# Presentation: 352594

##  Preface

* **Preface*

- E787 (which was the previous experiment of E949) had really excellent photon veto, and its performance was

- In order to overtake the performance even in the E949 severe condition (high beam intensity = more accidentals and false veto),  many works were done.

- What kind of modifications were done aiming  more rejection in E949.

- In this poster , we will reveal that :

- (2) How much improvement are gained actually ?

- (3) How we measured such a big rejection and
-      small acceptance in a reliable way ?

- Kp2 :

- Kp2 background rejection :
- Acceptance                        : 80 %

## Powerful  Hermetic Photon Veto System in E949 Experiment

*Powerful  Hermetic Photon Veto System in E949 Experiment*

- and _ Kentaro MIZOUCHI_
-            (Kyoto Univ.)

- 2004 RHIC & AGS Annual User’s Meeting
- (May 10-14, 2004 at Brookhaven National Laboratory.)

- E949 collaboration

## [Introduction]  E949 Experiment

- [Introduction]*  E949 Experiment*

- Measurement of Branching ratio :

-                     exploring an extremely !!!Rare!!! decay region.
- Powerful Background rejection is the key to perform this experiment successfully.

*Small theoretical uncertainty*  (~7%)

- Golden Mode :

- E949/E787 new result :

- Note : E949 is “stopped Kaon Experiment”.

## Role of Photon Veto

*Role of Photon Veto*

- Kp2 events
- (background)

- Signal region

- Br = ~ 10-10

- Event Signature  :

- Fig: _Final plot after opening the signal box._
-       (Charged particle energy vs range
-         distribution)

- Two important keys in this experiment:
-    (1) Accurate and redundant
-                measurement.
-    (2) Tight veto on any extra
-          activity except for       .

- (1)

- (2)

_**mis-detected**_

- _Dominant backgrounds :_

- (called _Kp2_)

- + nothing

- kinematical rejection : 10-4  (E/P/range)

- photon veto rejection should be > 106 .

- Note

## Photon Veto Related Detectors 

*Photon Veto Related Detectors *

***E949 detector***
***Side view***

- Red     :  photon veto system

- Orange: photon veto + charged track
-                                      measurement

- BV/BVL  : Pb + scintillator

- EC           : CsI + CCD readout

- AD          : Pb glass

- _Lots of photon veto systems :_

- Collar PV counter,
- Micro-Collar PV,
- Beam hole PV counter
- Up-stream PV counter
- Down-strem PV counter
- Range stack counter
- Target Scintillation Counter ...

- Detail descriptions are skipped, but
- _Pick out only interesting points._

## Efforts to gain more rejection and save acceptance loss !

*Efforts to gain more rejection and save acceptance loss !*

- Online Trigger modification : Mean-time calculation
- (2) Add more radiation length and cover the 45 degree hole
- (3) Reinforce solid angle coverage (especially around beam line.)

- In order to gain more rejection than E787, while keeping acceptance loss at the same level even under the twice high rate beam intensity,
-  ***A lot of excellent ideas and works  are taken in E949.***

- (rejection)

- (acceptance)

- _Three main improvements :_

- (rejection)

-  TYPE:

## Mean-Time Calculation in Online Trigger (BV/BVL)

*Mean-Time Calculation in Online Trigger (BV/BVL)*

- _Fig:  Online Photon  Hit Timing distribution _

-  E787
- (Normal Analog signal timing)

**E949**

_***Mean-timed***_** **signal arrival time

- Photon Hit Timing

- Case A

- Case B

- End-1

- End-1

- End-2

- End-2

- Propagation delay

- (c : speed of light
- in material)

- But,  Mean-Time

- Propagation delay is smaller than that of case-A.

- photon

- Veto time window is successfully narrowed. (20ns     10ns)

- short

## Add more radiation length and cover “45 degree region”

*Add more radiation length and cover “45 degree region”*

_*Fig :Radiation Length vs polar angle*_

- (polar angle)

- E787

- BVL

- Radiation length

- E949

***45 degree hole***

- (A)add radiation length  and, (B)cover 45 degree region.

-  BVL was installed in order to

- 45 degree line

## Reinforce Solid Angle Coverage (especially beam line region)

***E949 detector***
***end view (upper half)***

***E949 detector***
***side view (upper half)***

*Reinforce Solid Angle Coverage (especially beam line region)*

- Blue     : Updated Photon veto
-              (PV) in E949

- Yellow : PV used since E787

- _Note:_

## Analysis issues

- E949 ran for 12 weeks in 2002 and collect data with the updated photon veto system.

- (1) How to measure the rejection /acceptance accurately

- (2) How to control backgrounds :
-                           aggressive background control

- (3) Final E949 Result : rejection and acceptance.

- We reveal the improvement brought by the PV update/modifications and show ...

## Reliable Rejection/Acceptance measurements

*Reliable Rejection/Acceptance measurements*

- Kp2

- Km2

- Thanks to the accurate and redundant charged track measurement.

- Monochromatic Energy/Momentum/Range

- Two body decays can be tagged well.

- E949 has reliable measurement system of veto performance with real data (not from Monte Carlo).

- Can collect clean control samples !!!
- and measure acc, rej directly.

- (Kp2)

- (Km2)

-  Background rejection      :

- False veto and acc. Loss  :

## How to veto photon(s)

*How to veto photon(s)*

_*Veto algorithm :*_

- Toffset [ns]    Twin [ns]   Ethr [MeV]

- BV    0.50         4.50           0.20

- BL    0.75          2.00           0.00

- EC    0.25          2.25           3.80

- RD   -0.75         1.50           3.80

- ...      ...             ...             ...

-  (1) Not reconstruct

- Veto the event, if
-    the energy sum in the given
-    time window > threshold

- Critical if a photon escape to beam line direction.

- (2) Do not use charged
-      particle information.

- One example : parameter set used in actual analysis.

- _Noticeable points_

- Independency should be kept for correct background estimation.

##  Parameter scan and How to find the best parameter set 

* **Parameter scan and How to find the best parameter set *

*Rej vs Acc trade-off curve.*

*acceptance*

*rejection*

- Search boundary line.
- ( = can not gain further improvement )

- (A) Find the most efficient cut set.

- (B) Get a trade-off function
-        between acc and rej
-            used as **Aggressive** bkg control

- The reason why parameter scan
- is required :

- many accidentals.
-    But certainly
-     have photon hits

- _BV hit timing_

- _EC hit timing_

- Balance of tightness compare to other PV systems should be _***optimized.***_

- Clean photon peak.
-    but Photo-nuclear
-     interaction tail.

- more acc. w/ the same rej.

- Initial points

- more rej. w/ the same acc.

- Boundary
- line

## E949 acceptance and rejection

*E949 acceptance and rejection*

- Acc. vs Rej. Curve.

**E787**

**E949**

- Standard acceptance
_*x 2 better*_

_*x 5 better*_

- E787 (‘98)

- E949

- _Offline rejection_

- _Online rejection_

_**Total rejection**_

- ~50

- Note : Both rejection are measured at the same acceptance points (80%).

- Twice better rejection is achieved !

## Analysis background summary 

- Analysis background summary

_**Aggressive background control**_ :

- Background Type

- Total Background

- Kp2

- Km2

- ...

- ...

-  [@ BNL colloquium by David Jaffe]

*Well suppressed!*

- Loosen or tighten veto parameters based on the estimated/required background number.

- Actually, thanks to the powerful detector update, E949 photon veto had a enough rejection, therefore we loosened cuts and earned ~5 % acceptance finally.

## Remember that more rejection is gained compared with E787 at low acceptance region.

- => Can we do something with these excellent abilities ?

- Yes.

- Well,

- The strongest points in E949 is

- There are many available way/method to understand or know much about itself (detector performance, background measurement and so on.).

- AND

## Single photon inefficiency Study

*Single photon inefficiency Study*

- ?

_*Special trigger*_ :
-      collect the one photon
-       mis-detected kp2 events.

- Kinematics of the missing photon can be determined.  Note : stopped kaon experiment.

_**Photon Inefficiency map **_
- (as a function of  detector geometry and photon energy)

- Detector hole,  photon interaction with dead material
- Photonuclear interaction, giant delta resonance
-                                   (energy vs missing probability)

- A kind of X-ray test of our detector.

- This is pretty powerful study; We can reveal that

## And also have a Physics impact !

*And also have a Physics impact !*

- : spin 0

- Helicity suppression

- : (pure) left-handed

- But, if right-handed neutrinos exist and couple with Z0, this decay will occur.

- Decay into weak interaction (unknown) particles instead of

- : light photino

- [Sov. J. Nucl. Phys 47, 296 (1988)]

- (2) Discussion/Expectation from cosmology :

- Impact on some cooling models of universe

- World record :

- (**E787** experiment)

- There was a discussion on

- (Radiation)

- Can explore the decay :

## Advantage in                 analysis

*Advantage in                 analysis*

- _(3) Reliable Background subtraction _

- (1)Excellent tagging ability to Kp2 events

- decay chain

- Independent and accurate E/P/range measurement of charged particles.

- (2) Superior photon veto ability to E787.

-  Advantage is enhanced more in low acceptance region.

- _Now, E949 have the best sensitivity in the world; _

- Background estimation from the convolution of single-photon ineff,

- Which is much reliable than Monte Carlo expectation.

- _Analysis Method_

- (1)Tag

- events.

- (2)Search for the events have
-     no activity except for       .

- (1)

- (2)

## Summary

*Summary*

- Powerful hermetic veto (covering 4pi solid angle)

- Reject any extra activity.

- (2) Highly reliable rejection/acceptance measurement with real data.
-      And aggressive background control.

- (3) Big improvement : More than twice rejection at 80 % acceptance even in the severe beam condition.

- (4) Have the best sensitivity in the world for

## Appendix :Newly installed detector subsystems

- scattering

*Target*

*Beam line*

_*Region-2*_

*Appendix :Newly installed detector subsystems*

- (B) Beam line PV