Tuesday, July 28, 2015

Antiprotons Interact With Each Other Just Like Protons, As Expected


In the Standard Model, the strong force of QCD and the electromagnetic force of QED should cause antiprotons (i.e. spin-1/2 baryons composed of two anti-up quarks and one anti-down quark) to interact with other antiprotons precisely the way that protons interact with other protons.

This includes the nuclear binding force that holds protons and neutrons together in atomic nuclei which is a "spillover effect" of the QCD strong force interactions between quarks by gluons within a proton or between antiquarks within an anti-proton, and is mediated by primary by pions.

Any gravitational interactions between an antiproton and another antiproton should likewise be identical to interactions between protons and other protons, although gravitational interactions should be too negligibly strong to matter at short distances and small masses involved in this experiment.

There is difference between the way that the weak force affects particles and antiparticles due to CP violation in the CKM matrix. But, since protons are stable and do not experience weak force decay, that difference should not cause any difference between the way that antiprotons interact with other antiprotons, and the way that protons interact with protons.

Thus, the symmetry between protons and their antipartricles and the stability of the proton, which should also apply to the antiproton so long as it doesn't interact with ordinary matter (a non-trivial task in a matter dominated universe), makes what otherwise might be very difficult calculations to determine how protons interact with other protons compared to how antiprotons interact with other antiprotons, facially obvious.

Previous experiments have directly measured the mass of the antiproton (which is exactly the same as a proton to within experimental limits), the charge of an antiproton (which is -1, exactly the opposite of a proton and the same in magnitude as an electron) and the mass some other simple anti-atoms (such as antihelium-4), but had not measured the strength of the strong nuclear force between two anti-protons. "Antinuclei produced to date include antiprotons, antideuterons, antitritons, antihelium-3, and the recently discovered antihypertriton and antihelium-4[.]" (The quotation is from the link below.)  All measured anti-nuclei masses correspond to the masses of their ordinary matter counterparts to within experimental limits.

The fact that this antinuclei had been produced that were seemingly identical in all respects, except charge which was opposite, to their ordinary matter counterparts, strongly suggested that antiproton interactions with each other were identical to proton interactions with each other. But, that was only indirection evidence of this fact.

The New Experiment

However, while the theoretical expectation was clear and was also supported by indirect evidence, until now, nobody had actually been able to directly measure the interactions between two anti-protons.

The Star Collaboration in Brookhaven, New York, however, has now direct determined from an analysis of 500 million collisions of two gold atoms that "the strong interaction is indistinguishable within errors between proton-proton pairs and antiproton-antiproton pairs." Statistical errors in this experiment grew dramatically when the momentum of the interaction was less than about 0.02 GeV/c in a set of measurements looking at momenta from 0.1 GeV/c to about 0.15 GeV/c due to the small number of events at those energy scales in this set of collisions.

Two constants are used to parameterize nuclear force strength between atoms, the singlet s-wave scattering length, f0, and effective range, d0, of the interaction.

With respect to these parameters, direct measurement established that: "Within errors, the f0 and d0 for the antiproton-antiproton interaction are consistent with their antiparticle counterparts – the ones for the proton-proton interaction."

The error bar in the measurement of f0 is about 0.5 fm (1 fm=10-15 meters), and the anti-proton measurement is within about 0.5 fm of the proton measurement. The error bar in the measurement of d0 is about 1.5 fm, and the anti-proton measurement is again within about 0.5 fm of the proton measurement. The measured radii for protons and antiprotons were within 0.05 fm of each other and were also consistent within the margin of error. More precisely:
[T]he singlet s-wave scattering length and effective range for the antiproton-antiproton interaction to be f0 = 7.41±0.19(stat)±0.36(sys) fm and d0= 2.14±0.27(stat)±1.34(sys) fm, respectively. The extracted radii for protons (Rpp) and that for antiprotons (Rp¯p¯), are 2.75 ± 0.01(stat) ± 0.04(sys) fm and 2.80 ± 0.02(stat) ± 0.03(sys) fm, respectively

Admittedly, these measurements aren't extremely precise. The proton and antiproton radius measurements are accurately to +/- 2%, the f0 measurement is accurate to two significant digits, and the d0 measurement is accurate only to about +/- 75%. But, it is obviously better than having no direct measurement.  Calculations of proton properties from first principles using QCD itself rarely get more precise than 1% or so, due mostly to uncertainties in the up and down quark masses and the strong force coupling constant.

The methodology used involving momentum correlations dates to the 1950s, but previous studies have not had instrumentation and collision numbers sufficient to make this measurement.


These results are not at all surprising.

Indeed it would have been profoundly shocking if they were otherwise, it is reassuring that a result predicted more than 45 years ago has now finally been confirmed with a direct measurement of antiproton-antiproton interactions.  No serious beyond the Standard Model theories of physics proposed any thing different, although this experiment places new constraints on any theory that would argue otherwise.

This is yet more experimental evidence for the existence of CPT symmetry and the absence of any meaningful CP violation in the strong force, which makes previous experimental conclusions more robust because it employs a new methodology to test it and is conducted with a different apparatus by a different set of investigators than those who did prior state of the art collider experiments at LEP, Tevatron and the LHC.

The fact that predictions made by the Standard Model in the 1970s are routinely being confirmed for the first time in 2015 says volumes for its accuracy as a true description of how nature acts within the limits of our ability to observe it.

Off Topic

In other physics news, the first new data since it was reactivated this year at 13 TeV energies has been released.  The LHC was turned back on in May and is a bit behind schedule.  This result is based on about two months of data at the new higher energies.

The newly released report shows that the number of top quark pairs produced at 13 TeV energies, a very well understood process with an unmistakable decay signature is easily measured with a small amount of data, confirms the Standard Model expectation of about 875 top quark pairs per billion collisions at 13 TeV energies.  Top quark pairs have a combined mass of about 346 GeV.

Top quark pairs were produced only about 6 times per billion collisions at the 2 TeV energies that were available at Tevatron which proceeded the LHC.  But, higher energies non-linearly increases the number of very high energy events which are observed.  The 13 TeV run ought to be able to discern meaningful numbers of events producing particles with combined masses of 2 TeV or more (e.g. pairs of 1 TeV mass particles, which would be about six times as heavy as a top quark).

As physicist Tommaso Dorigo explains, this well understood process is basically a calibration test to confirm that the higher energy phase of the LHC experiment is working as expected before conducting measurements where there is any real uncertainty regarding what will be seen.

In a few months, in the coming fall and winter, we should start to see more interesting experimental data out of the LHC which was previously operating at 7 TeV and 8 TeV energies.

Implications Of New Particle Discoveries

Any new fundamental particle seen at the LHC would involve beyond the Standard Model physics, although a variety of new heavy hadrons could be produced without any BSM physics.

The Standard Model has no fundamental particles heavier than the top quark, and there are good reasons in light of prior experiments to expect that no "fourth generation" Standard Model-like fermions exist.

For example, if there were a heavy b' quark (excluded up to masses of 675 GeV as of 2013, compared to a top quark mass of about 173 GeV and a b quark mass of about 4.2 GeV), production of b' quark pairs would be expected to decay almost instantaneously to top quark pairs and produce an excess number of top quark pairs in the 13 TeV energy scale experiments that would also be notable for their energy scale in their own right.  A fourth generation top quark is currently excluded up to masses of 782 GeV as of 2014, but that limit too will quickly be extended by the 13 TeV run.  A t' or b' quark would presumably have a shorter mean lifetime (and hence greater width) than all currently known quarks, but that limitation runs up against the mean lifetime of the W boson which governs quark flavor transitions which is only slightly shorter than that of the top quark.  It would be paradoxical for a t' or b' quark to have a shorter mean lifetime than the W boson, which argues for the proposition that they do not exist.

Fourth generation charged leptons are excluded up to 100.8 GeV as of 2001 (compared to 1.776 GeV for the tau charged lepton).  A variety of measures exclude fourth generation "fertile" leptons as well, directly up to about 39.5 GeV as of 2001 (when the other three neutrino mass eigenvalues are all under 1 eV and are probably each under 0.1 eV) and indirectly through cosmology and oscillation measurements, which tend strongly to show the existence of only three neutrino varieties.

A heavy charged gauge boson W' is excluded up to masses of 2.9 TeV and a heavy neutral gauge boson Z' is excluded up to masses of 2.59 TeV.  Heavy neutral Higgs bosons are excluded up to a little less than 1 TeV and heavy charged Higgs bosons are excluded over a fairly similar mass range.

Failure to observe new fundamental particles drives up the masses at which extra beyond the Standard Model Higgs bosons and superpartners of Standard Model particle must have if they exist, narrowing the parameter space of beyond the Standard Model theories like supersymmetry (SUSY) to increasingly unnatural scales.

Implications Of Coupling Constant Measurements

The 13 TeV scale interactions will also provide a solid test of the accuracy of the "beta functions" of the Standard Model which explain how particle masses and the three gauge coupling constant strengths change as a function of the energy scales involved.  As I noted a year and a half ago, these beta functions provide one of the most experimentally accessible ways to compare the Standard Model to supersymmetry.

Observations of the running of the fine structure constant of electromagnetism and of the weak force coupling constant at the LHC in this run should start to allow researchers to discriminate between Standard Model and SUSY predictions for the running of those constants at high energies.  The differences between the predictions of the Standard Model and SUSY for the running of the fine structure constant are subtle, but the extreme precision with which they can be measured makes it plausible that the two hypothesizes can be distinguished.  The weak force coupling constant is harder to measure precisely, but the differences between the Standard Model expectation and the SUSY expectation for the running of this constant at high energies is much greater (indeed, the direction in which this constant runs is different between the two theories).

Of course, any SUSY model can escape these concerns by pushing the energy scale at which SUSY phenomena appear higher by adjusting its parameters.  But, the indirect measurement of this scale made using the running of the gauge coupling constants can probe higher energies than the direct measurements based upon the detection or non-detection of the myriad of new particles predicted by SUSY.

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