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n
What Neutrino Experiments
Need to Know
Kevin McFarland
University of Rochester
ECT*
16 May 2012
n
Outline
• Neutrino scattering vs. electron scattering
• Goals of neutrino oscillation experiments
• “Narrow” and Broad Beam Experiments
• What Needs to be Modeled
• Current Practices
• Possible Paths to Progress
16 May 2012 K. McFarland, Needs for Neutrinos 2
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16 May 2012 K. McFarland, Needs for Neutrinos 3
Neutrinos vs. Electrons
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16 May 2012 K. McFarland, Needs for Neutrinos 4
Neutrino Dictionary for
Parity Violators Electron Physics Concept
APV ~10-6
s |APC|2+2ReAPC*APV+negligible
Polbeam a limiting systematic
Target
Detector
Ebeam a number you
choose
n
16 May 2012 K. McFarland, Needs for Neutrinos 5
Neutrino Dictionary for
Parity Violators Electron Neutrino Physics Concept
APV ~10-6 1
s |APC|2+2ReAPC*APV+negligible negligible
Polbeam a limiting systematic 1-mn2/En
2
Target
Detector (see “Target”)
Ebeam a number you
choose
a distribution you
barely know
n
Neutrino Facts of Life
• Neutrino experiments require massive targets to
carry out goals
Few 104 or 105 kg of target material of current and
“near future” experiments
• We only know what we see in the final state
• Targets are large nuclei
Carbon, Oxygen, Argon, Iron are all being used in
current or near future experiments
• Detectors have severe limitations
Need to measure interactions throughout target
Must balance expense vs. capability
16 May 2012 K. McFarland, Needs for Neutrinos 6
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16 May 2012 K. McFarland, Needs for Neutrinos 7
Neutrino Oscillation Goals (at lightening speed)
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Neutrino Flavor • Neutrinos were discovered by
the final state positron is no accident!
we’ve seen neutrinos
produce all three charged
leptons in weak interactions
• The Z boson decays into
three (and only three) neutrino states
16 May 2012 K. McFarland, Needs for Neutrinos 8
p e nn
ee
n
n
n
n
n
Neutrino Flavor Mixing
• The defining question of the field
today turns out to be from an unusual
conjecture (Pontecorvo)
• Are these neutrinos “of definite flavor”
the eigenstates of the neutrino
mass matrix
• Or are we looking at neutrino puree?
16 May 2012 K. McFarland, Needs for Neutrinos 9
f la v o r f la v o r ,
m a ss e ig e n s ta te s ,
i i
i
Un n
ee
n
n
n
n Neutrino Flavor Mixing
(cont’d) • If neutrinos mass states mix
to form flavors
• and the masses are different…
flavors of neutrinos can change in flight
• Explains Davis’ “solar neutrino puzzle”
since only electron flavor neutrinos are
detected ν+n→p+e-
16 May 2012 K. McFarland, Needs for Neutrinos 10
e
n
n
n
f la v o r ,
m a ss e ig e
f
n
la v
s ta te s ,
o r
i i
i
U nn
n
Neutrino Flavor Oscillation
• Each neutrino wavefunction
has a time-varying phase in its rest frame,
• Now, imagine you produce a neutrino of definite
momentum but is a mixture of two masses, m1, m2
• so pick up a phase difference in lab frame
16 May 2012 K. McFarland, Needs for Neutrinos 11
/iE te
2
2 2 1
1 1 2
2
2 2 2
2 2 2
1
1
mE p m p
p
mE p m p
p
2 2
1 2 1 2( ) ( )
L ci E E i m m
p
n
Neutrino Oscillation (cont’d)
• Phase difference leads to interference
effect, just like with sound waves of two frequencies
frequency difference sets period of “beats”
16 May 2012 K. McFarland, Needs for Neutrinos 12
ν2
ν3
νμ
n
• Phase difference
• Analog of “volume disappearing” in beats is
original neutrino flavor disappearing
and appearance
of a new flavor
more generally, mixing need not be maximal
Neutrino Oscillation (cont’d)
16 May 2012 K. McFarland, Needs for Neutrinos 13
c o s s in
s in c o s
i
j
nn
nn
only two
generations
for now!
2 2
1 2 1 2( ) ( )
L ci E E i m m
E
n
16 May 2012 K. McFarland, Needs for Neutrinos 14
Neutrino Oscillation (cont’d)
• For two generations…
Oscillations require mass differences
Oscillation parameters are mass-squared differences, dm2, and
mixing angles, .
• One correction to this is matter… changes , L dep.
E
LmmP
4
)(sin2sin)(
2
1
2
222nn
Wolfenstein, PRD (1978)
22
22
2
2
)2cos(2sin
)2cos(2sin
2sin2sin
xLL
x
M
M
n
m
EnGx
eF
2
22
e- density
appropriate units
give the usual
numerical factor
1.27 GeV/km-eV2
n
16 May 2012 K. McFarland, Needs for Neutrinos 15
Solar Neutrinos: SNO
• D2O target uniquely observed:
charged-current
neutral-current
• The former is only
observed for ne (lepton mass)
• The latter for all types
• Solar flux is consistent
with models
but not all ne at earth
X Xd pnn n
ed ppen
n
16 May 2012 K. McFarland, Needs for Neutrinos 16
KAMLAND
• Sources were
Japanese
reactors
150-200 km
for most of
flux. Rate uncertainty ~6%
• 1 kTon scint. detector in
old Kamiokande cavern
overwhelming confirmation
that neutrinos change flavor
in the sun via matter
effects
n
16 May 2012 K. McFarland, Needs for Neutrinos 17
Atmospheric Neutrinos
• Neutrino energy: few 100 MeV – few GeV
• Flavor ratio robustly predicted
• Distance in flight: ~20km (down) to 12700 km (up)
n
16 May 2012 K. McFarland, Needs for Neutrinos 18
Super-Kamiokande
• Super-K
detector has
excellent e/
separation
• Up / down
difference: L/E
• Muons distorted, electrons not; so mostly
old, but
good data!
2004
Super-K
analysis
n n
n
16 May 2012 K. McFarland, Needs for Neutrinos 19
MINOS 735km baseline
5.4kton Far Det.
1 kton Near Det.
Running since early 2005
Precise measurement of
n disappearance energy
gives dm223
n
16 May 2012 K. McFarland, Needs for Neutrinos 20
CNGS Goal: n appearance
• 0.15 MWatt source
• high energy n beam & 732 km baseline
• handfuls of events/yr
e-, 9.5 GeV, pT=0.47 GeV/c
n interaction, En=19 GeV
fiugres courtesy A. Bueno
3kton Pb
Emulsion layers
n
1 mm
1.8kTon
figures courtesy D. Autiero
n
16 May 2012 K. McFarland, Needs for Neutrinos 21
Two Mass Splitings:
Three Generations
• Oscillations have told us the splittings in m2, but nothing
about the hierarchy
• The electron neutrino potential (matter effects) can
resolve this in oscillations, however.
figures courtesy B. Kayser
dmsol2 dm12
2≈8x10-5eV2 dmatm2 dm23
2≈2.5x10-3eV2
n
16 May 2012 K. McFarland, Needs for Neutrinos 22
Three Generation Mixing
• Note the new mixing in middle, and the phase, d
slide courtesy D. Harris
n
16 May 2012 K. McFarland, Needs for Neutrinos 23
Are Two Paths Open to Us?
• If “reactor” mixing, 13, is small, but not too
small, there is an interesting possibility
• At atmospheric L/E,
n
dm232, 13
dm122, 12
ne
2 2
2 2 2 1( )
( ) s in 2 s in4
e
m m LP
E
n n
SMALL LARGE
SMALL LARGE
n
16 May 2012 K. McFarland, Needs for Neutrinos 24
Implication of two paths
• Two amplitudes
• If both small,
but not too small,
both can contribute ~ equally
• Relative phase, d, between them can lead to
CP violation (neutrinos and anti-neutrinos differ)
in oscillations!
n
dm232, 13
dm122, 12
ne
n
13 in 2011
• T2K, an accelerator experiment,
showed a signal of 6 events
1.5 expected if 13=0
• Consistent, but less significant,
indication from MINOS shortly after 16 May 2012 K. McFarland, Needs for Neutrinos 25
n
13 in 2012
• Two reactor experiments recently showed
overwhelming evidence for large 13.
Both place detectors near and far (~1km) from reactors
Look for a small
rate difference
between two
locations
16 May 2012 K. McFarland, Needs for Neutrinos 26
n
13 in 2012: Daya Bay
16 May 2012 K. McFarland, Needs for Neutrinos 27
Figures from K. Heeger
n
13 in 2012: RENO
16 May 2012 K. McFarland, Needs for Neutrinos 28
Figures from S.B. Kim
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Implications of Large 13
• If 13 is large, then one of the two paths
is larger than the other.
• This implies large signals, but small CP
asymmetries
16 May 2012 K. McFarland, Needs for Neutrinos 29
n
dm232, 13
dm122, 12
ne
n
Implications of Large 13
• Quantitative analysis to
illustrate this expected
behavior
Fractional asymmetry
decreases as 13 increases
• We live here
• Statistics are (relatively)
high, so the challenge will
be controlling systematic
uncertainties.
16 May 2012 K. McFarland, Needs for Neutrinos 30
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16 May 2012 K. McFarland, Needs for Neutrinos 31
Current and Future Experiments
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16 May 2012 K. McFarland, Needs for Neutrinos 32
Narrow Band Beam
• “CP violation” (interference term) and matter
effects lead to a complicated mix…
• Simplest case:
first oscillation
maximum, neutrinos and
anti-neutrinos
• CP violation gives ellipse
but matter effects shift
the ellipse in a
long-baseline accelerator
experiment…
Minakata & Nunokawa
JHEP 2001
n
16 May 2012 K. McFarland, Needs for Neutrinos 33
Broadband Beam
• See different mixture of solar/interference “CP” term,
matter effects at different oscillation maxima
• This shows En. Recall argument of vacuum oscillation term is ~L/En
( )e
P
n n
FNAL-
DUSEL
L=1500km
n
4 February 2009 K. McFarland, Neutrinos at Accelerators 34
Beam Design Options
• All experiments will want to see first oscillation
maximum, L/E ~ 400 km/GeV
• Then one has a choice…
Narrow Band Beam at
First Oscillation Peak
Broad Band Beam Covering
Multiple Oscillation Peaks
• Because there are many
parameters, need
neutrino and anti-
neutrino measurements
(minimally)
• Perhaps multiple
baselines
• In principle, can measure
everything with one
experiment!
• However require much
larger L/E and L
• Also need good energy
resolution at low neutrino
energies
En
n
4 February 2009 K. McFarland, Neutrinos at Accelerators 35
• First Suggested by BNL-889 proposal
• Take advantage of Lorentz Boost and 2-body kinematics
• Concentrate n flux at one energy
• Backgrounds lower: – NC or other feed-down
from highlow energy
– ne (3-body decays)
• Generally optimal if only accessing “first maximum”
Narrow Band Beam:
Off-axis Techinque
figure courtesy D. Harris
n
4 February 2009 K. McFarland, Neutrinos at Accelerators 36
Narrow: T2K • Tunable off-axis beam from
J-PARC to Super-K detector
beam and n backgrounds are
kept below 1% for ne signal
~2200 n events/yr (w/o osc.)
d=0, no matter effects
figures courtesy T. Kobayashi
n
4 February 2009 K. McFarland, Neutrinos at Accelerators 37
Narrow: NOnA
• Use Existing NuMI beamline
• Build new 15kTon Scintillator Detector
• 820km baseline--compromise between reach in 13 and matter effects Assuming m2=2.5x10-3eV2
ne+A→p p+ p- e-
figure courtesy M. Messier
figures courtesy J. Cooper
Goal:
ne appearance
In n beam
n
Broad(er): LBNE
16 May 2012 K. McFarland, Needs for Neutrinos 38
33 kTon (fiducial)
Liquid Argon TPC
figures courtesy M. Diwan
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16 May 2012 K. McFarland, Needs for Neutrinos 39
Needs for Modeling
n
Illustration: T2K
• Backgrounds are significant
Primarily from neutral current neutral
pion production
• Neutrino energy is a powerful
background discriminant, but has
little other information about
oscillations
• No official plot yet, but I can
guarantee you that the
backgrounds in neutrino and anti-
neutrino beams are different.
16 May 2012 K. McFarland, Needs for Neutrinos 40
n
Illustration: LBNE
• Maximum CP effect is range of red-blue curve
• Backgrounds are significant, vary with energy and are
different between neutrino and anti-neutrino beams
Pileup of backgrounds at lower energy makes 2nd maximum only
marginally useful in optimized design
• Spectral information plays a role
CP effect may show up primarily as a rate decrease in one beam
and a spectral shift in the other
16 May 2012 K. McFarland, Needs for Neutrinos 41
n
Generic Features
• Physics goals require comparing neutrino
and anti-neutrino transition probabilities
• Backgrounds are significant and different
• Reconstructing the neutrino energy is key
For T2K, this is quasi-elastic states
For NOvA, LBNE, need to reconstruct neutrino
energy for inelastic final states
16 May 2012 K. McFarland, Needs for Neutrinos 42
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16 May 2012 K. McFarland, Needs for Neutrinos 43
Challenges
n Energy Reconstruction:
Quasi-Elastic • Sam and Juan covered this extensively in the context of
MiniBooNE data.
• Inferred neutrino energy changes if target is multinucleon.
16 May 2012 K. McFarland, Needs for Neutrinos 44
ex: Mosel/Lalakulich 1204.2269, Martini et al. 1202.4745,
Lalakulich et al. 1203.2935, Leitner/Mosel PRC81, 064614 (2010)
Lalakulich, Gallmeister, Mosel,1203.2935
n Energy Reconstruction:
Inelastic
• Here the problem is actually worse
• Detector energy response varies
Neutrons often exit without interacting
Proton and alpha ionization saturates
π- capture on nuclei at rest, π+ decay, π0 decay to
photons and leave their rest mass in detector
• Any detector, even liquid argon, will only
correctly identify a fraction of the final state
Need to know details of final state in four vector and
particle content to correct for response
16 May 2012 K. McFarland, Needs for Neutrinos 45
n
Modeling Backgrounds
• νe appearance is very sensitive
signal rate is low so even rare
backgrounds contribute!
• Current approach is to measure
the process elsewhere and
scale to the oscillation detector
But data constraints on neutral current from neutrino
scattering can’t tell us the cross-section as a function of
energy (missing final state neutrino)
So there is always an unknown correction that comes…
from a model, of course.
16 May 2012 K. McFarland, Needs for Neutrinos 46
p0 background
from En>peak
signal
n
16 May 2012 K. McFarland, Needs for Neutrinos 47
Current Practices
n
The Essential Tension
• Ulrich Mosel’s brilliant observation at NuINT11:
Theorist’s paradigm: “A good generator does not
have to fit the data, provided [its model] is right”
Experimentalist’s paradigm: “A good generator does
not have to be right, provided it fits the data”
• Most of the generators currently used by
oscillation experiments (NUANCE, GENIE,
NEUT) are written and tuned by experimentalists
See above! Our generators are wrong. WRONG!
• Models do not fit (all) the data, although they
provide insight into features of this data 16 May 2012 K. McFarland, Needs for Neutrinos 48
n
Neutrino Generators
• GENIE, NUANCE, NEUT are the generators
currently used in neutrino oscillation and cross-
section experiments
• Share same approach, with minor variations
Relativistic Fermi Gas in Initial State
Free nucleon cross-sections
o Llewllyn Smith formalism for quasi-elastic scattering
o Rein-Sehgal calcluation/fit for resonance production
o Duality based models for deep inelastic scattering
Cascade models for final state interactions
o Roughly, propagate final state particles through nucleus and allow them to interact. Constrained by πN, NN measurements.
16 May 2012 K. McFarland, Needs for Neutrinos 49
n What is Useful about
Generators?
• This approach gives a set of four vectors for
every particle leaving the nucleus
Essential for oscillation experiments where limited
detectors have responses that vary wildly depending on
final state particle
• Many tunable parameters, and it is always easy
to add more
Why? Initial model isn’t self-consistent anyway, so
experimenters just tune knobs to make data agree
o Which of course only applies to data we have and may or
may not be predictive for the future.
16 May 2012 K. McFarland, Needs for Neutrinos 50
n What is Deadly About
Generators?
• No way to know a priori if range of tunable
parameters that external data seems to
allow is really spans difference between
generator and truth
• Difficult or impossible to put in a complete
calculation for a single exclusive or semi-
inclusive final state.
Even if that calculation is better, it may not be
clear how to factorize from the ensemble of
reactions and effects in the generator. 16 May 2012 K. McFarland, Needs for Neutrinos 51
n What to do when models and
data don’t agree?
• Most of these models give absolute predictions.
So how to make them agree with data?
• MiniBooNE oscillation
analysis approach:
Modify the dipole axial
mass and Pauli blocking
until model fits data.
But there is nothing
fundamental behind this
approach. It’s a mechanical convenience. Dipole form
factor is unlikely to be right, and changes in Pauli
blocking are masking deficiencies in models! 16 May 2012 K. McFarland, Needs for Neutrinos 52
n What to do when models and
data don’t agree? (cont’d) • Here’s another example from T2K
• Want to tune multiple data sets that should have similar
physics, e.g., CC1π0 and NC1π0, using similar methods
E.g., should be able to modify parameter X or Y and fit both
16 May 2012 K. McFarland, Needs for Neutrinos 53
Good fit to all kinematic
distributions for CC by increasing
dipole mass and normalization
But the same tune in the NC makes
a large enhancement, not seen in
data, at high pion momentum!
n
Multi-Nucleon Correlations
• One “solution” to the high MiniBooNE CCQE
cross-section is enhancement of scattering due
to correlations among nucleons in nucleus
Could alter kinematics and rate in a way that would
make a better fit to the data
• How to implement?
Microphysical models
don’t (yet) give complete
final state description
Then what is advantage
over “ad hoc” scaling
from electron scattering?
(Bodek, Budd, Christy) 16 May 2012 K. McFarland, Needs for Neutrinos 54
n What to do if Experiments
“Don’t Agree”?
• As Omar conjectured, perhaps we can describe this with
a single (multinucleon) model. But this it is not evident.
16 May 2012 K. McFarland, Needs for Neutrinos 55
* NOMAD Fermi Gas (MA=1.35 GeV)
Fermi Gas (MA=1.03 GeV)
Fermi Gas (MA=1.35 GeV)
Fermi Gas (MA=1.03 GeV)
30%
• MiniBooNE data is well above
“standard” QE prediction
(increasing MA can reproduce s)
• NOMAD data consistent with
“standard” QE prediction
(with MA=1.0 GeV)
• MiniBooNE
* NOMAD
G.P. Zeller
n Exclusive Resonance
Models and Duality Models
• Duality models, as argued
before, fit data by construction
However, in a generator context,
have to add details of final state
• Typical approach (GENIE,
NEUT and NUANCE) is to use
a resonance model (Rein & Sehgal) below W<2 GeV,
and duality + string fragmentation model for W>2 GeV
Almost the worst possible solution!
Discrete resonance model (probably) disagrees with total cross-
section data below W<2 GeV and is difficult to tune
Average cross-section at high W does agree with data, but final
state simulation is of unknown quality and difficult to tune also.
16 May 2012 K. McFarland, Needs for Neutrinos 56
n
Final State Interactions
• Most generators implement a semi-classical cascade
model of transport for FSI. E.g., NEUT:
• But attempts to retune still don’t reproduce precise data.
Is it nucleon-level model or is it simplicity of FSI model?
How can we distinguish between the two problems?
16 May 2012 K. McFarland, Needs for Neutrinos 57
MiniBooNE
CC1pi+ Data
Figures and analysis from P. dePerio, NuINT11
Pion-C scattering data compared
to NEUT’s tuned model
n
16 May 2012 K. McFarland, Needs for Neutrinos 58
Next Steps?
n Approaches to Final State
Interactions
• Currently propagate final state particles through
the nuclear medium with varying degrees of
sophistication where they interact according the
measured cross-sections or models
• Issues:
Are the hadrons modified by the nuclear medium?
Are hadrons treated as only on-shell or is off-shell
transport allowed?
How to cleanly separate the initial state particles from
their final state interactions?
• (Olga’s talk) 16 May 2012 K. McFarland, Needs for Neutrinos 59
n One Direction:
Microphysical Models
• These may do a better job describing the
underlying physics
• To fit in a generator, however:
Must give complete description of final state
Must be able to be integrated with other parts of
generator without “double counting”, e.g., final state
interactions
• This is extremely challenging, and we have not
yet succeeded with any model more complex
than Rein & Sehgal resonance model.
16 May 2012 K. McFarland, Needs for Neutrinos 60
n
16 May 2012 K. McFarland, Needs for Neutrinos 61
Another Direction: Duality
• Governs transition
between resonance and
DIS region
• Sums of discrete
resonances approaches
DIS cross-section
• Bodek-Yang: Observe in
electron scattering data;
apply to n cross-sections
Low Q2 data
DIS-Style PDF prediction
1
1222
xQMW
T
n
Duality’s Promise and Trap
• In principle, a duality based approach can be applied
over the entire kinematic region
• The problem is that duality gives “averaged” differential
cross-sections, and not details of a final state
• Microphysical models may lack important physics, but
duality models may not predict all we need to know
How to scale the mountain between the two? 16 May 2012 K. McFarland, Needs for Neutrinos 62
n
Conclusions
• “Big picture” goals in neutrinos require improved
knowledge of neutrino interactions.
• We are working with a very primitive set of tools,
and we need a modern machine shop.
• There are many barriers to improving the tools,
and it is not obvious (to me) which approaches
will be the most productive.
• New neutrino data (e.g., MINERvA, μBooNE,
T2K, NOvA) will help us. But…
• … only if we also improve modeling.
16 May 2012 K. McFarland, Needs for Neutrinos 63
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