# Presentation: 536296

## Future Prospects for K Decay Experiments

- L. Littenberg
- BNL

## Outline

- What should we be doing?
- Is (American) history any guide?
- The leading opportunities
- K++
- KL0
- KL0l+l-
- Conclusions

## What should we be doing?

- KL0 & K++
- NP governed by a single effective operator
- Determine the IP_x0006__x0006_ & modulus of any NP contribution
- Can be calculated in almost any model
- Sensitive to high mass scales
- KL0e+e- & KL0+-
- In SM, same information as KL0
-  NP can contribute to >1 operator: richer but harder to interpret
- Recent developments show these more accessible!
- LFV processes, T-violation, medium rare
- Some pion decays

**Notes:**
At the end, introduce the U.S. K ->pi,nu,nubar intiatives

## BNL AGS E949 K++ 

## FNAL CKM K++ Experiment

## FNAL P940 K++  

- FNAL P940 K++

## KAMI KL0 at FNAL

- KAMI KL0 at FNAL

## KOPIO KL0 at BNL

## Experimental considerations for K++

- BR ~ 8  10-11
- 3-body decay, only 1 visible
- + common K decay product
- Backgrounds:
- K++()
- K+ + 0
- K+nK0p; KL  + l-, l- missed
- Ke4
- Beam (stopped-K configuration)
- Beam + mis-ID as K+, then fakes K decay at rest
- K+ decay in flight
- 2 beam particles
- Beam (in-flight)
- Beam + mis-ID as K+, then interacts
- 2 beam particles

- _

- pnn1

- pnn2

## E787/949 

- Solenoidal detector at the end of a stopped K+ beam

##  E787/949 Technique

-  Incoming 700MeV/c beam K+: identified by Č, WC, scintillator hodoscope (B4). Slowed down by BeO

-  K+ stops & decays at rest in scintillating fiber target – measure delay (2ns)

-  Outgoing +: verified by IC, VC, T counter. Momentum measured in UTC, energy & range in RS and target           (1T magnetic field parallel to beam)

-  + stops & decays in RS – detect  ++e+ chain

-  Photons vetoed hermetically in BV-BVL, RS, EC, CO, USPV, DSPV

** **E787/949 Technique

## E787/949 Analysis Strategy

-  Measure background level with real data
-  To avoid bias,
-    1/3 of data  cut development
-    2/3 of data  background measurement
-  Characterize backgrounds using back-
-   ground functions
-  Likelihood Analysis

- Signal region “the BOX”

- Background sources

- Analysis Strategy

- Identify *a priori. *at least 2 independent cuts to target each background:

**K****+**********************

- PNN1: p > p(K++0) = 205MeV/c

-  K++0
-  muon background (K++(),...)
-  Beam background
-  etc.

## E787/949 Events

- B(K+******+******) =
-  (1.47+1.30-0.89)10-10

**_**

## Combined E787/949 Result

- (68% CL interval)
- E787 result:

## Status & prospects for E949

* *E949 detector worked well
-  Obtained ~2/3 sensitivity of E787 in 12 weeks (1/3 pnn1+1/3 pnn2)
-  Found one new pnn1 candidate
-  pnn2 analysis currently in progress – looks promising
-   AGS & beamline problems cost a factor ~2 in sensitivity/hour
-   DOE cut off experiment after 12 of 60 promised weeks

## J-PARC K++ LOI

-  Stopped K+ experiment
-  Builds on E787/949 experience
-  Lower energy separated beam
-  Higher B spectrometer
-  More compact apparatus
-  Better resolution
-  Finer segmentation
-  Improved  veto (crystal barrel)
-  Aims for 50 events
-  Not an early experiment for J-PARC
-  Needs longer spill than planned
-  beamline
-  place on the floor
-  $ for detector

## P326 (NA48/3) K++ 

- Proposal submitted to CERN for ~100 events

## P326 Technique

- Detection in-flight
- High energy (75 GeV/c) unseparated beam (800 MHz!)
- Careful design to keep halo to ~7MHz
- Measure all beam tracks (“Gigatracker”)
- Differential Cerenkov (“CEDAR”) for K ID
- Redundant measurement of pion momentum
- Two-stage magnetic spectrometer (straws in vacuum)
- Require large missing momentum
- Redundant pion I.D.
- Magnetized hadron calorimeter (“MAMUD” + RICH)
- (Almost) hermetic photon veto system
- NA48 liquid Krypton calorimeter
- Small angle charged & neutral vetoes (beam bent out of the way)
- Wide-angle frame anti’s

## Kinematics

- + momentum cut requires a huge momentum mismeasurement to mistake a beam + for a final state particle (7535 GeV/c).  Also guarantees large missing momentum, *e.g.* so that there’s plenty of  energy from *K**+* +0

- Assumption of pion mass spreads out *K**+*+ peak
- 0.3% resolution on pK and
- 1% resolution on p allows >10% acceptance

## Plan for P326

- 2005
- Gigatracker R&D
- Vacuum tests
- Technical design & cost estimate
- 2006
- Detector tests in present beam
- Construction & installation 2007-8
- Construct new beamline
- Construct & install detectors
- 2009-10
- Data taking
- Expect ~80 events with S:B ~ 8:1

## Stopped vs In-flight K+

|  | E949 (prop.) | P326 |
| --- | --- | --- |
| beam/spill | 25M | 2500M |
| K+/spill | 30M | 125M |
| purity | 0.8 | 0.05 |
| duty factor | 4.1/6.4 | 4.8/16.8 |
| inst. beam rate | 7.2MHz | 800MHz |
| ‘decay’ factor | 26% | 10% |
| DKs/clock-sec | 0.81M | 0.74M |
| DAQ/veto livetime | 1/1.7 | 1/1.7 |
| eff. DK/clock-sec | 0.48M | 0.44M |
| acceptance | 0.3% | 17% |
| acc. DK/clock-sec | 1400 | 74000 |

## Stopped vs In-flight K+

|  | E949 (prop.) | P326 | J-PARC |
| --- | --- | --- | --- |
| beam/spill | 25M | 2500M | 41M |
| K+/spill | 30M | 125M | 31M |
| purity | 0.8 | 0.05 | 0.75 |
| duty factor | 4.1s/6.4s | 4.8s/16.8s | 1.7s/4.4s |
| inst. beam rate | 7.2MHz | 800MHz | 24MHz |
| ‘decay’ factor | 26% | 10% | 40% |
| DKs/clock-sec | 0.81M | 0.74M | 2.8M |
| DAQ/veto livetime | 1/1.7 | 1/1.7 | 1/1.9 |
| eff. DK/clock-sec | 0.48M | 0.44M | 1.5M |
| acceptance | 0.3% | 17% | 0.6% |
| acc. DK/clock-sec | 1400 | 74000 | 8800 |

- How far can stopped exp. be pushed?

## How to pursue K++?

- In-flight has the “appeal of the new”
- The only way to get >100 events
- But requires 11 O.M. leap!
- Watch out for tails, acceptance losses, the unexpected
- Stopping experiment very well understood
- Technique shown to have sufficient S/B
- Any further improvements can increase acceptance
- Note acceptance of 787/949 is ~0.002-0.003
- Plenty of room for improvement!
- Trick is to increase the beam rate w/o losing acceptance
- Could ***really know*** if 50-100 events possible
- But so far very little support for such an experiment

## The Challenge of KL0

- B(KL0) ~ 310-11, need intense flux of K’s
- rates inevitably rather high
- hard to minimize both random vetoing & veto-blindness
- Kinematic signature weak (2 particles undetectable)
- Backgrounds with 0 up to 1010 times larger than signal
- Veto inefficiency on extra particles, both charged particles and photons, must be 10-4
- Self-vetoing is a problem
- shower spreading makes it hard to maximize both signal efficiency and veto power
- Huge flux of neutrons in beam
- can make 0 off residual gas – requires high vacuum
- halo must be tiny
- hermeticity requires photon veto in this beam
- Need convincing measurement of background

**_**

- _

## 1st dedicated KL0 experiment - E391a

- 1st dedicated KL0 experiment - E391a

- Pencil
- beam

-  KEK 12 GeV PS
-   4° “pencil” beam
-   <pK> ~ 2 GeV/c
-   CsI calorimeter

## Pencil Beam

- 5 stages of collimators made of heavy metal (tungsten)
- 2 stages of sweeping magnets
- Thermal neutron absorber
- Pb/Be plug for control of  /neutron flux
- Fine alignment using telescope
**GEANT3 M.C. agrees well with the measurements**

## 2 analysis

- 2 analysis

- Data without tight veto (from Run 1)

- Reconstructed vertex (cm)

- PT(GeV/c)

- M.C. for KL decays ( Without Normalization)

- o produced at CC02

## Veto Optimization                         ~Main-barrel timing (low E sample)~

- KL pure sample

- B.G.sample

- upstream

- downstream

- Backsplash  should NOT veto!

- Real photon hit  should veto.
- Backsplash  should NOT veto.

**1**

**2**

- early

- late

## E391a Result from 10% of Run I

- No events observed/expected background of 0.030.01 events (mainly K2)
- 1.14109 KL decay, 0.0073 acceptance  s.e.s of 1.1710-7
- B(KL0)<2.86 10-7 @ 90% CL (c.f. 5.9 10-7 from KTeV)

- Z(cm)

- PT(GeV/c)

## Better quality of data (online plots)

- Reconstructed vertex (cm)

- PT(GeV/c)

- Run-I

- Run-II

- Run-II analysis

- Run-III (starting now)

## E391a status & prospects

- First physics run Feb-June 2004
- 2.21012 12 GeV _x0013_POT, 50% duty factor
- 5 105 KL/pulse
- Detector worked well
- Nominal s.e.s. 410-10
- But acceptance ~ 15 lower than in proposal (0.0073)
-  first sight of the enemy
- Halo neutrons, self-vetoing, etc.
-  Analysis of 10% of data  B(KL0)<2.86 10-7
- Run II, Feb-March 2005
- Many problems fixed, 60% of Run 1
- Run III, Nov-Dec 2005, starting now

## JPARC Phase I Beamlines

## KEK-PS to J-PARC

-  1014 interacting 30 GeV protons/cycle, 5sr beamline @ 16°
-  22MHz KL @ 20m, <pK> = 2.1 GeV/c, 9%/5m decay
-  4% acceptance
-  23 events in 3 Snowmass years (competition from )
-  S:B~1:1 (optimization studies in progress)_x0013_

## Step by step at JPARC

- “Step by step” approach, learning as they go
- Different beam angles, lengths
- Larger detector
- Eventual goal – few 100 evts

| Tgt     z-det | Beam size | KL flux (MHz) | Decay prob | Decay rate |
| --- | --- | --- | --- | --- |
| 16°      20m | 10cm | 27MHz | 25% | 6.8MHz |
| 5°        50m | 10cm | 63MHz | 11% | 6.9MHz |
| 8°        70m | 15cm | 28MHz | 14% | 3.9MHz |

## KL0 Experiment

- prod.
- tgt

- veto

- calor.

- beam veto

## KL0 Experiment

- prod.
- tgt

- veto

- calor.

- beam
- veto

- prerad

## In the KLCoM

- In the KLCoM

-  Bckgnd mainly in discrete areas
-  Obvious for KL00 “even”
-  But even “odd” case not ubiquitous

-  K3 infests slightly different area

-  Even after all bckgrnds accounted for, still some clear space for signal
-  Can get factor 50-100

## In the KLCoM

- In the KLCoM

-  Bckgnd mainly in discrete areas
-  Obvious for KL00 “even”
-  But even “odd” case not ubiquitous

-  K3 infests slightly different area

-  Even after all bckgrnds accounted for, still some clear space for signal
-  Can get factor 50-100

## KOPIO Technique

- High intensity micro-bunched beam from the AGS
- Measure everything! (energy, position, angle, time)
- Eliminate extra charged particles or *photons*
- KOPIO: 0 inefficiency < 10-8
- Suppress backgrounds
- Predict backgrounds *from data*:dual cuts
- Use “blind” analysis techniques
- Test predictions “outside the box”
- Weight candidate events with S/N likelihood function

## Is KOPIO completely dead?

- AGS E926 is certainly completely dead
- But we asked whether Fermilab might be interested
- Would use 8 GeV proton beam from Booster
- Answer - not immediately, but maybe later
- Agreed to have accelerator physicists help
- Some studies were done ....

## First Surprise

- Useful K flux not much impacted!

## How to get the protons

## Dave McGinnis’ Scenarios

- NUMI and 4/23 batches to KOPIO
- Accelerator cost not including slow extraction ~10-12 M$
- Constraints
- Cycle time – 1.5 Seconds
- C.O.D 6 mm
- _Loss 550W_
- Booster Notching  - 3 / 84 bunches
- Improvement comes because notching is not needed for momentum stacking KOPIO beam
- NUMI – 12 batches @ 4.8e12 protons/batch
- KOPIO – 4.6e16protons/hr
- NUMI and 8/23 batches to KOPIO
- Constraints
- Cycle time – 1.5 Seconds
- C.O.D 6 mm
- _Loss 600 W_
- Booster Notching  - 2.4 / 84 bunches
- Improvement comes because notching is not needed for momentum stacking KOPIO beam
- NUMI – 12 batches @ 4.8e12 protons/batch
- KOPIO – 9.2e16protons/hr

## Second Surprise

- To run NUMI and KOPIO at 4.6e16 protons/hour:
- Decrease the closed orbit distortion by 40% from present
- Decrease the notching loss by 43% from present
- Increase the permitted loss in the Booster tunnel by 20% from present
- KOPIO spill length 82% of cycle

- From Mc Ginnis’ Summary:

- KOPIO is VERY sensitive to duty factor (67% @ AGS)
- Inst. K rate a little lower than at AGS
- Bottom line - Sensitivity/hour = 93% of AGS

## Third Surprise

- When E926 (KOPIO) proposed, asked for 3000 hours per year for three years.
- That was thought aggressive but possible in the “AGS-2000” era
- When RSVP plan finally set, we were given 6240 hours over three years
- But not all at 100 TP, equivalent was 5684 hours
- Fermilab seems to assume ~6000 hours/year

## Discovering/Constraining New Physics

## Discovering/Constraining New Physics        at FNAL

- Discovering/Constraining New Physics        at FNAL

## How many K’s needed to see KL0+-?

- From thesis of M. Sadamoto
- Osaka, 1999
- (KTeV)

## How many K’s needed?

-  In 1999, thought it would require two further generations of experiments to see KL0+- at 3 (KL0e+e- similar)
-  Also, most people thought in terms of measuring SM , so high precision needed
-  **Situation today is completely different**

## New Situation for KL0l+l-

- Now that NA48 observed KS0l+l- at higher end of expectation
- & arguments for constructive interference between mixing & direct CP-violating components strong as ever
- SM expectation for KL0l+l- rather large:
- B(KL0e+e-) = (41)10-11
- B(KL0+-) = (1.50.3)10-11
- Compare with KLl+l- background (worst one):

|  | KL0e+e- (‘99) | KL0+- (‘97) |
| --- | --- | --- |
| KTeV s.e.s.. | 10.410-11 | 7.510-11 |
| KLl+l- evts | 0.99 | 0.37 |
| B(KLl+l- )effective | 10.310-11 | 2.810-11 |
| S:B | 1:2.5 | 1:1.9 |

## Motivation for KL0l+l-

- Add to this, now  we are interested in bigger game than  Imt.
*E.g.*, from Isidori, *et al*., hep-ph/0404127      

-  Take KaMI as example of a next-generation experiment with sensitivity to KL0+-. In 3 years, KaMI would have reached a s.e.s of 410-13.
-  In the example above, would collect 110±13 signal events (with 70 events of background) compared with a SM expectation of 37 events.
-  Similar sensitivity experiment possible for KL0e+e-

## Conclusions

- K experiments extinct in US
- State of the art in 4 of the 5 most interesting decays
- Some data analysis continuing
- Experiments could be mounted at FNAL (KOPIO?)
- But K experiments seem low on DOE priority list
- Continuing K program in Japan
- K++ proposal at CERN
- Could use a KOPIO-type KL0 experiment
- or at least one with photon pointing.
- KL0l+l- experiment(s) now seem very worthwhile
- Could be done!
- But no proposals yet