LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 1 LVL1 Calorimeter Algorithm Updates Changes since the TDR: Greater “integration” of e/  and  /h triggers:  Numbers of em and tau thresholds now adjustable  Common RoI algorithm for e/  and  /h algorithms  Split e/  hadronic isolation into two regions “Evolution” rather than “Revolution”: Main effects are to:  Increase flexibility  Simplify implementation Changes approved by the ATLAS T/DAQ Steering Group in July ‘99

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 2 TDR e/  Algorithm Object accepted if:  1 trigger cluster  cluster thr  em isolation E T  em isol thr  had isolation E T  had isol thr  RoI cluster E T = local maximum Tau algorithm based on same 4  4 tower window (next slide)

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 3 Current e/  Algorithm Object accepted if:  1 trigger cluster  cluster thr  em “ring” E T  em isol thr  had “ring” E T  outer had isol thr  had “core” E T  inner had isol thr  RoI cluster E T = local maximum Tau algorithm same except:  trigger clusters = 2 em + 2  2 had towers  only the 2 “ring” sums are used for isolation (Tau algorithm unchanged from TDR)

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 4 Adjustable Numbers of e/  and  /h Triggers TDR design had fixed numbers of em and tau selections  Proposed N em = N  = 8 Optimum allocations hard to predict  Will be luminosity-dependent  May depend on what we find Better if we could make these numbers adjustable  Hard to vary total ( N tot = N em  N   )  Can adjust N em & N  within this total  Requires small increase in functionality within CP chip

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 5 Adjusting Numbers of e/  and  /h Triggers Em & Tau algorithms very similar  See Table  To switch between the two:  Use multiplexors to select either em or tau trigger clusters  Do not set “hadronic core” isolation for tau triggers Don’t need total flexibility: We propose:  8 sets of em thresholds  8 sets adjustable em/tau (Even this not fixed if FPGAs used)

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 6 Hadronic Isolation: “Core” & “Ring” Performance Issues Imbalance between the two regions  “core” provides greater part of jet rejection  “ring” is more vunerable to noise Separation allows different weighting of the two  same (or slightly better) overall performance  more flexible response to conditions & requirements E T in 2 regions (signal & background)

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 7 Core & Ring Isolation: Signal vs Background Signal: Look at electrons + 48 mbias  Pessimistic pileup model  Parameterize BCID More E T in “ring” Two sums largely uncorrelated Background: “Core” E T typically larger Correlations weak  “Ring” contributes to rejection

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 8 Effect of two-region hadronic isolation Compare rates for same isolation efficiency  look at electrons with “pessimistic” pileup  compare jet background rates for cuts giving 95%, 98% & 99% isolation efficiency find some improvement when very high efficiency required  look also with “more realistic” pulse-shape modelling improvement smaller (few %) no situation in which rate is worse Cluster > 15 GeV, em & had isolation

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 9 A Common RoI Algorithm? Motivations  Simplify implementation only 1 set of RoI clusters to form & test makes “selectable” em/tau triggers easier to build  Simplify RoI data single object produces unique RoI coordinate for both em and tau algorithms Which algorithm?  Em RoI for taus? Much of tau E T may be in HCAL May degrade shower containment Not ideal  Tau RoI for e/  ? Isolated e/  should deposit little E T in HCAL Objects for which it makes a difference will fail hadronic isolation anyway Worth investigating

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 10 Possible Effect: Most Extreme Case Possible Effects: RoI coordinate may shift by 0.1  Rare (< 1% of e/  )  Clusters for which this happens will mostly fail isolation anyway Possible effects of this:  Moves hadronic E T between “ring” and “core” sums  Very rarely may increase em isolation or reduce trigger cluster Size of effects:  Changes in em sums can be no larger than hadronic E T deposit

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 11 Effect on Efficiency: 30 GeV e - + pileup What effects do we see: (with pessimistic pileup model)  No effect in  99% of e  RoIs  Small differences in isolation E T have no effect on efficiency. See same story for:  Different electron p T “ noise/BCID assumptions “ pileup levels em RoIem+had RoI

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 12 Effect on Jet Rejection More effect seen in background:  O(10%) of background clusters with E T > 15 GeV have RoIs shifted (cf < 1% e/  ) Does it affect jet rejection?  Use pessimistic pileup (as should maxmize size of any effects)  Choose isolation cuts to give 95%, 98% and 99% isolation efficiency  Compare jet rates for 2 RoI algorithms small reduction in rate seen Cluster > 15 GeV, em & had isolation

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 13 Efficiency in Physics Events Look at effect in physics events  In case more sensitive than single e/  events  Look at a range of processes containing e/  plus jets What do we see?  For most processes, no effect  Greatest effect in t  e  b, jjb  1% events have shifted RoIs Small changes in Had Isol n. No difference in efficiency Had isol n sums, e  from top events

LHCC Review, CERN, 19/10/99Paul Bright-Thomas, for Alan Watson 14 Summary of Changes N em and N tau variable  Increases flexibility of system  No possible performance drawback Two-region Hadronic Isolation  May increase flexibility/robustness of performance  Slight improvement in jet rejection. Common RoI Algorithm  Technical simplification  No adverse effect on performance (maybe very small gain)