The most promising means to achieving the unprecedented jet energy resolutions desired for the ILC is through particle-flow algorithms (PFA). A PFA attempts to separately identify in a jet its charged, electromagnetic, and neutral hadron components, in order to use the best means to measure each. On average, neutral hadrons carry only ~11% of a jet's total energy, which can only be measured with the relatively poor resolution of the HCal. The tracker is used to measure with much better precision the charged components (~64% of jet energy), and the electromagnetic calorimeter (ECal) to measure the photons with about sigma(E)=0.15*sqrt(E) (~24% of jet energy). A net jet energy resolution of sigma(E)=0.3*sqrt(E) is thus deemed achievable by using the HCal only to measure the neutral hadrons with sigma(E)=0.6*sqrt(E). However, this will certainly require extensive and simultaneous optimization of detector design and tuning of algorithm parameters.

A calorimeter designed for PFAs must be finely segmented both transversely and longitudinally for 3-d shower reconstruction, separation of neutral and charged clusters, and association of the charged clusters to corresponding tracks. This requires realistic simulation of parton shower evolution and of the detector's response to the particles passing through it. Accurate simulation relies heavily on analysis of data from beam test of prototype modules.

Very large numbers of events will have to be simulated to evaluate competing detector designs, in terms of both technology and geometry, vis-a-vis ILC physics goals. Characterization of signatures arising from processes predicted by some extensions of the SM will require simultaneous coverage of broad ranges of undetermined parameters. Parametrized fast simulation programs will thus have to be developed once the algorithms have stablized. Parametrization of PFAs will require much work, and is one of our key objectives.

Members of NIU became involved in developments of PFAs and simulation software development efforts in January, 2002. Toward the optimization of the HCal design, the NIU team has been investigating both analog (cell energy measurements) and digital (hit density measurements) methods as functions of the cell size. Indeed, the first fully digital PFA was developed by our team.

Our preliminary findings suggest that with sufficiently small cells, the digital method yields a more precise measurement of the hadron energy, i.e., fluctuations in hit density are smaller than those in the sampled energy of a hadronic shower. Use of local hit density in lieu of the deposited energy to weigh the calorimeter hits results in superior energy resolution and lateral profiling of single hadron showers. It should be noted, however, that use of local energy densities and gradients, instead of just individual cell energies, can also lead to similar improvements.

We have used our PFA (several variants) to perform full jet reconstruction and achieved similar results (in terms of Z->jj mass resolutions) as other groups, e.g. those at ANL and DESY. Presently, our primary focus is twofold: 1. to reduce the uncertainty arising from incorrect assignment of calorimeter hits to the parent particle (the so-called ``confusion term'' which limits a PFA-based jet energy resoution), and 2. to improve the understanding of the differences in energy deposition patterns between different types of particles (e.g. neutrons vs. K^0_L vs. anti-neutrons).

The NIU group has also made significant contributions to ILC detector simulation software since 2002. We have developed, in close collaboration with our colleagues at SLAC, a stand-alone GEANT4-based simulation package called ``LCDG4'', which was the first to fully comply with the model put forth by the ALCPG simulation group, and added several useful functionalities to it. Virtually all PFA development studies in North America from mid-2003 to mid-2005 relied on LCDG4.

Further, as members of the CALICE collaboration (CAlorimeter for the LInear Collider with Electrons,\cite{calice}), we contributed to the production of a GEANT4-based simulator, ``TBMokka'' for the detector prototype module that is expected to be exposed to test beams at Fermilab or CERN over a period of 3-4 years starting in mid-2006.

In another endeavor, we have developed a package called ``DigiSim'', which parametrically simulates the conversion of energy deposits produced by GEANT4 to electronic read-outs. This package offers the user a simple, flexible, and standard way to simulate the effects of thresholds, noise, cross-talk, inefficiencies, attenuation, and timing, that are involved in signal collection, propagation, and conversion (digitization). In essence, it allows the user to model an arbitrary transfer function from the energy deposited at the cell to the corresponding ``raw data''. DigiSim can be used either in a stand-alone mode to produce a persistent output, or as an on-the-fly preprocessor to the reconstruction program. In stand-alone mode, it produces output in the same format as that envisaged from the real detector (except, of course, the simulation output also contains the ``Monte Carlo truth'', which the real data does not). No high claim on the performance of an algorithm can be substantiated without a realistic accounting of the above-mentioned detector effects. Thus, DigiSim</nop plays a vital role, and has been warmly welcomed by the user community worldwide. We expect it to be used for the simulation of both the various test-beam prototypes and full-detector designs.

Among the members of our group we have adequate experience in calorimeter hardware, electronics, reconstruction software, and algorithm development. We anticipate close collaboration with other groups with similar interests. Active links have been established with ANL, SLAC, FNAL, DESY, and several university groups including the CALICE member institutions.

Our plans for the next 3 years:

During the first year we will concentrate on perfecting the usage of the DigiSim</nop package so the algorithms can be tuned on inputs that closely resemble real data. Beam tests will provide an opportunity to understand not only the detector hardware, but the simulation and reconstruction software as well. The first year deliverable will be a complete DigiSim</nop package that converts the energy deposits simulated by GEANT4 into raw data format, allowing for such detector effects as non-linearities, inefficiencies, noise, cross-talk etc. The latter are not easily simulated by GEANT4, but may well prove crucial to design decisions and algorithmic choices.

We also expect to have by the end of the first year a first version of a class of particle-flow algorithms based on full simulation and reconstruction of the central (barrel) region. Both analog and digital versions (for the hadronic section) of the algorithms, which give encouraging preliminary results, will be further investigated and optimized. In addition, the standard GEANT4-based simulation facility (farm+server) will be available for to the entire ILC community through a web-based request form.

In the second year, we will address the design issues that must be taken into account in order to extend PFAs to the forward regions. We also plan to design and implement the PFAs in such a way that they can be easily ported across detector design details. Algorithms will continue to be tuned, as simulations become more detailed and refined (the latter from analysis of test beam data).

Comprehensive studies of critical physics processes will have to be carried out in order to understand the impact of the calorimeter performance on the physics program of the Linear Collider. These studies will employ both the analog and digital versions of our PFAs. The second year deliverables will be further development of PFA-based jet-reconstruction and a partial assessment of physics reach vs calorimeter performance for the ILC.

In the third year we will complete the physics assessment with a clear statement on the desirability of a digital or analog option for the hadronic calorimeter. This will, of course, depend to a large extent on the test beam experience as well. If all goes well, we will also start the development of parameterized simulations of the particle-flow algorithms. The technology and geometry are expected to have been narrowed down by that time, thus setting the stage for such parametrized fast simulation for extensive physics studies. By the end of the third year we expect to produce, in collaboration with other groups, a fast simulation program based on PFAs. In addition, extensive benchmarking of critical physics processes, as well as evolution of pattern-recognition and reconstruction algorithms will continue.

The steady progress that we have achieved so far has been made possible by funding received received for this purpose during the past 3 fiscal years from DOE and NSF, in addition to generous, but less specific, funding from the Department of Education. However, the pace of progress has been limited by the level of support. The course of action is well laid out. An boost in available personpower will help ensure timely completion of the challenging task in our hands.

Please address the following questions in your statement.

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Topic revision: 22 Dec 2005, DanPeterson
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