1. USCMS Hans Wenzel and Nick Leioatts
When Bunches Collide
USCMS
Hans Wenzel and Nick Leioatts

2. Terminology Pt – momentum transverse to the beam axis
Pseudorapidity – angle of a particle relative to the beam axis
Rapidity – angle and momentum of a particle relative to the beam axis

3. Background Beam width 20um Arbitrary detector coordinate system
Other methods of reconstruction
must interrupt beam

4. Purpose (Beam Position)
Find the origin of collisions without having to reconstruct the primary vertex
Align/Measure Resolution of Detectors
Important Input for many physics analyses
B lifetime
Improvement of momentum measurement (beam constrained track fit)
Input for the trigger
(SVT, HLT)

5. Purpose (Beam Profile)
Useful feedback for the accelerator group (Measurement of beta-function)
Used for alignment of the Detector
Important resolution input for physics analyses

6. Methods Impact Parameter: D – closest approach of track to (0,0)
Phi(φ) – angle at minimum approach
Z0 – Z value at minimum approach

7. D-φ Correlation
D(φ0,Z0) = x0(sinφ0) + ax(sinφ0 )(Z0) – y0(cosφ0) - ay(cosφ0 )(Z0)
ax, ay : x and y slope of beam
X0, Y0 : position of beam at Z = 0

8. How many tracks?
1000 tracks needed for 1 micron precision

9. (longer luminous region)
1000 tracks give 0.5 micron/cm precision

10. β function Width of the beam given Z “Hourglass Effect”
Negligible in LHC

11. Data Generation Reconstruction Fit Beam Position D-phi fit
Generation (MC)
Pythia generated minimum bias events
Pt filter (1.5 GeV/c)
Distribute according to betafunction
Calculate track parameters
Smear track parameters with resolution function
Displace beam
Generation (Accelerator)
Not done yet
Reconstruction
Fit Beam Position
D-phi fit
Adjust track parameters to fitted beam position
Fit Beta Function and longitudinal beam distribution

12. Rapidity and Pt of Pythia MC events
Pseudorapidity : η = -ln(tan(θ/2))
Rapidity : y = (ln/2)*((E+pz)/(E-pz))
Pt = √(ρx2+ρy2)

13. Beam Profile

14. Average abs(D0) vs Z correlation
closest impact parameter (D0) given various errors in z

15. What are we fitting? Fit Parameters: ε: Emittance β* Z0: Longitudinal
beam Displacement
σz: width of
Longitudinal beam
distribution

16. σDtr(Pt) = 10μm + (10/Pt)
10000 tracks give 2cm (6%) precision

17. Conclusions
Beam properties can be reconstructed from their tracks (no vertex fitting necessary)
This allows position and width to be measured without disrupting the beam
Important input for various physics analysis
Measurement tool for aligning and checking the alignment of tracking detectors and measuring the tracking IP resolution function.

18. Future
Create accurate models of SVX and CMS detectors for analysis (Acceptance)
Apply to actual data (CDF)