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SiD preliminary answers to initial questions from IDAG

SiD preliminary answers to initial questions from IDAG. Dear John and Harry, As explained by Michel Davier minutes ago, the IDAG has completed a preliminary reading and discussion of the LoI of SiD,  and has prepared a first short list of questions.

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SiD preliminary answers to initial questions from IDAG

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  1. SiD preliminary answers to initial questions from IDAG

  2. Dear John and Harry, • As explained by Michel Davier minutes ago, the IDAG has completed a preliminary reading and discussion of the LoI of SiD,  and has prepared a first short list of questions. • They address rather general issues, which you will be able to address  during our meeting in Tsukuba. •    Looking forward to meeting you, •       Sandro Palestini • ---------- • Questions: • The choice of beam pipe radius and vertex detector inner radius are driven by machine background (mainly incoherent pairs from the IP). Can you provide additional information on your assumptions on background rates, on safety margins and on impact on performance if the background would be higher?    • The detector is expected to be read-out separating each bunch crossing (mainly by means of the KPiX circuit). Is this assumption going to be valid also for the vertex detector? • How extensive a study has been made of the robustness of the tracking against failure of one or more detector planes? The vertex detector is glued, replacement of parts seems unlikely. Similarly how much impact does the loss of one or more planes make on PFA performance.. • Can you provide more details concerning the choice of 4.5 interaction lengths for the depth of the HCal? How sensitive is it to assumptions on PFA algorithm? How much can be obtained from the Muon system used as a tail catcher for the hadronic showers, which is mentioned as an option? • Current PFA analysis provides rms90 = 4.0 GeV in M(Z->qq) from ZZ at 500 GeV, with most of the uncertainty due error in tracks/clusters matching. The Gaussian width of the Z(jj) appears significantly wider in the studies of benchmark channels.  The LoI mentions that the performance of the algorithm is expected to be improved, can you provide some more details about it?

  3. Question 1 The choice of beam pipe radius and vertex detector inner radius are driven by machine background (mainly incoherent pairs from the IP). Can you provide additional information on your assumptions on background rates, on safety margins and on impact on performance if the background would be higher?   Question The SiD beampipe was designed assuming 5 Tesla solenoid and the Nominal option, and a 2 mm radial allowance. As long as the beampipe is outside the pair edge, the detector background is negligible thanks to the single bunch crossing sensitivity. The pair background would double in the Low P option. In the RDR Low P option, the pair edge will interfere with the beampipe, and the detector background would become intolerable. However, the Low P option is a moving target, and there is a new Low P option, which is compatible with the current beampipe as shown in the attached plot. With the Low P option, the detector background would double, but the detector performance will not change (Pair background at the single bunch crossing level is negligible). Since the number of bunches per train is half for the Low P option, the total background hits per train will be the same for the Nominal and Low P options.(courtesy Takashi Maruyama)

  4. Question 1; cont’d The choice of inner radius is of course a provisional choice. The greatest unknown is what effect actual beam tails will have on synchrotron radiation backgrounds and off -energy electron backgrounds. The pair backgrounds are proportional to luminosity, so will not be larger unless the machine exceeds its design luminosity, a problem we would be happy to deal with. There is margin. Present studies have already indicated that tracking doesn’t degrade with 10 bunch crossing’s worth of backgrounds. Higher rates will be studied.

  5. Question 2 Question 2: The detector is expected to be read-out separating each bunch crossing (mainly by means of the KPiX circuit). Is this assumption going to be valid also for the vertex detector? Answer: The vertex detector sensor has not been selected. SiD favours a sensor that can tag the bunch, and both the Chronopix and the 3D sensors will have this feature. However, the sensor choice so far seems to have difficult trade-offs among power, pixel size, time resolution, and thickness. It is possible that the final choice will involve more than one sensor technology.

  6. Question 3 Question 3: How extensive a study has been made of the robustness of the tracking against failure of one or more detector planes? The vertex detector is glued, replacement of parts seems unlikely. Similarly how much impact does the loss of one or more planes make on PFA performance. Answer: To the best of our knowledge, once a vertex detector has been installed, no experiment has ever exchanged a module that was not functioning. Vertex detectors are buried deep inside the detector and exchanging single modules carries more risks than outweigh the benefits. A detector would never be installed if it had a non-working layer. The fact that the detector is glued does not increase the risk for failure during installation. Since we require 7 hits on a track, track finding should not be compromised due to the lack of a single module, or even the lack of a single layer, if there were unforeseen accidents during operation.

  7. Question 3; cont’d Question 3: …….. Similarly how much impact does the loss of one or more planes make on PFA performance.. Answer: We require 7 hits on a track. The absence of one hit will not affect the track finding. The momentum determination will hardly be affected. Therefore PFA performance will not be compromised. Even if the track were not found, the energy would be measured by the calorimeter with adequate resolution.

  8. Question 4 Question 4: Can you provide more details concerning the choice of 4.5 interaction lengths for the depth of the HCal? How sensitive is it to assumptions on PFA algorithm? How much can be obtained from the Muon system used as a tail catcher for the hadronic showers, which is mentioned as an option? Answer: -> Limiting the inner radius of the solenoid was an initial SiD design/cost constraint. This limits the HCal depth. -> The HCal could be made deeper, if needed for very high energy jets, if the physics return justified the additional cost. -> Our HCal, with active gap materials included, is 4.8. Together with our 1 ECal, we have a total depth of 5.8. -> With this depth, we are slightly past the knee of cost vs. performance (on the Higgs triple coupling) – so we have made a somewhat conservative choice.

  9. Question 4; cont’d -> We selected 40 layers to give 4.5 from the absorber alone based on a 2-dimensional plot of nlayers vs. n (see attached plot)

  10. Question 4; cont’d -> A study of incorporating information from the endcap muon systems showed a small gain in performance. A similar study for the barrel is yet to be carried out, but marginal, if any, improvement is expected given the placement of the solenoid between the barrel HCal and the barrel muon system. This addition is not easy to incorporate in the PFA. -> A small performance gain can be realized, at the price of a reduction in statistics, by vetoing on events with significant activity in the last few layers of the barrel HCal.

  11. Question 4; related…. Only use numbers from LOI

  12. Question 5 Question 5: Current PFA analysis provides rms90 = 4.0 GeV in M(Z->qq) from ZZ at 500 GeV, with most of the uncertainty due error in tracks/clusters matching. The Gaussian width of the Z(jj) appears significantly wider in the studies of benchmark channels.  The LoI mentions that the performance of the algorithm is expected to be improved, can you provide some more details about it? Answer: Next page

  13. Question 5 • For a standalone Z a90 ~ 4 GeV, gaussian fit of the same distribution gives s~6 GeV • For SUSY analysis • s~8 GeV before KinFit (similar s for mmqq) • s~4 GeV after KinFit • Possible explanation of difference between 6 and 8 GeV: Physics analysis includes • Boson natural width • All quark species (light, c, b) – neutrinos from c, b can worsen the resolution • Forward CAL • Effects of jet clustering (FSR, jet confusion and combinatorics). Events are forced in 4 jets. Vector boson invariant mass for chargino and neutralino2 signals After KinFit, normalized to same # of events Y.Li Before KinFit, normalized to same # of events Before KinFit, normalized to same luminosity

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