Neutron-Antineutron Oscillations: Can CurrentNeutron-Antineutron Oscillations: Can CurrentExperimental Limits Be Improved?Experimental Limits Be Improved?

what is it?why is it interesting? (B-L violation)PhenomenologyCold neutron beamsPossibility of UCN

M. SnowM. SnowIndiana University/IUCFIndiana University/IUCF

Thanks for slides: Rabi Mohapathra, Yuri Kamyshkov, Albert Young, Yasuhiro Masuda, …

Proto-collaboration for R&D StudiesProto-collaboration for R&D Studies

Ayman Hawari, Bernard Wehring, Albert Young, NC StateAlexandre Kozlov, Anatoli Serebrov, PNPIKenneth Ganezer, Brandon Hartfield, Jim Hill, CSUDHChen-Yu Liu, W. M. Snow, IndianaGeorge Flanagan, Jeffrey Johnson, Gary Mays, ORNLH Lee Dodds, Yuri Efremenko, Geoff Greene, Y. Kamyshkov, TennesseeJeff Nico, Pieter Mumm, NISTY. Pokotilovskiy, DubnaRabindra N Mohapatra, MarylandTakeyasu Ito, LANLWilliam Bugg, SLACH. Shimizu, KEK

n n transitions — “too crazy”?But neutral meson |qq〉 states oscillate -

K0, B0 K0, B0

2nd order weak interactions

And neutral fermions can oscillate too -

νµ ντ…

So why not -n nNew

physics?

Such systems are interferometers, sensitive to small effects. Neutron isa long-lived neutral particle (qn<10-21e) with a distinct antiparticle andso can oscillate. No oscillations have been seen yet.

Need interaction beyond the Standard Model that violates Baryonnumber (B) by 2 units. Why should such an interaction exist?

B conservation in SM is ApproximateFrom SM point of view, both B and L conservation are “accidental”

global symmetries: given SU(3)⊗SU(2)⊗U(1) gauge theory andmatter content, no dimension-4 term in Lagrangian violates B or L.No special reason why SM extensions should conserve B.

No evidence that either B or L is locally conserved like Q: where is themacroscopic B/L force? (not seen it in tests of equivalenceprinciple).

Nonperturbative EW gauge field fluctuations called sphalerons, presentin SM, VIOLATE both B and L, but conserve B-L. Rate iscompletely negligible in our vacuum, but should be faster thanexpansion rate at the electroweak phase transition in early universe.

Sakharov conditions for Baryon Asymmetry inBig Bang starting from B(t=after inflation)=0

A.D. Sakharov, JETP Lett. 5, 24-27, 1967Baryon number violation

– Not allowed in SM in perturbation theory.– Permitted in nonperturbative tunneling processes in SM at EW

phase transition, but these conserve B-LDeparture from thermal equilibrium

– Expansion of Universe– Phase transitions– Should not be a problem

T violation– SM CP/T violation from CKM phase seems to be orders of

magnitude too small to explain observed B asymmetry – weseem to need new physics here.

Impact of B-L-conserving SM interaction onImpact of B-L-conserving SM interaction on B asymmetryB asymmetry

Sphaeleron Sphaeleron mechanism in Standard Model lead to violation of leptonmechanism in Standard Model lead to violation of leptonand baryon number and baryon number ((’’t Hooft, 1976)t Hooft, 1976)

•• ““On anomalous electroweak baryon-number non-conservation in theOn anomalous electroweak baryon-number non-conservation in theearly universeearly universe”” (Kuzmin, Rubakov, Shaposhnikov, 1985)(Kuzmin, Rubakov, Shaposhnikov, 1985)

Sphaelerons Sphaelerons conserveconserve ((BB−−LL) but violate ) but violate ((BB+L+L). ). Rate of (Rate of (B+LB+L)-violating)-violatingprocessesprocesses at T > TeVat T > TeV exceeds the Universe expansion rate. If exceeds the Universe expansion rate. If B=LB=L≠≠ 0 0 is isset at GUT scale dueset at GUT scale due toto BB−−LL conserving conserving process, process, B asymmetry can beB asymmetry can bewiped out by (wiped out by (B+LB+L)-violating)-violating processprocess

Thus, for the explanation of universe B asymmetry, Thus, for the explanation of universe B asymmetry, B-L-violatingmechanisms (leptogenesis, NNbar, some nucleon decay modes…) seemto be required

“Proton decay is not a prediction of baryogenesis” [Yanagida’02]

What happens to B-L symmetry beyondstandard model ?

• Exact symmetry or broken ? what is the mass scale of itsbreaking and what physics ?

• Two expts that probe it directly are and N-Nbar• If neutrino is Majorana fermion (as expected in seesaw

mechanism), it breaks B-L by two units• observation of decay will confirm this but by itself

will not tell us much about new physics associated withB-L breaking.

• Observation of n-nbar oscillations canaddress the question of mass scale

!""0

!""0

Neutrino mass/quark-leptonunification/NNbar connection• Majorana neutrino -> implies B-L=2;• In quark-lepton unified theories B and L get connected

and L=2 implies B =2 and hence N-N-bar oscillation.• There are reasons to think that there may be quark

lepton unification at high energies.

• Majorana neutrino mass thereforeprovides a very strong motivation tosearch for N-N-bar oscillation, which canbe new window to Q-L unification aswell as B-L symmetry.

Questions for N-N-bar oscillation• Can the possible reach of N-N-bar oscillation

time in existing facilities probe interestingrange for new physics indicated by neutrinomass ?

- Are there decent theories EMBEDDING smallneutrino masses where scale of N-N-baroscillation is observable?

• Is it cosmologically safe to have observable N-N-bar oscillation ?

• The answer to all these questions is YES.For more details see:For more details see:Baryon and Lepton Number Violations WorkshopBaryon and Lepton Number Violations WorkshopSept. 20-22, 2007, LBLSept. 20-22, 2007, LBL http:http://inpa//inpa..lbllbl..gov/blnv/blnvgov/blnv/blnv..htmhtm

Neutron-Antineutron transition probability: quasifree condition

For H =E +V !

! E "V

#

$%&

'( P

n)nt( ) =

! 2

! 2 +V 2* sin2 ! 2 +V 2

ht

+

,--

.

/00

where V is the potential difference for neutron and anti-neutron.

Present limit on ! 1 10"23eV

For ! 2

+V2

ht

"

#$$

%

&''

<<1 ("quasifree condition") Pn(n

=!

h) t

*+,

-./

2

=t

0nn

*

+,-

./

2

Contributions to V:<Vmatter>~100 neV, proportional to density<Vmag>=µB, ~60 neV/Tesla; B~10nT-> Vmag~10-15 eV<Vmatter> , <Vmag> both >>α

NT2Figure of merit= N=#neutrons, T=“quasifree” observation time

HFR @ ILL

57 MW

Cold n-source

25! D2

fast n, " background

Bended n-guide Ni coated, L ~ 63m, 6 x 12 cm 2

58

H53 n-beam~1.7 10 n/s. 11

(not to scale)

Magnetically shielded

95 m vacuum tube

Annihilation

target #1.1m$E~1.8 GeV

Detector:Tracking&

Calorimetry

Focusing reflector 33.6 m

Schematic layout ofHeidelberg - ILL - Padova - Pavia nn search experiment

at Grenoble 89-91

Beam dump

~1.25 10 n/s11

Flight path 76 m< TOF> ~ 0.109 s

Discovery potential :

N tn% = %2 915 10. sec

Measured limit :

&nn' %8 6 107. sec

N-Nbar search at ILL (Heidelberg-ILL-Padova-Pavia)

with L ~ 90 m and t = 0.11 sec

measured Pnn< 1.6 !10

"18

# > 8.6 !107sec

No GeV background No candidates observed.Measured limit for a year of running:

Baldo-Ceolin M. et al., Z. Phys. C63,409 (1994).

Better Cold Neutron Experiment (Horizontal beam)Better Cold Neutron Experiment (Horizontal beam)•• need cold neutron source need cold neutron source at high flux reactor,at high flux reactor, close access ofclose access of neutron focusing reflector to cold source, free flight path of >~300mneutron focusing reflector to cold source, free flight path of >~300m

Detector

100 MW

HFIR

reactor

Cold

Neutron

Moderator

reactor

core

vacuum tubemagnetic shield

focusing

reflector

beam

dump

annihilation

target

! 2

.3 m

L ~ 200 - 500 m

New Experiment at Existing Research Reactor?New Experiment at Existing Research Reactor?

•• No luck so far No luck so far(requests to all >20 MW(requests to all >20 MW

research reactorsresearch reactorswith cold neutronwith cold neutron

sources)sources)

Cutaway view HFIR reactor at ORNL

•• would be verywould be verydesirable!desirable!

Scheme of Vertical N-Nbar experiment

· ~3 MW TRIGA researchreactor with vertical hole andcold neutron moderator ® vn ~1000 m/s

· Vertical shaft ~1000 m deepwith diameter ~ 4-5 m atDUSEL

· Large vacuum tube, focusing reflector, magnetic shielding

· Detector (similar to ILL N-Nbar detector) at the bottom of shaft

Letter of intent to DUSELsubmitted

1 m

10 m

Annular Core

TRIGA Reactor

3.4 MW with

convective

cooling.

3E+13 n/cm2/s

central thermal flux

LD2 CM

Focusing

Reflector

L~150 m

Vacuum

tube

L~1000 m

dia ~ 4 m

Annihilation

target

dia ~ 2 m

Beam dump

Annihilation

detector

Neutron

trajectory

Scheme of neutron-antineutron search experiment with vertical layout

Approximate

scales

X

Transition

point

Sources of x1000 Improvement on ILL ExperimentSources of x1000 Improvement on ILL Experiment with Cold Neutronswith Cold Neutrons

-increased phase space acceptance of neutrons from source(using m=3 supermirrors): x~60

-increase running time: x~3

-increase neutron free-flight time time (t2):x~100 (vertical), ~10 (horizontal)

-source brightness :x~1/20 (vertical 3.4 MW TRIGA)X~1/2 (horizontal, 20-60MW research reactor)

For horizontal experiment: greater source brightness~counteracted by (dispersive) gravitational defocusing ofMaxwellian neutron spectrum

Prototype supermirrors with m~6 produced

Useful neutron flux scales roughly as m2

Image Courtesy; H. Shimizu

Annular core TRIGA reactor (General Atomics)Annular core TRIGA reactor (General Atomics) for N-Nbar search experimentfor N-Nbar search experiment

annular core TRIGA reactor 3.4 MWwith convective cooling, vertical channel, and large cold LD2 moderator (Tn ~ 35K).

Courtesy of W. Whittemore (General Atomics)

~ 1 ft

• GA built ~ 70 TRIGA reactors 0.01¸14 MW (th)• 19 TRIGA reactors presently operating in US (last commissioned in 1992)• 25 TRIGA reactors operating abroad (last commissioned in 2005)• some have annular core and vertical channel

Well-established technology

µBt<<ћ ILL achieved |B|<10 nT over 1m diameter, 80 mbeam,one layer 1mm shield in SS vacuum tank, 1% reduction inoscillation efficiency (Bitter et al, NIM A309, 521 (1991).

Quasifree Condition: B Shielding

If nnbar candidate signal seen, easy to “turn it off” by increasing B

Control Room& Electronics

Detector Hall

Neutron Dump

Neutron shaft

Access Tunnel

Door

Voptt<<ћ: Need vacuum to eliminate neutron-antineutron opticalpotential difference.P<10-5 Pa is good enough,much less stringent than LIGO

Quasifree Condition: Vacuum

Possible apparatus for the “neutrons in thebottle” experiment using UCN

CalorimeterMuonveto

Pressure vessel,magnetic shield

Tracker

Neutron reflector

Typical n-nbar event:Evis ~ 1.5 GeV

Npions ~ 5Epion ~ .3 GeV

Neutron guide

n

n

Geometry and Estimates

Estimates give improvements of 10-50over ILL experiment in transition rateusing reasonable assumptions (A. Young)

NNbar NNbar SummarySummary

If discovered:

• n→nbar observation would violate B-L by 2 units, establish a newforce of nature, and illuminate beyond SM physics

• may help to understand matter-antimatter asymmetry of universe

If NOT discovered:• will set a new limit on the stability of “normal” matter via antimattertransformation channel

New physics beyond the SM can be discovered by New physics beyond the SM can be discovered by NNbar NNbar searchsearch

Expected improvement in Expected improvement in N-Nbar N-Nbar transition probability is a factor of ~1,000transition probability is a factor of ~1,000

ConclusionsConclusions•With the discovery of neutrino mass the case for N-Nbaroscillations is a lot stronger now than before. N-Nbar discoverywill completely change the thinking on grand unification.

Nucleon decay detectors can improve limits by factor of ~5,atmospheric neutrinos already producing background.

Sensitivity of cold neutron experiment for n-nbar transition ratecan be improved by factor of ~1000. Combination ofimprovements in neutron optics technology, longer observationtime (from vertical arrangement @DUSEL), and larger-scaleexperiment

Sensitivity of UCN experiment for n-nbar transition ratecan be improved by factor of ~10-50 using sources underconstruction.

http:http://inpa//inpa..lbllbl..gov/blnv/blnvgov/blnv/blnv..htmhtm

Scientific Reach: Discussed in Recent LBL Workshop