## Discovering new physics at LHC: a paper worth a close look February 11, 2008

Posted by dorigo in physics.

I just finished reading an enlightening paper by Michelangelo Mangano, a theorist at CERN and longtime colleague in the CDF experiment. Michelangelo is one of the few theorists who have consistently helped CDF from the inside, falling just short of pulling cables during Run II commissioning but other than that doing his share of dirty work as a full collaborator.

Michelangelo’s insight in the problems and the pitfalls of extracting a signal of new physics in hadron collider data is well known to me, since the time when we were both members of the oversight committee charged with assessing the soundness of Paolo Giromini’s analyses of the “superjets” – 13 weird events isolated in a sample of b-tagged W+jet events collected by CDF during Run I (I described the saga of the superjets and the following signal of a scalar quark in a series of posts about a year ago). Working with him in that committee did teach me one thing or two back then.

The paper (titled “Understanding the Standard Model, as a bridge to the discovery of new phenomena at the LHC“) contains no formulas -everybody can read it and understand it. It is a refreshing read, which tries to put in perspective what it will mean to “discover new physics” with CMS and ATLAS.

Michelangelo in fact focuses on the very incremental process whereby data is analyzed, gradually better understood as the combination of known effects, to the point when discrepancies with the models arise, and an interpretation is required.

Hic sunt leones: a discrepancy is not a sign of new physics, regardless of whether there are indications that this or that model, cunningly tailored around the effect or unsuspectingly formulated years before, explains the discrepancy perfectly. Not until every possible mundane explanation has been ruled out. Michelangelo makes the point quite clearly:

“As we move away from the default Higgs scenarios, into the territory of new physics beyond the SM, life becomes more difficult. One should think of two phases for a discovery: establishing the deviation from the SM, and understanding what this deviation corresponds to. It is crucial to maintain these two phases separate. The fact that a given anomaly is consistent with one possible interpretation does not increase its significance as an indication of new physics. If we see something odd in a given final state, it is not by appealing to, or freshly concocting, a new physics model that gives rise to precisely this anomaly that makes the signal more likely or more credible. The process of discovery, namely the detection of a deviation from the SM by more than, say, 5 standard deviations of combined statistical and systematic uncertainties, should be based solely on the careful examination of whether indeed the signal violates the SM expectation. Assigning this discrepancy to a slot in the space of possible BSM scenarios is a subsequent step.”

In the paper Mangano considers different examples of where a discrepancy might build up. Mass peaks are of course one of the clearest indications of the creation of a new particle or final state, but anomalous shapes of kinematical distributions are no less intriguing to the hunting physicist. Indeed, one of the very first things that many will look in LHC data will be the distribution of missing transverse energy – a possible “smoking gun” for the production of supersymmetric particles. Here is a sobering cautionary remark:

“There is no question, therefore, that unless each of the background components can be separately tested and validated, it will not be possible to draw conclusions from the mere comparison of data against the theory predictions.

I am not saying this because I do not believe in the goodness of our predictions. But because claiming that supersymmetry exists is far too important a conclusion to make it follow from the straight comparison against a Monte Carlo. One should not forget relevant examples from the colliders’ history […]”.

Then a third “clear” signature of new physics is considered. The murkiest: counting experiments. In a counting experiment one relies solely on the numerical excess of the data in a given region of phase space, and in that, they are similar to searches for shape anomalies. Here a positive example is given, only for the purpose of showing just how complex it is to use with confidence a numerical excess as an indication of a discovery: the case of the top quark discovery. Here, a number of checks were necessary in order to make the excess rise from the status of a mere anomaly to that of a genuine new signal. But here too lies a caveat. Michelangelo compares the observation of the $\Omega^-$ particle, which was based on just one, very striking, event, with an anomalous event collected by CDF in Run I, the infamous $e^+ e^- \gamma \gamma \not E_T$ – one which has a probability of fractions of one part in a million to be due to known processes.

“In terms of pure statistics, the $e^+ e^- \gamma \gamma \not E_T$ is (still today, after 30 times more luminosity has been collected by CDF and D0) even more significant as a deviation from the background (whether caused by physics or instrumental) than the first W obsertvations at UA1 and UA2. Why do we not consider it as evidence of new physics ? Because consensus built up in the community that, in spite of the “$5 \sigma$“, the evidence is not so compelling. On one side plausible BSM [Beyond the Standard Model] interpretations have been ruled out by the LEP experiments […]. On the other, doubts will always remain that some freaky and irreproducible detector effect may be at the origin of this event.”

Michelangelo is optimistic on the way the review process works and prevents incorrect claims from being accepted by the physics community in large experiments:

“All apparent instances of deviations from the SM emerged so far in hadronic or leptonic high-energy collisions have eventually been sorted out, thanks to intense tests, checks, and reevaluations of the experimental and theoretical systematics. This shows that the control mechanisms set in place by the commonly established practice are very robust.”

I agree with him… But if I think of the effort it took eight years ago to our oversight committee to perform those intense tests, checks, and reevaluations, in the case of the superjet affair… Well, I hope it will be somebody else’s job this time around. I believe Michelangelo concurs!

1. Andrea Giammanco - February 12, 2008

your link to eeggmet.pdf is forbidden (or maybe the file is missing?)

“Forbidden
You don’t have permission to access /~dorigo/eeggmet.pdf on this server.”

2. dorigo - February 12, 2008

Hi Andrea, that’s right. I do not know why. Try the ps file instead.

Cheers,
T.

3. Luboš Motl - February 12, 2008

Dear Tommaso,

this approach of Michelangelo – it’s not the painter, is he? – could be classified as Standard Model fundamentalism. Of course that one must be careful before he claims that he found new physics. But insisting that one must always say that the explanation is a mundane one – one that hasn’t been tried yet – is just another form of bias.

When new physics is found and when it becomes a part of the new standard picture, his words will sound ludicrous. And the probability that this will happen even with the LHC is comparable to 50%.

Best
LM

4. dorigo - February 12, 2008

Hi Lubos,

I do not think that Mangano is claiming one should insist on mundane explanations. Maybe I did a poor service to his paper… I think he is saying that new physics cannot and should not be on the table when assessing whether a particular effect can be interpreted in terms of known physics and statistics alone.

I am happy, though, to see you are rather reasonable in your assessment of the likelihood that some new physics will be seen. I have seen much, much steeper claims in the past – the very fact that we both bet on the matter, and in opposite direction, shows that we are considering that losing our bet is a possibility.

The difference between me and you, of course, is that I would be happy if you win your bet and I lose mine!

Cheers,
T.

5. michelangelo mangano - February 13, 2008

Hi lubos, it’s not a matter of being a SM fundamentalist. I spend my life studying SM backgrounds not because I care about the SM, but because I care about what’s beyond, and because I want to make sure that we do not deceive ourselves. We are not playing here, we are exploring what nature TRULY is about. We have earned an immense credit of credibility because we never failed to properly assess the meaning of what we observe. This is what, in course of the decades, allowed to pursue our research with the support of public funds and the respect of the scientific community. We have to protect this respect, and before we go out and claim that we discovered some new fundamental law of nature we have to be absolutely sure that we are not falling victim of over enthusiasm. I am simply arguing in favour of preserving the highest standards of scrutiny, following established and successful (as I document in my paper) practices. I have witnessed many times false discoveries being announced on the basis of irrational enthusiasm and overconfidence. At a time when our field is the subject of great funding pressure, and great competition with other areas of science, the last thing we want is to embarrass ourselves with premature announcements of discoveries based on incomplete studies of the signal. I am more convinced than anyone else that the LHC will give us great discoveries. But I cannot bet on what precisely will be discovered. And I want to make sure that when we say we’ve discovered something, it will stay on the books for years to come.
I don’t believe my words will sound ludricous once new physics will be discovered, but I can guarantee to you that we shall all look ridiculous if we claim we discovered supersymmetry, and it turns out not to be true. This happened before. After UA1 “discovered” supersymmetry with the monojets, to find out few months later that it was just an underestimated background, people stopped using monojets as a tool for discovery for over 10 year. I remember spending a lot of time in the mid-90’s convincing my colleagues in CDF that monojets, if properly used, are a valuable mean of discovery, that should be pursued. This led to the first analysis of monojets in CDF, with the goal, at the time, of setting limits on light gravitinos. This effort turned out to be very timely, since it could then be used immediately to study the possible signals of large extra dimensions, after the paper by Dimopoulos and friends came out in 1997. It was just a matter of doing things well, that’s all I’m arguing for. And I can guarantee you that this is not easy, and not something that you do by just running a Monte Carlo!

6. dorigo - February 14, 2008

Dear Michelangelo,

thanks for visiting! Lubos is a fundamentalist of his own kind – I do not even know whether he thinks fundamentalism is per se a bad idea 😉

Cheers,
T.

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