Saturday, February 18, 2017

Have Atoms of Antihydrogen Really Been Discovered?

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Illustration of the CERN Alpha 2 apparatus which putatively has detected anti-hydrogen atoms.

Consider that in the original event called the Big Bang, matter and antimatter particles theoretically should have been create in equal numbers. In such a case, all the particle-antiparticle pairs ought ot have been annihilated with the universe left with no matter or antimatter at all, viz.

[M+]   +   [M-]   -> 0

where [M+] refers to the total of all (+) charged matter pairs, and [M-] refers to all antiparticle pairs. The essential point being the two classes behave identically except for the difference in sign (+ or -) disclosing difference in electric charge.

In retrospect, it wouldn't have actually taken much of a matter-antimatter imbalance to see the predominant matter universe we behold today.  One extra proton per billion proton- antiproton pairs would have been sufficient. But alas, even that tiny excess production is currently unexplainable. Note the matter preponderance doesn't specifically require matter v. antimatter manifest any difference other than sign, but if such a difference was discovered it would be critical in understanding the asymmetry of the early universe.

Some efforts in the past have already been made, namely, one postulate requiring a violation of what is called "CP symmetry invariance". Fitch and Cronin's (1963-64) discovery of a violation of CPT invariance. (C for charge conjugation, P for parity (spatial reflection) and T for time reversal set the original scene. Up until their 1960s investigations, it was widely accepted by physicists that nature played no favorites where charge conjugation, parity and time reversal were concerned. The discovery of a fundamental violation (Fitch and Cronin found a tiny fraction:  45 out of 22,700 - K2 mesons, spontaneously disintegrate into 2 pions, e.g. π mesons, (instead of the usual 3) changed all this.

It was suggested by them that this CPT invariance violation might also - in some way - account for the apparent asymmetry in the distribution of matter with respect to antimatter. Since then experiments have disclosed T-invariance can be subsumed by CP symmetry invariance. Trouble is, the existence of so little antimatter still violates CP invariance.

Beyond the CPT invariance, the fact is very little antimatter -matter imbalance in the primordial cosmos would have been needed to produce the matter universe we observe today. In fact, only one extra proton per billion proton-antiproton pairs would have been sufficient. to manifest the cosmos we have.  But never mind, even that tiny excess remains unexplainable.

Enter now the need for experiments which might detect a difference between matter and antimatter other than sign.  If such experiments found a difference this could be the key to understanding why the asymmetry existed.  In fact, such experiments have been going on  for the past thirty years at CERN where researchers have attempted to trap and study antihydrogen atoms.  The main line of inquiry has been to use precision spectroscopy to compare the known structure of hydrogen with antihydrogen.

Fast forward and it is now known that one of the CERN teams (the ALPHA collaboration) has achieved the first spectroscopic evidence for the transition between the 1s and 2s states of antihydrogen.  Recall the probability regions associated with hydrogen tabbed to the principal quantum number n:
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In the above image, the far left n=1 image with its spherical shape denotes the 1s hydrogen orbital. The middle ring image with n=2 denotes the 2s orbital. If we zoom in’ on the 1s  configuration, and the probability for the electron in this region we end up with the graphs below:
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This diagrams shows that the hydrogen electron occupies no definite position. Instead, it’s confined someplace within a “cloud” or probability space (b) but that probability can be computed as a function of the Bohr radius (ao = 0.0529 nm).  The probability P1s for the 1s orbital is itself a result of squaring the “wave function” for the orbital.  If the wave function is defined:

y (1s) = 1/Öp (Z/ ao) exp (-Zr/ ao),

and the probability function is expressed:

P = ½y (1s) y (1s) *½

 Where y (1s) * is the complex conjugate, then the graph shown in (a) is obtained.   Now, as far as we know antihydrogen displays the same probability space distribution for its 1s  state, and likely the 2s as well.  Indeed, the ALPHA results up to now reveal no differences between hydrogen and antihydrogen, but the physicists hope that their precision (currently 200 parts per million) will improve and ultimately show the difference they suspect.

How does the ALPHA technique work? Basically, a laser is used to excite atoms in samples containing up to 1012 atoms. (For comparison, a single milliliter of water contains about 6 x 1012 atoms.)  The antimatter atoms are much more difficult to obtain and entail use of CERN's Antiproton accelerator. This device produces about 3 x  107   antiprotons every 100s. These can then be manipulated using cryostats made of matter.

The decelerated, "raw" antiprotons travel at about 7 percent of the speed of light (e.g. v = 0.07c) but the velocity is further slowed by directing the beams at thin aluminum foil.  The downside is that some 99 % of the antiprotons are lost in the process. The one percent or so that survive annihilation (and are scattered by the foil to a lower energy) can be combined with positrons  (generated from radioactive decay) to produce antihydrogen. The technique produces about 25,000 antiatoms per trial.

Of course, this is only the initial phase of the experiment. Because one needs to "trap" the antihydrogen then it must be rendered "cold" (less than 0.5K).  Given the generated atoms are charge neutral trapping them electromagnetically  requires exploiting their magnetic moment, e.g. 

m=  ½  [mv 2 /B]  

so that one uses an inhomogeneous magnetic field whereby a fraction of the atoms are drawn to the lower intensity B-field (in the middle ) with high field at the edges.. So in many respects it resembles the magnetic mirror machine we saw in the plasma physics posts (from January last year).   The point is the atoms can only be confined if the energy is low enough to yield temperatures less than 0.5K.

How much progress has been made with this trapping technique? In 2010, ALPHA's first successful trapping yielded 38 atoms trapped across 335 trials, with each atom trapped for a fraction of a second.  Now, up to 14 atoms are trapped per trial and held for many minutes.  Because of the low trapping success of the 2010 trials they were mainly focused on just that - trapping. Now with the improvement in trapping rates and times spectroscopy has come front and center.

The new "ALPHA 2" arrangement is depicted in the drawing at top of this post. The laser power is amplified by an optical cavity bound by highly reflective mirrors places outside the trapping region. This maximizes the sparse samples of antihydrogen.  The laser circulates roughly 1 W through the trapping region.  When an atom is excited by the laser it doesn't always return to its original state say 1s. On occasion it absorbs an additional photon and is ionized (absorption of energy E = hf causes a positron to be lost). At other times it reverts to a spin flipped version of the ground state.  Then the spin-flipped atom (or antiproton in the case of ionization) drifts to the wall of the apparatus, gets annihilated and produces a signal that can be detected.  When the given trial ends, the B-field for the trap is turned off and the remaining atoms are counted.

So far, the CERN physicists have probed the transition between antihydrogen's 1s or ground state and the first excited, or 2s state. Fortunately, the excitation's long lifetime -roughly  1/8 second- enables ample  time to absorb another photon before decaying back to the 1s state. (N.B. The long excitation time arises because of the 'forbidden transition' for a single photon. Hence, instead of tuning the laser to 121 nm - the full energy of the transition - the wavelength of 243 nm is used. This relies on two photons simultaneously absorbed to make the transition.)  When the two photons arrive from opposite directions  their Doppler shifts nearly cancel so there is no significant line broadening. (Recall the three important types of line broadening are Doppler effect, natural and pressure broadening.)

What has been the end result thus far? Well, after employing laser frequencies at resonance and detuned by -200 kHz, there is a mixed outcome but not one diverging from the null hypothesis. Specifically, 27 annihilation events have been observed during the off resonance trials - but this is consistent with the background effects from cosmic rays. By contrast, in the eleven "on resonance" trials - looking at 146 atoms - a total of 79 escaped the magnetic trap.  These results imply that any fractional differences between the transition frequencies of hydrogen and antihydrogen are less than  2 x 10-10 .  

But  this is kind of re-assuring. Look at it this way, if there was some discrepancy of behavior observed different from charge- it would actually undermine the symmetry between particles and antiparticles. This symmetry (i.e. we can't tell them apart other than by charge) is underpinned by both quantum mechanics and general  relativity. So....if there were differences observed we'd have to go back to the drawing boards to revise those theories.  In other words, we'd require major changes to the two primary theories of modern physics.

No one wants to go there, and if they do have to, the evidence for differences had better be very solid and fairly scream for attention.




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