B -> s µµ decays: Current status

In summary, the current status of the B->s mu+ mu- decays, specifically B^0 -> K^0 mu mu and B^+ -> K* mu mu, shows a potential deviation from the Standard Model predictions. These measurements point towards fewer muons in the decays, and the angular distribution analysis of B^0 -> K* mu mu also shows an interesting deviation at intermediate q-values. Theoretically, the deviations could be explained by new interactions between four particles or new heavy particles, but these explanations are not yet conclusive. Other measurements, such as the rare decay B_s -> mu mu, could help shed light on the situation. The LHCb seminar on Tuesday may provide more updates on these measurements.
  • #1
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Something curious is going on with these decays. LHCb gives a seminar talk Tuesday next week, a livestream will be available.
Edit: See results discussed starting here

I'll summarize the current status here. I tried to keep most at the advanced (I)-level, but I don't think that always worked. (B)-level summary: We might have found signs of new unexpected physical effects, but the situation is still unclear.##b \to s \mu^+ \mu^-## is a rare process in the Standard Model, it involves a flavor-changing neutral current. New physics could introduce a coupling to these particles and alter the frequency or the dynamics of the process. We cannot see isolated quarks of course, so the experiments measure the decays of various B mesons to a hadron containing a strange quark plus the muons, an example is shown in the following image.

decays.png
An important test is the frequency of these decays ("branching fraction"). To get more precise theoretical predictions, the branching fraction is often compared to the equivent decay with electrons instead of muons. Based on the production processes, the branching ratios for these decays should be nearly identical, only the slightly different phase space leads to a (well-predicted and tiny) difference.

##B^0 \to K^0 \mu \mu## and ##B^+ \to K^* \mu \mu## -> too low by 2.0 sigma and 2.2 sigma, respectively (the other measurement has been superseded, see the following entry)
##B^+ \to K^+ ll## -> muons are too rare by 2.6 sigma
##B_s^0 \to \phi ll## -> muons are too rare by 3 sigma

Individually, all these measurements look like statistical fluctuations. But they all point in the same direction, towards fewer muons.

##B^0 \to K^{*}(\to K^+ \pi^-) \mu \mu## is a 3-body decay, but due to the quick decay of the ##K^*## 4 particles are produced in total, that makes it interesting to look at the angular distributions. Typically they are studied as function of the invariant mass of the muon pair. Going into all the details would be beyond the scope of this thread (you can read the papers), in summary many parameters are measured. LHCb did the most precise measurement of them so far. One of them, called P'5, shows an interesting deviation at intermediate q-values, see the figure below. How to interpret this?

Pprime5.png


Wilson coefficients are a set of parameters describing a generic new interaction between four particles via heavy particles. Here is an introduction. LHCb did a fit to these parameters based on the angular analysis of ##B^0 \to K^{*}(\to K^+ \pi^-) \mu \mu##. ##C_9## showed a shift of 3.4 standard deviations relative to the SM value. A new spin-1 particle could lead to such a deviation.

This triggered a lot of interest, so other collaborations measured the same decay as well or updated their results for the recent Moriond conference.
Belle result - a similar deviation in P'5, 2.6 sigma
ATLAS result - a similar deviation in P'5, 2 sigma due to the larger uncertainty.
CMS result - no visible deviation, but also with a significant uncertainty

Individually, all these measurements look like statistical fluctuations. But they all point in the same direction, at the same point that is closely linked to ##C_9##.
Theorists combined all these results (and a few more with larger uncertainties) to global fits to the Wilson coefficients: Status of the B->K*µ+µ- anomaly after Moriond 2017.
The result? The best fit value for ##C_9## differs from the Standard Model expectation by 4.9 sigma, supported by both the lower number of muons in the decays and the P'5 measurements. Another option is a deviation in both ##C_9## and ##C_{10}## with opposite sign. This also fits well to the experimental results (with a slightly lower significance), and would correspond to new heavy particles only coupling to left-handed leptons.
LHCb did a similar analysis just based on their own measurements, and got results consistent with the global fit.

Theoretical flavor physics is complicated. It could be that some effects related to QCD were neglected that will turn out to be larger than expected, and explain at least parts of the deviations seen in P'5. It is unlikely that they explain the observed deviations in the decay probabilities, however.What is next?
There will certainly be more work on the theory side to see how the observed deviations can be explained - either by SM effects or by some plausible new physics model. Personally I am more waiting for updated measurements, either showing this was all a weird statistical fluctuation, or establishing the deviations beyond reasonable doubt. There are deviations inconsistent with plausible new physics models, in this case it is a problem of our understanding of the SM. If the deviations are consistent with new physics models, then these models will make predictions for other measurements. One important example is the rare decay ##B_s \to \mu \mu## - it has the same particles contributing. So far, the uncertainty on its decay frequency is too large to contribute notably in global fits, but that will change soon.

There is the LHCb seminar on Tuesday, and we can expect new results. So far the collaboration has shown the results for ##B^0 \to K^{*} \mu \mu## based on Run 1 data (2011-2012), the dataset collected in 2015 to 2016 should have a similar size. They might double their statistics. I expect more big updates in the next two years based on the 2017 and then 2018 datasets, and ATLAS and CMS can improve their measurements as well.

Edit: See results discussed starting here. More branching fractions with missing muons.There is another related measurement: The ratio of ##B^0 \to D^* \tau \nu## to ##B^0 \to D^* \mu \nu## (and equivalent with ##D^0## instead of ##D^*##). As the ##\tau## has a large mass, it is smaller than 1: The SM prediction is 0.25. Belle, BaBar and LHCb measured it. All experimental values are higher, with a combined significance of 3.9 sigma. Again fewer muons...As various 3-5 sigma excesses in the past showed, new physics is always the most unlikely explanation, unless all possible alternatives have been ruled out. It is probably not new physics. But at least it is a promising place to look. And we'll know more next week.
 
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mfb said:
There is the LHCb seminar on Tuesday, and we can expect new results. So far the collaboration has shown the results for ##B^0 \to K^{*} \mu \mu## based on Run 1 data (2011-2012), the dataset collected in 2015 to 2016 should have a similar size. They might double their statistics. I expect more big updates in the next two years based on the 2017 and then 2018 datasets, and ATLAS and CMS can improve their measurements as well.

I should say that the result being presented on Tuesday is Run 1 only. We have not shown this particular measurement before.
 
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  • #3
Why would those effects appear in loops and not tree-level decays? (originates from your mention to the BD*τν vs BD*μν excess)...
 
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  • #4
ChrisVer said:
Why would those effects appear in loops and not tree-level decays? (originates from your mention to the BD*τν vs BD*μν excess)...
If it is a coupling to ##b##,##s##,##\mu##,##\mu##: There is no SM tree-level diagram.

In general, processes without SM tree-level mechanism are more promising places to search for new physics as the SM amplitudes are smaller.

@dukwon: A completely new measurement sounds interesting.
 
  • #5
There are previous measurements from the B factories, I should add. Their error bars are much larger though.
 
  • #6
Well, here are the slides: https://indico.cern.ch/event/580620/attachments/1442409/2226501/cern_2017_04_18.pdf

The quantity being measured is $$R(K^*) \equiv \frac{B^0 \to K^{*0} \mu^+ \mu^-}{B^0 \to K^{*0} e^+ e^-}$$
Results are on slides 32 and 33:
##R(K^*)=0.660^{+0.110}_{-0.070}\pm0.024## in ##q^2 \in [0.045,1.1]\text{ GeV}^2/c^4## (2.2~2.4σ below SM)
and
##R(K^*)=0.685^{+0.113}_{-0.069}\pm0.047## in ##q^2 \in [1.1,6.0]\text{ GeV}^2/c^4## (2.4~2.5σ below SM)

Attached is a plot comparing the results to SM predictions:
lhcb-public_RKstar_sm.png


This result agrees with the B-factory measurements, but their errors were ~30%.
 
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  • #7
Ratio of ##B^0 \to K^* \mu \mu## to ##B^0 \to K^* ee##.
2.2-2.4 sigma below the different SM predictions in the lowest bin of dilepton invariant mass.
2.4-2.5 sigma below the different SM predictions in the second lowest bin of dilepton invariant mass.

Again missing muons...

Edit: dukwon was faster.

5/fb expected in Run 2, together with higher energy and better triggers this could give a 5 times larger dataset. As the analysis is limited by statistics, the uncertainty should reduce by more than a factor 2. If the central value stays the same, it would give a 5 sigma deviation in both bins.

An interesting detail about the electrons:
The LHCb electron energy measurement is mainly based on the tracking system, which means bremsstrahlung emitted before passing the magnet is a problem. LHCb developed a system to add bremsstrahlung photons to the reconstructed electrons. This is different to ATLAS and CMS which mainly rely on their calorimeters.
 
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  • #8
@dukwon collected some theory papers that popped up.

Patterns of New Physics in b→sℓ+ℓ− transitions in the light of recent data
Interpreting Hints for Lepton Flavor Universality Violation
Flavour anomalies after the RK∗ measurement
RK and RK∗ beyond the Standard Model
Towards the discovery of new physics with lepton-universality ratios of b→sℓℓ decays
On Flavourful Easter eggs for New Physics hunger and Lepton Flavour Universality violation

Different flavours sorry of the same interpretation: 3.5 to 5 sigma tension with the Standard Model depending on what exactly you consider. Could be explained by a variation of C9, or potentially C9 and C10.

A larger C10 would make the decay Bs -> μμ more frequent, but even the recent LHCb measurement is not yet precise enough to contribute notably to fits.

Leptoquarks are a viable model.
A Z' could explain it.
Even with more precise measurements, if the deviation gets more significant, it will be challenging to figure out what exactly is correct.
 
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  • #9
mfb said:
There is another related measurement: The ratio of ##B^0 \to D^* \tau \nu## to ##B^0 \to D^* \mu \nu## (and equivalent with ##D^0## instead of ##D^*##). As the ##\tau## has a large mass, it is smaller than 1: The SM prediction is 0.25. Belle, BaBar and LHCb measured it. All experimental values are higher, with a combined significance of 3.9 sigma. Again fewer muons..
LHCb added another measurement

Guess the direction of the deviation.
Fewer muons than expected. How did you guess that?

A naive average puts the new significance at 4.1 sigma.

It is expected that the full Run 2 dataset (including data up to 2018) will lead to an LHCb measurement more precise than the current world average. If the central value stays the same, we would expect more than 5 sigma from LHCb alone, and even more as world average. The analysis is challenging, it will probably take until late 2019 or 2020 until we see the result.

Belle II is expected to start taking data in 2018, but their 2018 dataset will probably be too small to beat LHCb's precision. The dataset size will increase rapidly in 2019-2020.
 
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  • #10
Well, I believe "physics beyond the Standard Model" when it's really discovered. So from this measurement, it'll take about 2 years :-(.
 
  • #11
Well, when is it discovered? The LHCb datasets are growing continuously, and we get an update once in a while. BaBar/Belle are still working on some analyses, but I wouldn't expect too many updates from them.
If it is new physics, then this trend will just go on. We will get more and more measurements with odd 2-3 sigma effects, that slowly get 3-4 sigma effects and eventually 4-5 sigma effects, while the combined significance grows as well - hitting 5 sigma before individual measurements do that, but with different results from different theorists. There won't be a single date where we go from "this is curious" to "this has to be new physics".

In ATLAS and CMS, we had several years where everything was possible - the first 7 TeV data in 2010, the first large 7 TeV dataset in 2011, the first 8 TeV data in 2012, the first 13 TeV data in 2015, the first large 13 TeV dataset in 2016 - all could have had some 5 sigma effect out of nowhere (and searches with 2016 data are still ongoing). That is not the case for flavor physics at LHCb, where you just accumulate more and more B-mesons over time - LHCb exceeded its design luminosity long ago.
 
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  • #12
Gamaliel`s principle :rolleyes:

if this counsel or this work be of men, it will come to nough
 
  • #13
What measurements, if any, can ATLAS and CMS do to shed some light on these discrepancies? The LHCb presentation mentioned charged Higgs, which I assume means a multiple Higgs model, is that something that would/should show up in ATLAS and CMS?
 
  • #14
Charged Higgs bosons could be found by ATLAS and CMS.

For ##B_s \to \mu \mu##, they can contribute a lot to the precision.

Everything involving kaons should be out of reach as the big experiments cannot distinguish them from the much more frequent pions.
I don't see much hope for the other decays either. B decays are very low-energetic for these detectors - two muons are rare enough to trigger on them despite the low energy, but hadronic or semileptonic decays are way too frequent to record them (or even read them out).
 
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  • #15
Yet another LHCb measurement
This time ##R_{K^{*0}} = \left( B(B \to K^{*0} \mu \mu) \right) / \left( B(B \to K^{*0} ee \right)## (ignoring some technical details) in two bins of the dilepton invariant mass.
Muons are too rare by 2.2 and 2.4 standard deviations, respectively, again the same direction.

lhcbrkstar.png


Edit: Forgot the plot. Note the tiny theory uncertainties, and how the measurement is dominated by statistical uncertainties - more data will make it more accurate.

An LHCb member gave a talk about the current status at EPS, the topic discussed here starts at slide 15.CMS updated its P'5 measurement. The result is very close to the SM prediction - sometimes above sometimes below it. They also show a more recent theoretical prediction, which estimates the parameter to be closer to the LHCb/Belle/BaBar measurements (slide 13).

Belle https://indico.cern.ch/event/466934/contributions/2588875/attachments/1487735/2315899/slides.pdf (slides 20 and 21), but the uncertainty is large. The value is ~0.5 sigma above the SM prediction and 1 sigma below the world average.

Edit: More updates, done now.Edit: I missed an older measurement, ##\Lambda_b^0 \to \Lambda \mu \mu##. Here it is - see figure 5 on page 13. Same trend as observed everywhere else.
 
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  • #16
And one more LHCb measurement

##B(B^+_c→J/\Psi \tau^+ \nu_\tau)\,/\,B(B^+_c→J/\Psi \mu^+ \nu_\mu)## (including the charge conjugated modes).

The SM prediction is about 0.25 to 0.28, the experimental result is 0.71±0.17(stat)±0.18(syst), or about two standard deviations higher than expected, corresponding to more taus or fewer muons.

The pattern continues...
 
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  • #17
There is a nice non-mathematical article on this stuff ("Measuring Beauty" by Guy Wilkinson, who works on LHCb) in the November issue of Scientific American.
 
  • #18
I know of mechanisms that enhance the 3rd generation couplings and can lead to non-universalities, but I don't know of any mechanism that predicts less muons?
 
  • #19
Why only 2011-2012 for this? (if i read that right) Does it take that long to process that mountain of data?
 
  • #20
LHCb collected large datasets in 2011, 2012, 2016 and 2017, and another one is expected for 2018. Afterwards the LHC will shut down for two years for upgrades. While there are some results based on 2016 data alone and a few more are probably in preparation, in most cases it is more useful to prepare the analysis now and complete it once the 2018 dataset can be included. That increases the statistics a lot, and it is a useful step before the upgrade comes and the detector changes a lot.

Apart from that: Precision measurements take time. A few months if you have a big team and if you want to get it done quickly, but 1-2 years is more typical. While you can do some things in advance, there are many things that can only be done (or have to be repeated) once the full dataset is available.
Searches for new particles are much faster as they don't need the precision. It doesn't matter much if you have 10% uncertainty somewhere if your main result is basically binary ("we found nothing" or "we might have found something").
 
  • #21
Upcoming seminar: New results on theoretically clean observables in rare B-meson decays from LHCb
3.1 sigma away from the SM prediction of lepton universality in the ##B^+ \to K^+ \mu \mu## vs. ##B^+ \to K^+ e e ## comparison. Again the same direction, muons are less common than electrons.
9/fb, i.e. the whole LHCb dataset so far.
Preprint on arXiv

This is an updated measurement of the 2.6 sigma result I linked in the first post. A bit more luminosity, a bit more significance.
 
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  • #22
So, where do we stand, as of now? Is there some recent review of all the anomalous results?
 
  • #23
I think, it's this preprint, we are talking about:

https://arxiv.org/abs/2103.11769

According to the abstract the violation of lepton universiality in ##\mathrm{B}^+ \rightarrow \mathrm{K}^+ + \ell^+ + \ell^-## decays is violated in comparing ##\ell=\text{e}## to ##\ell=\mu## at ##3.1 \sigma## significance now.

It's getting more significance for some "beyond the standard model physics", but on the other hand we've seen many ##3\sigma## signals gone with getting more data before...
 
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  • #24
I miss some explanation about why the decays towards particles with different masses, as muon and electron are, should be expected to have the same branch ratio. Same coupling, yes. But branch ratio should depend on the energy and momentum avalaible for each decay.
 
  • #25
The Q of the two decays is very similar: muons are light compared to that.
 
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  • #26
And, of course, the "trivial" effect of the different phase space due to the different masses of the electrons and muons is taken into account too.
 
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  • #27
It is taken into account, but it is quite small. Unmeasurably small. I don't know the number off the top of my head, but it's like a fraction of a percent. Maybe 0.1%?
 
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  • #28
LHCb says "1.00+-0.01" (page 7) as theory expectation. Their statistical uncertainty is 4% and the central value for the ratio is 0.846, and that small 1% theory uncertainty comes from higher order effects, not from the lepton masses.

Gudrun Hiller et al updated their numbers on the theory side: Flavorful leptoquarks at the LHC and beyond: Spin 1
 
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  • #29
Ah yep I see that it is because of the high Q, that then theory expectation is near 1. In the theory papers it is mentioned that LHCb has also low-medium Q experiments, for which experimental result is about 0.6 and theory expectation is about 0.8
 
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  • #30
The Q of the decay (total Q) is not the same as the Q of the window in which they look. Both channels can go low in decay Q. Only the very highest Qs are accessible to electrons and not muons. But this effect is tiny.

I haven't looked at the calculation in a long time (and I should - Gudrun was once my office-mate at Kavli) but I expect the effects are due to what used to be called "vector meson dominance" in photoproduction. You can replace the photon with a virtual omega (e.g.) or phi(1020) and the relative phases can be slightly different for e's and mu's.

That said, there is zero chance that this is a real effect due to SM miscalculation.
 
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  • #31
Vanadium 50 said:
I expect the effects are due to what used to be called "vector meson dominance" in photoproduction
[...]
That said, there is zero chance that this is a real effect due to SM miscalculation.
I want to understand this comment (I'm interested in vector dominance), so just to be sure:

are you referring in the first part, to @mfb's higher order effects (#28), and in the second part, to the B-meson decay anomalies?
 
  • #32
vanhees71 said:
I think, it's this preprint, we are talking about:

https://arxiv.org/abs/2103.11769

According to the abstract the violation of lepton universiality in ##\mathrm{B}^+ \rightarrow \mathrm{K}^+ + \ell^+ + \ell^-## decays is violated in comparing ##\ell=\text{e}## to ##\ell=\mu## at ##3.1 \sigma## significance now.

It's getting more significance for some "beyond the standard model physics", but on the other hand we've seen many ##3\sigma## signals gone with getting more data before...

That is why I am somewhat hesitant of calling them " [itex]3\sigma[/itex] signals "... The probability distribution out of which those numbers appear is calculated given the null hypothesis, [itex]p( x>x^* | !M)[/itex] , and doesn't correspond to the probability of the signal hypothesis , [itex]P(M)[/itex]. In that respect I am not very fond of the way the abstract is typed (although they call the evidence of LFU a 3σ effect and not the LFU)
 
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  • #33
I am referring to higher order effects in the calculations

ChrisVer said:
That is why I am somewhat hesitant of calling them "3σ signals "...

Then you should be happy that the authors don't call them that. Further, it is well understood that significance refers to the probability of the null hypothesis alone producing an effect as large or larger and nothing like "the probability the signal is real". (Which is not well-defined.)
 
  • #34
Vanadium 50 said:
Then you should be happy that the authors don't call them that. Further, it is well understood that significance refers to the probability of the null hypothesis alone producing an effect as large or larger and nothing like "the probability the signal is real". (Which is not well-defined.)

I agree up to the last parenthesis. However, I think they could have phrased the abstract better to not give any false impression.
 
  • #35
Which false impression? This is what they write:
This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations
Papers are written by experts for experts and this is a very concise and clear summary of what they measure. But even if we consider non-experts, everyone who can understand what "significance of 3.1 standard deviations" means at all should know that "the probability the signal is real" is not a thing the analysis can answer.
 
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<h2>1. What is the significance of B -> s µµ decays in the field of particle physics?</h2><p>B -> s µµ decays are important because they provide a way to study the behavior of the fundamental particles known as B mesons. These decays involve the transformation of a B meson into a strange quark and a pair of muons, which are relatively rare processes that can provide valuable insights into the Standard Model of particle physics.</p><h2>2. What is the current status of experimental studies of B -> s µµ decays?</h2><p>Experimental studies of B -> s µµ decays are ongoing at various particle colliders, such as the Large Hadron Collider (LHC) at CERN. These experiments have observed a small number of these decays, but more data is needed to fully understand their properties and implications.</p><h2>3. What challenges do scientists face in studying B -> s µµ decays?</h2><p>One of the main challenges in studying B -> s µµ decays is the extremely low probability of these decays occurring. This means that a large amount of data is needed to observe and analyze these processes, which requires sophisticated experimental techniques and high-energy particle colliders.</p><h2>4. What do the current results of B -> s µµ decay studies tell us about the Standard Model?</h2><p>The current results of B -> s µµ decay studies are consistent with the predictions of the Standard Model. This provides further evidence for the accuracy of this theory, but also highlights the need for more precise measurements and potential new physics beyond the Standard Model.</p><h2>5. How do B -> s µµ decays contribute to our understanding of matter-antimatter asymmetry?</h2><p>B -> s µµ decays are related to the phenomenon of matter-antimatter asymmetry, which is one of the biggest mysteries in particle physics. By studying these decays, scientists hope to gain a better understanding of why the universe is made up mostly of matter and not antimatter.</p>

1. What is the significance of B -> s µµ decays in the field of particle physics?

B -> s µµ decays are important because they provide a way to study the behavior of the fundamental particles known as B mesons. These decays involve the transformation of a B meson into a strange quark and a pair of muons, which are relatively rare processes that can provide valuable insights into the Standard Model of particle physics.

2. What is the current status of experimental studies of B -> s µµ decays?

Experimental studies of B -> s µµ decays are ongoing at various particle colliders, such as the Large Hadron Collider (LHC) at CERN. These experiments have observed a small number of these decays, but more data is needed to fully understand their properties and implications.

3. What challenges do scientists face in studying B -> s µµ decays?

One of the main challenges in studying B -> s µµ decays is the extremely low probability of these decays occurring. This means that a large amount of data is needed to observe and analyze these processes, which requires sophisticated experimental techniques and high-energy particle colliders.

4. What do the current results of B -> s µµ decay studies tell us about the Standard Model?

The current results of B -> s µµ decay studies are consistent with the predictions of the Standard Model. This provides further evidence for the accuracy of this theory, but also highlights the need for more precise measurements and potential new physics beyond the Standard Model.

5. How do B -> s µµ decays contribute to our understanding of matter-antimatter asymmetry?

B -> s µµ decays are related to the phenomenon of matter-antimatter asymmetry, which is one of the biggest mysteries in particle physics. By studying these decays, scientists hope to gain a better understanding of why the universe is made up mostly of matter and not antimatter.

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