lepton universality tests in kaon decays at na62

Lepton Universality Tests in Kaon Decays at NA62

Francesca Bucci, INFN Sezione di Firenzeon behalf of the NA62 Collaboration

The XIth International Conference onHeavy Quarks and Leptons

June 11-15, 2012, Prague, Czech Republic

F. Bucci 2

Introduction

Precise measurements of FCNC processes in the B sector have severely restricted the parameters space of new-physics (NP) models

Experiments at the LHC have started a direct exploration of the physics in the TeV range

In this scenario, the study of Lepton Flavor Universality (LFU) in kaon decays is equally interesting and fully complementary to search for NP effects

Heavy Quarks and Leptons 2012, 13 June

F. Bucci 3

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 )


222

222

||18

)lK( usKK

llKF VfMMMMG

40 tanβGeV, 50010 3

HSM

M

F. Bucci 4

RK=(K→e)/(K→m) in the SM

In the ratio RK =(K→e/K→m) 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 →eg process


5corr. rad.K

2

2μ

2K

2e

2K

2

μ

eSMK 10001.0477.21R

R

mmmm

mm d

[V. Cirigliano and I. Rosell Phys. Rev. Lett. 99 (2007) 231801]

helicity suppression factor

F. Bucci

Radiative K→eg Decays In K→eg (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.

Heavy Quarks and Leptons 2012, 13 June 5

K±

e±

e

gIB

K±

e±

e

gSD

6

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

Limited by recent Bs m+m- measurement

2

2

22

SM

βtan1lK

lK

H

K

su

s

mm

mmm

βtan1RR 62

13

2

e

τ

4

H

KSMK

LFVK m

mmm

[A. Masiero, P.Paradisi, R. Petronzio Phys. Rev. D74 (2006) 0011701]

slepton mixing[Fonseca, Romão, Teixiera, arXiv:1250.1411]

F. Bucci 7

The NA62 Experiment

The NA62 Collaboration:Birmingham, Bratislava, Bristol, CERN, Dubna, Fairfax, Ferrara, Florence, Frascati, Glasgow, Liverpool, Louvain, Mainz, Merced, Moscow, Naples, Perugia, Pisa, Prague, Protvino, Rome I, Rome II, San Luis Potosì, SLAC, Sofia, TurinHeavy Quarks and Leptons 2012, 13 June

NA62 (RK)

2007-2008: Ke2 ,K

m2 data taking

after the long CERN accelerators shutdown

NA62 (K++ )

2007-2013: design & construction

2014 ? : K++ data taking

F. Bucci 8

NA62 (RK) Detector Primary SPS protons (400 GeV/c) 1.81012 /SPS spillUnseparated secondary positive and/or negative hadronic beam (p=74.01.4 GeV/c)Composition: K () = 5% (63 %)K decaying in vacuum tank 18%


Main subdetectors: 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: Fast trigger for charged particle and timing for the event (st 150ps)

Magnetic spectrometer:sp/p = (0.48 0.009 p)% (p in GeV/c)

Helium filled tank

Magnet

beam

F. Bucci 9

NA62 (RK) Data Taking

Data taking: 4 months in 2007 2 weeks in 2008 (used for systematics studies)

Data samples: K+, K- samples with different background conditions : with lead wall (Pb), without lead wall (noPb) (see next)

K+ (noPb), K+ (Pb), K- (noPb), K- (Pb)Heavy Quarks and Leptons 2012, 13 June

The data taking strategy has been optimized to measure the two main backgrounds in the Ke2 sample: Beam halo muons Km2 decays with a muon misidentified as an electron

F. Bucci 10

Measurement Strategy Ke2, Km2 collected simultaneously:

No dependence on K flux measurement; Cancellation of several systematic effects in the ratio at first order

MC simulations used to a limited extent: Acceptance correction and the geometrical parts of the acceptances for most bkg

processes PID, trigger, read out efficiencies and muon halo bkg are measured directly

from data Analysis performed in bins of the reconstructed lepton momentum


LKReeeB

eBeK fKKAf

KKAfKNKNKNKN

DR 11

22

22

22

22

mmm

mm

# signal events

# background events acceptance

measured PID efficiency

LKr trigger efficiency

LKr readout efficiency

downscaling factor of Km2

F. Bucci 11

Trigger LogicMinimum bias (high efficiency, but low purity) trigger configuration used


Ke2 condition: Q1ELKr 1Trk Km2condition: Q11Trk/D, downscaling factor D=150

F. Bucci 12

Ke2 vs Km2 Selection Geometry

One reconstructed charged track 13 < p < 65 [GeV/c] Geometrical acceptance cut Cut on K decay vertex position Photon veto using LKr

Kinematics Missing mass: Mmiss

2(l)=(PK-Pl)2

Sufficient Ke2/Km2 separation up to 30 GeV/c

Particle ID (ELKR/pspectr) e : (0.9 to 0.95) < E/p < 1.1 m : E/p < 0.85m suppression in e sample: 106


F. Bucci 13

Km2 Background in Ke2 SampleMuon ‘catastrophic’ energy loss in or in front of the LKr is the largest bkg source

The probability of mis-identify a muon as an electron Pme 310-6 (and momentum dependent)


m

e

Measurement of Pme on data To avoid electron contamination from muon

decay (10-4) a 9.2 X0 thick lead wall covering 18% of the acceptance was installed between the HOD planes for 50% of the run time

Km2 candidates : track traversing Pb, p>30GeV/c, E/p>0.95 (positron contamination < 10-8)

Pme is modified by the Pb wall due to: ionization losses in Pb (low p) bremsstrahlung in Pb (high p)

F. Bucci 14

Muon Misidentification

The correction fPb= Pme/ PmePb evaluated with a dedicated Geant4-based simulation


MC precisiondPme

/Pme = 10%

MC precisiondfPb/fPb = 2%

Pme Pb (Pb wall installed) vs momentum Correction for Pb: fPb = Pme / Pme

Pb

Result : B/(S+B) = (5.640.20)%

F. Bucci 15

Muon halo background measured on the data K+ (K- ) only sample used to measure muon halo background in K- (K+ ) sample

Control samples normalized to the data in the Mmiss2 region populated mainly

by beam halo events (-0.3 < Mmiss2 < - 0.1 (GeV/c2)2 )

Strongly depends on decay vertex position and track momentum

Regions delimited by solid line are excluded Probability to reconstruct a Ke2

candidate due to a decay of an opposite sign kaon has been taken into account

Muon Halo Background in Ke2 SampleThe muon sweeping system was optimized for the positive beam

z vertex (m)

Ke2- (noPb)

p e- (G

eV/c

)


p e+ (G

eV/c

)

z vertex (m)

Ke2+ (noPb)

Result : B/(S+B) = (2.110.09)%

F. Bucci 16

K±→e± g Background in Ke2 Sample

SD contribution of the K±→e± g decays must be carefully estimated and subtracted


The SD+ (positive photon helicity) component peaks at high electron momentum in the K rest frame and is therefore kinematically similar to the Ke2 decay

SD- decays and the interference between the IB and SD processes are negligible

The SD+ background contribution has been estimated by using MC simulation based on the measured K±→e±g (SD+) differential decay rate

E e (M

eV)

Eg (MeV)

SD+

SD-

IB

[J. Bijnens, G. Colangelo, G. Ecker, J. Gasser, arXiv:hep-ph/9411311]

[F. Ambrosino et al., Eur. Phys. J. C 65 (2010) 703]

Result : B/(S+B) = (2.600.11)%

F. Bucci 17

Ke2 Sample

145,958 K±e ± candidates

B/(B+S) = (10.950.27)%

e± ID efficiency = (99.28 0.05)%


Ke2 candidates

F. Bucci 18

Ke2 BackgroundKe2 candidates and backgrounds in momentum bins


Decay B/(S+B)K± m± (5.64 ± 0.20)%K± m± (m e) (0.26 ± 0.03)%

K± e±g (SD+) (2.60 ± 0.11)%

K± 0De± (0.18 ± 0.09)%

K± ±0D (0.12 ± 0.06)%

Wrong sign K (0.04 ± 0.02)%Beam halo (2.11 ± 0.09)%Total (10.95 ± 0.27)%

K+ (noPb)

F. Bucci 19

Km2 Sample and BackgroundThe only significant background source in the Km2 sample is the beam halo


42.817 106 K±m ± candidates (collecting with downscaling factor D=150)

B/(S+B) = (0.50 ± 0.01)%

K+ (noPb)

F. Bucci 20

NA62 Result (full data set)Fit over 40 RK measurements (4 data samples 10 momentum bins)

including correlations: c2/ndf=47/39


RK=(2.488 ± 0.007stat ±0.007syst ) x 10-5=(2.488 ± 0.010) x10-5

21

Uncertainties on RK


Source dRK 105

Statistical 0.007

Km2 background 0.004

K±e ±g (SD+) background 0.002

K±0e ±, K±0 backgrounds 0.003

Muon halo background 0.002

Matter composition in the spectrometer 0.003

Acceptance correction 0.002

Spectrometer alignment 0.001

Electron identification efficiency 0.001

1-track trigger efficiency 0.001

LKr readout inefficiency 0.001

Total Systematic 0.007

Total 0.010

F. Bucci 22

RK World Average

For non-tiny values of 13

the sensitivity to H± in RK is strong


July 2011 world average : RK= (2.488 ± 0.009) 10-5 (dRK/RK= 0.4%)

PDG 2010 (KLOE result): RK= (2.493 ± 0.031) 10-5 (dRK/RK= 1.3%)

b sg

B→t [HFAG 2010]Rm23 [Eur. Phys. J. C 69 (2010) 399 ]b→sg [HFAG 2010]

F. Bucci 23

Future Prospects

NA62 (K++) data taking foreseen in 2014after the long CERN accelerators shutdown

Hermetic veto (large-angle and small-angle veto counters) will strongly decrease the Ke2g background.

Beam spectrometer (beam tracker plus Cherenkov beam tagger) will allow time correlation between kaons and decay products. Beam halo background expected to be reduced to negligible level.

Required statistical uncertainty 0.05%1 month of data taking sufficient for such RK measurement


F. Bucci 24

Conclusions

The analysis of the full data sample taken by NA62 is completed The measured value is RK=(2.488 ± 0.010) x10-5, reaching a record level of

accuracy of 0.4% The combined experimental accuracy has improved by an order of

magnitude since 2008, but is still an order of magnitude bigger than the SM prediction precision

Excellent prospects for future measurements of RK within the NA62 experimental program


lepton universality tests in kaon decays at na62

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