Observation of CP Violation in Charm Decays

[Wrichik Basu]

New results from the LHCb experiment have identified a difference in the way charm quarks ($c$) behave as compared to anticharm quarks ($\bar{c}$), when a spatial inversion is taken into account. This so-called charge-parity (CP) violation had previously been observed in strange ($s$) and beauty ($b$) quarks.

R. Aaij et al. (LHCb Collaboration), Phys. Rev. Lett. 122, 211803 – Published 29 May 2019
https://doi.org/10.1103/PhysRevLett.122.211803

 

[eys_physics]

Thanks for sharing.

 

[mathman]

It is such observations which hint at an explanation for anti-matter matter disparity.

 

[mfb]

CP violation in the charm sector is small (as expected) - even smaller than in the B meson system. It won't explain the observed matter/antimatter asymmetry in the universe.

 

[mathman]

CP violation in the charm sector is small (as expected) - even smaller than in the B meson system. It won't explain the observed matter/antimatter asymmetry in the universe.

I agree. My point is that similar (undiscovered) processes will eventually give an explanaton.

 

[mfb]

Hopefully. That needs CP violation beyond what we have in the Standard Model. Neutrinos maybe.
LHCb had an interesting (3.5 sigma) hint for larger than expected CP violation in the charm sector a while ago but that turned out to be a statistical fluctuation.

 

[ohwilleke]

Basically, this is just one more confirmation of a Standard Model prediction, made possible by improved experimental detection capacity at the LHCb.

The introduction of the Letter notes that:

The noninvariance of fundamental interactions under the combined action of charge conjugation (C) and parity (P) transformations, so-called CP violation, is a necessary condition for the dynamical generation of the baryon asymmetry of the universe. The standard model (SM) of particle physics includes CP violation through an irreducible complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. The realization of CP violation in weak interactions has been established in the K- and B-meson systems by several experiments, and all results are well interpreted within the CKM formalism. However, the size of CP violation in the SM appears to be too small to account for the observed matter-antimatter asymmetry, suggesting the existence of sources of CP violation beyond the SM. The observation of CP violation in the charm sector has not been achieved yet, despite decades of experimental searches. Charm hadrons provide a unique opportunity to measure CP violation with particles containing only up-type quarks. The size of CP violation in charm decays is expected to be tiny in the SM, with asymmetries typically of the order of 10^−4 − 10^−3, but due to the presence of low-energy strong-interaction effects, theoretical predictions are difficult to compute reliably.

The observed amount of CP violation of just the magnitude that the Standard Model predicts:

(−15.4 +/- 2.9) × 10^−4

The paper notes in its conclusion that:

The result is consistent with, although in magnitude at the upper end of, SM expectations, which lie in the range 10^−4 − 10^−3. In particular, the result challenges predictions based on first principle QCD dynamics. It complies with predictions based on flavor-SU(3) symmetry, if one assumes a dynamical enhancement of the penguin amplitude.

The researchers belief that it is likely that there will be BSM phenomena that will explain the baryon asymmetry of the universe is a case of hope triumphing over experience. Every single bit of available empirical and observational evidence suggests that the baryon asymmetry of the universe was part of its initial conditions, and these initial conditions do not violate any requirement theoretically necessary for a consistent cosmology model. But, because a Big Bang made of "pure energy" that is then deviated from due to CP violation somehow seems prettier than a non-zero baryon number of the universe to start with, researchers have presumptuously convinced themselves that they must be missing something.

 

[mfb]

Every single bit of available empirical and observational evidence suggests that the baryon asymmetry of the universe was part of its initial conditions

Please name such a piece of evidence, because I think that statement is blatantly wrong (unless you say “0 out of 0 is 100%”).

 

[ohwilleke]

Please name such a piece of evidence, because I think that statement is blatantly wrong (unless you say “0 out of 0 is 100%”).

1. There has never been an observation of non-conservation of baryon number. This has been tested in multiple processes, e.g. proton decay, flavor changing neutral currents, etc. The experimental bounds on proton decay and neutron oscillation are both very strict. "No baryon number violating processes have yet been observed." Lafferty (2006) citing S. Eidelman et al. (Particle Data Group), Phys. Lett. B592 (2004).

"Despite significant experimental effort, proton decay has never been observed. If it does decay via a positron, the proton's half-life is constrained to be at least 1.67×10^34 years." Yet, the universe is roughly 1.4*10^9 years old. This experimental result has been a leading means by which GUT theories are ruled out.

Similarly, neutron-antineutron oscillation is not observed but if baryon asymmetry involves this process there "is an absolute upper limit on the n − n¯ oscillation time τn−n¯ of 5 × 10^10 sec. irrespective of the B − L breaking scale, which follows from the fact that we must generate enough baryon asymmetry via this mechanism (according to the linked 2013 paper). The limit on neutron-antineutron oscillation as of 2009 was τn−n¯ ≥ 10^8 sec. See also confirming the experimental result here.

Exclusions for flavor changing neutral currents at the tree level have also not been observed although the measurements are less precise:

In the SM, flavor-changing neutral currents (FCNC) are forbidden at tree level and are strongly suppressed in loop corrections by the Glashow–Iliopoulos–Maiani (GIM) mechanism with the SM branching fraction of t → qH predicted to be O(10^−15). Several extensions of the SM incorporate significantly enhanced FCNC behavior that can be directly probed at the CERN LHC.

In top quark decays they are excluded to a branching fraction of not more than about 0.47% (per the link above).

2. Likewise, there are no processes which have ever been observed which do not conserve lepton number (e.g. there is no observational evidence of neutrinoless double beta decay). These bounds are very strict already.

The universe is roughly 1.4*10^9 years old, so the current limit from GERDA (from 2015) means that no more than one in 3.8*10^16 of hadrons that could have done so have actually experienced neutrinoless double beta decay since the formation of the universe.

3. There has never been an observation of a sphaleron interaction (which would not conserve baryon number or lepton number) but the energies at which sphaleron interactions would take place (about 10 TeV and up) and the rates at which they would occur in the SM (whose parameters and equations are well tested) if they do exist are too small, particularly in light of the small CP violating phase in the CKM matrix (which has been carefully measured). See also, e.g., Koichi Funakub, "Status of the Electroweak Baryogenesis" ("[W]e find that the sphaleron process is in chemical equilibrium at T between 100 GeV and 10^12 GeV.")

4. It is widely accepted and has been proven that with SM physics (and the linked article below acknowledges), that (1)-(3) imply that the baryon number of the initial conditions is positive and non-zero in the absence of BSM physics of particular baryon number and lepton number violating, CP violating processes that occur (only) out of equilibrium.

These are known as Sakharov’s conditions (Yoshimura is also sometimes given credit for them). This source also notes that:

Another way to view things consists in assuming that the primordial Universe developed through interactions of gravity and other fundamental forces, e.g. through the amplification of vacuum fluctuations. In such a case, gravity being blind to the difference between matter and antimatter, equal initial numbers of baryons and antibaryons are expected, and the current unbalance must be induced by subsequent interactions. . . . We only mention for completeness the possibility that the observed baryon excess is a local artefact, and that the Universe is constituted with domains with either baryon or antibaryon excess. The gamma rays arising from annihilation at the boundary of such domains would be a tell-tale sign, and the fact that they have not been observed rejects such a possibility to the limit of the observable Universe.

See also Paolo S. Coppi, "How Do We Know Antimatter Is Absent?" (2004) (reviewing the evidence against spatial anti-matter domains).

Thus, no theory of quantum gravity alone can solve the problem unless it has CP violation which no leading theory of quantum gravity does. There is no experimental evidence of CP violation in gravity at the local level.

In the Standard Model, neither the strong force nor the electromagnetic force have any CP violation either.

The sole source of CP violation in the Standard Model is the weak force, a force in which the coupling constant gets smaller, not larger, a higher energies (as shown in the famous MSSM gauge coupling constant unification illustration below in the left hand panel; no anomalies in the running of any of the SM coupling constants with energy scale has been observed at the LHC so far), which is the opposite of the direction needed if it is to provide a source of CP violation sufficient to explain the baryon asymmetry of the universe ("BAU") given an assumption that aggregate baryon number at the time of the Big Bang was zero.

See also Wikipedia articles on Baryon asymmetry and Baryogenesis.

5. The Higgs boson mass and the associated beta function for it, imply that the SM maintains unitarity up to Big Bang energies. There is nothing that would cause the SM to break down in terms of mathematically if there were no new physics at all at any scale above what is measured and the universe is at least metastable up to the mass (the Higgs boson and top quark masses haven't been measured precisely enough to determine if the universe is stable or metastable if there are no laws of physics other than the Standard Model). See also, e.g., Koichi Funakub, "Status of the Electroweak Baryogenesis" (noting that Higgs boson masses with more than 120 GeV are problematic for models creating BAU from a starting point of zero, when the global average measured value as of 2019 is 125.10 ± 0.14 GeV).

We know empirically that the SM laws of physics remain valid at least up to Big Bang Nucleosynthesis energy scales (see below) and Large Hadron Collider ("LHC") energy such as those necessary to create a quark-gluon plasma.

6. No one disputes that the aggregate mass-energy of the universe at the Big Bang was non-zero so it isn't as if there is a precedent that every aggregate parameter of the universe had to be zero at time equals zero (there is dispute, however, over whether gravitational energy is conserved globally in general relativity).

7. Another quantity that is conserved in the Standard Model locally, and in the aggregate, is electromagnetic charge (for example, e → νe γ and astrophysical limits [m] >4.6 × 10^26 yr, CL = 90%), which still indistinguishable from zero in the aggregate in the universe now, and at all observable times in the history of the universe, and hence, aren't subject to wildly different laws of the universe from the SM if they were zero in the aggregate at the time of the Big Bang.

This is particularly notable because aggregate baryon number, is equal three times the number of quarks in the universe minus the number of anti-quarks in the universe, and all quarks also electromagnetically charged. Thus, any baryon number violating or lepton number violating process must also be electromagnetic charge neutral.

8. There are no traces in the predictions of Big Bang Nucleosynthesis that imply that there was not baryon asymmetry in the initial conditions of the universe. Indeed J.-M. Fr`ere, "Introduction to Baryo- and Leptogenesis" (2005) notes that:

based on nucleosynthesis (which occurs late in the history of the Universe and is therefore not too sensitive to the various scenarios – even if it can be affected by the number of neutrino species and the neutrino background) indicate a stricter, but compatible bound: 4 10^−10 < nB/nγ < 7 10^−10.

Any baryon number violating process must take place at T > 200 MeV (the QCD phase transition temperature), otherwise the success of nucleosynthesis will be spoiled. This temperature is about 400,000,000 times the temperature of the Sun and is believed to correspond to a time one microsecond after the Big Bang in the conventional chronology of the universe. One microsecond is about the time it takes a muon to decay. BBN itself is assumed to take place 10 to 1000 seconds after the Big Bang. This temperature is in the ballpark of the highest temperatures arising at the Large Hadron Collider (a temperature scale at which the Standard Model continues to perform as expected in myriad experimental tests).

Put another way, even advocates of a zero baryon number initial condition (and this would be a majority of theoretical physicists and cosmologists notwithstanding the lack of empirical or observational evidence for it) pretty much agree based upon observation and empirical evidence and well established SM equations and reasonable extrapolations beyond the Standard Model, that the baryon asymmetry of the universe had to be in place around one microsecond after the Big Bang.

The main reason we can't rule out baryon number violation prior to one microsecond after the Big Bang is that we have no way to observe it.

9. There are no traces in the CMB that imply that there was not baryon asymmetry in the initial conditions of the universe. (This is unsurprising given that BBN happens much earlier in the cosmology timeline than the CMB traces at t=100,000 years or so). See also the implications of the CMB for inflation.

Indeed, both of these windows into the very early universe (8) and (9) imply that if there was not baryon asymmetry at time equal zero, that baryon asymmetry had to completely arise very, very quickly.

10. As of 2019, all experimentally measured CP violation observed in Nature (apart from neutrino oscillations data where the experimental uncertainties are too great to saying anything more than that CP violation occurs in these oscillations which may be possible to characterize with a single parameter of the PMNS matrix), is defined by a single parameter out of four parameters in all, in the CKM matrix, which as noted above, is insufficient in magnitude to explain the baryon asymmetry of the universe.

The present consistency of global CKM fits is displayed in Fig. 4. Each coloured band defines the allowed region of the apex of the unitarity triangle, according to the measurement of a specific process. Such a consistency represents a tremendous success of the CKM paradigm in the SM: all of the available measurements agree in a highly profound way. In presence of BSM physics affecting the measurements, the various contours would not cross each other into a single point. Hence the quark-flavour sector is generally very well described by the CKM mechanism, and one must look for small discrepancies.

There are also no experimentally measured deviations from CPT symmetry in non-gravitational physics. See generally, Thomas Mannel "Theory and Phenomenology of CP Violation" (2006).

But see Belfatto, et al. (pre-print 2019) (arguing that there is a 4 sigma tension with unitarity in the measurements of the CKM elements involving the up quark, although such a tension, even if it is more than a fluke measurement wouldn't be remotely sufficient to explain BAU and primarily involves the non-CP violating parameters of the CKM matrix).

11. Attempts to fit cosmology data to inflation theories allow for only a reasonably narrow number of e-fold (ca. 20-80 at the outside with 40-60 cited more often as consistent with the data), which implicitly imposes strict minimum boundaries on the amount of baryon asymmetry that has to emerge per e-fold since the available time in which cosmological inflation must occur and the available time in which baryon asymmetry must occur if you start from zero aggregate baryon number are basically the same. But, we don't have any indication whatsoever that there is a process that is both baryon number violating and CP violating to the necessary degree, or anything remotely close to that.

12. We don't need it to get dark energy. Indeed, while the conventional cosmological constant starts with a near zero dark energy and then has it grow proportionately to the volume of space over time, a transition of zero baryon number to massive baryon asymmetry would imply a huge surplus of energy very close to time zero that is not needed to make the Lambda-CDM model (a.k.a. the Standard Model of Cosmology) work.

13. We don't need it to gethttps://www.cfa.harvard.edu/research/rg/21-cm-cosmology-observations to coincide with what is observed. These measure conditions at ca. 300,000 years after the Big Bang so we wouldn't expect them to so signs of baryon symmetry violating processes.

14. We don't need an initial condition of baryon number equal to zero to get Hubble's constant or a particular amount of dark matter. Assuming a dark matter particle paradigm, according to a pre-print by Yang (2015) subsequently published in Physical Review D, the lower bound on the mean lifetime of dark matter particles is 3.57×10^24 seconds. This largely rules out the possibility that dark matter could bear baryon number and serve as an escape valve around baryon number conservation that is hard or impossible to measure directly.

There really are no observed phenomena in astronomy or the SM which we need an initial baryon number of zero to explain. Even if the phenomena searched for were "just beyond" current experimental limits, the rates of phenomena like proton decay, neutron oscillation, neutrinoless double beta decay and CP violation beyond the Standard Model, and sphaleron interactions were all observed, so long as the existing experimental results remained accurate, none of these could explain the BAU from a hypothesis that aggregate baryon number was zero at T=0.

A non-zero baryon number as an initial condition is the null hypothesis. It is the conclusion that we reach when we follow the available experimental and observational data, and all experimentally validate laws of physics to their logical conclusion, and assume no modifications of those laws of physics not motivated by empirical or observational evidence.

To be clear, it isn't impossible that the laws of physics could deviated wildly from the Standard Model at energy scales well in excess of those at the LHC. Lots of respectable physics believe that someday, somehow, we will discover something like this, and almost all published articles in the field of baryogenesis consider the hypothesis that the initial aggregate baryon number of the universe to be "well motivated". Some physicists even assume, without any evidentiary or theoretical consistency support that the initial conditions of the universe must have included a zero baryon number. For example: "the CP violation in the standard model is a small effect. In particular it is too small to create the observed matter-antimatter asymmetry of the universe, for which CP violation is an indispensable ingredient.", in Thomas Mannel "Theory and Phenomenology of CP Violation" (2006) (emphasis added).

But, we have no meaningful positive evidence to indicate that not only does this happen, but that the deviation from the Standard Model violates baryon and lepton number, is strongly (basically maximally) CP violating, and only occurs in out of equilibrium systems. Indeed, some of the strongest experimental exclusions in all of physics involve searches for baryon number violating processes and lepton number violating processes.

Arguing for a non-zero aggregate baryon number at the Big Bang isn't glamorous or fun. It's like arguing that the Electoral College is a good idea, or that coal needs to be phased out gradually rather than immediately to prevent the economy from collapsing. But, all existing empirical and observational evidence to date supports this conclusion.

 

[mfb]

1. There has never been an observation of non-conservation of baryon number.
2. Likewise, there are no processes which have ever been observed which do not conserve lepton number
3. There has never been an observation of a sphaleron interaction
8. [...] Any baryon number violating process must take place at T > 200 MeV
9. There are no traces in the CMB that imply that there was not baryon asymmetry in the initial conditions of the universe.
10. [CP violation in CKM matrix is too small]

There are plenty of processes expected to exist but known to be out of range of our experiments. Black holes should violate both lepton and baryon number, for example, if they are small enough to produce them. As a more accessible example we have never seen double Higgs production either, but there is no doubt that such a process is possible, the HL-LHC will probably see good evidence for it.

4. It is widely accepted and has been proven that with SM physics [...]

It is universally accepted that the SM is incomplete. The lack of C violation in electromagnetism isn't showing that the universe has C-symmetry. It just means you have to look further.

5. The Higgs boson mass and the associated beta function for it, imply that the SM maintains unitarity up to Big Bang energies.

But this is all about the "Big Bang energies". No new physics beyond the SM at accessible energies but new physics beyond that is exactly what would make 1-3 and 8-10 not surprising.

6. No one disputes that the aggregate mass-energy of the universe at the Big Bang was non-zero so it isn't as if there is a precedent that every aggregate parameter of the universe had to be zero at time equals zero
7. Another quantity that is conserved in the Standard Model locally, and in the aggregate, is electromagnetic charge

I don't see how mass-energy would be equivalent in any way. Baryon number can be positive or negative, energy probably not. Electric charge is a much better comparison. Within experimental uncertainties the total electric charge of the universe is zero. It doesn't have to be - why should the electric charge be so close to zero but baryon number and lepton number be different?

11. Attempts to fit cosmology data to inflation theories allow for only a reasonably narrow number of e-fold

We don't have an upper bound on the amount of baryon number violation during this process. "How can you possibly walk 5 km tomorrow, there are only 24 hours in a day" - and then you learn about typical walking speeds of a person.

12. We don't need it to get dark energy.
13. We don't need it to get 21cm observations to coincide with what is observed.
14. We don't need an initial condition of baryon number equal to zero to get Hubble's constant or a particular amount of dark matter.

We also don't need it to have lunch, but I don't see how this would be relevant.

There really are no observed phenomena in astronomy or the SM which we need an initial baryon number of zero to explain.

There is also no observed phenomena that needs a non-zero initial baryon number to explain. That's what I meant by "0 out of 0 is 100%". We simply don't know because we have no idea what happens at higher energies. All the observations you listed were done at lower energies. Yes, we know baryon number violation has to be zero or extremely rare there, but that doesn't mean anything for higher energies.