Slides

Searches for Lepton Number Violation
and resonances in the K→ decays
by NA48/2 at CERN
Karim Massri – University of Liverpool
on behalf of the NA48/2 Collaboration
(email: [email protected])
Outline:
● The NA48/2 experiment
● Theoretical Motivations
● Search for LNV K →∓ decay – Majorana neutrinos
● Search for resonances in K →– decays
● Prospects for the NA62 experiment
51st Rencontres de Moriond (EW session) La Thuile, Italy – 13/03/2016
Introduction – Kaon physics facilities
Kaon Physics Facilities
FNAL:
KTeV
BNL:
E777, E787, E865, E949
CERN:
NA48, NA62
IHEP:
ISTRA+, OKA, KLOD
LNF:
KLOE, KLOE-2
KEK/J-PARC:
E391a, KOTO, TREK
A variety of experimental techniques:
K decay­in­flight (CERN, IHEP, FNAL), stopped K (BNL, JPARC) and a  factory (LNF)
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Introduction – The NA48/NA62 experiments @ CERN
The NA48/NA62 experiments @ CERN
NA62 is the last from a long tradition of fixed­target Kaon experiments in the CERN North Area [G. Ruggiero –  Form Factor and K+→+ at NA62]
History of NA48/NA62 experiments
1997

2001
2002
This talk
NA48 (KS/KL)
Re ε′/ε
Discovery of
direct CPV
NA48/1
(KS/hyperons)
Rare KS and
hyperon decays
2003

NA48/2 (K+/K-)
2004
Direct CPV,
Rare K+/Kdecays
2007
NA62-RK (K+/K-)

2008
RK = Ke2/K2
2015

-
K+→+,
Rare K+ and 0
decays
NA62 (K+)
NA62: currently ~ 200 participants, 29 institutions from 12 countries
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Introduction – The NA48/2 experiment
The NA48/2 detector
Narrow momentum band Kbeams:
PK = 60 GeV/c, PK/PK ~ 1% (rms)
Nominal Kdecay rate: ~ 100 kHz
Main triggers: 3­track vertexK±→±
Simultaneous K+/K– beams
22% of kaons decay in 114m­long vacuum tank
upstream the detector
Principal sub­detectors:
Spectrometer (4 DCHs)
p/p = 1.02%  0.044% p(GeV)
4 views/DCH: redundancy→efficiency
● Scintillator Hodoscope
Fast trigger, time measurement t ~ 150 ps
● LKr EM calorimeter
High­granularity, quasi­homogeneous
E/E = 3.2%/√E(GeV) 9%/E(GeV)  0.42%
●
Helium-filled tank
x = y = 4.2mm/√E(GeV)  0.6mm (1.5mm @ 10 GeV)
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Beam
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Theoretical Motivations – Majorana Neutrinos
Majorana Neutrinos
Asaka­Shaposhnikov model (MSM) [Asaka and Shaposhnikov, PLB 620 (2005) 17]:
Dark Matter + Baryon Asymmetry of the Universe (BAU) + low mass of SM 
can be explained by adding three sterile Majorana neutrinos N i to the SM
Gorbunov & Timiryasov, PLB 745 (2015) 29
N1 is the lightest O(keV) → Dark Matter candidate
● N , N are nearly degenerate (100 MeV to few GeV)
2
3
to tune CPV­phases and extra­CKM sources of baryon asymmetry. N2, N3 produce standard neutrino masses through seesaw with a Yukawa coupling of ~ 10 ­8
●
Active­sterile neutrino mixing (U­matrix):
Effective vertices involving the sterile neutrinos N i,
the W, Z bosons and SM leptons
This talk 
[ℓ =  N2,3 production in K decays:
K →ℓ  N, K→0ℓN, …
N2,3 decays for m2,3< mK–mℓ:
BR(K→NBR(N→∓) ~ |U|4
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N→ ℓ ∓, N→0
N→ℓ1ℓ2∓2, N→1ℓ2+ℓ2–
N→ℓ 
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Theoretical Motivations – Inflatons
Inflatons
Shaposhnikov­Tkachev model [Shaposhnikov and Tkachev, PLB 639 (2006) 414]:
MSM + a real scalar field (inflaton ) with scale­invariant couplings
Explains Universe homogeneity and isotropy on large scales/structures on smaller scales
­Higgs mixing with mixing angle 
● ­Higgs coupling → Universe reheating
●  is unstable:  ~ (10­8­10­12) s
Bezrukov and Gorbunov, PLB736 (2014) 494
●
Bezrukov and Gorbunov, PLB736 (2014) 494
This talk
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 in Kaon decays [m < 354 MeV/c2]
2|p⃗χ| 2
BR( K ± → π± χ)=1.3×10−3 (
)θ
MK
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Search for resonances in K→ decays – Same-sign muons sample
The NA48/2 same­sign muons sample (LNV)
Basic principles of the searches:
● Fully reconstructed final states, 3­track vertex topology
● Similar topology to normalisation channel K ±→±

DCH1 DCH2
→First­order cancellation of systematic effects (trigger inefficiency, etc)
Search for Lepton Number Violation – Majorana neutrinos
● Method: exclusive search for the K →∓ decayN
4
DCH3 DCH4


B
p mis-reconstruction
Main background: K±→ ±   with 2  ±→ ± decays (one within the Spectrometer)
● Sensitivity: UL on BR(K→∓)N
4
1
●
Sensitivity: UL on BR(K→N4BR(N4→∓) ~
N K∗Acceptance
K decays in the fiducial volume: NK ~ 21011
(from reconstructed K±→± decays)
Mass and lifetime dependence of the signal acceptance from dedicated MC simulation of the signal
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Search for resonances in K→ decays – Same-sign muons sample
The same­sign muons selection (LNV)
Blind analysis: K→∓ selection based on o K→∓ MC simulation
2
− Uniform phase­space (|M | = 1)
fi
− Resonant Majorana neutrino model
o K±→± MC simulation (10 10 events)
o Control Region: M(∓) < 480 MeV/c2
● Event selection:
o One well­reconstructed 3­track vertex
o 2 same­sign muons, 1 odd­sign pion
o Total P consistent with zero
T
●
o
Signal Region: |M(∓) – MK| < 5 MeV/c2
Expected background: Additional K±→± MC sample (1010 events) used to evaluate number of expected K±→± events in Signal Region
●
Events in Signal Region observed after finalising K →∓ selection → Nobs = 1
Expected background (from MC simulation): N exp = 1.163±0.867stat±0.021ext±0.116syst
Rolke­Lopez statistical treatment to get UL(N sig)→ BR(K→ ∓  ) < 8.6 10-11 @ 90% CL
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Search for resonances in K→ decays – Opposite-sign muons sample
The NA48/2 opposite­sign muons sample (LNC)
Basic principles of the searches:
● Fully reconstructed final states, 3­track vertex topology
● Similar topology to normalisation channel K ±→±
→First­order cancellation of systematic effects (trigger inefficiency, etc)
Search for resonances in K→   – decays
● Method: exclusive search for the decay chains K→N (N →∓), K→X(X→–)
4
4
Main background: K→   – (irreducible) → Limited sensitivity
● Sensitivity: UL on BR(K→N )BR(N →∓)
√ BR( K ± →π± μ+ μ – )
4
4
●
Sensitivity: UL on BR(K→X)BR(X→–) 13/03/2016
~
√ N K∗Acceptance
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√
σ( M res )
mK −(mπ +2 mμ )
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Search for resonances in K→ decays – Opposite-sign muons sample
The opposite­sign muons selection (LNC)
Event selection:
o Minimal changes with respect to same­sign
o One well­reconstructed 3­track vertex
o 2 opposite­sign muons, 1 pion
o Total P consistent with zero
T
●
o
Signal Region: |M() – MK| < 8 MeV/c2
3489 K±→± candidates in Signal Region
K±→± background: (0.36 ± 0.10)%
To be scanned searching for peaks in M(∓) and M() invariant masses
Improved selection with respect to previous NA48/2 K±→± analysis [PLB 697 (2011) 107]
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Search for resonances in K→ decays – Mass scan framework
The mass scan framework
Basic principles:
● Based on selected K → candidates. Variable step = 0.5(M
) and window =  2(Mres)
res
● For each M
: Observed events in data (Nobs) vs Expected events from MC (Nexp) → UL(Nsig)
res
● Rolke­Lopez statistical treatment used in each mass hypothesis M
to get UL(Nsig)
res
Search for Lepton Number Violation – Majorana neutrinos
● 284 mass hypotheses M
tested
res
●
2 possibilities in building M(∓) [same­sign s]: closest invariant mass to Mres considered
Search for resonances in K→   – decays
● The distributions of both invariant masses M(∓) and M() are probed
● 267 hypotheses for M(∓)
● 280 hypotheses for M()
● K→– MC simulation uses form factors extracted from the selected data sample to obtain best data/MC agreement
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Search for resonances in K→ decays – Final results
Search for K→ N4(N4→ ∓ ) decays
Same­sign muons sample (LNV)
UL(BR ( K ± →μ ± N 4 )BR (N 4 → π ∓ μ ± ))=
UL(N sig )
N K∗Acceptance
Acceptance ~ 1/ for > 1ns
Statistical significance
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z=
N obs −N exp
σ(N obs )⊕σ (N exp )
never exceeds +3: no signal observed
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Search for resonances in K→ decays – Final results
Search for K→ N4(N4→  ∓) decays
Opposite­sign muons sample (LNC)
±
±
±
∓
UL(BR( K →μ N 4 )BR (N 4 →π μ ))=
UL(N sig )
N K∗Acceptance
Acceptance ~ 1/ for > 1ns
Statistical significance
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z=
N obs −N exp
σ(N obs )⊕σ (N exp )
never exceeds +3: no signal observed
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Search for resonances in K→ decays – Final results
Search for K→ X(X→  –) decays
Opposite­sign muons sample (LNC)
±
±
+
–
UL(BR( K → π X ) BR( X →μ μ ))=
UL( N sig )
N K∗Acceptance
Acceptance ~ 1/ for > 1ns
Statistical significance
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z=
N obs −N exp
σ(N obs )⊕σ (N exp )
never exceeds +3: no signal observed
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The NA62 experiment – Prospects
Prospects for the new NA62 experiment
NA62 will collect the world­
largest K+ decay sample: ~ 1013 decays in 3 years of data taking
(~ 50 times more than NA48/2)
Kaon and  0 LNFV decays
Mode
UL at 90% CL Experiment
NA62 acceptance*
K+→++e−
1.3 × 10−11 BNL 777/865
~ 10%
K+→+–e+
5.2 × 10-10 BNL 865
~ 10%
K+→e+
5.0 × 10-10 BNL 865
~ 10%
K → e e
6.4 × 10
K+→    +
8.6 × 10-11 NA48/2
K+→e+e+
2.0 × 10-8
+
 + +
K+→e++
0→e
-10
BNL 865
Geneva Saclay
no data
3.6 × 10-6
~ 5%
~ 20%
~ 2%
~ 10%
KTeV
~2%
0→e
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* From fast MC with flat phase­space distribution.
Single­event sensitivity:
1/(NK × acceptance)
This talk
NA62 Sensitivities:
~10−12 for K+ decays
~10−11 for 0 decays
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Conclusions
Conclusions
The NA48/2 experiment at CERN was exposed to ~ 21011 K decays in 2003­2004
New NA48/2 results presented for the first time today:
− Search for LNV K → ∓     decay:
●
●
●
−
Search for K→ N4(N4→ ∓ ) decays [Majorana neutrinos]
●
−
Limits on BR products of the order of 10­9 for neutrino lifetimes < 100 ps
Search for K→ X(X→  –) decays [Inflatons, …]
●
●
Limits on BR products of the order of 10­10 for neutrino lifetimes < 100 ps
Search for K→ N4(N4→  ∓) decays [LNC heavy neutrinos]
●
−
BR(K→ ∓  ) < 8.6 10­11 @ 90% CL
[World Best Limit]
Factor of 10 improvement with respect to previous best limit [1.1  10­9 @ 90% CL]
Limits on BR products of the order of 10­9 for resonance lifetimes < 100 ps
Prospects for the new NA62 experiment:
● Major beam and detector upgrades for K +→+ improved performances
● NA62 will collect the world­largest K + decay sample (~ 1013) in 3 years of data taking
● Potential sensitivities ~ 10 ­12 for K decays, ~ 10­11 for 0 decays
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