Search for heavy neutrino in K‾→ µ‾ ν γ decay at ISTRA+ setup

Viacheslav Duk, INR RAS

ISTRA+ collaboration

ISTRA+

IHEP U-70 (Protvino, Russia)

Plan

• LSND/KARMEN/MiniBooNE anomaly and heavy sterile neutrino νh

• Search for νh in kaon decays• ISTRA+ setup• Event selection for K‾→ µ‾ ν γ • Signal extraction• Limits for |Uµh|2

• Conclusions

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Kaon decays: motivation

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Relatively easy to get kaon beamsPossibility to do precise measurements

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Check SM predictionsSearch for NP

3

experiment theory

Low uncertainties in calculations within Standard Model (SM)New Physics (NP) contributions

Motivation for this work

Paper by S.N.Gninenko (INR RAS)Resolution of puzzles of LSND, KARMEN and

MiniBooNE experiments Phys.Rev.D83:015015,2011. arXiv: 1009.5536

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Neutrino oscillations: LSND

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Neutrino oscillations: KARMEN

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Neutrino oscillations: MiniBooNE, neutrino mode

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Neutrino oscillations: MiniBooNE, anineutrino mode

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Above 475 MeV:Event excess475-1250 MeV: 20.9±14.0475-3000 MeV: 24.7±18.0

Below 475 MeV:Event excess200-475 MeV: 18.5±14.3

LSND/KARMEN/MiniBooNE anomalies: summary

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Possible explanation of experimental results (S.Gninenko, INR RAS)

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Origin of excess

Possible explanation (S.N.Gninenko)

New weakly interacting particle νh:• Produced in NC• Mixing with νμ ( must be in CC, e.g. kaon

decays) or separate vertex (may be in NC only)• Decays radiatively via μtr

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Properties of a new particle νh

• m > 40 MeV: no event excess in KARMEN (threshold effect)

• m < 80 MeV: νh production in LSND suppressed by phase space factor for m > 80MeV

• τ(νh) > 10-11 sec: from LEP constraints: BR(Z→ννh) x BR(νh → ν γ) < 2.7 x 10-5

• τ(νh) < 10-9 sec: νh‘s decay within MiniBooNE detector volume

• 10-3 < |Uμh|2 < 10-2 : from event excess in MiniBooNE experiment

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New weakly interacting particle νh

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Decays mostly as νh →νγ

40 MeV < m (νh) < 80 MeV 10-3 < |Uμh|2 < 10-2

10-11 sec < τ(νh) < 10-9 sec

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νh : limits from pion and kaon decays

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Muon energy in 2-body kaon decay

Search for νh in kaon decays

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K→μνh : peak in Eμ (cms) background from K→μνμ insensitive to low masses of νh because of resolution

K→μνh , νh →ν γ: peak in Eμ (cms) signature the same as for K→ µ ν γ no background from K→μνμ

sensitive to low masses of νh

secondary decay vertex

Suitable for ISTRA+

Muon energy in 2-body kaon decay

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ISTRA+ collaboration

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Institute for High Energy Physics, Protvino (IHEP) Institute for Nuclear Research, Moscow (INR) Joint Institute for Nuclear Research, Dubna (JINR)

ISTRA+

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ISTRA+ setup

C1-C4 – thresh. cherenkov counters; S1-S5 – scintillation counters; PC1-PC3 – proportional chambers; SP2 – veto calorimeter; SP1 – lead-glass calorimeter; DC – drift chambers; DT-drift tubes; MH – matrix scintilation godoscopeQFTHEP-2011

T0=S1 . S2 . S3 . S4 . C0 . C1 . C2 . S5 (prescaled by a factor of ~10)

T1=T0 . (∑SP1 > MIP)

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ISTRA+ setup: beam part

C1-C4 – thresh. cherenkov counters; S1-S5 – scintillation counters; PC1-PC3 – proportional chambers; SP2 – veto calorimeter; SP1 – lead-glass calorimeter; DC – drift chambers; DT-drift tubes; MH – matrix scintilation godoscopeQFTHEP-2011

T0=S1 . S2 . S3 . S4 . C0 . C1 . C2 . S5 (prescaled by a factor of ~10)

T1=T0 . (∑SP1 > MIP)

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ISTRA+ setup: decay volume

C1-C4 – thresh. cherenkov counters; S1-S5 – scintillation counters; PC1-PC3 – proportional chambers; SP2 – veto calorimeter; SP1 – lead-glass calorimeter; DC – drift chambers; DT-drift tubes; MH – matrix scintilation godoscopeQFTHEP-2011

T0=S1 . S2 . S3 . S4 . C0 . C1 . C2 . S5 (prescaled by a factor of ~10)

T1=T0 . (∑SP1 > MIP)

vacuum

He

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ISTRA+ setup: magnetic spectrometer

C1-C4 – thresh. cherenkov counters; S1-S5 – scintillation counters; PC1-PC3 – proportional chambers; SP2 – veto calorimeter; SP1 – lead-glass calorimeter; DC – drift chambers; DT-drift tubes; MH – matrix scintilation godoscopeQFTHEP-2011

T0=S1 . S2 . S3 . S4 . C0 . C1 . C2 . S5 (prescaled by a factor of ~10)

T1=T0 . (∑SP1 > MIP)

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ISTRA+ setup: ECAL, HCAL

C1-C4 – thresh. cherenkov counters; S1-S5 – scintillation counters; PC1-PC3 – proportional chambers; SP2 – veto calorimeter; SP1 – lead-glass calorimeter; DC – drift chambers; DT-drift tubes; MH – matrix scintilation godoscopeQFTHEP-2011

T0=S1 . S2 . S3 . S4 . C0 . C1 . C2 . S5 (prescaled by a factor of ~10)

T1=T0 . (∑SP1 > MIP)

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K→µνh (νh→νγ) event reconstruction: primary and secondary vertex for signal

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K

K

μ

μ

νμ

ν

γ

γνh

A

A

B

B

Pγ calculated using A, B

Pγ calculated using A, B:additional energy smearing

Eνh ~ 240 MeV , mνh ~ 40–80 MeV

smearing not crucial

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K→µνh (νh→νγ): primary and secondary vertices

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Zνh - ZK

(Zνh – ZK)/(ZECAL – ZK)

τ=10-11 sec τ=10-10 sec

τ=10-9 sec

τ=10-9 sec

τ=10-10 secτ=10-11 sec

dz, cm dz, cm dz, cm

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K→µνh (νh→νγ): Eγ smearing

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τ=10-11 sec τ=10-10 sec τ=10-9 sec

dE, GeV dE, GeV dE, GeV

dE = Etrue - Emeasured

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K→µνh (νh→νγ): kinematics in kaon rest frame

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νh ν

γ

μ

Eνh ~ 240 MeV , mνh ~ 40–80 MeV Eγ > 50 MeV

kaon decay vertex

Pγ: kaon rest frameP*γ: νh rest frame θ – (γ-νh) angle

cos θμγ ~ (-1)

peak sharper for smaller mh

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*

general caseassumed isotropic

*

K→µνh (νh→νγ) event selection: K→µνγ signature• Track requirements (one primary track, one secondary track,

cuts on track quality)• Veto requirements (no signals above threshold)• Vertex requirements (400 < z < 1600 cm, cut on vertex fit

probability)• Particle ID : Photon: isolated shower in ECAL Muon: 1) MIP in ECAL 2) ADC sum in HCAL < 200 3) relative energy deposition in last 3 layers of HCAL > 0.05

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K→µνγ : decay rate and kinematical variables

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Kinematical variables:x=2*Eγ(cms)/Mk y=2*Eµ(cms)/Mk

3 main terms: IB – dominant SD±, INT± - most interesting (→ Fv , FA)

x

y

IBDalits-plot

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K→µνh (νh→νγ): background rejection and signal observation• Main background:• K→ µ ν γ (Kµ2γ)• K→ µ ν π0 (Kµ3)with 1 gamma lost (from π0→γγ) • K→ π π0 (Kπ2)with 1 gamma lost (from π0→γγ) and π misidentification• Signal observation: peaks in y and cos θμγ where θμγ is the angle between pµ and pγ in kaon rest frame. θμγ peaks at (-1) for signalBackground rejection procedure: scanning over (y, x) Dalits-plot and looking for a peak in cos θμγ

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K→µνh (νh→νγ): (y, x) Dalits plot

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Kµ2γ (MC)

Kµ3 (MC)Kπ2 (MC) X

XX

Y

signal (MC)X

Y

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data

X

X

Y

Y Y

main background: Kπ2

K→µνh (νh→νγ): signal extraction

• (y, x) dalits-plot is divided into stripes with Δx=0.05 width (x-stripes)

• cut on y is put in each x-stripe: 1 < y < 1.2• Simultaneous fit of y and cos θμγ is done in x-stripes

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X

y

signal (MC)

7 x-stripes selected for further analysisin the following region:1 < y < 1.20.2 < x < 0.55

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Possible signature for νh in x-stripes; |Uµh|2=0.01, m=60 MeV, τ=10-10 sec

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Stripe 1: 0.2 < x < 0.25

Stripe 4: 0.35 < x < 0.4

Stripe 7: 0.5 < x < 0.55

cos θµγ

cos θµγ

cos θµγ

Y

Y

Y

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magenta: signalgreen: K→µνγblue: Kμ3red: Kπ2

peak sharper for large x

Possible signature for different masses of νh ; |Uµh|2=0.01 , τ=10-10 sec

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m=80 MeV m=60 MeV m=40 MeV

cos θµγ cos θµγ cos θµγ

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peak sharper for small mh

Possible signature for different lifetimes of νh ; |Uµh|2=0.01 , m=60 MeV

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τ=10-9 sec τ=10-10 sec τ=10-11 sec

cos θµγ cos θµγ cos θµγ

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peak sharper for large τh

Signal efficiency

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mh τ(lab) because of Lorentz boost

low efficiency for small mh

2 effects

mh E(cms)

cut on y (y>1) kills signal

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τ=10-9 sec

τ=10-11 sec

τ=10-10 sec

mνh, MeV

mνh, MeV

mνh, MeV

K→µνh (νh→νγ): simultaneous fit in x-stripes

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fitting cos θμγ and y simultaneously is more reliable

Signal and background shapes taken from MC

magenta – signal, green – Kμ2γ, blue – Kμ3, red – Kπ2

0.4 < x < 0.45

0.3 < x < 0.35

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Y cos θµγ

Y cos θµγ

|Uµh|2 calculation

• BR(νh) measured from

BR(νh)/BR(Kμ2γ)• BR(Kμ2) taken from PDG• BR(Kμ2γ) taken from theory• f(mh) contains chirality flip

and phase space factors

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blue (chirality flip): 1+(mh/mμ)2

red (total): f(mh , mμ)

f(mh , mµ)

f = 1.1 – 1.5

mνh, GeV

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|Uµh|2 calculation

• |Uµh|2 is calculated for each x-stripe

• Nexp(νh)/ Nmc(νh) obtained from simultaneous fit

• Values |Uµh|2 for x-stripes are averaged

• Upper limit is set for averaged |Uµh|2

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Averaging |Uµh|2

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|Uµh|2 = (6.6 ± 3.9)*10-6

m=50 MeV, τ=10-10 sec

X

|Uµh|2

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|Uµh|2

X

1σ interval

|Uµh|2 for τ=10-9 , 10-10 and 10-11 sec

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|Uµh|2|Uµh|2|Uµh|2

mνh, MeVmνh, MeV mνh, MeV

τ=10-9 τ=10-10 τ=10-11

effect does not exceed 2σ

Main sources of systematics

• Fit (shape) systematics• bin size in cos histogram• x-stripe width (bin size in the final fit)• Cut on x (number of x-stripes in the final fit)• Cut on y in x-stripes (study in progress)

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Main sources of systematics

• Fit (shape) systematics• bin size in cos histogram• x-stripe width (bin size in the final fit)• Cut on x (number of x-stripes in the final fit)• Cut on y in x-stripes (study in progress)

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Fit (shape) systematics

• MC shape is not perfect• Errors of simultaneous fit scaled to χ2=1• New |Uµh|2 has larger error • Additional error is treated as shape systematics• Dominant source

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m = 80 MeV τ = 10-10 sec

x

|Uµh|2

|Uµh|2 = (0.9 ± 0.5)*10-5 |Uµh|2 = (0.7 ± 0.8)*10-5

|Uµh|2

x

Main sources of systematics

• Fit (shape) systematics• bin size in cos histogram• x-stripe width (bin size in the final fit)• Cut on x (number of x-stripes in the final fit)• Cut on y in x-stripes (study in progress)

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Bin size in simultaneous (cos histogram) and final (x-stripe width) fits

• Varying bin size in cos histogram: results are compatible

• Varying x-stripe width: results are compatible• No systematics found

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Main sources of systematics

• Fit (shape) systematics• bin size in cos histogram• x-stripe width (bin size in the final fit)• Cut on x (number of x-stripes in the final fit)• Cut on y in x-stripes (study in progress)

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Systematics of a cut on x

• Varying number of stripes in the final fit• Fitting dependency of |Uµh|2 on x• slope multiplied by stripe width gives error

estimation• εsyst < 0.2 εstat

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x-stripe number

|Uµh|2

Setting UL on |Uµh|2

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upper line – total errorbottom line – statistical error only

τ=10-11 τ=10-10

τ=10-9

UL (95% C.L.) UL (95% C.L.) UL (95% C.L.)

mνh, MeV mνh, MeV mνh, MeV

Comparison with Gninenko’s prediction

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blue stripe: predictions from LSND, KARMEN. MiniBoonE

Black line: ISTRA+ upper limit @ 95% C.L.

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mνh, MeVmνh, MeV mνh, MeV

|Uµh|2 |Uµh|2|Uµh|2

Preliminary results• |Uµh|2 < (4-6) x 10-5 (95% CL) for τ=10-9 sec• |Uµh|2 < (1-2) x 10-5 (95% CL) for τ=10-10 sec• |Uµh|2 < (1.5-2) x 10-5 (95% CL) for τ=10-11 sec

• More detailed scan of (m, τ) and study of systematics is in progress

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conclusions Heavy sterile neutrino νh is proposed for LSND/KARMEN/MiniBoone anomaly

explanation:

40 MeV < m(νh) < 80 MeV, 10-11 sec < τ(νh) < 10-9 sec, 10-3 < |Uμh|2 < 10-2

νh can be effectively searched for in kaon decay K→µνh (νh→νγ)

First preliminary limits on |Uµh|2 are obtained from K‾→ µ‾ ν γ decay at ISTRA+ setup:

|Uµh|2 < (4-6) x 10-5 (95% CL) for τ=10-9 sec |Uµh|2 < (1-2) x 10-5 (95% CL) for τ=10-10 sec |Uµh|2 < (1.5-2) x 10-5 (95% CL) for τ=10-11 sec

more detailed study is in progress

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THANK YOU!

Back-up slides

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LSND: oscillation interpretation

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Neutrinos in LSND and KARMEN

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Full list of cuts

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General formula for decay rate

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Taken from: D.Gorbunov, M.Shaposhnikov. JHEP 0710:015,2007

(х, cosθμγ) correlations

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τ=10-9 sec

τ=10-9 sec

τ=10-11 sec

τ=10-11 sec

m=40MeV m=40MeV

m=80MeV m=80MeV

Kπ2Kμ3 cos θ ~ 1/Eγ for large νh mass (similar to π0)

х

х

х

х

х

х

cosθμγ

cosθμγcosθμγ

cosθμγcosθμγ

cosθμγ

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