1. UNIVERSITÀ DI PAVIA Dipartimento di Fisica Nucleare e Teorica 12 Gennaio 2006 Alessandro Menegolli Dottorato di Ricerca, XVIII ciclo Study of the Low and Intermediate Energy Electron Samples Energy Electron Samples with the ICARUS T600 Detector

2. 2 Contents  The ICARUS programme  Event reconstruction tools  The low energy electron sample  The intermediate energy electron sample  Conclusions

3. 3 The ICARUS programme The ICARUS programme is based on the technology of the Liquid Argon Time Projection Chamber (LAr TPC), a suitable device for a completely uniform imaging with high accuracy of massive volumes LAr TPCs find their main application in studies concerning some of the major issues of particle and astro-particle physics: Study of solar and atmospheric neutrino interactions Study of neutrino oscillations with neutrino beams Detection of Supernova neutrinos Search for nucleon decay

4. 4 Detection of ionizing events The simplest detector of ionizing events consists of a pair of electrodes immersed in a dielectric liquid and connected to a power supply The electron-ion pairs formed after the passage of an ionizing particle are pushed towards the electrodes by a suitable electric field Since the electron mobility is orders of magnitude larger than the one of the positive ions, only electrons will contribute significantly to the current The signal observed from a single electron starting at x position x within the gap is then:

5. 5 Imaging of ionizing events Imaging of ionizing events in the LAr volume is possible because of: 1. the long lifetime of drift electrons (~ 1 ms) 2. the sensitivity of the modern low noise amplifiers (~ 10 3 electrons) A non-destructive read-out, necessary to ensure the imaging of different views, can be realized by replacing the plane electrodes with wire planes, which are highly transparent to the electrons of the event

6. 6 LAr is easy to get with a very high purity as a by product of air liquefaction LAr is easy to get with a very high purity as a by product of air liquefaction LAr is a homogeneous medium which acts both as a detector and as a target LAr is a homogeneous medium which acts both as a detector and as a target Interesting physical properties for a tracking device: Interesting physical properties for a tracking device: The passage of charged particles in LAr induces: The passage of charged particles in LAr induces: - Relatively abundant ( ~ 0.9% of air) - Cheap ( ~ 0.5 Euro/Kg) - High density: 1.4 g/cm 3 - High density: 1.4 g/cm 3 - Radiation length: 14 cm, Interaction length: 80 cm - Radiation length: 14 cm, Interaction length: 80 cm - Ionization → high mobility of drift electrons - Scintillation → VUV spectrum,  = 128 nm, useful for trigger Why the Liquid Argon as active medium? The TPC medium must be a noble gas in order to permit long electron drifts

7. 7 Liquid Argon (87 K) Cathode Anode: 3 wire planes (at ±60° and 0°) E (500 V/cm) ICARUS T600 (half) detector (inner view) A particle crossing the detector ionizes the atoms of Argon…  e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- …electrons are guided by an electric field towards three wire planes… …on which an electric signal, to be treated by electronics, is induced. ICARUS T600

8. 8 T600 detector overview The T600 detector is composed by 2 half modules, each of 300 tons (T300). Each T300 houses an inner detector made of: two LAr TPCs (wire chambers): 3.6 (width)  3.9 (height)  19.6 (length) m 3 the field shaping system monitors and probes a PMT array for the LAr scintillation light detection surrounding thermal insulation layers (passive Nomex honeycomb) Outside the detector are located: The read-out electronics A Liquid Nitrogen cooling circuit A system of purifiers to achieve the required LAr purity ( < 0.1 ppb O 2 equivalent )

9. 9 Wire chambers three vertical, parallel wire planes  Each LAr TPC consists of three vertical, parallel wire planes (17.95  3.16) m 2, 3 mm apart from each other  The distance between the two TPCs is 3 meters, with a cathode plane placed 1.5 meters of drift length in the middle → 1.5 meters of drift length (Left and Right chambers) 1. Induction I (0°, 2112 wires) and Induction II (+60°, 5728 wires) face the LAr drift volume and work in induction mode 2. Collection (-60°, 5728 wires) works in charge proportional collection mode (charge collected is proportional to the ionizing track energy to the ionizing track energy)  Wire directions in the three planes run 0° and ±60° w.r.t. the horizontal at 0° and ±60° w.r.t. the horizontal: PMTs directly immersed in LAr detect the prompt (< 2  s) VUV scintillation photons prompt (< 2  s) VUV scintillation photons, triggerzero time definition used for trigger and zero time definition

10. 10 The technical run in Pavia T600 detector was fully tested in a technical run held in a surface laboratory in Pavia during summer 2001. The test was performed with one T300 half module fully assembled, instrumented and filled with LAr ~ 3  10 4 cosmic ray events of different typologies were collected during the 104 days of run, demonstrating the full capability of the LAr TPC technique to detect and image a wide class of ionizing events: Long muon tracks crossing full detector length Extensive air showers with muon bundles Muon stopping and decaying inside the LAr volume Hadronic interaction with secondary particle production Large electromagnetic shower 18 meters

11. 11 Massive LAr TPC physics …Supernova neutrinos… …solar neutrinos… Atmospheric neutrinos… …neutrinos from CERN to LNGS… …proton decay

12. 12 To understand the capability of LAr TPCs in detecting and reconstructing such wide range of events, T300 data were analyzed in order to:  develop suitable tools for LAr TPC data analysis: managing of great amount of data imaging of ionizing events ionizing event spatial reconstruction ionizing event energy reconstruction  study the limit of the LAr TPC technique in detecting low energy events (electrons), which is a crucial point for the solar and SN neutrino physics  verify if a LAr TPC can usefully work as a calorimeter and in this case study its performance, which is fundamental for neutrino beam physics 1. Study of the low energy electron sample (E < 10 MeV) 2. Study of the intermediate energy electron sample (E ~ 50 ÷ 1000 MeV)

13. 13 Event reconstruction tools 1.The position in the wire planes (2D coordinates) 2.The deposited energy Each wire of the T600 read-out planes records the information on the energy deposited in a well delimited segment of the ionizing track hit The basic element of a track is called hit, defined as the portion of the track whose energy is read by a given wire of the three wire planes The reconstruction tools must extract the relevant physical information from the read-out wire output signal, that is:

14. 14 hit finding The first step of the reconstruction is the hit finding, where hits are searched in every wire in a region of a certain width where signals rise above a baseline Hit finding algorithms are based on the geometrical features of the signal, output intended as the energy output (digitized in ADC counts) as a function of the drift time drift time, which is sampled 4096 times by electronics (0.4  s per sample) High frequencylow frequency High frequency and low frequency noises affect the output of a read-out wire: they can be removed by suitable signal filters

15. 15 Specific geometrical features of the output in its develop along the 4096 by channels of the drift time coordinate are looked for. Hit is searched by imposing minimal requests imposing minimal requests on the values of the parameters which characterize it and by maximizing the signal-to-noise ratio:

16. 16 fine hit reconstruction It follows a phase of fine hit reconstruction, where the parameters containing the physical information of the original segment of track are extracted: Hit baseline, position, height, width and area hit baseline In this phase the hit baseline, that is the output value above which the hit area is computed, is precisely determined, as well as the hit left and right drift time coordinates In this way it is possible to determine: the hit position in the drift direction, that is associated to the drift coordinate of the hit peak; the peak height, that is the difference between the peak output value and the baseline; sum of all output values which lie above the the hit area, that is the sum of all output values which lie above the baseline baseline, from the left to the right bound of the hit; the error on the area, which is a dominant contribution in the overall error on the track energy. Three are the contributions to this parameter:  B 1.the error  B on the determination of the baseline  n 2.the error  n due to the wire noise  A 3.the instrumental error  A coming from the digitization of the signal

17. 17 The algorithms described above are implemented in Fortran 77 language and then embedded in a GUI HIGZ-based program, which allows to open and visualize selections of events from the T300 data. The program is interactive, that is the user can directly work on the selected views, because it associates the local window coordinates to the real wire plane and time drift coordinates through proper HIGZ routine calls

18. 18 The low energy electron sample A study of the very low energy region of the T300 data has been carried out: electrons from few hundreds keV to some MeV were studied The main goal is the study of the backgrounds and the comparison with the Montecarlo results obtained by considering: Decays of 238 U and 232 Th contaminants of the aluminium Decays of 238 U and 232 Th contaminants of the aluminium walls in the inner detector walls in the inner detector Environmental neutron capture events in 40 Ar and 36 Ar Environmental neutron capture events in 40 Ar and 36 Ar spectral shape of the background  information on the spectral shape of the background, that affects a number of physics items in ICARUS, like solar and SN neutrinos; low energy calibration point  a low energy calibration point for the T600 detector; trigger rate  an estimation of the trigger rate for low energy events both in surface and in underground laboratory (LNGS); upper limit  an upper limit on the 39 Ar isotope activity in the T600 detector. The search provided:

19. 19 Geant3 Montecarlo simulations f For the simulations of neutron captures in 40 Ar and 36 Ar, the capture rates f due to neutrons on both isotopes were evaluated from the capture cross  N sections , the primary neutron flux  and the number N of target Ar atoms: f =  ·  · N 2635 Hz A total capture rate of 2635 Hz was found. A single electron energy spectrum from a 10 seconds acquisition time (26350 captures) was obtained: In the computation of 238 U and 232 Th contaminants of the aluminium walls in the inner detector were used the results obtained by ICARUS Collaboration A spectrum of single electrons from 10s acquisition time (~ 5.4 x 10 5 photons entering in the LAr volume, ~ 5.2 x 10 4 electrons with a 300 keV threshold) has been obtained:

20. 20 The 511 keV peak If one selects only electrons produced just by photoelectric effect in LAr, a small peak set at 511 keV is evident in the low energy spectra, both for neutron captures and for radioactive contaminants The 50 Hz of photoelectrons in such peak are thought as due to photons from the annihilation of e + coming from  → e + e - conversion

21. 21 To obtain a clean sample of low energy electrons, reducing as much as possible the cosmic ray background, a visual scanning of the T300 events have been performed, selecting for the reconstruction regions free from: - electromagnetic showers - single tracks (mostly crossing muons) Measurement Routines of hit finding and reconstruction were performed on filtered signals for all wires belonging to the selected regions, with the following constraints: ~ 300 keV minimum hit area = 80 ADC counts (~ 300 keV); 8 hits maximum of 8 hits on adjacent wires; minimum peak height = 5.5 ADC counts over the baseline 2066 A sample of 2066 selected electrons has been found from ~ 2.6 x 10 5 ~ 0.1 seconds scanned wires, corresponding to a total acquisition time of ~ 0.1 seconds

22. 22 set at 140 ADC counts, In real data a small peak, set at 140 ADC counts, is present. Using the calibration factor measured in the T600 for mip muons (3.3 keV/ADC), one founds an energy of about 500 keV, as in the Montecarlo, but… In the photoelectric interpretation, a calibration at low energies has been found: 3.64 ± 0.15 E(keV) = 3.64 ± 0.15 ADC (counts) only the 5%~ 1 KHz …the comparison between MC with the real data shows that the events in the photoelectric peak due to n captures and to detector contaminants are only the 5% of the total (~ 1 KHz), the remnant being due to cosmic e.m. shower tails Broad bump of events not interpreted yet

23. 23 1.0÷3.5 MeV MC well reproduces the surface integral trigger rate in the range 1.0÷3.5 MeV Rate still surviving at higher energy is interpreted as due to cosmic e.m. showers A threshold of ~ 9 MeV for low energy electron detection seems to be accessible, also in this very pessimistic case (surface measurements)

24. 24 Recent measurements have estimated an 39 Ar activity in natural Argon of the order of 1 Bq/Kg: ~ 1.1 Bq/kg Activity measured below the 39 Ar  -decay spectrum end-point energy resulted ~ 1.1 Bq/kg In this range of energies the cosmic ray contribution cannot be easily upper limit estimated: 39 Ar activity can be only given as an upper limit 39 Ar upper limit

25. 25 Remarks energy below 10 MeV  A search for electrons with energy below 10 MeV in T300 surface data has been carried out in order to get information about the rate and the spectral shape of the background  In real data (~ 2000 electrons in 0.1 s acquisition time) a clear bump of events was photoelectric peak at 511 keV found: this was interpreted as a photoelectric peak at 511 keV: the rate of events in ~ 1 kHz the peak is ~ 1 kHz.  The same peak was found in Montecarlo simulations, but the rate of MC events in the just 50 Hz peak is just 50 Hz → the 95% of these probably comes from cosmic rays  With 50 Hz of events underground (no cosmic rays) a good calibration point at very low C = (3.64 ± 0.15) keV/ADC energies can be obtained in few minutes of dedicated run: C = (3.64 ± 0.15) keV/ADC energy threshold at ~ 9 MeV  Background rates extrapolated for LNGS provides an energy threshold at ~ 9 MeV for low energy electron detection (useful for solar and SN neutrino detection)

26. 26 The intermediate energy electron sample  0 production A search for hadronic interaction of cosmic rays in LAr with  0 production has been carried out: after the scanning of T300 data, a sample of events with the signature of the  0 decay in two photons has been collected and measured The motivation of this work is twofold: 1. The development of a calorimetric method to measure  0 mass in LAr TPC events. This could be useful in order to separate electron from  0 for distinguishing e CC interactions from  NC interactions: e + n → e - + p (CC) e + n → e - + p (CC)  + n →  +  0 + X (NC)  + n →  +  0 + X (NC) 2.The test of the calorimetric performance of a LAr TPC in measuring intermediate energy electrons, that is electromagnetic showers ranging roughly from some tens to several hundreds MeV

27. 27 Wire coordinate (3mm pitch) Drift time coordinate A scanning of T300 data was fulfilled in order to find and select  0 →  events, to be analyzed by the hit finding and reconstruction routines two clear The signature of the decay event is given by the presence of two clear electromagnetic showers electromagnetic showers (X 0 ~ 14 cm in LAr) pointing to the main vertex of an hadronic interaction in both Collection and Induction views

28. 28 Measurement of ( ,  ) systems only Hits inside the green polygonal line only are selected for energy reconstruction M  For the evaluation of the invariant mass M  of ( ,  ) systems are needed: E 1 E 2 1.The two shower energies, E 1 and E 2  12 2.The angle  12 between the two showers Shower energy is given by: C = calibration factor converting from ADC to MeV (after MC simulations) ADC to MeV (after MC simulations) A k = k-th hit area in ADC counts e (t k -t 0 /  e ) = correction for drift electron finite life time  e finite life time  e 1+k Q (dE/dx) = electron-ion recombination factor (Birks’ law) factor (Birks’ law) main vertex The angle  12 is computed from the coordinate of the main vertex starting points of the interaction and from the starting points of the two showers

29. 29 Montecarlo simulations Algorithms for shower energy measurement and  0 mass reconstruction (M  0 = 135 MeV/c 2 ) have been tested and optimized through the measurement of several MC (FLUKA) events with  0 production :  0  (at rest)   + 40 Ar n +  0 + X (2 GeV)  0  0 + p (at rest)

30. 30 60  systems from 2 GeV  - interactions in LAr  systems from  0 at rest  systems from  +  →  0  p decay

31. 31 Number of events Angle  (  )Angle  (  ) ( MeV)RMSE MC / 1. Photon – 50 MeV 106060120492 1,02 ± 0,06 2. Photon – 100 MeV 456060120995 1,01 ± 0,06 103090964 1,04 ± 0,04 10641161005 1,00 ± 0,03 10060985 1,02 ± 0,03 10120180964 1,04 ± 0,04 3. Photon – 300 MeV 1060601202929 1,03 ± 0,02 4. Photon– 1000 MeV 10606012099422 1,01 ± 0,02 Other MC-FLUKA simulations: photons and electrons 1. Photons 2. Electrons

32. 32 Results from T300 data 247 real hadronic events 247 real hadronic events with at least two e.m. showers pointing to the main vertex of the interaction were selected: 374 combinations The plot shows 374 combinations of two showers from all the events after quality cuts: above the background a rather broad bump of events it is visible

33. 33 E.m. showers at low angle between them can overlap → it is expected that large angles between showers the mass values are more precise at large angles between showers Average invariant mass as a function of a low cut on the shower angle Event cut at 60° (243 combinations surviving) M  = 129.7 ± 29.7 MeV/c 2 (23%)

34. 34 A more precise determination of the invariant mass can be obtained one pair of e.m. showers only by selecting events with production of one pair of e.m. showers only, where most of the wrong combinations are cut away 101 combinationsmost of background removed 101 combinations survive after quality cuts → most of background removed Again, it is present a distortion of the invariant mass at low angles low cut of 60° between showers: a low cut of 60° in the angle has been chosen Average invariant mass as a function of a low cut on the shower angle Event cut at 60° (59 combinations surviving) M  = 135.5 ± 29.1 MeV/c 2 (21%)

35. 35 Data also allowed to evaluate the energy resolution of the detector in the reconstruction of e.m. showers of some hundreds MeV  E /EE The energy resolution  E /E as a function of the e.m. shower energy E has 594 showers been studied for all e.m. showers with energy below 1 GeV (594 showers)

36. 36 Remarks  0 production  A systematic search for hadronic interactions of cosmic rays in LAr with  0 production has been accomplished scanning T300 surface data  The electromagnetic  0 →  decay has been studied in order to develop a procedure to from the two e.m. showers from  conversion reconstruct  0 mass from the two e.m. showers from  conversion 247 hadronic events  247 hadronic events with the production of at least a  0 were selected from T300 data: ( ,  ) invariant mass has been evaluated both for multi-shower and for two-shower events  In both cases, the best fit value is in agreement with  0 hypothesis: the result for two- M  = (135.5 ± 29.1) MeV/c 2 shower events is M  = (135.5 ± 29.1) MeV/c 2  The energy resolution of the detector in reconstructing e.m. showers of some hundreds ~ 600 e.m. showers MeV has been evaluated from ~ 600 e.m. showers

37. 37 Conclusions (1) Results here presented concerned the analysis of two particular samples of events extracted from the data collected by ICARUS T600 LAr TPC in Pavia during a test run with cosmic rays held in summer 2001 The detector fully demonstrated the feasibility of the Liquid Argon TPC technique both for the detection and for the imaging of ionizing tracks crossing the sensitive LAr volume It has been shown that LAr TPC data can be properly worked out in order to extract information concerning a wide range of topics of particle physics Results on low energy electron analysis have been presented, showing the features of a LAr detector in reconstructing low energy events (solar and SN neutrino physics)

38. 38 By the end of 2006 the ICARUS T600 detector will be operative in Gran Sasso underground laboratories, being the very first time a LAr detector of such mass would be used for astro-particle and neutrino physics, with the 200 keV ÷ 100 TeV possibility to investigate an enormous range of energies (200 keV ÷ 100 TeV) Conclusions (2) It has been shown that a LAr TPC can calorimetrically reconstruct ionizing events (precise measurement of  0 mass with real data) Methods presented in this work will be in the next future extended in order to develop tools for the analysis of the data coming from larger LAr TPCs (tens of kilotons) Multi-kiloton LAr TPCs will be the most important laboratory for the study of astro-particle and neutrino physics, and hopefully for the discovery of new phenomena beyond the Standard Model of Particle Physics