A Quantum Diaries Survivor

Just a link to a post by Gianandrea Giacoma on the site of the sci.bzaar.net workshop, an event about which I wrote here, here and here.

In the post, Gian uses very kind words to introduce a video on my thoughts on the need of horizontality in scientific blogs. I already posted a link to my video yesterday (beware, it is in Italian - I will try to find the time for an English version though, or at least provide a transcript in English), but the one on the sci.bzaar.net site does not need to be downloaded before playing - a huge bonus since you might get bored halfway through (oh well, damned if you do. It’s just 7 minutes).

On Saturday, May 15th, a conference called “sci.bzaar.net” will take place in Milano. It will bring together a restricted group of researchers, psychologists, bloggers, designers, physicists, writers, philosophers, computer scientists and web experts, who will discuss scientific divulgation, production of knowledge, and open culture in the academic world.

I will not be there in person, but a video I produced for the event will be shown - and I will connect with skype or some other means to take questions. You can see the agenda of the workshop here.

In addition, I produced for the web site of the event another short video where I discuss the importance of horizontality in a blog aimed at scientific divulgation. Unfortunately, I only have a version in Italian so far (the event is aimed at an italian public). I will paste below a writeup as I have the time, but if you are interested you can see me in the 7-minutes video here (beware though, it is kind of heavy - 500 Mbytes!).

Here is an excerpt of the latest LHC schedule for the following few months, as agreed in a meeting at CERN chaired by the Director-General, with the experiments and LHC machine heads.

Based on the good progress for the cool down of the LHC sectors, and on the powering tests from two sectors, the following planning was arrived at:

1. End of June: The LHC is expected to be cooled down. [...]
2. Mid of July: The experimental caverns will be closed [...]
3. End of July: First particles may be injected, and the commissioning with beams and collisions will start.
4. It is expected that it will take about 2 months to have first collisions at 10 TeV.
5. Energy of the 2008 run: Agreed to be 10 TeV. The machine considers this to be a safe setting to optimize up-time of the machine util the winter shut-down (starting likely around end of November).[...]
6. The winter shut-down will then be used to commissioning and train the magnets up to full current, such that the 2009 run will start at the full 14 TeV design energy.

The above means that the machine will deliver collisions from the end of September on, for at most nine weeks in 2008. More safely, one can assume 6 full weeks of data-taking. What luminosity do we expect to collect ?

A state-of-the-art estimate was made by a colleague, who used his past experience with LEP as well as the information on the current limitations of the RF system -which will make the proton bunches shorter than planned (RMS of 5.4 cm), and with a transverse size of 46 microns. At the lower energy the low-beta squeeze will also be loosened from 2 to 3 meters. These figures reduce the instantaneous luminosity, and the estimate for 6 weeks of collisions are of about 40 inverse picobarns of data in 2008.

If ATLAS and CMS will be fully on during the weeks of collisions, these 40 inverse picobarns will fruit, in my opinion:

• A top pair production cross section with 10-15% accuracy
• A sizable sample of vector boson decays to leptons, very useful for calibrations and checks of lepton efficiency studies
• The first estimates of b-tagging and tau-tagging capabilities of current algorithms
• no information on the Higgs
• no SUSY discovery (of course!)

All the above will have a chance of being ready for the 2009 winter conferences, if all goes well…

Here is a selected list of interesting links from blogs I read:

• Bee at Backreaction has the most complete list of reasons why you should not be bothered by the LHC destroying the Earth. Instructive, entertaining, to the point. With useful furthering of the matter in the comments thread.
• Peter at Not Even Wrong has two interesting posts out. In one he reports about Witten’s take on dark energy. In the other the question on what string theorists would do if their pet theory was proven wrong is discussed. Don’t miss the comments thread.
• Carl at Mass explains in detail why the current cosmology does not explain the angular correlations in the fluctuations of cosmic microwave background for large angles, while a changing speed of light would fit the data better. Controversial!
• Lubos at the Reference Frame discusses whether a theory that makes no predictions is to be preferred or disfavored, in relation to one that is more predictive. He also has a poll. Let’s all ask him to add a bullet, “A and B are equally unlikely because they are both favored by Lubos”,
• Jester at Resonaances has a short but poignant post on how to be a good crackpot. Recommended.
• Kea at Arcadian Functor has reached lesson 182 in category theory. Her explanations make you believe you know those things, and there are a bunch of graphs you cannot miss. Esthetically pleasing.
• Chad at Uncertain Principles has one of his imperdible dog dialogues out. Highly recommended.

Although ten short meaningless posts won’t outvalue a longer thoughtful one, for today I stick with the former. So let me just paste here a link to a post about me at sci.bzaar.net, the site of a workshop I will attend virtually next week.

In a few days I plan to provide the site owner, Gianandrea Giacoma, with a couple of short videos where I discuss some limits of blogs in the context of scientific outreach. If I am not too lazy I will produce an English version of those (the event is for an italian audience).

Upon coming back from a shor vacation on the Alps, I rushed to connect my laptop to the internet. And one of the first things I did was to check for recent results by CDF. The experiment has been producing new beautiful results at an impressive pace during the last few months: it is as if the work of years of preparations, refining algorithms, tools, thinking hard at new methods, and a parallel strong push for the collection and processing of data had converged to a singularity, and now results are popping up like flowers in a garden.

My interest in the new analyses is boosted by the fact that in less than a month I will be describing them at PPC2008, a conference in Albuquerque where I am going to give a talk on new CDF results. So it is about time for me to start thinking about the organization of my talk.

As I browsed the recent talks in the Higgs Discovery Group, I found a new blessed (i.e., internally approved for public consumption) result that warmed my heart. It is the first Run II limit on associated Higgs boson production based on the 4-jet signature of WH or ZH decay. This signature arises when the Higgs boson is produced by the process called “higgs-strahlung” off a virtual W or Z boson, and both bosons then decay in a pair of hadronic jets (see picture). The Higgs, if it is lighter than 135 GeV, most of the times decays to a pair of b-quarks (in red), while W and Z bosons decay to all available quarks (in blue) more democratically.

Hadronic decays of vector bosons are the most common ones: W bosons decay to two quark jets 66% of the time, and Z bosons 70% of the time. So, with a large fraction of Higgs bosons also materializing into two jets, looking for four-jet final states to see a WH or ZH signal might look like a no-brainer. Quite the contrary!

Indeed, the 4-jet final state has always been considered absolutely hopeless. 4-jet events are among the most common final states of a proton-antiproton collision, and the kinematic handles one can use to try and discriminate associated WH or ZH production from generic QCD 4-jet production are absolutely insufficient. One can consider the invariant mass of pairs of jets, in the knowledge that W, Z, H all have a well-defined mass, while QCD produces jet pairs without any constraint on their common mass.

Hopeless, in particle physics, is a very attractive word for some of us. Out-smarting our colleagues is one of the highest forms of satisfaction in a scientific workplace… So, after my group demonstrated against all odds the possibility to see top pair decays in their 6-jet final state (one that arises when both W bosons emitted in the chain decay to jet pairs), in 1996, we started thinking at what would be the best way to exploit the experience we had formed in reconstructing high-mass states with jets.

One branch had already born fruit: my PhD was already in full swing, and I would show a first signal of decays soon thereafter. But that is another long story. Instead, in 1998 we started working at the idea of reconstructing the WH or ZH signal in events with four hadronic jets. In Run I the analysis had already been undertaken by Juan Valls and Jorge Troconiz, and they had indeed produced a fine piece of physics, with a limit on Higgs production which challenged those in the “golden” leptonic channels.

We aimed at Run II, and started working at the most critical issue: the one of triggering on 4-jet events with b-quarks. The multijet trigger which had been the basis of both the and the analyses was very inefficient on the latter signal, because of inefficiencies in the online jet reconstruction.

Enter the SVT (silicon vertex tracker), a fantastic device which measures online the impact parameter of tracks, allowing the collection of B-decays with high efficiency. SVT had been designed for B-physics purposes and was thus aimed at low-energy events, so we needed to verify it would work fine for 4-jet events too. This implied determining that those complicated, high-track-multiplicity events were reconstructable in the 20 microseconds available for a trigger decision at Level 2; and then designing a set of selection cuts that would allow the maximum efficiency on signal events while keeping the data acquisition rate at an acceptable level. In parallel, we also studied alternative strategies involving the semileptonic decay of B-hadrons, by combining jet signatures with soft lepton detection.

This job kept us busy for three years, and fruited a graduation to Giorgio Cortiana, a PhD to Luca Scodellaro and Mario Paolo Giordani (and I am certainly forgetting some other students). But as Run II started for real, and multijet events started being collected with high efficiency, we gradually lost interest: Luca Scodellaro’s analysis had shown that the signal was really, really hopeless. Too hopeless even for us - or maybe we were already growing old and disillusioned ?

The recent analysis by Song-Ming Wang, Rong-Shyang Lu, and Ankush Mitra (Academia Sinica), Daniel Whiteson (UC Irvine), and Aart Heijboer and Joe Kroll (University of Pennsylvania) shows otherwise. Sure, they do not reach a sensitivity sufficient to exclude Standard Model production of WH and ZH events in any region of Higgs masses, but they nevertheless extract an excellent result which will be successfully combined with the other searches, improving the global Tevatron limits on Higgs production. Since this post has become much longer than I wanted, I will only describe it shortly, and jump to the results.

The analysis selects events with four jets, two of which have to contain a signal of B-hadron decay, and then uses a Matrix-Element approach to determine the probability that the observed final state is the result of the decay of a WH or ZH pair, and the probability that it is instead due to background processes. The information is merged in a discriminant which separates the processes on a statistical basis. One thus ends up fitting the distribution of the discriminant as a sum of background and signal, as in the plot below.

To put in evidence the small contribution from top pair production (in blue), diboson and single top (in green), and WH/ZH processes (in red), a logarithmic plot is appropriate:

As you see, the signal would contribute mainly in the right part of the distribution, but with a tiny fraction of the events: Standard Model predicts a contribution of less than 10 events in a sample of more than 20,000.

The maximum amount of signal allowed by the fit determines a limit on the production cross-section of Higgs and vector bosons. The limit on the cross-section depends on the Higgs boson mass for two reasons: one is the increase in collection efficiency as the Higgs mass grows, and the other is the decrease in Higgs branching fraction to b-jet pairs. In the end, one obtains a limit on the ratio between cross section and SM expected cross section, as a function of Higgs mass. The limit is always larger than 1 -it actually is higher than 30- so no Higgs mass is excluded by this search. It is shown below with a red line; the limit the analysis would predict to set, based on pseudo-experiments, is shown by the hatched black line and 1-sigma and 2-sigma yellow and green bands.

This result really makes me feel that the work we did eight years ago was not wasted!

During the last ten years I have graduated 11 undergraduate students in Physics, plus tutored four PhD students through to their title. Despite this variety of personalities that have crossed my path in forming their credentials as physicists, there is one single example of “my student” which stands above all, for continuity and results, and that example is Giorgio Cortiana.

Giorgio joined our group in 2000 as a summer student at Fermilab, and he worked during the months of August and September with me at the design of a trigger we were putting together to collect Higgs bosons in the forthcoming Run II. Following the positive experience, he asked our group for a thesis in CDF, and worked with me at the same topic, a multijet trigger for Higgs events.

He graduated with the highest score, and entered Padova’s PhD program at the end of 2002. CDF data was just starting to pour in in reasonable amounts, and Giorgio’s PhD time span was well-placed to allow us to invent something new. We started working at a search for top pair decays including tau leptons and jets, a channel nobody had ever considered due to its apparent trouble -a huge background from QCD events. We, however, were soon convinced our search could yield a pleasant surprise.

And indeed we struck gold when, in early 2004, we found out that by extending the search to an inclusive signature of missing transverse energy and jets -which allowed to include events where one of the top quarks decayed to an electron or a muon which failed the tight lepton identification criteria- we soon obtained a large signal of events that other searches had totally ignored.

With the data we had selected, Giorgio and I obtained CDF’s third-best measurement of the top pair production cross-section, and we soon published a paper on Physics Review Letters. In the meantime, Giorgio also obtained his PhD, which was soon followed by a research grant to continue working with our group in Padova. The plan of the grant was to measure the top quark mass with the decays he had collected in the inclusive missing Et plus jets search: he did it very effectively, and he published another nice paper in record time. While he was doing that, he also had his hands full in a new re-design of the CDF calorimeter trigger, again focused on a more efficient collection of Higgs events. He took an important role in the project as responsible for the monitoring of the trigger, and his group completed the task in due time: CDF now has a much more effective identification of jets at trigger level 2, and this means a sizable increase in Higgs sensitivity.

Despite these successes, we had to witness once again how Italy is not generous with young researchers. Bright, young and able, with the highest academic title in his pocket Giorgio -as hundreds like him- is deprived of job security, and has to accept a salary which in other countries would be refused by a graduate student. So he recently started looking for a better position outside Italy, and he of course found one very soon. He gave a farewell seminar in Padova last week (if I have a chance I will describe his interesting talk here), and he is now off to Munich, where he is joining the ATLAS group. ALAS, I would say, since I at least hoped he would end up in a CMS group instead: that would have allowed me to continue collaborating with him…

The best of luck to Giorgio then. I am sure he will be appreciated in his new group. In the meantime, I have to reckon with a thinning group of collaborators: Julien left three months ago… To ATLAS too!

This just to post a couple of links concerning yesterday’s talk by Ray Orbach at the Fermilab Ramsey auditorium: an article on the event and a video of his presentation.

In twenty minutes the mini-workshop on Dark Matter at LHC that we organized for Physics students will start in Aula B at our Physics Department “Galileo Galilei”, here in Padova. I will be closing the workshop with a talk named as this post: which is both a good and a bad thing. It is good to have the last word, but it is not good to see other unmoderated talks straggling past their allotted time and the audience leaving to catch the last train before you had a chance of hypnotizing them.

In any case, I have prepared a reasonably light-weight presentation. The slides are unfortunately in Italian, but I will give a transcript here as an update, later this evening or tomorrow. They are tightly packed - a feature which I call a annoying defect in other people’s presentations, but I find always excusable in my own. No, really - the reason for filling the slides up with text is to make the slides usable without the speaker: a commendable, unselfish reason, you will agree.

So, please find the slides here. I will remove the link once I manage to put together a transcript, since I am running short of space in the public area where I store my stuff. Incidentally, I will have to find a solution for that. Does anybody have an advice on free sites offering permanent access to a Gig of disk space ?

Due and happy thanks to a friend for pointing me to the following sentence, appeared minutes ago at the Cosmic Variance site in a guest post by Juan Collar:

“Thanks DAMA, for cheapening the level of our discourse to truly imbecilic levels. (Sean, if you edit this I will scratch the paint off your car. I may not write blogs, but I do read them: I know how to hurt you).”

No, I think Sean will not edit it - by now it is on record. In any case, I have two comments. The first is that I am happy that a comment I recently made in this blog about the presentation of the new DAMA result sounds polite and positive if compared with the above. The second is that I think we should all back off and realize that no matter whether an experiment will one day win the Nobel prize or be proven laughably wrong, every scientist who works in our field deserves our respect until proven an imbecile. Doing otherwise harms the whole field, and ourselves.

Oh, and - I still thank Sean for linking to my own commentary of the DAMA-LIBRA signal.