Kaon Experiments at CERN: Recent Results and Prospects Francesca Bucci, INFN Sezione di Firenze...
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Transcript of Kaon Experiments at CERN: Recent Results and Prospects Francesca Bucci, INFN Sezione di Firenze...
Kaon Experiments at CERN: Recent Results and Prospects
Francesca Bucci, INFN Sezione di Firenze Korea Institute for Advanced Study (KIAS), October 29, 2013
F. Bucci 2
Introduction
Kaon decays have played a key role in the shaping of the Standard Model (SM)
Parity violation GIM mechanism CP violation
Kaon decays continue to have an important impact on flavor dynamics
ChPT Tests Constraining physics beyond the SM
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F. Bucci 3
NA48/n and NA62Fixed target experiments at the CERN Super Proton Synchrotron (SPS)
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F. Bucci 4
NA48/n History
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199719981999200020012002200320042005
:2013
::
NA48Main goal: Search for direct CPVMeasurement of ’/Beams: KL+KSNA48/1
Main goal: Rare KS decays, hyperon decays, CPV testsBeams: KS
NA48/2Main goal: Search for direct CPV Charge asymmetry measurementBeams: K+ + K-NA62
Main goal: Measurement of the K+ + decaysBeams: K+
F. Bucci 5
Outline
Recent results: NA48/2 and NA62 (RK phase)
ChPT Tests Lepton Flavor Universality
Prospects: NA62
K++ Other rare and forbidden decays studies
29/10/2013
F. Bucci 6
NA48/2 Beam(s)
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SPS protons at 400 GeV/c Simultaneous, unseparated, focused positive and negative hadronic beams Kaon momentum: 603 GeV/c (NA48/2), 741 GeV/c (NA62-RK)
F. Bucci 7
NA48/2 Detector
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LKr electromagnetic calorimeter: sE/E = (3.2/√E 9.0/E 0.42)% (E in GeV)
sx =sy = (4.2/ √E + 0.6)mm (E in GeV)
Hodoscope: st 150ps
100 m long decay region in vacuum Similar acceptance for K+ and K- decays
Magnetic spectrometer:sp/p = (1.0 0.044 p)% (p in GeV/c) NA48/2sp/p = (0.48 0.009 p)% (p in GeV/c) NA62-RK
Momentum kick: 120 MeV/c NA48/2 265 MeV/c NA62-RK
beam
F. Bucci 8
Outline
Recent results: NA48/2 and NA62 (RK phase)
ChPT Tests Lepton Flavor Universality
Prospects: NA62
K++ Other rare and forbidden decays studies
29/10/2013
F. Bucci 9
Chiral Perturbation TheoryThe Chiral Perturbation Theory (ChPT) is the effective field theory of quantum
chromodynamics (QCD) at low energies
In the ChPT the eight lightest hadrons (,0,K,K0,K0,) are the Goldstone bosons (GB) due to the spontaneous breakdown of the chiral symmetry
The chiral Lagrangian can be organized in terms of the increasing number of GB fields derivatives or, equivalently, in terms of the increasing powers of their momentum (chiral powers)
In the chiral expansion, the intrinsic hadronic uncertainties are parametrized by low energy constants (LECs) whose value must be determined by phenomenology.
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4eff
2eff
0effeff LLLL
F. Bucci 10
NA48/2 and NA62-RK ChPT Tests
Radiative kaon decays:
K 0 EPJC 68 (2010) 75-87 K e+ e- PLB 659 (2008) 493-499 K e (SD) K
Non radiative kaon decays:
K 0 e , K 0 K + - e EPJC 54 (2008) 411-423 , EPJC 70 (2010) 635-657, PLB 715 (2012) 105-115 K 0 0 e K 0 0 EPJ C64 (2009) 589
K + - PLB 6495-6 (2007) 349-358 K e+ e-, K + - PLB 677 (2009) 246-254, PLB 697 (2011) 107-115 K 0 e+ e-
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ChPT is the ideal framework to describe kaon decays
F. Bucci 11
K Decay Theory
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In the ChPT framework the differential rate is:
22
2222223
2
)(,,14
1,,ˆ
8zDzBrzyzCzByzcAz
m
zyK
D’Ambrosio, Portolés, PLB 386 (1996) 403-412
The leading contribution is at O(p4) Rate and spectrum in z=m2
/m2K at O(p4) depend on a single unknown
parameter
N and L are fundamental ChPT parameters
181514109
2
233
128ˆ NNNLLc
weak chiral lagrangian
QCD loops and counterterms
F. Bucci 12
K Decay Theory
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Predictions at O(p4) and O(p6) differ: may be tested experimentally !D’Ambrosio, Portolés, PLB 386 (1996) 403-412Gerard, Smith, Trine, NPB 730 (2005) 1
The dominant amplitude A is responsible for a cusp at m = m2Ecker, Pich, De Rafael, NPB 303 (1988) 665-702
13
K Fit
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Fit the z distributions to extract the value of the ĉ parameter in the framework of the ChPT O(p4) and ChPT O(p6)
F. Bucci
NA48/2 NA62-RK
Data support the ChPT prediction of a cusp at the di-pion threshold
O(p6) O(p6)
14
K Fit Results
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Compute the BR in the full kinematic range assuming the ChPT O(p6) BR6 (K) = (1.010.06)10-6
F. Bucci
ChPT O(p4) vs O(p6) models cannot be discriminated within the current exp. sensitivity
BR(K) vs ĉ d/dz vs z
PRELIMINARY
F. Bucci 15
Scattering and K Decays
The S-wave scattering lengths aI in the isospin I=0 and I=2 states are precisely predicted by ChPT
The DIRAC collaboration at CERN produced +- atoms to measure its lifetime and obtained
The scattering lengths can also be determined by precise measurements of the kaon decays with pions in the final state
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syst.stat.2533.0 0078.00073.0
0080.00078.020
aa
NPB 603 (2001) 125, PRL 86 (2001) 5008
F. Bucci 16
Cusp Effect in K00 Decays
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Change of slope observed in the m2p0p0 distribution at the region near m2
p0p0= (2mp+)2
was interpreted as due to the strong pp rescattering in the final state K+-
Cabibbo PRL 93,2004
(2mp+)2
m2p0p0 (GeV/c2)2
(2mp+)2
zoom
m2p0p0 (GeV/c2)2
0149.00096.00129.00241.0 0088.00029.00048.02571.0 220 aaa[a0,a2 values in units of 1/m+]
F. Bucci 17
Phase Shift in K+-e(Ke4) DecaysVery clean hadronic environment since there are only two pions in the final state
The shift () between the phases of the S-wave, I=0 and P-wave, I=1 form factors is measured as a function of the +- invariant mass [EPJC 70 (2010) 635-657].
Roy equations allow to connet measured phase shifts to scattering lengths [PLB 36 (1971) 353] .
29/10/2013 [a0,a2 values in units of 1/m+]
best ChPT prediction
68% CL contours
F. Bucci 18
Scattering: Theory and Experiments
Combine both the cusp [EPJ C64 (2009) 589] and Ke4 results [EPJC 70 (2010) 635-657] from NA48/2
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Impressive agreement with ChPT
[a0,a2 values in units of 1/m+]
F. Bucci 19
Outline
Recent results: NA48/2 and NA62 (RK phase)
ChPT Tests Lepton Flavor Universality
Prospects: NA62
K++ Other rare and forbidden decays studies
29/10/2013
F. Bucci 20
K+l+ Sensitivity to New Physics Leptonic kaon decays within the SM are mediated by a charged current at
tree level
The natural size of the non-standard contributions depends on the particular BSM scenario
In Models with 2 Higgs Doublet (2HDM-II including SUSY) sizable charged Higgs (H) exchange contributions
obstructed by hadronic uncertainties (fK )
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222
222
||18
)lK( usKK
llKF VfM
MMMG
40 tanβGeV, 50010 3
HSM
M
F. Bucci 21
RK=G(K→en)/G(K→mn) in the SM
In the ratio RK =G(K→en/K→mn) hadronic uncertainties cancel The SM prediction of RK has reached <0.1% precision
dRK is the correction due to the Inner Bremsstrahlung part of the radiative K →eng process
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5corr. rad.K
2
2μ
2K
2e
2K
2
μ
eSMK 10001.0477.21R
R
mm
mm
m
m
[V. Cirigliano and I. Rosell Phys. Rev. Lett. 99 (2007) 231801]
helicity suppression factor
F. Bucci 22
RK beyond the SM Charged Higgs bosons (H) appearing in any model with two Higgs doublets
(including SUSY case) can contribute at tree level
Such contribution does not affect the ratio RK
LFV couplings, present at one loop level, can contribute to RKSM at the % level
Recent measurements of BR(B s+-) and BR(Bu ) significantly lower the SUSY contribution to RK
Sensitive to SM extensions with 4th generation, sterile
2
2
22
SM
βtan1lK
lK
H
K
su
s
m
m
mm
m
βtan1RR 62
13
2
e
τ
4
H
KSMK
LFVK m
m
m
m
A.Masiero, P.Paradisi, R.Petronzio Phys. Rev. D74 (2006) 0011701, J. Girrbach and U. Nierste, arXiv:1202.4906
slepton mixing
Fonseca, Romão, Teixiera, arXiv:1250.1411
29/10/2013
Lackers, Menzel, JHEP 1007 (2012) 006
F. Bucci 23
Measurement Strategy Ke2, Km2 collected simultaneously
MC simulations used to a limited extent
PID, trigger, read out efficiencies and muon halo bkg are measured directly from data
Analysis performed in bins of the reconstructed lepton momentum
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LKReeeB
eBeK fKKAf
KKAf
KNKN
KNKN
DR
11
22
22
22
22
# signal events
# background eventsacceptance
measured PID efficiency
LKr trigger efficiency
LKr readout efficiency
downscaling factor of Km2
F. Bucci 24
RK Final ResultFit over 40 RK measurements (4 data samples 10 momentum bins)
including correlations: c2/ndf=47/39
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RK=(2.488 ± 0.010) x10-5 , RK/RK = 0.4%PLB 719 (2013) 326
F. Bucci 25
RK World Average
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2013 world average : RK= (2.488 ± 0.009) 10-5 (RK/RK= 0.36%)
PDG 2010 (KLOE result): RK= (2.493 ± 0.031) 10-5 (RK/RK= 1.3%)
Consistency with older measurements and with the Standard Model
Experimental accuracy still one order of magnitude away from the SM prediction Motivation for improved precision measurement
2013 average
F. Bucci 26
Outline
Recent results: NA48/2 and NA62 (RK phase)
ChPT Tests Lepton Flavor Universality
Prospects: NA62
K++ Other rare and forbidden decays studies
29/10/2013
27
K in the SMFlavour Changing Neutral Current (FCNC) process forbidden at tree level in the SM
Highest CKM suppression very sensitive to New Physics High theoretical cleanness :
• Dominated by short distance dynamics• In case of K+→p+ nn small effects of long distance contributions due to charm• Hadronic matrix element extracted by K+→p0e+n
SM predictions BR(KL0)=(2.43 0.39 0.06)10-11
BR(K++)=(7.81 0.75 0.29)10-11
BR proportional to |V*tsVtd| theoretically clean Vtd dependance
Parametric error dominated by Vcb ,
Pure theoretical error mostly LD contribution
F. Bucci 28
K++ Previous MeausrementsE949/E787 experiment (BNL):
7 candidates observed in the two allowed kinematics regions
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Low energy separated K+ beam K decays at rest Hermetic coverage
1015.105.1 1073.1 :'08] [E787/E949
KBR
Experimental uncertainty > 50%
NA62 aims at measuring the BR with an accuracy of 10%
F. Bucci 29
K beyond the SMSeveral SM extensions predict sizable deviations for the BR
E949/787experimental uncertainty
NA62expected precision
RSc: Randall-Sundrum, LHT: Littlest Higgs with T-parity, SM4: SM with 4th generation
(hep-ph/0906.5454, hep-ph/0812.3803,hep-ph/0604074)
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30
F. Bucci
LAV
CHANTI
GTKKTAG-CEDAR
Target
protonshadron beam
75 GeV/c
Vacuum <10-5 mbar
STRAWTracker
RICH
CHOD
LKr
MUV
SAV
Signal signature: Incoming high momentum (75 GeV/c) K+
Outgoing low momentum (<35 GeV/c) p+ in time with the incoming K+
The NA62 Beam and Detector
50 MHz 800 MHz
10 MHz
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Unseparated positive hadron beam (K+ 6%)
F. Bucci
F. Bucci 31
Experimental Technique
High K momentum Low momentum to allow enough missing energy (p0 40 GeV/c) to be
detected by hermetic veto detectors (LAV,IRC,SAC,LKr)
Particle identification to separate and (RICH, MUV)
Kinematical rejection with lightweight spectrometers (GTK, STRAW)
Beam particle identification and inelastic event suppression (KTAG, CHANTI) Fast timing
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F. Bucci 32
Backgrounds 92% of kaon decays separated from signal by kinematic cuts
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Decay BR
K++ 63.5%
K++0 20.7%
K+++- 5.6%
K++00 1.8%
2πK2miss PPm
K++0 splits the signal region in 2
(m2miss ) < 10-3 GeV2/c4
(pK)/pK 0.2%, (p)/p 1 %
keep multiple scattering as low as possible
t=100 ps on + , t=150 ps on K+
F. Bucci 33
Backgrounds
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8% of kaon decays not separated from signal by kinematic cuts
Decay BR
K+0e+ 5.1%
K+0+ 3.4%
K++ 5.510-3
K++0 1.510-3
K++-e+ 410-5
K++-e+ 110-5
identification inefficiency < 10-5
veto inefficiency 10-5
- separation at 10-3 level
F. Bucci 34
Expected Sensitivity
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Simple cut and count estimation with no optimizationBackground to be evaluated on data to reach the 10% accuracy
F. Bucci 35
Tracking Detectors: GTKComposed of 3 hybrid silicon pixel stations mounted around four achromat magnetsInside the vacuum and subjet to a high and non-uniform beam rate (750 GHz in total)
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1 sensor, 10 bump-bonded chips
Provide precise momentum ((p)/p 0.2%), time (t 175 ps at 300V bias) and angular measurements (() 16 rad)
30 m
m
60 mm
GTK1 Pixel 23GTK2 Pixel 23
245 ps 2 175 ps
F. Bucci 36
Tracking Detectors: STRAWSConsists of 4 chambers intercepted in the middle by a dipole magnet
Minimize multiple scattering: ultra-light material, integration in the vacuum tank
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Measurement of coordinates ( < 130 m), and momentum ((p)/p 0.3%)
of charged particles originating fromthe decay
Each chamber equipped with 1800 straw tubes, positioned in 4 views (u-v,x-y)
F. Bucci 37
PID Detectors: KTAG-CEDARDifferential Cherenkov counter filled with H2 for positive K (50 MHz) identification
Without kaon tagging vacuum should be better than 610-8 mbar
29/10/2013
Upgraded form of the CEDAR built for the SPS secondary beam
New PMTs and electronics, modified mechanics /optics
Excellent time resolution required (t =100 ps)
F. Bucci 38
PID Detectors: RICHRing Imaging Cherenkov detector composed of a cylindrical vessel (17 m long)
filled with Ne slightly above atmospheric pressure
Project validated by strong R&D17 m long radiator, 1 mirror, 400 PMTs
Requirements: / separation in 15< p <35 GeV/c with
mis-id probability <10-2
Measure crossing time with t =100 ps Level 0 trigger for charged tracks
Momentum (GeV)
m M
isID
Pro
babi
lity
Momentum (GeV)
Tim
e Re
solu
tion
(ps)
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F. Bucci 39
Photon Veto SystemTo suppress the dominant decay K++0 (BR 21%): 0 rejection inefficiency at 10-8 level ( detection inefficiency at 10-4) Hermetic coverage up to 50 mrad
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3 different detectors to cover 3 different angular regions Large Angle Vetoes (LAV): 8.5 – 50 mrad The NA48 Liquid krypton calorimeter (LKr): 1 – 8.5 mrad Small Angle Vetoes: 1 mrad
F. Bucci 40
Large Angle Veto (LAV)12 stations formed by 4 to 5 overlapping rings of OPAL lead glasses
29/10/2013
The first eleven are part of the vacuum decay tube, the last one is located outside the vacuum tank
Inefficiency < 10-4 for 100 MeV < E < 35 GeV
F. Bucci 41
Muon Veto3 muon veto (MUV) stations (partially re-use of the NA48 hadron calorimeter)
29/10/2013
MUV 3
MUV 1MUV 2IRON
MUV1+MUV2 24 (MUV1) and 22 (MUV2)
iron/scintillator layers reach a factor 106 in muon rejection
(combined with the RICH)
MUV3 After 80 cm iron, scintillator tiles
with direct photon detection Fast muon trigger (L0), 1 ns
resolution (10 MHz muon rate)
F. Bucci 42
Detector Status
Detectors installed: KTAG, LAV(8/12), LKr, SACUnder construction/installation: CHANTI, STRAWS, RICH, IRC, MUV
Installation completed by October 201429/10/2013
F. Bucci 43
Outline
Recent results: NA48/2 and NA62 (RK phase)
ChPT Tests Lepton Flavor Universality
Prospects: NA62
K++ Other rare and forbidden decays studies
29/10/2013
44
LFV/LNV modesHigh fluxes and PID/veto capabilities of NA62 are well suited to look for Lepton Flavor/ Lepton Number Violation mode both in kaon and pion decays
Mode UL at 90% CL Experiment NA62 acceptanceK+++e- 1.310-11 BNL 777/865 10%K++-e+ 5.210-10 BNL 865 10%K+-+e+ 5.010-10 BNL 865 10%K+-e+e+ 6.410-10 BNL 865 5%K+-++ 1.110-9 NA48/2 20%K+-e+e+ 2.010-8 Geneva Saclay 2%K+e-++ no data 10%0+e- 3.810-10 KTeV 2%0-e+ 3.410-9 KTeV 2%
Expected decays in Fiducial Volume in 2 years of data taking: 1.21013 K+ decays, 2.5 1012 0 decays
NA62 single-event sensitivities: 10-12 for K+ decays, 10-11 for 0 decays
F. Bucci 45
Others
Decay Physics Present result NA62
K++h,h Heavy neutrino Limits up to mh= 350 MeV
RK LU and NP (2.4880.010)10-5 > 2 better
K++ ChPT <500 events 105 events
K+00e+ ChPT 66000 events O(106)
K+00+ ChPT - O(105)
29/10/2013
F. Bucci 46
Conclusions
• More than 60 years after their discovery, kaons provide a unique playground for testing our ideas on fundamental physics
• NA48 and NA62 collaborations are analyzing data taken in past years and producing very precise results for leptonic, non-leptonic and semileptonic kaon decays
• The new generation NA62 apparatus will start its data taking in fall 2014. Its unprecedented statistics and its powerful detector features will allow to push further our knowledge of rare (and forbidden) kaon decays
29/10/2013
F. Bucci 4729/10/2013
Thank you for your attention
Additional Slides
F. Bucci 49
The ChPT weak chiral lagrangian The basic S=1 O(p4) chiral lagrangian can be written as:
37 poorly known Ni coefficients and Wi operators Combination of these couplings are accessible by measuring kaon decays
branching ratio and form factors
29/10/2013
L
iiSSS WNFGUDUDFGLLL
286
48
41
211
F. Bucci
Radiative K→eng Decays In K→eng (Ke2g), g can be produced via internal bremsstrahlung (IB) or
direct-emission, the latter being dependent on the hadronic structure (SD)
RK is defined to be inclusive of the IB, ignoring however the SD contributions To compare data with SM prediction the SD contribution must be carefully
estimated and subtracted.
29/10/2013 50
K±
e±
ne
gIB
K±
e±
ne
gSD
KL0→p0nn
The charm contribution can be fully neglected since it proceeds in theSM almost entirely through direct CP violation → determination of h
Need a huge number of KL decaysNA48 KL flux corresponding to 31010/yearNA62 possible KL flux 5-10 times NA48 oneAfter SPS upgrade 100 times more
Exp Machine Meas. or UL 90% CL Notes
KTeV Tevatron <5.710-7(0→eeg)
E391a KEK-PS <2.610-8
KOTO J-PARC Aim at 2.7 SM evts/3y
KOPIO Opportunity at Project X ?
F. Bucci29/10/2013 51