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Cosmics at far position P.Sala , M.Antonello , A.Ferrari , D.Stefan , R. Sulej

This report discusses the background effects caused by cosmic rays in the far position of the experiment, including cosmic photons and cosmic events inside the drift time. It also presents simulations and results of underground measurements.

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Cosmics at far position P.Sala , M.Antonello , A.Ferrari , D.Stefan , R. Sulej

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  1. Cosmics at far position P.Sala,M.Antonello, A.Ferrari, D.Stefan, R. Sulej LNGS SC

  2. From report to PAC LNGS SC

  3. Few trivial things • Three years of data tacking  6.6 1020 pot  1.4 108 spills  220 s • 4 105νCC events collected at the far position in positive focusing mode one every 400 spills • Signal events are of the order of 600 over three years • Intrinsic νe order of 2000 events over three years, νO 200000 Background “types” and effects In a shielded situation, practically ALL background is (re-)generated by muons • Cosmic photons in coincidence with beam spill • Direct background to nue search, reduce through reconstruction and/or veto (identify associated muon, distinguish nue from photon) • Cosmic photons inside the drift time, trigger given by another cosmic event • Reduce via a light system capable of “segmenting” the LAr volume and veto • Cosmic events inside drift time, trigger given by neutrino event • Can harm event reconstruction, reduce via light and veto LNGS SC

  4. Simulations Two-step FLUKA simulations: Step 1: showering of primary cosmic rays in atmosphere, at the FNAL location . Double differential particle fluxes scored at various quotes in atmosphere and at ground. Step 2: re-sample these particle distributions on a sphere around the detector and propagation around and inside the detector

  5. Negative muons at floating altitudes: CAPRICE94 Open symbols: CAPRICE data Full symbols: FLUKA primary spectrum normalization ~AMS-BESS Astrop. Phys., Vol. 17, No. 4 (2002) p. 477 Paola Sala, HSS06

  6. L3 Muons (S.Muraro, PhD thesis Milano) exp. data Vertical Horizontal FLUKA simulation Paola Sala, HSS06

  7. Comparison with AMS data Protons and leptons below the geomagnetic cutoff have been measured by the AMS experiment at altitudes 370-390 Km, latitude ±51.70 Astrop. Phys. 20,221 (2003) Downgoing proton flux, simulation(solid line) AMS data(triangles). M is the geomagnetic latitude in radians Paola Sala, HSS06

  8. 1st step : fluxes particles/cm^2/primary at FNAL elevation (225 m) Neu Pro Pio+ Pio- K0 Muo+ Muo- Pho Ele Total 5.9E-03 5..6E-04 3.4E-06 4.4E-06 4.9E-08 1.1E-02 8.7E-03 7.4E-03 1.8E-03 E>100 MeV 3.3E-03 4.4E-04 3.2E-06 4.0E-06 4.9E-08 1.1E-02 8.6E-03 2.2E-03 6.9E-04 E>200 MeV 1.5E-03 3.1E-04 2.9E-06 3.5E-06 4.8E-08 1.1E-02 8.4E-03 9.1E-04 3.4E-04 E>1 GeV 1.2E-04 6.8E-05 1.4E-06 1.4E-06 4.0E-08 8.3E-03 6.6E-03 7.7E-05 4.2 E-05 The photon flux listed here is relevant for un-shielded detectors only LNGS SC

  9. Step 2 Simulation restarted at 250m for different particle types, with thresholds at 100 MeV except for muons --> 30 MeV Simulated exposure equivalent to about 150s for each particle type (less for muons) Configuration 1: detector on surface. Included: non-active LAr, Al, cryo, thermal insulation and its support, ``passerelle'' on top. Configuration 2: detector in a pit, covered by 3m of rock. For this configuration, for the moment we have only the contribution from primary MUONS and NEUTRONS, that we expect is the dominant one. the rest is coming. Events have been recorded in the usual T600 full simulation, plus some auxiliary data for quick retrieval of information. LNGS SC

  10. Results underground The acquisition and event processing will have to deal with 3 millions of events, out of which less than one tenth are neutrinos Every neutrino event will have >5 muon tracks superimposed in the same chamber (4 chambers) There will be 56000 photons ON TIME with the spill , 140 of them isolated There will be 1.5 million photons with E>200 MeV collected “in drift” (0.5 in drift times 3 millions triggers) , out of which 3000 haveno associated muon

  11. How do neutrino events look like? Angular distribution of emitted leptons in the Booster beam Distribution is wide  angle wrt beam is not a good cut for cosmics Warning for internal veto: 45% of  events are NOT contained, need 2 anticoincidence hits to discriminate cosmics Energy distribution of electrons produced in “signal like” e events. Dashed is the cumulative distribution. 200 MeV corresponds to about 10% loss in efficiency WARNING: Simulation of  interactions at these energies is subject to large uncertainties

  12. Zenith of primary Zenith angle distribution of primary particle in coincidence with a background event in the detector , underground location. Dashed line (right axis) is the Cumulative distribution. Average cosine is 0.778 , Corresponding to 40 degrees NOT all the muons are vertical

  13. Deposited energy Spectrum of energy deposited by background events in the detector (cut at 100 MeV) LNGS SC

  14. Energy of muons entering the detector About 15% are stopping inside dN/d(logE) GeV

  15. Cosmic Photon Energy Energy distribution of the most energetic photon in each event, threshold at 100 MeV . GeV

  16. Photon conversion distance Distance of photon conversion vertex from parent muon, in cm, for cosmic background events in t600. Distance is perpendicular to the muon TRACK: it can be used to define a cylindrical “fiducial volume cut” around muon tracks. On the right the cumulative distribution cm cm

  17. Photon conversion distance Distance of photon conversion vertex from parent muon, in cm, for cosmic background events in t600. ONLY photons with E>200 MeV Distance is perpendicular to the muon TRACK: it can be used to define a cylindrical “fiducial volume cut” around muon tracks. On the right the cumulative distribution At 30 cm 2% are left  About 1000 photons “on time” survive, to be reduced through dE/dx or other cm

  18. Examples One background event: isolated photon LNGS SC

  19. Example Background event with one muon + one pion- entering Pion interacts and produces pizero Collection 6630823 Pion - Induction 2 Muon

  20. Example Wire chambers Cathode Muon entering from top, accompanied by photon generated ouside Note the dispersion of the em shower the “first” photon is not always the only one to be considered 7379926

  21. Example LNGS SC

  22. Example One muon can have more than one accompanying photon 9897540 LNGS SC

  23. Muon entering from side and crossing, many small photons around 7003983

  24. Photon identification : preliminary work • We used the same algorithm that was optimized for the signal type events (FNAL and CERN). The procedure is exactly the same. (see later) • We generated photons 0-1 GeV along Z direction • Look for • Pair conversions that can mimic a neutrino interaction (activity at the vertex) • Pair conversions with a “one-mip” like energy deposition • Comptons A semi-automatic procedure was used, where the conversion point and the shower direction are taken from MC, the shower reconstruction, topology and dE/dx are automatic NEXt : apply to simulated background events

  25. OK • Example of event with: • no activityat the vertex • 2 mip Initial part of the cascade is marked as dark blue: 2.5 cm in 3D, dE/dx: 3.97 MeV/cm

  26. Example of event classified as „activity at the vertex” • activity at the vertex • > 2 mip Coll Ind 1 Ind 2 3 cm 2 clusterswithin 3 cm (10 wires) Collection enlarged part of the cascade dE/dx from 2.5 cm of the initial part of the cascade: 7.48 MeV/cm Induction 2 enlarged part of the cascade

  27. Two other examples classified as events with activity at the vertex. Ind2 Collection Zoom – Ind2 Zoom - Collection dE/dx from 2.5 cm of the initial part of the cascade: 3.96 MeV/cm Ind2 Collection dE/dx from 2.5 cm of the initial part of the cascade: 4.8 MeV/cm Zoom – Ind2 Zoom - Collection

  28. photons p > 0.2 GeV/c - photons along z dir. • for photon momentum > 0.2 GeV/c: • 87% eventshave no activityat the vtx. • dE/dx > 3.5 MeV/cm: 708events/765events: 93% • dE/dx < 3.5 MeV/cm && activityat the vtx: 21 events/765 events: 3% (0 comptons) 3.5 MeV/cm Info. dE/dx is measured along the higher electron momentum

  29. photons p > 0.3 GeV/c - photons along z dir. • for photon’smomentum > 0.3 GeV/c: • 88% eventshave no activityat the vtx. • dE/dx > 3.5 MeV/cm: 610events/650events: 94% • dE/dx < 3.5 MeV/cm && activityat the vtx: 16 events/650 events: 2% (0 comptons) 3.5 MeV/cm

  30. photons p > 0.2 GeV/c Examples of dangerous photons (slide 7, slide 8, slide 9): dE/dx < 3.5 MeV/cm && activity at the vtx. • In total there are 21 events • 18 are asimmetric as on the event on the left • 2 photons converted in such a way that the electron with higher momentum goes along collection wire, so they have dE/dx = 0 MeV/cm • 1 has electromagnetic activity close enough to have more than one cluster at the beginning and dE/dx lower than threshold (3.5 MeV/cm) (slide 9). Collection Induction 2

  31. Induction 2 Collection dark blue: hits taken to compute dE/dx : 2.1 MeV/cm Collection - zoom Collection Collection - zoom Induction 2

  32. Two other interesting examples photon with momentum 0.36 GeV/c e+: 281 MeV/cm and e-: 76 MeV/cm Only one event this type: 2 clusters within 3 cm and dE/dx lower than 3.5 MeV/cm. 3.3 MeV/cm dE/dx = 0 becauseinitialdir of electron with higherenergyisalongcollectionwire x primary vtx

  33. LNGS SC

  34. Preliminary results from the visual scanning of MC ne events As a first hint only ~59% of ne CC MC events with vertex inside the fiducial mass and activity at vertex can be recognized with the previous selection criteria Three independent scanners. Preliminary results on the first 100 events.

  35. Results from the scanning of low energy MC ne CC events • only ~50% of ne CC MC events with vertex inside the detector fiducial volume have activity at vertex and can be recognized with the previous selection criteria • 25 % of the events in the fiducial volume shows no detectable hadronic activity at vertex! Intrinsic ne spectrum with En < 1.3 GeV (56 %) Preliminary results on the 175 scanned events.

  36. MC event 3 (NO) Collection view 43 cm 44 cm 44 cm Induction1 view Incoming neutrino 62 cm 94 cm 142 cm Induction2 view En = 0.51 GeV Edep = 0.49 GeV A ne q.e. without activity at interaction vertex.

  37. MC event 8 (OK) 60 cm 74 cm 64 cm Incoming neutrino Collection view 64 cm 94 cm 95 cm En = 1.16 GeV Edep = 0.50 GeV Induction1 view Induction2 view DIS event with backward going electron.

  38. LNGS SC

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