Ke2/Kmu2 and LFV tests

Crimea2008 Yalta 27/09/08 – 04/10/08

Andrey Zinchenko JINR, Dubna

On behalf of NA62 Collaboration:Bern, Birmingham, CERN, Dubna, Fairfax, Ferrara,

Florence, Frascati, Mainz, Menlo Park, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome,

Saclay, San Luis Potosi, Sofia, Triumf, Turin

K+→e+νe in the Standard Model

In the Standard Model(SM), the decay K+→e+νe (Ke2) is strongly helicity-suppressed compared to the muonic channel K+→μ+νμ(Kμ2). The positron e+ must be left-handed due to angular momentum conservation – thus the coupling of the W+-Boson is suppressed by (1– β). In the SM, the ratio of the decay rates can be predicted very precisely and is given as

5

2

22

222

10)001.0477.2()1()(

)(

QEDK

eKeeK R

mm

mm

m

m

K

eKR

where δRQED= -3.6% is a correction due to Kl2γ(IB) and virtual photon processes (Cirigliano, Rosell, JHEP 0710:005, 2007).

K+→e+νe in Supersymmetry

In Supersymmetry (SUSY), K+→e+νe can proceed via exchange of a charged Higgs H+ instead of W+, but in this case, at tree level, the ratio Γ(K+→e+νe)/Γ(K+→μ+νμ) remains unchanged.

However, loop effects are predicted to lead to lepton flavour violating (LFV) couplings lH±ντ, which noticeably change only the rate of K+→e+νe

(Masiero, Paradisi, Petronzio, PRD 74, 2006):

The LFV parameter Δ13 should be of o(10-4 – 10-3) (similar to LFV in the neutrino sector). For given values of MH

± and tan β, deviations up to a few percent on RK are possible.

As a result of the helicity suppression of the SM contribution, the Ke2 decay rate is sensitive to New Physics.

Tree-level diagram for K+→l+ν

Loop diagram for K+→e+ντ

62

132

2

4

4

tan1eH

KSMK

SUSYK M

m

M

mRR

Entering the precision realm for RK

Main actors (experiments) in the challenge to push down precision on RK:KLOE

Preliminary result with 2001 – 2005 data: RK = (2.55 ± 5stat ± 5syst)×10-5

from 8000 Ke2 candidates (3% accuracy), perspectives to reach 1% errorafter analysis completion.NA48/2

Preliminary result with 2003 data: RK = (2.416 ± 43stat ± 24syst)×10-5, from~ 4000 Ke2 candidates, statistical error dominating (2% accuracy).

Preliminary result with 2004 data: RK = (2.455 ± 45stat ± 41syst)×10-5, from~ 4000 Ke2 candidates from special minimum bias run (3% accuracy).NA62Collected ~ 100,000 Ke2 events in dedicated 2007 run, aims at breakingthe 1% precision wall, possibly reaching < ~ 0.5 %.

Experimental Status

PDG2006 + three new preliminary measurements by NA48/2 and KLOE give 1.3% total accuracy and are in agreement with SM prediction:

RK=(2.475±0.032)×10-5

SUSY limits by RK measurements

(MH,GeV)

NA48/2 Experiment

Unseparated, simultaneous K± highly collimated beams , designed to precisely measure K±→π+π-π± (π0 π0 π±) dalitz-plot density:· pK ~ 60 GeV/c, σp ~ 3 Gev/c (3.8% p-bite)· spot of ~ 5 mm width DCH1 entranceTrack decay products with 4 DCH’s:· PT kick of 121 MeV/c after DCH2· σp/p ~ 1.02% 0.044% p [GeV/c]

NA48/2 Experiment

Scintillator hodoscope:

• establish event time (σ~150 ps), initiate trigger

LKr calorimeter: efficient vetoing, e.m. energy resolution

– σE/E = 3.2%/ √E[GeV] 9%/E[GeV] 0.42%

– σx,y = 4.2mm/√E[GeV] 0.6 mm, granularity of ~13,000 2×2 cm2 cells

Hadron calorimeter, Muon veto system

Analysis starting samples:

– 1-track minimum bias trigger, acquired during 12-hour run in 2003

– 1-track triggers, acquired during 56-hour special run in 2004

Ke2 trigger: 1 track (hodoscope) + ELKr > 10 GeV

Kμ2 trigger: 1 track (hodoscope), possibly downscaled

Analysis of Ke2/Kμ2 at NA48/2

Main analysis steps for Ke2, Kμ2 selection:

One track with 15 < p < 50 GeV, passing acceptance + quality cuts

Data MC Ke2

p (GeV) p (GeV)

Analysis of Ke2/Kμ2 at NA48/2

Main analysis steps for Ke2, Kμ2 selection:

One track with 15 < p < 50 GeV, passing acceptance + quality cuts

Data MC Ke2

Zvtx (cm) Zvtx (cm)

Analysis of Ke2/Kμ2 at NA48/2

Main analysis steps for Ke2, Kμ2 selection:Associate electron track to LKr cluster, compute E/PVeto events with extra energy in LKr above 1.5 GeVCalculate missing mass squared, assuming electron

hypothesis

Data (2004)Data (2003)

Ke2 Ke2

E/P E/P

Analysis of Ke2/Kμ2 at NA48/2

Main bkg to Ke2 selection due to Kμ2 with μ faking e cluster, O(5×10-6):– At high momentum, p > ~30 GeV/c, can’t reject Kμ2 using kinematics– Evaluate probability of “catastrophic release” from selected μ sample– Statistics of μ sample induces a systematic error for bkg subtraction

For the analysis of 2004 data:– 1.59% systematic error due to subtraction of Kμ2 bkg for Ke2– 1.85% statistical error due to Ke2 counts

p (GeV) p (GeV)

Nμ bkg (Evts)MM2(e) (GeV/c2)

RK – Present experimental status

Recent (preliminary) results improved greatly with respect to 2006 PDGWorld average, RK= (2.457±32)×10-5, agrees with SM

NA62 Physics Run 2007

Beam + Detector:

• Charged kaons are produced by 400 GeV/c protons impinging on a Be target.

• An achromatic system of dipole magnets selects charged particles ( ~ 90 % π±, 10 % K±) with momentum of (75±2) GeV/c.

• The decay volume is housed in a ~ 110 m long vacuum tank. The decay products are measured in the detector of the former NA48/2 experiment.

Data taking:

• 120 days run period, June – October 2007.

• Minimum Bias-Trigger: Require minimum number of hits in drift chambers and hodoscope + energy deposition in the LKr calorimeter > 10 GeV.

> 120000 K± → e±ν decays recorded

Goal of NA62: Measurement of RK with accuracy < 0.5 %

Ke2 Identification and Background Rejection

Rejection of K±→μ±ν:

Events with ptrack < 35 Gev/c (40 %):

Kinematic separation by “Missing

mass” M2 = (PK – Ptrack)2

Events with ptrack > 35 GeV/c (60 %):

Separation by ratio E/p:

Electrons have E/p ~ 1,

Muons E/p << 1.

P

Most relevant background: Decays of K± → μ±ν (Br = 63 %).All other decays and processes are excluded kinematically or measured in data (e.g. beam halo background).

“Katastrophic Bremsstrahlung”

Problem: “Katastrophic Bremsstrahlung”A very small fraction of muons deposit all energy in the calorimeter by emission of a high energy bremsstrahlung photon. A few 10-6 of muons with E/p > 0.95 being mis-identified as electrons.

Solution: Cover part of the calorimeter with lead plate (see sketch), allowing only muons to pass. Direct measurement of muon background by explicit E/p measurement for

muons.

Background from bremsstrahlung photons

Number of events

Ptrack (GeV/c)

Ke2 Identification and Background Rejection

Use MC for checks of any bias due to the presence of Pb wallCan use Pb wall data to validate results of G4 simulationData-MC agreement looks good

Total Background Estimation

Will subtract background from beam halo directly measuring it:Use K-less data normalized in side-band of negative Mmiss

2

Expect ~ 7% total background to Ke2, dominated by Kμ2

Analysis of Ke2/Kμ2 at NA62

Data taking lasted 4 months:NA62 has the world largest data set, exceeding 100000 Ke2 eventsLargest homogeneus period (K+ data), lasted ~1 month: > 40000 Ke2 events

Conclusions

Golden LFV observable is RK:

• Preliminary mmt 2% from NA48/2, 3% from KLOE

• Results from preliminary measurement in agreement with SM

Huge effort to enter the precision realm for RK:

• KLOE expects to push error to ~1.3%, after analysis completion

• NA62 dedicated 2007 run aims at reaching <0.5% error