HiRes 5Y Operations – Program and Context What Physics Will be Done? How Does it Compare With Other Projects?

General Goals Stereo Spectrum from 1018 eV on – Study ankle, pileup and GZK cutoff Confirmed energy resolution using E pull ( E1-E2/Eav) Confirmed atmospheric parameters using energy balance and detailed laser studies. Statistical strength and energy reach for 5 years

Stereo Aperture and Dependence on Atmospheric Parameters

Monocular Spectrum

Preliminary Look at Stereo Data – Energy Resolution

Event Energy Distribution (> 10 EeV) – 800 hours of stereo operation

Preliminary Stereo Spectrum above 10 EeV

Predicted Event Statistics for 5Y OP

Event Rate Prediction – Based on Current Reconstructed Event Rates (conservative) 5Y op = ~ 5000 hours Expect ~ 500 events > 1019eV Expect ~ 5 events > 1020 eV (based on NSF event) ~ E-3 spectrum Expect ~ 30 events if AGASA flux is correct.(~ 5 sigma discrepancy) > 1020 eV are “golden” – high signal, highly constrained.

NSF Event

“Golden” Events Very strong signals ( aperture cut-off due to track-length, not S/N at Xmax). If Xmax seen in both eyes, Smax balance can constrain/test atmospheric parameters. Events typically seen with one Rp much smaller than other – atmospheric effects mitigated. If fluorescence efficiency well understood, very hard to significantly shift event energies.

MEASURING FLUORESCENCE AT SLAC Extensive Air Showers (EAS) are predominantly a superposition of EM sub-showers. Important N2 transition (2P) not accessible by proton excitation; only e-beam can do it. FFTB beam-line provides energy equivalent showers from ~1015 to ~1020 eV. 108-1010 electrons/pulse at 28.5 GeV. 2% of electron pulse bremsstrahlung option.

Significance of pileup No-pileup ( E-3) flux implies order of magnitude decrease in integral flux between 1019 and 3 x1019 eV. (81 events – 8.1x1.7 =13 events expected ) Observe 29 Stereo events above 3 x1019 eV. 2.9 sigma. After 5Y, expect 500 events above 1019. Expect 50x1.7=85 events above 3 x1019 eV if ( E-3) flux. If present data is correct, would see 179 events. This is a 7 sigma effect.

Significance of ankle ( dip structure) Dip structure can be used to cross-calibrate energy scale of various experiments. What is the physics of this structure? e+e- energy loss? Appearance of extragalactic spectrum? Expect ~ 3000 events above 1018.3 eV and ~10,000 events above 1018 eV. Statistics will not be an issue.

Second Knee Second knee seen in many experiments. Simple energy scale shift can bring them into good agreement. Physics of transition between galactic and extragalactic flux? Composition is changing in this energy region. Can be accessed with HiRes II mono data. Propose to push thresholds down to near 1016 eV. Continuous measurement of composition over second knee, ankle and pileup region of spectrum is then possible.

Second Knee, showing correlation between knee energy and spectral normalization

Second Knee Spectrum, Shifted to make knee come out at same energy

General Goals, cont. Composition of Cosmic Rays from 1018 eV Xmax method – preliminary stereo results confirm earlier Fly’s Eye results. Preliminary stereo results consistent with hadronic model predictions – sensitivity to composition. Expected statistics for one and two view Xmax determination. Xmax resolution confirmed by Xmax pull for two view events. Search for gamma ray flux using LPM and mag. Brem. Expected statistical reach.

Resolution After All Cuts 542 w/database atm, 181 RGA Energy Xmax 723 Events

Pull Distribution – Data and MC ) (XmaxI-XmaxII)/(XmaxI+XmaxII

Comparison of gamma ray Xmax distribution with LPM and magnetic brem effects taken into account

General Goals, cont. Anisotropy Small-scale anisotropy – stereo data has symmetric error boxes and excellent angular resolution ~ 1 deg. Is the AGASA auto-correlation result correct? HiRes I monocular data does not support it, but suffers from assymetric errors Sensitivity to putative point sources for 5 Y of operation.

Xmax viewing efficiency Integrated over energy range ~60% of triggered events have Xmax well reconstructed in at least one view. ~40% of triggered events have Xmax well reconstructed in both views. Composition above 1019 eV will be based on ~ 300 events. Resolution function based on ~200 events.

HiRes Monocular Error Boxes

HiRes monocular simulated data

HiRes Stereo simulated data

Akeno/AGASA result on anisotropy near 1018 eV

General Goals, cont. Measurement of total inelastic p-Air cross-section. Composition above few x 1018 appears to be proton dominated. Cut on deeper events to increase proton fraction. Xmax distribution decrement is sensitive to p-Air. Because result comes from tail of distribution, understanding of resolution and reconstruction is essential- requires tightly cut stereo data.

General Goals, cont Neutrino search Search for unusual shower development

Other Experiments and Proposals AGASA Pierre Auger SOUTH Telescope Array Pierre Auger NORTH EUSO OWL

AGASA 100 km2 ground array. Ran for ~ 12 years. To be decommissioned ( another two years possible). Quoted accumulated aperture ~ HiRes I mono. No further significant data expected.

Pierre Auger SOUTH 3000 km2 ground array + Fluorescence detector. Ground array aperture is 7000 km2 str near 1020 eV. Probable completion data 2005. Hybrid aperture is 10% or ~700 km2 str . If HiRes runs for 5 years and stops in 2007, Auger hybrid will have equal statistics in ~7 years, or 2012. Southern and Northern spectra may be different

Pierre Auger SOUTH, cont. Ground array will have similar statistics to HiRes in 2006. Issues of ground array vs. fluorescence energy scale will not be settled until hybrid data becomes significant, perhaps in 2007.

Telescope Array Full – scale proposal for 8 stations + 2 HiRes stations still pending at Monkasho. If approved, five year construction is expected. TA phase I ( three partial eyes + ground array) proposal submitted to …. HiRes group is collaborating on both proposals.

Pierre Auger North Originally proposed for Millard County, UT. Current status unclear – may be in Europe. Unlikely to start construction before 2006. MOU exists between HiRes, Auger and TA to work together if built in Utah.

EUSO Phase A ESA approval for deployment on ISS in ~ 2009. Aperture ~ order of magnitude larger than Auger ground array. Two to three year operational life. Similar to Auger over expected life of detectors. Detector threshold near 1019 eV.

OWL Next generation ( post-EUSO) proposal. Two 1000 km orbit free-flyer satellites. Stereo observation – ~106 km2str instantaneous aperture. Unlikely to fly before 2014.