New physics, sigma>3.1?
March 23, 2021 10:59 AM Subscribe
LHCb has produced evidence for an unknown mechanism in muon decay. (The Guardian). At a confidence level of 3.1 sigmas, meaning a roughly one-in-a-thousand chance of being due to normal statistical fluctuations, the results currently fall short of the 5 sigma gold standard used in particle physics, but are nevertheless a tantalizing hint that there may be new physics beyond the Standard Model, within reach of our current particle accelerators.
I said sigmas come
And sigmas go
So what are you going to do about it
That's what I'd like to know
posted by CheeseDigestsAll at 11:50 AM on March 23, 2021 [2 favorites]
And sigmas go
So what are you going to do about it
That's what I'd like to know
posted by CheeseDigestsAll at 11:50 AM on March 23, 2021 [2 favorites]
the [3.1 sigma] results currently fall short of the 5 sigma gold standard used in particle physics
Math people help me out here, but I think this means we need 1750 more posts before we can agree it's true?
posted by ryanrs at 12:01 PM on March 23, 2021 [4 favorites]
Math people help me out here, but I think this means we need 1750 more posts before we can agree it's true?
posted by ryanrs at 12:01 PM on March 23, 2021 [4 favorites]
These are two really nice results from the full Run II data-set from LHCb (9 fb-1) for B meson decays into final states containing muons (or electrons). The Guardian article links to the slides from the talks with the technical details.
One is a sweet measurement of the branching ratios of B_s to mu+mu- and B_d to mu+mu- (the latter being an upper limit), nicely consistent with the Standard Model. B_s to mu+mu- is a nice clean observable that constrains various new physics (and I should check to see if I need to update my code...).
The other is an also-sweet measurement of the ratio of branching ratios of B+ to K+ mu+mu- versus B+ to K+ e+e- (sometimes called R_K). This OUGHT to be equal to 1, with very very little theoretical uncertainty. Instead, it's coming out around 0.85, with increasingly tight error bars -- this is the 3.1 sigma deviation that the Guardian reported on. The so-called R_K anomaly has been around for several years and is a real "who ordered that???" kind of result, because it implies that physics affecting muons and physics affecting electrons is different -- i.e., a violation of lepton flavour universality.
Nobody's favourite model predicted lepton flavour universality violation before this anomaly, and another B-decay anomaly involving taus versus muons, surfaced several years ago, because lepton flavour universality violation is a pain in the butt to accommodate without screwing up very tightly constrained flavour observables. You have to resort to ugly not-particularly-well-motivated stuff like leptoquarks or a Z-prime that is the gauge boson of spontaneously broken "L_mu minus L_tau" (muon number minus tau number) or things like that, which there would be no compelling reason to put in your model if it weren't for these weird anomalies. That's why you can have anomalies, that when combined in a certain way are above a 5-sigma signal for certain underlying operators, that have been around for literally years and you've never heard of them -- because they aren't a smoking gun for something really compelling that theorists naturally get excited about.
So it's really interesting to see the R_K anomaly confirmed with the full Run-II LHCb data-set: it could have gone away (in which case *shrug* statistical fluctuation), but it didn't! We will learn more from (1) Run-III of the LHC, during which LHCb will collect even more data, and (2) the recently-begun Belle-II experiment (e+e- collider in Japan, the upgrade of Belle whose data-points you can see in some of the linked slides). This will take a few years, but these are already-built experiments and they are guaranteed to measure these observables more precisely. If the anomalies do persist, finding out what the underlying physics is will be somewhat more challenging because it'll require actually discovering the new particle(s), but LHC Run-III may be able to do that. I think the viable proposed models that can explain these anomalies are already fairly constrained by direct LHC searches, which means that more direct LHC searches will either discover or rule out more of the possible parameter space. A definite signal of new physics in these B-meson decays kind of puts an upper bound on the mass and a lower bound on the interaction strength of whatever is causing it, so you can't hide it forever.
posted by heatherlogan at 6:17 PM on March 23, 2021 [23 favorites]
One is a sweet measurement of the branching ratios of B_s to mu+mu- and B_d to mu+mu- (the latter being an upper limit), nicely consistent with the Standard Model. B_s to mu+mu- is a nice clean observable that constrains various new physics (and I should check to see if I need to update my code...).
The other is an also-sweet measurement of the ratio of branching ratios of B+ to K+ mu+mu- versus B+ to K+ e+e- (sometimes called R_K). This OUGHT to be equal to 1, with very very little theoretical uncertainty. Instead, it's coming out around 0.85, with increasingly tight error bars -- this is the 3.1 sigma deviation that the Guardian reported on. The so-called R_K anomaly has been around for several years and is a real "who ordered that???" kind of result, because it implies that physics affecting muons and physics affecting electrons is different -- i.e., a violation of lepton flavour universality.
Nobody's favourite model predicted lepton flavour universality violation before this anomaly, and another B-decay anomaly involving taus versus muons, surfaced several years ago, because lepton flavour universality violation is a pain in the butt to accommodate without screwing up very tightly constrained flavour observables. You have to resort to ugly not-particularly-well-motivated stuff like leptoquarks or a Z-prime that is the gauge boson of spontaneously broken "L_mu minus L_tau" (muon number minus tau number) or things like that, which there would be no compelling reason to put in your model if it weren't for these weird anomalies. That's why you can have anomalies, that when combined in a certain way are above a 5-sigma signal for certain underlying operators, that have been around for literally years and you've never heard of them -- because they aren't a smoking gun for something really compelling that theorists naturally get excited about.
So it's really interesting to see the R_K anomaly confirmed with the full Run-II LHCb data-set: it could have gone away (in which case *shrug* statistical fluctuation), but it didn't! We will learn more from (1) Run-III of the LHC, during which LHCb will collect even more data, and (2) the recently-begun Belle-II experiment (e+e- collider in Japan, the upgrade of Belle whose data-points you can see in some of the linked slides). This will take a few years, but these are already-built experiments and they are guaranteed to measure these observables more precisely. If the anomalies do persist, finding out what the underlying physics is will be somewhat more challenging because it'll require actually discovering the new particle(s), but LHC Run-III may be able to do that. I think the viable proposed models that can explain these anomalies are already fairly constrained by direct LHC searches, which means that more direct LHC searches will either discover or rule out more of the possible parameter space. A definite signal of new physics in these B-meson decays kind of puts an upper bound on the mass and a lower bound on the interaction strength of whatever is causing it, so you can't hide it forever.
posted by heatherlogan at 6:17 PM on March 23, 2021 [23 favorites]
Metafilter: sigmas come and go.
(Interesting; I think we're getting close to some bigger breakthroughs in physics.)
posted by blue shadows at 9:39 PM on March 23, 2021
(Interesting; I think we're getting close to some bigger breakthroughs in physics.)
posted by blue shadows at 9:39 PM on March 23, 2021
It's worth noting that in 2015 there was an even larger anomaly at the LHC that simply evaporated upon gathering more data in 2016. So don't pop the champagne just yet.
posted by Johnny Assay at 4:57 AM on March 24, 2021
posted by Johnny Assay at 4:57 AM on March 24, 2021
The LHC measures way more than a thousand different observables, so you expect some nonzero number of those observables to be measured as "different" from their "real" values with one-in-a-thousand probability just from statistical fluctuations. But that's also exactly what the new discoveries look like. Excellent explanation by heatherlogan about why this particular observable is interesting.
posted by fantabulous timewaster at 7:56 AM on March 24, 2021
posted by fantabulous timewaster at 7:56 AM on March 24, 2021
(Apologies to any leptoquark lovers out there for calling leptoquarks ugly. If they are real they will immediately become the most exquisitely beautiful thing I have ever seen. And gauged L_mu minus L_tau miiiiiight have something to do with the pattern of neutrino mixing but I'm no expert on that.)
posted by heatherlogan at 10:23 AM on March 24, 2021 [4 favorites]
posted by heatherlogan at 10:23 AM on March 24, 2021 [4 favorites]
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