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| 1 | +--- |
| 2 | +title: "Implementing AD in CMS Combine" |
| 3 | +layout: post |
| 4 | +excerpt: "A CERN Summer Student 2025 project aiming at the integration of |
| 5 | +automatic differentiation (AD) into the CMS Combine tool to accelerate statistical |
| 6 | +inference by leveraging RooFit's AD support and LLVM-based gradient generation." |
| 7 | +sitemap: false |
| 8 | +author: Galin Bistrev |
| 9 | +permalink: blogs/2025_galin_bistrev_introduction_blog/ |
| 10 | +banner_image: /images/blog/gsoc-banner.png |
| 11 | +date: 2025-07-05 |
| 12 | +tags: cern cms root combine c++ rooFit automatic-differentiation |
| 13 | +--- |
| 14 | + |
| 15 | +### Introduction |
| 16 | +Greetings! I’m Galin Bistrev, a fourth-year student specializing in Nuclear and |
| 17 | +Particle Physics at the University of Sofia "St. Kliment Ohridski". |
| 18 | +As part of the CERN Summer Student Programme 2025, I’m working on a project to |
| 19 | +integrate automatic differentiation (AD) |
| 20 | +into the CMS Combine tool. |
| 21 | + |
| 22 | +### Project description |
| 23 | + |
| 24 | +This project focuses on implementing automatic differentiation (AD) into the CMS |
| 25 | +Combine tool, which the primary statistical analysis framework used by the CMS experiment |
| 26 | +at CERN. Combine is built on top of RooFit, which recently introduced AD |
| 27 | +technology to support minimization methods. |
| 28 | +By generating computationally efficient gradients through AD, RooFit enables significant |
| 29 | +performance gains. RooFit’s implementation of AD works by converting internal |
| 30 | +likelihood representations into standalone C++ code, from which gradient code is |
| 31 | +produced. This approach not only accelerates fitting but also improves |
| 32 | +the portability and shareability of likelihood models, making them accessible |
| 33 | +even to users without deep knowledge of RooFit or Combine internals. |
| 34 | + |
| 35 | +### Brief description of the CMS Combine engine |
| 36 | +Combine is a statistical analysis tool designed to compare a model of expected |
| 37 | +observations with real data. It’s commonly used for tasks such as discovering |
| 38 | +new particles or processes, setting limits on potential new physics, and measuring |
| 39 | +physical quantities like cross sections.While Combine was developed with High |
| 40 | +Energy Physics (HEP) applications in mind, it contains no built-in physics |
| 41 | +knowledge,thus it remains completely general and independent of the interpretation |
| 42 | +of any specific analysis. This flexibility allows it to be used across a wide range |
| 43 | +of statistical problems |
| 44 | + |
| 45 | +### Project goals |
| 46 | +The main goals of this project are: |
| 47 | + |
| 48 | +- Support external users in working with Combine-generated models without needing |
| 49 | +to dive into RooFit or Combine internals. |
| 50 | + |
| 51 | +- Optimize performance in high-complexity workflows within the Combine framework. |
| 52 | + |
| 53 | + |
| 54 | +### Implementation strategy |
| 55 | + |
| 56 | +- Refactor Combine to use standard RooFit primitives where possible, and add AD support to custom components. |
| 57 | + |
| 58 | +- Integrate RooFit's AD- generated gradients into Combines's likelyhood |
| 59 | +evaluation and minimization workflows to improve performance and enable external |
| 60 | +gradient use. |
| 61 | + |
| 62 | + |
| 63 | +- Developing benchmarks to quantitevly demonstrate the performance improvements AD brings to Combine overflows. |
| 64 | + |
| 65 | +##Conclusion |
| 66 | + |
| 67 | +By integrating automatic differentiation into the CMS Combine tool, we aim to make |
| 68 | +statistical inference not only faster but also more accessible to the broader |
| 69 | +community. I’m excited to contribute to this effort during my time at CERN and |
| 70 | +look forward to sharing progress, insights, and benchmarks as the project evolves. |
| 71 | + |
| 72 | +### Related Links |
| 73 | +- [CMS Combine GitHub page]https://cms-analysis.github.io/HiggsAnalysis-CombinedLimit/latest/ |
| 74 | +- [ROOT official repository]https://github.com/root-project/root |
| 75 | +- [My GitHub profile]https://github.com/GalinBistrev2 |
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