Transcript New results for K+

New results for K+→+ at low
+ momentum from BNL E949
Artur Shaykhiev
(for E949 collaboration)
Institute for Nuclear Research
Moscow
New trends in high-energy physics (experiment, phenomenology, theory),
Yalta, Crimea, Ukraine, September 27 - October 4, 2008
Outline





Motivation
Experiment E949
Backgrounds: suppression and estimation
Results
Future
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
2
Motivation
VCKM
 Vud

  Vcd
V
 td
Vus Vub 

Vcs Vcb 
Vts Vtb 
 1   / 2





1


/2

 A  (1    i )  A 2

A  (   i ) 

2
A


1

Goal: Independently define
the Unitarity triangle/ CKM matrix (A, λ, η, ρ
Wolfenstein parameters)
A better determination of Vtd from K+ π+νν
will provide a sensitive test of the SM by comparing
the results from the K and B sector and probe new
physics
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
3
Sensitive probe to new physics
B(KL0)4.4B(K++)
Grossman – Nir Bound
BSM (K     )  (0.85  0.07) 1010
J. Brod and M. Gorbahn
arXiv:0805.4119
Because of the strong
suppression of the
sd short-distance
amplitude in the SM,
rare K decays are the
most sensitive probes
of new physics.
SM prediction
Excluded by E787/E949 results
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
4
Previous results
PNN1
PNN2
[211,229]
[140,195]
1995-98(E787) and 2002(E949)
1996-97(E787)
Stopped K+
7.71012
1.71012
Candidates
3
1
Background
0.450.06
1.220.24
P(+)(MeV/c)
Years
BR(K+ + )
Crimea08, Sept 27 - Oct 4
(147
. 10..3089 )  1010 (68%CL)
A.Shaykhiev, INR
< 2210-10 (90%CL)
5

Arbitrary Units
The experimental signature of K+ + is single + + “nothing”
K


Momentum (MeV/c)
Momentum spectra of charged particles from K+ decays in
the rest frame
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
6
E949 Detector
 Incoming ~700 MeV/c beam K slowed down
by BeO and AD
 K+ stops and decays at rest in scintillating
fiber target – measure delay (at least 2 ns)
 Outgoing + momentum is measured in UTC,
energy & range in RS and target
 + stops & decays in RS – detect
++e+ chain
 Photons veto in BV – BVL, RS, EC, CO,
USPV, DSPV
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
7
E949 analysis strategy





A priori identification of background sources
Use at least two uncorrelated cuts to estimate each background
using information outside of the signal region
Set cuts using 1/3 of data then measure backgrounds with
remaining 2/3 sample
Verify backgrounds estimate by loosening cuts and comparing
observed and predicted rates
“Blind” analysis: DON’T examine signal region until all backgrounds
verified
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
8
Backgrounds
Background
PNN2
Crimea08, Sept 27 - Oct 4
BR (·10-3)
K+→π+πo (Kπ2)
212
Beam particles
-
K+→π+π-e+νe (Ke4)
0.039
K+→π+πoγ (Kπ2γ)
0.275
K+→πoμ+νμ (Kμ3)
31.8
K+→μ+νμγ (Kμνγ)
5.5
CEX (K+n→Kop,
KoL → π+μ-νμ or
KoL → π+e-νe )
Prob=0.0015
0.135
0.194
A.Shaykhiev, INR
9
Main background is K++0
Regular   0
  Target Scatter
  Target Scatter


K
K
Target

Target


  and  0 are back to back so photons
  scatters in the target, loses energy
are directed to efficient part of the
photon veto
and the photons lose directional
correlation with the  
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
10
Suppression of K2- scatter events


Photon veto of 0 . ’s may go to beam region => photon
detection in this region is important
Identification of + scattering in the target:
 kink in the pattern of the target fibers
 + track that doesn’t point back to the K+ decay point
 energy deposits inconsistent with an outgoing +


K
Target

Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
11
Identification of + scattering
Kink in the pattern of target
fibers
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
12
Two cuts used for suppression +-scatter events are
Photon veto
photon veto and target-scatter cuts
B
D
C+D
A
B
C
target
Signal region
photon veto is inverted,
target cuts are applied.
Background
A = BC/D
C
target cuts are inverted,
photon veto is applied.
Sample “B” also contains K2-scatter in range stack and K++0 events.
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
13

Ke4 process forms a background
when the - and e+ interact in
the target without leaving a
detectable trace

Ke4 background could not be
distinguished from the larger
K2-scatter background based
solely on the + track, and we
cannot make a purely databased background estimate
 + momentum (MeV/c)
K++-e+(Ke4) background
Use both data and Monte Carlo
to estimate this background
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
   e  kinetic energy (MeV)
14
K++-e+(Ke4) background
Use target pattern recognition to
isolate Ke4 sample
Crimea08, Sept 27 - Oct 4
Estimate the rejection power of the
target pattern recognition using
simulated data supplemented by the
measured   energy deposit in
scintillator
A.Shaykhiev, INR
15
Charge exchange (CEX) background
Target
K  n  K L0 p
KL0   l  l
This background is suppressed by K+ decay time condition
(Ks lifetime is only 0.1 ns ) and target pattern recognition
to identify gaps between K+ and + tracks. Additional
suppression is provided by detecting the negative lepton.
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
16
Due to kinematics constrain only multi-body
decays K++0 and K++ are important
This backgrounds are suppressed
kinematically and by identification of
+ decay
Range (cm)
Muon background
Decay chain:
+
+
Momentum (MeV/c)
e+
Muons have different kinematics
parameters than pions
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
17
Beam backgrounds
Double-beam
Single-beam
Suppressed by time delay
Crimea08, Sept 27 - Oct 4
Suppressed by looking for any extra
activities that are coincident with the
charged track in the beam instrumentation
A.Shaykhiev, INR
18
Total background in pnn2
Compared to E787 pnn2 analysis, our total background was decreased by 24%
and total acceptance was increased by 63%
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
19
Division of signal region
Use four cuts to divide the signal region into nine cells
with differing the relative signal-to-background levels
Photon Veto

K Decay Time
K


  e 
Decay Sequence


Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR

e
  Kinematics


20
Examine the signal region
The probability of
all 3 events to be
due to background
only is 3.7%
Solid line represents signal
region, dashed shows
tightened kinematics cuts.
Gray points are simulated
From these three new candidates alone
10
B(K     )  (7.899.26
)

10
5.10
K++
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
21
Combined E787/E949 results
10
B(K     )  (1.731.15
)

10
1.05
Branching Ratio (x10-10)
3.5
 Standard
Model
3
2.5
 PNN1
Only
2
1.73
1.5
1
1.47
 ALL
E787/E949
0.85
0.5
0
The probability that all seven events
were due to background only is 0.1%
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
Consistent with SM prediction
22
The future




NA62 (formerly NA48/3) in preparation at CERN.
The use of kaon decay-in-flight to measure
K++ has not been attempted before.
NA62 proposes to observe  65 K++ events
There is a letter of intent for a stopped kaon
decay experiment in Japan.
Analyses of E949/E787 for other K+ decay
modes still continue.
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
23
Thank you! Questions?
Special thanks to David Jaffe and Joss Ives for the
resources that made this talk possible
Crimea08, Sept 27 - Oct 4
A.Shaykhiev, INR
24