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Slow but sure January 19, 2009

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Just a quick note – I am getting slow in answering comments these days. That is probably the outcome of the few threads which have developed under my controversial posts on the Gaza war. The comments left there by supporters of the israeli military action on one side, and by supporters of peace on the other side, have been piling up in my “comments managing” page, hiding the good comments on physics posts under them.

I am taking care of answering all comments now, but it takes some time… If you left a comment here a few days ago and checked back later finding it unanswered, check back now.

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The Corfu 2005 proceedings online April 10, 2008

Posted by dorigo in astronomy, books, games, humor, internet, language, mathematics, music, news, personal, physics, politics, science, travel, Uncategorized.
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Just a note to post here the permanent link to the proceedings of a conference I attended in Corfu (Greece) three years ago. This is a long (32 pages) report on “High-P_T Physics: from the Tevatron to the LHC“, now published in the Journal of Physics: Conference Series [Tommaso Dorigo 2006 J. Phys.: Conf. Ser. 53 163-194]. I think I did post a draft of the paper on this blog a couple of years ago, but then I forgot to post the final version as well.

The paper is a bit dated in some parts, where the most recent (back then) results from the Tevatron are discussed; however, some parts -especially a discussion of the usefulness of Tevatron data for LHC physics- are still readable IMHO. Also worth noting is the fact that the acknowledgments section mentions the late Riqie Arneberg, a friend who passed away last fall, who had accepted the offer I had made to all readers of this blog to proofread the manuscript, and contributed in several places to the clarity of the text.

The publisher has now made available online all its 100 open access volumes through the JPCS home page. Of course I salute this contribution to the free diffusion of science with enthusiasm.

Confused March 29, 2007

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I am sitting in the conference room just before the start of the Thursday morning session, and I have just been handed a revised program of the day. In it, there is no mention of Louise Riofrio’s talk on her exotic formula for space-time.

This sounds quite strange – I had thought that the issue with her talk had been resolved. Or maybe this is connected to the fact that Louise  wrote yesterday in her blog that she is being detained at Gatwick for some problems with her visa ?

With Louise not showing up, the talk being cancelled, and  anonymous posters here casting doubts on Louise’s identity, the plot has thickened considerably…

Testing the CMS tracker March 13, 2007

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At the core of CMS, a giant detector which will be assembled this year inside one of the underground caverns of the Large Hadron Collider at the CERN laboratories in Geneva, lies a marvelous device, a silicon tracking chamber.

Tracking chambers are detectors for charged particles. Their aim is to reconstruct the particle paths using the ionization trails left in their midst. Until a few years ago, trackers were almost invariably designed as large vessels filled with a gas mixture (typical was argon/ethane, but the right recipe could involve some black magic depending on the construction details) and lined with high-voltage wires which collected the ionization and provided position measurements along the track.

Recently, a paradigm shift has occurred, due to the explosion of silicon microstrip detectors and the improved production capabilities of these devices in large quantity. Silicon detectors are 300-micrometer-thick wafers of silicon, usually instrumented with thinly spaced conductive “read-out” microstrips on one side. If a proper voltage is applied to the wafer, the silicon of the wafer becomes void of carriers of electric charge, and the ionization charge left by the crossing particle along its path is collected and read out with ease in the strips.  

Crafting silicon microstrip detectors is a quite complex business, so due to the high precision of the position measurement guaranteed by the microstrips, these detectors have so far been used in collider experiments to instrument small volumes close to the beam line. CMS has instead taken that cutting-edge technology to mass-production mode,  designing its whole tracker out of these wafers. But will these tens of square meters of silicon microstrips work ?

Apparently, yes. In a integration test that is taking place at CERN, last Friday the TIB and the TOB (acronyms for, respectively, Tracker Inner Barrel and Tracker Outer Barrel) have been taking data together for the first time, and they collected the signals of hundreds of thousands of muon tracks from cosmic rays in a day-long exposure.

Here is how a track reconstructed in the full TIB+TOB looks like (thanks Lino Demaria for providing the picture):

In the drawing you see the organization of TIB (inner 8 layers) and TOB (outer 6 layers) silicon detectors, seen in the plane transverse to the beam. Well, the beam is not there yet, but you got the idea. These are thus 14 concentrical cylinders of silicon sensors. The blue points represent position measurements for one cosmic ray event, and the red line shows the fitted path of the track which originated them.

Our job these days is to extract as much information as possible from the data that the integration test is collecting. Indeed, one gathers information not only on operation of the detector itself, but also on the performance of the track reconstruction software.

The CMS Tracker web page has a whole slide show of cosmic ray events, thanks Domenico Giordano and Giuseppe Zito. Enjoy!

Short summary of graviton and gravitino searches February 11, 2007

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CDF and D0 have performed several searches for these exotic particles in the recent past. While answering a question by Helge in the comment column of a post below I ended up putting together links to most of the produced papers by the Tevatron experiments on gravitino or extra dimension searches. I think the links are useful so I replicate the information below…

Here is Helge’s question:

Just a little off-topic question. I wonder, if anybody thinks of discovering “gravitons” at LHC,…. It didn’t show up at https://dorigo.wordpress.com/2007/02/02/what-will-be-the-next-discovery/.

And my answer: 

Well, I think gravitons will be sought with enthusiasm at the LHC, especially since a few of their production signatures are important also in searches for SuperSymmetry. The reason you do not see the graviton in the pie of future discoveries is that it is hidden in the “large extra dimensions” slice.

Both the graviton (either a Kaluza-Klein excitation in n>1 extra dimension models, or the so-called Randall-Sundrum graviton) and the supersymmetric gravitino can be detected, if they exist, in hadronic collisions of high enough energy. In fact they have already been sought for at the Tevatron; one straightforward way to do so is to look for events with a high-energy jet recoiling against “missing” energy. The production processes foresee in the final state a gluon recoiling against the gravit-on (-ino), which may leave the detector unseen, thereby resulting in a large imbalance in the transverse energy measured by the calorimeter.

See here for the gravitino and here for the RS or here for the KK graviton searches performed by CDF in that final state, for instance. Or this paper for a search in the final state of a high-energy photon recoiling against, again, missing energy.

As for D0, they published their latest graviton searches in PRD 90 (2003), 251802 (sorry, no link) and PRL 95, 091801 (2005); for large extra dimension searches in the dimuon final state see PRL 95 (2005) 161602.

The LHC discovery reach for these particles is of course much higher. A graviton can be seen as a high-mass resonance of two photons, for instance, or an electron-positron pair. CMS and ATLAS have a discovery reach well in excess of 2 TeV for the graviton mass in these decay modes, provided 100 fb-1 of integrated luminosity are collected, and the coupling of the graviton to standard model particles is not too weak.

A note by CMS describing some of these searches is available here.

Mercury February 10, 2007

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Well, it looks like today I am in browsing rather than writing mood… I have been checking most of my colleagues’ blogs and left lots of comments around. And I got to read a great deal of interesting stuff, too!

Anyway, Mercury. The element, not the planet. Jennifer Ouellette has a great post on the history of mercury poisoning and a few satellite stories at her site. Check it out!

Misspelled! February 6, 2007

Posted by dorigo in Blogroll, humor, internet, italian blogs, news, personal, physics, science, Uncategorized.
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My recent rant  about growing old is still hot in the press, and here I am, posting another rant. Maybe I am indeed growing old and bitter.

The fact is, everybody loves to be quoted. In the blogosphere, sure, but even more so if it is in a respected, widely known magazine on scientific divulgation . That is, unless they get your name wrong!

It is TOMMASO! T-o-m-m-a-s-o! There. Not that hard, huh ? Why is it that most English-speaking johns end up writing it down and even speaking it up as Tomasso ?

The fact that Tomasso sounds like Tom-ass-**** only adds insult to injury, of course.

However, if the NS journalist who interviewed me on the phone the other week should happen to read this: please don’t feel bad or apologize… You are only the last of a long list. I have sort of metabolized it, until I saw it written on tens of thousands of printed pages.

Oh well. I guess I will get it over with. That is, until they’ll start misquoting me. For that I retain a reserve of indignation.

PS: thanks to my friend David for bringing it up to my attention, and even  writing about it.

Bowling December 10, 2006

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Today we ate at a nice restaurant in Mirano (“I molini”) and after lunch we spent some time at a gaming place nearby where Filippo and I had a couple of games of bowling – something he’s liked a lot since he discovered it last summer in Austria.

Of course, the balls weigh about a fifth of his total body weight (he’s not quite 8 years old) so one cannot really pretend him to be able to impart it a lot of speed. But he played a few good shots nonetheless, and we had quite some fun.

And I had my share of fun as well: in the second game I beat my own record, with four strikes – two in the last two frames. No, I am not the kind of father that will play to let his son win! I did encourage him a lot, and Filippo well realizes he still has to grow a lot stronger before he can play it at a competitive level…

Below are a couple of pictures of Filippo playing, and – for the record – a screen shot of the last game.

Radiation hazard October 26, 2006

Posted by dorigo in personal, physics, science, Uncategorized.
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Ionizing radiation, we have come to know, is a potentially life-threatening hazard.

Radioactivity was discovered in the late nineteenth century, when Bequerel (1896) was studying phosphorescence of different kinds of minerals, and he found that even if kept in the dark, uranium salts would blacken photographic emulsions.

For a while it was thought that radioactivity bore no harm to the living beings – in truth, it was even offered as a revitalizing potion some eighty years ago, in the form of radium and thorium traces dissolved in water. Radithor was its name, and its use brought death to its inventor as well as to many others in the twenties.

Now, after a hundred years of studies of the forces behind radioactive decay, two nuclear bombs dropped on Japan, a few more tested in the pacific killing or injuring nearby fishermen, and other medical evidences, we know pretty well that a radioactive dose has the potential to kill human beings, by causing irreparable damage to cells, modifying DNA, and producing hard-to-heal burns.

We do not know all about the damage of radioactivity to flesh and bones – we know much better the interaction of radiation with matter, since that is important for the design of particle detectors! But what we know gives us limitations to the amount of radiation a human being can be subjected to before it becomes really risky.

Radiation dose to man is measured by a unit called rem . It takes into account the energy released by ionizing radiation as much as the different damage that different kinds of radiation produce on the human body. To give you a scale, during a year on the surface of our planet your body gets a dose of about 360 millirem, something your cells are able to cope with and whose damage are able to repair. That dose comes mostly from radon emitted by rocks, which we inhale, and cosmic rays.

The US Department of Energy allows its workers to get a dose as high as 5 rem per year. It is estimated that few or no effect is apparent on the human body for doses below 15 rem per year.

At Fermilab, radiation hazard is a potential issue. Accelerators produce radiation and the beam can activate (make radioactive) materials that are usually non-radioactive. Moreover, radioactive sources are used widely for calibration of instruments and detectors. To cope with that, a whole division at Fermilab (the Radiological control organization) is in place to keep the risk to users and employees at a minimum.

The facility is owned by the US Department of Energy, but safety being one of the commitments of the lab, the administrative limit is lowered to 1.5 rems per year. Of course, for Fermilab to be able to monitor the amount of radiation its employees receive through their daily activities in the lab, there needs to be a monitoring system.  When employees have to perform work activities in an area where radiation exposure is possible, they have to wear personal dosimeters.

There is a complex policy for the management of radioactive waste – mostly the dust that gets activated by beam products inside the accelerator tunnel, plus other materials used in pumps or magnets. The disposal of radioactive waste is complex and costly, so a lot of care is put to prevent the creation of unnecessary radioactive waste. Moreover, if radioactive waste of unknown origin or composition is created, or materials that are hazardous even if not radioactive are activated, the costs for characterization and proper disposal become huge.

Employees as well as visitors are required to attend a course if they have to work in controlled areas or in places where dosimeters are needed. I just attended such a course yesterday, for the third time in my long career in CDF. It was a useful refreshment of my memory on several issues everybody here should be aware of.

Overall, Fermilab is a very secure environment for radiation. Contamination of individuals is an extremely rare occurrence and it usually boils down to having the subject shower under monitoring, and nobody gets hurt. The lab has to be careful because otherwise the DoE could even decide to shut us down…

The world chess championship continues October 2, 2006

Posted by dorigo in chess, news, Uncategorized.
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After a forfeit on game 5 by Vladimir Kramnik – caused by his refusal to play in protest of FIDE’s decisions concerning the complaint by Topalov’s team – the World Chess Championship in Elista was stopped for a few days, but today play finally resumed. Kramnik is playing game 6 but he is contesting the decision to give him a forfeit.

After 30 moves, the position is drawish, despite Topalov’s attempts at forcing the position. Not an overly exciting game today, but at least chess enthusiasts around the world had their share of fun.

The game is now declared drawn at move 31.