“Other” UCN Physics Jeff Martin University of Winnipeg TRIUMF Townhall Meeting Ultracold Neutron Working Group Aug Topics: 1. n-nbar oscillations (Kamyshkov, Young, Snow) 2. n to “mirror neutron” oscillations (Pokotilovski, Ban et al, Serebrov et al) 3. surface physics (Steyerl, Korobkina, et al) physics – experiment - TRIUMF

1. n-nbar oscillations ● A search for baryon number violation ● predictions in various models show possibilities just beyond current limit – supersymmetry (Mohapatra) – extra dimensions (Shrock) ● baryon number violation required to explain BAU (but doesn't necessitate nnbar oscillations) ● current limit for free neutrons (CN beam) – tau > 8.6e7 seconds

Vertical Cold Beam

Snow, 2003.

UCN-nbar oscillation experiment ● create a bunch of UCN ● put them in a bottle ● search for 5 pions appearing from the edge of your bottle, signifying annihilation of an nbar. ● magnetic shielding (magnetic moment of nbar is opposite that of n) ● previous best experiment was done with cold neutrons on a long beamline at ILL.

~few metres

UCN-UCNbar experiment issues: big detector B = 0 “resetting” of UCN wavefunction at surfaces => discovery potential linear w/ meas't time if there were no walls (or infinitely large vessel) would go as t^2. need lots of UCN

UCN-UCNbar history ● discussed extensively in recent workshops – IU, 2002 – Berkeley, 2007 (the week after our UCN workshop) ● considered in great detail in a set of two long papers by Yoshiki and Golub. – magnetic field tricks might maintain coherence at walls, partially, if UCNbar-nucleus interaction well known

Comments on nnbar for TRIUMF ● Very high physics rating. ● Requires high production and high density, which has never been created, anywhere, yet. ● Experiment alone would cost $10M. ● A good future goal to shoot for as UCN production increases.

2. n-n' oscillations: “mirror neutrons” ● Maybe there is a mirror universe with identical particle content to our own. ● A way to restore global parity symmetry. (Kobrzarev, Okun, Pomeranchuk, 1966) ● Recently: interesting implications for cosmology and particle physics – mirror universe as dark matter candidate – in 2005, no experimental limit on n-n' oscillations. Fast oscillations (tau << nlife) could erase GZK cutoff (PRL 2006, Berezhiani, Bento) – relationship to supersymmetry (Mohapatra, 2005)

Experiments ● Berezhiani, Bento derived tau > 1 s based on the old ILL nnbar experiment. ● So, any lab that had any UCN (ILL) and some magnets went searching... – Pokotilovski suggested a bunch of experiments (2006) – Ban et al (2007) tau > 103 s – Serebrov et al (2007) tau > 414 s

Basic experimental setup (Serebrov paper) ● Put in UCN ● See how many come out ● Compare experiments – B = 0 – B ≠ 0 B = 0 => degenerate => oscillations, B ≠ 0 => nondegenerate => no oscillations

Comments on Mirror Neutrons for TRIUMF ● Physics interest originally centered around short oscillation times. ● The easiest experiments have already been done. ● Possibly there would still be interest in several years time. ● Seems risky to bank our program on it.

3. Surface and Materials Physics with UCN ● Are mentioned in the UCN book (Golub, Richardson, Lamoreaux, 1991) – limited by flux – clever pioneering experiments by Steyerl ● Most modern work seems focused on interactions of UCN with surfaces, usually in relation to understanding the interaction so that better UCN- lifetime measurements can be made – Steyerl et al, EPJB 28, 299 (2002) – Korobkina et al, PRB 70, (2004)

E.g. Korobkina et al PRB article ● Most losses of UCN on surfaces are due to hydrogen on the surface – but what kind of hydrogen (tightly, weakly bound? 2D gas?) and how? ● Studied low-temperature (<77K) dependence of UCN loss rates on Cu surface with different surface qualities in oil-free vacuum. ● Discovered loss rate decreases with temperature

Interpretation ● Cu-H bond model disagees with data ● H2O ice in surface clusters agrees with date (or “hydrogenated surface clusters” C-H and/or O-H bonds) ● model based on previous measurements of theirs on (n,gamma) on surfaces, also done using UCN, and on measurements of surface contamination in first 10 nm of surface (showed 8% hydrogen contamination)

Relevance for surface physics ● Korobkina et al comment that – UCN very sensitive to hydrogenated films – can be used to monitor low frequency excitations in ~nm thick films with V_F < E_UCN (e.g. polymers, ice,...) – two methods: ● (n,gamma) using UCN ● UCN losses – sensitivity complementary to ● NIS (neutron inel. scatt.) (sensitive to bulk and higher energy excitations) ● electrons (thin films, but not H)

Comments on surface physics for TRIUMF ● Need more UCN to be useful (even Korobkina says that) ● Details of this type of surface physics have not been understood for 30 yrs – “anomalous losses” of UCN ● Possible to draw parallels with success of CN case, where instruments have been developed over time. ● A good goal to have, but nothing terribly concrete.

Summary and relevance to white paper 1. n-nbar oscillations ● good future goal 2. n to “mirror neutron” oscillations ● might be interesting if we had a source right now 3. surface physics ● interesting now, but need good ideas - in all three cases, if you have any UCN, people will be interested. - Personally, I do not see a (fast, initial) flagship experiment, here.