Plot Of The Week: A Lovely Dimuon Mass Spectrum
    By Tommaso Dorigo | July 27th 2010 08:21 AM | 17 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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    While everybody is busy discussing the latest Tevatron results on the Higgs boson searches -is that the light-mass excess the internet was abuzz, is it consistent with a signal as we expected it, how long will it take to confirm it is not a fluke, etcetera, etcetera, etcetera- I think I have a different plot with which to enthuse you.

    If you do not like the figure below, courtesy CMS Collaboration 2010, you are kindly requested to leave this blog and spend your time reading something else than fundamental physics. I do not know what will ever make you believe particle physics is beautiful, if not what is shown here.

    The figure shows, using a logarithmic scale on both axes, the reconstructed mass of pairs of muon candidates of opposite charge, collected by CMS in its first 280 inverse nanobarns of 7-TeV proton-proton collisions collected until a week ago. Nothing fancy has been done to prettify this graph: these are honest-to-god muon pairs, as Nature (the bitch, not the magazine) has produced them in the core of CMS. True, the intercession of a detector and a reconstruction software were needed to go from ionization clouds to event counts; but this is the absolute minimum of manipulation you can ever expect from particle signals.

    Now, what should enthuse you about the graph is the following. The distribution reveals, clearer than a million words could describe, the structure of all the most important bound states decaying by electroweak interactions into pairs of muons which we can produce in hadron collisions. We immediately spot the Z boson on the far right, and the towering peak of J/Psi mesons; but we also see Upsilon mesons, and at lower energy, we detect the ligher resonance decays of rhos, omegas, and phi mesons. What a spectroscopist's delight! This figure is tremendously informative! If we sent it to outer space, without labels or units, no intelligent race could ever mistake its meaning!

    You also notice that these jewels stand atop a background of unidentified muon pairs. Muons can be produced singly by the weak decay of kaons and pions, for instance, or even more massive states like bottom and charm. Occasionally, pairs of muons of opposite charge can emerge that do not have the same parent: the frequent production of these uncorrelated pairs creates the significant backgrounds you see in the picture. Note, however, how these backgrounds die out for large dimuon masses: the Z boson is basically background-free, a fact I have noted in my previous posting here.

    CMS and ATLAS have presented scores of interesting physics results at the international conference in Paris this week. None of those were groundbreaking ones; a few were significant advances, though, and many others were just meant to demonstrate that the experiments are ready for big challenges, such as discovering new physics, the Higgs, measuring the top mass better than the Tevatron, etcetera. The presented results took about a hundred man-years to produce, and I have a lot of respect for them -not to mention the fact that I did my little bit to contribute. But it is my humble opinion that the graph shown above could well be the one to single out and attach on the bulletin board of all the universities and institutes participating in the LHC experiments!


    OK, it looks like these are all the J,P=1- states. I think I almost understand that, a muon and antimuon in an s-wave can be 1- or 0-. But why don't we see the 0- states? Something to do with helicity?

    the bitch


    That plot is beautiful – makes me burst out in song!

    Not my usual Welsh or Neapolitan songs, though.   Rather the driving melody and rhythm of Stefani Joanne Angelina Germanotta (of whose music Hank does not approve.) 
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    "But it is my humble opinion that the graph shown above could well be the one to single out and attach on the bulletin board of all the universities and institutes participating in the LHC experiments!"

    I absolutely agree with you on this, and it will only become more beautiful with more integrated luminosity. I will need to keep an eye on the doors and bulletin boards of the CMS people down the hall to see if it goes up, and otherwise shaaaaame ;)


    Actually something has been done to this graph, as the quantized yield of counts surrounding the Z has a slope.

    Hi Ics,

    well, these are events per GeV. If the binning varies, so does the amount corresponding to one event.

    these are events per GeV
    Would that mean that between 20 and 30 GeV, say, there are 10 times as many bins as between 2 and 3 GeV?  With the logarithmic x-axis, were the y-axis linear, in order to provide an equal-area view** one would rescale to (events/GeV)*GeV; perhaps even here it would be better, so that (were the graph plotted on paper) a strip represented 1 mm on the x-axis here could be validly compare with a strip of 1 mm there.

    When I really boil over is where the scaling of the black body radiation curve is such that one cannot do an area comparison.

    ** I’ve been into map projections since I was a teenager
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Hi Robert,

    the way the binning is chosen is not easy to determine from the graph, although I suspect that it scales with the logarithm of the mass.  Here equal area means really little, since the production rate of these different states follows complicated laws. The log-log form is useful because it allows to plot on the same graph resonances with quite different shapes and rates (the width of the Z is equal to the spacing between the rho and the J/psi, to be clear).

    Is there also a tau pair curve? An electron-positron pair curve?

    Hi Claudia,

    excellent question! No, so far I have not seen any wide-range electron-positron mass distribution, only signals around the Z and J/Psi. Electrons at low energy are harder than muons to reconstruct with small backgrounds... Even harder are tau leptons!! For them, I have never seen a similar spectrum, and I suspect it is only possible for lepton colliders.

    and I suspect it is only possible for lepton colliders.
    Perhaps this sort of information may be found in work done with DORIS at DESY.
    Almost 300 metres in length, the DORIS storage ring (Double Ring Store) has been in operation at DESY since 1974. Originally designed as an electron-positron storage ring in which electrons collide with their antiparticles, the positrons, DORIS was used for particle physics research until 1992. In 1975, “excited charmonium states” were detected for the first time at DORIS – a discovery that gave birth to the study of the physics of heavy quarks. In 1987, physicists at DORIS discovered that particles called B mesons can transform into their antiparticles, at a surprisingly high rate. From this observation it could be deduced that the mass of the sixth quark, the top quark which was still missing at the time, must be much higher than previously assumed. The top quark was eventually detected for the first time in 1995 at Fermilab in the USA.
    Dear old DORIS is, however, now used as the (mainly X-ray) light source DORIS III
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Tommaso, thanks for posting the figure.

    As to the low-statistics high-energy region from about 100 GeV to about 200 GeV,
    there seem to be 4 peaks, which I have enlarged in a figure I put on the web at

    What is your interpretation of those 4 events?

    To me, number 4 around 160 GeV might be W+W- pairs,
    1 and 2 are around 120 GeV: maybe they are the same ?
    maybe my light Tquark ? - maybe Higgs?
    3 is around 130 GeV: maybe my light Tquark? - maybe Higgs?)

    As to the absence of any peak around 170 for the central Tquark mass value,
    maybe at that energy and for those processes there are too few events to show up ?
    Comparing the height of 4 around 160 for W+W- which might be produced in much
    greater quantities than the Tquark around 170 makes me think that any Tquark peak
    around 170 might be too low to show up.


    Hi Tony,

    there are no peaks. There are only events compatible with background, mostly due to drell-yan production in that energy range. For sure not top (cannot decay to two muons!).

    Hi Adrian,

    welcome here. If you dig in this blog, you will find about 400 posts, some of which are useful. In my older blog (, which I ran from 2006 to april 2009, there are 1400 more. Enjoy!

    And thanks for the link...

    ... And now that I've seen your article, let me make you my congratulations! You have quite an audience there. Keep up with the good work -doing outreach in particle physics can be rewarding and fun!

    Adding the corresponding ATLAS plot:

    Apparently CMS does a better job capturing low pT Muons, but the amount of Z found is almost equal.


    Thanks for the contribution! Cheers, T.