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Scott Menary Faculty of Science
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Physics Research

"If I knew what I was doing, it wouldn't be research."

D.M. Winnicott

Presently I Am Concentrating On:

Testing Fundamental Symmetries with Trapped Antihydrogen

A primary reason for making measurements with antimatter is to try to find clues to one of the central mysteries of our time - namely, "Where is all the antimatter?" That is, in the Big Bang, matter and antimatter were created in equal amounts but the universe is now dominated by matter. This is a good thing as matter and antimatter annihilate each other so we wouldn't be around if this wasn't the case. But it still begs the question as to the mechanism that led to the disappearance of the primordial antimatter. You can hear me talk about this on the "Big Questions" Quirks & Quarks show broadcast June 21, 2014 as well as my "the star spot" podcast Where Have All the Antimatter Gone? broadcast on June 15, 2014. A few years ago I also gave a public lecture about this using as a central theme the movie "Angels & Demons" where antimatter is used as bomb (trailer alert: not possible, so relax). By the way, while there isn't a lot of it, it is still true that there is antimatter all around us - even coming from you ... and bananas! A nice discussion of antimatter is given in Ten things you might not know about antimatter (April, 2015) although, come on, admit it, you know at least 5 of them.

The ALPHA (Antihydrogen Laser PHysics Apparatus) experiment ( animation) is done at CERN's Antiproton Decelerator. The AD has been upgraded with a new deceleration ring ELENA which is to come online in 2021 (LS2 Report: waiting for antiprotons... - CERN News, October, 2020). ALPHA managed to produce and hold ("trap") antihydrogen atoms for as long as 1000s (2011), made the first measurment of microwave transitions in antihydrogen (2012), and measured the charge of antihydrogen (2014). The Canadian group on ALPHA was awarded the 2013 NSERC Polanyi Prize.

Not to toot our own horn or anything but in 2010 ALPHA along with another AD experiment called ASACUSA were named the Physics World Breakthrough of the Year. We were one of the finalists for the 2021 Breakthrough of the Year but ultimately Quantum entanglement of two macroscopic objects is the Physics World 2021 Breakthrough of the Year. You win some, you lose some.

Spectroscopic Measurements with ALPHA

Spectral lines can be measured with great precision. Comparing the spectral lines of antihydrogen with those of hydrogen allows for one of the most precise tests of the CPT Theorem (a kind of symmetry between matter and antimatter) which requires the lines to be exactly the same. CPT invariance is a fundamental requirement of a quantum field theory and the Standard Model, the theory that describes the interactions of elementary particles, is a quantum field theory. Finding deviations from CPT invariance would have enormous implications for physics and would require a rethink of our mathematical description of nature. A clear, detailed discussion of testing CPT (and gravity, see below) using trapped antihydrogen is given in the 2015 PhD thesis by Andrea Capra.

Here are some recent results:

We were on the cover of Nature!

Testing Gravity with ALPHA-g

Lately I've been concentrating on efforts to use trapped antihydrogen to test the gravitational interaction between matter and antimatter (so-called "antigravity"). Einstein's General Theory of Relativity was published (1916) seventeen years before the first observation of an antiparticle (the anti-electron or positron) and so, naturally, did not include any considerations of whether antiparticles would gravitate differently than particles. There have been many attempts over the years to implement antigravity although most (see for example, Schiff's 1958 paper) found it problematic. Nevertheless, as always, it is an experimental question! As said by Shel Silverstein in his poem "Falling Up"

I tripped on my shoelace
And I fell up -
Up to the roof tops,
Up over the town,
Up past the tree tops,
Up over the mountains,
Up where the colors
Blend into sounds.
But it got me so dizzy
When I looked all around,
I got sick to my stomach
And I threw down.

Anyway, towards this end, we designed and built a second optimized apparatus, called ALPHA-g. You can watch Timelapse videos of the construction in 2018 of the ALPHA-g apparatus in the ALPHA pit. It's really amazing how the pit got filled up with stuff!

Our first ALPHA-g paper - Observation of the effect of gravity on the motion of antimatter (nature, September 27, 2023) - detailed our finding for the free fall acceleration of antihydrogen of (0.75 ± 0.13 (statistical + systematic) ± 0.16 (simulation))g. This garnered a lot of attention around the world with headlines like;

While we found that antihydrogen is certainly not gravitationally repelled by the Earth, I believe it is technically incorrect to say that this is equvalent to the statement that antimatter is not gravitationally repelled by matter. The reason has to do with the mass constitution of the proton (and, therefore, due to CPT Conservation, of the antiproton). The 2018 Lattice QCD calculation of Yang et al. found that 32(4)(4)% of the mass of the proton is due to the kinetic energy of the quarks with the rest associated with gluon dynamics. The masses of the two up quarks and one down quark in the proton add only another 1% so the quark mass-energy contribution to the proton mass is ~33% and, therefore, due to CPT Conservation, only around one-third of the antiproton mass is actually due to antiquarks, i.e., is actually "antimatter". I explored the consequences of this fact in the following arXiv preprints;

One of the results presented in the third preprint is that it follows quite naturally from antigravity that the free fall acceleration of antihydrogen would be (0.78+0.11−0.08)g. The improbably good agreement with the ALPHA-g result shouldn't be taken terribly seriously but it is true that I found that the antihydrogen free fall acceleration would be positive (i.e., antihydrogen would fall down just like hydrogen) but more importantly it is constrained to be definitively less than g. It will be interesting to see if ALPHA-g can get the systematic errors down low enough to test this.

The part of ALPHA-g that I specifically worked on was antihydrogen detection using a tracking chamber (a so-called radial Time Projection Chamber or rTPC) and a scintillating cosmic ray veto detector. Here is some information about the rTPC:

The rTPC at CERN (without its cover) before installation into ALPHA-g

Long-baseline Neutrino Experiments using Liquid Argon Time Projection Chambers (LArTPC)

Muon Colliders

  • One large advantage of lepton colliders such as the e+e- colliders LEP and SuperKEKB over hadron colliders like the proton-proton collider LHC is that all of the beam energy goes into the primary collision. That is, in a 91 GeV per beam collider like LEP, there is 182 GeV of energy available for new particle production since the collision is a point-particle on point-particle collision. At the LHC, on the other hand, the protons may have a kinetic energy of 7 TeV or so but the proton is not a point-particle and so that energy is shared among the constituents of the proton, the quarks and gluons. The actual point-particle collisions (the so-called "hard collision") are between constituents of the colliding protons and so the energy available for production of new particles is often considerably less than 14 TeV. Why not then just build a 14 TeV center-of-mass energy e+e- collider you ask? The answer is synchrotron radiation, the EM radiation emitted by charged particles when they are accelerated (as happens for a charged particle travelling in a circular path as in a collider). The problem is that for a given kinetic energy and accelerator radius, the power (energy/time) of the emitted radiation is inversely proportional to the 4th power of the mass of the particle being accelerated. Since the proton is around 2000 times as massive as the electron, then amount of synchrotron radiation emitted for an electron is 20004 times greater than for a proton. You can reduce this loss by increasing the radius but it puts a major limit on e+e- colliders and is the reason why LEP basically set the energy limit for circular colliders while the LHC uses the same tunnel but has beams with almost 100 times the energy.
  • One could get around this to some extent (i.e., have the advantages of leptonic point-particle collisions but with much less synchrotron radiation loss) if there were a heavier lepton than the electron. Luckily there is - the muon. I have for many years (as in several decades) thought that muon colliders were the way to go. Oddly enough there are quite a number of other crazy souls out there as well as evidenced by this August, 2023 article from Scientific American - Particle Physicists Dream of a Muon Collider. There's also the November, 2021 article from APS News - Muon Colliders Hold a Key to Unraveling New Physics. Ah, to sleep, perchance to dream …
  • Another the nice thing is that with a muon collider you also get a Neutrino Factory essentially for free. A Neutrino Factory was historically seen as a precursor to a Muon Collider since you don't need two (colliding) beams but just one. Here's a small sample of the various design reports, etc., over the years:

Past Experimental Collaborations: