Ke2/Km2

1
Ke2/Km2

• Riunione di Gruppo I
• Napoli 16/12/08
• Paolo Massarotti
• 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

2
K?e?e in the Standard Model

• In the Standard Model(SM), the decay K?e?e
  (Ke2) is strongly helicity-suppressed compared to
  the muonic channel K?µ?µ(Kµ2).
• The ratio of the decay rates can be
  predicted very precisely and is given as

where dRQED -3.6 is a correction due to
Kl2?(IB) and virtual photon processes
(Cirigliano, Rosell, JHEP 0710005, 2007).
3
K?e?e in Supersymmetry

• In Supersymmetry (SUSY), K?e?e can proceed via
  exchange of a charged Higgs H instead of W, but
  in this case, at tree level, the ratio
  G(K?e?e)/G(K?µ?µ) remains unchanged.
• However, loop effects are predicted to lead to
  lepton flavour violating (LFV) couplings lH?t,
  which noticeably change only the rate of K?e?e
  (Masiero, Paradisi, Petronzio, PRD 74, 2006)
• The LFV parameter ?13 should be of o(10-4
  10-3) (similar to LFV in the neutrino sector).
  For given values of MH and tan ß, 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?
Loop diagram for K?e?t
4
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 error
• after 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).
• NA62
• Collected 100,000 Ke2 events in dedicated 2007
  run, aims at breaking
• the 1 precision wall, possibly reaching lt 0.5
  .

5
NA48/2 Experiment

• Scintillator hodoscope
• establish event time (s150 ps), initiate trigger
• LKr calorimeter efficient vetoing, e.m. energy
  resolution
• sE/E 3.2/ vEGeV 9/EGeV
  0.42
• sx,y 4.2mm/vEGeV 0.6 mm, granularity of
  13,000 22 cm2 cells
• Hadron calorimeter, Muon veto system

• Unseparated, simultaneous K highly collimated
  beams , designed
• to precisely measure K?pp-p (p0 p0 p)
  dalitz-plot density
• pK 60 GeV/c, sp 3 Gev/c (3.8 p-bite)
• spot of 5 mm width DCH1 entrance
• Track decay products with 4 DCHs
• PT kick of 121 MeV/c after DCH2
• sp/p 1.02 0.044 p GeV/c

6
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 p, 10 K) with
  momentum of (752) 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 gt 10 GeV.
• gt 120000 K ? e? decays recorded
• Goal of NA62 Measurement of RK with accuracy lt
  0.5

7
NA62 Physics Run 2007
Design of NA62 run optimized for Ke2/K?2, major
parameters tuned PK 60 GeV ? 75
GeV Momentum bite 3.8 ? 2.5 P? kick 120 MeV
? 263 MeV
MM2 resolution improved Better separation for Ke2
and K?2
?(MM2) (GeV2)
Data 2007
2003/4 pars 2007 pars
Expected
MM2(e) (GeV2)
P? (GeV)
Resolution contributions PK, P?, ?K?
8
Analysis method selection algorithm

• Basic principles for data reduction
• Filter Ke2 (Km2) from events triggered by 1TRKLM
  ELKr gt 10GeV (1TRKLM, downscaled by 150)
• Common to Ke2 and Km2 selections
• 1 track reconstructed by DCH, 15 lt p lt 65 GeV
• No cluster of significant energy unconnected to
  track
• For Ke2, one connected cluster with 0.95 lt E/P lt
  1.1
• For Km2, require E/P lt 0.2 or no cluster connected

9
Ke2 Identification and Background Rejection
Most relevant background Decays of K ? µ? (Br
63 ). All other decays and processes are
excluded kinematically or measured in data (e.g.
beam halo background).
P

• Rejection of K?µ?
• Events with ptrack lt 35 Gev/c (40 )
• Kinematic separation by Missing
• mass M2 (PK Ptrack)2
• Events with ptrack gt 35 GeV/c (60 )
• Separation by ratio E/p
• Electrons have E/p 1, (NAPLES)
• Muons E/p ltlt 1.

10
M2Miss(l) Analysis

• Data taking lasted 4 months
• NA62 has the world largest data set, exceeding
  100000 Ke2 events
• Largest homogeneus period (K data), lasted 1
  month gt 40000 Ke2 events

11
E/P 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 gt 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.

12
E/P muons contamination
Check probability for ? to fake e, directly
measuring it Subsample of data taken with Pb
wall between HODs Use HOD pulse heights to
select ?s with MIP energy loss in Pb
Number of events
Ptrack (GeV)
13
Muons contamination Data-MC comparison
Use MC for checks of any bias due to the presence
of Pb wall Can use Pb wall data to validate
results of G4 simulation Data-MC agreement looks
good
, 10?6 units, Pb wall present
Data
MC
Ptrack (GeV)
14
Backgrounds subtracion
Will subtract background from beam halo directly
measuring it Use K-less data normalized in
side-band of negative Mmiss2 Expect 7 total
bkg to Ke2, dominated by K?2
15
RK Present and future sensitivity to NP
Sensitivity shown as 95-CL excluded regions in
the tan??- MH plane, for fixed values of the 1-3
slepton-mass matrix element, ?13
10-3,0.5?10-3,10-4
Present world avg RK 2.457(32)?10-5
tan?
tan?
MH (GeV)
MH (GeV)
16
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 lt0.5
  error

17
E/P evaluation for high momentum electron using
the KL?p e neruns 20433 - 20435
F. Ambrosino, P. Massarotti, F. Perfetto.

18
Event section
We use the KL runs because the KL momentum is
between 30 until 100 GeV. The electon momentum
(KLe3) is comparable with the Ke2 one Our
goal is to select the KLe3 decay. The main
backgrounds are due to KLm3 and KL3p decays
g
.
KL
Lkr
e
p-
19
Signal selection KLe3

• The signal is obtained asking
• one particle with 0.2 E/p 0.7,
• the other is the electron/positron candidate
  track-cluster.
• Track Muon status -1
• No clusters in the LKr or only one in the
  direction of the not track (big box)
• Associated cluster quality status lt 4

20
Searching for a photon from external
bremsstrahlung KLe3
Only one cluster in the LKr in the direction of
the not deflected elctron candidate track
We define two boxes The BIG one The small
one DX lt 2 cm DX lt 0.75 DY lt 2 cm
DY lt 0.8
The first is used for the selection of the
global signal, the second one is used the
selection of a pure KLe3g distribution
(external bremsstrahlung)
21
Pure electrons selection KLe3 with e?eg

• The signal is obtained asking
• one particle with 0.2 E/p 0.7,
• the other is the electron/positron candidate
  track-cluster.
• Track Muon status -1
• One neutral cluster in the direction of the not
  deflected track in the small box
• Associated cluster quality status lt 4

22
Pions selection KLp3

• The pions distribution signal is obtained asking
• Track Muon status -1
• Two neutralclusters in the LKr
• p0 invariant mass between 125 and 145 MeV
• KL invariant mass between 480 and 510 MeV

23
Mouns Selection KLm3

• The muons distribution is obtained asking
• one particle with 0.2 E/p 0.7,
• the other is the muon candidate
  track-cluster.
• Track Muon status 1

24
Signal selection KLe3

• The signal is obtained asking
• one particle with 0.2 E/p 0.7,
• the other is the electron/positron candidate
  track-cluster.
• Track Muon status -1
• No clusters in the LKr or only one in the
  direction of the not track (big box)
• Associated cluster quality status lt 4

m
e
p
25
Strategies background evaluations

• Using the shape of the different decays (KLe3g,
  KLp3, KLm3) we can
• fit whole the region before the peak and the peak
  in E/P using the 3 contributions
• fit the region before the peak, between 0.0 and
  0.5 in E/P, using only the
• background contributions (KLp3, KLm3)
• fit whole the region before the peak and the
  peak, between 0.6 and 1.0 in E/P
• using the KLe3g and KLp3 contributions
• subtract formally the two backgrounds (KLp3, KLm3)

26
Fit results very good agreement!

• fit whole the region before the peak and the peak
  in E/P using the 3 contributions
• (KLe3g, KLp3, KLm3)
• fit the region before the peak, between 0.0 and
  0.5 in E/P, using only the background
  contributions (KLp3, KLm3)
• fit whole the region before the peak and the
  peak, between 0.6 and 1.0 in E/P using the KLe3g
  and KLp3 contributions
• subtract formally the backgrounds (KLp3, KLm3)

!!!!Agreement for the four methods in 0.1-0.2
!!!!
27
Conclusions

• Result good agreement between the different
  procedures
• Our goals
• extend these fits to the very high momentum
  region (55 to 65 GeV)
• evaluate the E/P inefficiency in the semileptonic
  charged channel (Ke3)

28
SPARES
29
Background from bremsstrahlung photons
Number of events
Ptrack (GeV/c)
30
Total Background Estimation
Will subtract background from beam halo directly
measuring it Use K-less data normalized in
side-band of negative Mmiss2 Expect 7 total
background to Ke2, dominated by Kµ2
31
Analysis of Ke2/K?2 at NA62
Data taking lasted 4 months NA62 has the world
largest data set, exceeding 100 K Ke2
evts Largest homogeneus period (K data), lasted
1 month gt 40 K Ke2 evts
N(Ke2) (Kevts)
Data taking shifts (8 hours)
Ke2
K?2
MM2(e) (GeV2)
MM2(?) (GeV2)
32
Ke2 Identification and Background Rejection
Use MC for checks of any bias due to the presence
of Pb wall Can use Pb wall data to validate
results of G4 simulation Data-MC agreement looks
good
33
Searching for a photon from external
bremsstrahlung
Only one cluster in the LKr in the direction of
the not deflected elctron candidate track
34
Searching for a photon from external
bremsstrahlung
35
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(2.4750.032)10-5

(MH,GeV)
SUSY limits by RK measurements
36
Sources of systematic error eLKr(Ke2)

• Cluster efficiency for electrons
• Control check independent efficiency evaluation
  from data KLe3 (Mainz. Independent analysis from
  Neaples is in progress)
• Difference between Ke3c and Ke3n correction
  factors is 0.025