov, Phys. Lett. B 594,333 (2004.)

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