Brief Discription of what the various job types are doing, please.


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Profile Ray Murray
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Message 15655 - Posted: 12 Nov 2013, 20:59:03 UTC
Last modified: 12 Nov 2013, 21:00:25 UTC

The job types I have seen or seen mentioned are;

Comix
Sherpa
Herwig
Vincia
Pythia 6
Pythia 8
Epos

Crystal Pellet has copied a description of EPOS, but I don't know where that has been taken from.
Could we get a similar brief description, please, (dumbed down for the layman if possible) of what the other job types are doing.

Profile Ray Murray
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Message 15658 - Posted: 13 Nov 2013, 23:26:26 UTC
Last modified: 13 Nov 2013, 23:37:39 UTC

The best guy to deal with this, Peter Skands, is not back in the office until 30th November. Hopefully, if his desk hasn't been piled too high during his absence, he will be able to give us some idea of what's going on inside the different flavours of model.

Profile Peter Skands
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Message 15762 - Posted: 14 Dec 2013, 12:52:17 UTC - in response to Message 15658.

Hi Ray,

Apologies for the long wait. Your question is very good. In fact, I think we should write something about this on the T4T homepages, so I've started putting a little page together about that, hopefully ready within a few days. I'll post a link here when it's up!

Peter

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Message 15768 - Posted: 17 Dec 2013, 9:42:50 UTC - in response to Message 15762.

Related to this: are T4T simulations LHC-specific or are they useful for other colliders, including future colliders like CLIC and ILC?

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Message 15769 - Posted: 17 Dec 2013, 10:04:27 UTC - in response to Message 15768.

Hi yo,

In brief, they are useful for both LHC and for future collider studies. Similarly, we run comparisons not only to LHC but also to a number of experiments at older accelerators, like Tevatron, SPS, LEP, and SLAC. For the specific purpose of making guesses about what things might look like at future colliders, we look at the energy scaling between the previous ones, and extrapolate, so this is quite an important part to get right.

Ray, sorry I didn't get the page up yet. Working on it ...

Peter

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Message 15770 - Posted: 17 Dec 2013, 13:25:52 UTC - in response to Message 15769.

Hi Peter,

Thanks for the fast reply :)

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Message 15833 - Posted: 7 Jan 2014, 7:08:06 UTC
Last modified: 7 Jan 2014, 7:12:07 UTC

Hi there

Like to add one to the above list:

Phojet

(appears rather seldom, just seen one after several months)

Greetings,
Bylo

Profile Peter Skands
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Message 15899 - Posted: 27 Jan 2014, 13:07:26 UTC - in response to Message 15833.

Hi,

Time to apologize again for the slow motion in getting these descriptions ready for you. Christmas and a busy January filled with travel are my excuses this time. All of February I will be at CERN though, working away in my office, so should get time to write these. Just to let you know I did not forget my promise to give some slightly more elaborate explanations about each of the sim packages.

Peter

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Message 15900 - Posted: 27 Jan 2014, 20:13:04 UTC - in response to Message 15899.

Thanks for that update, Peter.
It's possibly a tough assignment, to get the wording right to be of interest to more learned contributors while not being too far over the heads of the interested but not qualified lay-person, like myself.

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Message 16043 - Posted: 25 Feb 2014, 11:54:46 UTC

Peter,I just saw your name on a job running in my old BOINC_VM window and this reminded me of your promise to explain us what all those jobs are doing. I am running also VBoxHeadless on another PC but there I see nothing. Maybe it is my fault, since it is running in a Ubuntu 10.30 Virtual Machine, upgraded to 11.1 during installation on my main Linux box with SuSE 13.1, whose VirtualBox 4.3.6 is not seen by T4T, and I am still a Ubuntu dummie.
Tullio

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Message 16614 - Posted: 28 May 2014, 8:36:34 UTC

I have downloaded a CERN Openlab 56 pages PDF document, a Whitepaper on future IT challenges in scientific computing but it had no information on what we are trying to do.
Tullio

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Message 16615 - Posted: 28 May 2014, 9:03:17 UTC - in response to Message 16614.

I have downloaded a CERN Openlab 56 pages PDF document, a Whitepaper on future IT challenges in scientific computing but it had no information on what we are trying to do.
Tullio


Hi Tullio,

This document is supposed to suggest joint projects for CERN and its Openlab industrial partners to work on over the next 5 years, with possible collaborations with some other European labs. It doesn't seem to be a suitable environment for work on volunteer computing, due to the industrial component. (I talked to its main author yesterday about this).

Thanks for your post,

Ben

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Message 16616 - Posted: 28 May 2014, 9:21:38 UTC



CERN openlab publishes a whitepaper on future IT challenges in scientific research


____________

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Message 16617 - Posted: 28 May 2014, 12:39:49 UTC - in response to Message 16615.

Ben, I know that industry is going the cloud way. In France, Atos has just bought my old employer Bull.
Tullio

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Message 17011 - Posted: 23 Jul 2014, 5:11:10 UTC - in response to Message 16617.

Hi,

Here is a link to a post that (hopefully) answers the original question....

http://lhcathome2.cern.ch/vLHCathome/forum_thread.php?id=1325&postid=15098

Perhaps this could be copied to the main web page to act as the introduction to the project..??

Thanks, Jay

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Message 17012 - Posted: 23 Jul 2014, 7:34:47 UTC - in response to Message 17011.
Last modified: 23 Jul 2014, 7:35:08 UTC

Yes, but we wanted something more specific. I am seeing all kinds of CERN jobs, such as sherpa, herwig, etc, and I would like to know something about them. Peter Skands had promised to give us at least a summary.
Tullio

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Message 17027 - Posted: 26 Jul 2014, 1:32:16 UTC
Last modified: 26 Jul 2014, 1:32:31 UTC

I see that FastJet is using a release 3.0.3 instead of 2.4.4. Any information on this change would be welcomed.
Tullio

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Message 17029 - Posted: 26 Jul 2014, 19:04:02 UTC - in response to Message 17027.

I see that FastJet is using a release 3.0.3 instead of 2.4.4. Any information on this change would be welcomed.
Tullio

Exactly 1 year ago Peter Skands wrote:

With respect to FastJet, which is an analysis package (it contains nice algorithms that cluster particles into "jets"), we currently use version 2.x (2.4.4), and there has indeed been a major upgrade to 3.x. Again, due to the nontrivial nature of the changes, it will take a bit of development work on our side to integrate the new version. Now, for our current needs, 2.x is still providing all the functionality we require for all the analyses we run in T4T, so upgrading is not urgent. But eventually, say a year from now, there are likely to be new experimental analyses which will make use of the additional functionality that was introduced in FastJet 3.0. At that point, we would not be able to run those analyses in T4T unless we upgrade. So this is something we are aiming to do over a longer timescale, say over the next year.

Great clairvoyance, Peter!

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Message 17035 - Posted: 27 Jul 2014, 10:40:05 UTC - in response to Message 15900.

Time for the promised and long overdue post about what the various job types are doing.

All the jobs we run in T4T have two main components: an event generator (which simulates particle collisions) and an analysis framework (which emulates the measurements performed on each simulated event). In T4T, the analysis framework is in all cases RIVET, which contains a large library of encoded analyses most of which correspond to real-world measurements that have been performed in real experiments, and which RIVET allows us to repeat on simulated events, for comparison. (In addition, we also run a few analyses that represent pure theory-to-theory comparisons.) RIVET is therefore essential to what we call data and analysis preservation, in that it allows us to compare old measurements to the newest state-of-the art calculations, encoding all the subtleties of the original measurement in a unified framework preserved for the future. That's all I will say about RIVET here, as this post is mainly going to be about the event generators. Currently, we run ones called ALPGEN, EPOS, HERWIG++, PHOJET, PYTHIA 6, PYTHIA 8, SHERPA, and VINCIA.

We distinguish between 'general-purpose' event generators and more specialized ones. As the name implies, general-purpose generators are supposed to be able to handle many (ideally all) different physical process types, and be able to take care of all aspects of the event simulation (apart from the analysis step, handled by RIVET). These are thus the main workhorses for particle physics simulations. The three most widely used general-purpose generators in high-energy physics are HERWIG, PYTHIA, and SHERPA. As you see above all of them are represented in T4T, and one of the main uses of the http://mcplots.cern.ch web site which relies on T4T for its computing is cross-comparisons between these generators. To give an idea of just how widely used these generators are, a recent check of physics-journal publications from the LHC experiments ATLAS and CMS revealed that fully 80% of them cite PYTHIA, for instance, with smaller but still substantial fractions quoting HERWIG and SHERPA (see also this link: http://www.symmetrymagazine.org/article/may-2013/the-top-40-physics-hits-of-2012). What these generators actually do is somewhat complicated to describe, but here goes.

Starting from a fundamental scattering process, like the production of a Higgs boson or the like, the generators contain algorithms for 'initial- and final-state radiation' (bremsstrahlung, also called 'parton showers', caused by the violent acceleration or deceleration of charged particles taking part in the fundamental scattering process), particle decays (like the decay of a Higgs boson into two photons, one of the most important discovery channels for it), additional interactions between the 'beam remnants' (called 'multiple interactions'), and hadronization (the process by which 'partons' - quarks and gluons produced in the fundamental scattering or by parton showering - turn into hadrons like pions or other mesons and baryons).

Each generator contains slightly different approximations to these processes, and had slightly different historical origins, as follows: the oldest of the programs is PYTHIA, which started in 1978 and was then called JETSET (now obsolete and replaced by the PYTHIA label). It has been developed mainly by a research group based at Lund University in Sweden, together with researchers at Fermilab and CERN. The first version was a FORTRAN program small enough to fit on a couple of printed pages and contained an algorithm that described the 'fragmentation' of a 'fast parton' into a 'jet of mesons', corresponding to the 'hadronization' step mentioned above. Although the program has grown vastly since then (the newest version, PYTHIA 8, is about 100,000 lines of C++ code, though the last version of its FORTRAN predecessor, PYTHIA 6, is also still quite widely used), the modern-day PYTHIA generator still places a main emphasis on hadronization, and it is probably fair to say that it contains the most sophisticated model for that aspect, based on the so-called Lund String Model, which combines concepts from classical string theory (not superstrings!) with quantum tunneling.

Next on the scene came the HERWIG generator, started in the early eighties by researchers based in the UK (mainly Cambridge, Durham, and Manchester), also originally written in FORTRAN but now succeeded by a C++ version hence the name HERWIG++. The advent of HERWIG was tied to significant advances in the theoretical understanding of the process of 'parton showering', another of the steps mentioned above. Notably, the HERWIG model was constructed to give an improved description of coherence effects between different charged partons (such as when a positive and negative charge 'screen' each other), and was combined with a different (simpler) model of hadronization, called the 'Cluster Model'. For most of the eighties and nineties, any difference in the results obtained with PYTHIA and HERWIG was taken as a measure of the theoretical uncertainty surrounding each particular distribution. The modern equivalent of this is why it is so important that we don't just run a single generator in T4T.

Finally, improvements in the 00s on our ability to calculate the fundamental-scattering process itself (and the slow pace of moving from FORTRAN to C++ by the other generators) led to the advent of the SHERPA model, a brand new generator written from scratch in C++ and including a sophisticated algorithm for incorporating systematic quantum corrections to the hard-scattering process. This was initially combined with quite simple models for parton showering and hadronization, but SHERPA has now since quite a few years been developed to a full-fledged generator with state-of-the-art algorithms for all aspects of the physics modeling. This group was initially based in Germany (it originated in Dresden) but is now also strongly represented at Durham University, in the UK.

In addition, there is a very large number special-purpose generators, which focus on just one aspect of the physics modeling, with interfaces to the general-purpose ones for handling the remaining aspects. In T4T, the ones represented are ALPGEN, EPOS, PHOJET, and VINCIA.

ALPGEN is developed mainly by a research team based at CERN and in Italy. It focuses on improvements to the hard-scattering process similar to those done by the SHERPA generator, but with somewhat different underlying calculational methods. It is interfaced to either HERWIG or PYTHIA for parton showering and hadronization.

EPOS is a relatively new generator, developed mainly by French research teams, and it has a much broader range of application than 'just' the kinds of collisions represented in T4T. In particular, EPOS is developed to be able to handle also 'heavy-ion' collisions and cosmic-ray interactions in the atmosphere, both of which entail modeling not only proton-proton collisions but also nucleus-nucleus collisions. Nevertheless, it also does apply to proton-proton (pp) collisions, which is a main calibration point for it, hence it is interesting to compare it with pp measurements from the LHC and earlier accelerators, as done in T4T. Its main hadronization model is string-based, similarly to PYTHIA, but it also incorporates hydro-dynamics inspired models required to describe the so-called quark-gluon plasma phase of heavy-ion interactions.

PHOJET is another generator that has very old roots, mainly from the German physics community. It is written in FORTRAN and uses a special kind of description of relatively low-energy ('soft') interactions called Regge Theory, which also has roots in string-based models, and it is interfaced to PYTHIA for hadronization.

VINCIA is the youngest of the generators, having started in 2007, and is written as a C++ plugin to PYTHIA 8. It began as a collaboration between researchers at Fermilab (Chicago) and Saclay (Paris), and now also has a main base at Monash University in Melbourne. It focuses on the combination of the fundamental-scattering process with parton showers, and aims to simultaneously improve the precision of both using a different scheme than those of ALPGEN and SHERPA. It interfaces PYTHIA 8 for hadronization.

OK, that was a brief summary of the main emphasis and historical/geographical origins of each project. Going into details about each simulation component would require a (much) longer post, but I hope nonetheless these comments may be useful to people wanting to understand a little better which programs we run, and why there are so many of them. All contain different approximations, and attempt to reach the state-of-the-art through slightly different paths. Some do 'everything' while others only do specific aspects.

All the best ,
Peter

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Message 17036 - Posted: 27 Jul 2014, 11:54:32 UTC

Thanks Peter. I see them running on 2 Virtual Machines and they rarely fail, so they must be good. I have only a problem with BOINC on this host, which houses another Virtual Machine, Ubuntu, and when this runs the BOINC task is suspended while the WM still runs, so MCPlot increases but I get no credits, but this does not matter.
Tullio

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Message boards : Science : Brief Discription of what the various job types are doing, please.