Workshop Summary , BNL May 2008

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Tuesday morning plenary

The workshop began with a plenary session on Tuesday morning, starting with a summary by Peter Jacobs on the history and goals of the TECHQM initiative. He explained that, for the purpose of getting the work of the Collaboration started, this first TECHQM workshop would concentrate on two key issues in RHIC and LHC heavy-ion collisions, collective flow and parton energy loss, even though the scope and long-term goals of the TECHQM Collaboration are much more comprehensive. A short prose version can be found on the meeting web site This introduction was followed by a report by Derek Teaney on the progress made during a very successful 2-week workshop program on viscous relativistic hydrodynamics at BNL that had just ended (see links on the page Bulk Evolution). Three talks by Francois Gelis, Abhijit Majumder, and Pasi Huovinen reviewed the present status and listed key open issues in the areas of early evolution and initial conditions, parton energy loss and jet quenching, and collective flow and relativistic hydrodynamics, respectively. These talks provided starting points for the afternoon discussions (see below). Finally, Urs Wiedemann discussed the purpose of the two afternoon breakout sessions. Urs also reviewed some questions, which - due to their complexity - have turned out to lie beyond the capacity of individual research groups, though the major elements to their solution are widely known. TECHQM is intended as a platform for work on these wider questions, which require large scale collaboration. In this way, TECHQM is expected to increase the visibility of individual research.

Tuesday afternoon breakout sessions

The main goal of the two afternoon breakout sessions on Collective Flow and Parton Energy loss was to develop a draft work plan for the TECHQM Collaboration for the next 6 months, adding also a slightly more longer term perspective on how to follow up during the second half of the first year of operation of TECHQM. Each session had two parts, one before, the other after the coffee break. In the first part, members from the community made short contributions to discuss what they thought were key open issues that required collaboration between several research groups, that were ripe for a solution and should be attacked by a collaborative effort within TECHQM. Links to the slides shown during the sessions (including some transparencies submitted by prospective TECHQM members who could not attend the workshop in person) can be found further above on this page. Each presentation was followed by a discussion among all attending working group members. After the coffee break, the two working groups began to draft a working plan. The convenors kept a record of this plan, to be presented in the plenary session next morning, including a list of names of people who, during the discussion, indicated that they were interested in contributing to specific items in the plan. Only people who were present in person were listed, but others who had expressed to workshop participants before the meeting there interest to contribute were mentioned. The discussions were characterized by an atmosphere of excitement and optimism and moved forward very constructively.

Wednesday morning plenary: Working Group reports

The Wednesday morning plenary session started with a report by the convenors of the two breakout sessions.

Collective Flow working group report

Ulrich reported on the Collective Flow session and presented a matrix of physics issues, concrete tasks, and interested people, subdivided in two parts corresponding to a 6-month and 12-month time horizon. Note that the listed names reflect expressions of interest voiced at the meeting (those with ? were transmitted to the convenor before the meeting) and not actual commitments. The question how to join TECHQM and formally commit to its activities was discussed later that morning (see below). The matrix lists a possible subdivision of the Collective Flow activities into three working groups [Hydro-WG (focussing on relativistic hydrodynamics, ideal and viscous), Post-Hydro-WG (focussing on hybrid algorithms where hydrodynamics is followed by a hadronic cascade to calculate spectra and HBT correlations), and Initial-Conditions-WG (focussing on microscopic models for early evolution to provide initial conditions for the hydrodynamic stage)], but at the meeting it was decided for practical reasions to begin with a single Bulk Evolution Working Group, postponing a subdivision to a later time if and when the complexity of the work and size of the work-load requires it.

Also listed in the matrix as an "Opportunity" is the need for developing a strategy for the statistical analysis of data using the dynamical model output. This complex issue involves a large number of observables and model parameters (such a transport coefficients, saturation momentum, critical temperature, etc.) and requires modern Bayesian techniques with which our community is not familiar. While we were not able to include this task in the draft work program due to a lack of dedicated manpower among those present, several (including the BNL ALD for RHIC, Steve Vigdor) stressed its importance and the need for developing these tools as soon as possible, preferably before the first model results for particle spectra and correlations from TECHQM become available. The call for joining the TECHQM will explicitly point out this "opportunity to contribute", and Mike Lisa, Scott Pratt, and Ron Soltz said they would help anyone interested in this project by investing some fraction of their time to help it move along.

Hard Probes working group report

Urs Wiedemann and Brian Cole reported on the breakout session of the Hard Probes working group. The hard probes working group agreed to define several benchmark test problems. The first is the “static QGP brick” problem, which will be completed within the coming months. This assumes a static uniform slab of equilibrated QGP matter with a thickness either L=2 and 5 fm and a fixed temperature varied over a range including T=200 and 300 MeV. Groups which do not characterize the medium in terms of a temperature should use their model parameter(s) instead, vary them over a wide range and specify how they are related to temperature. Either running alpha or fixed alpha=0.3 can be assumed but fixed alpha is preferred to simplify comparison without introducing extra assumptions on alpha_max etc. A light parton jet with energy 10 GeV and 100 GeV is assumed to pass through the slab assuming it was created at the entrance point of the slab (not at t=-infinity). Groups with ability to treat heavy bottom quarks are asked to compute generate results also for these jets, using a bottom quark mass of 4.3 GeV. Each group will calulate the energy loss probability distribution P(Delta E; E,L,T) for either the radiative, elastic or total fluctuating energy loss, DeltaE(E,L,T,MQ). All contributed P curves will be compared side by side and differences will be explained clearly in the writeup.

Upon successful completion of this first task, the more demanding benchmark test of jet energy loss and deflection through a boosted slab with transverse velocity , beta_perp=0.25 and 0.5 , will be attempted. some groups will have to generalize their theory to accomodate flowing matter. Thus partly this will require innovations beyond published papers. Each group may independently first publish their results before the working group compiles all results. The goal of both benchmark tests is to clearly define a baseline how each theory behaves to different temperature, flow velocity field, and geometric configurations. Once these two benchmark tests are completed and compiled, the more intresting jet energy loss problem though a realistic hydrodynamic temperature and flow field from the hydro working group can be attempted. The third benchmark problem would be a b=0 and b=7 fm AuAu 200 AgeV collision computed in a "standardized" hydrodynamic model with initial conditions and freezeout, for which the Collective Flow WG will provide hydrodynamic model output. For this collaborative effort between the Collective Flow and Parton Energy Loss groups an output standard needs to be defined, and one must agree on the hydrodynamical model parameters (collision system and energy, impact parameter, etc.). Beyond the 6-month horizon, the selfconsistent inclusion of collisional energy loss and hadronic matter effects (below T_c) is anticipated, as well as a generalization of the hard-soft-interface such that it allows to feed back the parton energy loss into the collective flow models for the medium.

TECHQM Organizational Issues

Following the presentations from the breakout sessions, the plenum began under the guidance of Berndt Muller to discuss issues of logistics. A lengthy discussion of the structure of the Working Groups led to the consensus that we would start, for the time being, with just two working groups. A roll call for interest in participating in one or the other showed 19 hands for the Collective Flow WG and 24 hands for the Parton Energy Loss WG. This was considered healthy, but more participants are expected to join once the call for contributions goes out. It was decided that for the time being the organizers of this TECHQM workshop would act in some fashion as interim WG convenors, until the WG membership has firmed up, at which time each WG would elect two convenors from within their ranks.

The discussion about WG organization flowed into a more general exchange of opinions about the formal structure of the TECHQM Collaboration. There was consensus that TECHQM should be open to anyone interested in and willing to contribute to its goals. During a debate of what it means to "join TECHQM" and what level of commitment should be required from prospective members, several participants emphasized the "grass-roots spirit" of TECHQM and voiced preference for a self-organization process that is driven by physics interests, with as few barriers for membership as possible.

TECHQM: comments and concerns

Extensive discussion addressed concerns voiced by participants. Theorists are not used to working in a large collaboration and have in general only limited understanding of how, for example, an experimental collaboration works. Here is an attempt to paraphrase some of the questions that were asked and the answers given (note that the following are not established rules of engagement in TECHQM, but reflect the spirit of the discussion how a working collaboration might function):

Q: How can I publish work I have contributed to the TECHQM effort? A: Individual research papers will be published individually, as usual. TECHQM focuses on questions outside the capacity of an individual research group. Papers resulting from such work are published by those who have contributed to the paper in question; co-authorship requires a significant contribution as is customary, so most papers will show only a (small) subset of TECHQM members as authors. However, in order for the paper to receive a TECHQM preprint number, it has to be vetted by the Collaboration: anybody can ask any reasonable question about the work presented in the paper, and every question receives an honest answer. The paper is shepherded through this process by a "godfather committee" for the paper. Unresolvable disagreements are dealt with by appropriate comments in the text, in the same way as an author would deal with a critical comment by a referee. In essence, the paper is refereed internally by TECHQM before being receiving its stamp of approval (i.e. a TECHQM preprint number).

Q: How can I keep control over numerical codes that I contribute to the TECHQM effort? A: While all codes contributed to TECHQM should be fully verified and accompanied by appropriate documentation, the author of the code is free to specify any rules how others can use the code. Examples: 1. The author(s) of the code must be listed as coauthors on all papers that use results obtained with the code. (Do you really want to check every paper that inherits your name in this way in sufficient detail to be certain that you want to sign it?) 2. The author(s) of the code require that certain standard references where this code is first described or where results obtained from it are first published are cited in any paper that uses the code or results obtained from it. (This ensures that a useful code collects appropriate numbers of citations for its author, but you are not required to approve the contents of all papers that use your code, nor can you prevent its abuse). 3. The author(s) require that proper credit is given to them by citing a specified list of references for any "tricks" or algorithms that are gleaned from their code and transported to another code each time results obtained from the daughter code are published.

Q: Won't theorists doing modeling work for TECHQM instead of developing novel theoretical concepts decrease their chances for getting tenure at a university? A: Past experience suggests that this can indeed not be excluded, but examples exist for theorists on tenure-track or tenured faculty positions whose main claim to fame results from numerical modeling. In other fields (e.g. numerical astrophysics) the number of such cases is larger than so far in Nuclear Physics. By validating concepts developed by individual researchers and embedding them into the modeling effort, TECHQM is expected to enhance the visibility of these individual efforts. We hope that after TECHQM establishes itself as a quality trademark (see above answer on publishing and internal refereeing), having made valuable contributions to TECHQM will serve as an asset rather than a liability on the candidate's CV.

Q: What are the benefits of bi-weekly or monthly phone meetings? Can't I just discuss with my collaborators by email or phone as I am used to and when I need/want to? A: Phone meetings can always be cancelled when there is nothing to report or discuss, but they are difficult to arrange on short notice when an important question arises that may benefit from broader input. This simple argument favors a bi-weekly over a monthly schedule. Regular meetings serve as an incentive for students to clean up their results and post them on the wiki page in a form that facilitates discussion. This is an important clarification process. Regular meetings keep all participants abreast of what is going on elsewhere in the Collaboration even if they themselves have nothing new to report; those who present results benefit from the comments of everyone and from different angles, not just from their supervisors and immediate collaborators. Phone meetings identify problems worth thinking about and dead ends. Rather than being a waste of time, they help to avoid wasting your time by focussing your thoughts.

There was considerable resistance from members in the audience to bi-weekly phone meetings. It was decided that each WG will establish its own meeting schedule, but recommended that each group should meet at least once a month to keep the momentum going.

Comments from DOE

The last two presentations were by Ted Barnes, detailee for Nuclear Theory in the Office of Nuclear Physics at the Department of Energy, and by Dima Kharzeev, Head of the BNL Nuclear Theory Group. Barnes discussed the present funding situation in his office, and cited possible avenues for providing support for TECHQM activities. He had been present for the entire workshop, and expressed support for the planned TECHQM activities. Barnes stated that in the present constrained funding environment it may be somewhat less difficult to find a mechanism to support a novel collaborative effort such as TECHQM than to increase or newly fund individual research grants. He cautioned, however, that it may be beneficial to defer funding requests until TECHQM has been able to establish a "track record" of research, showing that it is able to function as a collaboration (not just as a conglomerate of individuals and individual research groups) and produce the desired results. He expressed confidence that the TECHQM work plan adopted at the meeting, once successfully addressed, will provide that track record. Barnes discussed two specific group funding mechanisms, SciDAC projects and nuclear theory Topical Collaborations, in more detail. The novel Topical Collaborations were recommended by NSAC in previous Nuclear Physics Long Range Plans, and he noted that a first call for proposals may be issued in FY09.

CATHIE Proposal

Dima Kharzeev presented a draft proposal for a "Center of Analysis and Theory for Heavy Ion Experiments (CATHIE)" located at BNL. The purpose of CATHIE is to coordinate collaboration between theory and experiment on issues related to detailed analysis and numerical simulations of heavy ion collisions. Its day-to-day work on quantitative modeling and simulation of heavy-ion collisions would be coordinated by a staff scientist who would maintain a depository of simulation codes, run tests, and perform quantitative data analysis. CATHIE would provide a natural interface with the TECHQM Collaboration. The Center's activities would be directed by a representative Advisory Board, including major theory groups and experimental collaborations, both in the US and internationally, working on RHIC, LHC and FAIR heavy ion physics.

Future meetings and planning

Participants decided that we shall have the next TECHQM meeting in about 6 months, probably in the first half of December 2008 in Berkeley. A third TECHQM meeting has been tentatively scheduled for middle of March 2009, in time before Quark Matter 2009 (March 29 - April 4, 2009), so that a report on the first year of TECHQM activities can be presented at that conference.

The meeting closed with the announcement that a summary of the results of this workshop and a call for joining TECHQM will be widely distributed as soon as possible, and that a first phone meeting for the members of the Working Groups will be held at the beginning of June, before the Hard Probes 2008 Conference.