What New Experiments, If Any, Would Help Determine Light Quark Masses?

[fresh_42]

I think that disregard of ...

Since quarks can never be observed as asymptotic free states, it's not clear, how to define their "mass".

For non-confined objects there is a particular mass scheme that has a simple connection to a more "classical" picture of what the mass of a free particle is, but for objects subject to confinement we don't have this.

... is the actual problem with this thread.

 

[arivero]

The OP has already rejected the "just a parameter" concept.

But he is not setting for Newtonian concept of mass neither. I guess all of us could agree in "increasing precision of the parameters of the standard model".

 

[arivero]

... is the actual problem with this thread.

Indeed one would go with more detail here, about renormalisation schemes and how they preserve or transform the measurement errors and the relationships (proportions, absolute max) between parameters. Perhaps too advanced for PF?

 

[Reggid]

While everything being said in this thread about the issues with the definition of quark masses, different renormalizaton schemes, confinement, etc... is true, I still don't see why it was brought up in the first place and what it has to do with the OP question.

The question was simply

What new experimentally data, if any, would be most helpful in advancing the cause of more precisely determining the light quark masses?

so if we make this a bit more precise one could say:

The PDG gives here ( https://pdg.lbl.gov/2022/reviews/rpp2022-rev-quark-masses.pdf ) an estimate for the strange quark mass from lattice QCD in the MSbar scheme at a renormalization scale of $\mu=2\,\rm{GeV}$ in a $N_L = 4$ flavor scheme as

$\bar{m}_s(\mu=2\,\rm{GeV}) = (93.1 \pm 0.6)\,rm{MeV}$ (Eq. 60.5 on page 5)

What can be done not on the theory side, but instead on the experimental side to bring that $\pm 0.6\,\rm{MeV}$ down? (There are other methods than lattice QCD to determine light quark masses, but the equivalent question can be asked for them too)

This seems a well-posed question to me that should be answerable (just not by me, because I simply don't know the answer).

 

[malawi_glenn]

I still don't see why it was brought up in the first place

if we make this a bit more precise one could say

Lack of precision in the question perhaps?

What can be done not on the theory side, but instead on the experimental side to bring that ±0.6MeV down?

Feed LQCD with more precisce hadron masses so that those masses can be computed. Probably the same experiments that have already been performed, but upgraded detectors and higher statistics.

And also

measuring the η→3π decay since it provides a value for md - mu

I forgot to link to this paper, will do it now
https://arxiv.org/pdf/1610.03494.pdf

 

[ohwilleke]

While everything being said in this thread about the issues with the definition of quark masses, different renormalizaton schemes, confinement, etc... is true, I still don't see why it was brought up in the first place and what it has to do with the OP question.

The question was simply
so if we make this a bit more precise one could say:

The PDG gives here ( https://pdg.lbl.gov/2022/reviews/rpp2022-rev-quark-masses.pdf ) an estimate for the strange quark mass from lattice QCD in the MSbar scheme at a renormalization scale of $\mu=2\,\rm{GeV}$ in a $N_L = 4$ flavor scheme as

$\bar{m}_s(\mu=2\,\rm{GeV}) = (93.1 \pm 0.6)\,rm{MeV}$ (Eq. 60.5 on page 5)

What can be done not on the theory side, but instead on the experimental side to bring that $\pm 0.6\,\rm{MeV}$ down? (There are other methods than lattice QCD to determine light quark masses, but the equivalent question can be asked for them too)

This seems a well-posed question to me that should be answerable (just not by me, because I simply don't know the answer).

Exactly! I'm really not trying to ask anything more profound than that.

I don't know the answer, and I suspect contrary to post #26 that more precise hadron mass measurements wouldn't help much for light quark masses since so many hadrons with only light quarks (u, d, s) already have extremely precise mass measurements.

 

[malawi_glenn]

I don't know the answer, and I suspect contrary to post #26 that more precise hadron mass measurements wouldn't help much for light quark masses since so many hadrons with only light quarks (u, d, s) already have extremely precise mass measurements.

Before you created this thread, what was your own thoughts regarding your question?
What did your own investagtion regarding this question yield?

I have same personal opinion, that I do not think Hadron masses are that important anymore to push LQCD calcs of light q-masses, it was something that one of my friends who is hadron physics researcher mentioned. But, perhaps I misinterpred her.

Tau lepton decays can provide insight on s-quark mass, but there you have neutrinos involved so those taus need to be produced in such a way that you can do kinematical fits in order to "reconstruct" the missing four-momenta from the neutrino and thus get a better handle on the decay rate
https://arxiv.org/pdf/hep-ph/9701305.pdf
https://cds.cern.ch/record/237795/files/th-6422-92.pdf

Btw: shouldn't this thread be tagged with "A"?

 

[jim mcnamara]

Tagged with "A", please consider using the report facility for changes to the thread metadata.

 

[ohwilleke]

Btw: shouldn't this thread be tagged with "A"?

Threads are tagged in part based upon the level of depth and technicality expected in an answer.

I generally assume that a thread tagged with "A" is looking for something like help with actually making a detailed and difficult calculation requiring graduate level knowledge, for example, rather than just describing the gist of what calculation is needed, what it does, and what that implies.

 

[arivero]

I find that generically the expanded version of the particle data group report is a good hint to answer this kind of questions, as well as looking at the year of each reference. But sure the OP has already done this.

 

[ohwilleke]

Noted that this is a "featured thread". Interesting choice.

 

[ohwilleke]

Before you created this thread, what was your own thoughts regarding your question?
What did your own investagtion regarding this question yield?

I have same personal opinion, that I do not think Hadron masses are that important anymore to push LQCD calcs of light q-masses, it was something that one of my friends who is hadron physics researcher mentioned. But, perhaps I misinterpred her.

I don't think that hadron masses themselves ought to matter for LQCD calculations since they are already so precise, although I could imagine that additional precision data on more pairs of heavier hadrons whose mass differences can be isolated to be primarily due to quark mass differences due to sum rules or some other form of cancellation due to symmetries or similarities in structure could help.

One of the least accurately measured fundamental observables is the mean lifetime of a free neutron (which, obviously, is a hadron with all light quarks as valence quarks that is well understood in many other respects including its mass), with big tensions between two different kinds of measurement methodologies, that basically involve inclusive and exclusive type measurement differences, but not all that impressive precision even for each method separately (setting aside methodology issues leading to systemic error in one of them, which is something the PDG entry in question on neutrons already acknowledges). I could imagine that greater precision there could help.

Another possibility is that greater precision in the strong force coupling constant measurement as proposed at a next generation CEPC collider experiment derived from measurements of top quark pairs could help, by reducing wiggle room in that pervasive factor in LQCD calculations. See https://arxiv.org/abs/2207.12177

But, mostly, I just don't know.

Tau lepton decays can provide insight on s-quark mass, but there you have neutrinos involved so those taus need to be produced in such a way that you can do kinematical fits in order to "reconstruct" the missing four-momenta from the neutrino and thus get a better handle on the decay rate
https://arxiv.org/pdf/hep-ph/9701305.pdf
https://cds.cern.ch/record/237795/files/th-6422-92.pdf

Good thought.

It also seems that LHCb, Belle II, and other collider experiments focused on hadron physics are doing lots of new precision work with B_s mesons, D_s mesons, and intermediate kaons in the decays of those mesons that have the potential to provide insight into the s quark mass.

But, again, I don't have enough of a quantitative grasp of what the uncertainties are in key quantities and what the critical issues are in the current calculations to really know where the existing uncertainties are common from and how to tame them.

 

[malawi_glenn]

the mean lifetime of a free neutron

Do you know of any paper with a theoretical calculation of the neutron decay rate with quark masses? I only know about calculations with neutron and proton masses as inputs. Would be happy to read actually.

 

[ohwilleke]

Do you know of any paper with a theoretical calculation of the neutron decay rate with quark masses? I only know about calculations with neutron and proton masses as inputs. Would be happy to read actually.

I've never seen the theoretical calculation of the neutron decay rate although I have a general familiarity with what goes into it. I've only seen experimental measurement papers on the subject, although they'd probably have references to a theoretical calculation.

 

[malawi_glenn]

I've never seen the theoretical calculation of the neutron decay rate although I have a general familiarity with what goes into it. I've only seen experimental measurement papers on the subject, although they'd probably have references to a theoretical calculation.

I have seen a few such paper, but quark masses never show up in there, only cabibbo matrix elements.

One of the least accurately measured fundamental observables is the mean lifetime of a free neutron

what is a "fundamental observable"? Are there observables that are not fundamental?

https://pdg.lbl.gov/2021/listings/rpp2021-list-n.pdf
879.4 ± 0.6 s
error is 0.07%

π⁰
8.43±0.13 × 10^-17s
error is 1.5%

 

[ohwilleke]

what is a "fundamental observable"? Are there observables that are not fundamental?

https://pdg.lbl.gov/2021/listings/rpp2021-list-n.pdf
879.4 ± 0.6 s
error is 0.07%

π⁰
8.43±0.13 × 10^-17s
error is 1.5%

Fundamental observable is sloppy wording where I started to say one thing and then said another and didn't proofread afterwards. I just mean observable.

Parts per mill is not great. The neutron mass is measured to parts per billion or better. And, the notes at the link explain that one of the methodologies producing irreconcilable values was just ignored. It says at the link:

We average seven of the best eight measurements, those made with ultracold neutrons (UCN’s). If we include the one in-beam measurement with a comparable error (YUE 13), we get 879.6 ± 0.8 s, where the scale factor is now 2.0.

For a recent discussion of the long-standing disagreement between inbeam and UCN results, see CZARNECKI 18 (Physical Review Letters 120 202002 (2018)). For a full review of all matters concerning the neutron lifetime until about 2010, see WIETFELDT 11, F.E. Wietfeldt and G.L. Greene, “The neutron lifetime,” Reviews of Modern Physics 83 1173 (2011).

The neutral pion mean lifetime, as you note, is only percent precision.

 

[malawi_glenn]

Parts per mill is not great.

If it is great or not depends on how sensitive other calculations which uses it as an input are. Like BBN.

The neutron mass is measured to parts per billion or better. And, the notes at the link explain that one of the methodologies producing irreconcilable values was just ignored

Yes, so? I am not that interested in the "exact" error here, just an order of magnitude, which is ~10^-3

Calculating quark masses are not one of them (I am pretty sure on that, but not at 5σ confidence level)

I can list many observables in the regime of subatomic physics which have worse experimental margin of errors.

BTW - You do realize these two paragraphs are THREE sentences? It was quite hard to read.

I don't think that hadron masses themselves ought to matter for LQCD calculations since they are already so precise, although I could imagine that additional precision data on more pairs of heavier hadrons whose mass differences can be isolated to be primarily due to quark mass differences due to sum rules or some other form of cancellation due to symmetries or similarities in structure could help.

One of the least accurately measured fundamental observables is the mean lifetime of a free neutron (which, obviously, is a hadron with all light quarks as valence quarks that is well understood in many other respects including its mass), with big tensions between two different kinds of measurement methodologies, that basically involve inclusive and exclusive type measurement differences, but not all that impressive precision even for each method separately (setting aside methodology issues leading to systemic error in one of them, which is something the PDG entry in question on neutrons already acknowledges). I could imagine that greater precision there could help.