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Bumps part II – more evidence for scalar quarks? January 30, 2007

Posted by dorigo in personal, physics, politics, science.
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As I discussed yesterday , the situation in CDF was red-hot in 1999, after Giromini’s group had shown they were seeing a bump which resembled a resonance in the dimuon mass distribution, and had proposed a fancy explanation which would fit the bump as well as two other intriguing results they had found in the previous years, and had been unable to get approved for publication.

It was in fact some kind of an accumulation point, reached after years of debates on obscure technical details affecting the measurement of the top quark cross section, maturated into a controversy on the origin of a handful of top candidate events (13) which were unexplainable with ordinary means, and saturated into little less than a garage fight between proponents of the analysis -who wanted to publish those 13 “superjet” events – and top group coordinators and spokespeople.

The funny thing was that, arguably, the scientific method had been followed after all – by a combination of Giromini’s intuition and the steering force of a skeptical collaboration.  

So, before I delve into discussing significance issues, let me make a flash-back and tell in a logical sequence the events that had gotten us in front of the dimuon mass bump.

The facts: following the top quark discovery by CDF and D0 in 1995, Giromini’s group had spent the years 1996-1998 measuring the top quark pair production cross section in the freshly collected Run I dataset, and had been in competition with another big group, mainly composed of American physicists who had more political support in the collaboration. The two groups had found different results, mainly because of the evaluation of something called the “b-tagging scale factor”, which Americans claimed to be unity (data and Monte Carlo jets would be tagged at the same rate), and Frascati claimed to be 1.25 (a higher Monte Carlo tagging rate). Frascati’s group had been silenced after many controversial debates in the group meetings, and the top cross section had been measured at (7.6 +1.8 -1.5) pb (submitted paper here ).

A higher scale factor meant a smaller cross section for tt production. And sure enough, after three more years of malignant external scrutiny of his work, stomped feelings, and embittered relationships, Giromini had been proven right (see the scale factor measurement as a function of jet transverse energy above), and an improved measurement was finally published at (5.1+-1.5) pb in September 2001 ( submitted paper here). For what is worth, the theoretical prediction at 1.8 TeV is today 4.9 pb if I remember correctly.

Now the plot thickens. For the sample used to measure top quark pairs in the single lepton final state (so-called “W+jets”, containing a leptonic W decay plus several hadronic jets) contained an excess of events with peculiar characteristics. A smaller top pair cross section meant this excess was even more prominent: 13 events were found with a jet of very large energy, containing both a secondary vertex (a “b-tag”, made by tracks which would not fit to the interaction vertex but were produced by the decay of a long-lived particle) and an electron or a muon. Electrons and muons are indeed produced by b-quark decay, but the 13 events with these “superjets” were way more abundant than what one would expect to get (4+-1 events) from normal processes. CDF had an excess of superjet events.

So what do you do when you observe an excess of events of a particular class ? Simple: you study their kinematic characteristics, to see if these are what you expect from any one of the processes you expect to have contributed to the data sample at hand.

The kinematics of those 13 events was weird. A statistical analysis examining a complete set of kinematic observables from the events showed they were utterly incompatible with known processes – at a level exceeding 5-sigma. One could indeed start to fancy what exotic process could produce such a weird signal when a jet with a secondary vertex always contained a charged lepton. A scalar quark with a 100% branching ratio to leptons was a possible candidate – if a bit nutty – explanation.

The proposed explanation by Giromini generated further more controversy, and additional endless scrutiny. I was then asked to be part of the “Oversite godparenting committee” which had the uncomfortable assignment of determining the soundness of the analysis. We did many tests, produced ancillary results, studied independent samples. No mistake was found, either in the background prediction (4+-1 events), or in the statistical tests. Below is a plot of the probability of observed kinematic distributions for the superjet sample and for a control sample of similar events containing no lepton in the “superjet”. It seemed really hard to find a sample with kinematics as weird as that of those intriguing events!

Then, Paolo Giromini had a brilliant idea. If those events were the result of some fancy scalar quark produced together with the W, one could seek it in other datasets. And the Frascati group looked in two samples they knew well: dijet events with an electron or muon in the jet. Those samples had been used to measure the scale factor mentioned above.

Sure enough, a careful investigation of the rate of multi-lepton events revealed a startling excess of jets with two leptons in the same jet, and with a very small angle between them. More weird stuff! And this time, the statistics were not small: it was hundreds of events with those characteristics, so one could study them in detail with advanced statistical means.

The publication process of these two anomalies took more years of pain. And while the review process was in full swing, Giromini had another idea: let’s see if this scalar quark makes a bound state! If it produces leptons at small angle, it must have spin zero, and maybe – just maybe – if it got bound in a squark-antisquark pair in a p-wave configuration, it would have the same behavior of spin-1 vector mesons such as the J/psi or Upsilon.  So let’s look in the sample where the latter show up: dimuon resonances!

Above you can see a dimuon mass distribution of opposite-signed pairs (in blue) and same-sign pairs (in red). The peaks are respectively the J/psi, the psi(2s), the Upsilon(1s), (2s), and (3s) resonances. Quite amazing, if you ask me. These are really bound states of two tiny things orbiting around each other, “resonating” for a tiny fraction of a second before disintegrating into two muons!

Giromini looked closer. He knew his alleged particle had to have a mass of something between 6 and 8 GeV – his multi-lepton signal had convinced him the mass of the scalar quark would be in the 3 to 4 GeV range. He knew what the width of his resonance had to be. And he could guess what the rate would be.

[To be continued…] 

Comments

1. Tony Smith - January 30, 2007

Tommaso said “… the situation in CDF … the years 1996-1998 … Frascati’s group had been silenced after many controversial debates in the group meetings, and the top cross section had been measured at (7.6 +1.8 -1.5) pb …
an improved measurement was finally published at (5.1+-1.5) pb in September 2001 …”.

When CDF announced in 1998 the cross section as ( 7.6 + 1.8 – 1.5 ) pb for mt = 175 GeV,
hadn’t D0 already announced in 1997 that their measurement was ( 5.5 +/- 1.8 ) pb for mt = 173.3 GeV ?

Did the D0 measurement and analysis have much impact on the internal debates etc at CDF ?

Was there any impact from the 1998 work of Buoncani (INFN) and Catani, Mangano, and Nason (CERN) in hep-ph/9801375 in which they said
“… We compute the effect of soft-gluon resummation, at the next-to-leading-logarithmic level, in the hadroproduction cross-section for heavy flavours. Applications to top, bottom and charm total cross-sections are discussed. … At sqrt(s) = 1.8 TeV and for m_t = 175 GeV, we get a fully resummed result of sigma_ttbar = 5.06 +0.13 -0.36 pb … The current experimental results from CDF and D0 are respectively:
sigma_ttbar = 7.6 +1.8 – 1.5 pb (CDF, at m_t = 175 GeV),
and
sigma_ttbar = 5.5 +/-1.8 pb (D0, at m_t = 173.3 GeV) …”

Tony Smith

2. dorigo - January 30, 2007

Hi Tony,

no, the D0 measurement I am sure did not have any impact in CDF’s internal affairs. Of this I am quite sure.

About the fact that the theoretical prediction was more in line with what the Frascati group was finding back in 1998, the answer is still no. However, Michelangelo Mangano was part of the oversite godparent committee on the superjet affair, and we should ask him whether he believed a b-tagging scale factor of 1.25 back then, due to his theoretical prediction being more in line with that rather than with the 7.6 pb measurement. Michelangelo, by the way, is a very bright physicist and I am quite skeptical that he may have been influenced in his judgement by 1-sigma preferences to one number with respect to another.

Cheers,
T.

3. Not Even Wrong » Blog Archive » Short Items - January 30, 2007

[…] Experimental HEP bloggers keep putting out gripping multi-part stories about what it’s like to be dealing with collider data that is not conclusive, but has anomalies that promise the possibility of something new and exciting. See the latest from John Conway and Tommaso Dorigo. […]

4. Andrea Giammanco - January 31, 2007

one of your best posts ever.

5. dorigo - February 1, 2007

Thank you Andrea, drink whatever you like – it’s on me.

By the way you never gave me the address to your blog!
Cheers,
T.

6. Bumps part III - how significant is that peak? « A Quantum Diaries Survivor - February 1, 2007

[…] February 1, 2007 Posted by dorigo in personal, science, politics, physics. trackback  In the previous part  of this story we left Paolo Giromini busy with the examination of the dimuon mass spectrum […]

7. Andrea Giammanco - February 2, 2007

ah, right! I’m going to send you an e-mail with the URL.

8. Discovering new physics at LHC: a paper worth a close look « A Quantum Diaries Survivor - February 11, 2008

[…] 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 […]

9. Do you remember the dimuon bump ? « A Quantum Diaries Survivor - July 18, 2008

[…] Part 2: More evidence for scalar quarks ? […]


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