Neutrino-nucleus cross-section measurements at T2K Callum Wilkinson - - PowerPoint PPT Presentation

Neutrino-nucleus cross-section measurements at T2K

Callum Wilkinson On behalf of the T2K collaboration

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Why neutrino cross sections?

• Event rate; Neutrino flux; Cross section; Detector

smearing; Oscillation probability

• Near/far ratios don’t fully cancel systematics:
• Dramatic Eν change
• ND is νμ dominated. Use to infer νe
• σ(Eν,x) relates observables x to Eν

Require few % cross-section systematics to fulfill design goals of future OA experiments

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Broad neutrino fluxes, must understand the entire nuclear response... … but no consistent theoretical description Integrate!

Why neutrino cross sections?

Nuclei Nucleons Quarks

Energy transfer

• Ann. Rev. Nucl. Part. Sci., 68, 2018

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The plot thickens

Final state particles do not correspond to initial interaction type (or energy transfer)!

(E.g. 2p2h)

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The T2K experiment

• Long-baseline accelerator neutrino
• scillation experiment
• Near detectors:
• Constrain flux and cross-section

model before oscillation

• Cross-section measurements in

unoscillated beam

• Far detector: oscillation analyses

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Near detector complex

ND280 (2.5º) INGRID (0º) WAGASCI (1.5º)

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• ND280 (2.5° off-axis):
• Plastic scintillator (C8H8) and water targets
• High resolution tracking + magnet for sign and momentum
• WAGASCI (1.5° off-axis): water and C8H8
• INGRID (on-axis): water, iron and C8H8 targets

Near detector complex

• Fluxes:
• On axis
• 1.5° off-axis
• 2.5° off-axis
• ν (FHC) and ν (RHC)

enhanced modes

• Phys. Rev. D88, 032002 (2013)

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Initially measure lepton kinematics Benefit from different fluxes/targets Add hadron kinematics over time

T2K cross-section strategy

θl Hadrons l± pl νl

( )

Build selections of interaction topologies by adding restrictions

• n outgoing hadrons:
• No model-dependent corrections
• Increasing Nπ≈ increasing energy

transfer

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• Measure CC0π on C8H8 by

selecting muon, allowing protons, and vetoing pions

• Control samples for CC1π &

CCNπ backgrounds

ND280 CC0π νμ & νμ

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Paper in preparation

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Simultaneous fit to measure cross section in 58 pμ,cosθμ bins for each mode, including correlations between the samples

νμ

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ND280 CC0π νμ & νμ

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• Correlations between

measurements can be used to uncover model differences

• Adds significantly more

power for model-building

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Paper in preparation

ND280 CC0π νμ & νμ

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ND280 νμ-CC0π C8H8 & H2O

FGD2

• Combined FGD1 (C8H8) and

FGD2 (C8H8 + H2O) analysis

• Difficult to reconstruct vertices

from passive water layers, so a joint fit is essential!

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• Fully correlated C8H8 and water measurements also produced
• Next step is a fully correlated νμ/νμ, C8H8/water analysis

ND280 νμ-CC0π C8H8 & H2O

(105.7) (145.9)

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INGRID CC-inclusive

• On-axis Eν spectrum
• Limited phase space due to detector design:

1μ-: pμ ≥ 0.4 GeV, θμ ≤ 45°

• Use pure C8H8, water+C8H8 and iron+C8H8 targets

→ A-scaling

arXiv:1904.09611

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• Water+C8H8 detector 1.5˚ off axis
• Limited phase space due to

detector design:

• 1μ±: pμ ≥ 0.4 GeV, θμ ≤ 45°
• 0π: pπ ≥ 0.2 GeV, θπ ≤ 70°
• 0p: pp ≥ 0.6 GeV, θp ≤ 70°
• Unmagnetized, so main result is

νμ+νμ to avoid model dependence

WAGASCI νμ-CC0π0p

(Tracking) C8H8 target H2O+C8H8 target

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WAGASCI νμ-CC0π0p

C8H8 H2O

• Results including correlations between C8H8 and H2O

measurements

• Future work will produce correlated INGRID (0º),

WAGASCI (1.5º), and ND280 (2.5º) measurements, to maximize model constraining power

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• Important to understand intrinsic

backgrounds for OA experiments

• Also important to control

potential νe/νμ differences

• Challenge to characterize and

constrain γ-backgrounds

ND280 electron neutrinos (νe)

FHC νe RHC νe RHC νe

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• Rare SM process, background

for νe-appearance in Cherenkov detectors

• Enhancements suggested as a

possibility to explain low energy excess in MiniBooNE

ND280 NC1γ

• Search for e+e- pairs with low

invariant mass

• Backgrounds from π0

producing processes

• J. Phys. G 46, 08LT01 (2019)

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• Neutrino cross-section measurements are critical for

current and future oscillation experiments

• T2K focuses on making unbiased, model-independent

cross-section measurements:

• Variety of nuclear targets
• Different fluxes
• Different observable channels
• Aim to provide high quality data to constrain various

cross-section models

Conclusions

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Backup

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Nuclear targets

• Free nucleon: the interaction level cross section, including

hadronization at high energy transfer

• Initial nuclear state: how nucleons behave inside the nucleus.

E.g., Relativistic Fermi Gas.

• Nuclear effects: additional effects due to the presence of multiple
• nucleons. E.g. np-nh interactions.
• Final State Interactions: subsequent interactions before

interaction products exit the nucleus.

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Free nucleon response

QE RES DIS

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Nuclear response

QE RES DIS Interactions with more than one nucleon contribute 2p2h

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QE RES DIS 2p2h Integrate! Can’t reconstruct ω, so no way to avoid poorly modelled regions!

Nuclear response

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• Can only measure for outgoing particle kinematics, as a

function of topology, not interaction channel

• Integrated over a broad neutrino flux
• Post-FSI, integrate out all degrees of freedom other than y:
• Direct theory comparisons to data are difficult
• Require Monte Carlo generator to do integrals numerically

ν-A cross section data

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• Forthcoming publication to add to the extremely

spartan literature

• Future work on νe-CC0π sample

Electron neutrinos (νe)

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