The neutrino mass hierarchy measurement with a neutrino telescope in the Mediterranean Sea: A feasibility study

The neutrino mass hierarchy measurement with a neutrino telescope in the Mediterranean Sea:

A feasibility study

Apostolos G. Tsirigotis, Dimitris Lenis, Spyros Tzamarias

Physics Laboratory, Hellenic Open University for the KM3NeT collaboration/ORCA WG

Introduction

KM3NeT and ORCA

Detector performance studies and first results Sensitivity studies

More studies on detector performance Plans for detector optimization

Summary

VLVnT13 - Very Large Volume Neutrino Telescope Workshop 2013 7 August 2013 – AlbaNova University Center

Introduction

All neutrino mixing parameters known (with good precision) θ₁₃ is non-zero

Missing – δ_CP phase

- neutrino mass hierarchy

neutrino mass pattern, origin of flavour

Important impact on LBL, ββ0ν experiments (also for δ_CP measurement)

Strategy: probe ν_μ <--> ν_e governed by Δm²₁₃ + matter effects

to resolve the sign of Δm²₁₃

Maximal enhancement at resonant energy

Introduction - Motivations

a few GeV for Earth densities

prospects for long-baseline and atmospheric neutrino experiments !

Introduction – Oscillation probabilities

Differences in (E_ν, θ_ν) patterns (oscillograms)

make it possible to identify the Neutrino Mass Hierarchy Example with PINGU-like detector:

(perfect resolution, large effective volume)

Akmedov, Razzaque, Smirnov JHEP 02 (2013) 082 BUT the effect is not so neat due to:

● Uncertainties: atmospheric neutrino fluxes oscillation parameters

earth matter effects

● Kinematic smearing ν_μ → μ (few degrees)

● Detector finite efficiency and resolution in E_ν, θ_ν

● Flavour identification uncertainties

Introduction

Introduction - Neutrino beam option

Clean ν_μ beam (<1% contamination from other flavours)

Most neutrinos between 1-6 GeV (Don't need energy reconstruction) Known neutrino direction

Count electrons to measure the neutrino mass hierarchy Needs flavour identification (tracks vs. cascades)

Track event rates

Cascade event rates

KM3NeT and ORCA

KM3NeT:

Focus on neutrino astronomy

KM3NeT phase 1 will proceed with the construction of a detector for Galactic sources

ORCA: Oscillation Research with Cosmics in the Abyss

F

Currently a feasibility study

Use agreed KM3NeT technology ORCA: Working Group

APC, CPPM, ECAP, HOU, IPHC, LNS, NIKHEF 52 people

Use multi-PMT optical module

31 3-inch PMTs in 17-inch glass sphere

each PMT covers a part of the full solid angle sensitive to the direction of the incident

photons

Detector performance studies and

first results

Detector performance studies – the major experimental questions

What is the optimum detector geometrical layout?

What are the trigger/event selection efficiencies?

How and how efficiently can we separate different event classes ?

How can we reconstruct these events and what resolutions can we reach on E

and θ ?

How can we control the backgrounds?

What are the dominant systematic effects and how can we control them?

What precision of calibration is needed and how can it be achieved?

Questions under investigation,

no firm conclusions yet

Detector performance studies – simulation/reconstruction chain

Detector performance studies – the first studied ORCA configuration

Feasibility study started with a detector:

50 strings, 20 OMs each 20 m horizontal distance 6 m vertical distance 1.75 Mton instrumented volume

The following detector performance results are for this example detector

Detector footprint

Detector performance studies – reconstruction efficiency

ν_μ CC events

● reconstructed as upgoing

● reconstructed vertex inside the detector instrumented volume Quality cuts applied for zenith angular resolution comparable to intrinsic ν-μ median angle

No background rejection cuts Effective mass > 1.6Mton for E_ν > 5GeV

Detector performance studies – zenith angle resolution

ν_μ CC events

● reconstructed as upgoing

● reconstructed vertex inside the detector instrumented volume Quality cuts applied for zenith angular resolution comparable to intrinsic ν-μ median angle

No background rejection cuts Zenith angle resolution < 10º for E_ν > 5GeV

Median of zenith angle difference between reconstructed direction and neutrino direction

Detector performance studies – muon energy resolution

Muon energy reconstruction from track length estimation

● ν_μ CC events reconstructed as upgoing with reconstructed vertex inside the detector

instrumented volume

● Projection of selected hits on reconstructed track direction

● Find first/last emission points Muon energy is underestimated for E_μ >~ 10 GeV, due to the muon escaping from the

16% and 84% quantiles as a function of E_μ^true

Sensitivity studies

Sensitivity studies - A toy analysis

- Neutrino interactions generated in detector volume - Require at least 15 PMT hits

- Use true muon direction for zenith (realistic) - Assume 20% Gaussian uncertainty on

- No backgrounds, flavour misidentification etc.

- Assume hierarchy (NH or IH), pick oscillation parameters within experimental uncertainties, generate “toy experiment”

- Perform log-likelihood fit

(free parameters: ), assuming both NH and IH

- Investigate log-likelihood ratio NH/IH

Sensitivity studies - A toy analysis

- Neutrino interactions generated in detector volume - Require at least 15 PMT hits

- Use true muon direction for zenith (realistic) - Assume 20% Gaussian uncertainty on

- No backgrounds, flavour misidentification etc.

- Assume hierarchy (NH or IH), pick oscillation parameters within experimental uncertainties, generate “toy experiment”

- Perform log-likelihood fit

(free parameters: ), assuming both NH and IH

- Investigate log-likelihood ratio NH/IH

Sensitivity studies - Results of toy analysis

• Neutrino vertex in detector volume, true µ direction,

• Distribution of log-likelihood ratio NH/IH for toy experiments

• Experimental determination

of mass hierarchy at 4-5 σ level requires

~20 Mton-years

• Improved

determination of

seems possible

Sensitivity studies - Results of toy analysis

• Neutrino vertex in detector volume, true µ direction,

• Distribution of log-likelihood ratio NH/IH for toy experiments

• Experimental determination

of mass hierarchy at 4-5 σ level requires

~20 Mton-years

• Improved

determination of seems possible

significance

More studies on detector performance

Hadronic shower energy reconstruction (ECAP, HOU)

Atmospheric muon background studies (Bologna & INFN)

Flavour and interaction identification studies (APC, LNS, HOU) Track vs shower discrimination (ECAP)

Trigger studies (Demokritos, ECAP)

Hadronic shower energy reconstruction – Intrinsic variations in Hadronic Showers – Limits (ECAP)

What is the best energy resolution of hadronic showers we can reach (limit)?

If one could separate shower from muon hits (effectively assuming perfect reconstruction) The reference detector is not limited by its density but by physical limits

Shower reconstruction

studies needed

Hadronic shower energy reconstruction (ν

CC) – Estimation of neutrino energy (HOU)

16% and 84% quantiles as a function of E_h^true

ν_μ CC upgoing events with the reconstructed vertex inside the detector

instrumented volume

Muon reconstruction (direction+energy)

Expected number of pes from muon, N_μ

Number of pes from hadronic shower (Ν_tot- N_μ-N_bckgr)

Hadronic energy estimation using

parametrizations of N_pe vs E_h

Hadronic shower reconstructed energy vs the MC true energy

PRELIMINARY

E νreco

PRELIMINARY

Neutrino reconstructed energy vs the MC true neutrino energy

Shower energy reconstruction (ν

) (HOU)

ν_e CC+NC upgoing events with the reconstructed vertex inside the detector

instrumented volume

Neutrino direction and vertex position

reconstructed with the same procedure as ν_μ events

Number of pes from shower (Ν_tot-N_bckgr) Neutrino energy estimation using

parametrizations of N_pe vs E_ν

16% and 84% quantiles as a function of E_ν

Reconstructed neutrino energy vs. the MC true energy

PRELIMINARY

Atmospheric muon background studies (Bologna)

Reject miss-reconstructed atmospheric muons

Take into account reconstructed vertex position (R

) Quality cut based on likelihood value (Λ)

Cut on zenith reconstruction error (β)

1% contamination from atmospheric muons

What are the optimum cuts that

Atm. muons Atm. neutrinos

Under

Flavour and interaction identification studies

Impact of ν

miss-reconstructed as ν

evaluate contamination rate

evaluate impact on Neutrino Mass Hierarchy sensitivity

8.34GeV 9.7GeV

ν

event ν

event

Hit time Hit time

Projection distance from ν vertex along neutrino track

Flavour and interaction identification studies – Track-like vs. shower-like discrimination (ECAP)

Random Decision Forest classification technique (no ⁴⁰K background for these results)

PRELIMINARY

Flavour and interaction identification studies – Identification of ν

CC events by looking for signal from the Michel electron (muon decay secondary) (HOU)

Detected number of photons created by the Michel electron (~40 MeV energy)

● Muon decays inside the detector instrumented volume ORCA detector with 1000 OMs/1.75 Mton

instrumented volume Detector 70 times more dense

Michel electron can give detectable signal (over the ⁴⁰K background) for very dense instrumentations (Optimization?)

Signal too weak to be distinguished from 40K noise

Plans for detector optimization

Full simulation for a detector with 3x3x3 m spacing

“Switch off” some OMs for the optimization study

Ignore shadowing effect in first stage

Advantages:

Allows for studying various configurations

Allows for (non-)

containment/veto studies Concentrate on “premium

events” (with most of produced light contained in the detector) due to CPU time consumption

Detector footprint

111,231 OMs in 2.85 Mton

instrumented volume

Summary & Outlook

• Neutrino telescopes in deep water have demonstrated that low-energy measurements are possible (some 10 GeV).

• Even lower energies could be studied with densely instrumented configurations.

• A determination of the neutrino mass hierarchy with atmospheric neutrinos may be in reach but is

experimentally difficult.

• If possible, this approach will be significantly faster and cheaper than any alternative.

• The first results for a 1.75Mtot detector show that the reconstruction efficiency and angular resolution is

adequate for measurement of the neutrino mass hierarchy.

• Studies for adequate energy reconstruction and

flavour/interaction identification techniques are ongoing,

while the detector optimization is under way

Sensitivity studies – Influence of ν

-ν

miss-identification on sensitivity

ν_μ - perfect detector resolution, no electron neutrino contamination

+ 50% of electron neutrino background misidentified as muon neutrinos,

while 50% of muon neutrinos is rejected. Electron neutrino resolution 1 degree The same as the black line, but with 5 degrees electron neutrino resolution