Lepton universality in K l n decays R

Lepton universality in K → l n decays R. Fantechi INFN – Sezione di Pisa on behalf of the NA 48 Collaboration HEPP EPS 2007 – Manchester, July 19 th 2007

Outline – Physics motivations – Results from NA 48 data collected in 2003/2004 – Run 2007 – Conclusions R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 2

Physics motivations Well predicted by the Standard Model, including radiative corrections (M. Finkemeier, PLB 387 (1996)) RK(SM) = (2. 472 ± 0. 001) · 10 -5 0. 04% precision!! Caveat: radiative corrections model dependent (VMD). c. PT study underway PDG average = (2. 45 ± 0. 11) · 10 -5 From three experiments from the 70’s, with kaons at rest R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 3

Physics motivations • The value of RK could be different in case of SUSY LFV – Masiero, Paradisi, Petronzio, Phys. Rev. D 74 (2006) 011701 • The difference wrt SM could be as large as 3% in the hypothesis of tanb~50 e MH+~500 Ge. V/c 2 – It scales as tanb 6 • Kaon sector is favoured wrt to heavier mesons – Golden mode to search for SUSY LFV effects – NA 48 has measured RK in 2003 and 2004 and it is running for a measurement with an error down to 0. 3% R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 4

The NA 48 Beam and Detector Simultaneous K+/K- beam (60 ± 3) Ge. V/c momentum LKr Calorimeter: (E)/E 3. 2%/√E 90 Me. V/E 0. 42% Spectrometer: (P)/P 0. 48% 0. 009 P[Ge. V/c]% R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 5

2003 and 2004 runs • 2003 sample collected during 1 month – During the asymmetry run – Downscaled trigger – Level 2 trigger for mass cut Trigger Km 2 1 track in the hodoscope downsc Ke 2 1 track + energy in the calorimeter + L 2 mass cut • 2004 data collected in a special run – – 56 hours of data taking with a minimum bias trigger Trigger Reduced beam intensity K 1 track in the hodoscope downsc No Level 2 trigger K 1 track + energy in the calorimeter Get rid of L 2 inefficiency m 2 e 2 • Similar statistics R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 6

Analysis • Similar kinematics – Keep at maximum the same cuts for both modes • 1 track topology • z of decay vertex • Geometric acceptance at chambers and Lkr calorimeter • Track momentum – Particle identification using ELKr/Pspectr • Muons : E/P < 0. 2 • Electrons: E/P > 0. 95 – Cut on the missing mass with the right lepton mass assignment R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 7

Background • The dominant background is Km 2 Data MC – A muon loses almost all its energy in the calorimeter, giving the E/p of the electron – It is measured from the data in momentum bins • Ke 3 contribution is obtained by MC R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 8

2003 and 2004 results • 2003 data • 2004 data – 4670 ± 77 st +29 -8 sy events – Trigger efficiency is the bigger systematics – 3407 ± 63 st ± 54 sy events – The bigger systematics is the Km 2 background subtraction RK = (2. 416 ± 0. 043 ± 0. 024) • 10 -5 RK = (2. 455 ± 0. 041) • 10 -5 • Cfr. KLOE -T. Spadaro, this conference – ~ 8000 events from 1. 7 fb-1 – Different systematics RK = (2. 55 ± 0. 05) • 10 -5 R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 9

P 326/NA 62 (former NA 48) 2007 run • A dedicated run to measure RK has been approuved for 2007 – 4 months of data taking – Collect ~150000 Ke 2 – Goal to reach an error of 0. 3% • Optimized beam parameters wrt 2004 – Kaon momentum from 60 Ge. V/c to 75 Ge. V/c – Kaon momentum bite from 3 Ge. V/c to 2. 5 Ge. V/c – p. T kick from the spectrometer magnet from 120 to 263 Me. V/c • All these improve the kinematic separation of Ke 2 R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 10

The Run • Running conditions – Same minimum bias trigger as in 2004 • Ke 2: 1 track in the hodoscope + energy in the LKr • Km 2: 1 track downscaled • From 2004 experience, all systematics below 0. 2%, except background to Ke 2 In 2004 limited by the statistics of the control sample – Event filtering performed in L 3 • A factor 2. 4 reduction • Ke 2, Km 2 streams + control triggers and few additional decay modes R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 Tabella di venelin, con freccia error is statistic 11

Background to Ke 2 • It is mainly Km 2 • Below 35 Ge. V/c use kinematic separation – Build M 2 miss with electron assumption • Above 35 Ge. V/c use ELKr/ptr to identify the electron • But 5 • 10 -6 of muons deposit all the energy in the LKr and are identified as electrons Montecarlo simulation equal samples of Ke 2 and Km 2 R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 12

Background to Ke 2 • Measure this background during the run • Put a 9 X 0 lead bar between the hodoscope planes in front of the LKr – – Lose 18 % of acceptance All electrons are killed Only muons pass The E/p distribution of the muons is measured R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 13

Expectations • Expected precision on Rk – Statistical 0. 28% with 150000 Ke 2 decays – Systematic 0. 2% • Thanks to the measurement of the Km 2 background and to the improved beam and spectrometer conditions • The Run – – It is going on since ~ the end of June The detector has restarted well after 3 years Data taking is quite smooth As a reference, a good shift (8 hours) is giving around 500 Ke 2 R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 14

Conclusions • An accurate measurement of RK is capable to explore the parameter space of SUSY LFV • NA 48 has presented two measurements with data collected in 2003 and 2004 compatible with the SM prediction • A dedicated run is under way to collect ~150000 Ke 2 and measure RK with 0. 3% accuracy. • Stay tuned for the result! – May be in the next HEPP-EPS conference… R. Fantechi - HEPP- EPS 2007 Manchester 19/07/2007 15