Detector Backgrounds in the HERA ep-Colldider F. Willeke, BNL October 9, 2012

HERA Overview ZEUS HERMES HERA-B

HERA Provided collisions with 920GeV p beams and 27.5 GeV longitudinally spin-polarized lepton (e+/e-) beams for colliding beam experiments H1 and ZEUS. Two 6 km circumference rings in underground tunnel (20m). Proton ring with superconducting magnets, lepton ring with superconducting RF. Luminosity: cm -2 s -1 Lepton ring and proton rings also provided independently beam to fixed target experiments (HERMES and HERA-B)

HERA Chronology 1984 Construction Project authorized in Commissioning Complete 1992 Start Operation 820GeV p+ & 27.5GeV e GeV p & 27.5 GeV e add HERMES IR and spin rotators 1996 add HERA-B IR, 920GeV p – 27.5GeV e reach design luminosity IP  NEG, Hardware upgrade GeV p & 27.5GeV e-/e+ productive runs >50 pb -1 y Luminosity Upgrade, Factor 3-5, H1 & ZEUS Spin rotator Upgrade brings machine components into detectors to 2m from IP including S.C. magnets Synchrotron radiation from beam separation magnets passes through entire IR and is absorbed far (30m) from IP 2001 Fall Re-commissioning Resolving Detector Background Issues Productive luminosity runs >100pb -1 y -1, p-e+/e-,long. Polarized leptons Runs with varying proton energy: 450GeV-620GeV (F L ) HERA I HERA II

HERA Accelerator Parameters protons per bunch N p / normalized emittance  N /  m Proton  functions   y,x /m Leptons Current I e /mA Lepton emittance  e /mA number of coll. Bunches n b E vert. bb tune shift/ip  ye hor./vert. beam size at IP  x,y,p,e /  m specific LuminosityLs /10 30 cm -2 s -1 mA -2 Luminosity L Luminosity L / cm -2 s (15-20) 0.18, x30 112x Design Achieved

80k beampipe ! Detector

Vacuum clean, minor traces of Methane, Ethane after warmup IR vacuum improving rapidly and in steps after warm up of sc. Magnets with 80K beam pipe IR Pumping: Distributed NEG Lumped Ion pump Ti-Subl.Pumps

H1 Detector protons leptons Lq Ar Hadron Cal EM Cal  Chambers Drift Ch  VD Forward Cal Backward Cal Solenoid coil yoke

Background Issues after Upgrade of the Interaction Region Synchrotron radiation from far away (100m) bend  resolved immediately Faulty Design of Backscatter mask for MVX and drift chambers, conceptually  straight forward fix, but required large shut down Poor collimator design in both ZEUS and H1 detectors: no cooling, poor geometry and large HOM heating: high gas load generated during injection affecting beginning of each run:  straight forward fix, but required a large shut down Proton backgrounds from collisions of 920GeV p with desorbed gas generated by e beam synchrotron radiation on vacuum component of p-side of IP  Severe problem which disappeared only slowly over several years of running

Issues worried about which did not cause problems e particle backgrounds:  High energy leptons after rest gas collisions get bent into detector by separation magnetic fields: This was only a noticeable contribution to background shortly after machine was turned on Impact of 80K beam pipe of SC IR magnets:  There was never evidence of any detrimental effect, on the contrary, 80K surface acted as cryo-pump for CO2, Methane … Direct Synchrotron radiation from the lepton side IR quads and separation magnets  This required careful adjustments of IR beam orbits and was not really a problem Synchrotron Radiation Back Scattering from keyhole-masks of the IR vacuum system on the hadron-side of the IP  This required good IR orbit control but did not cause a major background issue Synchrotron radiation backgrounds in drift chambers and radiation dose to micro vertex detectors:  Always critical but usually under control by careful tuning of the IR orbits, the beam parameters (tunes, chromaticity, coupling) Small angle low energy synchrotron radiation desorbed gases in the detector region  Vacuum inside the detector beam pipe indeed worse than in the rst of the IR. This effect might have contributed in addition to to lack of pumping (one ion pump at 3.6m) and limited conductance

Issue: High occupancy of calorimeters reduces the fraction of useful data. It is caused by debris from proton rest gas interaction. Debris scatter off the vacuum components and the masks against back scattered synchrotron radiation from the crotch absorber Background from 920GeV p scattering on desorbed Gases in the IP

Secondary Vertices detected in the H1 detectors

Background from 920GeV p scattering on desorbed Gases in the IP Study of Background Multiplicities: No evidence of vacuum contamination Date explained by CO 2 in residual gas HERA I HERA II

Background from 920GeV p scattering on desorbed Gases in the IP Why was this a major problem for HERA II and not for HERA I (before 2000)? (This was never satisfactorily answered ) -Crotch Absorber only 11m from IP, more intense desorption from back scattered photons, however vacuum in IR 1E-10- 1E-11 -Methan production in IR quadrupole NEG Pumps: no evidence for Methane or Ethane in beam vacuum -HERA I started with small proton and lepton currents (30mA/15mA) and intensity was increased gradually to 70mA/45mA) between no venting of IRs, whereas HERA II started with 100mA proton currents and 40mA lepton currents, there were several IR vacuum leaks which always caused a set back in vacuum conditions

Hadron Backgrounds cont’d The Hadron induced backgrounds reduced slowly and by 2004, reached levels were, while still being important, they did not constitute a major obstacle for data taking Improvements occurred in steps after each warm up of the superconducting IR magnets and subsequent reactivation of the IR NEG pumps. Interpreted as effect of more efficient NEG and cryo pumping There were several vacuum leaks in the IR which required vent of the IR vacuum. This was followed by increased background levels which recovered quickly and had the tendency to drop below the levels before the leak: interpreted as an effect of flushing the IR vacuum with N 2

Conclusions HERA backgrounds from synchrotron Radiation and Hadronic background were found at unexpected high levels after an upgrade of the IR regions which brought machine elements much closer to the IP (2m). Synchrotron radiation backgrounds were soon under control after some flaws with synchrotron radiation masks were fixed and after beam tuning became routine The most problematic backgrounds were caused by scattering of high energy protons on the rest gas desorbed by synchrotron radiation from the lepton beam. While the geometric conditions after the upgrade favored somewhat higher backgrounds, the main reasons for the problem was that extremely good IR vacuum conditions are required to operate a high luminosity and high energy ep collider with tolerable background conditions.

Some HERA Highlights