Interactions of Neutrinos
What Neutrino ExperimentsNeed to KnowKevin McFarlandUniversity
of RochesterECT*16 May 2012nnOutlineNeutrino scattering vs.
electron scattering Goals of neutrino oscillation experimentsNarrow
and Broad Beam ExperimentsWhat Needs to be ModeledCurrent
PracticesPossible Paths to Progress16 May 2012K. McFarland, Needs
for Neutrinos2n16 May 2012K. McFarland, Needs for
Neutrinos3Neutrinos vs. Electronsn16 May 2012K. McFarland, Needs
for Neutrinos4Neutrino Dictionary for Parity
ViolatorsElectronPhysics
ConceptAPV~10-6s|APC|2+2ReAPC*APV+negligiblePolbeama limiting
systematicTargetDetector
Ebeama number you choosen16 May 2012K. McFarland, Needs for
Neutrinos5Neutrino Dictionary for Parity
ViolatorsElectronNeutrinoPhysics
ConceptAPV~10-61s|APC|2+2ReAPC*APV+negligiblenegligiblePolbeama
limiting systematic1-mn2/En2TargetDetector(see Target)
Ebeama number you choosea distribution you barely knownNeutrino
Facts of LifeNeutrino experiments require massive targets to carry
out goalsFew 104 or 105 kg of target material of current and near
future experimentsWe only know what we see in the final
stateTargets are large nucleiCarbon, Oxygen, Argon, Iron are all
being used in current or near future experimentsDetectors have
severe limitationsNeed to measure interactions throughout
targetMust balance expense vs. capability
16 May 2012K. McFarland, Needs for Neutrinos6n16 May 2012K.
McFarland, Needs for Neutrinos7Neutrino Oscillation Goals(at
lightening speed)n
Neutrino FlavorNeutrinos were discovered by
the final state positron is no accident! weve seen
neutrinosproduce all three charged leptons in weak interactionsThe
Z boson decays into three (and only three) neutrino states16 May
2012K. McFarland, Needs for Neutrinos8
nNeutrino Flavor MixingThe defining question of the field today
turns out to be from an unusual conjecture (Pontecorvo)Are these
neutrinos of definite flavorthe eigenstates of the neutrino mass
matrix Or are we looking at neutrino puree?
16 May 2012K. McFarland, Needs for Neutrinos9
nNeutrino Flavor Mixing (contd)If neutrinos mass states mixto
form flavors
and the masses are differentflavors of neutrinos can change in
flightExplains Davis solar neutrino puzzlesince only electron
flavor neutrinos aredetected +np+e-16 May 2012K. McFarland, Needs
for Neutrinos10
nNeutrino Flavor OscillationEach neutrino wavefunctionhas 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 2012K. McFarland, Needs for Neutrinos11
nNeutrino Oscillation (contd)Phase difference leads to
interference effect, just like with sound waves of two
frequenciesfrequency difference sets period of beats
16 May 2012K. McFarland, Needs for Neutrinos12
23
n
Phase differenceAnalog of volume disappearing in beats is
original neutrino flavor disappearingand appearance of a new
flavor
more generally, mixing need not be maximal Neutrino Oscillation
(contd)16 May 2012K. McFarland, Needs for Neutrinos13
only two generations for now!
n16 May 2012K. McFarland, Needs for Neutrinos14Neutrino
Oscillation (contd)For two generations
Oscillations require mass differencesOscillation parameters are
mass-squared differences, dm2, and mixing angles, q.One correction
to this is matter changes q, L dep.
Wolfenstein, PRD (1978)
e- density
appropriate units give the usual numerical factor 1.27
GeV/km-eV2n14standard oscillation formula when you put in hbars and
cs get the usual formulas that push you to long baselines. 10^3
eV^2 means L/E 1000km/GeV
matter effects modify this picture somewhat significantly if
electrons are dense in matter16 May 2012K. McFarland, Needs for
Neutrinos15Solar Neutrinos: SNOD2O target uniquely
observed:charged-currentneutral-currentThe former is onlyobserved
for ne(lepton mass)The latter for all typesSolar flux is
consistentwith modelsbut not all ne at earth
n15SNO added a new capability see all active neutrino flavors.
and in fact they find they can account for all neutrinos we think
should originate from the sun its just that few remain as electron
flavor neutrinos!16 May 2012K. McFarland, Needs for
Neutrinos16KAMLAND
Sources wereJapanesereactors150-200 kmfor most offlux. Rate
uncertainty ~6%1 kTon scint. detector inold Kamiokande
cavernoverwhelming confirmationthat neutrinos change flavorin the
sun via mattereffectsn16now, fits suggested that dm2 was not from
naive L/E (earth-sun distance)/(energy of neutrinos), but rather
that matter effects in sun were important. therefore dm2 big enough
to see oscillations if L~200km, E~2 MeV
16 May 2012K. McFarland, Needs for Neutrinos17
Atmospheric NeutrinosNeutrino energy: few 100 MeV few GeVFlavor
ratio robustly predictedDistance in flight: ~20km (down) to 12700
km (up)n17atmospheric neutrinos. nature provides another generous
source ~GeV neutrinos, well understood flavor ratiocan compare
upward going (produced far away) against downward (produced
nearby)
16 May 2012K. McFarland, Needs for
Neutrinos18Super-KamiokandeSuper-Kdetector hasexcellent
e/mseparationUp / down difference: L/E
Muons distorted, electrons not; so mostly
old, but good data!2004 Super-K analysis
n18Super-K has the gold standard data. Note the muon sample
deficit at upward going zenith angles and fit to oscillations.
16 May 2012K. McFarland, Needs for Neutrinos19MINOS
735km baseline5.4kton Far Det.1 kton Near Det.Running since
early 2005
Precise measurement ofnm disappearance energygives dm223
n19MINOS has as its primary goal high statistics disappearance,
and therefore precise delta-m23.key for future experiments
optimization16 May 2012K. McFarland, Needs for Neutrinos20CNGS
Goal: nt appearance 0.15 MWatt source high energy nm beam &
732 km baseline handfuls of events/yr
e-, 9.5 GeV, pT=0.47 GeV/c interaction, E=19 GeV
fiugres courtesy A. Bueno3kton
PbEmulsion layersnt1 mm
1.8kTonfigures courtesy D. Autieron20CNGS is (primarily)
checking that neutrinos do go to taus as expected, at low
statistics16 May 2012K. McFarland, Needs for Neutrinos21
Two Mass Splitings: Three GenerationsOscillations have told us
the splittings in m2, but nothing about the hierarchyThe electron
neutrino potential (matter effects) can resolve this in
oscillations, however.figures courtesy B. Kayserdmsol2
dm1228x10-5eV2dmatm2 dm2322.5x10-3eV2
n21two splittings very different. implies three generations
worth of neutrinosthe hierarchy is not defined though. two heavy
and one light (inverted) or vice versa?matter effects at
accelerator beams can resolve16 May 2012K. McFarland, Needs for
Neutrinos22Three Generation MixingNote the new mixing in middle,
and the phase, d
slide courtesy D. Harrisn22those three generations mix. heres a
decomposition of the matrix. note that one element, Ue3 has its
size set by s13 parameter with the phase delta.
16 May 2012K. McFarland, Needs for Neutrinos23Are Two Paths Open
to Us?If reactor mixing, q13, is small, but not too small, there is
an interesting possibility
At atmospheric L/E, nmdm232, q13 dm122, q12 ne
SMALLLARGESMALLLARGEn23so why not keep rolling the dice?heres a
winner. if mixing is small (but not zero!) then we see the
following two competing pathsupper suppressed by angle, lower by
dm2 L/E
16 May 2012K. McFarland, Needs for Neutrinos24Implication of two
pathsTwo amplitudes
If both small,but not too small,both can contribute ~
equallyRelative phase, d, between them can lead toCP violation
(neutrinos and anti-neutrinos differ) in oscillations!nmdm232, q13
dm122, q12 nen24if both non zero and not small, and if have a
relative phase, delta, can see CP violation in oscillationsq13 in
2011T2K, an accelerator experiment, showed a signal of 6 events1.5
expected if q13=0
Consistent, but less significant, indication from MINOS shortly
after16 May 2012K. McFarland, Needs for Neutrinos25
nq13 in 2012Two reactor experiments recently showed overwhelming
evidence for large q13.Both place detectors near and far (~1km)
from reactorsLook for a smallrate differencebetween twolocations 16
May 2012K. McFarland, Needs for Neutrinos26
nq13 in 2012: Daya Bay16 May 2012K. McFarland, Needs for
Neutrinos27
Figures from K. Heeger
nq13 in 2012: RENO16 May 2012K. McFarland, Needs for
Neutrinos28Figures from S.B. Kim
nImplications of Large q13If q13 is large, then one of the two
paths
is larger than the other.This implies large signals, but small
CP asymmetries16 May 2012K. McFarland, Needs for
Neutrinos29nmdm232, q13 dm122, q12 nenImplications of Large
q13Quantitative analysis to illustrate this expected
behaviorFractional asymmetry decreases as q13 increasesWe live
hereStatistics are (relatively) high, so the challenge will be
controlling systematic uncertainties.16 May 2012K. McFarland, Needs
for Neutrinos30
n16 May 2012K. McFarland, Needs for Neutrinos31Current and
Future Experimentsn16 May 2012K. McFarland, Needs for
Neutrinos32
Narrow Band BeamCP violation (interference term) and matter
effects lead to a complicated mixSimplest case:first
oscillationmaximum, neutrinos andanti-neutrinosCP violation gives
ellipsebut matter effects shiftthe ellipse in along-baseline
acceleratorexperiment
Minakata & Nunokawa JHEP 2001n32this is the old wonder of
the quark sectora complication naively want to compare just nu and
nubar oscillations, but different matter effects mean you have to
know about and correct for that.
16 May 2012K. McFarland, Needs for Neutrinos33Broadband BeamSee
different mixture of solar/interference CP term,matter effects at
different oscillation maximaThis shows E. Recall argument of vacuum
oscillation term is ~L/E
FNAL-DUSELL=1500kmn4 February 2009K. McFarland, Neutrinos at
Accelerators34Beam Design OptionsAll experiments will want to see
first oscillation maximum, L/E ~ 400 km/GeVThen one has a
choiceNarrow Band Beam at First Oscillation PeakBroad Band Beam
Covering Multiple Oscillation PeaksBecause 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 LAlso need good energy resolution at
low neutrino energies
En4 February 2009K. McFarland, Neutrinos at Accelerators35First
Suggested by BNL-889 proposalTake advantage of Lorentz Boost and
2-body kinematicsConcentrate nm fluxat one energyBackgrounds
lower:NC or other feed-downfrom highlow energy ne (3-body
decays)Generally optimal if onlyaccessing first maximumNarrow Band
Beam:Off-axis Techinque
figure courtesy D. Harrisn35one ideal for the next generation of
experiments can control backgrounds with off-axis beam4 February
2009K. McFarland, Neutrinos at Accelerators36Narrow: T2KTunable
off-axis beam from J-PARC to Super-K detectorbeam and nm
backgrounds are kept below 1% for ne signal~2200 nm events/yr (w/o
osc.)
d=0, no matter effectsfigures courtesy T. Kobayashi
n36send that beam to Super-K. off-axis and tunable to vary
energy depending on MINOS dm2.electron neutrino appearance beam
power -> high statistics4 February 2009K. McFarland, Neutrinos
at Accelerators37
Narrow: NOnAUse Existing NuMI beamlineBuild new 15kTon
Scintillator Detector 820km baseline--compromise between reach in
q13 and matter effects
Assuming Dm2=2.5x10-3eV2
ne+Ap p+ p- e-figure courtesy M. Messierfigures courtesy J.
CooperGoal:ne appearanceIn nm beamn37NOvA.. build a near detector.
same techniques as T2K. more from Gary Feldman later, so now Ill
pick on it for a bit. this, like J-PARC is a big projectsBroad(er):
LBNE16 May 2012K. McFarland, Needs for Neutrinos38
33 kTon (fiducial) Liquid Argon TPC
figures courtesy M. Diwan
n16 May 2012K. McFarland, Needs for Neutrinos39Needs for
ModelingnIllustration: T2KBackgrounds are significantPrimarily from
neutral current neutral pion productionNeutrino energy is a
powerful background discriminant, but has little other information
about oscillationsNo official plot yet, but I can guarantee you
that the backgrounds in neutrino and anti-neutrino beams are
different. 16 May 2012K. McFarland, Needs for Neutrinos40
nIllustration: LBNEMaximum CP effect is range of red-blue
curveBackgrounds are significant, vary with energy and are
different between neutrino and anti-neutrino beamsPileup of
backgrounds at lower energy makes 2nd maximum only marginally
useful in optimized designSpectral information plays a roleCP
effect may show up primarily as a rate decrease in one beam and a
spectral shift in the other
16 May 2012K. McFarland, Needs for Neutrinos41
nGeneric FeaturesPhysics goals require comparing neutrino and
anti-neutrino transition probabilitiesBackgrounds are significant
and differentReconstructing the neutrino energy is keyFor T2K, this
is quasi-elastic statesFor NOvA, LBNE, need to reconstruct neutrino
energy for inelastic final states16 May 2012K. McFarland, Needs for
Neutrinos42n16 May 2012K. McFarland, Needs for
Neutrinos43ChallengesnEnergy Reconstruction: Quasi-ElasticSam and
Juan covered this extensively in the context of MiniBooNE
data.Inferred neutrino energy changes if target is multinucleon.16
May 2012K. McFarland, Needs for Neutrinos44ex: Mosel/Lalakulich
1204.2269, Martini et al. 1202.4745, Lalakulich et al. 1203.2935,
Leitner/Mosel PRC81, 064614 (2010)
Lalakulich, Gallmeister, Mosel,1203.2935nEnergy Reconstruction:
InelasticHere the problem is actually worseDetector energy response
varies Neutrons often exit without interactingProton and alpha
ionization saturates- capture on nuclei at rest, + decay, 0 decay
to photons and leave their rest mass in detectorAny detector, even
liquid argon, will only correctly identify a fraction of the final
stateNeed to know details of final state in four vector and
particle content to correct for response16 May 2012K. McFarland,
Needs for Neutrinos45nModeling Backgroundse appearance is very
sensitivesignal rate is low so even rare backgrounds
contribute!Current approach is to measurethe process elsewhere and
scale to the oscillation detectorBut data constraints on neutral
current from neutrino scattering cant 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 2012K. McFarland, Needs for Neutrinos46
p0 backgroundfrom En>peaksignaln16 May 2012K. McFarland,
Needs for Neutrinos47Current PracticesnThe Essential TensionUlrich
Mosels brilliant observation at NuINT11:Theorists paradigm: A good
generator does not have to fit the data, provided [its model] is
rightExperimentalists 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 experimentalistsSee above! Our generators are
wrong. WRONG!Models do not fit (all) the data, although they
provide insight into features of this data16 May 2012K. McFarland,
Needs for Neutrinos48nNeutrino GeneratorsGENIE, NUANCE, NEUT are
the generators currently used in neutrino oscillation and
cross-section experimentsShare same approach, with minor
variationsRelativistic Fermi Gas in Initial StateFree nucleon
cross-sectionsLlewllyn Smith formalism for quasi-elastic
scatteringRein-Sehgal calcluation/fit for resonance
productionDuality based models for deep inelastic scatteringCascade
models for final state interactionsRoughly, propagate final state
particles through nucleus and allow them to interact. Constrained
by N, NN measurements. 16 May 2012K. McFarland, Needs for
Neutrinos49nWhat is Useful about Generators?This approach gives a
set of four vectors for every particle leaving the nucleusEssential
for oscillation experiments where limited detectors have responses
that vary wildly depending on final state particleMany tunable
parameters, and it is always easy to add moreWhy? Initial model
isnt self-consistent anyway, so experimenters just tune knobs to
make data agreeWhich of course only applies to data we have and may
or may not be predictive for the future.16 May 2012K. McFarland,
Needs for Neutrinos50nWhat 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
truthDifficult 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
2012K. McFarland, Needs for Neutrinos51nWhat to do when models and
data dont agree?Most of these models give absolute predictions. So
how to make them agree with data?MiniBooNE oscillationanalysis
approach:Modify the dipole axialmass and Pauli blockinguntil model
fits data.But there is nothingfundamental behind this approach. Its
a mechanical convenience. Dipole form factor is unlikely to be
right, and changes in Pauli blocking are masking deficiencies in
models!16 May 2012K. McFarland, Needs for Neutrinos52
nWhat to do when models and data dont agree? (contd)Heres
another example from T2KWant to tune multiple data sets that should
have similar physics, e.g., CC10 and NC10, using similar
methodsE.g., should be able to modify parameter X or Y and fit
both16 May 2012K. McFarland, Needs for Neutrinos53
Good fit to all kinematic distributions for CC by increasing
dipole mass and normalizationBut the same tune in the NC makes a
large enhancement, not seen in data, at high pion
momentum!nMulti-Nucleon CorrelationsOne solution to the high
MiniBooNE CCQE cross-section is enhancement of scattering due to
correlations among nucleons in nucleusCould alter kinematics and
rate in a way that would make a better fit to the dataHow to
implement?Microphysical modelsdont (yet) give completefinal state
descriptionThen what is advantageover ad hoc scaling from electron
scattering? (Bodek, Budd, Christy)16 May 2012K. McFarland, Needs
for Neutrinos54
n
What to do if Experiments Dont Agree?As Omar conjectured,
perhaps we can describe this with a single (multinucleon) model.
But this it is not evident.16 May 2012K. McFarland, Needs for
Neutrinos55* NOMADFermi 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. ZellernExclusive Resonance Models and Duality ModelsDuality
models, as arguedbefore, fit data by constructionHowever, in a
generator context, have to add details of final stateTypical
approach (GENIE, NEUT and NUANCE) is to use a resonance model (Rein
& Sehgal) below W2 GeVAlmost the worst possible solution!
Discrete resonance model (probably) disagrees with total
cross-section data below W scanning in W2can see resonance behavior
modulating around a prediction from continuous DIS
PDFsFACTORIZATION (that defines PDFs) is not operative here. But on
average it works.Bodek-Yang: take electron scattering data to tell
you about wiggles in W2; take DIS limit to tell you about continuum
that the wiggles modulateDualitys Promise and TrapIn principle, a
duality based approach can be applied over the entire kinematic
regionThe 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 knowHow to scale the mountain
between the two?16 May 2012K. McFarland, Needs for Neutrinos62
Microphysical models of exclusive processesDuality models based
on data of inclusive ratesnConclusionsBig 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, BooNE, T2K, NOvA) will
help us. But only if we also improve modeling.16 May 2012K.
McFarland, Needs for Neutrinos63n
