Rare kaon physics from 30,000’ and other perspectives

L. Littenberg29 April 2013

Outline

• Some things can best be appreciated from 30,000 feet• What do we have to do to get some respect?• What does N events buy us?• Lightening review of K ® pnn experimental situation• Real ghosts• Any alternatives?• A challenge• Cycling back to the beginning

Some things you can only appreciate from 30,000’

Like the Nasca lines in Peru

It’s clear they were trying to perfect them

Could have been a contender• Almost as old as the Nasca pix:

• Green dashed line is supposed to be the usual unitarity triangle• Red solid line is a triangle from rare kaon information alone• If SM is the whole story the vertices at the upper left should meet

• Evolution in what qualifies as a discovery. • Now 5s demanded before people believe something.

– It’s kind of interesting, 3s means there’s a 1 in 770 chance of being wrong

– But that’s not good enough, we demand a 1 in 3.5M chance!– Note that in the US, there are 1.1 deaths/108 highway miles

• So if you drive 10,000 miles/year, your chance of dying is 1 in 9000.• So you will bet your life on 3.7s, much less than you demand of a physics

result!– Seems like we don’t trust our own systematic errors

• Let me pose the question – how close could one come to the Standard Model, and have a five sigma deviation from it?

What do we have to do to get some respect?

Even though we experimentalists couldn’t possibly make a >5s mistake!

• Evolution in what qualifies as a discovery. • Now 5s demanded before people believe something.

– It’s kind of interesting, 3s means there’s a 1 in 770 chance of being wrong

– But that’s not good enough, we demand a 1 in 3.5M chance!– Note that in the US, there are 1.1 deaths/108 highway miles

• So if you drive 10,000 miles/year, your chance of dying is 1 in 9000.• So you will bet your life on 3.7s, much less than you demand of a physics

result!– Seems like we don’t trust our own systematic errors

• Let me pose the question – how close could one come to the Standard Model, and have a five sigma deviation from it?

What do we have to do to get some respect?

How many events you need to have 5s?

Equivalent Events

Ratio

to S

M @

5s

Zeroing in

D.M. Straub arXiv:1012.3893

Zeroing in – 10 event experiments

Zeroing in – 100 event experiments

Zeroing in – 100 event experiments

Zeroing in – 1000 event experiments

More realistically …

A. Buras et al., General MSSM, tanb = 20 hep-ph/0408142

_

_

1000 evts

More recent theory example

“Trivially Unitary Model” from A. Buras et al. arXiv:1301.5498v1

Primary K ® pnn experimental technique

Veto System

Vetoed Event

Ghosts of Experiments Past: E949 @ BNL

• Proved K+ ® p+nn could be done in the “traditional” way• Designed to be a 10-event experiment by my criterion• But killed in the prime of life so ended up a two-event experiment

Ghosts of Experiments Past: E391a @ KEK

• First dedicated KL ® p0nn experiment• High energy approach performed at low energy• Need another factor 1000

Ghosts of Experiments Present: KOTO @ J-PARC

• High energy technique for KL ® p0nn at even lower energy• Scheduled for 1st long physics run any minute• Much experience, improved technology• First stage is a 1.5 event experiment by my criterion• Step-by-step approach has a lot going for it• Next step, get rid of some unnecessary constraints

Ghosts of Experiments Present: NA62@ CERN

• First to use in-flight technique for K+ ® p+nn • Physics run scheduled for 2014• Designed to leap 11 orders of magnitude in a single bound• 75-event class experiment

Ghosts of Experiments Future: ORKA@FNAL

• Lineal descendent of previous K+ ® p+nn experiments• 10x the exposure of E949• 10x better acceptance • Scientific approval December 2011• Would be a 400-event experiment by my criterion

Ghosts of Experiments Future: KOPIO-like experiment for Project-X2

• True 1000-event class experiment• Really requires 3 GeV Stage 2 of Project-X

Once and future K experiments

E39

1a

KO

TO

E94

9

NA

62

OR

KA

P-X

K0

Even

tsEvents

LL Evts

True Ghost Experiments

E949 - killed in its prime by DOE KOPIO – NSF pulled the plug on RSVP

CKM - murdered by P5 KaMI - rejected by the Fermilab PAC

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Another Look at KL®p0l+l-• KL®p0l+l- has several kinds of bad-luck and trouble.• NA48 observed KS®p0l+l- at higher end of expectation• & arguments for constructive interference between mixing &

direct CP-violating components strong as ever• SM expectation for KL®p0l+l- rather large:

– B(KL®p0e+e-) = (41)10-11

– B(KL®p0+-) = (1.50.3)10-11

• Compare with KL®l+l- background (worst one):KL®p0e+e- (‘99) KL®p0+- (‘97)

KTeV s.e.s.. 10.410-11 7.510-11

KL®l+l- evts 0.99 0.37

B(KL®l+l- )effective 10.310-11 2.810-11

S:B 1:2.5 1:1.9

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Motivation for KL®p0l+l-

Add to this, now we are interested in bigger game than Imt.

E.g., from Isidori, et al., hep-

ph/0404127

• Take KaMI as example of a next-generation experiment with sensitivity to KL®p0+-. In 3 years, KaMI would have reached a s.e.s of 410-13.

• In the example above, would collect 110±13 signal events (with 70 events of background) compared with a SM expectation of 37 events.

• KL®p0e+e- case left for homework

Why not KL ® +-?K ® pnn

• BR ~ few 10-11

• Very poor signature

• Background 10% or worse

KL ® +-

• BR ~ 7 10-9

• Beautiful signature

• 1% background

Here’s Why

The fundamental irony of rare kaon physics: The very interactions that make the the process detectable, introduce obfuscating long-distance effects!

The above process introduces an absorptive part that is many times larger than the short-distance contribution plus a dispersive part that can interfere with it.

A long distance to go• We start off with what I’d classify as a 3700 event experiment.• The absorptive piece can be well-determined by measuring KL ®

– Subtracting it gives Bdisp(KL®+-)= (3.2±1.2) 10-10

– I.e. a number ~20 times smaller than the absorptive part– And ~3 times smaller than SM fits to the short-distance part!

• This corresponds to a 7 event experiment! Nature is a real killjoy.• And this is before trying to untangle the dispersive interference.

– Where there’s agreement from all sectors of the theory world that this is very difficult!

• What could possibly be done?• The experiment can be done better – I never imagined it would be worth it.• Could imagine 5 smaller errors, would correspond to a ~200 event

experiment for the dispersive part.• Then the theorists could do their part. If they were perfect, and the true

value was ~ the SM-predicted one, could get to 500 event equivalent. So it might be worth considering

– But theorists really have to do well. A 10% error on the dispersive amplitude degrades it to a 100 equivalent event experiment

What about KS ® +-?

• In this case the short distance piece is CP-violating!– In the SM µh2.– BR = 1.7 10-13, exp as sensitive as the KL one would see 1.5 evts

• Moreover the long distance parts are tractable – Don’t interfere with the SM– Well-calculable – maybe the lattice could improve further

• But LD parts are still 25 bigger than SD (~510-12).• Note Isidori & Unterdorfer (hep-ph/0311084) point out that

K+ ® p+nn currently limits SD BR to ~10-11 in most BSM models. – So any sensitivity exceeding that limit is interesting.– Starts to limit K ® pnn, also Grossman-Nir bound

• Present limit, 9x10-9 @ 90%CL, is from LHCb! (arXiv 1209.4029)• How much better could a fixed-target experiment do?

KS ® m+m-

Limits on the K unitarity triangle (Isidori & Unterdorfer)

Corresponds to B(KS® +-)<1.710-11

Evts

/0.1t S

KL®+-

KS + KL (SM)

K®+- events for a 10-13/evt experiment. At the SM level, KL dominates. Green curve corresponds to the BR at which KS®+- becomes a stronger constraint on KL®p0nn than the Grossman-Nir bound. Could get >6s separation from SM for green curve at this sensitivity.Time (tS)

Is the Unitarity Triangle Melting?

Is the Unitarity Triangle Melting?

For a 100 evt KL experiment this distance is >6s!

Take-away Thoughts

• The virtue of the golden twins is safe for at least another generation. There’s a huge range of BR in which to prospect for BSM effects.

• If new physics shows up, can further refine its properties by studying other rare modes.

• But what’s really strange is that the golden twins may be essential for cleaning up what’s now considered the SM CKM triangle, for which they were proposed 25 years ago!

• Editorial: rare process experiments thrive in a programmatic, rather than a project environment.

• Homework: beat LHCb in KS ® +-

• Appreciate that your work may be have uses you never thought of…

Limit on heavy photons from K+ ® p+X0

Backup Slides

What I mean by “equivalent events”

• If you had a perfect experiment, with no background and no systematic errors, a given number of events gives a fractional uncertainty = 1/sqrt(S), where S is the number of signal events.

• So I’m really talking about a certain sensitivity• If one had a perfectly determined background B, the equivalent

number of events would be S/(1+B/S).• If the background had an uncertainty sB, then the equivalent

number of events would be S/(1+B/S + sB2/S)

• This gets increasingly complicated as more realism is inserted, but the basic idea should be clear.