Update EventSelection.md

Author: altsybee

docs/analysis-tools/EventSelection.md

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 * One has to apply offline selections in O2 explicitly in contrast to AliPhysics where these selections were applied together with trigger alias selection.
 * EvSel table might be also useful in user tasks relying on beam-beam and beam-gas decisions in forward detectors, e.g. in UPC tasks.
 
+
+
+## Occupancy estimators and selection bits
+
+
+In ALICE Run 3 Pb–Pb collisions, **occupancy** in the TPC refers to the contamination of an event’s TPC clusters by signals from other nearby collisions within the TPC drift time window.  
+
+- The TPC has a long drift time (~100 µs), so tracks from multiple collisions can overlap in the detector.  
+- Higher occupancy worsens:  
+  - **Tracking efficiency**  
+  - **PID performance (dE/dx shifts, peak broadening)**  
+- Occupancy can be estimated using:  
+  - the number of ITS tracks with ≥5 clusters from other collisions in a time window around a given event. In the analysis it can be retrieved as:
+      ``` c++
+      int occupancyByTracks = col.trackOccupancyInTimeRange(); // range is from 0 up to ~15000
+      ```
+  - the summed FT0C amplitude from other collisions:
+     ``` c++
+      float occupancyByFT0C = col.ft0cOccupancyInTimeRange(); 
+      ```
+  In the occupancy calculation, multiplicities of nearby collisions are "weighted" according to their time separation from the collision-of-interest.
+  Both occupancy estimators are calculated per event in [EventSelectionModule.h](https://github.com/AliceO2Group/O2Physics/blob/daily-20251029-0000/Common/Tools/EventSelectionModule.h#L1361).
+  Estimators return the value of -1 if a collision is close to Time Frame borders (so, not enough information for the occupancy calculation; we need information -40 µs...+100 µs time range wrt a given collision).
+
+
+---
+In addition to the occupancy estimators described above, several additional event selection bits are implemented for a better cleanup of various nearby effects from other collisions (related not only to the TPC, but also to the ITS, e.g. to high occupancies in the ITS Readout Frames).
+The following table summarizes the event selection bits used to mitigate occupancy effects in ALICE Run 3.
+
+| **Bit** | **Definition** | **Strictness** | **Typical Effect / Event Loss** |
+|---------|----------------|----------------|--------------------------------|
+| `kNoCollInTimeRangeNarrow` | Rejects events if another collision within **±0.25 µs** | Narrow veto | Useful to suppress residual BC mis-associations; minimal loss |
+| `kNoCollInTimeRangeStandard` | Rejects if: (1) another coll. within ±0.25 µs, or (2) multiplicity > threshold in −4…+2 µs | Medium | Further suppression of effects from nearby collisions; ~3-10% event loss depending on IR |
+| `kNoCollInTimeRangeStrict` | Rejects events if another collision is within **±10 µs** | Very strict | Strongly reduces effects from nearby events; large loss of statistics at high IR (can exceed 30–40%) |
+| `kNoCollInRofStrict` | Rejects events if >1 collision in the same **ITS Readout Frame** (~15 µs in Pb-Pb) | Very strict | Removes in-ROF pileup; at 38 kHz Pb–Pb cuts ~35% of events |
+| `kNoCollInRofStandard` | Allows >1 collision per ROF but rejects if another has multiplicity > threshold (default: FT0C amplitude >5000 a.u. ≈ 500 tracks) | Medium | Retains more stats, but protects against large in-ROF pileup |
+| `kNoHighMultCollInPrevRof` | Vetoes event if **previous ROF** has high multiplicity (FT0C >5000 a.u.) | only for cross-ROF ITS reco | Removes cases where previous ROF “steals” clusters; few % loss, but improves ITS tracking quality |
+
+These bits can be used as follows:
+``` c++
+if (col.selection_bit(o2::aod::evsel::kNoCollInTimeRangeStrict)) { /* do analysis */ }
+```
+---
+
+More details can be found in the [report at the APW 2024](https://indico.cern.ch/event/1462154/#7-occupancy-effects), where the concepts and observations are explained (while some figures and indicated values might be outdated).
+
+Tight cuts on occupancy improve quality (better S/B, cleaner PID, less bias in kinematics), but reduce event statistics. 
+Sensitivity to the occupancy effects depends on analysis. Therefore, the suggested approach is to study how results of a given analysi change as a function of occupancy (one may try several occupancy "bins", e.g. [0,500), (500, 2000), (2000-4000)), 
+and in addition apply occupancy selection bits, e.g. `kNoCollInTimeRangeNarrow` to eliminate the bc-collision mismatches, and 'kNoCollInTimeRangeStandard' to make a further cleaunup.
+
+Note that the TPC-related occupancy effects are mostly pronounced in Pb-Pb runs, however, one may use the tools described above for occupancy studies also in pp and light-ion runs. 
+
+
+
+## Usage of the RCT flags
+
+(to be updated)
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