# Presentation: 789012

## Absolute Luminosity Measurement in PP RUN 2002 at PHENIX

- Sergey Belikov, Iowa State University,                 for PHENIX collaboration

- S. Belikov, SC Meeting

- 10/22/2002

## Content

- Introduction
- Simulation results and accuracy
- BBCLL1 efficiency
- Results

- S. Belikov, SC Meeting

- 10/22/2002

## Introduction

- Determine total inelastic cross section in PP interactions at S=200 GeV.
- During van der Meer scan PHENIX supplied signals from 3 independent detectors:
- Zero Degree Calorimeter (~ 1-2% efficiency, too low statistic for analysis).
- Normalization Trigger Counters (too sensitive to a background).
- Beam Beam Counters with Local Level 1 trigger subsystem (effectively cuts background, efficiency ~ 50%).
-    In this analysis only BBC LL1 trigger was used.

- S. Belikov, SC Meeting

- 10/22/2002

## Beam – Beam Counters

- Two arms at ± 1.5 m from the PHENIX center.
-  64 PMTs in each arm.
- PMT has 3 cm thick quartz window – detects Cherenkov light.
- Flash ADCs convert signals to digits on each beam crossing.
- Timing data are processed by the BBC Local Level 1 Trigger System.

- S. Belikov, SC Meeting

- 10/22/2002

## BBC LL1 operation

- Converts input bits into channel time values
- Masks off bad channels
- Checks time range for each channel
- Determines number of hits in each arm
- Calculates average arm time
- For each arm checks if number of hits is in range
- Checks if Vertex is in range
- Result of the last two checks and FEM Unreliable signal are used as an input into      4->2 Look Up Table that creates 2 output bits for GL1.

- S. Belikov, SC Meeting

- 10/22/2002

## BBC Efficiency calculation

- In principle it is possible to simulate both BBC and LL1, but we decided to split the task:
- Simulate BBC and find out their efficiency to all PP inelastic processes.
- Determine BBC LL1 efficiency directly from data accumulated during van der Meer/vernier scan. This permits to avoid systematic error introduced by pedestals drift, etc.

- S. Belikov, SC Meeting

- 10/22/2002

## Efficiency dependence on Vertex position

- 	PYTHIA + PISA simulation shows dependence of BBC efficiency on vertex position. But within  ± 30 cm (PHENIX analysis region) the dependence is negligible In this range BBC “sees” 51% of min bias triggers.  For vernier scan ± 75 cm cut of vertex position was applied.

- S. Belikov, SC Meeting

- 10/22/2002

## BBC efficiency to different processes

- BBC efficiency behaves differently for  different processes. (QCD – qq,qg, gg,... processes)

- S. Belikov, SC Meeting

- 10/22/2002

## BBC efficiency dependence on bunch width 

- S. Belikov, SC Meeting

- 10/22/2002

## Comparison with experimental data

- In PHENIX by selecting events triggered by ZDC we can measure BBC efficiency and can compare it with our simulation. As  you can see we have good enough agreement in +/- 150 cm region, but our simulation definitely exaggerates BBC efficiency outside this region. Integral (+/- 300 cm range) BBC efficiency extracted from the data is 5% lower then that obtained from simulation. Reason: inaccuracy in the simulation of the Cherenkov light propagation when charged particle hit PMT from back.
- Would be better to use narrower vertices range, ± 140 cm. Advantages: less sensitive to background. Disadvantage: more sensitive to the bunch length.
- Integral BBC efficiency in the ± 140 cm is **BBCeff=0.437**

- S. Belikov, SC Meeting

- 10/22/2002

## Dependence on Parton Distribution Function modelFive functions were used to estimate systematic error. PDF variation introduces 2% systematic error.

- S. Belikov, SC Meeting

- 10/22/2002

## LL1 efficiency

- To determine BBC LL1 efficiency two triggers were used:
- Clock trigger – distributed randomly in time (not biased, but low statistic)
- BBC LL1 without vertex cut (high statistic, but biased)
- 	To estimate BBCLL1 trigger background we sorted all events triggered by BBCLL1 into 4 groups according to beams fill patterns:
- Yellow empty, Blue empty
- Yellow empty, Blue filled
- Yellow filled, Blue empty
- Yellow filled, Blue filled.
- First 3 groups were used for background estimation. It is worth to mention that the background to BBCLL1 triggered events in most runs is ~ 0.1%

- S. Belikov, SC Meeting

- 10/22/2002

## BBCLL1 efficiency

-     The BBCLL1 efficiency to events  with "BBC off-line  reconstructed Vertex" depend on beam width,  and we had two beam conditions

- “Corrected" means that background (non PP interaction events) was subtracted from BBC LL1 events statistic.Two beam settings are definitely seen.
- Two our vernier scans were done with beam width > 70 (77 cm  during the first scan and 82 cm during the second scan).

- S. Belikov, SC Meeting

- 10/22/2002

## Clock and BBC_no_Vcut triggers

- We used two types of triggers to estimate BBCLL1 efficiency: Clock Trigger and BBC_no_Vcut Trigger Clock  trigger is unbiased, but had too low statistic. BBC_noVcut trigger had many times higher statistics, but is biased -  it depend on BBC acceptance. To estimate what is the level of BBC_noVcut  biasing I plotted the ratio of BBC efficiencies obtained from analysis of events triggered by  those triggers.

- If you compare the previous plot with these two, you'll se that on the above figures there are no two peaks that would correspond to two beam width settings. It means that ratio does not depend on the beam width. So effect of BBC_no_Vcut biasing can be in average estimated as: Effclock/EffBBCnoVcut=**0.913****±0.003**. During vernier scan we may say that  BBCLL1 efficiency was **0.720 ****±**** 0.003** if we use BBC_no_Vcut trigger, and, after correction of this trigger biasing: **BBCLL1efficiency =   0.72*0.913 ****±**** 0.018 = 0.66 ****±**** 0.018**. So the total cross section reconstructed from BBCLL1 may be estimated as:  **12.6mb/********BBC****/ ********BBCLL1**** =12.6/0.44/0.66 = 43.4 ****±**** 4.4 mb **(assuming 10% error in the luminosity measurement) **.**** **Compare** **it** **with 42 mb predicted by PYTHIA.

- S. Belikov, SC Meeting

- 10/22/2002

## Conclusions

**Total inelastic cross section of PP interaction at ********S= 200 GeV, measured with PHENIX Beam Beam Counters, is equal to 43.4 ****±**** 4.4 mb**
**What to do next:**
**Tune BBC response in PISA simulation.**
**Tune ZDC response and compare BBC/ZDC efficiencies calculated from PISA and from real data.**
**Estimate PYTHIA simulation accuracy. **

- S. Belikov, SC Meeting

- 10/22/2002