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_episodes/01-introduction.md

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teaching: 15
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exercises: 0
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questions:
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- "What is the physics motivation for measuring B<sup>0</sup><sub>s</sub>→μ<sup>+</sup>μ<sup>−</sup>?"
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- "What is the physics motivation for measuring $B_s^0 \to \mu^+\mu^-$?"
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- "What is the overall analysis strategy?"
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objectives:
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- "Understand why B<sup>0</sup><sub>s</sub>→μ<sup>+</sup>μ<sup>−</sup> is a sensitive probe of new physics."
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- "Understand why $B_s^0 \to \mu^+\mu^-$ is a sensitive probe of new physics."
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- "Know the key ingredients of the branching fraction measurement."
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- "Understand the role of the normalization channel."
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keypoints:
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- "B<sup>0</sup><sub>s</sub>→μ<sup>+</sup>μ<sup>−</sup> is a FCNC decay heavily suppressed in the SM — new physics can enhance it."
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- "$B_s^0 \to \mu^+\mu^-$ is a FCNC decay heavily suppressed in the SM — new physics can enhance it."
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- "The branching fraction is extracted from a simultaneous fit across BDT categories."
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- "B<sup>+</sup>→J/ψK<sup>+</sup> serves as the normalization channel to cancel many systematic uncertainties."
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- "$B^+ \to J/\psi K^+$ serves as the normalization channel to cancel many systematic uncertainties."
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---
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## Physics motivation
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<!-- TODO: paste/expand from TWiki introduction section -->
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The decay B<sup>0</sup><sub>s</sub>→μ<sup>+</sup>μ<sup>−</sup> is a Flavour-Changing Neutral
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The decay $B_s^0 \to \mu^+\mu^-$ is a Flavour-Changing Neutral
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Current (FCNC) process. In the Standard Model it is loop- and helicity-suppressed, giving a
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branching fraction of:
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**BF(B<sup>0</sup><sub>s</sub>→μ<sup>+</sup>μ<sup>−</sup>) ≈ 3.66 × 10<sup>−9</sup>**
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$\text{BF}(B_s^0 \to \mu^+\mu^-) \approx 3.66 \times 10^{-9}$
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Many beyond-SM scenarios (SUSY, leptoquarks, extra dimensions) predict significant deviations
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from this value, making it one of the most sensitive indirect probes of new physics at the LHC.
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The measurement follows the strategy of the CMS Run-2 paper [BPH-21-006](https://cms-results.web.cern.ch/cms-results/public-results/publications/BPH-21-006/index.html):
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1. Select B<sub>s</sub>→μμ candidates and classify them into **8 BDT categories** based on signal/background discrimination.
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1. Select $B_s \to \mu\mu$ candidates and classify them into **8 BDT categories** based on signal/background discrimination.
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2. Model the **signal PDF** using a double Gaussian + Crystal Ball shape fitted to MC.
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3. Model **background PDFs**: combinatorial (Bernstein), peaking (KDE from MC), semileptonic (KDE from MC).
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4. Fit the **normalization channel** B<sup>+</sup>→J/ψK<sup>+</sup> in data to extract the observed yield and efficiency.
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5. Perform a **simultaneous unbinned maximum likelihood fit** across all 8 categories to extract BF(B<sub>s</sub>→μμ).
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4. Fit the **normalization channel** $B^+ \to J/\psi K^+$ in data to extract the observed yield and efficiency.
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5. Perform a **simultaneous unbinned maximum likelihood fit** across all 8 categories to extract BF($B_s \to \mu\mu$).
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## Branching fraction formula
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\text{BF}(B_s \to \mu\mu) = \frac{N_{B_s}}{N_{B^+}} \cdot \frac{\varepsilon_{B^+}}{\varepsilon_{B_s}} \cdot \frac{f_u}{f_s} \cdot \text{BF}(B^+ \to J/\psi K^+)
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$$
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where f<sub>u</sub>/f<sub>s</sub> is the ratio of B<sup>+</sup> to B<sup>0</sup><sub>s</sub> production fractions.
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where $f_u/f_s$ is the ratio of $B^+$ to $B_s^0$ production fractions.

_episodes/02-signal-mc-fit.md

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- "How do we model the B<sup>+</sup>→J/ψK<sup>+</sup> signal peak?"
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- "How do we model the $B^+ \to J/\psi K^+$ signal peak?"
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- "How do we apply MC-derived shape parameters to data?"
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objectives:
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- "Fit a double Gaussian model to B<sup>+</sup>→J/ψK<sup>+</sup> MC."
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- "Fit a double Gaussian model to $B^+ \to J/\psi K^+$ MC."
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- "Understand mean shift and resolution scale corrections."
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- "Fit the full signal+background model to data."
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keypoints:
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- "The signal shape is fixed from MC, with a floating mean shift and resolution scale fitted in data."
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- "The combinatorial background uses an exponential; the J/ψ<sup>+</sup>X tail uses an error function."
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- "The combinatorial background uses an exponential; the $J/\psi^+X$ tail uses an error function."
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---
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## Task 2.1 — Double Gaussian fit to MC (category 0)
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Fit a double Gaussian model to the B<sup>+</sup>→J/ψK<sup>+</sup> MC in category 0.
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Fit a double Gaussian model to the $B^+ \to J/\psi K^+$ MC in category 0.
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The invariant mass range is 5.0–5.8 GeV.
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<!-- TODO: add figure from task_2_1 output -->
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## Task 2.2 — Fit data with fixed signal shape
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Use the MC-derived signal shape (fixed parameters) and fit the B<sup>+</sup>→J/ψK<sup>+</sup>
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data with a signal + combinatorial + J/ψ<sup>+</sup>X background model.
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Use the MC-derived signal shape (fixed parameters) and fit the $B^+ \to J/\psi K^+$
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data with a signal + combinatorial + $J/\psi^+X$ background model.
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<!-- TODO: add figure from task_2_2 output -->
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> Run `task_2_4.py`. Note that the MC parameters are different for category 1.
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{: .challenge}
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## Task 2.5 — B<sub>s</sub>→J/ψφ signal fit
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## Task 2.5 — $B_s \to J/\psi\phi$ signal fit
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Repeat the MC+data fit for the B<sub>s</sub>→J/ψφ channel (mass peak near 5.37 GeV).
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Repeat the MC+data fit for the $B_s \to J/\psi\phi$ channel (mass peak near 5.37 GeV).
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<!-- TODO: add figure from task_2_5 output -->
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> ## Task 2.5
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>
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> Run `task_2_5.py`. Note the different mass peak position and the simpler background
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> (no J/ψ<sup>+</sup>X tail needed for the B<sub>s</sub>→J/ψφ channel).
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> (no $J/\psi^+X$ tail needed for the $B_s \to J/\psi\phi$ channel).
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{: .challenge}

_episodes/03-normalization.md

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teaching: 10
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questions:
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- "How do we fit the normalization channel B<sup>+</sup>→J/ψK<sup>+</sup>?"
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- "How do we extract the B<sup>+</sup>→J/ψK<sup>+</sup> yield and efficiency?"
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- "How do we fit the normalization channel $B^+ \to J/\psi K^+$?"
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- "How do we extract the $B^+ \to J/\psi K^+$ yield and efficiency?"
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- "How do we compute the fs/fu production fraction ratio?"
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objectives:
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- "Fit the B<sup>+</sup>→J/ψK<sup>+</sup> data and MC to extract signal yield and shape parameters."
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- "Fit the B<sub>s</sub>→J/ψφ channel to extract the Bs yield."
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- "Fit the $B^+ \to J/\psi K^+$ data and MC to extract signal yield and shape parameters."
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- "Fit the $B_s \to J/\psi\phi$ channel to extract the Bs yield."
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- "Compute fs/fu from the ratio of the two channel yields."
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keypoints:
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- "The normalization channel B<sup>+</sup>→J/ψK<sup>+</sup> cancels many systematic uncertainties."
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- "fs/fu is measured from data using B<sub>s</sub>→J/ψφ and B<sup>+</sup>→J/ψK<sup>+</sup>."
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- "The normalization channel $B^+ \to J/\psi K^+$ cancels many systematic uncertainties."
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- "fs/fu is measured from data using $B_s \to J/\psi\phi$ and $B^+ \to J/\psi K^+$."
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---
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## Task 3.1 — B<sup>+</sup>→J/ψK<sup>+</sup> normalization fit
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## Task 3.1 — $B^+ \to J/\psi K^+$ normalization fit
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<!-- TODO: describe the B<sup>+</sup>→J/ψK<sup>+</sup> fit in data across all BDT categories -->
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<!-- TODO: describe the $B^+ \to J/\psi K^+$ fit in data across all BDT categories -->
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> ## Task 3.1
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> Run `task_3_1.py` to fit the B<sup>+</sup>→J/ψK<sup>+</sup> invariant mass distribution in data.
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> Run `task_3_1.py` to fit the $B^+ \to J/\psi K^+$ invariant mass distribution in data.
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> Record the signal yield and efficiency for each category.
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>
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> ```python
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> python task_3_1.py
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> ```
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{: .challenge}
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## Task 3.2 — B<sub>s</sub>→J/ψφ yield fit
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## Task 3.2 — $B_s \to J/\psi\phi$ yield fit
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<!-- TODO: describe the B<sub>s</sub>→J/ψφ fit used to measure fs/fu -->
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<!-- TODO: describe the $B_s \to J/\psi\phi$ fit used to measure fs/fu -->
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> ## Task 3.2
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>
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> Run `task_3_2.py` to fit the B<sub>s</sub>→J/ψφ invariant mass distribution.
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> Run `task_3_2.py` to fit the $B_s \to J/\psi\phi$ invariant mass distribution.
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> This gives the Bs yield needed to compute the fs/fu ratio.
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{: .challenge}
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index.md

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The exercise is performed on data collected during Run 2.
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### Facilitators CMSDAS LPC 2026
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<img src="fig/loading_circles_set.jpg" alt="" style="height:40%; width:auto">
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<img src="fig/loading_circles_set.jpg" alt="" style="width:60%; height:200px">">
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* [Chris Cosby](mailto:ccosby@fnal.gov) (FNAL)
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* Andrew Melo (Vanderbilt)

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