SiD Simulation Studies at UCSC/SCIPP ECFA Linear Collider Workshop Palacio de la Magdalena Santander, Cantabria, Spain May 30 – June 5, 2016 Bruce Schumm UC Santa Cruz Institute for Particle Physics

2 The SCIPP BeamCal Simulation Group The group consists of UCSC undergraduate physics majors (and one engineering major) Christopher Milke (Lead) * Heading to SMU’s doctoral program in fall Jane Shtalenkova, Luc D’Hauthuille, Spenser Estrada, Benjamin Smithers, Summer Zuber, Cesar Ramirez Alix Feinsod Led by myself, with technical help and collaboration from Jan Strube, Anne Schuetz, Tim Barklow VXD Occupancy / BeamCal Performance / Anti-DiD Field Determining ILC IP parameters with the BeamCal SUSY in the degenerate limit Bhabha events and the two-photon physics veto

3 Study of KPiX Channel Occupancy in the Forward EMCal

4 Forward EMCal Readout Buffer Depth Study Issue: EMCal read out by KPiX chip KPiX chip has limited number of buffers (currently 4). This limits the number of hits that can be recorded per pulse train Study backgrounds to determine if buffer depth needs to be extended, and if so, by how much.

5 BhaBha Gamma-gamma to Hadron Pair Backgrounds Low Cross-section (down to 0.1 events/train) Event Types Included

6 Hit Number Distribution (Integrated over a full train)

7 Fraction of Hits Lost During the Train as a Function of KPiX Buffer Depth

8 But: Are there “Hot Spots”? Fraction of Hits Lost, By EMCal Layer

9 Fraction of Hits Lost, By Radius (Distance from Beam Line)

10 Using the BeamCal to Obtain Information about Collision Parameters (Earliest results from new MDI initiative)

11 Contributors Luc D’Hauthuille, UCSC Undergraduate (thesis) Anne Schuetz, DESYGraduate Student Christopher Milke, UCSCUndergraduate With input from Glen White, Jan Strube, B.S. Idea is to explore the sensitivity of various beamstrahlung observables, as reconstructed in the BeamCal, to variations in IP beam parameters. The sensitivity will be explored with various different BeamCal geometries. Goal

12 Of these, we believe the following can be reconstructed in the BeamCal: Total energy and its r, 1/r moment Mean depth of shower Thrust axis and value (relative to barycenter; could also use mode of distributions. What is wise choice though? Maybe just (0,0)?) Mean x and y positions Left-right, top-bottom, and diagonal asymmetries

13 IP Parameter Scenarios Thanks to Anne Schuetz, GuneaPig expert Relative to nominal: Increase beam envelop at origin (via  -function), for electron and positron beam independently, by 10%, 20%, and 30% Move waist of electron and positron beam (independently) back by 100  m, 200  m, 300  m. Change targeting angle of electron and positron beam (independently) by 5 mrad and 20 mrad (in retrospect, isn’t this a bit much?) Details at

14 First (Early) Results Luc has coded the following observables: Deposited energy, mean depth of shower, L/R and up/down asymmetries, thrust (relative to barycenter) value. He has taken eight beam crossings (working on larger sample soon!) and explored the following “trajectories”: Beam envelope for electrons Beam envelope for positrons Electron waist position Following are a core-dump of plots of these observables over those trajectories.

15 Beam Envelope Scan (Electrons and Positrons)

16 Total Deposited Energy e + and e - beam envelope scan

17 e + and e - beam envelope scan Mean Depth

18 e + and e - beam envelope scan L/R Asymmetry

19 e + and e - beam envelope scan Up-Down Asymmetry

20 e + and e - beam envelope scan Thrust Value

21 Waist Scan (Electrons Only)

22 e - waist scan Total Deposited Energy

23 e - waist scan Mean Depth

24 e - waist scan L/R Asymmetry

25 e - waist scan Up-Down Asymmetry

26 e - waist scan Thrust Value

27 Summary and Conclusions First look at BeamCal observables and IP parameter dependence Need to finish coding observables (thrust definition question) Need to increase statistics (~100 pulses generated; working on simulation) Need to develop some more interesting IP parameter variations (discussion!) Need to explore sensitivity to BeamCal geometry But this should be a good foot in the door for now…

28 Degenerate SUSY (Another new initiative)

29 Degenerate SUSY and Electron Tagging SUSY has a cosmologically-motivated corner where a weakly-coupled particle (stau) is nearly generate with the LSP (  0 ) We have generated events at E cm = 500 GeV with M  ~ = (100, 150, 250) GeV  ~ -  0 splittings of (20.0, 12.7, 8.0, 5.0, 3.2, 2.0) GeV Concern: Two-photon events provide greater and greater background as splitting decreases. Hope: We can tag the scattered electron or Positron in the Beamcal and veto. But: If photons are from Beamstrahlung, electron/positron do not get a p T kick (is this right?)

30 Two-Photon Event Rate Thanks to Tim Barklow, SLAC, we have ~10 7 generator- level two photons events, with electron/positron photon fluxes given by the Weizsacker-Williams approximation (W) and/or the Beamstrahlung distribution (B). Events have been generated down to M  = 300 MeV. For this phase space, the ILC event rate is approximately 1.2 events/pulse.  1 year of  events corresponds to (1.2)x(2650)x(5)x(10 7 ) events, or about 1.6x10 11 events per year. How do we contend with such a large number of events in our simulation studies?

31 Two-Photon Approach Convenient data storage in 2016: ~5 TB Tim Barklow: 5 TB is about 10 9 generated (not simulated!) events events requires day jobs Jan Strube: Don’t worry about CPU (really?)  Proposed approach: Do study at generator-level only. Except: Full BeamCal simulation to determine electron-ID efficiency as a function of (E,r,  ) of electron. Parameterize with 3-D function and use in generator-level analysis Devise “online cuts” applied at generation that reduce data sample by x100 (can this be done?) Store resulting 10 9 events and complete analysis “offline”

32 In Search Of: “Online Selection” For now, looking at three observables: M: mass of  system S: Sum of magnitudes of p T for all particles in  system V: Magnitude of vector sum of p T for particles in  system Each of these is done both for McTruth as well as “reconstructed”  detector proxy Detector Proxy: Particles (charged ot neutral) detected if No neutrinos |cos(  )| < 0.9

33 ISO “Online Selection”:  Mass (M)  0 Mass SUSY Signal;  ~ Mass = 150 GeV 2 GeV Splitting Two-photon background Seems like a clean cut, but what is seen in the “detector”?

34 “Detected”  Mass (M) Two-photon background SUSY Signal;  ~ Mass = 150 GeV  0 Mass For 2 GeV splitting, even a cut of 0.5 MeV removes some signal

35 S Observable: “Detected” Fairly promising as well; but is it independent of M? Two-photon background SUSY Signal;  ~ Mass = 150 GeV  0 Mass

36 V Observable: “Detected” Not as promising; looks better for “true”, but even for V true = 0, reconstructed V has significant overlap with SUSY signal (save for “offline” part of study?) Two-photon background SUSY Signal;  ~ Mass = 150 GeV  0 Mass

37 Cut flow for S, M Distributions News is not the best: S and M observables very correlated 3% loss of signal (at 2 GeV!) reduces background by only ~2/3 Other discriminating variables?

38 BhaBha and BeamCal Electron Tagging (Yet another new initative)

39 Bhabha Events Issue: Degenerate SUSY has background from two-photon events Hope to reduce by detecting scattered primary e +/- in BeamCal and vetoing the event If a SUSY event is overlain with a Bhabha event with an e +/- in the BeamCal, we will reject SUSY  What is the rate of Bhabha events with e +/- in the Beamcal? Bhabhas with virtuality  -Q 2 > 1 GeV (~ 4 mrad scatter) available at with cross section  = 278 nb  Raw rate of 0.76 Bhabha events per beam crossing ftp://ftp-lcd.slac.stanford.edu/ilc4/DBD/ILC500/bhabha_inclusive/stdhep/bhabha_inclusive*.stdhep

40 Event TypeFraction of Q 2 > 1 Bhabhas Fraction of Beam Crossings Miss-Miss23%18% Hit-Miss14%11% Hit-Hit63%48% Bhabha Event Classes Bhabha events fall into three classes Miss-Miss: Both e - /e + miss the BeamCal; not problematic Hit-Hit: Both e - /e + hit the BeamCal; should be identifiable with kinematics (need to demonstrate) Hit-Miss: One and only one of e - /e + hit the BeamCal; background to two- photon rejection. Naively, 11% of SUSY events would be rejected due to Hit-Miss events, plus whatever fraction of the 48% of Hit-Hit crossings aren’t clearly identified based on e +/- kinematics.

–Johnny Appleseed “Type a quote here.” Hit/Hit events: e + -e - angular correlation

42 After cut of  < 1.0 Mrad, 33% of Hit/Hit Bhabhas remain (16% of crossings). Can possibly eliminate with energy cut (need to balance against two-photon and SUSY events)

43 For Hit/Miss events, there may well be useful kinematic handles… but again, need to compare to two-photon and SUSY signal distributions

44 Summary of Simulation Studies Study of KPiX channel occupancy combining all expected sources of background suggests that as many as 8 buffers may be needed to avoid information loss at smallest radius Beginning study of BeamCal observables that may provide fast monitoring of collision parameters. As study evolves, it can inform IP design as well as BeamCal design. Starting to explore BeamCal tagging for degenerate SUSY scenarios. Grappling with challenge of simulating the two- photon background. Looking at effect of Bhabhas on BeamCal tagging. Probably not a problem but needs to be confirmed.

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47 30 x 30 5 BX Vertex Occupancy Dependence on L* Configuration L* occupancy differences appear to depend on backscatter deflection angle 3.5 m L* 4.1 m L* x 10 -4

48 30 x 30 5 BX Vertex Occupancy Dependence on Anti-did Field Anti-did field generally improves occupancy in barrel and consistently improves occupancy in endcap Base Anti-did Plug is in place! x 10 -4

49 Occupancy Dependence on Plug Geometry As expected, occupancy gets progressively lower as more of the BeamCal plug is cut away x Base Circle Wedge

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51 Forward EMCal Readout Buffer Depth Study Issue: EMCal read out by KPiX chip KPiX chip has limited number of buffers (currently 4). This limits the number of hits that can be recorded per pulse train Study backgrounds to determine if buffer depth needs to be extended, and if so, by how much.

52 BhaBha Gamma-gamma to Hadron Pair Backgrounds Low Cross-section (down to 0.1 events/train) Event Types Included

53 Hit Number Distribution (Integrated over a full train)

54 Fraction of Hits Lost During the Train as a Function of KPiX Buffer Depth

55 But: Are there “Hot Spots”? Fractions of Hits Lost, By EMCal Layer

56 Fractions of Hits Lost, By Radius (Distance from Beam Line)

57 Event Types Included

58 Event Types Included

59 Tiling strategy and granularity study Constant 7.6x x x3.5 Variable Nominal Nominal/  2 Nominal/2

60 Parting Thoughts The SCIPP simulation group is active on a number of fronts. In addition to expanding our BeamCal efforts, we are also looking into the forward EMCal occupancy. We have a number of studies in mind, largely related to answering design questions about the BeamCal. We continue to be open to suggestion or refinements. Support from Norman and others will remain essential.