Jet Studies 1. Changes in seed finding for cone jets reconstruction Emmanuel Busato 1. Changes in seed finding for cone jets reconstruction 2. t42 and jets

1. Changes in seed finding for cone jets reconstruction - technical description - effect on cone R=0.5 jets reconstruction - what's the best value for the new parameter « n » - conclusion

? Technical description List of towers (pT ordered) Take the highest pT tower in the list « T » PT > seedET = 500MeV Current preclustering (Simple Cone) yes no Go to clustering around T (with R=coneSize=0.3). Remove progressively towers from the list. Stop preclustering

? List of towers (pT ordered) Modified preclustering Yes no yes no Take the highest pT tower in the list « T » ? pT > seedET =500MeV Yes Take highest ET cell in T : « C » if C is CH or ECMG and if C has E < n * sigma  pT = pT  ETC no pT > seedET =500MeV ? yes Stop preclustering no Go to clustering around T (with R=coneSize=0.3). Remove progressively towers from the list.

Effect on cone R=0.5 jets reconstruction Data sample :  Run #169894 (30 Dec) and #172603 (16 feb) Reconstrution version : p13.06.01  Quality cuts : 0.05<emf<0.95 chf<0.4 hotf<10 n90>1  No trigger selection Event by event comparison Number of jets/event  5.1% of events have more jets with old seeding 1.4% of events have more jets with new seeding

Jets not found with new seeding (R>0.5) mean = 0.43 mean = 2.22 Jets found in both cases Jets not found with new seeding Jet not found with new seeding : - mostly low pT jets - in ICR - big number of merge

New Jets found with new seeding (R>0.5) - First, an example : Old preclustering : 1 “normal” seed + 1 CH seed  2 low pT clusters reconstruted with cone algo, individually above threshold New preclustering : 1 “normal” seed  1 low pT cluster reconstruted with cone algo, above threshod __________________________ Before split/merge __________________________ Splitting : Assign shared cells to nearest “protojets”  clusters below threshold (or fail quality cut) No jets left One jet left __________________________ After split/merge __________________________ No split/merge

- What are those jets ?  Look like low pT good jets

What's the best value for the new parameter « n » ? nb1 nb2 nb1 nb2

Conclusion - Jets not found with new seeding look like noise jets - New jets found with new seeding look like low pT good jets Looking at f90 distribution as a function of n, we decided to remove all CH seed (n=1000sigma) This new seeding scheme is in p14 with n=1000sigma

2. t42 and jets - jets properties (emf, chf, f90 ...) without and with t42 - jets reconstruction without and with t42

Jets properties (emf, chf, f90 ...) without and with t42 Top group’s alljets skim (reco version p13.06.01) : * Passes the 4JT10 trigger. * At least 4 jets (JCCB). * HT > 100 GeV (just plain sum of uncorrected JCCB jets).   20000 events Jets are JES corrected. No quality cuts applied because jet id distributions change significantly. Color code : t42 no t42

Color code : t42 no t42 New seeding not applied, its effect is small once t42 is applied

Warm zone around phi ~ 0.8 disappears with new seeding !

Summary - id distributions change significantly with t42 - t42 reduces the number of jets by ~ 11% - mostly in -2 < eta < 2 - mostly low pt jets - mostly high f90 jets - merge > 0 - t42 doesn’t affect jets with 0 merge

Jets reconstruction without and with t42 There are jets, close to a track jet (R<0.5) ( probably good), not found with t42 !  Part of these jets can be explained simply • low pT jets : - threshold effect (the same jet is less energetic with t42) • high pT jets : there is still 0.9 of jets with not found with t42 - threshold effect up to 25 GeV - unphysical cases : - split/merge effect : pT same jet R(cal jet, track jet) < 0.5 pT> 25

JETS PT=64 GeV merge=2 split=0 PT=42 GeV merge=1 split=1 PT=31 GeV merge=0 Without t42 With t42 This particular example is not isolated, approximately 40 of high pT jets close a track jet not found with t42 can be explained by this effect.

 Another simple reason could be that we remove good jets  Look at MC sample without noise : - no calorimeter noise - no minimum bias Single top events – schannel production mode JETS without t42 : Number of jets = 3568 with t42 : Number of jets = 3540  Reduction of  0.8

t42 not42 Same number of Ws ! ELECTRONS W selection : Hmx8 < 20 isolation < 15 EMF>0.9 no track match PT (electron)>20 Gev and MET>20 GeV not42 t42 Same number of Ws !

Conclusion and to do  Looking at single top events without noise reconstructed without and with t42, we find that the number of jets is reduced by  0.8.  look what happens on a QCD sample without noise  understand lost jets Number of Ws not reduced with t42  In data, good jets (and all objects) are polluted with noise, we saw that split/merge effects changes jets multiplicity.  make sure that it goes in the good direction