V0-AND trigger cross section at 7 TeV p+p collisions by van der Meer scan method

1. V0-AND trigger cross sectionat 7 TeV p+p collisionsby van der Meer scan method ALICE Physics Forum, Sep. 1, 2010 Ken Oyama University of Heidelberg

2. Introduction • Interaction cross section of “process” fires V0-AND (V0-A & V0-C) trigger has been determined • Method: Van der Meer scan (Vernier scan) • measure rate of the trigger with different displacements of two beams • V0-AND trigger has been chosen because • back-ground free ( < few Hz ) • high efficiency ( > 80 % of total p+p cross section) • stable detector operation • Usage: • cross section normalization • luminosity measurements ALICE Physics Forum

3. Basic of the Method beam intensities • Luminosity and cross section: • We don’t know beam profile but by displacing two beams by Dx and Dy  trigger rate directly gives the square average beam sizes number of bunches and machine freq. beam profiles Gaussian shape approximation beam sizes (g: normalized Gaussian) top rate ALICE Physics Forum

4. Extracted Raw Data from DCS DB • Right: systematic decrease of luminosity and Left: magnified in time • Straight gray bold line (fit) is considered to be blow-up and correction has been performed (make it flat) ALICE Physics Forum

5. Beam Condition • Single_2b_1_1_1one collision in ALICE • beta* = 2 m • Zero crossing angle • Colliding bunch intensities: N1 = 17.4 x 109 protons N2 = 20.6 x 109 protons • Intensity systematic error • measurement done by: • BCT (beam current transformer): bunch-by-bunch relative intensity • DCCT (DC current transformer) : absolute scale but for DC current • Systematic uncertainty is governed by offset of DCCT ~ 5% for each beam • LHC-wide systematic error • Intensity decrease … negligible (this is different from emittance increase) ALICE Physics Forum

6. Pile-up Correction • At 1 kHz trigger rate : average number of “firing” times per bunch crossing is 1kHz / frev ~ 0.1  this is the  (Poisson average) Interaction rate Trigger rate  • Correction is as much as 5% at around top luminosity where R(0,0) ~1 kHz • V0-AND trigger is slightly biased as it is essentially multiplicity trigger however effect of that to the rate measurement is <<1% ALICE Physics Forum

7. Corrected Data • Three different correction types tested • nc: no correction • pc: pile-up corrected • bc: pile-up + blow-up corrected ALICE Physics Forum

8. Fitting Methods (Single & Double Gaussians) • In analysis note, of course, all fit results are presented • For y-scan, tail needs double gaussian, but for x-scan, it is not needed ALICE Physics Forum

9. Comparison of All Combinations • More comparisons are given later stages • For double-Gaussian, “area sum method” (shown later) was used ALICE Physics Forum

10. Area Sum Method • Not assume Gaussian for the beam shape (robust because of it) • Assume beam profile is factorized in x and y: • Rate: where sharpness is defined: • Scan area:  • because top rate R(0,0)=kbfrevN1N2QxQy ,  Luminosity and cross section can be obtained only by top-rate R(0,0) and Sx, Sy =1 ALICE Physics Forum

11. Numerical Area Sum Method Results • Statistical error is over-estimated due to correlation between Sx,y and R(0,0) actual error is estimated to be below 1% ALICE Physics Forum

12. Comparing All Methods • Different methods and corrections give as much as 1.8% of differences In RMS of the deviation from chosen center (sum. pc.). • chosen as final central value • no shape assumption • blow-up corr. makes nodifference • but also has no strong reason tells this is reasonable large statistical error is to be corrected (due to strongly correlated parameter for double-Gaussian fit for x-axis) ALICE Physics Forum

13. Systematic Errors and Final Value • The largest uncertainty is the beam intensity (55%) • The second largest is the separation calibration (can be improved future) • Result V0-AND trigger  = 62.3 [mb] stat. uncertainty < 1% syst. uncertainty 8% Remarks • All other LHC experiments are dominated by the same beam intensity errors • STAR: syst. = 5.7% (Au+Au), dominated by beam current 22% [A.Drees et.al.] • Simulation gives ~61 [mb] … to be reviewed !! value may be updated with further scans ALICE Physics Forum

14. Summary and Outlook • V0-AND trigger cross section has been measured using van der Meer scan method • Analysis note written (being checked by experts now) • Simulation needed to check the results • Value might be updated after all checks and also new scan • New scan being prepared • with CMUS1 and possibly EMCal triggers • with more systematic study (xu – yu – xd - yd, wire scan, and etc) • BCNWG (beam current normalization working group) is working for beam current, hopefully we get less systematic (Martino as behalf of ALICE) • DCCT baseline error can become small if beam current is larger • Larger intensity gives more pile-up (being discussed) • Large beta* helps • Acknowledgements G. De Cataldo, A. Di Mauro, C.Garabatos, and F. Antinori ALICE Physics Forum

15. Backup … Bump Calibration 1 • Both beam-1 and beam-2 were displaced by ~80 m and even vertex displacement has been measured (Davide) ALICE Physics Forum

16. Backup … Bump Calibration 2 • Fit result gives 2% of deviation in Dx,y scale put as systematic error [Davide, Antonello] ALICE Physics Forum

17. Backup … To be prepared for new scan ALICE Scalars • Continue with V0-DCS scalar. • Single muon trigger (DMUS1) to be monitored • Other triggers? (EMC, INT1?) • Putting some triggers In interaction record? • V0 has no after pulse masking, but we can mask it offline • Can have 2 triggers • Can have multiple colliding bunches Beam Intensity (measured by BCT and DCCT) • This is the largest uncertainty • Dominated by DCCT (DC current transformer) offset More bunches or more bunch intensity decreases relative uncertainty • But more bunch intensity gives pile-up problem ALICE Physics Forum

18. Backup … For Further Scans Requirements sent to LHC by Antonello Non-Invasive scan (after technical stop?), at least • One colliding bunch in ALICE • Double scan time at tail and increase steps around top (if possible) • Scan range: ± 3σ for each beam (± 5σ or more if possible) • Introduce background check with displaced beams before collapsing bumps • Introduce longitudinal scan • Separation calibration: ≥5 point, ~100K events/point • Wire scan then X-Y-X-Y (second scan in reverse direction), then separation, then new X-Y when emittance has grown (lower mu), or after wire scan? For dedicated scan, in addition to above • Better than 5% on intensity. N1,2= ~ 2x1010 protons • No x-angle, dipole ON • beta*=10 m or as large as possible (if scan range relative to beam size is ok) • Dump after last scan for DCCT baseline check  better uncertainty on intensity ALICE Physics Forum

19. Backup … Treatment of Double-Gauss Fits In previous analysis • If you assume 2 Gauss as beam profile, it creates 2x2 Gauss overlapping area gives 3 different Gaussian due to crossing term  not consistent to double fit anymore • Proper way is to use the same as sum method (see last presentation) ALICE Physics Forum