The Rare K-decay Experiment at BNL and Its Accomplishments

L. Littenberg20 Oct 2010

A very interesting target• The kaon rare decay program at the BNL AGS was in full

swing by the end of the 1980’s. – Most of the experiments searched for good-signature processes

like KLμe and had reached sensitivities ~10-10/event.

– But one experiment, E787, was different, pursuing K++, which has a very challenging signature.

• Why take this on?– Unlike most of the other rare decays, it had a Standard Model

prediction that seemed to put it almost within reach.– It was also firmly predictable in terms of the fundamental

parameters of not only the SM, but almost ANY theory.

K+ +

2 invisible particles1 charged particle of a type very common in K+ decay. No peak. Occurs once in ten billion times!

_

A very rare decay• The K+ lives 12 billionths of a second. If + were its

only final state, the K+ would live almost 2 hours!• This allows other very rare phenomena to manifest

themselves • If something new contributes to K++, non-SM heavy

particles, or almost anything else, the process can speed up.

t’_

˜W

˜Wt̃

˜

_

__

E787 had reached:

1989-91 result:B(K++)<2.410-9

estimated background ~ 0.5 evt

A natural Japan-US collaboration• The first modern calculation of B(K++) was done in Japan (Inami

& Lim).• The most recent previous experiment on this process had been done

at KEK (#10).• The US experiment was very similar in concept to the Japanese

predecessor – the most important differences were the larger acceptance and the introduction of a magnetic field.

• E787 had reached a background limit at ~10 -9/evt (see plot). To go further something major needed to be done.

• Japan could provide components and expertise for the upgrade not available in the US (e.g. high-field tolerant PMTs, YAlO calibration sources).

• Japan could also supply physicists with rare-K skills

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• Serious discussions began late in 1990, principally with Professor Shojiro Sugimoto on the Japanese side.

• US-Japan approval came in 1992 • With the Japanese participation, the

E787 (stopped K+) beam & detector underwent a major upgrade

Early history of the collaboration

E787 K++Stopped K+ ~10M/spill, purity ~75% ~1/4 stop in tgt

CM device looks like collider detector

Measure + p,T,R, lifecycle

Hermetic veto

Early history of the collaboration-2

• The upgraded beam & detector had a commissioning run in 1994, whose main physics output was a measurement of K++.– This was the first run in which the Japanese

component of the collaboration took part. – It resulted in the first clear measurement of the

structure-dependent radiation from this process

• In 1995 the first extended run of the upgraded beam and detector took place. An very large data set was collected including the first example of K++. _

The First Event

Attention!

The later runs of E787

• Further data was collected in 1996, 7, and 8.• A second event was found in the 1998 data.• Much attention!• Rate twice that predicted by the SM (but statistically

consistent).• Very exciting situation• But a big problem!

– With the advent of RHIC, the AGS was slated to become an injector, and DOE OHEP was no longer the landlord of BNL

– This meant many constraints on further high intensity proton running.– Experiments would have to be approved by DOE on an individual

basis.

The later runs of E787

• Further data was collected in 1996, 7, and 8.• A second event was found in the 1998 data.• Much attention!• Rate twice that predicted by the SM (but statistically

consistent).• Very exciting situation• But a big problem!

– With the advent of RHIC, the AGS was slated to become an injector, and DOE OHEP was no longer the landlord of BNL

– This meant many constraints on further high intensity proton running.– Experiments would have to be approved by DOE on an individual

basis.

The Miracle• Realization that since E787 was instantaneous rate-limited, if the AGS spill could be lengthened enough, we could use all the available beam• The AGS was needed only a very short fraction of the time to feed RHIC• If no other experiment competed with us for protons, we could gain sensitivity at an unprecedented rate.• Fortuitously an idea developed for the KOPIO proposal allowed the AGS spill to be lengthened almost without limit.• A proposal was written to use this idea along with modest improvements to the detector.• Due to a tri-partite deal between BNL, DOE and Fermilab, one AGS experiment was granted a life after the transfer of the AGS to Nuclear Physics

Enhanced veto, beam instrumentation Much higher proton flux (65 TP) Improved tracking and energy resolution Higher rate capability due to DAQ, electronics and trigger improvements

E787 E949

+ Momentum from K + +→

E787

E949 at2x inst. rate of

Improved UTC Zσ

E949

E787

Range Stack StrawChamber trackingImproved by 5 x

2-10 better

0 efficiency

E949 Upgrade Performance

E949

E787

E949 - 2002 Performance

• Lower beam duty factor (Siemans Westinghouse)• Lower proton energy (by 10%, cost 10% in flux)• Problematic separators, worse K/ ratio (4 3), fewer K/proton (factor ~ 1.5)• Total sensitivity cost 2• All could have been fixed in 2003!• But AGS operations were suspended after 2002 due to budget problems

E949 Event

(68% CL interval)

c.f. SM prediction: (0.85±0.07) ✕ 10-10

1.30 10

0.89( ) (1.47 ) 10BR K + + + −

−→ = ×1.73+1.15

-0.105

7 events

Final Combined E787/949 Result

Products of the E787/949 Program

• 20 publications in peer-reviewed journals (so far)– Several cited hundreds of times– Three important new decay modes discovered

• Existence proof that one can access 2nd order GIM-suppressed decays with good S:B even if they have rather poor signatures – As a result both K+ + & KL 0 are actively being pursued.

• Several new techniques developed that have been adopted by others– Highly evolved blind analysis– Fine-mesh phototubes– Fiber stopping targets

• Several of the young people (students & postdocs) who grew up on the experiment have gone on to be leaders in the field

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The Future

• Encouraged by Professor Sugimoto, Takashi Nakano and others proposed to take the E949 detector to Japan– Approved by the DOE in 2008

• This is underway at the moment. Some elements have already been shipped. The major work, disassembling the magnet iron and coils is about to start.

• It will be used first in photoproduction experiments at SPring-8.

• Eventually it will be taken to J-PARC where it will be used in various ways, including service as the basis of a new K++ experiment.

• Thus the cycle will be completed.

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BACKUP

u

A weak decay

K+

s

The s antiquark gives birth

W+W+

t

u

The W decays to a positive muon and a neutrino

t

μ+

u

The tbar decays to a W- and a dbar

W-

u

μ+

The W- absorbs the + and becomes an anti-neutrino

u

The dbar combines with the u to form a +

+

July 2005 L. Littenberg – Varenna 28

• 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 Technique

July 2005 L. Littenberg – Varenna 29

• Blind Analysis• 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 Analysis Strategy

E949 Detector

E787 in 1991

E787 in 1999

1.30 10

0.89( ) (1.47 ) 10BR K + + + −

−→ = ×

BR K( ) ( . )..+ +

−+ −→ = × 157 1 821 75 1

(68% CL interval)

E787 result:

Combined E787/949 Result