Lepton Universality Test
with K+→l+ν Decays at CERN NA62
Evgueni Goudzovski(University of Birmingham)
for the NA62 collaboration(Bern ITP, Birmingham, CERN, Dubna, Fairfax, Ferrara, Florence, Frascati,IHEP Protvino, INR Moscow, Louvain, Mainz, Merced, Naples, Perugia, Pisa,Rome I, Rome II, Saclay, San Luis Potosí, SLAC, Sofia, TRIUMF, Turin)

Outline:1) Motivation & experimental status;
2) Beam, detector and data taking;3) Backgrounds & systematic effects;
4) Result and prospects.
Kaon 2009 • Tsukuba, Japan
June 10, 20091Kl2 and πl2 decays in the SM
Radiative correction (few %)due to the IB part of theradiative K→eνγ process,by definition included into RK

Helicity suppression (V–A couplings):
enhances sensitivity to non-SM effects

Standard Model:• excellent sub-permille accuracy of RP (P=K,π) due tocancellation of hadronic uncertainties in the ratio;• strong helicity suppression of the electronics
channel enhances sensitivity to non-SM effects.
+K

SM uncertainties well below 10–3

RKSM = (2.477±0.001)×10–5
RπSM = (12.352±0.001)×10–5ue +, μ+W+

νe , νμV. Cirigliano and I. Rosell,
Phys. Lett. 99 (2007) 231801

2E. Goudzovski / Kaon09, 10 June 2009

Kl2 decays beyond the SM
Possible scenario in MSSM:
charged Higgs mediated SUSY LVF
contribution with emission of τ neutrino
can be strongly enhanced.
A. Masiero, P. Paradisi and R. Petronzio,
PRD74 (2006) 011701 and JHEP 0811 (2008) 042

A few percent effect in large (not extreme)tanβ regime with massive charged Higgs.

SUSY limits fromKLOE (preliminary)and NA48/2 (preliminary)combined results:
RK=(2.457±0.032)×10–5Example:(Δ13=5×10–4, tanβ=40, MH=500 GeV/c2)
lead to RKLVF = RKSM(1+0.013).
NB: analogous SUSY effect in pion decay
is suppressed by a factor (mπ/MK)4 ≈ 6×10–3
E. Goudzovski / Kaon09, 10 June 20093

Kl2 & πl2: experimental status
Kaon decay:
Æ PDG’08 average (1970s measurements):
RK=(2.45±0.11)×10–5 (δRK/RK=4.5%)
Æ NA48/2: two preliminary results
based on 2003 and 2004 data sets
RK=(2.416±0.049)×10–5 (δRK/RK=2.0%)
RK=(2.455±0.061)×10–5 (δRK/RK=2.5%)

NA48: valuable for development of
the NA62 method, however
analyses are not completed.
No plans to finish themeasurements.

L. Fiorini, PoS (HEP2005) 288,
V. Kozhuharov, PoS (KAON) 049

Æ Recent improvement: final KLOE result
RK=(2.493±0.031)×10–5 (δRK/RK=1.3%)
Mario Antonelli, La Thuile ’09

Pion decay:
Æ PDG’08 average (1980s, 90s data):
Rπ=(12.30±0.04)×10–5 (δRπ/Rπ=0.3%)
Æ Future plans: TRIUMF proposal S1072,
δRπ/Rπ=0.06% precision foreseen
Toshio Numao, PANIC ’08

E. Goudzovski / Kaon09, 10 June 20094

NA48/NA62: kaons at CERN
1997: ε’/ε run KL+KS

Jura mountains

1998: KL+KS

FranceNA48

NA48/NA62
Switzerland

SPS
LHC
NA48/1

Geneva airport

Dedicated to Ke2 (aiming to collect 150K events),
40% partial data set presented here
Talk by Giuseppe Ruggiero
E. Goudzovski / Kaon09, 10 June 2009

N

NA48/2

NA62(phase I)

NA62(phase II)

1999: KL+KS

KS HI

2000: KL only

KS HI

2001: KL+KS

KS HI

2002: KS/hyperons
2003: K+/K–
2004: K+/K–
2007: K±e2/K±μ2 tests
2008: K±e2/K±μ2 tests
2006–2010:
design & construction
2011:
5++start of K →π νν run

NA62 data taking: 2007/08
Data taking:• Four months in 2007 (23/06–22/10):~400K SPS spills, 300TB of raw data
(90TB recorded); reprocessing &data preparation finished.
• Two weeks in 2008 (11/09–24/09):
special data sets allowing reduction ofthe systematic uncertainties.

Principal subdetectors for RK:• Magnetic spectrometer (4 DCHs):
4 views/DCH: redundancy ⇒ efficiency;
Δp/p = 0.47% + 0.020%*p [GeV/c]
Vacuum beam pipe:• Hodoscope
non-decayed kaons
fast trigger, precise t measurement (150ps).
• Liquid Krypton EM calorimeter (LKr)
High granularity, quasi-homogenious;
σE/E = 3.2%/E1/2 + 9%/E + 0.42% [GeV];
He filled tank,atmospheric pressure
σx=σy=0.42/E1/2 + 0.6mm (1.5mm@10GeV).
E. Goudzovski / Kaon09, 10 June 2009

Decay volume
is upstream

6Kaon beams
Improvement of Ke2/Kμ2
kinematic separation

NA48/2 beam line: capable of delivering
simultaneous K+/K– beams (74 GeV/c in 2007)
Kinematic ID of the Kl2 candidates:

Optimization of Mmiss2 resolution:
narrow momentum band
not measured in every event (average used)beams (ΔPKRMS/PK=2%)
Kaon sign
3× 10
Beam halo background much higher for 350
Ke2– (~20%) than for Ke2+ (~1%):300~90% of data sample: K+ only.
~10% of data sample: K– only.

Beam halo directly measured
with the K– only sample

250

Data

200

Collection of K+ ONLY and K– ONLY sets 150
allows direct “cross-measurements” of 100
beam halo background with50
excellent precision.
E. Goudzovski / Kaon09, 10 June 2009

Kμ2 decay Z vertex

0-20Lower cut(low Ptrack)

0Lower cut(high Ptrack)

2040

Kμ2 MC
76080

Z vertex , mTrigger logic

LKre

ELKr: energy depositof at least 10 GeV

HODee

Q1: coincidence
in the two planes

Ke2 condition: Q1×ELKr×1TRK.
Purity ~10–5.

1TRK: very loose conditionon activity in DCHs
against high multiplicity events

DCHs
x 10

Kμ2 condition: Q1×1TRK/D,
downscaling (D) 50 to 150.
Purity ~2%.

3ELKr efficiency vs energy

Control & ELKr triggers

3500113000• Efficiency of Ke2 trigger: monitoredwith Kμ2 & other control triggers.

0.80.82500

Kμ2 & control triggers

2000
1500

0.60.60.40.4

ELKr triggers

1000

0.20.2

5000Minimum bias
(high efficiency, but low purity)
trigger configuration used

Energy deposit, GeV
0202040406060000• ELKr inefficiency for electrons measured

10 GeVthresholdto be (0.05±0.01)% for ptrack>15 GeV/c.

• Different trigger conditions for signal
and normalization!
Energy deposit, GeV
8202040406060

Measurement strategy(1) Ke2/Kμ2 candidates collected simultaneously:• the result does not rely on kaon flux measurement;• several systematic effects cancel at first order(e.g. reconstruction/trigger efficiencies, time-dependent effects).

(2) A counting experiment in track momentum bins:

N(Ke2) – NB(Ke2)RK = N(K ) – N (K )μ2

Bμ2

A(Kμ2) × fμ × ε(Kμ2)
A(Ke2) × fe × ε(Ke2)

N(Ke2), N(Kμ2):
numbers of selected Kl2 candidates;
numbers of background events;
NB(Ke2), NB(Kμ2):
MC geometric acceptances (no ID);
A(Ke2), A(Kμ2):
fe, fμ:measured particle ID efficiencies;ε(Ke2)/ε(Kμ2)>99.9%: ELKr trigger condition efficiency;
fLKR=0.998:
global LKr readout efficiency.

1fLKR
main source ofsystematic errors(3) MC simulations used to a limited extent:• acceptance correction (only for geometry, not for particle ID);
• simulation of “catastrophic” bremsstrahlung by muon.
E. Goudzovski / Kaon09, 10 June 20099

Ke2 and Kμ2 selection
Large common part(due to topological similarity)• one reconstructed track;• geometrical acceptance cuts;• limit on LKr extra energy deposition;• track momentum: 15GeV/c<p<65GeV/c;• decay vertex defined as closest approachof track & nominal kaon axis.

Missing mass vs momentum, data
electron hypothesis…poor separation at high p

Kinematic separation
missing mass
: average measured with K3π decaysÆ Excellent Ke2/Kμ2 separation at ptrack<25GeV/c

Separation by particle ID
E/p = (LKr energy deposit/track momentum).
0.95<E/p<1.10 for electrons,
E/p<0.2 for muons.
Æ Powerful muon suppression by f~106
E. Goudzovski / Kaon09, 10 June 2009

Log scale10Muonic background in Ke2 sample
Problem:“Catastrophic” energy loss by muons in LKr.
Muons with E/p>0.95 are identified as electrons.
P(μ→e) ~ 3×10–6 (and momentum-dependent).

P(μ→e)/RK ~ 10%:Kμ2 decays represent a major background
Need a direct measurement of P(μ→e)
with pure muon samples to validate
theoretical bremsstralung cross-sectionin the very special high Eγ region.

Thickness:
Width:Height:
Area:
Duration:

Solution:Pb wall (~10X0) between the HOD planes.
Tracks traversing the wall & with E/p>0.95
are pure muon samples (electron contamination <10–7),
with the μ→e decay component (initially ~10–4) suppressed.
E. Goudzovski / Kaon09, 10 June 2009

~10X0 (Pb+Fe)
240cm (=HOD size)
18cm (=3 counters)~20% of HOD area
~50% of RK runs,
special muon runs

11

Muonic background (2)P(μ→e): measurement (2007 special muon run) vs Geant4-based simulation
Mis-ID P(μ→e) vs track momentum
model validationExcellent data/MC agreement
for the Pb wall case!
P(μ→e) modified by the Pb wall via
two main competing mechanisms:
1) ionization losses in Pb (low p);2) bremstrahlung in Pb (high p).

Used forbackground subtraction
analysis region

Result: B/(S+B) = (6.28±0.17)%(uncertainty is due tothe limited size of the data sample
used to validate the simulation)

Prospects:• The 2008 special muon sample is twice as large as the 2007 one;
• Use muons from Kμ2 decays in good Ke2/Kμ2 separation region (p<25GeV/c).
E. Goudzovski / Kaon09, 10 June 200912

Kμ2 with μ→e decay in flight
For NA62 conditions(74 GeV/c beam, ~100 m decay volume),cosΘMichel distribution

P(Kμ2, μ→e decay)/RK ~ 10Kμ2(μ→e) naïvely seems a major background

Muons from K± decay are fully polarized:
Michel electron spectrum
d2Γ/dxd(cosΘ) ~ x2[(3–2x) – cosΘ(1–2x)]x = Ee/Emax ≈ 2Ee/Mμ,Θ is the angle between pe and the muon spin,(all quantities are defined in muon rest frame).

Result: B/(S+B) = (0.23±0.01)%
Important but not dominant background
E. Goudzovski / Kaon09, 10 June 2009

x=Ee/Emax

Only energetic forward electrons(passing Mmiss, E/p, vertex CDA cuts)
are selected as Ke2 candidates:(high x, low cosΘ),
configuration highly suppressed
according to the Michel distribution.
13

K+→e+νγ (SD+) background• Background by definition of RK• Rate similar to that of Ke2• Known with poor precision of ~15%
Ee, GeV

0.25

0.2

0.15

0.1

0.05

Ke2γ (SD+) Dalitz plot distribution

000.05

0.1

0.15

0.2

• Experiment: BR=(1.52±0.23)×10–5[1970s measurements]

0
Only
energetic electrons (Ee*>230MeV)
0 are compatible to Ke2 kinematic ID
0This region of phase space isaccessible for direct BR and0
form-factor measurement0(being outside the region Ee*<227 MeV
0populated by the Ke3 background).
0000
Backgroundestimate (ChPT phase space)
00.25
B/(S+B) = (1.02±0.15)%

Eγ, GeV

Ke2γ (SD–) background is negligible,
as this decay is suppressed at high Ee
E. Goudzovski / Kaon09, 10 June 2009

[model-dependent form factor]

×0e )l3beKiss byec dca tet naoN miod(ChPT O(p4) form factor• Theory: BR=(1.12–1.34)×10–5(uncertainty from PDG BR,to be improved by NA62&KLOE) 14

Backgrounds: summary
Statistics in momentum bins

Background summary
SourceKμ2Kμ2 (μ→e)
Ke2γ (SD+)
Beam halo
Ke3K2πTotalx5x5x25(selection criteria, e.g. for Zvertex and Mmiss2,
are optimised individually in each Ptrack bin)
E. Goudzovski / Kaon09, 10 June 2009

B/(S+B)(6.28±0.17)%(0.23±0.01)%(1.02±0.15)%(0.45±0.04)%
0.03%
0.03%
(8.03±0.23)%

Record Ke2 sample:
51,089 candidates
with low background
B/(S+B) = (8.0±0.2)%15Ke2: 40% of data set
Ke2 candidates

104

103

102

101

Log scale

51,089 K+→e+ν candidates,
99.2% electron ID efficiency,
B/(S+B) = (8.0±0.2)%
cf. KLOE: 13.8K candidates (both K+ and K–)~50% electron ID efficiency, 16% background
E. Goudzovski / Kaon09, 10 June 2009

NA62 estimated total Ke2 sample:~120K K+ & ~15K K– candidates.
Proposal (CERN-SPSC-2006-033):
150K candidates
16Kμ2: 40% of data set
Kμ2 candidates

15.56M candidates
with low background
B/(S+B) = 0.25%(Kμ2 trigger is
pre-scaled by D=150)

The only significant
background sourceis the beam halo.

17
E. Goudzovski / Kaon09, 10 June 2009

Electron ID efficiency fe (99.2%)
measured with samples of pure electrons• K±→π0e±ν from main K data taking(limited momentum range p<50GeV/c);• KL→π±e±ν from a special 15h KL run(wider track momentum range,
due to broad KL momentum spectrum).

Good agreement between the twomeasurements, precision better than 0.1%.

Radius at LKr plane, cm

Other systematic effects
120100Electron ID inefficiency = f (Ptrack, RLKr)%

Average = 0.8%
cf. KLOE: ~50%

1.4
1.3
1.2

80

1.1
160
0.9

40

0.8
0.7

20Acceptance correction
0.6
15 20 25 30 35 40 45 50 55 60 65• ptrack-dependent, A(Kμ2)/A(Ke2)~1.3;
Track momentum, GeV/c• strongly affected by the radiative (IB)
Trigger efficiency correction
corrections to Ke2;
• ELKr efficiency directly affects RK;
IB process simulated according toV. Cirigliano and I. Rosell,• monitored with control trigger samples;
Phys. Lett. 99 (2007) 231801• conservative systematic uncertainty• conservative systematic uncertainty
for preliminary result: δRK/RK=0.3%
for prelim. result: δRK/RK=0.3%, due to (dead time generated by accidentals).
approximations used in IB simulation.
18E. Goudzovski / Kaon09, 10 June 2009

Preliminary result (40% data set)––55RRKK == (2.500±0.011)
×10(2.500 ±± 0.012
0.012stat±0.011)
×10syststatsyst–5
RRKK == (2.500(2.500 ±± 0.016)0.016) ×× 10
10–5

Independent measurements
in track momentum bins

SM(New, June 09)

Uncertainties
SourceStatisticalKμ2
Beam halo
Ke2γ (SD+)
Electron ID
IB simulation
Acceptance
Trigger timing
Total

δRK×105
0.012
0.004
0.001
0.004
0.001
0.007
0.002
0.007
0.016

(0.64% precision)

E. Goudzovski / Kaon09, 10 June 2009

The whole sample will allow
a statistical uncertainty ~0.3%,and total uncertainty of 0.4–0.5%. 19

Comparison to world data
March 2009

June 2009

δRK×105

World average
Precision
2.467±0.024 0.97%
March 20092.498±0.014 0.56%
June 2009(NA48/2 preliminary results
excluded from the new average:
they are superseded by NA62)

20
E. Goudzovski / Kaon09, 10 June 2009

Conclusions & prospects• Due to the helicity suppression of the Ke2 decay, the measurementof RK is well-suited for a stringent test of the Standard Model.

• NA62 data taking in 2007/08 was optimised for RK measurement.
The NA62 Ke2 sample is ~10 times the world sample.
Powerful Ke2/Kμ2 separation (>99% electron ID efficiency and~106 muon suppression) leads to a low 8% background.
• Preliminary result based on ~40% of the NA62 Ke2 sample:
RK = (2.500±0.016)×10–5, reaching a record 0.7% accuracy andcompatible to the SM prediction. A timely result, as direct searches
for New Physics at the LHC are approaching.
• With the full NA62 data sample of 2007/08, the precision is
expected to be improved to better than δKR/RK=0.5%.
21E. Goudzovski / Kaon09, 10 June 2009