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Supernova Relic Neutrinos Topical Group Report

Supernova Relic Neutrinos Topical Group Report. Mark Vagins IPMU, University of Tokyo/UC Irvine. LBNE Mini-Workshop at the Institute for Nuclear Theory Seattle, WA August 9, 2010. Status report in a nutshell:. The supernova relic neutrino [SRN]

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Supernova Relic Neutrinos Topical Group Report

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  1. Supernova Relic NeutrinosTopical Group Report Mark Vagins IPMU, University of Tokyo/UC Irvine LBNE Mini-Workshop at the Institute for Nuclear Theory Seattle, WAAugust 9, 2010

  2. Status report in a nutshell: The supernova relic neutrino [SRN] section of the Physics Working Group Interim Report is just about ready. Here’s a quick tour…

  3. Along with a variety of other references, two very recent review articles proved particularly useful in assembling this section: “Diffuse Supernova Neutrinos at Underground Laboratories,” C. Lunardini, July 2010, 57pp arXiv:1007.3252 “The Diffuse Supernova Neutrino Background,” J.F. Beacom, April 2010, 25pp arXiv:1004.3311

  4. Constant SN rate (Totani et al., 1996) Totani et al., 1997 Hartmann, Woosley, 1997 Malaney, 1997 Kaplinghat et al., 2000 Ando et al., 2005 Lunardini, 2006 Fukugita, Kawasaki, 2003(dashed) Solar 8B (ne) Solar hep (ne) Reactor n (ne) So, first the section briefly introduces the range of SRN flux predictions, expected physics backgrounds, and scientific motivations for doing the measurement. SRN predictions (ne fluxes) Atmosphericne

  5. What can we learn by observing the Supernova Relic Neutrinos? • Understanding supernovae, central to understanding many aspects of the present physical universe, requires the detection of their neutrino emissions. More supernova neutrino data is strongly needed; the SRN will provide a continuous stream of input to theoretical and computational models • The shape of the SRN spectrum will provide a test of the uniformity of neutrino emissions in core-collapse supernovae, determining both the total and average neutrino energy emitted. Was SN1987A a “normal” explosion or not? The sparse, 23-year-old data concerning a single neutrino burst cannot say, but the SRN data will, as long as we have spectral information. • How common are optically dark explosions? No one knows.  Comparing the SRN rate with optical data of distant SN’s can tell us.

  6. Allowed regions from the SN1987A data compared with the excluded region from the current relic flux limit.

  7. How the fraction of invisible SN’s affects the relic spectrum.All lines are currently allowed.

  8. What can we learn by observing the Supernova Relic Neutrinos? • N.B.: Contrary to what you may have heard before, measuring the total SRN flux will NOT serve to uniquely determine the cosmic core-collapse (and hence star formation) rate. • This key factor in cosmology, stellar evolution, and nucleosynthesis is currently uncertain at the ±40% level, but by the time of LBNE it will have been quite well-determined – to around 5% – by the coming generation of large scale astronomical sky surveys. However, measuring the SRN flux will provide a new and independent probe of this rate.

  9. Then, a thumbnail sketch of how the relic fluxes are predicted (and why they have a large range) is presented: • 1) Pick a supernova explosion model (Livermore, Garching, Arizona, etc). Assumptions about total emitted neutrino energy and average neutrino energies enter here. • 2) Allow the neutrinos to oscillate and self-interactwithin the star. Is hierarchy normal or inverted? How big is sin2q13? This modifies the mix and energies of the n flavors which arrive at Earth. • 3) What’s the rate of stellar collapse? This ±40% normalization uncertainty will soon be reduced by synoptic surveys to the 5% level.

  10. After turning the crank, here are the rangesof SRN fluxes we need to consider for different detector designs: WC = factor of 12 (small spectral window) WC+Gd = factor of 6 (larger spectral window) LAr = factor of 7 (similar spectral window as WC, but less variation in nue than nuebar)

  11. Next, we reviewed the current state-of-the-measurement, and considered where things could stand 15 years from now.

  12. Super-Kamiokande 50000 tons ultra-pure water 22500 tons fiducial volume 1 km overburden = 2700 m.w.e. Our only real competition for a timely SRN measurement

  13. SK-I,III,IV: 40% PMT Coverage SK-II: 19% PMT Coverage

  14. Energy spectrum of SK-Iand SK-II SK-I (1496days) SK-II(791 days) Total background 90% CL limit of SRN Atmosphericnm → invisible m → decay e Atmospheric nm → invisible m → decay e Events/4MeV Energy (MeV) Atmosphericne Atmosphericne Spallation background Observed spectrum is consistent with estimated background. Search is limited by the invisible muon background.

  15. SK flux limit vs. SRN flux predictions Getting very close to some… but Super-K is background limited.

  16. Expect between 0.25 and 2.8 SRN events/yr on top of 14 background events/yr (atm n, sub-threshold m) in SK Fifteen years from now, Super-K will be 29 years old! Ignoring the five year period between mid-2001 and mid-2006, and applying an 80% typical usable livetime factor means SK can expect to have accumulated 269 background events compared with: Best case (flux near limit) = 54 SRN events  3.3 s  SK beats LBNE to discovery Top of Lunardini’s range = 27 SRN events  1.6 s  No SK discovery in 29 years

  17. Then we work our way through all of the detector configurations in order to asses their relative sensitivities to discovering the relic flux. Goal 1: Discover the SRN flux Goal 2: Determine its spectral shape

  18. In water Cherenkov detectors the relicswould be detected via the inverse beta reaction: ne + p e+ + n • In general we can compare with Super-K to get these numbers, with two main differences: • Because the 4850 level at DUSEL is deeper than Super-K’s 3300 feet, the spallation rate is 15X less per unit volume for LBNE. • The atmospheric neutrino flux at Homestake is 50% higher than that at Kamioka, due to South Dakota’s higher latitude.

  19. For 15% HE PMT coverage, the detector will behave a lot like SK-II did with 19% non-HE PMT coverage. The spallation leakage will be mostly eliminated, and the invisible muon rate will be increased by 50%, so figure an SRN energy window very similar to SK’s with a bit more background. Therefore, for one live year in one such 100 kton fiducial module, expect Between 1 – 13 SRN events and 93 background events  SRN flux must be near top of range for discovery 

  20. For 30% HE PMT coverage, the detector will behave a lot like SK-I did with 40% non-HE PMT coverage. Due to lower spallation and higher atmospheric backgrounds than at Kamioka, figure an SRN energy window somewhat wider (starting 2.5 MeV lower) than SK’s but with a bit more background. Therefore, for one live year in one such 100 kton fiducial module, expect Between 1.5 -- 17 SRN and 107 background events  SRN flux must be in top half of range for discovery 

  21. [Beacom and Vagins, Phys. Rev. Lett., 93:171101, 2004] Adding Gd allows opening the relic energy window down to 11 MeV, plus atm backgrounds and spallation are reduced

  22. ne can be identified by delayed coincidence. Neutron tagging in Gd-enriched WC Detector Possibility 1: 10% or less n+p→d + g n g ne 2.2 MeV g-ray p p Gd e+ g Possibility 2: 90% or more n+Gd →~8MeV g DT = ~30 msec Positron and gamma ray vertices are within ~50cm. [reaction schematic by M. Nakahata]

  23. For 30% HE PMT coverage with gadolinium, the energy window extends from 11 to 30 MeV, and the atmospheric neutrino-related backgrounds are suppressed by about a factor of five. . Therefore, for one live year in one such 100 kton fiducial module, expect Between 4 – 25 SRN and 21 background events Limiting the window to 11 MeV – 19 MeV improves S/N Between 3 – 12 SRN and 3 background events  Near certain discovery plus spectrum 

  24. The primary DSNB reaction in liquid argon is: This cross section has about a 30% uncertainty. Since there is little high-exposure experimental LAr data available to study, a number of crucial assumptions must be made to predict the response of LAr detectors to the relic neutrinos…

  25. • No nuclear recoils from fast neutrons will be able to produce an event which looks like a single electron in the energy window. • Unlike in water Cherenkov detectors, liquid argon detectors do not suffer from sub-Cherenkov muons decaying into electrons and faking the SRN signal, as no muons (or evidence of their decays) should escape detection in the detector. • No spallation products will be produced which generate electrons in the energy range of interest without clear evidence of their parent muon allowing the event to be removed from consideration. The full family of spallation daughters of argon does not seem to be known, but it must include all possible oxygen spallation products, e.g. 11Li, a b- emitter, Q = 20.6 MeV. • No radioactive background or impurity, electronic effect in the detector, track-finding inefficiency, particle misidentification, or failed event reconstruction will ever be able to lead to a signal in the energy range of interest.

  26. The solar hep neutrinos determine the lower energy threshold (18 MeV) for LAr. Zero spallation is assumed, along with all other critical assumptions. [from Cocco et al.]

  27. For one live year in one 17 kton fiducial LAr detector with a photon trigger on the 4850 level, and using the optimal window of 18 – 30 MeV, we can expect Between 0.2 – 1.1 SRN events and 0.1 atm n events • SRN flux exceeds atm n background in all cases  But statistics will be very low. As with any rare search, for a conclusive detection the rate of fake events must be aggressively and convincingly controlled. The figure of merit to keep in mind is <1 false event/kiloton LAr/century.

  28. For one live year in one 17 kton fiducial LAr detector with no photon trigger on the 300 level, and using the optimal window of 18 – 30 MeV, we can expect Between 0.1 – 0.9 SRN events and 0.1 atm n events But muon rate at 300 feet is 32600 times that at 4850! There will always be several lines of charge being drifted at once, potentially complicating/confusing reconstruction, especially with no photon trigger.Will no spallation or other stuff make it through? • Convincing SRN detection unlikely  High-exposure proof-of-principle LAr data is needed!

  29. For one live year in one 17 kton fiducial LAr detector with a photon trigger on the 800 level, and using the optimal window of 18 – 30 MeV, we can expect Between 0.2 – 1.1 SRN events and 0.1 atm n events But muon rate at 800 feet is still 8000 times 4850 rate! Will no spallation or other stuff make it through? One year of false-background-free running here is equivalent to 136 Mton-years without a single fake event at 4850… a bit hard to accept without data. • Convincing SRN detection unlikely  High-exposure proof-of-principle LAr data is needed!

  30. SRN Rank: 1b 4b 3b . . .

  31. Personal opinion: Sure, I’d love to see 300 kton of Gd-loaded water, but this (Config 1b) strikes me as, ahem, rather unlikely. Therefore, I feel the best path would be one of the blended options of WC, WC+Gd, and LAr (Configs 4b or 3b). This would provide a solid SRN discovery with spectral information, plus the needed real-world running (and ne’s) of a large LAr detector at modest depth.

  32. So, that’s the story of the supernova relic neutrinos. We do have a opportunity to make a rapid discovery, but we will need the right technology choices to do it.

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