What Neutrino Experiments Need to Knoksmcf/for-debbie/Trento Neutrino Interactions.pdfNeutrino Flavor Oscillation •Each neutrino wavefunction has a time-varying phase in its rest

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What Neutrino Experiments

Need to Know

Kevin McFarland

University of Rochester

ECT*

16 May 2012

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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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Neutrinos vs. Electrons

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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

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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

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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


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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


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?


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-


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

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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


/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

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Neutrino Oscillation (cont’d)

• Phase difference leads to interference

effect, just like with sound waves of two frequencies

frequency difference sets period of “beats”


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



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



• 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

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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

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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

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Atmospheric Neutrinos

• Neutrino energy: few 100 MeV – few GeV

• Flavor ratio robustly predicted

• Distance in flight: ~20km (down) to 12700 km (up)

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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


MINOS 735km baseline

5.4kton Far Det.

1 kton Near Det.

Running since early 2005

Precise measurement of

n disappearance energy

gives dm223

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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

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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

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Three Generation Mixing

• Note the new mixing in middle, and the phase, d

slide courtesy D. Harris

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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,

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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

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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

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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


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13 in 2012: Daya Bay


Figures from K. Heeger

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13 in 2012: RENO


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


n

dm232, 13

dm122, 12

ne

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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.


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Current and Future Experiments

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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

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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

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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


• 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

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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

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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

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Broad(er): LBNE


33 kTon (fiducial)

Liquid Argon TPC

figures courtesy M. Diwan

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Needs for Modeling

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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.


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


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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


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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.


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


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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.


p0 background

from En>peak

signal

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Current Practices

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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

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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.


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.


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


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!

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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.


* NOMAD 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.


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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?


MiniBooNE

CC1pi+ Data

Figures and analysis from P. dePerio, NuINT11

Pion-C scattering data compared

to NEUT’s tuned model

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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.


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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

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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

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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.


Documents

What Neutrino Experiments Need to Knoksmcf/for-debbie/Trento Neutrino Interactions.pdfNeutrino Flavor Oscillation •Each neutrino wavefunction has a time-varying phase in its rest