Low-energy (anti)neutrino physics with Borexinonow/now2014/web-content/TALKS/bTue/Plen/Mosteiro.pdf · Neutrinos from the primary proton-proton fusion process in the Sun Low-energy

Pablo Mosteiro, Princeton University*(Borexino Collaboration)

NOW – September 2014

Low-energy (anti)neutrino physics with Borexino

*Now at INFN-Roma1

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Neutrinos from the primary proton-proton fusion process in the Sun

Low-energy (anti)neutrino physics with Borexino

Nature 512, 383-386 (28 August 2014)

Pablo Mosteiro, Princeton University*(Borexino Collaboration)

NOW – September 2014

*Now at INFN-Roma1

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Solar Neutrinos

pp chain CNO cycle

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Solar neutrino spectrum

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The Borexino detector

[McCarty 2006]

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Scintillation

Ydet and kB are detector parameters

Photoelectrons picked up by photomultipliers (PMTs)

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7Be neutrinos in Borexino

[Bellini et al 2011]

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Other Borexino measurements

??



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Why search for pp neutrinos?

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[Los Alamos Science, 1982]

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Data acquisition

Threshold K=20-25 Position

reconstruction

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Energy estimators

Offline, each trigger is analyzed to look for clusters, i.e., scintillation event candidates

After the beginning of each cluster, count the number of PMTs hit within time ДT=230ns

Multiple photons on same PMT count as 1 hit

Analytical formula for variance as well(variations in time, event position, Poisson fluctuations)

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Neutrinos and backgrounds

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Data selection: cuts

Remove muon-related eventsSelect events within r<3 m; |z|<1.7 m (Fiducial Volume)

Other technical, detector-specific cuts

~450 keV

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pp neutrinos: challenges

pp end point 420keV (recoil energy < 261keV; PMT dark noise)

14C beta-decay spectral shape (end point 156keV)

14C pile-up

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Challenge 1: dark noise

Trigger gate start

16.5 us

…

230 ns

Count number of PMTs hit in each of these windows

Trigger gate end

We can estimate the contribution from dark noise by using a new variable based on random triggers

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Trigger gate start

16.5 us

…

Count number of PMTs hit in each of these windows

Trigger gate end

Spectrum in that variable

Challenge 1: dark noise

230 ns

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Challenge 2: 14C rate

When a trigger contains two events, the second one is not subject to trigger threshold; so the shape is preserved better.

Result used to constrain rate of 14C in final fit

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Challenge 3: 14C pile-up

Rate of 14C in scintillator → RC ~ 40 Bq/100 t

Borexino mass → M = 300 t

Rate of 14C pile-up → (M RC) * RC * 230 ns ~ 100 cpd/100 t

Expected rate of pp ~ 130 cpd/100 t

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Pile-up may come from 14C but also from other detector events

Synthetic pile-up: overlap uncorrelated data with regular events

Result used to constrain rate of pile-up in final fit

Trigger gate start

…

230 ns

Trigger gate end

After-pulsingUncorrelated

data

Challenge 3: 14C pile-up

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Fit results

1) Calculate energy estimator, position, etc, for all events

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Fit results

1) Calculate energy estimator, position, etc, for all events2) Apply cuts

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Fit results


3) Introduce dark noise

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Fit results


3) Introduce dark noise4) Constrain 14C

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Fit results



5) Constrain pile-up

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Fit results



5) Constrain pile-up6) Perform spectral fit

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Fit results

Nature 512, 383-386 (28 August 2014)

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Fit results

Nature 512, 383-386 (28 August 2014)pp = 144 ± 13 (stat) cpd/100 t29

Alternative fitting method: full-spectrum convolution

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Main sources of uncertainty

Pile-up

85Kr rate

Fiducial Volume

Energy Estimator

Synthetic vs. Convolution

Statistics vs. Background

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Systematics estimation

Values obtained by varying fit conditions.The distribution shown is peaked at ~144 cpd/100 t.

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Final resultpp detection rate: 144 ± 13 (stat) ± 10 (syst) cpd/100 t

HM-SSM + LMA-MSW: 131 ± 2 cpd/100 t

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Interpretation 1:Survival probability measurement

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Interpretation 2:pp neutrino flux measurement

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Check the time stability of the Sun (time scale 105 years), which is a crucial assumption in the Standard

Solar Model

Interpretation 3:Solar (in)variability


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the Borexino Collaboration

UMass Amherst

Milano

Perugia

Princeton

Virginia Tech

Genova

JINR Dubna

HeidelbergMünchen

Kurchatov Moscow

Jagiellonian Kraków

St. Petersburg

Paris

Hamburg

Gran Sasso

Houston

Los Angeles

Moscow

Mainz

TU DresdenNapoli

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Thank you!

Thank you all for listening and please let me know if you have any questions

Thanks to the Borexino collaboration; in particular, to the pp working group

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Back-up slides

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Maximum pp-induced electron recoil kinetic energy

ν (E,p)e (m,0) e (ER,k)

ν (E’,p’)

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Why two nylon spheres

[McCarty 2006]

1) Convection2) Diffusion

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Spectral fitter

Versatile tool for fitting multiple species in multiple configurations, within the Borexino detector

Validated against known Monte Carlo simulations and data

Takes into account energy response, detector geometry, etc

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• Why 230 ns?

• It is the minimum time needed to separate two clusters, below which the characteristic time of the scintillator (~100 ns) makes it very hard to tell apart.

• More time, more dark noise, and not so much scintillation light.

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Solar (in)variability


The solar neutrino problem could be explained by the MSW effect, but alternatively it could be explained by

variations in the solar photon luminosity in the past 105

years.

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• Event at given position and energy -> what is distribution of npmts_dt1 variable?

107 MC events at center

Scintillation line shape

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Simulation Validation

Using GEANT4 and a detailed Borexino-specific package for geometry and detector effects

Source: 139Ce (166keV gamma)

Location: 12cm from center

kB=0.0099 cm/MeV

kB=0.0104 cm/MeV

kB=0.0109 cm/MeV

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Pile-up check against simulations

Assuming that most pile-up comes from 14C overlapping with itself

Generate sample of 14C events and randomly overlap events with a time displacement equal to 1/(14C rate)

Rate consistent with estimate from “synthetic method”

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Checks: 210Bi spectral shape

No effect48

Parent Daughter Decay Energy Half LifeMode [MeV]

238U 234Th ↵ 4.27 4.47⇥109 yr234Th 234Pa � 0.273 24.1 d234Pa 234U � 2.20 6.70 hr234U 230Th ↵ 4.86 2.45⇥105 yr230Th 226Ra ↵ 4.77 7.54⇥104 yr226Ra 222Rn ↵ 4.87 1.60⇥103 yr222Rn 218Po ↵ 5.59 3.82 d218Po 214Pb ↵ 6.12 3.10 min214Pb 214Bi � 1.02 26.8 min214Bi 214Po � 3.27 19.9 min214Po 210Pb ↵ 7.88 0.164 ms210Pb 210Bi � 0.0635 22.3 yr210Bi 210Po � 1.43 5.01 d210Po 206Pb ↵ 5.41 138 d206Pb stable

238U chain: 214Bi-Po coincidences

Efficiency: 89% [Bellini et al 2014]

Diffusion

Long half-life

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Direct? Real-time?

Direct: the measurement is made by detecting electron recoils triggered by pp neutrinos. Indirect measurement is made by measuring other neutrinos, then inferring the pp neutrino flux by assuming the Standard Solar Model is valid.

Real-time: the neutrinos are detected one-by-one, via their interactions with electrons, and their energies can be inferred. Other experiments are not able to count individual events and integrate over a range of energies.

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Systematics: 85Kr

Effect ~ 7%

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Systematics: Fiducial Volume

Effect ~ 8%

Some freedom in choosing the fiducial volume

Smaller volume will have reduced statistics

Larger volume will include radioactivity from the vessels and end caps

But some variation should be tolerated

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Systematics: Energy Estimator

Effect ~ 8%

Entire analysis can be re-done with a time window of 400 ns, instead of 230ns

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Synthetic pile-up: uncorrelated hits

# of hits in tail consistent with dark rate

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Checks: 14C rate

Rate = 40.0 ± 0.7 cpd/100 t55

Checks: soft α/β cut

Bellini et al 2014

Data

Pile-up

No effect56

Checks: kB, binning

No effect57

Stainless Steel SphereExternal water tank

Nylon Inner VesselNylon Outer Vessel

Fiducial volume

InternalPMTs

Scintillator

Buffer

WaterRopes

Steel platesfor extrashielding

Borexino Detector

MuonPMTs

P>1 = 0.004

Checks: threshold for photoelectron detection

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Solar abundance problem

Low-metallicity model [Asplund, Grevesse, Sauval & Scott 2009] → more recent calculations, inconsistent

with observations

High-metallicity model [Grevesse & Sauval 1998] →

outdated calculations, consistent with observations

Difference in pp flux is ~1% → out of reach for

Borexino at present59

Solar abundance problem

With 14C rate of ~40 /s/100 t, and scintillator time constant of ~100 ns [Bellini et al 2014], must separate pile-up events by hit time profiles (soft α/β cut).

Considerable work on simulations to reproduce data very accurately at low energies.

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Checks: 87Rbβ emitter with Q-value of 283.3 keV

Endpoint of pp-induced e- recoil spectrum: 261 keV

Assumption: relative abundances as in crust (both alkali metals)

A(40K) < 0.4 cpd/100 t [Bellini et al 2010]

A(87Rb) < 0.1 cpd/100 t61

Checks: 87Rb (continued)Recent measurement of purified NaI has 87Rb enriched by

~200 with respect to 40K [Calaprice et al unpublished]

A(40K) < 0.4 cpd/100 t [Bellini et al 2010]

A(87Rb) < 0.1 cpd/100 t x 200 = 20 cpd/100 t

Δpp=8 cpd/100 t~6%

Not included because our purification is

done in liquid

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Low-energy (anti)neutrino physics with Borexinonow/now2014/web-content/TALKS/bTue/Plen/Mosteiro.pdf · Neutrinos from the primary proton-proton fusion process in the Sun Low-energy