Limited Effectiveness of Timing Cuts to Reduce Backgrounds in a Muon Collider Higgs Factory

Limited Effectiveness of Timing Cuts to Reduce Backgrounds in a Muon Collider Higgs Factory Detector

T.W. Markiewicz, T. Maruyama, L. Keller, U. Wienands, J.P. Delahaye, T. Fieguth

SLAC National Accelerator Laboratory 2575 Sand Hill Road Menlo Park CA 94025

Abstract:

Introduction

Higgs Factory Muon Collider Parameters and Consequent Background Loads due to Muon Decay

Ring Circumference 300m Ring Revolutions per Muon Lifetime 1300 Ring Revolutions per fill 1000 Time per fill 1 msec Fill Frequency 30 Hz Bunches per beam 1 Muons per bunch 2E12 Energy/beam at injection 20 kJ Average Energy/beam over fill 14.2 kJ Peak to Average Conversion factor over fill 0.70 Average electron energy in lab from muon decay 22.1 GeV Power in electrons/beam at injection 5.4 MW Decays/m/beam at Injection 5.1E6 Average # muon decays/m/beam over fill 3.56E6 Average power loss/m/beam 500 W/m

Ring Studies

The beam transport code “Decay Turtle” was used to study the decay electro hit rate near the IP and the power loads on the Beampipe and W shielding. A comparison was made to corresponding FLUKA results then FLUKA was used to characterize the backgrounds entering the detector volume.

We first focus on the location where muon decays originate that can cause a hits in the 7m long region spanned by the W shielding cones and the beam pipe. For this comparison the so-called version 2 nozzle presented at the June 2013 MAP Collaboration Meeting was used. Figure: E_dep/m vs. s

FLUKA Studies

Figure: The geometry coded into the FLUKA simulation. Muons are transported from z=22.5m CCW to the IP. The IP shielding “nozzle” is the so-called version 2 nozzle presented at the June 2013 MAP Collaboration Meeting.

Figure: E_dep in Nozzle and Beampipe

Figure: Origin of particle backgrounds that make it into the IR

Subsection: Irreducible backgrounds emanating from QD0 to QD0 versus backgrounds entering detector volume from ring

Subsection: Neutrino background issues

Subsection: ANSYS (or rule of thumb engineering): Cooling for beampipe and Nozzle and consequences (increased interaction rate on beampipe; lower effective density of nozzle)

Subsection: Mechanical Support of Nozzle 1. Detector Issues

Figure: The MAP SiD-Like Detector as defined in the MCDRCAL00 detector model xml file (http://www.lcsim.org/detectors/???)

A figure of merit to evaluate the importance of background: The Peak Hit Density per readout element per readout time

Creating an optimized Muon Collider Higgs Factory Detector and fully simulating it in GEANT4 was beyond the planned scope of this study. Instead, detector challenges were explored by tracking the FLUKA backgrounds using GEANT3 in a uniform 5 Tesla magnetic field through a series of nine 320m thick silicon layers placed at the positions of the first layers of the barrel and endcap VXD, Tracker and ECAL of the MAP SiD-Like Detector.

Procedure:

The parent FLUKA particles were classified as photons, e±, neutrons or charged hadrons. Photons and charged particles interacted in the silicon and the secondary particles produced in the interaction tracked. When >8keV was deposited in a silicon plane, a “hit” was recorded. As GEANT3 cannot simulate hadronic interactions, for neutrons the silicon layers were treated as massless scoring planes. A separate simple FLUKA study of the interaction probability of the parent neutrons, with the correct energy distribution in 320m silicon indicated a 0.1% probability that a neutron would deposit 8 keV and cause a recordable hit; this factor was applied to the neutron distributions of the VXD and Tracker scoring planes. For the ECAL silicon layers, presumed to measure the energy flowing into some form of yet-to-be specified calorimeter, all parent particles are scored (100% conversion efficiency) and the energy they carry accumulated.

The Timing Cut:

A PYTHIA simulation of →H was used to generate distributions of the time of arrival of “physics” particles on each of the silicon layers. These were used to define a timing gate for each layer. The number of background particles within each timing gate was recorded.

Figure: The time of flight as a function of z (for barrel detectors) or radius (for endcap detectors) required for physics generated particles to arrive at the silicon layers used in the GEANT simulation. The timing gates required to efficiently collect these physics hits are indicated by red horizontal lines and the values listed.

Peak Hit Density:

The z and phi distribution at each layer is not uniform. We consider both the average hit density per layer and the peak hit density.

Barrel Half Radius Area Gate Detecto Length Readout Element (cm) (cm2) (ns) r (cm) VXD 7.7 28.7 2.78E+0 0-1 20m x 20 m pixel 3 Tracker 50.0 91.0 5.72E+0 1.5-4 50m x 1cm strip 4 ECAL 131.0 200.0 3.29E+0 4-10 1cm x 1cm pad 5

Endcap Inner Outer Z Area Gate Detecto Radius Radius Readout Element (cm) (cm2) (ns) r (cm) (cm) VXD ±30.0 6.7 18.4 9.23E+02 0.5-1.5 20m x 20 m pixel Tracker ±92.0 25.3 53.3 6.91E+03 3-4 50m x 1cm strip ECAL ±200.1 38.0 130.9 4.93E+04 6.5-9 1cm x 1cm pad

Table: The location, dimensions, area, timing gate and assumed readout element shape and size used for each of the nine 320m thick silicon layers in this study.

Photon Backgrounds

Figure: The z distribution and timing distribution from the photon hits from one bunch of one of the two colliding muon beams at a scoring plane corresponding to the first layer of the VXD barrel detector. The maximum hit density corresponding to hits from both positive and negative muon beams would be ~(17+3)E5 hits/cm2. 8.1% of the hits lie within 10ns of the beam crossing time (75ns in this plot). The usual conversion efficiency to produce a hit in a silicon detector is 1%. Thus the peak charged hit density of in-time background at this location is 1620 hits/cm2.

Figure: The z distribution and timing distribution from the photon hits from one bunch of one of the two colliding muon beams at a scoring plane corresponding to the first layer of the TRACKER barrel detector. The maximum hit density corresponding to hits from both positive and negative muon beams would be ~(3+1)E4 hits/cm2. 1.6% of the hits lie within 10ns of the beam crossing time (75ns in this plot). The usual conversion efficiency to produce a hit in a silicon detector is 1%. Thus the peak charged hit density of in-time background at this location is 6.4 hits/cm2.

Figure: : The z hit distribution and z energy distribution from the photon hits from one bunch of one of the two colliding muon beams at a scoring plane corresponding to the first layer of the ECAL barrel detector. The maximum hit density corresponding to hits from both positive and negative muon beams would be ~2x4500 hits/cm2. 1.09% of the hits lie within 10ns of the beam crossing time (75ns in this plot). The photons will all deposit their energy in the ECAL. Thus the peak charged hit density of in-time background at this location is 98 hits/cm2. The average energy of these photons is 1.3 MeV. The average (as opposed to peak) in-time hit density is 58 hits/cm2, corresponding to 46 MeV/ cm2 or 15 TeV total in- time energy deposited. An energy threshold per cell will need to be applied to have the ECAL function.

Charged Hadron Backgrounds

Figure: The above figure shows the time distribution of charged hadron hits on a scoring cylinder at r=10cm. On the left the charged hadrons are those produced in the FLUKA simulation (again, 75ns corresponds to the t=0); on the right the charged hadrons are produced by PHYTHIA. Note the different scales of the ordinate.

Figure: Shows the hit density on a r=10cm scoring plane due to one beam after a z dependent 2.5ns timing cut, sqrt(102+z2)/30 < 2.5 ns, where z is in cm. The peak time cut hit density is ~(0.4+0.2) hits/cm2, or 600 hits/cm2/train. Subsection: Hit densities in SiD-like detector

Readout Issues:

Subsection: Required pixel sizes for 4 hits/ train in KPIX architecture The KPIX readout architecture developed for the ILC can buffer 4 hits per train of colliding bunches, which are then read and cleared before the next train of bunches arrives 200ms later. The muon collider Higgs Factory will store beam for 1000 crossings at the rate of 30 Hz. In the example of the first layer of the VXD, the 1000 beam crossing would produce a hit density of 1.62E6 hits/cm2, degraded by a factor of 70% to account for the muons that have decayed, or 1.13E6 hits/cm2. If this peak hit rate is to be contained in 4 buffers, the area of each sensor should be 19 m or less, consistent with current VXD pixel sizes.

In the table below this analysis in extended to other detectors and particle types. Rather than calculate the sensor area to hold 1000 bunch crossings of data in 4 buffers, we calculate the number of buffers that would be needed to contain the data for sensors of typical dimensions.

In Time Hits/cm^2 Within Characteristic Nbuffers Detector Particle Hits/cm^2 Corresponding Area Typical Dimensions /BX 10ns dimensions needed /Train Barrel VXD Converted Photons 20000 8.10% 1134000 8.82E-07 cm^2 88.18 um^2 9.39 um 9.39 um 20 um 20 um 4.536 Barrel Tracker Converted Photons 400 1.60% 4480 2.23E-04 cm^2 22321.43 um^2 149.40 um 149.40 um 50 um 10000 um 22.4 Barrel ECAL Converted Photons 9000 1.09% 68670 1.46E-05 cm^2 1456.24 um^2 38.16 um 38.16 um 10000 um 10000 um 68670 Barrel VXD Charged Hadrons 600 1.67E-03 cm^2 166666.67 um^2 408.25 um 408.25 um 20 um 20 um 0.0024

Subsection: Figures of Merit for detector

Subsection: Discussion of “CMS-like Detector”

Conclusions

If built, would it be useable for physics (probably need physics analyses which we will not do).