Muon Capture in Double Chooz

Matthew Strait, on behalf of the Double Chooz collaboration

University of Chicago

5 August 2015

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 1 / 18 Outline

Introduction to Double Chooz Muon capture Search for βn isotopes produced by stopping muons Search for other isotopes produced by stopping muons Future prospects

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 2 / 18 Double Chooz Location Double Chooz Site

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 3 / 18 Double Chooz Detectors Detectors

Three-zone inner detector: Target, Gamma Catcher, Buﬀer . . . surrounded by an Inner Veto . . . and topped with an Outer Veto

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 4 / 18 Double Chooz Neutrino Detection Reactor Neutrino Detection

+ Detectν ¯e via inverse beta decay:ν ¯e p → e n Prompt positron (1–8 MeV) Delayed neutron capture on Gd (8 MeV) or H (2.2 MeV)

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 5 / 18 Muon-induced Backgrounds Muon-induced Backgrounds

13 Hz of muons through the Target and Gamma Catcher Produce backgrounds, including 9Li βn decay Same signature asν ¯e We veto most 9Li using: Number of neutrons following µ Distance from muon to candidate Analysis assumes through-going muons. What about stopping muons? Look for correlation in time and space between stopping muons and 9Li-like events

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 6 / 18 Stopping Muons Muon Capture Physics

A stopped µ− quickly cascades to the 1s atomic orbital, then: High Z, µ− usually captures Low Z, usually decays: capture probability 7.7% for carbon In compounds, no general rule for how much each element captures µ− converts proton to neutron Hydrogen a special case: pµ neutral Often this neutron recoils enough to escape Muon gets picked oﬀ by Remaining nucleus may de-excite through γ, heavier elements n, p, deuteron, α, etc. emission So hydrocarbons can be Typically 15–20 MeV is transferred to the considered pure carbon nucleus, remaining to ν

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 7 / 18 Stopping Muons Possible Signals

In principle, signal can be as much as a seven-fold coincidence:

1 Muon track 2 Muonic x-rays: 1 ns after stop (buried in track’s light) 3 Capture nuclear recoil: ∼2 µs after atomic cascade (severely quenched) 4 γ/charged particles from nuclear de-excitation: usually 1 ns after capture Gammas sometimes visible: low eﬃciency Protons/d/α/etc. severely quenched

5 Capture of neutrons from nuclear de-excitation: 10–100 µs after µ capture 6 β or βn decay of daughter nucleus: 10 ms–10 s after µ capture 7 Capture of neutron from βn decay: 10–100 µs after βn decay

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 8 / 18 Analysis Stopping Muon βn Selection βn Selected Events

Select βn events using pre-existing inverse beta decay analysis Require distance to stopped muon < 300 mm Possible due to high-precision muon reconstruction (NIM A764:330–339, 2014) Eﬃciency determined using common reaction 12C µ− → 12B ν

Double Chooz Preliminary 103 12B Background 102

10 Events/50mm

0 1 2 3 4 5 µ-12B distance (m) 12B production probability known to 4% (Phys.Rev, 133:B663–B675, 1964) So 12B also used to normalize capturing µ− rate ⇒ 1.8 × 105 captures on 12C in 490 days of livetime

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 9 / 18 Analysis Beta-n Decays Selection of βn Decays

Clear excess associated with stopping muons in space and time

16 16 Double Chooz Preliminary 14 Signal 14 12 Background 12 10 10 8 8 Events/bin

6 Events/0.2s 6 4 4 2 2 0 0 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1 1.5 2 2.5 3 50 100 ∆r3 (109 mm3) Time since stopping muon (s) < 0.4 s after a stopping muon < 300 mm from a stopping muon

Flat background =ν ¯e events

Not enough to contribute signiﬁcantly toν ¯e background, but still interesting 9 What is it? Lifetime looks long for Li (t1/2 = 178.3 ms). . .

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 10 / 18 Analysis Beta-n Decays How Much 9Li?

Turns out there are many possible βn isotopes given 12,13C and 16,17,18O 400 kg of oxygen in acrylic vessels plus a bit more in scintillator

βn isotopes

Half-lives range from 8.75 ms (11Li) to 4.2 s (17N) Simultaneously ﬁt ∆t distribution for all using an unbinned likelihood

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 11 / 18 Analysis Beta-n Decays Fit Results

Same plot as before: Zoomed in: 16 Double Chooz Preliminary Double Chooz Preliminary 10 14 Data Data, no neutrons 12 Fit with 1σ error 8 Fit with 1σ error 10 6 8 Events/0.2s

6 Events/0.05s 4 4 2 2 0 0 0 0.5 1 1.5 2 2.5 3 50 100 0 0.1 0.2 0.3 0.4 0.5 Time since stopping muon (s) Time since stopping muon (s) Overall βn signiﬁcance 5.0σ 2.9σ evidence of 8He (119 ms) and/or 9Li (178 ms) 8 9 +0.9 −4 No power to separate He/ Li. Probability/capture: (2.4−0.8 ± 0.1) × 10 No evidence of shorter-lived isotopes Long-lived isotopes included as nuisance parameters with pull terms Data does not signiﬁcantly move their input values.

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 12 / 18 Analysis Beta-n Decays 9Li with a Neutron

Originally reasoned that 9Li should be produced with a neutron Requiring a neutron capture dramatically reduces the background Post-muon neutron eﬃciency is 75%, so expect about 10 9Li events

16 Double Chooz Preliminary 14 Data, 1 neutron 12 Fit with 1σ error 10 8

Events/s 6 4 2 0 0 0.5 1 1.5 2 2.5 3 50 100 Time since stopping muon (s)

Instead ﬁnd about2 (consistent with zero) Plus a longer-lived component, probably 17N

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 13 / 18 Analysis Beta-n Decays 9Li from 13C

What happened? 13C(µ−, να)9Li In retrospect, more reasonable than 12C(µ−, ν p n)9Li 13 +0.9 So probability/capture on C: (2.4−0.8 ± 0.2)% Interpretation is not deﬁnitive, but is in line with other measured isotopes:

Isotope P(µ−, να) 12C 0.59 ± 0.05% Later in this talk 13C2 .4 ± 0.9 % 23Na 1.10 ± 0.15% Nucl.Phys.A 294 (1978) 278 27Al 0.76 ± 0.11% Nucl.Phys.A 294 (1978) 278

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 14 / 18 Analysis Plain Beta Isotopes Other Isotopes: 8Li

With this analysis framework, it is easy to extract other results These are only mildly interesting as ν backgrounds, more so as nuclear physics

Double Chooz Preliminary 250

Data, 0 neutrons 200 Fit with 1σ error 150

Events/0.5s 100 Li 12B 16N 50

0 2 4 6 8 10 12 14 Time since stopping muon (s)

Flat background is accidental single events 12C(µ−, να)8Li probability: (0.59 ± 0.04 ± 0.03)% Never before measured, as far as I can tell In fact, not much at all measured in carbon Inconvenient for beam experiments. Low capture rate means backgrounds in small detectors are a serious problem.

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 15 / 18 Analysis Plain Beta Isotopes Long-lived Isotopes: 15C, 16N, 11Be

Half-lives of 2.4 s, 7.1 s and 13.8 s Major uncorrelated background: 12C(n,p)12B caused by through-going muons Reduced 40% using likelihood analysis, adapting method used for 9Li 16O(µ−, ν)16N probability already measured (11 ± 1%). Used as input. Solid line is ﬁt with only 16N. No evidence for 15C or 11Be.

Double Chooz Preliminary Double Chooz Preliminary 140 0.4 Data Best fit Fit without 11Be, 15C 120 0.3 68% CL Fit with 11Be, 15C Be (%) 11 90% CL 100 0.2

Events/4s 80 0.1 60 Probability of 8Li 40 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 16 Time since stopping muon (s) Probability of 15C (%)

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 16 / 18 Conclusions Results Table – Preliminary

Parent Daughters Probability/capture 12C 6He, 0nX < 0.45% 12C 6He, 1nX < 0.49% 12C 6He, 2nX < 0.08% 12C 6He, 3nX < 0.09% 12C 8B, 4n < 3.4 × 10−5 12C 8Li, α (0.59 ± 0.04(stat) ± 0.03(syst))% 12C 11Be, p < 0.23% natC 8He, X < 7 × 10−4 nat 9 +0.9 −4 C Li, X (2.4−0.8(stat) ± 0.1(syst)) × 10 13 8 +1.1 C Li, nα (5.3−1.0(stat) ± 0.4(syst))% 13 9 +0.9 C Li, α (2.4−0.8(stat) ± 0.2(syst))% 13C 11Li, 2p < 0.6% 13C 12Be, p < 0.24% 13C 12B, n (47 ± 2(stat) ± 4(syst))% 13C 13B < 75% 14N 9C, 5n < 3.7% 16O 14B, 2p < 0.44% 16O 9C, p6n < 0.10% 16O 15C, p < 9% 16O 12N, 4n < 9 × 10−4

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 17 / 18 Conclusions Conclusions

Discovered βn-isotope (probably 9Li) production by stopping µ− Many other reactions with no previous data also investigated Results useful for: Understanding neutrino backgrounds Tests of nuclear models Analysis improvements in the pipeline: Use new measurements of our 13C fraction Decouple from 1964 12B analysis Analysis of gamma lines from 12B excited states Paper in preparation Double Chooz near detector, now online, will collect stopping muons at 20× the rate of the far detector

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 18 / 18 Backup

Backups

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 19 / 18 Backup Comparison of Experiments

Reactor Inner Scint Overburden PMTs/ Power Mass (m.w.e.) detector 140 near Double Chooz 8.4 GWth 2 × 27 t 300 far 390 120 near RENO 16.4 GWth 2 × 30 t 450 far 354 250 near 192 + Daya Bay 17.4 GWth 8 × 40 t 860 far mirrors

For stopping muons, want fewer¯νe and less overburden (to a point) Beneﬁt disproportionately from position resolution Non-βn: RENO-near & DB-near should be similar to DC-near & DC-far

βn: RENO-far probably next best after DC, others swamped byν ¯e

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 20 / 18 Backup Stopping Muon Reconstruction

Tentatively tag stopping muons by low Inner Veto energy

Constrain track length by energy deposition Endpoint resolution: 12 cm — compare to multiple scattering: O(10 cm)

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 21 / 18 Backup 12B with a neutron

Like 13C(µ−, ναn)8Li, we can look at 13C(µ−, νn)12B

104 Double Chooz Preliminary

Data: 0 neutron 103 Data: 1 neutrons

102

Events/10ms 10

1 100 200 300 400 500 Time since stopping muon (ms)

Probability per 13C capture: (47 ± 2(stat) ± 4(syst))% Dominant error is systematic: due to not knowing the 13C fraction very well Highly variable and depends where you got the carbon!

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 22 / 18 Backup 8Li with a neutron

Double Chooz Preliminary 25 Data, 1 neutron 20 Fit with 1σ error 15

Events/0.5s 10

0 2 4 6 8 10 12 14 Time since stopping muon (s)

13 − 8 +1.1 C(µ , ναn) Li probability: (5.3−1.0 ± 0.4)% Also never measured before Higher probability for αn (vs. α) consistent with previously measured isotopes Rate consistent with 14N, 27Al, etc. for (µ−, ναxn)

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 23 / 18 Backup Isotopes produced with lots of neutrons

Previous experiments have shown that multiple neutron emission is: Common in heavy isotopes (e.g. 4n from 209Bi: 5%) Rare in light isotopes — But quantitative results are lacking

Parent Daughters t1/2 Probability/capture 12C 8B, 4n 770 ms < 3.4 × 10−5 16O 12N, 4n 11 ms < 9 × 10−4 16O 9C, p6n 127 ms < 0.10%

These searches are very nearly background-free No signal events selected for any

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 24 / 18 Backup 6He

6He has an intermediate half-life of 0.8 s and low endpoint of 3.51 MeV Can, in principle, be produced with 0, 1, 2 or 3 neutrons:

12C(µ−, ν α d)6He — Unlikely 12C(µ−, ν α p n)6He — Not too bad: can go through 7He resonance 12C(µ−, ν 3He p 2n)6He — Unlikely 12C(µ−, ν 2p 3n)6He — Seems crazy

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 25 / 18 Backup 6He with one neutron

Double Chooz Preliminary

Data 10 Best fit All bkgd Time cut with ﬁt in energy Accidental bkgd Try to extract 6He shape from PMT 1 gamma background Events/0.25 MeV Small excess: 2.7σ signiﬁcance 0.1 If taken as signal, probability per capture is (3.4 ± 1.5) × 10−3 15 2 4 6 8 10 12 0, 2, and 3 neutron cases also 10 checked: no signal 5

Signal - bkgd 0

−5 2 4 6 8 10 12 Energy (MeV)

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 26 / 18 Backup With very high energy β decay: 14B

In general, anything produced with a lifetime similar to 12B is buried by 12B But 14B has a β endpoint of 20.6 MeV and a 12.5 ms half-life. Take a look at the ﬁrst 5 half-lives:

103 Accept

Data 14B MC, arb norm 102

10 Events/0.5MeV

1 5 10 15 20 25 Energy (MeV)

No events: probability of 16O(µ−, ν2p)14B < 0.44% at 90% CL

Matthew Strait (University of Chicago) Muon Capture in Double Chooz 5 August 2015 27 / 18