What’s That at the Top of This Page?

The graphic chosen years ago as the header for this blog is an event display from the UA1 detector in 1982, of historical importance since it was the first event found with a W candidate. To be honest, the reason it’s there is that I was looking for something quick to use at the interactions.org Imagebank, figuring these were graphics I could steal without getting sued. What I ended up with is a cropped, lower-resolution version of the much better image available here.

Even stripped of identifying info, UA1 experimentalist Jim Rohlf of course recognized it, and recently wrote me to tell me some more about it. He also tells me that he will soon be blogging at Quantum Diaries, and I look forward to seeing that. So, here’s the story behind that image:

The collision is Run 2958, Event 1279 and was the very first W candidate that was found. It was recorded in the Fall of 1982 with UA1. As a newly minted junior faculty member and CERN scientific associate, I was resident at CERN and the first round of the W event selection and analysis was completed during the CERN holiday shutdown. On 23 January 1983 we submitted the W discovery paper for publication (Phys. Lett. 122 B, 103 (1983)). The details of the events were given in this paper. Within a few days, I got a letter from Lev Okun who had become a good friend of mine due to his frequent visits to CERN and his great interest in the working details of our experiment. In this letter which was several pages long he referred to this event as a “monster” because it decayed in the “wrong direction” and asked if we could have made a measurement error. Then the obvious hit me instantly- nobody had thought of this before- we don’t measure the longitudinal momentum of the neutrino due to the singularity in the direction of beam pipe but we can solve it to a quadratic ambiguity knowing the W mass. Furthermore, I saw that the kinematics of a 80 GeV object being produced with a relatively low cm energy of 540 GeV gave a remarkable result: often one of the 2 solutions was kinematically forbidden and when it wasn’t, the two solutions were often close together. Therefore, we could solve for the longitudinal momentum of the neutrino and be able to transform to the rest frame of the W. Since the W was polarized because it was produced in proton-antiproton collisions, we could measure the angle of the decay wrt the spin direction. Very simple idea, but be the first to do it and it becomes interesting and fun. I immediately wrote this up as a UA1 internal note in which I acknowledged the contribution of Okun. This technique subsequently became a standard at the Tevatron and now at the LHC.
In the following months, we collected more data and the next international conference to come along was at Fermilab and I was told by Rubbia to give the talk which was published (J. Rohlf, “Physics at the Proton-Antiproton Collider,” Proceedings of the 12th International Conference on High Energy Accelerators, Fermilab, 619 (1983). I reported the first measurement direct observation of parity violation in (real) W decay and measurement of the spin. I attach a slide from a talk I gave at Fermilab 20 years later in 2003 where I pulled up some of my 1983 slides. (Notice I fit the W mass to 2% and got the right answer.) You can see the “monster” event in the bin at cos(theta) =-1. This W decayed in the wrong direction. We went on to collect about 300 W events in UA1. We never saw another one go in the wrong direction. We also could not find anything wrong with that original event 1279. So you see the event was “not even wrong”.

Update: A copy of the talk slide that Jim Rohlf refers to is here.

Update: Jim Rohlf’s blog at Quantum Diaries is now up here. His first blog entry is great, it’s about, independent of the Higgs issue, the fundamental problem the LHC hopes to investigate: what is causing electroweak symmetry breaking? He emphasizes that one way to study this is to try and see the self-interactions of Ws and Zs, which become strong at the TeV scale.

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14 Responses to What’s That at the Top of This Page?

  1. Chris Austin says:

    With reference to solving for the longitudinal momentum of the neutrino up to a quadratic ambiguity, I raised this recently here after Tommaso said that we only know the transverse component of the neutrino momentum. But from Jim Rohlf’s message, apparently the longitudinal momentum of the neutrino is sometimes measured this way. Is it possible to clarify when this can be done?

  2. B Experiment At Small Theta says:

    Very interesting! I recognized the image as one of the UA1 W events too, although I did so from having previously dug around on CDS for images from the infamous megatek
    (I wasn’t born when that event was read out!). You can see the beautifully rendered event displays these things produced here: http://cdsweb.cern.ch/record/1049887

    would it be possible for you to host the slide Jim sent you (pending his permission?) I’d love to see more.

  3. Peter Woit says:

    B experiment,

    He didn’t send me the image, it’s always been hosted at Imagebank, and you can access it there. There may be some other source of UA1 event display images, but I don’t know anything about this.

  4. Jim Rohlf says:

    Directly we measure only the transverse component of missing energy because hundreds of GeV can escape down the uninstrumented beam pipe. However, for the case of W –> e nu, mass of the parent particle is known and we have m**2 = E**2 – p**2 (where E is electron plus neutrino energy and p is electron plus neutrino vector momentum). This is a quadratic equation for p_z , the unknown longitudinal momentum of the neutrino which has TWO solutions. Now visualize where the neutrino can go to see the two solutions conceptually. If the electron has energy nearly half the W mass and is produced nearly at right angles to the colliding beams, then the neutrino must recoil opposite the electron and have very little p_z. If on the other hand the electron is going forward (like the monster event), then to make the W mass the neutrino could be also going forward and have large energy so that p_z plus the electron p_z exceed the proton energy (kinematically forbidden) or it can go at large angles and have smaller energy.

    I think B Experiment was asking about the W angular distribution I sent you which you may post if you want.

    B Experiment:
    I have tons of events displays because I never throw anything out. I am at Fermilab now but when I return to Boston I may find some time to dig up some more pictures of this famous event.

  5. Chris Austin says:

    Jim, thanks very much for the explanations. To summarize when the two solutions coincide, (I think you described a special case for simplicity), if M is the W mass, p and q are the neutrino and electron transverse momenta, and c is the cosine of the angle between the neutrino and electron transverse momentum 2-vectors, the solutions coincide when M^2 = 2 p q ( 1 – c ). I guess with enough statistics, “lucky” events, where this is approximately satisfied, could be selected to improve the mass resolution of some of the Higgs searches.

  6. Peter Orland says:

    Hi Peter,

    You should substitute “what’s” for “what”.


  7. Peter Woit says:

    Thanks Peter! Fixed.

  8. Bernhard says:

    This story is really interesting, is nice to know more about how it happened and the REAL persons involved with it. I had enormous trouble reading “The God particle” from Leo Lederman, because he gives the impossible impression he almost did everything by himself, had all the ideas, and therefore got the Nobel. I immediately thought “but who was the postdoc (or “newly minted junior faculty member “) who really did the work anyway?”. Same thing with the W discovery. Only now, I know who did it. Not to say John did it alone too, but reminds us how giving Nobel prizes to individuals for experimental collaboration work is totatally bogus.

  9. Bernhard says:

    Sorry, I meant Jim, not John.

  10. Pingback: A massa do W… o início. « Ars Physica

  11. SpearMarktheSecond says:

    Great to hear from Jim Rohlf, he is a gem.

    `Nobel Dreams’ by Taubes describes the subsequent discovery of Supersymmetry by UA1 and UA2.


  12. Jim Rohlf says:

    What Higgs? Only joking (well, sort of). At the LHC, Higgs searches in fact do select “lucky” decays by asking for the charged lepton + neutrino transverse mass to be an appreciable fraction of the W mass. These lucky decays BTW are not so improbable because a 2-body decay makes the so-called Jacobian peak which is just a solid-angle effect. This is the reason the W mass can be determined so well from only measuring the charged lepton. I was truly thrilled when I first understood this!

  13. Jim Rohlf says:

    The problem with the Taubes book is he arrived way too late to document the W and Z… at least I got to be the Prince. For the record, UA2 was not part of the SUSY false alarm which was largely propagated by theorists. I stand behind the two major talks I gave on the subject at ICHEP Leipzig 1984 and APS Eugene 1985.

  14. Anon says:

    The header image contains a good unintentional example of the parallel lines illusion http://www.moillusions.com/2006/04/parallel-lines-illusion.html

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