Analysis of CMS HCAL testbeam performance

Analysis of CMS HCAL testbeam performance Philip Harris

CMS HCAL(a brief look) • Designed to survive large magnetic fields • Consists of crystals conducive to Parton Showers • Major form of detection is amplified Cerenkov light • Light signals are amplified by HPDs • An HPD consists of a photocathode with a small gap (with a voltage potential) that is then connected to a piece of Silicon. • If positioned correctly no Lorentz force

Major Issues for the Testbeam • The most critical issue in the testbeam involves the behavior of the HPDs • Last year It was discovered that timing is non linear with energy. • Simulations showed that the detector would still be fine in the LHC • Still this remains a critical issue and is fundamentally rooted in the behavior of the HPDs

A Quick look at What’s in the beam • The tests over the summer concern various energies of 3 different particles

Very Low Energy • One of the most important tests that are being done this summer involve an analysis of the Very Low Energy (VLE) response of the detector • In order to do this we need a good trigger system • External separation of Muons, Pions and Electrons is essential • Many external triggers were added for this purpose

VLE Beam characteristics • The problem with VLE is that it is difficult to separate out the beam perfectly • Pions in particular decay into muons and screw up the data royally • Below are a series of plots that describe this for 10 GeV we expect ~10% contamination

VLE beam examples • Here are actual plots showing the VLE beam (calculations agree with reality)

General detector Problems • One problem with HPDs is that they release background emission thereby changing the time. • The plot below shows a strange high Energy bump

Is it a problem • Previous results showed timing shifts resulting from this effect • Plots show a shift on the order of the a ns (within statistical shifts)

Pedestal Analysis • Another very interesting thing is the shift in the Pedestals when a signal comes in • Reasoning: Silicon is imperfect • 2d plots of Pedestals after signal show little correlations

Energy Resolution • A major debate concerns the optimal pedestal signal • Pedestal subtraction event by event or during the whole beam is debated. Plot below might yield answer.

Overall resolution • Plot below shows the whole Energy scale

Other Issues • During the VLE beam gains were changed on HPDs to increase resolution. • So far no statistical change is seen in calibration pulses.

Long Term • Most data has been looked at for short term, but long term remains to be fully analyzed. • Plots below show detector seems reasonably stable.

Along with Analysis • There many other things to be done at a testbeam • Calibration of the detector can be tricky. • At present a code has been written to carefully select muons and ratio scale this so as to check the calibration • This will be checked with radioactive source data to see if changes in Energy have occurred.

Future • Multiple particle beams as well as a thorough pedestal analysis remain to and will be looked at, to insure that the detector has reasonable behavior.

Conclusions • CMS HCAL behaves quite well, but there remain minor issues that need to be settled. • There is a lot of analysis even on a simple testbeam like this. • All these plots were generated by an analysis code package that I created and might be used for realtime user analysis at CMS

Acknowledgements • H2 testbeam people • Dragoslav Lazic • Pawel De Barbero • University of Michigan • CERN • The Number of Neutrino flavors

Analysis of CMS HCAL testbeam performance