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STUDY OF THE NEUTRINO BACKGROUND IN THE T2K NEAR DETECTOR
Jose Luis Alcaraz Aunion
Treball de recerca de tercer cicle en física
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OUTLINE
● PART I: NEUTRINO PHYSICS
● PART II: T2K EXPERIMENT
● PART III: THE BACKGROUND SIMULATION
● PART IV: ANALYSIS
● PART V: SUMMARY & CONCLUSIONS
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PART I: NEUTRINO PHYSICS
● Neutrino oscillation theory.
● Neutrino oscillation experiments:
– Solar neutrino experiment.
– Atmospheric neutrino experiment.
– Artificial neutrino sources experiments.
4
mixing matrix (MNSP)
Mass eigenstates
Weak eigenstates
● If the neutrino has mass, the flavour eigenstates could be different from the mass eigenstates.
where the evolution in time of each i is:
NEUTRINO OSCILLATION THEORY I(in vacuum)
~ atmospheric oscillation (
23)
~ solar oscillation (12
)(13 ,
) ~reactors
(23
, 12
, 13 ,
) are the oscillation mixing parameters
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NEUTRINO OSCILLATION THEORY II(in vacuum)
...then oscillation is .....In the two neutrino species:
the survival probability can be different from 1
There experiments where L and E can be under control.
is the mixing angle; m2= m22 m2
1;
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NATURAL NEUTRINO SORUCES
(Phys. Rev. Lett. 89:011301, 2002)
● SuperKamiokande (1998):
– Measurements: flux dependency with the zenith angle.
– Results: Evidence for oscillation in atmospheric neutrinos.
● Sudbury Neutrino Observatory (SNO, 2001/2002)
– Measurements: flux from the 8B process into the sun.
– Results: Direct evidence for neutrino flavour transformation.
Sensible to all neutrino flavours:
SOLAR NEUTRINOS ATMOSPHERIC NEUTRINOS
hepexp/9810001
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ARTICIFICAL SOURCES
● ACCELERATORS: large long baseline experiments (LBL)
Neutrino oscillation evidence using manmade neutrino sources.
● REACTORS: KamLAND (Japan): found evidence of antie deficit.
● K2K (L=250 Km, Japan): found evidence for
disappearance
– MINOS (L=735 Km, USA ) : found evidence for
disappearance
– T2K (L=295 Km, Japan): will measure
disappearance, e
appearance
L
Far Detector
Near Detector
Accelerator
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PART II: T2K EXPERIMENT
● The T2K experiment. ● The Off Axis Configuration● Physics motivations.● Background in the measurements. ● The ND280 near detector.
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THE T2K EXPERIMENT● Tokai to Kamioka (T2K) Experiment: next generation of long
baseline neutrino oscillation experiment in Japan.
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T2K: neutrino beam● JPARC :
– protons E= 40/50 GeV.
– I = 3.3 x 1014 p/spill. ● Target collision ( graphite) .
● Focusing elements (3 e.m. horns will focus the pions).
● Decay pipe (130 m length)
● Beam dump (to stop all the charged particles).
● Muon monitor: to obtain the profile of neutrino beam.
● spill width = 5.6 s. (is the temporal width of the neutrino pulse). T = 3.5 s;
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PHYSICS MOTIVATIONS● disappearance measurements: Improve the precision measurements
(23
, m223
) . In 5 years of full operation :
● 23
~ 1.00 0.01 (precision of 1%).
● m223
: uncertainty of 105 eV2 (2 orders of magnitude better than K2K results.)
.
● eappearance measurements: to determine 13
(subdominant oscillation
> e )
– Precision: an order of magnitude better than CHOOZ limit. (oscillation excluded at sin2(2) > 0.17 )
– Important for CPviolating phase > Related with the matter asymmetry of the universe.
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T2K: off axis configuration
2, 2.5, 3 degrees
●ADVANTAGES ( with respect to on axis):
● A narrow neutrino beam peaked at the energy of the oscillation maximum (0.75 GeV) for the distance of 295 Km.
● Reduction of the e contamination because of the different kinematics.
● Reduction of the high energy neutrinos.
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BACKGROUND ● The
e contamination from
beam. (it will be measured by the Time
projection chamber and ECAL.)
● The NC0 interactions. (it will be measured by the Pi0 Detector).
SK: in e appearance measurements
NEAR DETECTOR: in all the measurementsn
n
p beam
● Skyshine background: neutrons coming from atmosphere.
● interactions into the rocks of the
cavern, magnet and other passive materials.
.
MOTIVATION OF THIS WORK: A study of this kind of background, analysing the effect over the signal in the near detector.
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THE ND280 NEAR DETECTOR
Designed to measure the flux, the energy spectrum, the flavour and the cross section before the neutrino can oscillate.
Onaxis
Offaxis
Experimental Hall
R = 9.5 m
37 m
beam
The off axis detector
The on axis detector
● to measure the beam
direction.
● Composition:
– iron
– scintillator
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● 4: 0 DETECTOR (P0D).
– Measure NC0 in interactions.● 5,6,7: TIME PROJECTION CHAMBER
– 3D tracks.
– Identify particles.
– Measure the neutrino spectrum ● 8,9: FINE GRAINED DETECTORS
– Target for the interactions.
– Also to measure particles from CC interactions.● 1,2 : YOKES, COILS (UA1 Magnet)
– B = 0.2 Tesla.● 3: ELECTROMAGNETIC CALORIMETER .
– Measure the e.m. energy coming from interactions which escapes from P0D or TPC.
~ 8m
~ 6m~5m
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PART III: THE BACKGROUND SIMULATION
● Description.● The near detector in the simulation.● in GEANT4 .
– get interactions.– get a continuous flux.
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DESCRIPTION
● Simulation software package:
– GEANT4.● to simulate the interaction of the particles through the matter.
– NEUGEN. ● Is a neutrino generator event ( 100 MeV < E < 10 GeV).
● The neutrino beam:
– It is used an off axis neutrino beam. (extracted from T2K MC simulation.)
– neutrino flux of 5 years of exposure.
– Energy range 1 MeV 20 GeV.
An off axis neutrino beam is transported throughthe ND280 cavern and “forced” to interact.
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The near detector in the simulation
Groud
Walls Cavern
Offaxis detector
Yokes
Coils
Scintillator (Active Volume [A.V.])
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IN GEANT4: get interactions
●Interaction lengths in some materials ((E
max= 1 GeV), F= 8 cm):
● L(air
)= F* x
max /
airF*m ;
● L(quartz
)= F* x
max /
quartzF*m
● L(iron
)= F* x
max /
ironF cm
where max
=[max
(E= 20 GeV,
iron= 7.87 g/cm3] = 1.41 x 1036 cm2;
We “force” the neutrino to interact:
Pint
[(E
max= 20 GeV,
max =
iron )]
max *
iron= 1
Then , the interaction length is calculated as follows:
L(i)
= F* x
max /
i; Where F is a free control parameter. Define
the range of the interaction length.
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IN GEANT4: get a continuous flux● As the neutrino is “forced” to interact and an uniform
background want to be produced, the same neutrino is propagated again to the next interaction.
● Only the offaxis neutrino flux can interact several times> In GEANT4, that neutrinos are redefined as Jnu.
F = 8 cm, > Nint ~ 3 /Jnu
Uniform background
Not uniform background
Interactions .vs. Energy
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PART IV: ANALYSIS● Previous concepts.● Neutrino flux at ND280/ Normalization.● Neutrino background
– Spacial distribution– Neutrino energy spectrum– Neutrino background estimation.
● Particle background
– Classification and distribution.● Charged particles.● Neutral particles.
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Previous concepts● Neutrino inducing background: neutrinos which, interacting outside of
the active volume, generate activity inside of it.
– variables to analyse:● Kinetic energy and interaction vertex.
● Particle background: particles produced by neutrino background which interact inside of the active volume.
– variables to analyse:● Kinetic energy just when it enters into the active volume.● Spatial vertexes where particles were created.● Time: Defined as the interval since a neutrino start to be
propagated until the particle background reach the active volume.
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Neutrino flux at ND280/Normalization
NORM* N int
sim = N int
CDR ;
N int
sim (A.V.) = [2.54 ± 0.06 (stat) ± 0.2(sys*)] x 1016 /year/ton
N int
CDR = 1.73 x 105 /year/ton
flux
NORM = 6.91 x 1012
*(The systematic errors come from the uncertainty in the neutrino cross section of 10%)
flux expected at near detector On Axis (dashed lines) and Off Axis (solid line)
Neutrino interactions at near detector
The values of the neutrino interactions are normalized with respect to theT2KCDR values for an easy interpretation of our results.T2KCDR: contains the MC that predicts the number of neutrino interactions
into the offaxis detector.
T2KCDRSimulation
Used to normalized the background values
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Neutrino Inducing Background: Spatial distribution
● Contribution to the total neutrino background:
– Walls (~80%).– Magnet (~20%).
beam ( x~ 3 m)
beam ( y ~ 7 m)
top view
front viewside view
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● The background generated into the magnet contains the low energy neutrinos ( the offaxis neutrinos).
● The background generated into the walls contains the high neutrinos (onaxis neutrinos).
Neutrino Inducing Background: Energy spectrum
Total/Cavern/Magnet
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Neutrino Inducing Background: Estimation
N bg
= [2.74 ± 0.00 (stat) ± 0.3(sys)] x 108 /year
N sig
= 1.73 x 105 /year/ton
● It is obtained an estimation of the background because the simulation adopts several simplifications:
– Composition of Earth: not only quartz.
– Detector geometry: Only magnet simulated. Not the metallic basket or the metallic structure where the magnet sits. Also the others detectors.
r = N(bg) / N(sig) = [1.6± 0.2(stat) ± 0.4(sys)] x103 x (Tons Det)
That means, for 1 neutrino interaction inside of the detector (10 tons), there are 100 neutrino interactions outside which could generate “some activity” into the detector.
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Particle background: Classification & distribution
● Classification: – Charged particles (~10%).
– Neutrons (~90%).
● Distribution: – ~70% neutrons produced in
cavern.
– Charged particles produced into the magnet.
Magnet /Cavern
*In terms of particles/spill. Better to compare with signal: ~1 interaction/spill
●Neutrons: lose few energy by elastic scattering, then can travel long distances.
●Charged particles: lose the energy by ionization, then only cross short distances.
*
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Charged Background: Time distribution● The charged particles are synchronous with the signal (ns).
● Charged particles produced at Cavern must be relativistic particles.Cavern/Magnet
Cavern/Magnet
Cavern/Magnet
Cavern/Magnet
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Charged background: Kinetic Energy Thresholds
● After physics thresholds:
– Np ~ 0.15 p/spill
– N 0.2 /spill
– N 0.35 /spill
– N 3.5 /spill
● The charged background can be detected by the external detectors like ECAL. The ECAL at the same time could become in another source of background. Further study.
f(Ek)=
j N
j(E > E
k)
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Neutron Background : Energy
● A high percentage of the neutrons are very low energetic neutrons (E < 0.5 MeV)
● These neutrons mainly come from the cavern (losing their energy by elastic scattering in the different materials).
ENERGY DISTRIBUTION:
Magnet/Cavern
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● Largest times: cavern neutrons. They cross large distances and are not relativistic.
● Shortest times: magnet neutrons.
Neutron Background: Time TEMPORAL DISTRIBUTION:
Magnet/Cavern
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Neutron Background: Selecting neutron background
● Cut in times: neutrons inside of spill width (t < 5.6 s)
● Cut in energies: select neutrons which can produce detectable signals (specially interested in production, E
cut > 140 MeV).
Magnet/Cavern
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Neutron background: Interactions
Neutrons can interact inside of the detector giving a signal which could be not distinguished from the neutrino signal.
● Neutrons can interact inside of the detector giving a signal which could be not distinguished from the neutrino signal.
– Specially interested in neutrons generating 0's. ● Then, a brief study of the neutron background interaction is done.
N(neutrons) = 97 neutrons/spill
Particles produced by background neutron:
All reactions with: particles/spill0 0.05 0,18protons 0.17
Particles produced by interactions:
From T2KCDR paper.
Magnet/Cavern
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Particles produced in neutron background interactionsMomentum distribution
Photons coming from nuclear excitations. With energies smaller than the 0 mass.
● The physic threshold (~200 MeV ) reduces the protons the can be observed.(105 protons/spill)
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Par
ticle
s pr
oduc
ed b
y ne
utro
n ba
ckgr
ound
Par
ticle
s pr
oduc
ed b
y ne
utr in
o in
tera
ctio
n s
The shape of the momentum distribution for the particles produced by neutron background are shifted to low energies with respect to those particles produced in neutrino interactions.
Particles produced in neutron background interactions
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Summary & Conclusions● One of T2K main goals: e appearance measurements.
– background for SK is measured into the near detector (P0D, TPC).
● But near detector has background too: we have study that, focusing the analysis into the neutron background.
● The simulation has shown that there is many activity around the cavern (~104 interactions in 10 Tons Detector);
● 90% background are neutrons (97 n/spill)
– after temporal and energetic cut (0.88 n/spill)– particles produced by these neutrons, including 0 are of the
order of 105 particles/spill. (compared with signal ~1 )● But, background muons (~3.5 part/spill) should be taken into
account.
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END
Thanks for coming
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ADDITIONAL SLIDES
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● Systematic uncertainties in 13
– Background sources:
● e contamination in
● single 0 production in NC.
– Reduction for different cuts:● Nu reconstruction
Energy around maximum oscillation: (0.35 < E < 0.85 ) GeV.
● mass of events reconstructed = m(0).
BACKGROUND IN SK
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NORMALIZATIONN
int sim (A.V.) = [2.54 ± 0.06 (stat) ± 0.2(sys)] x 1016 /year/ton
N int
sim = * sim
simi (E) =
i
N int
sim = * i
~ 1012
The neutrino flux of the simulation is given as follows:
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IN GEANT4: get interactions
P(i(E
i)) = C exp(L/);
where max
/i
max
=[max
(E= 20 GeV,
iron= 7.87 g/cm3] = 1.41 x 1036 cm2;
● As it does GEANT4, we calculate the interaction length for the neutrino.
● We “force” the neutrino to interact:
– Pint[(E
max= 20 GeV,
max =
iron )] =1
Li= F*
max /
iwhere F is a free parameter to control the
number of interaction inside the cavern.
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NEUTRINO INTERACTIONS
Quasielastic scattering(QE): It is an elastic body scattering between neutrino an nucleon.
Resonance (RES):The neutrino excites the nucleon to a resonance state.
Deep Inelasticscattering:The neutrino interacts with quarks of nucleon, breaking.
Coherent production:The neutrino interacts with whole the nucleus transferring low momentum.
The neutrino can interact through charged or neutral currents:
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● P0D:
– ¾ parts of the detector: Water+Scintillator+lead foils.
● Water> to have the same target for the neutrino interactions that SK.
– Measure the Ncpi0 neutrino interactions:
● The pi0 momentum distribution is similar > It will can predict the Ncpi0 in SK.
–
THE ND280 NEAR DETECTORP0D
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● Tracker of the offaxis detector– 3D tracks:
● 2D from pad plane● 1D from measurements of e drift velocity.
● Technique dE/dx to identify particles:● very important to measure the e contamination.
● Neutrino energy spectrum: measuring the momentum.
THE ND280 NEAR DETECTORTPC
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On axis detector● To measure the neutrino profile
● There is only a counter detector (not energies measured)
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Particles/spill
● 1 year ~ 107 seconds.● 1 spill each T ~ 3.5
seconds● 5 years = 5*107 / 3.5 ~
107 spills● Interactions = 1.7x105
/year/ton– 12 tons > 0.7
interactions/spill ~ 1 int/spill
●Charged particles: lose the energy by ionization, then only arrive those close to the A.V.
●Neutrons: loss few energy by elastic scattering, then can travel long distances.
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● 1930: Wolfgang Pauli postulated the neutrino ( ) .ν
● 1934: Enrico Fermi incorporated the in his formulation of the weak ν
interactions theory.
● 1956: First detection of the : Cowan and Reines (Project Poltergeist). ν
● 1957: Bruno Pontecorvo proposed the ν ν oscillation ( analogously to
K K).
● 1962: Maki, Nakaga and Sakata introduced the flavour mixing and
flavour oscillation.
HISTORICAL INTRODUCTION I
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● 1968: Solar neutrino observations by Davis and Bahcall found “
solar neutrino anomaly “ .
● 1985: “Atmospheric neutrino anomaly” observed at Kamiokande
experiment.
● 1998: SuperKamiokande confirmed the muon neutrino oscillation
in atmospheric neutrinos.
● 2002: SNO provides evidence that neutrino oscillation cause the
solar neutrino defficit.
HISTORICAL INTRODUCTION II
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● Davis' experiment goal: confirm the SSM. They found ~ 1/3 of the neutrino flux predicted theoretically.(called “solar nuetrino deficit”)
Sudbury Neutrino Observatory (SNO, 2001/2002) Measure the solar ν flux coming from the 8B process Solved the solar neutrino deficit.
Ro=12 Ri= 6m of D20
SOLAR NEUTRINO
(Phys. Rev. Lett. 89:011301, 2002)
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ATMOSPHERIC NEUTRINO ● Kamiokande (1985) : [3kt,~1000 PMT] found an anomaly into the
atmospheric neutrino flux 3 kt, and ~ 1000 PMT .
● SuperKamiokande (1998): [55 kt, ~ 11000 PMT] found Evidence for oscillation in atmospheric neutrinos.
(hepex/9805006)
R= ν/ ν
e ~2 R= ν
up/ ν
down
~ 1
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The Neutrino Beamz=280 m
y=8,7 m
S. R.:“Target” S. R.: Centre Cylinder
● 1) z= 280 m > z' = 29,5 m;
The original beam correspond to the expected flux at the offaxis detector in 5 years of exposure.
● 2) get off axis configuration:
● x= 0m > x'= +3 m;● y= 8.7 m > y' = 8.7 m;
CHANGE SYSTEM REFERENCE: “Target” > “Centre Cylinder”
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Neutron Interactions
● Brief study of the neutron interactions in scintillator.
● Scintillator box: ~ 100 m3, ~ 100 Tons.
● Neutron sample Ntot
= 10000 with energies 1 MeV to 10 GeV.
● To have estimative numbers for the background neutron:
– each neutron is weighted:
● Wi = N
n(E
i)/N
tot; where N
n(E
i) is the number of background
neutros with energy Ei .
● We are only interested in the primary particles (0, ±) -> They are
not tracked.
Neutrons can interact inside of the detector giving a signal which could be not distinguished from the neutrino signal.
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Summary● T2K motivations
– disappearance
– e appearance:
● It is crucial to know all the background sources:– e contamination from beam (SK)
– NC0 interactions (SK)● There will be the P0D and TPC at the near detector to measure
these two sources but the near detector has background too:
– skyshine.
– 0 produced by neutron background interactions could contaminate the measurements of P0D.
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Conclusions● Many activity of neutrino
interactions around the cavern.
● Main background contribution (90%): Neutrons.
● After an energetic and temporal cut:
r = N(bg) / [ N(sig) / Mdet] ~ 105 x (Tons Det)
0.88 n/spill
● The number 0's produced by neutron background are negligible (105 0/spill). Therefore they will not represent a problem for the P0D.
● However, this background could appear as some hits into the TPC but this need a further study.
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Why the dimensions of cavern● We want to select the dimensions such as all the background can be
contained inside.
● Simulation with dimensions:
– walls: 150m (portion of earth crossed by neutrinos)
– ground: 100 m (enough)