# Presentation: 083292

## RHIC Zero Degree Calorimeter

**RHIC Zero Degree Calorimeter**

- M. Chiu, S. White, A. Denisov, M. Csanad, P. Hidas, A. Ster, T. Csorgo, M. Murray, A. Makeev, J .Katzy, E. Garcia, A. Drees

## Luminosity/Cross-Sections
R = L
Event Characterization
Centrality (Au+Au, d+Au)
Event Vertex and Start-Time
Hadronic/Non-Hadronic (Au+Au, d+Au, p+p)
Clean Triggering
Ultra-Peripheral Coherent Interactions
Transverse Spin Asymmetries
Local Polarimeter
Relative Luminosity
Crucial for ALL

- R = L
- Event Characterization
- Centrality (Au+Au, d+Au)
- Event Vertex and Start-Time
- Hadronic/Non-Hadronic (Au+Au, d+Au, p+p)
- Clean Triggering
- Ultra-Peripheral Coherent Interactions
- Transverse Spin Asymmetries
- Local Polarimeter
- Relative Luminosity
- Crucial for ALL

- Luminosity/Cross-Sections
- R = L
- Event Characterization
- Centrality (Au+Au, d+Au)
- Event Vertex and Start-Time
- Hadronic/Non-Hadronic (Au+Au, d+Au, p+p)
- Clean Triggering
- Ultra-Peripheral Coherent Interactions
- Transverse Spin Asymmetries
- Local Polarimeter
- Relative Luminosity
- Crucial for ALL

**The ZDC is a Good Place to Be..**

## Location, Location, Location

**Location, Location, Location**

- There is a lot of physics at forward rapidities
- In a collider, you need to have a DX magnet to steer bunches so they collide
- Spatial Distribution of Charged Particles shown below
- Large Separation = Easy Timing = Very Clean Trigger against Beam Gas and Beam Scrape

## Collider Processes

- Au-Au --> X		         6.8 barns
- -:
- AuAu --> AuAu + e+e-        33 kbarns
- AuAu --> AuAu + 2(e+e-)   680 barns
- AuAu --> AuAu + 3(e+e-)     50 barns
- -N:   L(-N )=1029 cm-2s-1 2<E<300GeV
- AuAu --> Au+Au*                92 barns
-                                 X+neutrons
- AuAu --> Au*+Au*             3.670.26 barns
-                                 X+neutrons
-                                 Y+neutrons

**Collider Processes**

- You’re probably familiar with the “Hadronic Interactions”
- When colliding large nuclei, the Z creates a large photon flux
- ZDC exploits this for a luminosity measurement
- Also interesting physics (UCP program)

- Hadronic Interaction

- Peripheral Interaction

## Total Cross-Section

**NIM A 417:1-8,1998**, **nucl-ex/9801002**

- Use Weiszacker-Williams to calculate equivalent photon flux
- ie, Fourier Transform EM field to get photons
- idea first put forth by Fermi
- Take measured photon dissociation data
- Key Point: Integrate through the nuclear radius to reduce uncertainty in lower cutoff for peripheral collisions
- Calculated Cross-Section good to 5%
- 10.7 barns at root-s = 130 GeV
- 11.0 barns at root-s = 200 GeV
- But measurement is pristine

## Energy and Spatial distribution in ZDC ford+Au @ 100Gev

- Single neutron

- background

- Single neutron

- _X-Y scatter plot for single neutron events in North SMD_

- New in Run03

## SMD assembled with a preproduction ZDC module

- LED flasher

- 16-channel PMT M16

- 21 scintillator strips (180x5x5mm**3 ea)

- 21 WLS fibers (BCF-91)

- 7 (of  3 WLS fibers) bundles

- 7 clear fibers PMT<-->LED

- Beam

## Transverse Spin Asymmetries

- Not found but they did measure a neutron asymmetry
- Perhaps understood (?), but look for upcoming paper
- Large asymmetry gives good local (PHENIX) polarimetry.

**Transverse Spin Asymmetries**

- “Left-Right” asymmetry measured for different slices in phi:

## Rotator Commissioning at PHENIX

- Spin Rotators OFF

**Blue**

**Yellow**

- Spin Rotators ON, Current Reversed

**Yellow**

**Blue**

**Blue**

**Blue**

**Yellow**

**Yellow**

- Spin Rotators ON, Almost...

- Spin Rotators ON, Correct!

- |P|=30%, PT=0%  PL=30%)

- |P|=37%, PT=24% PL=28%)

- PB=35.5%

- PB=37%

**Run-03**

- We are extremely glad that we have this local polarimeter at hand to do these checks...Without all this, our gluon polarization measurements would have been completely blind and meaningless! – Abhay Deshpande

## Longitudinal Double Spin Asymmetries

- To determine G, look at ALL:
- R is the relative luminosity, and can be measured (to some accuracy) at PHENIX.
- Our Goal: R/R < 0.1% for each fill ALL < 2×10-3(expected ALL for pions ~ 3×10-3 @PT=3 GeV/c)

## Relative Luminosity

- In order to investigate our ability to measure the relative (++ vs. +-) luminosity:
-  look at ratio of 2 detector scalars crossing-by-crossing:
- a(i) = NA(i)/NB(i)
- Ratio should be the same for all crossings (constant) if:
- NA(i) = LAεA and NB(i) = LBεB
- B is always BBCLL1A is one of the others (CH2-8).
- Fit this by the expected pattern:
- a(i) = C[1+ALLPB(i)PY(i)]
- C, ALL are the fitting parameters.
- Precision of relative luminosity can be estimated by:
-  C/C
- If 2 of the fitting is bad, look for other factors in N(i).

**Ratio of Zero-Degree Counter scalars to Beam-Beam Counter  scalars, sorted by bunch crossing and fit to a constant.**

## Correction Factors

- What other factors could play a role in the determination of the scalar rate besides the luminosity?
- Vertex width
- Vertex width also measured crossing by crossing.
- Look for a correlation of the ZDC/BBC ratio with the vertex width:
- Good correlation
- Can correct ratio for this factor.

## Limit on Relative Luminosity Measurement

- After correction for (measured) vertex width, the ratio of counts in the two detectors is consistent with constant up to our level of statistics
- This means that if we apply correction for this the precision on R goes from:0.11%           0.06%
- (syst. limited)        (stat. limited)

- ZDC/SMD (Local Polarimeter) saved the day in
- two ways for the “Hot Off the Presses” PHENIX
- Preliminary Results for G/G

## Correlation between ZDC/SMD measurements and beam displacement during vernier scan

- Count Rate vs Beam Displacement
- SMD Position vs Beam Displacement

## Mass Splitting Phenomenon

- Identified particles show a split in the mass2 for positive and negative tracks
- Input to this calculation is only track angle at DC and TOF.
- TOF is charge independent, so it must be track angle
- Beam motion measured by ZDC-SMD
- Beam x-y position correction applied.
- Splitting gone.

- Before Beam Position Correction

- After Beam Position Correction

## ZDC has worked (phenomenally successful)
Luminosity Monitor & Cross-section Measurement
Clean Trigger
Bunch by Bunch Relative Luminosity Correction
Transverse Spin Measurement
p+p and Heavy Ion Event Characterization
More not foreseen...
The ZDC was almost an afterthought at RHIC
Good to see that it’s almost de rigueur now at colliders

- Luminosity Monitor & Cross-section Measurement
- Clean Trigger
- Bunch by Bunch Relative Luminosity Correction
- Transverse Spin Measurement
- p+p and Heavy Ion Event Characterization
- More not foreseen...
- The ZDC was almost an afterthought at RHIC
- Good to see that it’s almost de rigueur now at colliders

**Summary and Conclusions**

## Backup Slides

## ZDC Physics: Hadronic Interactions

**ZDC Physics: Hadronic Interactions**

- Since the ZDC sees only neutral, forward going particles, they are affected by any processes which produce or remove free neutrons
- Coalescence of spectator nucleons into light nuclei (d, triton, alphas)
- Evaporation of free neutrons from unstable fragments

- Coalescence volume

- fragment

- 1. Incoming Heavy Ions
- 2. After collision, spectators are free neutrons, protons, and fragments
- 3. Free neutrons and protons can coalesce, removing neutron deposition in ZDC
- 4. Fragments can boil off neutrons, increasing neutron deposition in ZDC

- 1.

- 2.

- 3.

- 4.

- n