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

BBC

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

e,beam

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

Fiber response vs. angle

(deg) (deg)

figure: courtesy S. White

The PHENIX BBC

NorthSouth

144.35 cm⊿η = 3.1 ~ 4.0

⊿φ = 2π

slide from T. Nakamura

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.

red triangles: STARblue 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 exhibitssignificant 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

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?