Transcript SRN_PWG_INT

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]
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, 25pp
arXiv:1004.3311
Reactor n (ne)
Constant SN rate (Totani et al., 1996)
Totani et al., 1997
Woosley, 1997
Solar 8B (ne) Hartmann,
Malaney, 1997
Kaplinghat et al., 2000
Ando et al., 2005
Lunardini, 2006
Solar hep (ne) Fukugita, Kawasaki, 2003(dashed)
SRN predictions
(ne fluxes)
Atmospheric ne
So, first the section
briefly introduces
the range of SRN
flux predictions,
expected physics
backgrounds, and
scientific motivations
for 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, 23year-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
compared
with the
excluded
region from
the current
relic flux limit.
How the
fraction
of invisible
SN’s affects
the relic
spectrum.
All lines are
currently
allowed.
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.
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-interact
within 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.
After turning the crank, here are the ranges
of 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.
４１．４ｍ
Super-Kamiokande
４０ｍ
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
SK-I,III,IV: 40% PMT Coverage
SK-II: 19% PMT Coverage
Energy spectrum of SK-I and SK-II
SK-I (1496days)
Atmospheric nm →
invisible m → decay e
Atmospheric nm →
invisible m → decay e
Events/4MeV
Total
background
90% CL limit
of SRN
SK-II(791 days)
Atmospheric ne
Atmospheric ne
Energy (MeV)
Spallation background
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… but
Super-K is background limited.
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
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
In water Cherenkov detectors the relics
would 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:
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 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 
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 
[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
Neutron tagging in Gd-enriched WC Detector
Possibility 1: 10% or less
ne
n
p
e+
p
Gd
g
n+p→d + g
2.2 MeV g-ray
g
Possibility 2: 90% or more
n+Gd →~8MeV g
DT = ~30 msec
Positron and gamma ray
vertices are within ~50cm.
ne can be identified by delayed coincidence.
[reaction schematic by M. Nakahata]
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 
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…
• 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.
[from Cocco et al.]
The solar hep
neutrinos
determine the
lower energy
threshold
(18 MeV)
for LAr.
Zero spallation
is assumed,
along with all
other critical
assumptions.
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.
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!
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!
SRN
Rank:
1b
4b
3b
.
.
.
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.
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.