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    The Slides Of My Seminar For LHCb - Part I
    By Tommaso Dorigo | June 15th 2009 12:21 PM | 18 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    Tomorrow I am traveling to CERN, where I have been invited to give a seminar at a meeting of the LHCb experiment. My talk will discuss the issue of the energy calibration of b-quark jets, a topic to which I have devoted a good part of my research time for the last thirteen years. The talk will of course be centred on the explanation of the analysis Julien Donini and I, together with a few colleagues, performed in CDF a few years ago, the search for Z boson decays to b-quark jet pairs.

    I have in the pipeline a simpler article than the present one, where I explain why b-quark jets are special, and what makes them so important at hadron colliders (it is all about the Higgs, the top quark, and new physics searches, as you will learn in the next piece). Here, however, I wish to get away with just a slide show: I am offering to you a preliminary version of the slides of my seminar, with minimal commentary. Of course, many of you will find the slides unintelligible, and I cannot do much to prevent that. This is thus an "experts only" post, but if you hang around anyway you might find something interesting and understandable in it nonetheless... So here we go.

    The Talk Slides



    Slide three (well, yeah. Slides one and two are just a header and a summary...): the Tevatron collider. Here I want to give some information on the running performance of this incredible machine, and point out that if it continues to perform as it is presently doing, a first evidence of the Higgs boson might be at reach of CDF and DZERO.


    Slide four: Just a descriptive summary of the CDF detector and its history.


    Slide five: some detail of the important parts of CDF. From top to bottom the pictures show the silicon inner detector, a drawing of CDF with the inner region exposed (the tracking system is inside the red layer, which is the electromagnetic calorimeter), and the central drift tracker.


    Slide six: the trigger system of CDF. Here the three-level architecture of the system is discussed. The trigger, in case you are wondering, is the system which decided online which collisions to filter and save to mass storage.


    Slide seven: SVT is the heart of the Run II CDF trigger system. With it, b-quarks are collected with high efficiency. On the left a cartoon shows the function of the SVT associative memory bank, which matches patterns to the reconstructed track segments. The center plot shows the large sample of hadronic B meson decays, which among other things granted CDF the first measurement of B_s oscillations; on the right, the inventor of the device, Luciano Ristori. Incidentally Luciano has recently been awarded the Panofsky prize for the SVT, together with Aldo Menzione.


    Slide eight: just an introduction on hadronic jets and their reconstruction. The cartoon on the right shows what happens when protons and antiprotons collide: a parton jet arises on one side of the event, and particles are created, which then interact and are destroyed in the calorimeter, where we measure the energy of the jet, and try to figure out what the energy of the originating quark was.


    Slide nine: here I make a couple of points about the goals of jet clustering.


    Slide ten: this is a roll-out display of the CDF calorimeter. In the grid -so-called eta-phi plane, basically a two-dimensional plane constructed on two angles describing the outgoing direction of the particles- the towers show the energy deposits from an energetic event. As you see there are clusters of energy scattered around, and different clustering algorithms "see" -interpret- these clusters as jets in different ways.


    Slide eleven: some details of the algorithm of choice by CDF in Run II.


    Slide twelve: an explanation of the algorithm that corrects the measured jet energy to make it match as closely as possible the energy possessed by the parent quark which originated it.


    Slide thirteen: a few plots that describe how the response of the CDF calorimeter to jet energy is equalized for disuniformities as a function of angle of incidence (bottom left), and how the study of the calorimeter response to single particles (top right, single particles in data; and center right, pions from test-beam runs) allows to tune the simulation, which is then used to get an absolute correction function (bottom right).


    Slide fourteen: this graph shows the energy behaviour of the biggest sources of systematic uncertainty on jet energy measurement. As you can see, at low energy the out-of-cone uncertainty -the unknown fraction of energy that flows out of a cone of fixed R=0.7 radius- dominates all others.


    Slide fifteen: this introduces the issue of b-jets, and why they are different. The graph demonstrates that the fraction of energy of a b-jet which is measured in the calorimeter (in blue or red) is smaller than for generic jets (the black curve).


    Slide sixteen: some more detail on the various sources of uncertainty from the b-jet energy scale, and the impact on the uncertainty on the measured top quark mass (which uses the b-jet energy measurement, of course).


    Slide seventeen: still some detail on generator-level issues -herwig and pythia, the two simulation programs, possess some tunable parameters which have an impact on the measured b-jet energy.


    Slide eighteen: the way the b-quark originates a stream of hadrons as it "fragments" is well-known from studies performed at LEP; however, some uncertainties from the modeling of fragmentations still exist.


    Slide nineteen: the pattern of possible decays of B hadrons has a clear effect on the fraction of energy that we can measure from a b-quark jet: that is because we do not measure neutrinos -which escape unseen- and also muons provide a small calorimeter response; electrons, on the other hand, over-compensate.


    Slide twenty: the Z decay to b-quark pairs was found already in Run I at CDF -I should know it, I did it by myself for my PhD thesis! This is just an introductory slide of the problem. On the top right you can see a figure of historical value: it is the first publication which measured the top quark mass, by CDF in 1994 -when the top quark had not been discovered officially yet. On the picture below the plot, you can see the detector which made it possible, the SVX -a four-layer barrel of silicon microstrip sensors.


    Slide twentyone: this shows the signal I found in 1998 from the 100 inverse picobarns of Run I data. The plots on the right show two different means of extracting the signal: a counting experiment (top), and a unbinned likelihood fit (bottom).

    The other twentyone slides will be posted tomorrow...

    Comments

    You are boring sometimes, aren't you?

    Hank
    Science blogging needs more boring people who just know what they are talking about.   If you prefer controversy, hype, politics, ideology, cultural grandstanding and ridicule in your science writing, of course there are better places to go.
    I hope you're not talking about Cosmic Variance... ;)

    Hi Tommaso, could you please explain why the Tevatron apparently did all the B physics in the CDF (and D0?) detector while there is a new detector at the LHC? Why the change? What advantages the LHCb has relatively to ATLAS or CMS to do any of these things? Aren't these advantages also classifiable as differences that make your CDF experience much less relevant for the LHCb people?

    What do you think, how many readers of yours know the answers to the questions above (if I essentially don't), and consequently, how many readers may find the posting useful? Is the expectation value 2? 0.02? Or is your theory that the Survivor has superseded hep-ex on the arXiv and Nuclear Physics? ;-)

    dorigo
    Hi Lubos,

    LHCb will be able to study highly boosted B hadrons produced in fixed-target collisions. This grants a large advantage compared with collider mode, since B hadrons usually travel a centimeter or so before decaying, allowing a much more effective reconstruction. Also, the detector is optimized for b physics. All in all, LHCb is expected to provide more precise measurements of phases, angles, mixing parameters -all the hot topics in B physics- than Atlas and CMS. Not orders of magnitude better though, just a few times better.

    About why LHCb wants to hear about my CDF insight in b-jet energy scale, well... I think that is because LHCb will do other physics too, much like Atlas and CMS will do heavy ion physics (with a precision comparable to that of Alice, if not better). The strength of the LHC physics program is that it is slightly redundant, and this is quite healthy for investigations of an energy regime that, once SUSY is definitely shown to not be there, will be not accessed by other accelerators for decades.

    About the expectation value: this post was for experts... I do think it is useful to share things outside of the normal distribution channels. First of all, there are people outside HEP who do understand this stuff, and it is a useful thing to feed them with digestible information. Second, this blog is read by some colleagues, and not everybody knows what everybody else is doing...

    Best,
    T.
    Thanks for your answer, Tommaso. Redundancy is great, as long as no one has any problem to pay for it. ;-)

    "[T]his is quite healthy for investigations of an energy regime that, once SUSY is definitely shown to not be there, will be not accessed by other accelerators for decades."

    In other words, the spectrum of the detectors is optimized for incorrect (and also uninteresting) expectations about the physics beyond the Standard Model, such as the expectation of a non-existent SUSY? ;-) More invariantly, I think that if no SUSY and no physics of a comparable revolutionary charge is seen in the LHC energy range, the further investigation of the Standard Model will become literally pointless. I would certainly not recommend to pay billions of euros if it were clear that the only possible result would be a 3x smaller error margin in W mass or other SM parameters. The LHC will be fine just with the Higgs but running it for 5-10 more years after the Higgs is observed, while knowing that nothing qualitatively new will come out of it, looks like a waste of money.

    These precise numbers are not important for applied purposes, and they would only be interesting theoretically if we could calculate them from the first principles, which is likely to require a theoretical progress, not an experimental advance, and such a theoretical progress can only be stimulated by a qualitative experimental development, not by a marginal and structureless increase of the precision of known things.

    You don't have to defend the right to write for experts. Still, I am curious (also because of similar dilemmas that I often face) about your estimate of the number of experts who actually read this information here. You know, among the 1,000 mostly lay readers of yours a day, the previous sentence requires a couple of very stringent conditions to be satisfied at the same moment:

    1) they must be experts, probably professionals
    2) they must nevertheless know about these things less than you do
    3) they must be sufficiently interested in this particular topic from this perspective
    4) they must have seen or looked for no competing recent articles about a similar topic

    When it's computed, I guess that the resulting number is below 1. What's your estimate?

    LHCb isn't a fixed target experiment. The clue is in the name, dumbass, Large Hadron *COLLIDER*!

    Dear Anonymous, I am afraid that you shouldn't underestimate Tommaso that much - after all, CERN and the Fermilab are hiring hundreds of people who are really very similar to him. So many of his incorrect statements must have a seed of truth in them.

    If you Google search for three words, fixed target lhcb, the first link goes to CERN Courier

    http://cerncourier.com/cws/article/cern/35880

    which talks about the three projects of 1998 to extract B-physics from the LHC, two of which are fixed target - by stealing a part of the beam.

    On page 4/23 of this talk,

    http://www-zeuthen.desy.de/students/2008/lectures/lhc_physics_part1_LHC_...

    it is said that the LHCb looks like a fixed target experiment (although it is not) but only with the forward region covered. So I hope that Tommaso will re-learn all details that he is still missing and give us a final explanation of the LHCb strategy here.

    dorigo
    Yes, my comment was hurried and I merged two sentences. Of course it is true, LHCb sits at a point of intersection of the beams like the other three experiments, so it sees pp collisions, but it is built like a fixed target experiment, and it effectively works like one. That is because when you select collisions yielding stuff flying all in one direction, that is due to a very energetic parton in one proton hitting another one basically at rest.

    Lubos, sorry but today I have been traveling from Venice to Zurich, and then Geneva - so yes, my answer was deceiving since I did not take the usual care. About the strategy, I already told you. Normally collider detectors avoid handling the very forward direction, because the design of detectors optimized for central, high-Pt physics is totally different from the design of those doing forward physics.

    Maybe I will make a post about the LHCb if I find the time... It has a quite interesting program of B physics and CP-violation studies.

    Cheers,
    T.
    dorigo
    Oh, but I do not thing any of those statements hold, except maybe 3).

    1) as I said, I know for a fact that there are non-professionals who read even the toughest HEP posts. They are not very many, but they are very valuable to me. They give me the motivation to keep writing to them.
    2) that isn't true either. In fact, I love it when I manage to get a discussion started from which I learn something. This has happened several times in the past.
    4) usually people hang around blogs just because they like to do it, and if they have read about something recently, they want to compare what they learned with what others write...

    Cheers,
    T.
    And now Motl, who has been trumpeting about the imminent unavoidable discovery of susy, and who bet how muc?h, $1000 on that, admits that after all it's not guaranteed.

    dorigo
    I wonder what kind of personality would emerge if we considered all anonymous comments appeared in my blog threads together. Admitting for a second they all come from the same entity, they would have to be a schizophrenic genius with zero social skills, a history of violence in their youth, and a lot of time in their hands. Definitely a person worth spending some time with! That is why I think anonymous comments add value to a blog after all... Here, poor Lubos gets bugged because he said sensible things. Maybe they prefer Lubos the enfant terrible to this grown-up variation.
    T.
    Motl gets bugged not because he said sensible things now, but because he said nonsense in the past, and his change of opinion allows me to highlight that nonsense.

    And of course you have more than one anonymouses on your blog. for instance, it was not me who made the first comment about LHCb not being a fixed target experiment.

    dorigo
    Dear Anon, changing one's mind is hardly a blemish. In fact, only fools never do it.

    Cheers,
    T.
    let's just not jump from here to a conclusion that those who first form a poorly informed but strongly held opinion, and then change it, automatically are exempted from being fools.

    I'm aware of how bad in terms of netiquette it is to comment days after the last relevant comment, but you have to excuse me and the power of weekends...
    I'd like to repeat something I've written months ago about the "target audience"; I'd prefer to avoid it cause it's a bit awkward, but maybe this is the most appropriate time after all. For a field of science which is largely based on transfer of experience on a person-to-person basis and which even lacks standardised educational literature (yes, it's experimental high-energy physics I'm talking about), this blog is of enormous help to phd students - especially those who haven't been privileged with a well-structured program of relevant grad courses.
    So what can I say... keep it up!

    dorigo
    Well Tulpoeid, I think I remember you did say that some time ago, but it just feels good to hear it over again every once in a while. As strange as it sounds, sometimes I lose steam and need to regain some motivation to write insightful pieces rather than more "ordinary administration" ones.

    Cheers,
    T.