Wednesday, December 01, 2004

LHC is running

A rather large group of phenomenologists at Harvard, led by Nima Arkani-Hamed, spends one hour every Wednesday by analyzing the data from the LHC.



Today, they (or we) were making sure that the spins of the particles are what they should be. How can you tell that the Z boson - that has clearly been seen at the LHC, according to the graphs - is a spin one particle, as opposed to spin zero particle?

Well, the spin zero particles have a uniform angular distribution. The spin one particles, on the other hand, have different distributions as a function of the angles.

When the LHC creates a Z, it's almost never the "longitudinal" polarization parallel to the incoming momenta - it's because you must combine the opposite helicities from the two quarks contained in the proton. Consequently, the spin of the Z boson is always aligned with the quarks - their spin can never cancel. You can see the parabolic profile of the angular distribution even if you assume that 50% of the Z-bosons have one spin, and 50% have the other spin.

Analogously, we have also found some Z' (Z prime) particles, showing that they're spin one.

The next task was to identify some superpartners that the LHC has created for us - roughly 250 events. How can you say that the stop has spin zero? In fact, there are many differences between the bosons and the fermions.

The pair-production of the stop squarks has roughly 5 times larger cross section than for two top quarks - you may imagine that the fermions "repel" by the Pauli exclusion principle, and they correspondingly have a lower effective phase space.

Also, right above the production threshold, the cross section for producing the fermions goes like "beta" while the cross section for producing the bosons goes like "beta cubed", because of some extra derivative couplings for the boson.

The graphs showed relatively clearly that the stop squark was a spin 1/2 particle, and it decayed to the top quark plus a bino - which seems to be the lightest supersymmetric particle, as far as everyone sees. Some things are very easy to see - note that the LHC is producing roughly 6 jets per event, and their opening angle is roughly 10 degrees.

There have been some other graphs without much structure. They suck because it's hard to analyze what particles were exactly produced in these events.

Summary: the phenomenologists at Harvard are ahead of the experimentalists - I guess that the experimentalists won't be able to finish their first analyses of the LHC data earlier than 2007. ;-)

10 comments:

  1. Lubos said:

    >The next task was to identify some superpartners that >the LHC has created for us - roughly 250 events.

    Lubos, are you saying there is clear evidence in the LHC data for superpartners already? That would be the biggest experimental descovery in decades and you are mentioning it like it's a given. What's going on with you?

    Best, Dan

    ReplyDelete
  2. I was hoping that the joke would be understood, Dan. ;-)

    These meetings of the phenomenologists are - so far - just children games. They're getting ready for the LHC, inventing different scenarios what can be observed and how the new physics will be identified. In reality, the LHC has not started yet.

    Best
    Lubos

    ReplyDelete
  3. You got me! Thanks, it was fun.

    I might add, I would have believed you're insane before I would have believed the superpartner story...

    Best,
    Dan

    ReplyDelete
  4. Hey Dan, thanks! Although some people still believe that SUSY will be discovered *before* I get insane. ;-)

    ReplyDelete
  5. Lubos,

    The graphs showed relatively clearly that the stop squark was a spin 1/2 particle, and it decayed to the top quark plus a bino - which seems to be the lightest supersymmetric particle, as far as everyone sees. Some things are very easy to see - note that the LHC is producing roughly 6 jets per event, and their opening angle is roughly 10 degrees.

    There have been some other graphs without much structure. They suck because it's hard to analyze what particles were exactly produced in these events.
    Can you place images of the graphs for what they might look like in your post?

    ReplyDelete
  6. Plato, how can you still not get the joke? I didn't quite get it at first, but you are more than stubborn, my friend. ;-))
    --Dan

    ReplyDelete
  7. Hi Plato!

    Dan misunderstands that the graphs DO exist. Rakhi (mahbuban@physics.harvard.edu) printed some of them. These graphs describe events generated at the LHC - except that so far all these graphs are produced by Monte Carlo simulations.

    All the best
    Lubos

    ReplyDelete
  8. Thanks Lubos,

    Yes Dan, you would have to know that there would have to be test runs of the situations before they are actually tried, and I was well aware of Lubos humour taking advantage of this situation.:)

    A group gather cramming for possible outcomes, is a good enviroment for predictions to be thought of as well?

    Lubos, has spoken in this context before?:)

    October 9, 2004 11:51 AM on Peter woits blogAlternatively, theorists won't be that fast, and the revolution will start experimentally in 2007, most likely with the LHC. A rather simple pattern of masses of superpartners will be found, together with supersymmetry, and it will match one of the popular SUSY scenarios within string theory. Alternatively, small black holes or excited strings are gonna be seen, and their precise patterns will be used to reversely engineer the shape of branes and hidden dimensions.This is a good thing, as one indeed has to be prepared.

    ReplyDelete
  9. Tried following your link Lubos and found Nima prep work intro instead.



    From arkani@jordan.physics.harvard.edu Thu Oct 21 17:09:19 2004
    From: arkani@jordan.physics.harvard.edu (Nima Arkani-Hamed)
    Date: Thu, 21 Oct 2004 12:09:19 -0400 (EDT)
    Subject: [Affiliates] LHC prep at Harvard


    Hi all,

    As you all know, the turn-on date for the LHC is fast approaching. A number of us here are getting increasingly interested in focusing on the challenge of sorting out the new physics at the TeV scale from the actual data we will get from colliders, but many of us aren't tooled up in the basics of collider physics neccessary to do this. So, we are organizing an informal in-house seminar series to get going!

    This won't be like a course, but much more hands-on. We will start with the basics, but then jump into what we will actually be doing with LHC data, and learn together by working through concrete examples of both
    Standard Model and new physics signals, discussing and thinking about interesting ideas that can be pursued along these lines.

    Our new post-doc, Lian-Tao Wang, will kick things off with an introduction today, at 2:40, in Jefferson 356. Starting next week, we will have the seminars on ednesdays at lunch in our regular seminar roon. Every week someone will present. Obviously the more the merrier! As some background material, the textbook by Barger et. al. (``Collder Physics") is useful. Also, the ATLAS collaboration technical design report (TDR) has many worked scenarios included, and I suspect we will go through a lot of them. The TDR can be found at
    http://www.atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/TDR/access.html.

    Everyone is welcome! This should really be terrific fun. Incidentally, people shouldn't feel that they have to come to every seminar to get something out of this--since we aren't following a course structure and
    won't usually be building on what came the last week, you can get something interesting out of it whenever you come.

    All the best,

    Nima

    ReplyDelete
  10. Woo-hoo! Harvard physics professors grapple with particle kinematics.

    I hear the Harvard economics department is holding a seminar on how to balance your checkbook.

    ReplyDelete