1. Kaons Physics at CERN: an overview of NA48 and NA62 results
Giuseppina Anzivino
University of Perugia and INFN
on behalf of the NA62 and NA48/2 Collaborations
The 7th International Symposium on Science
Antananarivo, Madagascar, September 17-22, 2015

2. Outline Introduction: why Kaons?
Kaon Physics at CERN NA48/n & NA62: brief history
Tests of SM and search for New Physics
Measurement of RK
Recent result
The K± → p±p0g* → p±p0e+e- decay
Summary and Outlook
Giuseppina Anzivino

3. «minimal flavour laboratory»
Introduction
KAON
the lightest kind of flavoured matter
studied since 60’s to test fundamental properties of nature
Discovered in cosmic rays
The birth of flavour physics puzzle
The first failure of a discrete symmetry
The matter-antimatter asymmetry
K0 → p+ p-
Kaons exhibit a very rich (actually qualitatively the richest among mesons) phenomenology
«minimal flavour laboratory»
Giuseppina Anzivino

4. The K mesons Three description (bases) for K0 states I3=+½ I3=-½
They are produced by strong interaction
They decay by weak interaction in non-strange particles and/or leptons with lifetimes ~ s
Strangeness eigenstates
Three description (bases) for K0 states
CP eigenstates
mass eigenstates in vacuum
“…. there is scarcely a physical system which contains so
many of the elements of modern physics” (V. Fitch, 1980)

5. ε A short reminder TRV K0↔ K0 What we have learned from kaons?
CP Violation in the neutral K system e
Direct CP Violation in K0→ 2p e’/e
T Violation
CPT invariance → NO CPT Violation
(mixing & decay)
CP violation in Charged Kaons
no CP at the level of 10-4
CP search in Kaon radiative decays
Search for LFV
a lot more……
TRV
K0↔ K0
CPLEAR
Is there still something to be learned by using kaons?
Giuseppina Anzivino

6. precision frontier complementary to LHC energy frontier
High energy scales
Flavour sector probing extremely high energy scales:
precision frontier complementary to LHC energy frontier
Study processes suppressed in SM, sensitive to New Physics
RARE DECAYS
Four main reasons
study explicit Violations of SM, such as LFV
probe the flavour sector by means of FCNC
test of fundamental symmetries such as CP and CPT
study of strong interaction at low energy in exclusive processes
Giuseppina Anzivino

7. NA48/n history NA48 NA48/1 NA48/2 NA62-I NA62 1997 1998
1999
2000
2001
2002
2003
2004 :
2005
2007
2008
2014
2018
NA48
Main goal: Search for direct CPV
Measurement of e’/e
Beams: KL + KS
NA48/1
Main goal: Rare KS decays and
hyperon decays, CPV tests
Beams: KS
NA48/2
Main goal: Search for direct CPV
Charge asymmetry measurement
Beams: K+ + K-
NA62-I
RK measurement
NA62
Main goal: Measurement
of the decay K+ -> p+nn
Beam: K+
Giuseppina Anzivino

8. CERN
NA62
Giuseppina Anzivino

9. Theoretical predictions (SM)
Measurement of e’/e
Double Ratio Technique, final NA48 result
Re(e'/e) = (14.7 ± 2.2) x 10-4
Phys. Lett. B544 (2002) 97
Theoretical predictions (SM)
Expect 20% result in 3 years from now (*)
(*) choose your own favourite “now”
Measurement compatible with SM
Large hadronic uncertainty in the calculations
Improvement expected with lattice QCD
Consistent with SM? No! Yes! Maybe…
PDG World Average: (16.6 ± 2.3)x10-4
Giuseppina Anzivino

10. CP violation in K± 3 decays
Primary NA48/2 goal: Measure slope asymmetries
in “charged” and “neutral” modes with high accuracy (few 10-4)
|M(u,v)|2 ~ 1 + gu + hu2 + kv2
Measure Dg to extract the asymmetry Ag
Eur.Phys.J, C52 (2007) 875
Charged mode (3.11 x 109 selected K±→ p± p+ p-)
Ag = (-1.5 ± 1.5stat ± 0.9trig ± 1.1syst) x 10-4 = (-1.5 ± 2.1) x 10-4
Neutral mode (9.13 x 107 selected K± → p± p0 p0)
Ag = (1.8 ± 1.7stat ± 0.9syst) x 10-4 = (1.8 ± 1.8) x 10-4
No evidence for direct CP violation at the order of 10-4
Giuseppina Anzivino

11. From NA48/2………..
K±+-e± n [EPJC 70 (2010) , PLB 715 (2012) , EPJC 54 (2008) ]
determination of the s-wave pp scattering lengths
BR and form factors measurement
K±±0 [EPJC 64 (2009) 589]
00 invariant mass shows a cusp-like structure in the region M00 = 2 m
K±±0g [EPJC 68 (2010) 75-87]
the first measurement of the DE and INT terms
access to chiral parameters
K±±gg [PLB 730 (2014) (NA48/2); PLB 732 (2014) (NA62)]
BR measurement and access to chiral parameters
K±±ge+e [PLB 659 (2008) ]
first BR measurement and first determination of some chiral parameters
K±±e+e-, K±±m+m [PLB 677 (2009) , PLB 697 (2011) ]
access to some chiral parameters
K±00e± n [JHEP 1408 (2014) 159]
determination of the s-wave scattering length
K±±0e+e-
NEVER OBSERVED
Giuseppina Anzivino

12. Test of m-e universality
………..towards NA62
Test of m-e universality
Measurement of RK
Giuseppina Anzivino

13. RK - Detector and data taking
Kaon momentum
74 ± 1 GeV/c
Momentum kick
265 MeV/c
Data set:
NA62-RK: four months in
Main detector components:
Magnetic spectrometer (4 DCHs):
4 views: redundancy  efficiency
σ(p)/p 1.0% % p [GeV/c]
Hodoscope: fast trigger and
precise time measurement (150ps)
Liquid Krypton e.m. calorimeter:
High granularity, quasi−homogeneous
σ(E)/E = 3.2%/√E + 9%/E % [GeV]
Hadron calorimeter, photon vetos,
muon veto counters
Same detector used by NA48
Giuseppina Anzivino

14.
Giuseppina Anzivino

15. RK in the SM measure the ratio
Leptonic decays of light pseudoscalar mesons
not directly usable due to hadronic uncertainties
measure the ratio
hadronic uncertainties cancel
RK very well predicted within the SM, well below 10-3
Ke2 strongly helicity suppressed (V-A coupling)
enhanced sensitivity to non-SM effects
[V. Cirigliano and I. Rosell, Phys. Rev. Lett. (2007) ]
helicity suppression ~10-5
radiative corrections
radiative corrections RQED (few %) due to the IB part of the radiative decay
are included in RK definition and well computed in the SM
Giuseppina Anzivino

16. RK beyond the SM
In several SM extensions, the presence of LFV terms introduces extra contribution to the SM amplitude, enhancing the decay rate
2HDM – tree level
K± l±n can proceed via charged Higgs, H±,
exchange does not affect RK
2HDM – one-loop level
Dominant contribution to RK: H± mediated
LFV (rather than LFC) with emission of nt
RK enhancement can be experimentally accessible
[Fonseca, Romao and Teixeira, EPJC (2012) 2228]
[Masiero, Paradisi, Petronzio, PRD 74, 2006]
[Lacker and Menzel, JHEP (2010) 006]
[Abada et al., JHEP 1302 (2013) 048]
Giuseppina Anzivino

17. RK - Measurement Strategy
Ke2 e Kμ2 candidates collected simoultaneously
measurement independent of Kaon flux
many systematic effects cancel in the ratio, at first order
MC simulation used only to limited extent
acceptance correction (geometry)
correction for background from energetic bremsstrahlung by muons
Counting events in track momentum bins
N(Kl2) = number of selected
kl2 candidates
NBG(Kl2) = number of background events
D = downscaling factor (m)
A(Kl2) = geometric acceptance (MC)
e(Kl2) = trigger efficiency
f l = lepton ID efficiency
fLKr = global LKr read-out efficiency
Giuseppina Anzivino

18. Ke2 and Km2 selection Ke2 and Kμ2 collected simoultaneously
1 track in the spectrometer
common geometrical acceptance
decay vertex well reconstructed
track momentum 13 < p < 65 GeV/c
Ke2 and Kμ2 separated by
Kinematics
2 body decay M2miss = (pK-pl)2
< Mmiss2 < 0.015
Particle ID (E/p)
electron (0.9 < E/p < 1.1)
muon (E/p < 0.85)
Giuseppina Anzivino

19. RK data Ke2 145.958 events Km2 42.817 M events Ke2 candidates
Km2 candidates
Ke2
events
Background sources
Km2 (5.64±0.20)%
Km2 (m -> e) (0.26±0.03)%
Ke2g (SD) (2.60±0.11)%
Ke3 (D) (0.18±0.09)%
K2p (0.12±0.06)%
Opp. sign K (0.04±0.02)%
Beam Halo (2.11±0.09)%
Total (10.95±0.27)%
Km2
M events
with pre-scaled trigger
main background:
beam halo (0.50±0.01)%
Electron ID efficiency:
(99.28±0.05)%
Giuseppina Anzivino

20. RK final result
Phys Lett B 719 (2013) 326
World Ke2 statistics increased by a factor 10
In agreement with SM expectation, within 1.2 s
Motivation for improved precision RK measurements
World average
RK x 105
precision
PDG 2008
2.447 ± 0.109
4.5%
2014
2.488 ± 0.009
0.4%
Prospects: 1 M events (downscaled trigger) expected, statistical uncentainty ~ 0.1%
Total uncertainty expected ~ 0.2%
Giuseppina Anzivino

21. Test of Chiral Perturabation Theory First observation of the decay
K± p± p0 e+e-
Giuseppina Anzivino

22. The ChPT framework
The Chiral Perturbation Theory (ChPT) is the effective field theory of Quantum Cromodynamics (QCD) at low energies
It is the ideal framework to describe Kaon decays
The basis DS = 1 up to O(p4) chiral Lagrangian can be written as a combination of coupling factors
there are 37 Ni coefficients and Wi operators, poorly known!
Combinations of such coupling are accessible by measuring Kaon Branching Ratios and Form Factors
NA48/2 can access all charged Kaon decays with high precision
G. D’Ambrosio PoS(EFT09)061
Giuseppina Anzivino

23. The K± p± p0g*  p± p0 e+e- decay
The g* is produced by two different mechanisms: Inner Bremsstrahlung (IB) and Direct Emission (DE) (DE has electric and magnetic components)
It offers opportunities for chiral theory tests:
Electric Interference (ΓBΓE) can confirm the discrepancy in sign with the theoretical prediction observed by NA48/2 in K± p± p0 g [EPJC 68 (2010) 75-87]
Possibility to extract the sign of the magnetic term ΓM (impossible to extract in K± p± p0 g)
Charge asymmetry not contaminated by indirect CP violation (as in K0)
Never observed so far!
Giuseppina Anzivino

24. Data and analysis Data collected in 2003 Selection:
3 reconstructed tracks coming from one decay vertex
Electron/pion separation using E/P
2 reconstructed g clusters giving the p0 mass
Pe > 2 GeV/c
P (ppee) within 56 and 64 GeV/c
Main sources of background:
K± p± p0D (g) (K2pD), with an extra photon (accidental or radiative)
K± p± p0p0D (g) (K3pD) with a merged or lost g
Giuseppina Anzivino

25. Data sample and background estimate
preliminary
K2pD |meeg - mp0| > 5 MeV
K3pD mpp > 120 MeV
< Mppee < (GeV/c2)
Total number of ppe+e- candidates
BG candidates from K2pD ±5.1
BG candidates from K3pD ±5.5
Giuseppina Anzivino

26. Normalization channel
For normalization use the decay K± p± p0D (g), collected with the same trigger
Very large sample with known BR (PDG)
This gives the total number of Kaon decays:
Giuseppina Anzivino

27. Branching ratio measurement
The available statistics is not sensitive to DE and INT
A model dependent BR has been computed using a total acceptance in which the relative weight of the 3 components are obtained from Cappiello et al. [Eur. Phys. J. C (2012) 72:1872)]
Branching Ratio is computed as
Quantity
Value
Nppee
1916
NBG
55.8±7.4
Nppee - NBG
1860±51
Kaon flux
(7.97±0.25) x 1010
Total Acceptance
(0.583±0.0019)%
Trigger efficiency
(98.7±0.65)%
Giuseppina Anzivino

28. Branching Ratio Result
The preliminary BR value is
to be compared with (G. D’Ambrosio, private communication, unpublished)
preliminary
preliminary
Compatible with theoretical prediction without radiative corrections
Radiative corrections included in the NA48/2 simulation using PHOTOS
Eur. Phys. J. C (2012) 72:1872
NA48/2
2003 data
G. D’Ambrosio, O. Catà
private communication
Giuseppina Anzivino

29. Summary and outlook
Precision physics complementary to high-energy approach for NP search
Fundamental contribution to CP Violation studies (NA48 and NA48/2)
in neutral Kaon system measurement of e’/e
in charged Kaons measurement of charge asymmetry
Test of LFV The measurement of the RK at NA62
result in agreement with SM expectation, within 1.2 s
motivation for further measurement
Test of ChPT - The mesurement of the K±  p± p0 e+e- decay ( NA48/2)
first observation
preliminary Branching Ratio in agreement with theoretical expectations
final result expected by the end of the year
This is the past and (partial) recent past of Kaon Physics at CERN
Kaons cannot retire after a very successful career!!
talk by Michal Zamkowsky
Giuseppina Anzivino

30. NA62 Penguins at work
Thank you!
Giuseppina Anzivino

31. Additional information
Giuseppina Anzivino

32. Kaon decays
C. Smith
Giuseppina Anzivino

33. NA48/2 has access to all the charged Kaon decay
Giuseppina Anzivino