Monday, November 14, 2011

Top Quark Hadrons and CP Violation Rumors

Top Quark Hadrons

The conventional account states that top quarks don't hadronize because the time frame in which hadrons form is longer than the mean lifetime of a top quark before it decays pursuant to the weak force.

But, surely, this can't be true. The top quark lifetime is just that, a mean. Some top quarks live longer. If you make enough top quarks, some of them will live for arbitrarily long time periods. A few should live long enough to hadronize, most should not. The percentage of top quarks that hadronize should be a small percentage of all top quarks, but the difference between the characteristic width of the strong force and the decay width of the top quark is not so great that it should be undetectibly negligible in a particle accelerator that is generating large numbers of top quarks. We haven't seen a top hadron yet, but that doesn't mean that we shouldn't at some point observe one. After all, there are also a handful of hadrons without top quarks that we have not yet observed.

Of course, these hadrons should have very short lifetimes, on average. And, I don't know the QCD equations well enough to have a sense of the relative frequency of different species of mesons and baryons with top quarks in them, although I would think that they would rather closely mirror the spectrum one sees with mesons and baryons containing charm quark, although perhaps exacerbating the differences one sees between the spectrum of kaons, pions and other exotic hadrons that one sees that contain up quarks and down, strange or bottom quarks with those that contain charm quarks and down, strange or bottom quarks.

Conversely, some of the other five kinds of quarks, which usually live long enough to hadronize, should sometimes live lifetimes too short to do so and transform immediately into another kind of quark via the weak force before having an opportunity to hadronize.

Calculating the frequency of both events ought to be, if not precisely elementary, certainly less challenging that lots of the calculations that go into figuring out QCD backgrounds apart from these effects. It could be that hadrons with top quarks should be so rare that they are below current experimental sensitivities. But, the standard explanation of a lack of top quark hadrons shouldn't rule them out entirely, just make them very rare.

Back of napkin, in my head calculations would suggest that the suppression of top quark hadrons ought to be on the order of 80%-99%, a major impediment to finding them, but far from a complete decree of non-existence.

D Meson Rumors

Rumor has it (via Jester commenting at Woit's blog) that we will be treated before the end of the year to a report from the LHCb experiment (a b quark factory), that D meson to kaon-kaon decays and D meson pion-pion decays exhibit unexpected CP violations at a three sigma level.

D mesons are charm quark-(up/down/strange quark) mesons with antiparticle permutations. Kaons are up-strange mesons with antiparticle permutations. Pions are up-down mesons with antiparticle permutations.

The implication would be a flaw in the CKM matrix of the Standard Model, that sets forth the likelihood of a quark of one kind changing flavor to a quark of another kind, via W boson emission or absorbsion.

Conventionally the four parameters of the CKM matrix (any 3x3 unitary matrix can be fully described with four parameters, but there are many ways to go about doing it), involve three parameters that are CP symmetric and one CP violating phase.

To get an inconsistency from the Standard Model in CP violation phases between D meson to kaon-kaon decays and D meson to pion-pion decays, would suggest that it might take at least two parameters with a CP violating component (hence at least one that is a mix of a non-CP violating mixing angle and a CP violating phase) if there are only three generations of particles, or a CKM matrix that was more than 3x3 so that it would have more degrees of freedom, one or more of which could be devoted to an additional CP violating phase (e.g. in a four or more generation of fermions extension of the Standard Model, which has 9 rather than 4 degrees of freedom in the CKM matrix). Alternately, perhaps the one CP violating phase also has a non-CP violating element, while three completely non-CP violating parameters could be salvaged.

Experimental data have waivered. Some experiments have seemed to show two CP violating phases, while others have seemed to rule it out. Early reports from the LHC on B meson decay (which involve mesons that have b quarks in them), had seemed to be consistent with the absence of more than one CP violating phase in the CKM matrix as the Standard Model predicts and seemingly contrary to hints of more than one CP violating phase in earlier B meson decay experiments. But, rumor has it that the new data kick us back to a more than one CP violating phase situation at the three sigma level, which while not really a discovery, starts to arouse strong interest.

An early CDF finding of a 3.4 sigma top-antitop quark asymmetry, which has since been contradicted by the latest LHC data, produced a paper this past spring detailing various kinds of new physics (W' bosons, Z' bosons or axigluons) that could explain it.

UPDATE: More details on the rumor here: "LHCb observed direct CP violation in neutral D-mesons decay. More precisely, using 580 pb-1 of data they measured the difference of time-integrated CP asymmetries of D→ π+π- and D→ K+K- decays. The result is ΔA = (-0.82 ± 0.25)% [which is] 3.5 sigma away from the Standard Model prediction which is approximately zero! . . . the CP asymmetry estimated at the level of 0.01-0.1%. On the other hand, LHCb measured it is closer to 1%."

Another nice account here and referencing the power point slideshow for the announcement, with page 6 notable showing prior measurements of equal put opposite asymmetries, but at much lower levels than in this experiment and page 7 showing higher past direct CP violation measurements possibly consistent with this result. Also, notably, this is the first evidence of CP violation in D mesons (with charm quarks) although it has previously been seen in B meson (with bottom quarks) and kaons (with strange quarks).

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