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May 28 17 tweets 6 min read Read on X
A #thread on penguins. The ones in particle processes.

What's the difference between these two drawings? [1/17]
Image
Image
Both are Feynman diagrams, a tool used to compute probabilities of particle processes.

In both cases a beauty (b) quark becomes a strange quark (s) and two muons (μ). The difference is inside, which is the stuff we do not see. [2/17]
The penguin is that one. It involves three among the heaviest particles we know: the top quark (t), the W and Z bosons, that are responsible for the weak interaction.

They appear for a very very short time (more on that later) thanks to Heisenberg's uncertainty principle. [3/17] Image
The heaviest particles we know do their business without being spotted. But maybe other particles we do not yet know are involved. There could be a heavy Z boson (Z') or a leptoquark (LQ). [4/17]
Image
Image
How can we tell? By looking at the direction in which the muons fly. We can parametrise this in quantities like P'₅, which we plot versus the squared mass of the two muons, q².
(About measuring masses, see ) [5/17]
The result @LHCbExperiment got in 2020 is this. Here q²=1 would be about the mass (squared) of a proton.

Ref: [6/17] arxiv.org/abs/2003.04831
Image
The data points in black differ from the theory prediction in orange. That could either indicate
1. New particles in penguins
2. Something wrong in the experiment
3. Some effect not accounted for in theory. That's where the other diagram enters. [7/17]
The other diagram involves a charm quark (c) and its antiparticle, the anti-charm quark. They also appear for a short time and annihilate into a pair of muons. [8/17] Image
Both diagrams contribute to the overall b→sμμ process. Sometimes it's the one, sometimes the other, sometimes both at the same time. Like in the double-slit experiment. [9/17]
For theorists predicting the charm diagram is much harder because it's nonlocal: the charm loop is not at the same place as where the muon pair is produced, and thus happens later.

How much later? we are talking of less than 10⁻²⁰ seconds. That's long for particles. [10/17] Image
That nonlocality breaks the maths tricks theorists use (don't ask why).

Penguins on the other hand are local. All fine. [11/17]
But if nonlocal is hard to compute, maybe we can measure the effect. So instead of vetoing the regions of dimuon mass where the charm loops dominate, we use them. They are in grey in the 2020 plot. [12/17] Image
That's the @LHCbExperiment data: here are the number of processes versus dimuon mass squared. Note the weird vertical scale. There are many many dimuons at the mass of the J/ψ and ψ(2S) particles, that are made of a charm and and an anti-charm. [13/17] Image
Here's what we get in P'₅. In red the total fit and in pink only the local penguin contribution. The difference is mostly where the J/ψ and ψ(2S) are. [14/17] Image
Now we do a trick: let's replace the penguin parameters we determine by those we would expect from theory, while the nonlocal parameters are those from our fit. That's the blue band. It gets closer to theory. In absence of new particles the blue and red would overlap. [15/17] Image
But the blue band is still not overlapping with the (binned) theory prediction in orange by @DannyD82 and colleagues. That indicates that there may be nonlocal effects not fully accounted for. [16/17]
The paper posted today concludes that there's some mild tension between the theory expectation and the data.

But to me the most important is the input to theorists so they can work on their predictions. [17/17]

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More from @PKoppenburg

Aug 24, 2023
A few more comments on the new Belle II result on B⁺→K⁺νν̅ and the fact that it comes out above the Standard Model prediction. 🧵
First this is an amazing analysis. The signal B meson decays to a kaon and all the rest is invisible. Your signal is a single kaon. What saves Belle is that they know there is another B meson in the event. And they try to understand this as well as possible. Image
They measure a branching fraction that is a factor 4 above the SM. It's the first evidence and the new combined branching fraction is a factor 2 to 3 above the prediction, about 2 standard deviations.
Read 13 tweets
Oct 18, 2022
It's been a while since I last wrote about lepton universality. The Standard Model assumes that all charged leptons - electrons, muons, taus - couple identically to all other particles except to the Higgs (that likes mass). Today we revisit that. A #Thread.
[1/21 - so far] Image
There are some indications it may not be the case. In processes involving decays of b quarks to one or two muons we see discrepancies with the expectations. (Plots at nikhef.nl/~pkoppenb/anom…). Image
Today we are looking at B̅ mesons (with a b quark) that proceed to a D meson (with a c quark), an invisible antineutrino ν̅ and a lepton. That can be an electron, a muon or a tau. Electrons are not practical for that purpose so let's concentrate on μ⁻ and τ⁻. Image
Read 26 tweets
Apr 12, 2021
A #Thread about the most beautiful plot particle physics produces: The Bₛ⁰ meson oscillation plot: A showcase of quantum mechanics in action.
𝐖𝐡𝐚𝐭 𝐢𝐬 𝐢𝐭? The Bₛ⁰ meson is an unstable particle made of an anti-b quark and an s quark. It can transform into its anti-particle the B̅ₛ⁰ meson ("Bee-ess-bar"), made of a b quark and an anti-s quark.
𝐎𝐬𝐜𝐢𝐥𝐥𝐚𝐭𝐢𝐨𝐧𝐬: This process is called oscillation. In quantum mechanics everything that is not forbidden will happen if you wait long enough. And as we will see, you don't need to wait for very long.
Read 21 tweets
Mar 23, 2021
Today is a special day for @LHCbExperiment and for me. A measurement which I started long ago stays stubbornly away from the theoretical expectation.

Is it time to open this bottle? It says "discovery" 🧐. Let's be cautions.
The story started in October 2004. I was a post-doc at Belle in Japan and had been invited to come to SLAC to discuss the potential of a high-luminosity upgrade of the BaBar experiment. Ironically this upgrade would not happen, while @belle2collab would go ahead.
Read 33 tweets
Dec 10, 2020
Today @LHCbExperiment reports the first observation of the Bₛ⁰ → K⁻μ⁺ν process, in which a beauty-strange B meson decays to a muon and a strange kaon. It gives us access to Vub. A #thread.
Vub is the weak-interaction coupling of beauty (b) to up (u) quarks. It's one of the fundamental transitions of the weak interaction. Image
There are actually nine such parameters, one for each of three quark charged 2/3 going to one of the three quarks charged -1/3. They form a matrix called Cabibbo-Kobayashi-Maskawa. See nobelprize.org/prizes/physics… at @NobelPrize Image
Read 21 tweets
Mar 21, 2019
Exciting news! @LHCbExperiment is announcing the observation of CP violation in charm decays. indico.cern.ch/event/807176/ What does that mean? A somewhat long thread.
The laws of physics that affect us (gravity and electrodynamics) are left-right symmetric. One of the pictures below is mirror reversed. Physics doesn't let you tell which. (Social constructs may help: numbers, letters, watches, is the man left-handed?...)
Until 1956 that had not been tested for the weak nuclear interaction, responsible for radioactive decays. Lee and Yang proposed an experiment, that Wu (known as "Madame Wu") conducted.
Read 23 tweets

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