ATLAS has better fluctuations…

December 13, 2011 at 9:51 am 3 comments

and CMS has the better analysis.

Fabiola Gianotti made a masterful presentation of the ATLAS Higgs search, and Guido Tonelli made a highly insightful and detailed one for CMS. The presentations were both excellent with a wealth of information that will take time to digest. The bottom line, though, is that both collaborations observe a modest excess around 125 GeV which may – or may not – be the first signs of a standard mode-like Higgs boson.

This is an extremely exciting time, despite all the sober and cautious statements by the speakers, DG, audience and by nearly all colleagues – people are excited by these results. Few people would claim that the Higgs has been found, but its definitely not not there, if you know what I mean.

I will share some personal first impressions here – but I should state clearly that I am a member of the CMS Collaboration, not involved in any of the Higgs searches but heavily involved in electroweak topics, some of which have a direct bearing on the Higgs results. Anyway…

It is plain that the statistical significance of the Higgs results from ATLAS is higher, if interpreted as evidence (not really the right word…) for a signal. They report a combined significance of 3.6σ compared to the CMS combined significance of 2.4σ. To be clear – this does not mean that ATLAS has done a better job or that their analyses are more sensitive. Indeed, they used less integrated luminosity, employed fewer channels and did not utilize any multi-variate techniques. Monte Carlo studies indicate that the CMS search is more sensitive, a priori, but the ATLAS Collaboration was luckier with the roll of the dice – they were dealt a luckier set of cards.

Both Fabiola and Guido made interesting comments within the context of their talks.

Fabiola pointed out that 120 – 130 GeV is a very nice region for the Higgs boson. At first this may seem peculiar, because it is a most challenging one at the LHC. But her point is a very good one: only in this low-mass region can the LHC collaborations expect to see a signal in several different final states (WW, ZZ, 2-photon, ττ, bb, etc.) which would, eventually, allow us to learn a lot about relative branching ratios, mass, spin and couplings. This would not be the case if a Higgs boson showed up at 250 GeV, in which case only WW and ZZ channels would be fruitful. So if the present hint is confirmed, LHC physicists can enjoy years of data analysis and slowly reveal the properties of the Higgs boson signal (if, indeed, that is what they would be observing).

Guido made the clear point that the CMS constraints on the Higgs boson are distinctly weaker than they should be due to the excess in the data – that there is essentially no difference in the mass limit at 95% Cl and 99% CL. This “difficulty” is precisely what one would expect if a new signal indeed is present. In his words, “the excess in the CMS data prevent to go below 127 GeV.” The same is true, of course, for the ATLAS data. He also showed how there is a broad excess in the WW + ττ + bb channels “modulated” by the γγ + ZZ channels. This is a pithy way of describing the current preliminary result.

Anyway, to return to my opening statement (which is tongue-in-cheek of course). I can phrase my impression another way:

  1. CMS did more with less, (the apparatus is much smaller than ATLAS) while
  2. ATLAS got more for less (the results are “luckier”).

Now let’s see what next year brings!

Update: A very complete set of documents, plots, videos, etc. from CMS is available here:


Entry filed under: Particle Physics.

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3 Comments Add your own

  • 1. Kea  |  December 13, 2011 at 9:33 pm

    Good to see you blogging again, and your one line summary seems pretty accurate to me, even if you are biased. However, as a dumb theorist, I have to say that ATLAS has better graphs.

  • 2. Michael Schmitt  |  December 13, 2011 at 10:17 pm

    HI Kea, thanks! Many of us on CMS would agree that ATLAS has nicer plots and graphs, and we know that matters. Some have even suggested that we should have a committee to guide the production of public plots, but fortunately we haven’t taken that step — yet. 😉

  • 3. Mario E de Souza  |  December 20, 2011 at 9:55 am

    The Higgs boson does not exist simply because quarks are composite. You may say now ‘come on, we haven’t seen it’, and the truth is that we have seen several indications of it. The first one was found in 1956 by Hofstadter when he determined the charge distributions of both nucleons. (one can see them around p. 450 (depending on edition) of the Berkeley Physics Course, vol. 1 (Mechanics)). We clearly see that both nucleons have two layers of internal constituents. Unfortunately these results were put aside from 1964 on due to the success of the quark model and of QCD later on. From 1985 on we began to see more indications of compositeness, but we were so enthusiastic with the SM that we didn’t pay much attention to them. A partial list of them: 1) in 1983 the European Muon Collaboration (EMC) at CERN found that the quarks of nucleons are slower when the nucleons are inside nuclei; 2) in 1988 the SLAC E143 Collaboration and the Spin Muon Collaboration found that the three quarks of the proton account for only half of its total spin (other subsequent collaborations (EMC in 1989 and Hermes in 2007) have confirmed this result which is called the proton spin puzzle); 3) in 1995 CDF at Fermilab found hard collisions among quarks indicating that they have constituents (this was not published because CDF didn’t reach a final consensus); 4) Gerald Miller at Argonne (Phys. Rev. Lett. 99, 112001 (2007)) found that close to its center the neutron has a negative charge equal to -1/3e (inside the positive region with +1/2e); 5) new measurements of the EMC effect have been carried out by J. Arrington et al. at Jefferson Lab and they have shown that the effect is much stronger than was previously observed; 6) the ad hoc terms of the matrix of Kobayashi-Maskawa; etc.
    Gerald Miller wrongly attributed to d quarks the -1/3 charge at the neutron center, but as the neutron ia a udd system we know (from QCD) that none of the 3 quarks spends much time at the center.
    The relevant paper on this subject is Weak decays of hadrons reveal compositeness of quarks which can be accessed from Google (it is at the top of the list on the subjects Weak decays of hadrons, Decays of Hadrons and Weak decays).

    Therefore, we should go back and probe further the nucleons in the low energy scale, and carry on Miller’s experiment with the proton. TAKE A CLOSER LOOK AT HOFSTADTER CHARGE DISTRIBUTIONS OF THE NUCLEONS: BOTH NUCLEONS HAVE TWO SHELLS WITH A CENTRAL COMMON SHELL.THREE POINT-LIKE QUARKS DO NOT REPRODUCE SUCH DISTRIBUTIONS.


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