Comparing the ATLAS and CMS SUSY Searches

March 6, 2011 at 8:49 am 8 comments

The ATLAS jet+MET paper (arXiv:1102.5290) came out on 25-Feb, some weeks after the CMS jet+MET paper (arXiv:1101.1628) on 8-Jan. Remarkably, they are really quite different.

Neither paper reports evidence for supersymmetry, a fact that has been discussed on several other blogs, such as Not Even Wrong, Quantum Diaries Survivor and Backreaction among others. I’ll avoid the tedious debates about whether SUSY is in trouble or is a bad idea in the first place.

My interest is much more in what the experiments did with their data.

Someone made the pithy statement: CMS optimized for background rejection, while ATLAS optimized for the SUSY signal. There is some real truth in this statement. The CMS analysis is extremely robust, clever and safe. There is little chance of a false positive and a very conservative, even skeptical attitude toward the detector and reconstruction algorithms has been adopted. The ATLAS analysis follows the paths laid out at the Tevatron, grasping the SUSY signatures by the horns and simply going for it as if they had years of experience with multi-jet event topologies.

Keep in mind that both papers are looking at events with multiple hadronic jets and missing transverse energy (“MET”). They both veto events with leptons and the jet reconstruction is quite similar. The initial event samples from ordinary pp collisions are huge and looking for a signal for new physics means picking a promising corner of the haystack and hoping to find the needle there.

The CMS analysis works with a kinematic variable called αT, given as the ratio of the ET of the second jet to MT, the transverse mass of the event reduced to a di-jet system. The αT distribution is the crux of the analysis, because the QCD background should fall below 0.5 but a SUSY signal will have a (small) tail above 0.5. Look at these plots from the CMS paper:

alpha_T distribution from CMS

αT distribution from CMS

Notice the log scale. There is a tremendous amount of background below the cut αT<0.55 which is clearly very well controlled by the CMS reconstruction; almost nothing leaks above this cut. Unfortunately, most of the likely SUSY signal also falls below the cut so the CMS search is really trying to catch the cat by its tail.

The ATLAS analysis takes samples of 2-jet and 3-jet events and applies hard kinematic cuts on the MET and on an effective mass variable, meff which is just the sum of MET and the ET of the two most energetic jets. For the most part they end up counting Z+jets events in which the Z had decayed to neutrinos, as you can see from these two plots:

m_eff from ATLAS

meff from the ATLAS SUSY search

Of course, the agreement of the data with the simulation is excellent, as in the case of the CMS plot above. One should not under-appreciate how important such agreement is, given the coming challenges of finding (or not) supersymmetry in larger data samples.

One point of the CMS analysis which I particularly like is the way the backgrounds are estimated through several data-driven methods. You must read the paper for the details. There are two ways of extrapolating from one kinematic region dominated by background into the signal region. There is a direct measurement based on a W→μν + jets signal which is exploited in two different ways. Finally, Z→νν backgrounds are checked with γ+jets. One has no doubt that the background really is pinned at something like 10±3 events, somewhat below the 13 events observed; the CMS exclusion contour is weaker than one would have predicted in advance.

The ATLAS analysis, in contrast, relies on rates and shapes taken from Monte Carlo simulations. To be sure, they have made many tests and studies to convince themselves that the simulations are reliable. But one wonders whether they would be willing to claim a discovery with this kind of analysis. Out of four subsamples of events, three of them have fluctuated downward, so their limits are slightly better than predicted.

ATLAS published a good plot which includes the CMS contour for comparison:

ATLAS SUSY exclusion plot

ATLAS SUSY exclusion plot in the (m0,m1/2) plane

The red line shows the exclusion the ATLAS, which is far more impressive than the black line showing the CMS exclusion. As already pointed out, it is slightly better than the expected exclusion contour, given by the blue dashed line. Note that 1σ fluctuations given by the dotted lines are quite far off showing that small ordinary fluctuations will have a big impact on the contour obtained from the data. The CMS expected exclusion is only slightly better than the one achieved; overall the ATLAS expected exclusion is better than the CMS expected exclusion.

Thus, we have to return to that earlier pithy statement: CMS optimized for background rejection, while ATLAS optimized for the SUSY signal. As a result, CMS has complete supreme control of its background, while ATLAS achieved a more stringent exclusion in the (m0,m1/2) plane.

Of course, this is only the beginning, and both collaborations have more results in the pipeline.

(Notice: I am a member of the CMS Collaboration.)

Entry filed under: Particle Physics.

Where have I been? A puzzle concerning the underlying event

8 Comments Add your own

  • 1. Rainer W. Kühne  |  March 9, 2011 at 8:52 am

    Supersymmetry has been suggested independently in 1971 by Juri Gol’fand and Evgeni Likhtman, in 1973 by Dmitri Volkov and V. Akulov, and in 1974 by Julius Wess and Bruno Zumino. In 1976 Peter van Nieuwenhuizen, Sergio Ferrara, Daniel Z. Freedman, Stanley Deser, and Bruno Zumino suggested a local supersymmetry called supergravity. In 1981 Edward Witten has shown that supersymmetry can solve several shortcomings of Grand Unified theories. In 1984 Michael Green and John Schwarz have shown that string theory and supersymmetry can be combined. This is the superstring theory. In 1995 Edward Witten has shown that the membrane concept can agree the 11-dimensional supergravity with the 10-dimensional superstring theory. Both theories are limit cases of an 11-dimensional M-theory.

    Supersymmetric theories predicted that the elementary particles of the standard theory of particle physics (leptons, quarks, photon, gluons, W- and Z-boson, Higgs boson) have supersymmetric partners. This supersymmetric particles (called neutralinos, photino, gluinos, Winos, Zinos, squarks, and sleptons) were all predicted to have rest masses between 50 and 300 GeV (billion electron volts).

    Now the ATLAS Collaboration of the LHC (Large Hadron Collider) presented data (arXiv: 1102.2357) which do not confirm the gluino. It would have been detected if its rest mass were less than 700 GeV.

    I am not so surprised that signs of light supersymmetric particles have not been detected. I predict that supersymmetry will not be confirmed. My arguments are the following.

    (1) The main reason for supersymmetry is that it can explain some shortcomings of minimal Grand Unified Theories, i. e. the mass-hierarchy problem (i. e. the fact that W- and Z-boson do not have rest masses of 10^15 GeV, although they should have “eaten” (coupled to) the Higgs bosons of Grand Unification) and the non-observation of the proton decay (lower limit: mean proton lifetime of 10^33 years).

    But this argument requires that there is Grand Unification.

    In 1997 I suggested (Modern Physics Letters A 12, 3153 – 3159 = hep-ph/9708394) a generalization of quantum electrodynamics, called quantum electromagnetodynamics. This theory is based on the gauge group U(1) x U’(1). In contrast to QED it describes electricity and magnetism as symmetrical as possible. Moreover it explains the quantization of electric charge. It includes electric and magnetic charges (Dirac magnetic monopoles) and two kinds of photon, the conventional Einstein electric photon and the hypothetical Salam magnetic photon. The electric-magnetic duality of this theory reads:

    electric charge — magnetic charge
    electric current — magnetic current
    electric conductivity — magnetic conductivity
    electric field strength — magnetic field strength
    electric four-potential — magnetic four-potential
    electric photon — magnetic photon
    electric field constant — magnetic field constant
    dielectricity number — magnetic permeability

    Because of the U(1) x U’(1) group structure and the Dirac quantization condition e * g = h (unit electric charge times unit magnetic charge is equal to the Planck constant), this theory is hard to agree with Grand Unification. Although a group such as SU(5) x SU’(5) is in principle not impossible.

    (2) Another reason for supersymmetry is that it can explain the existence of (anti-symmetrical) fermions in an otherwise symmetrical theory (such as Special Relativity and General Relativity).

    However, it has long been known that a generalization of General Relativity which includes anti-symmetry is Einstein-Cartan theory. The affine connection of this theory includes not only the non-Lorentz invariant symmetrical Christoffel symbol but also the Lorentz invariant anti-symmetrical Torsion tensor.

    Within the framework of a quantum field theory, the Torsion tensor corresponds to a spin-three boson called tordion, which was introduced in 1976 by F. W. Hehl et al.: Reviews of Modern Physics 48 (1976) 393 – 416.

    In 1999 I discussed (International Journal of Modern Physics A 14, 2531-2535 = arXiv: gr-qc/9806026) the properties of the tordion. Moreover I sugested that the electric-magnetic duality is analogous to the mass-spin duality. This analogy reads:

    electric charge — magnetic charge – mass — spin

    electric field constant — magnetic field constant — gravitational constant — reduced Planck constant

    electric four-potential — magnetic four-potential — metric tensor — torsion tensor

    electric photon — magnetic photon — graviton — tordion

    (3) Supersymmetric theories including superstring and M theory have not much predictive power. For example, so far no one has shown that these theories predict the empirically obvious Naturkonstanten-Gleichung (fundamental equation of unified field theory, Modern Physics Letters A 14, 1917-1922 = arXiv: astro-ph/9908356):

    ln (kappa * c * H * M) = −1 / alpha

    where kappa is the Einstein field constant, c is the speed of light, H is the Hubble constant, M is the Planck mass, and alpha is the fine-structure constant. By using the WMAP−5 value

    H = (70.5 +/- 1.3) km / (s * Mpc)

    (E. Komatsu et al.: Astrophys. J. Suppl. Series 180 (2009) 330 – 376) the left-hand side yields

    ln (kappa * c * H * M) = – 137.025(19)

    which is within the error bars equal to

    – 1 / alpha = – 137.035 999 679(94)

    The Naturkonstanten-Gleichung predicts the Hubble constant to be

    H = 69.734(4) km / (s * Mpc)

  • 2. Looking for Exotic SUSY Signals « Collider Blog  |  March 14, 2011 at 12:38 pm

    […] look everywhere at the same time. This includes classical channels for SUSY like those I discussed earlier, (jets and missing energy with or without isolated, high-pT leptons) as well as strange signals […]

  • 3. Christopher Lester  |  March 15, 2011 at 9:33 am

    It seems to me that this “Comparing the ATLAS and CMS SUSY Searches” article is one of the very few early posts to have got to the root of the difference in strategies used by ATLAS and CMS.

  • 4. Michael Schmitt  |  March 15, 2011 at 9:45 am

    Hi Christopher,

    thanks, I try hard to get to the point. 😉

    I was rather surprised by the contrasts between the two papers. It is nice that both collaborations set aside the dozens of feasibility studies and thought carefully about what to do with their data. There is nothing programmatic about the ATLAS or the CMS paper, and this bodes well for discovery, if nature affords us one.

    To be sure, the issue of statistical methods is relevant and I hope ATLAS and CMS will converge during 2011. But I’m not so interested in offering inexpert opinions on that…


  • 5. addieup aleve ingredients  |  November 9, 2014 at 3:29 am

    It’s hard to find well-informed people in this particular subject, but you seem like you know what
    you’re talking about! Thanks

    • 6. Michael Schmitt  |  November 9, 2014 at 8:35 pm

      Hi, Thanks. I’m not as expert as some of my colleagues, though…

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March 2011

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