# Presentation: 328304

## Luminosity Monitoring Issues

-  ZDC
-  what’s the advantage?
-  problems
-  BBC
-  can they do it?

- A. Drees                                                                                                                   QCD critical point workshop, Mar 10 2006

## What do we want/need?

- a reliable detector for online monitoring and steering
- a reasonable signal/noise ratio
- calibration (cross section measurement) for book-keeping
- comparison between the different IRs
- small integration times
- independent from vertex distribution and location
- redundancy

## ZDC overview

- The ZDCs are sampling hadronic calorimeters with tungsten/fiber layers (27 per module)
- common to all IRs
- Since they are located at +/- 18 m, behind the DX magnets, they ‘see’ only neutrons
- the mutual forward neutron cross-section is much smaller than the single neutron cross section (about x10 @100 GeV)
- if used as a collision monitor in coincidence mode the signal/noise ratio is very good (typically very low background)
- they see all collisions, regardless of the vertex position and distribution

## ZDC Location

-  Peripheral Interactions have very low pt, so you need to put the detectors very forward
-  In a collider, you need to have a DX magnet to steer bunches so they collide
-  Spatial Distribution of Charged Particles shown below

- figure: courtesy S. White

## ZDC Design

_**ZDC Calorimeter construction:**_
- Tungsten absorber/ fiber (C)sampling
- 2 Lint/module, 3 modules total
- C sampling filters shower secondaries
- Uniform response vs. impact point

- NIM A 470:488-499,2001, nucl-ex/0008005

- figure: courtesy S. White

## The PHENIX BBC

**The PHENIX BBC**

- slide from T. Nakamura

**Notes:**
BBC is seated on about 145 cm from center of PHENIX detector for both North and South arm along with beam axis. The pseudo rapidity coverage of central arm is pulse minus 0.35 and 90 degree in azimuth for both West and East arm. In contrast, coverage of BBC is 3.1 to 4.0 in pseudo rapidity, and 2 pai in azimuth.

## STAR Beam-Beam Counters

- 1-cm thick scintillator hexagons with fiber-optic light collection tiling an annulus (2.1<||<5).  East/west annuli separated by 7.5 m.

- slide from L. Bland

## Luminosity from collision rates in the ZDC (calibration from 2002) divided by the luminosity calculated from beam parameters (size & bunch current). Beam parameters are derived from vernier scans. This ratio should be constant and about 1. Should not depend on luminosity. 

- Luminosity from collision rates in the ZDC (calibration from 2002) divided by the luminosity calculated from beam parameters (size & bunch current). Beam parameters are derived from vernier scans. This ratio should be constant and about 1. Should not depend on luminosity.

- red triangles: STAR
- blue circles: PHENIX

- jump?

## Why does it help to have redundant measurements?

- Luminosity from collision rates
- measured by the STAR (triangles) and PHENIX (circles) BBCs divided by the luminosity from the ZDCs. This ratio should not depend on the luminosity but should be constant and about 1. Calibration used is from 2002.

- Luminosity from collision rates (calibration from 2002) as measured by the BBC divided by the luminosity from beam parameters (calculated). Beam parameters are derived from vernier scans. This ratio should be constant and about 1.

## The ZDCs are less affected by background

- Vernier Scan in 2005 in PHENIX. ZDC data is “clean” while the BBC data exhibits
- significant background contribution for negative x.

- ZDC

- BBC

## Shortcomings of the ZDCs

- BBC

- ZDC

- Data from the 10 GeV run in 2001.
- Statistically the ZDCs are already at their limits, less than 10 Hz is difficult to use for steering and monitoring.
- The BBC seem to be still ok (2 tubes on either side only), full BBC would enhance rate.
- STAR BBC was not available in 2001.

## PHENIX and STAR luminosity signals from ZDC and BBC during a pp store in 2005

- The existing difference of ~20% could not be explained by beta* differences (STARs was actually smaller!). The gaps were aligned in IR4 & 10.

- BBC: PHENIX/STAR = 0.48

- ZDC: PHENIX/STAR = 1.2

## PHENIX and STAR luminosity signals from ZDC and BBC during a pp store in 2006. No deliberate changes were made. 

- BBC: PHENIX/STAR = 0.42

- ZDC: PHENIX/STAR = 0.75

- PHENIX and STAR luminosity signals from ZDC and BBC during a pp store in 2006. No deliberate changes were made.
- While the ratio of the BBC basically didn’t change by more than about 10% between the two years (for whatever reason), it changed by approx. 40% for the ZDCs.

## Calorimeter limitations

- limit of production of cherenkov light in the fibers:  > 1/n
- n for the fibers = 1.4 => 1/n = 0.7
- for  = p/E = 2.3/3.5 = 0.65
- assume coulomb dissociation of the neutrons: Eneutron ~ Ebeam~Eshow.prod.
- depending on the beam energy we are just at the limit
- increase tube voltage, decrease discriminator voltage => new calibration ...
- acceptance drop due to fermi motion 1/p2beam

## Conclusion

- ZDCs seem statistically, acceptance and light limited at low energies
- they can most likely not be used reliably for monitoring or steering (and not for triggering)
- we will have to give up the idea of two independent measurement systems
- can we use the exp. BBCs and calibrate them?