1. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 1 Toward a Measurement and Monitoring of pp Luminosity in the CMS Experiment Gregory R. Snow University of Nebraska, USA (for the CMS Collaboration) CMS Luminosity Goals and Constraints Complementary Techniques Considered Occupancy in Very Forward Calorimeter Counting Fast-Reconstructed Vertices Counting Z’s

2. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 2 The CMS Detector Totem T1 here Totem T1 here

3. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 3 Luminosity Goals and Constraints GOALS Aim for luminosity determination to  5% (absolute and relative) Bunch-by-bunch measurement desired Best to have redundant, cross-checkable measurements Provide real-time luminosity information to CMS and LHC CONSTRAINTS No new detectors; use calibrated rates in existing CMS subdetectors ORGANIZATION CMS/TOTEM luminosity working group formed after 2-day Luminosity Workshop in November 1999 Lumi Working Group under umbrella of Forward/Diffractive Physics Working Group

4. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 4 TOTEM Total Cross Section Measurement Measurement of total cross-section (  tot ) at 14 TeV Early in LHC running Low luminosity (  10 28 cm -2 s -1 ) Special high  insertion, zero crossing angle Fewer bunches (36 x 36) Luminosity independent method Elastic cross section to smallest possible During TOTEM running Calibrate monitors for high-luminosity measurements

5. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 5 Luminosity-Independent Method for  tot Optical theorem relates  tot to elastic scattering extrapolated to t = 0 Must count elastic (N el ) and inelastic (N inel ) interactions Elastic to as low-|t| as possible, to minimize extrapolation error Correction from (1 +  2 ) term is small at LHC Correction less than 2%, known to about 0.5% Aim: few % uncertainty on  tot

6. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 6 Importance of precise meas. of  tot and its components (D0 experience with various  tot measurements: CDF, E710, E811) Determined by Monte Carlo and, for CMS, cross-calibration during TOTEM  tot measurement Used successfully in D0 employing small-angle scintillator arrays on each side of I.P.

7. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 7 CMS Very Forward Calorimeters (HF) 20 o HF wedges

8. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 8 HF HAD (143 cm) EM (165 cm) 5mm To cope with high radiation levels (>1 Grad accumulated in 10 years) the active part is Quartz fibers: the energy measured through the Cerenkov light generated by shower particles. This is the cause of two of the peculiar features of this calorimeter: The visible energy is carried by relativistic particles, i.e. electrons: the calorimeter is sensitive to the EM component (  0 ) of the hadronic shower. Shower size depends on Moliere radius not i The light is generated preferentially at 45 degrees: light propagation is far from ‘usual’ meridian one. Iron calorimeter Covers 5 >  > 3 Total of 1728 towers, i.e. 2 x 432 towers for EM and HAD  x  segmentation (0.175 x 0.175) HF is fast and transverse shower size is small

9. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 9 HF Segmentation 3 < |  | < 5    = 0.175  10 o = 0.175  0.175 432 towers per side “Zeros” based on occupancy in HF towers above a fixed E T threshold Toy Monte Carlo, 2000 zero-bias events from DPMJET Outer and inner phi rings eliminated from M.C. study       ()()

10. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 10 Event Characteristics 0 Phi distribution 2  0 Theta distribution   distribution HF

11. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 11 Particle types in zero-bias events Neutrinos and muons eliminated from counting Log scale p ee   KK  KLKL KSKS     

12. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 12 Number of interactions/Xing vs. Luminosity Different Luminosity values set by number of interactions per crossing 10 34 10 33 10 32 Average no. of interactions/crossing Luminosity (cm -2 s -1 )

13. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 13 Poisson distributed number of interactions Different Luminosity values set by number of interactions per crossing Most probable value shown in each histogram Events overlaid by choosing randomly from DPMJET sample 5 8 12 18

14. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 14 Low Luminosity – 1 interaction per Xing 20% of events have zero HF towers with E T > 10 GeV These histograms will be accumulated in the Global Cal Trigger boards and read out on demand for online monitoring ++ -- HF occupancy distributions 0 0

15. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 15 Medium Luminosity – 5 ints. Per Xing HF threshold sensitivity not severe HF occupancy distributions vs. threshold in restricted region of HF 0 0 0

16. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 16 Medium Luminosity – 6 ints. Per Xing Threshold sensitivity small compared to variation of zeros with luminosity HF occupancy distributions vs. threshold in restricted region of HF 0 0 0

17. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 17 Low Luminosity 10 34 10 33 10 32 Use all HF towers

18. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 18 Medium Luminosity 10 34 10 33 10 32 Use 4 symmetric slices of HF towers

19. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 19 High Luminosity 10 34 10 33 10 32 Use 4 symmetric boxes of HF towers

20. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 20 1% Boxes Slices Nominally require >1% of bunch crossings to report a “zero” Calibrate boxes vs. slices for luminosities where both are above 1% Switch to boxes when slices fall below 1% Transferring Calibration to High L

21. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 21 Global Calorimeter Trigger Electronics The L1 Global Calorimeter Trigger (GCT) receives “trigger primitives” from the Regional Calorimeter Trigger Among the trigger primitives are the HF trigger tower (2 towers in azimuth, 3 towers in pseudorapidity) energies – each bunch crossing, no dead-time Histograms will be accumulated using the same FPGA-based Trigger Processing Modules as the rest of the GCT For each bunch crossing, 64 histogram bins (16 bits/bin) can be arranged to store required occupancy information

22. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 22 Linear Impact Parameter in XY based on 2 pixel hits IP < 2 mm Propagate track candidate to third layer in RZ and find Z exp  Z 3 =|Z exp -Z rec |<1.5 mm 4 cm 7 cm 0 cm XY 4 cm 7 cm 23 cm RZ Reconstruct P T based on 3 stereo hits and select P T > 0.9GeV/c A. Giassi & A. Starodumov / INFN-Pisa / Fast track reconstruction method Barrel pixel detector

23. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 23 40% events with P T > 1 GeV, |  |<1.6 Fast track reconstruction method QCD events by Pythia Momentum: P T > 50MeV/c Primary vertex:  xy =15  m,  z =53mm Acceptance: |  |<1.6 (barrel only) Samples: 1, 2, 5, 10 pile-up events

24. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 24 Preliminary Results Increasing luminosity Vertices found Interactions/ crossing Fast vertex reco. eff. N pv N tot  r (%)  (%) 1 2 5 10 40.0  1.4 39.7  1.5 40.0  0.9 41.6  1.3 2000 1000 2000 1000 1.0 1.17 2.0 4.16 40 68 97 100

25. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 25 L (cm -2 s -1 ) 10 26 10 27 10 28 10 34 RmRm 2/s20/s200/s2  10 8 /s Time (2000 events) 16min2min10s< 1s Scale factor 111 R m /R L1  R L1 /R tape Fast vertices reconstructed per second Preliminary Results

26. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 26 PixelLines (Danek) for PV reconstruction Poisson distribution of pile-up events Highest luminosity values need study Fake rate study Systematic error study Future work

27. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 27 Counting Z’s Improved theoretical cross section calculations for Z  l + l - are approaching the precision needed to use inclusive Z production as an absolute luminosity monitor V. Khoze et al. [Eur. Phys. J. C19, 313-322 (2001)] predict  BR(Z  e + e - or  +  - ) = 1.85 nb at 14 TeV with an accuracy of  4% (NNLO, input parton distributions,  s, splitting functions, …) Other theorists predict uncertainty to shrink to  1-2% by LHC turn-on Experimental advantage to monitor both Z  e + e - and Z   +  - (Different systematic uncertainties involved) Given 30 hour luminosity lifetime and nominal 20-hour run, how well can we do, statistically, in a day’s run at LHC design luminosity?

28. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 28 Counting Z’s Inclusive rate of Z   +  - at L = 1  10 34 cm -2 s -1 is 18.5 Hz Assume 50% recovered (acceptance |  |< 2.0, trigger efficiency, …) for monitoring 20 hour run (L = 5  10 33 by the end) yields 540,000 Z   +  - Negligible statistical uncertainty for full run Every 2.5 minutes, 1000 Z’s yield 3% statistical uncertainty Statistics not sufficient for bunch-by-bunch monitoring over full run (Only 200 Z’s per bunch crossing over full run) BUT, “monitoring” bandwidth may be restricted to 1-2 Hz out of 100 Hz Full run yields 144,000 Z’s divided into  +  - and e + e - 15 minutes to accumulate 1000 Z’s in each channel for 3% uncertainty Ongoing CMS study of evolution of trigger and reconstruction efficiencies over a wide range of pile-up events/luminosity values

29. LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 29 Summary The CMS Luminosity measurement is conceptualized and needs work for implementation Firmware for FPGA occupancy histograms on Global Calorimeter Trigger boards Online monitoring of luminosity for CMS and LHC Luminosity database for physics analysis Special run requests Luminosity Conceptual Design Report in preparation, to be followed by Technical Design Report