f.sanchez (uab/ifae)iss meeting, detector parallel meeting. jan 2006 low energy neutrino...

F.Sanchez (UAB/IFAE) ISS Meeting, Detector Parallel Meeting. Jan 2006

Low Energy Neutrino Interactions & Near Detectors

F.SánchezUniversitat Autònoma de Barcelona / IFAE


Outlook

• Near detector goals. • Cross-sections.• Nuclear effects.

– Fermi motion and Pauli blocking.– Nuclear reinteractions.

• Remarks.


Function of a near detector

• CP violation measurement involves the measurement of the asymmetry:

• To do that we need to know: – flux of muon (anti)neutrino.– Flux of electron (anti)neutrino.– Cross-section of electron (anti)neutrino (or ratio to cancel

uncertainties). – Background for electron (anti)neutrino detection. Large

backgrounds can reduce the cancellation for double ratio.

)()(

)()(

ee

ee

NN

NNA


Function of a near detector

ee

ee

EffBNEffBN

EffBNEffBNA

//)(//)(

//)(//)(

• Backgrounds are generated by electron neutrinos, neutrinos (or mis-reconstructed muon neutrinos whenever the energy reconstruction is important). Precise knowledge of electron and 0 production processes are crucial both for NC and CC reactions.

• If flux is measured with a particular channel: = N /it is not ensured that the term Bcancels the uncertainties in

• The efficiencies can be different for neutrinos and anti-neutrinos. Precise knowledge of basic kinematics behavior of the interaction is important (W, q2, E dependencies, particles properties in the hadronic half of the reaction).


Function of a near detector• In addition to that neutrino spectrum is (almost) never monochromatic. We have to play with the knowledge of:

• Dependency of efficiency with neutrino energy.

• Dependency of cross-section with neutrino energy difficult near threshold.

• Difference between the neutrino energy distribution of the near and the far detectors (even with no oscillation at all). This is true even for the case of a perfect spectrum knowledge like beta-beams and Neutrino Factories.

•We never measure the product cross-section times flux but a convolution:

)()()( jjj

jii EEMEN

e


Function of a near detector• Flux normalization.

– Difficult to carry out with neutrino-Nucleus interactions. T2K/K2K we quote always:

and normalize always to the same reaction. CCQE

in K2K and T2K. This approach is not optimal for energies larger than ~2GeV and it is limited if the flux differs from near to far. It relies on the MC implementation of the interaction.

– Other alternatives like elastic electron scattering?.

• Study Neutrino interactions:– Kinematical properties of the interaction. (MC tuning)– Neutrino induced background. (MC tuning)


Cross-sections

• From 0.2GeV to 5GeV/c neutrino cross-sections crosses several production thresholds: – CCQE (~200 MeV)– CCD (~700 MeV) – DIS (~2 GeV)

• CC Coherent pion production is very poorly known in the full energy rage.

Experimental measurements are also worse at low energies.


CC Quasi-elastic

From PCAC

The axial form-factor; the standard dipole form

MA is a free parameter (the only one) and it is obtained from experiments.

Limited by systematic errors… it is a tough measurement!.

M

QFqQF

M

QFqiQF P

A

)()(

2

)()(

2

52

5

222

1

22

222 )(2)(

Qm

QFMQF A

P

2

2

2

2

1

)(

A

AA

M

Q

gQF

CVC – use electron scattering data for the dipole and form factors.


CC Quasi-elastic• The electromagnetic form factors

and the knowledge of the axial mass have influence in: – The total cross-section. – q2 dependency of the cross-section.

• Low q2 is also influenced by nuclear effects.– It can vary from nucleus to nucleus.

• Detector effects also influence the measurement like in the case of the momentum scale uncertainty.


CC Quasi-elastic

• Knowledge of CC-QE cross-section is poor.

• It is also not covering the threshold. This could be important for low energy neutrino experiments.


nln

pln

plp0

3 CC channels for neutrino reactions:

Dominant contributions comes from:

nlln

plln

pllp0

CC-1

They can be related by isospin relations except for nuclear corrections.

Theory is built as a mixture of electron data, free parameter and theory as in CCQE.

nlln

nllp

pllp00

0


The overall cross sections for CC1 (with the W ≤ 2 GeV cut):

nln

pln

plp0

CC-1

It is not well measured and it depends on the W cut (higher mass resonances –up to 18- and non-resonant region)


Very little is known about NC pion production:

(NC π0 and +- production are important backgrounds!)

NC-1


CC-1 and NC-1 in T2K

50% of background in e appearance is due to NC-10


Nuclear effects

• Fermi motion. The nucleon is not at rest!. – Oxygen or Carbon target the neutron

can have up to ~220MeV/c. • Pauli blocking prevents the production of

nuleons below the Fermi level. • Final State Interactions, nucleons and

pions interact with the dense nuclear matter before leaving the nucleus: – Change in momentum – Change exchange via resonances.


Nuclear effects

Fermi Motion affects the intrinsic neutrino energy resolution.

Energy is reconstructed under the assumption of the target neutron being at rest.


Nuclear effects• Fermi motion and Pauli

blocking affect the cross-section at low q2

and also introduce and additional smearing in the reconstruction of neutrino energy.


Nuclear effects: n-n correlations

• Short range correlations also alter the Fermi-motion distributions.

• This correlations are needed to reproduce the data on Fermi motion.


FSI on pions

• It alters the momentum distribution. (mainly below 200Mev/c)

• Big effect on final state topologies and multiplicities.

• Big impact on reaction identification since it is based on the hadronic part of the reaction.

NEUT MC

Never tested in detail.


FSI on pionsNo pion is seen


FSI on pions

Negative pion is seen


Nuclear effects

• Nuclear effects are difficult to measure because they are convoluted with cross-section (un)knowledge.

• They are different for neutrons and protons they are different for neutrinos and anti-neutrinos. Isoscalar nucleus could help (?).

• The optimal case is always to measure the near detector with the same target as the far detector difficult in some cases like water target.


T2K

• In T2K: – Basic normalization reaction is CCQE. – Lepton used to reconstruct energy under

CCQE assumption.– Proton/ provide the reaction selection.– Backgrounds are related to single pion

production in neutral and charge current. – But:

• Energy regime is favorable: just above CCQE threshold!.

• …but not much: on the top of CC1 threshold!.


Remarks• The near detector is fundamental to achieve design goals of

Neutrino Factory and Beta Beams.• Personal prejudices:

– Similar target Far/Near.– Exclusive channel measurements. – Possibility of changing neutrino spectrum.– Measure exclusive reactions.– Flux, is it possible to measure it from elastic scattering on

electrons?.– The problem of energy scale!.– Problem of reaction threshold’s.

• Design is driven by Far detector needs: – WHAT ARE THE EXPECTED BACKGROUNDS? – HOW TO NORMALIZE FLUX?– HOW TO MEASURE ENERGY?. WHICH PRECISION?.– …

• Watch at improvements in neutrino interactions from theorist (NuInt conference series) and experiments (Minera & future T2K).

• Some of these conclusions do not apply if large energy neutrinos are studied (> 4/5 GeV).

f.sanchez (uab/ifae)iss meeting, detector parallel meeting. jan 2006 low energy neutrino...

Documents

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