Friday, April 26, 2019

Up-Down Asymmetry In Charmed Baryon Decays?

Baryons (see below for background) tend to be less quirky than mesons, with fewer exceptions to the plain vanilla general rules regarding what types are possible and how they decay. But charmed baryons decaying to form baryons and mesons where a down quark replaces a charm quark have now been observed to have one quirk.

While it isn't inherently obvious on the face of the Standard Model physical laws, theoretical calculations to date have predicted based upon the Standard Model's physical laws, have predicted that there should be no detectible asymmetry between up type and down type quarks in these decays. But, in fact, an asymmetry has been observed in at least two different experiments in the decays of charmed baryons with numerical values that are consistent with an absolute value of a 1.00 when an asymmetry of zero is expected. 

One of the prior data experimental values is inconsistent with zero at the 2.94 sigma level. Another is inconsistent with zero at the 8.55 sigma level (a significance that would normally count as the discovery of "new physics"). This study looked at twelve different kinds of charmed baryon decays where results consistent with 1.00 at the one sigma level were observed which were inconsistent with an asymmetry of zero at 6.14 sigma or more in each case.

The main reason that these results aren't making headlines, in addition to the fact that they involve a rather obscure and hard to understand property of baryon decays in the Standard Model, is not that there is really any question about what the experimental results show or the statistical significance of those results. Instead, the concern that has boded caution is that the theoretical predictions in the literature might be flaws in some way.

But, these results are notable, because if the theoretical predictions in the literature are correct, then these experiments are observing beyond the Standard Model physics that have to be explained with some tweak to the Standard Model regarding differences in a novel property other than mass and electric charge between the up type quarks and the down type quarks, that had previously never been observed or predicted (probably without the need for a new particle or force).

Charmed Baryon Weak Decays with Decuplet Baryon and SU(3) Flavor Symmetry

We study the branching ratios and up-down asymmetries in the charmed baryon weak decays of BcBDM with Bc(D) with Bc(D) anti-triplet charmed (decuplet) baryon and M pseudo-scalar meson states based on the flavor symmetry of SU(3)F. We propose equal and physical-mass schemes for the hadronic states to deal with the large variations of the decuplet baryon momenta in the decays in order to fit with the current experimental data. We find that our fitting results of (BcBDM) are consistent with the current experimental data in both schemes, while the up-down asymmetries in all decays are found to be sizable, consistent with the current experimental data, but different from zero predicted in the literature. We also examine the the processes of Ξ0cΣ0KS/KL and derive the asymmetry between the KL/KS modes being a constant.
Comments:13 pages, no figure
Subjects:High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Experiment (hep-ex)
Cite as:arXiv:1904.11271 [hep-ph]
(or arXiv:1904.11271v1 [hep-ph] for this version)
The conclusion of the paper explains that:
We have studied the decay branching ratios and up-down asymmetries in the charmed baryon weak decays of Bc → BDM based on the flavor symmetry of SU(3)F . It is interesting to emphasize that these Bc decays with the decuplet spin-3/2 baryon receive only nonfactorizable contributions. We have shown that our fitting results for B(Bc → BDM) are consistent with the current experimental data in both pm and em schemes. In particular, the em scheme leads to a much smaller number for the χ 2 fit than the pm one, resulting in that the predicted values of B(Bc → BDM) in the em scheme contain much less uncertainties than those in the pm one. We have demonstrated that the isospin relations for the decay branching ratios in Eq. (18) are scheme- and model-independent. It is also interesting to note that the vanishing rates for the Cabibbo allowed decays of Ξ+ c → Σ ′+K¯ 0 and Ξ+ c → Ξ ′0π + have not been supported by the experimental data yet. 
For the up-down asymmetries, we have found that they are sizable, which are different from the prediction of zero due to the vanishing D-wave contributions in the literature. In particular, we have obtained that α(Bc → BDM) = −1.00+0.34 −0 for all decay modes in the em scheme, while they range from −1 to −0.42 at 1σ level in the pm scheme, consistent with the current only available data of αex(Λ+ c → Ξ ′0K+) = −1.00 ± 0.34 [11] for the up-down asymmetry. To justify the SU(3)F approach, we have proposed to search for α(Λ+ c → ∆++K−), which is predicted to be −0.86+0.44 −0.14, in the future experiments, as the the decay has the largest branching rate among Bc → BDM. 
In addition, we have examined the processes of Ξ0 c → Σ ′0KS/KL, which contain both Cabibbo allowed and doubly-suppressed contributions. We have predicted the KL − KS asymmetry of R(Ξ0 c → Σ ′0KS/KL) is −0.106, which depends on neither model/scheme nor the data fitting. Clearly, this asymmetry is a clean result in the SU(3)F approach, which should be tested by the experiments.
Background Regarding Observed Baryons 

So far, 24 of the 40 possible spin-1/2 baryons and 19 of the 35 possible spin-3/2 baryons have been observed. Sixteen spin-1/2 baryons and sixteen spin-3/2 baryons have yet to be observed. All of the baryons that have not yet been observed have at least one bottom quark but not more than one charm quark (24), at least two charm quarks (6), or a bottom quark and two charm quarks (2).

We have observed ten different single bottom baryons (both kinds of bottom lambda, four of the six kinds of bottom sigma, three of the four kinds of bottom Xi and one of the two kinds of bottom Omega), and one kind of double charmed Xi.

We have not yet observed any double charmed bottom, double bottom, triple charmed, or triple bottom baryons.

This pattern of discovery is to be expected, because baryons with heavier quarks are created only in higher energy collisions and are rare even then.

The least massive observed baryon, and the only one that is stable in all circumstances is the proton which has a rest mass of 938.272046(21) MeV. Neutrons (which are the second lightest observed baryons) are stable when bound in a nucleus, but have a mean lifetime of a bit less than fifteen minutes as free particles.

As a result of their extremely short mean lifetimes and the high energies needed to create new baryons of these types, baryons other than the proton and neutron are almost never observed outside particle colliders.

The lightest observed baryon other than the nucleons is the electrically neutral lambda with a mass of 
1,115.683±0.006 MeV. The most massive observed baryon is the spin-3/2, electrically neutral bottom Xi at 5,945.5±0.8±2.2 eV. In theory, the most massive possible three quark baryon would be the triple bottom Omega with an expected mass on the order of 14 to 15 GeV.

The longest lived observed baryon with spin-3/2 is the Omega with a mean lifetime of (8.21±0.11)×10−11seconds. The spin-3/2, electrically neutral bottom Xi is the second longest mean lifetime for an observed spin-3/2 baryon of (3.1±2.5)×10−22 seconds. The most short lived observed spin-3/2 baryon, the Delta (all four types) has a mean lifetime of (5.63±0.14)×10−2seconds.

The longest lived observed baryon with spin-1/2 other than the nucleons is the electrically neutral Xi with a mean lifetime of (2.90±0.09)×10−10 seconds. The most short lived observed spin-1/2 baryon, the single positively charged charmed sigma has a mean lifetime of >1.43×10−22 seconds. 

No three quark baryons not predicted by the Standard Model have been observed.

Baryon Nomenclature

A baryon is a composite particle with either three valence quarks, or three valence anti-quarks. The most common types of baryons, by far, which are of the former type, are protons and neutrons.

Baryons have total angular momentum a.k.a. spin a.k.a. "J" of either 1/2 or 3/2. For each type of baryon made of quarks there is exactly one anti-baryon made of the corresponding anti-quarks. 

In all there are 75 possible types of baryons with three valence quarks (40 spin-1/2 and 35 spin-3/2) and 75 corresponding types of possible anti-baryons in their ground state. There are also more massive excited states of baryons with short lifetimes that are not well understood. Pentaquarks are strictly speaking baryons and have five valence quarks, but only a small number of not entirely certain observations have been made of them and they are not very well characterized yet.

The base name of a baryon is based upon how many quarks that are not up or down quarks are present. If none are present, it is an Omega baryon. If only one is present it is a Xi baryon. If two are present it is usually called a Sigma baryon unless the up or down quarks present have opposite spins, in which case it is called a Lambda baryon. If all three quarks are up or down quarks and it has spin-3/2, it is a Delta baryon. If all three quarks are up or down and it has spin-1/2, it is called a proton or a neutron (a.k.a. Nucleons). Anomalously a bottom Xi baryon is also known as a "Cascade B" baryon.

If a baryon has any bottom quarks the prefix bottom is added if there is one of them, double bottom is added if there are two of them, and triple bottom is added if there are three of them. If a baryon has any charm quarks, the prefix charmed is added if there is one of them, double charmed is added if there are two of them, and triple charmed is added if there are three of them. If both charm quarks and bottom quarks are present, the charm quark prefix is placed first and the bottom quark prefix is placed second. Symbolically this is indicated with lettered subscripts to right of the symbol.

In the case of the six possible spin-1/2 Xi baryon with three different types of quarks (i.e. a charmed Xi, a bottom Xi or a charmed bottom Xi) where there are two possible configurations with the same quark content, the suffix "prime" is added to the configuration that is expected to have the higher mass. (A Lambda baryon would have been the unprimed version of the corresponding Sigma baryon which would have been the primed version, if there wasn't a separate Lambda designation). A "prime" baryon is indicated symbolically with a ' mark in the superscript between the symbol and its electric charge.

The name also does not distinguish between spin-1/2 and spin-3/2 baryons with the same quark content, but a "*" in front of the electric charge (if any) denotes spin-3/2 baryons.

These names do not, themselves, reveal where the up or down quarks present are up quarks or down quarks. This is revealed symbolically, however, by noting the electric charge of the baryon for baryons that can have more than one electric charge which will be ++, +, 0, - or --. The charge is indicated in superscripts to the right of the symbol.

Excited states are indicated with the symbol for the ground state (or N for nucleons) followed by the mass in MeV rounded to the nearest integer in parenthesis.

1 comment:

andrew said...

This paper comes to a different theoretical prediction that is closer to the evidence.