ATLAS Level-1 trigger studies for the invisibly decaying Higgs boson produced in vector boson fusion

Transcription

1 ATLAS Level-1 trigger studies for the invisibly decaying Higgs boson produced in vector boson fusion Guilherme Nunes Hanninger Markus Schumacher (Uni Siegen) Maria Laach, Sept. 2007

2 Invisibly decaying Higgs boson If: There are new weakly interacting particles with mass less than half the Higgs mass (mh) and mh is smaller than 160 GeV ~2mW, so that the Higgs partial width into SM particles is very small, the Higgs boson might predominantly decay into these new weakly interacting particles. If these new particles are neutral and stable, the Higgs boson will decay invisibly! 2

3 Higgs Invisible Inv. decaying Higgs boson appears in several extensions of the SM: eg.: SUSY, Dark matter, theories with extra space dimensions, 4th generation neutrinos, general scalar gauge singlet added to the SM,... Vector Boson Inv. VBF Higgs signature: Fusion (VBF) two jets in opposite hemisphere production channel is the most promising if trigger is feasible! considerable missing transverse energy cross section: ~3.5 pb (mh=130 GeV/c2) Main source of background at LVL1 trigger: Multi-jet production via purely QCD processes (QCDjj - ~20mb; pt>8 GeV/c) 3

4 VBF Higgs Invisible Offline event selection: ATL-PHYS

5 Signal jets topology: Full simulation Central Jets ( η <3.2) Mixed 1 Jet ( η <3.2) 1 Jet (3.2< η <4.9) Forward Jets (3.2< η <4.9) mh = 110GeV/c2 19±3% 68±4% 13±3% mh = 130GeV/c2 14.2±2.0% 69.3±2.6% 16.5±2.1% mh = 200GeV/c2 23±3% 66±3% 11±2% Need to trigger on central and forward jets if 2 jets are required! Higgs mass has low influence in the jets topology -3.2<η< <θ< <η< <θ<

6 LHC: pp collisions: High Energy: s=14tev Luminosity 3 initial years: L= cm 2 s 1 after: 34 L=10 cm s 2 1 Bunch crossing rate: t =25ns 40MHz 23 events per bunch crossing (minimum bias) 6

7 The trigger challenge: rate total interaction rate Bunch Crossing: 40MHz Interaction rate: 1GHz 1PB/sec Affordable mass storage: 300MB/sec Storage rate: ~200Hz Online rejection: % storage rate Powerful trigger needed: discoveries Highly selective; as inclusive as possible maximize the physics coverage open to new and unforeseen signatures configuration must be flexible 7

8 ATLAS trigger: Overview software hardware Level-1: decision based on data from calorimeters and muon trigger chambers 2.5 µs Level-2: uses Regions of Interest (identified by the level-1trigger) data (ca. 2%) with full granularity from all detectors Event Filter: has access to full event ~ 10 ms and can perform more refined event reconstruction ~ 1 sec. Regions of Interest 8

9 LVL1 Trigger Studies Level-1 trigger simulation: Full GEANT Simulation (FullLVL-1) Fast Simulation (FastLVL-1) Important for background simulation!! 9

10 LVL1 Trigger Studies Evaluating the Central Jet ET ( η <3.2), Forward Jet ET (3.2< η <4.9), and ETmiss underestimation by the level-1 trigger: Full Level-1 simulation, Invisible VBF Higgs (mh=130gev/c2) ET95miss = ETmiss at 95% efficiency Level-1 Example: ETmiss Level-1 cut ET95miss (reco) 20 GeV 50 GeV 150 GeV 57 GeV 88 GeV 213 GeV Level-1 thresholds are significantly below offline values! 10

11 LVL1 Trigger Studies ETmiss FullLevel-1 trigger simulation: Central Jets Forward Jets 11

12 LVL1 Trigger Studies FullLVL-1 Jets FastLVL-1 Jets 12

13 Level-1 Trigger menu High signal acceptance Keep the trigger rate low low trigger thresholds high trigger thresholds trigger menu is a compromise! Goal: level-1 output rates for Invisible VBF Higgs (+BKGD): O(~100Hz) 13

14 LVL-1 trigger menu: 2 Jets ( <4.9) + ETmiss: QCDjj L=2 1033cm-2s-1 PYTHIA (18mb; pt>8 GeV/c) Signal events (mh=130 GeV) Accep.= Nr. of evt. passed LVL1trig. Offline selec. Number of evt. passed Offline Selec. QCDjj not so sensitive to jet ET thresholds Low threshold for better signal acceptance! 14

15 LVL-1 trigger menu: 2 Jets ( <4.9) + ETmiss: QCDjj L=2 1033cm-2s-1 PYTHIA (18mb) and HERWIG (14mb) Signal events (mh=130 GeV) Large uncertainty coming from the generator! 15

16 LVL-1 trigger menu: 2 Jets ( <4.9) + ETmiss: Invisibly decaying Higgs boson produced in VBF: 2 Jets ( <4.9) >20 GeV + ETmiss >? 2 Jets ( <4.9) >20 GeV + ETmiss >50GeV provides signal acceptance above 95% and LVL1 output rate of ~ 50Hz. 16

17 Level-1 trigger output rates estimation 2 Jets ( <4.9) + ETmiss>60GeV 2 Jets ( <4.9) in opposite hemisphere + ETmiss >60GeV 2 Jets (0.8< <4.9) in opposite hemisphere + ETmiss>60GeV 2 Jets (1.6< <4.9) in opposite hemisphere + ETmiss>60GeV QCDjj Best QCDjj reduction and high Level-1 acceptance! L=2 1033cm-2s-1 level-1 17

18 L=2 1033cm-2s-1 2 Jets ( <4.9) ET>20GeV + ETmiss>50 GeV are recommended for the LVL1 trigger condition for the Invisible VBF Higgs For this configuration: QCDjj Level-1 output rate ~ 50 Hz Signal Level-1 L=1034cm-2s-1 needs study with pile-up 2 Jets (1.6< <4.9) in opposite hemisphere + ETmiss might be useful for background suppression and maintain high level-1 trigger acceptance 18

19 Many Thanks!!

20 ATLAS Detector: 22m 44m 3

21 Higgs boson production at LHC GGF VBF WH tth Diplom Kolloquium 8/31

22 Branch Ratio (BR) Standard Model Higgs boson decay Why investigate the Invisibly decaying Higgs boson? Diplom Kolloquium 9/31

23 Higgs Invisible Inv. decaying Higgs boson appears in several extensions of the SM: eg.: SUSY, Dark matter containing a stable singlet scalar, theories with extra space dimensions, 4th generation neutrinos, general scalar gauge singlet added to the SM,... VBF seems to be most promising channel if trigger is feasible! ATL-PHYS-PUB

24 VBF Higgs Invisible Invisible VBF Higgs signature: two jets in opposite hemisphere considerable missing transverse energy cross section: ~3.5 pb (mh=130 GeV/c2) Main source of background at LVL1 trigger: Multi-jet production via purely QCD processes (QCDjj - ~20mb; pt>8 GeV/c) 7

25 VBF Higgs Invisible At low luminosity: cm-2s-1 At level-1 trigger: No dedicated trigger menu for invisible VBF Higgs So far, 1 JET ( <3.2) > 50GeV + ETmiss>60GeV (SUSY) (ATL-DAQ ) No forward jets trigger at LVL-2 yet Only QCDjj background considered (Zjj and Wjj have low rate at LVL1 trigger (<10Hz)) Goal: level-1 output rates for Invisible VBF Higgs (+BKGD): O(~100Hz) LVL-1 trigger menu: High signal acceptance Keep the trigger rate low low trigger thresholds high trigger thresholds trigger menu is a compromise! 9

26 LVL1 Trigger Studies Level-1 trigger simulation: Full GEANT Simulation (FullLVL-1) Fast Simulation (FastLVL-1) Important for background simulation!! Before the LVL-1 trigger menu studies, investigate the LVL-1 trigger performance: FullLVL-1 (Jet E, Fjet E, E miss) vs. Offline Rec. (GEANT) (Jet E, Fjet E, E miss) FastLVL-1 (Jet E, Fjet E, E miss) vs. Offline Rec. (GEANT) (Jet E, Fjet E, E miss) T T T T T T T T T T T T FullLVL-1 (Jet E, Fjet E, E miss) vs. FastLVL-1 (Jet E, Fjet E, E miss) T T T T T T 10

27 LVL1 Trigger Studies Level-1 trigger underestimates Jet ET, ETmiss Discrepancies between FastLVL-1 and FullLVL-1 trigger simul. 13

28 1 Jet ( <3.2)> 50GeV + ETmiss>60GeV (SUSY) (ATL-DAQ ) Provides 67.6±2.7% acceptance for signal events and a LVL-1 output rate for the QCDjj background of 18.4±1.6Hz at L=2 1033cm-2s-1 2 Jets ( <4.9) + ETmiss 1 FJet (3.2< <4.9) + ETmiss 1 CJet + 1 FJet + ETmiss Topology (e.g.: jets in opposite hemisphere) Eff.= Nr of events passed LVL1 trigger Total number of events Accep.= Nr. of evt. passed LVL1trig. Offline selec. Number of evt. passed Offline Selec. 16

29 LVL-1 trigger menu: 2 Jets ( <4.9) + ETmiss: Level-1 signal efficiency: FullLVL-1 vs. FastLVL-1 Signal events (mh=130 GeV) FastLevel-1 presented lower efficiency than FullLevel-1 QCD rates might be underestimated! 14

30 LVL-1 trigger menu: 1CJet + 1FJet +ETmiss: Signal events (mh=130 GeV) QCDjj L=2 1033cm-2s-1 PYTHIA (18mb; pt>8 GeV/c) Provides even lower acceptance than the previous menu! 19

31 LVL-1 trigger menu: 1FJet (3.2< <4.9)+ETmiss: Signal events (mh=130 GeV) QCDjj L=2 1033cm-2s-1 PYTHIA (18mb; pt>8 GeV/c) Provides lower acceptance than the previous menu! 18

32 Does topology information helps QCDjj reduction? 2 Jets ( <4.9) in opposite hemisphere + ETmiss 1 Central Jet + 1 Forward Jet in opposite hemisphere + ETmiss L=2 1033cm-2s-1 2 Jets in opposite hemispheres: no significant improvement in the LVL-1 output rate 20

33 And redefining the boundary between central and forward jets? Signal events (mh=130 GeV/c2) After offline cuts: 25

34 L=2 1033cm-2s-1 2 Jets ( <4.9) ET>20GeV + ETmiss>50 GeV are recommended for the LVL1 trigger condition for the Invisible VBF Higgs For this configuration: QCDjj Level-1 output rate ~ 50 Hz Signal Level-1 >95% If not possible... 1 FJet (3.2< <4.9) ET>20GeV + ETmiss>50 GeV with acceptance ~ 80% and output rate ~ L=1034cm-2s-1 needs study with pile-up 2 Jets (1.6< <4.9) in opposite hemisphere + ETmiss might be useful for background suppression and maintain high level-1 trigger acceptance 19

35 Backup Dijet events produced via color singlet exchange: =>New source of Background? Exchange of a color singlet > yield a rapidity gap; The observation of rapidity gaps between jets at both the Tevatron and the DESY implies the exchange of a hard color singlet; The fraction of such events is roughly 1% at Tevatron and 10% at DESY; G. N. Hanninger LVL 1 trigger studies for the VBF Higgs > Inv. ATLAS T&P Week, June

36 DiJets with color singlet exchange: Cross-section: 0.15 mb L=2 1033cm-2s-1 G. N. Hanninger LVL 1 trigger studies for the VBF Higgs > Inv. ATLAS T&P Week, June

37 Observation: distribution for the QCD jets (ATLFAST with FastShower) Entries 0 Entries 0 Entries 0 All Jets G. N. Hanninger 2 highest PT jets LVL 1 trigger studies for the VBF Higgs > Inv. Problem with the FastShower!! ATLAS T&P Week, June

38 Solution for the Asymmetry: Michael Duehrssen: There is a shift of 0.1 for particles with eta< 3.2. The reason is a cast from double to int that doesn't behave as wanted for particles with eta< 3.2. change in FastShowerUtils/FastShowerUtils X/src/GridletForger.cxx the definition of constants at the begin of the file: change : const double mineta = 3.2 to : const double mineta = 5.0 and in FastShowerUtils/FastShowerUtils X/src/Gridlet.cxx, in the constants, change : const double s_phistep = 0.1 to : const double s_phistep = All Jets G. N. Hanninger LVL 1 trigger studies for the VBF Higgs > Inv. 2 highest PT jets ATLAS T&P Week, June

39 Observation: ETmiss distribution for QCD background produced by ATLFAST (with FastShower) and FASTLVL1; ETmiss FastLVL1 harder than ATLFAST Same distribution, different scale. ETmiss =>10 bit range for the JET RoI!!! Such events with high ETmiss will fire the trigger. G. N. Hanninger LVL 1 trigger studies for the VBF Higgs > Inv. ATLAS T&P Week, June

40 G. N. Hanninger LVL 1 trigger studies for the VBF Higgs > Inv. ATLAS T&P Week, June