Monte Carlo Studies of the HERMES RICH in SBS—progress report Andrew Puckett 11/10/2010

Outline HERMES RICH in SBS Background rate calculation for SIDIS proposal Progress and results so far Next steps

The HERMES RICH in SBS

SIDIS proposal to PAC37 Update to conditionally approved PR SIDIS on a transversely polarized 3 He target; – transverse target SSA: Collins, Sivers moments for the neutron – 14 degrees + 30 degrees – Wide kinematic coverage in x, Q 2, z, p T – Full azimuthal coverage by varying target spin direction – pion/kaon identification using RICH detector

PAC34 report: Issues raised on feasibility of operating RICH at proposed luminosity: 5 orders of magnitude greater than HERMES Low-energy gammas (E>2 MeV) Compton-scatter electrons in aerogel, which emit Cerenkov radiation. This dominates the background rate. Also pair production? Back-of-the-envelope estimate used in original proposal suggested ~5% occupancy for PMTs in RICH, deemed reasonable operating condition Use Pavel Degtiarenko’s “MCWORKS” code for a realistic calculation of the background rate

GEANT geometry Magnet Target Aerogel HCAL “HODO” Use 60 cm 3 He target at 10 atm; SiO 2 glass cell, 100 μm endcaps, 1 mm side wall thickness, density 2.77 g/cm 3 Iron magnet with air gap, 25 kG field Aerogel: use material information from PDG; From HERMES RICH long NIM paper, obtain: n= rho =.1456 g/cm 3 Geometry: thickness = 5.65 cm or.82 g/cm 2 GEANT simulation: store every electron/positron produced in aerogel once per event; when first enters or is produced in the aerogel. Store trajectory/energy

Where can direct backgrounds from the target hit the PMT matrix? Seems to me only the back side is exposed. From the side, the PMTs are shielded by a steel plate, both surrounding the PMT matrix and the steel matrix itself. From the front, the PMTs are not in direct view of the target.

Parameters of 1 st Successful GEANT Run GeV beam electrons on target Mistakes in setup of first run: – Accidentally used configuration with SBS at a distance of 1.6 m from target; currently running with SBS at 2.45 m as in proposal: will reduce solid angle for photon backgrounds – Aerogel wall was located 4.0 m from target; currently running with aerogel at 4.85 m from target, will reduce solid angle for photon backgrounds – Accidentally set gamma cutoff energy too high at 2.0 MeV, need to lower to 1.6 MeV to capture all electrons above aerogel threshold. Will increase the background rate only slightly, as near-threshold electrons have low photon yield. Results on following slides

Electron/positron yield Overall electron+positron yield of about 60,000 events out of beam electrons; Distribution of beta (right) and Cerenkov angle (left) This translates to a rate of about microamps. About 25% of particles are positrons (next slide), suggest pair production is about equally prevalent to Compton scattering. How many PMT hits in the RICH detector?

Particle ID: 2=positron, 3=electron

Calculation of the photon yield: Make the (reasonable) simplifying assumption of a non-dispersive medium; n is constant; then we have: I used lambda1 = 250 nm, lambda2 = 700 nm for the integration, roughly the range of sensitivity of the PMTs

dN/dx (cm -1 ) vs. beta Max. number of photons for beta=1 * 5.65 cm ~200 photons

Monte Carlo Photon Generation Unlike high-energy electrons moving mostly along the axis of the detector at high energies, low energy electrons come not only from target but other locations, much more spread in trajectories (pair production, etc.). Therefore, we cannot assume that all our electrons pass through the aerogel along their initial trajectories undisturbed. We also cannot assume all photons emitted by our electrons will be detected. Monte Carlo study using an idealized mirror/detector geometry

Monte Carlo Photons: range of electrons in aerogel Between 1.5 MeV and 10 MeV kinetic energy, electron range varies from ~.8 grams to about 6 grams (our aerogel wall is about.8 grams)

Monte Carlo Photons Sample azimuthal angle randomly Cerenkov angle set by beta Sample emission vertex randomly between 0 and min(range, aerogel boundary) along electron path Project photons to mirror Calculate reflected photon trajectory if photon hits mirror (some assumptions made about size/geometry of mirror) Project reflected photons to detector plane (also some assumptions involved since I don’t have an exact geometry) Count the number of photons that hit mirror AND reflect to detector. These results represent an upper limit in the sense that we assume electrons emit Cerenkov photons as if they were moving with a constant velocity equal to their initial beta. In reality, they are slowing down! This will reduce the true yield relative to this crude calculation.

Distribution of the number of Ckov. photons hitting the mirror+detector

“Max” number of photons ~200 for beta=1 particle moving along axis of aerogel HERMES RICH NIM paper states that 8-10 PMT hits/track is typical for good electron tracks. This suggests a total transmission+detection efficiency on the order of 5% (in rough agreement with the plot on the left) Make a conservative estimate of 15% efficiency to normalize our background calculations Leads to about 200 kHz/PMT or: 1.9% occupancy for 100 ns window.19% occupancy for 10 ns window Upgrade readout electronics to tighten time window, gain more rate tolerance? HERMES RICH long NIM paper: total transmission+detection efficiency for aerogel, separated into its component parts.

Conclusions Initial exploratory work suggests that the original proposal “back-of-the-envelope” estimate of RICH background rates was in the right ballpark, and may in fact have been too pessimistic. Most of the mistakes made in the initial attempt tend to increase the background rate; proposal conditions even better. More studies in progress; draft proposal update shortly