Supernova Relic NeutrinosTopical 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]section of the Physics Working Group

Interim Report is just about ready. Here’s a quick tour…

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, 25pparXiv:1004.3311

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10-5

10-4

10-3

10-2

10-1

11010 210 310 410 510 610 7

0 10 20 30 40 50 60 70 80Neutrino Energy (MeV)

Neut

rino

Flux

(/cm

2 /s

ec /M

eV)

Constant SN rate (Totani et al., 1996)Totani et al., 1997Hartmann, Woosley, 1997Malaney, 1997Kaplinghat et al., 2000 Ando et al., 2005Lunardini, 2006Fukugita, Kawasaki, 2003(dashed)

Solar 8B e

Solar hep e

SRN predictions(e fluxes)

Reactor e

Atmospheric e

So, first the sectionbriefly introduces the range of SRNflux predictions, expected physics backgrounds, and

scientific motivationsfor doing the

measurement.

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.

Allowed regions

from the SN1987A

data comparedwith the excluded

region fromthe current

relic flux limit.

How the fraction

of invisible SN’s affects

the relic spectrum.

All lines arecurrentlyallowed.

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.

What can we learn by observing the Supernova Relic Neutrinos?

Then, a thumbnail sketch of how the relic fluxesare 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 sin13? This modifies the mix and energies of the 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.

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)

Next, we reviewed the current state-of-the-measurement, and considered where things could

stand 15 years from now.

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Super-Kamiokande 50000 tons ultra-pure water

1 km overburden = 2700 m.w.e.

22500 tons fiducial volume

Our only realcompetition for a timely SRNmeasurement

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

Energy spectrum of SK-IEnergy spectrum of SK-I and SK-IIand SK-II

Atmospheric → invisible → decay e

Atmospheric e

90% CL limit of SRN

Total

background

Energy (MeV)Atmospheric e

Atmospheric → invisible → decay e

Spallation background

SK-I (1496days) SK-II(791 days)

Eve

nts

/4M

eV

Observed spectrum is consistent with estimated background.Search is limited by the invisible muon background.

SK flux limit vs. SRN flux predictions

Getting very close to some… butSuper-K is background limited.

Expect between 0.25 and 2.8 SRN events/yr on top of14 background events/yr (atm , sub-threshold ) 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 haveaccumulated 269 background events compared with:

Best case (flux near limit) = 54 SRN events 3.3 SK beats LBNE to discovery

Top of Lunardini’s range = 27 SRN events 1.6 No SK discovery in 29 years

Then we work ourway 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

In water Cherenkov detectors the relicswould be detected via the inverse beta reaction:

e + p e+ + n

In general we can compare with Super-K to get these numbers, with two main differences:

1) 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.

2) The atmospheric neutrino flux at Homestake is 50% higher than that at Kamioka, due to South

Dakota’s higher latitude.

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 onelive 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

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 onelive 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

[Beacom and Vagins, Phys. Rev. Lett., 93:171101, 2004]

Adding Gdallows

openingthe relicenergywindowdown

to 11 MeV,plus atm

backgroundsand spallationare reduced

e can be identified by delayed coincidence.

e

e+

2.2 MeV -rayp

n

Possibility 1: 10% or less

n+Gd →~8MeV T = ~30 sec

Possibility 2: 90% or more

Positron and gamma ray vertices are within ~50cm.

n+p→d +

p

Gd

Neutron tagging in Gd-enriched WC Detector

[reaction schematic by M. Nakahata]

For 30% HE PMT coverage with gadolinium, theenergy window extends from 11 to 30 MeV, andthe 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

The primary DSNB reaction in liquid argon is:

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…

This cross section has about a 30% uncertainty.

• 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 - 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.

The solar hep neutrinos

determine the lower energy

threshold(18 MeV)for LAr.

Zero spallationis assumed,along with allother criticalassumptions.

[from Cocco et al.]

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 events

SRN flux exceeds atm 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.

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 events

Convincing SRN detection unlikely

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?

High-exposure proof-of-principle LAr data is needed!

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 events

Convincing SRN detection unlikely

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.

High-exposure proof-of-principle LAr data is needed!

SRNRank:

1b4b3b...

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 e’s) of a large LAr detector at modest depth.

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.