Neutrino2002 Poster Session

1
Neutrino2002Poster Session Large Volume Detector
at LNGSand neutrino oscillation Marco Selvi
Bologna University INFN On behalf of the LVD
collaboration
Physics with Supernovae
In spite of the lack of a standard'' model of
the gravitational collapse of a massive star,
some features of its dynamics and, in particular,
of the correlated neutrino emission appear to be
well established. At the end of its burning phase
a massive star (M gt 8 Mo) explodes into a
supernova (SN), originating a neutron star which
cools emitting its binding energy EB 3 x 1053
erg mostly in neutrinos. The largest part of
this energy, almost equipartitioned among
neutrino and antineutrino species, is emitted in
the cooling phase Eanti-ne Ene Enx EB/6
(nx denotes generically nm , anti-nm , nt ,
anti-nt). The energy spectra are approximatively
Fermi-Dirac, but with different temperatures,
since ne , anti-ne , nx have different couplings
with the stellar matter Tne, lt Tanti-ne lt Tnx .
These features are common to all existing
stellar collapse models, and lead to rather model
independent expectations for supernova neutrinos.
The observable signal is then sensitive to
intrinsic n properties, as oscillation of massive
neutrinos. Indeed, oscillations change
significantly the expected number of events in
LVD.
In the study of supernova neutrinos nm , nt are
indistinguishable, both in the star and in the
detector, because their energy is below the
charge lepton production threshold consequently,
in the frame of three-flavor oscillations, the
relevant parameters are just (Dm2sol , Ue22) and
(Dm2atm , Ue32). We will adopt the following
numerical values Dm2sol , 5 x 10-5 eV2 , Dm2atm
2.5 x 10-3 eV2 , Ue22 0.33 the selected
solar parameters (Dm2sol , Ue22) describe a LMA
solution, favored by recent analyses. For a
normal mass hierarchy scheme, neutrinos (not
anti-neutrinos) cross two resonance layers one
at higher density (H), which corresponds to
Dm2atm , and the other at lower density (L),
corresponding to Dm2sol , (for inverted mass
hierarchy, transitions at the higher density
layer occur in the anti-neutrino sector, while at
the lower density layer they occur in the
neutrino sector. Anyway, both in case of normal
and inverted mass hierarchy, the dynamics of
collapse is not affected, since these layers are
located far outside the core of the star). Given
the energy range of supernova neutrinos (5 MeV ?
En ? 50 MeV), and considering a star density
profile r ? 1/r3, the adiabaticity condition is
always satisfied at the L resonance for any LMA
solution, while at the H resonance, this depends
on the value of Ue32. When Ue32 ? 5 x 10-4 the
conversion is completely adiabatic, meaning that
ne are completely converted into the mass
eigenstate n3 (detected at the Earth mainly as nm
and nt). Therefore, the SN neutrino signal could
feel the effect of Ue32 (and could also help to
discriminate the type of mass hierarchy).
Detector description

• The Large Volume Detector (LVD) in the INFN Gran
  Sasso National Laboratory, Italy, consists of an
  array of 840 scintillator counters, 1.5 m3 each.
  These are interleaved by streamer tubes, and
  arranged in a compact and modular geometry. The
  active scintillator mass is M1000 t.
• There are two subsets of counters the external
  ones (43), operated at energy threshold eh ? 7
  MeV, and inner ones (57), better shielded from
  rock radioactivity and operated at eh ? 4 MeV.
• In order to tag the delayed g pulse due to
  n-capture, all counters are equipped with an
  additional discrimination channel, set at a lower
  threshold, el ? 1 MeV.
• Relevant features of the detector are
• good event localization
• accurate absolute and relative timing Dtabs 1
  ms, Dtrel 12.5 ns
• short dead time (2 ms for each counter)
• uptime greater than 99
• energy resolution s(E)/E 0.07 0.23
  (E/MeV)-0.5.

• We calculated the number of events expected in
  the various reaction in the cases of
  no-oscillation and oscillation, under the
  following hypotheses
• We assumed a supernova exploding at D10 kpc,
  with an energy release Etot 3x1053 erg, pure
  Fermi-Dirac time integrated spectrum, energy
  equipartition, and neutrinospheres temperatures
  as Tne Tanti-ne Tnx /2.
• We included the active mass of the detector and
  the energy thresholds. We used the following
  values of detection efficiencies above threshold
  e (ne p,n e) 95 and e (n p, d g) 50 ,
  e (ne 12C, 12N e-) 85 , e (ne 12C, 12B e)
  70 and e (nl 12C, nl 12C) 55 .
• In the oscillation case, we used two extreme
  values for Ue32 Ue32 10-2 and Ue32 10-6, and
  the above mentioned mixing parameters (normal
  mass hierarchy, LMA solution).
• We did not include Earth matter effects (open
  sky neutrino burst).

Neutrino interactions in scintillator Neutrino interactions in scintillator Energy threshold Number of events
ne p ?n e CC 1.8 390
ni e- ?ni e- CC-NC 13 (3)
ne 12C ?12N e- CC 17.8 1
ne 12C ?12B e CC 13.9 1
ni12C ? ni 12C 12C ? 12C g NC 15.11 22 (17)
(-) (-)
(-) (-)
ne p,n e
CC with 12C
NC with 12C
non adiabatic
adiabatic
adiabatic
no oscillation
non adiabatic

• Beyond material, mass and depth, a Supernova
  neutrino telescope must have
• buffers adequate to handle high throughoutput,
• short deadtime
• accurate absolute and relative timing
• good energy resolution
• low maintenance cost and a high duty cycle
• (A. Burrows, 1992)
• LVD detector fulfills all requests

no oscillation
F. Vissani, G. Nurzia LVD collaboration Referen
ce paper see TAUP 2001 Proceedings
(astro-ph/0112312)
The possibility to detect neutrinos in different
channels makes LVD sensitive to different
scenarios for n properties, such as normal or
inverted n mass hierarchy, and adiabatic or non
adiabatic MSW resonances associated to Ue3.
CNGS beam Monitor
The CNGS beam from CERN to the Gran Sasso
Underground Laboratory (LNGS), over a distance of
732 km, is a wide-band high-energy nm beam
optimized for t appearance experiments. Such a
beam provides a large number of interactions at
Gran Sasso (about 2600 CC/kt/year at nominal beam
intensity). In principle the experiments forseen
at LNGS could provide monitor informations by
counting the number of nm CC interactions.
Unfortunately, this could take months to
accumulate due to their limited mass. In order to
monitor the performance of the CNGS beam, it has
been suggested to implement, in one of the LNGS
halls, a wide area simple apparatus capable of
detecting the muons induced by neutrino
interactions in the upstream rock and emerging
into the experimental hall. In this analysis we
show the capabilities, in this respect, of the
LVD detector whose beam-orthogonal surface is 13
x 10 m2 , greately much larger than the other
forseen CNGS experiments.
Experimental background measured in the fiducial
volume of LVD and expected e spectrum. In a 10 s
burst, about 5-10 background events are expected.

• The angle in space between muons and the main
  hall axis is the convolution of 3 contributes
• The (fixed) beam angle w.r.t. the horizon
  (3.2O).
• The angle between n and m in the CC interaction.
• Multiple scattering in rock and other radiative
  processes (pair production, bremsstrahlung, ...).

Energy spectrum of muons when they reach LVD. The
energy loss in rock, from the interaction point
and the detector, has been subtracted using a
full 3-d MonteCarlo simulation in which
ionization, pair production and bremsstrahlung
are taken into account.
LVD is member of SNEW Supernova Early Warning
System

• Each cluster (sequence of k events, tk gt t1,
  duration Dt tk-t1) is taken into account (after
  m rejection)
• Unique request Dtmax 200 s (in order to be
  model-independent)
• Given a standard acquisition rate, we compute
  the probability for each cluster (k, Dt) to be
  generated from background.

A full simulation of the n interaction, the muon
transport in rock and the LVD detector response
has been developed, in order to estimate muon
tagging efficiencies (nm CC interactions have
been uniformly generated in a rock volume larger
than the transverse LVD dimensions, in order to
take into account also laterally impinging
m). The time-coincidence with the CNGS beam
spill, makes this measurement pratically
background free.
Rock Mass 1969 kt
Nominal CNGS intensity 4.5 x 1019 pot/year
CC interaction rate 5.85 x 10-17 CC/pot/year
nm CC in rock 5.18 x 106 / year
Number of m in LVD 33600 / year
LVD efficiency 72
Detected m in LVD 120 / day

• A complete analysis of selected clusters tests
  their consistency with a neutrino burst, based
  on
• the study of topological distribution of pulses
  inside LVD,
• the energy spectrum,
• the time distribution of delayed low energy
  pulses (due to neutron capture following the
  anti-ne interaction).

Background estimate The main background sources
are cosmic muons. The rate in the full LVD
detector (3 towers) is 8600 muons/day (6 per
minute). If we ask an energy loss greater than
200 MeV per tank, 22 of them survive, that is
1900 per day. The use of the informations from
the CNGS beam spill (10.5 ms of spill lenght and
50 ms inter-spill gap) allows a reduction of the
number of cosmic muons of a factor 104, i.e.
about 0.5 cosmic muons per day.
Thanks to its large area (130 m2) LVD could act
as a very efficient muon monitor for the CNGS
beam. A mean number of 120 muons per day is
expected at nominal beam intensity. A 3
statistical error is achievable in 9 days. The
measurement is pratically background free.
Event display of a CNGS muon in LVD. Side and top
view of the detector are shown. The green area
means energy loss in scintillator between 200 and
300 MeV (mean muon loss in LVD tanks), while
yellow area stands for E between 300 and 500 MeV
, i.e. muon has undergone some radiative energy
loss.
The mean m energy loss in LVD scintillator is
1.56 Mev/cm, the tank lenght is 1.5 m, so the
mean energy loss in each tank is about 230 MeV.
This allow to define a muon criteria, requiring
at least one tank with energy gt 200 MeV. The
resulting efficiency is 72.