A Quantum Diaries Survivor

Being lazy is a bad thing in itself, but being a lazy blogger is horrible: you feel guilty of leaving your site unattended. Therefore, since today I have nothing in the world to report about (well, except a book I was asked to review, a new algorithm taking care of track momentum calibration in CMS, and my godparenting of a paper to come out on squarks and gluinos - expect posts on those shortly), I will just paste here a comment I left at cosmic variance, where Sean boasts about his (really, really good) blog ranking fourth in a certain wishful ranking:

Okay, there seems to be a bug somewhere; we’re not really the fourth-largest blog on the Internets, by any plausible way of counting. Unless they are counting by awesomeness.

Tsk, tsk. Self-praise is not kosher. Here is what I answered to Sean in his comments section:

Hi Sean,

at the risk of sounding jealous, envious, and green with livor, I have to say I do not rate your blog as awesome. It does not supefy, it does not startle, it does not cause shots of adrenaline. It has many pluses, of course - otherwise it would not be where it is, in the list above or in other ranking systems around.

And you should be happy about that. A physics blog cannot, _cannot_, be a top ranking one. Beware. If you get there, you have mutated to something you might not have liked in the past. What are you, a guy with an opinion on everything ? A star writer ? Certainly both, but you first and foremost are a scientist, and if you forget your roots it gets dangerous.

Just my two pence… and congratulations.
Cheers,
T.

Alas, what I say above is what I really think: a science blog cannot get very high in any ranking system for the very reason that it deals with science. If it goes up, it means it has started dealing with something else. It is a drift that any science blogger has to counter with a rational approach: keep talking about science. I of course indulge in extemporaneous divagations into other topics, but let me mention my five top ranking posts below:

1. Lisa Randall: black holes out of reach of LHC
2. Glashow humilates Carlucci on Maiani’s appointment
3. I want to know about the multi-b excess too!
4. Physics made easy
5. The Higgs rumor spreads again

So, I do talk about other things, but I am quite happy if I am mostly read for the science. Maybe I did mutate myself - my blog was, at the start, in the spirit of Quantum Diaries: a report on the life of a scientist. Forget it. I will continue to report on my life, and I will continue to discuss politics, astronomy, chess, my hobbies - but blogging about science has much, much more meaning. No, I will never cheer up if my blog ranks as high as Cosmic Variance: of course I love it when I get lots of traffic, but only if I think I deserved it through the service I provided. I intend to keep the variance down.

I receive and gladly paste here, given the interest this topic has aroused (and as some sort of reward, given the fact that it is comment number 100 to the post where it appeared):

The New York Times
Saturday 29 March 2008

Asking a Judge to Save the World, and Maybe a Whole Lot More

by Dennis Overbye

More fighting in Iraq. Somalia in chaos. People in this country can’t afford their mortgages and in some places now they can’t even afford rice.

None of this nor the rest of the grimness on the front page today will matter a bit, though, if two men pursuing a lawsuit in federal court in Hawaii turn out to be right. They think a giant particle accelerator that will begin smashing protons together outside Geneva this summer might produce a black hole or something else that will spell the end of the Earth — and maybe the universe.

Scientists say that is very unlikely — though they have done some checking just to make sure.

The world’s physicists have spent 14 years and $8 billion building the Large Hadron Collider, in which the colliding protons will recreate energies and conditions last seen a trillionth of a second after the Big Bang. Researchers will sift the debris from these primordial recreations for clues to the nature of mass and new forces and symmetries of nature.

But Walter L. Wagner and Luis Sancho contend that scientists at the European Center for Nuclear Research, or CERN, have played down the chances that the collider could produce, among other horrors, a tiny black hole, which, they say, could eat the Earth. Or it could spit out something called a “strangelet” that would convert our planet to a shrunken dense dead lump of something called “strange matter.” Their suit also says CERN has failed to provide an environmental impact statement as required under the National Environmental Policy Act.

Although it sounds bizarre, the case touches on a serious issue that has bothered scholars and scientists in recent years — namely how to estimate the risk of new groundbreaking experiments and who gets to decide whether or not to go ahead.

The lawsuit, filed March 21 in Federal District Court, in Honolulu, seeks a temporary restraining order prohibiting CERN from proceeding with the accelerator until it has produced a safety report and an environmental assessment. It names the federal Department of Energy, the Fermi National Accelerator Laboratory, the National Science Foundation and CERN as defendants.

According to a spokesman for the Justice Department, which is representing the Department of Energy, a scheduling meeting has been set for June 16.

Why should CERN, an organization of European nations based in Switzerland, even show up in a Hawaiian courtroom?

In an interview, Mr. Wagner said, “I don’t know if they’re going to show up.” CERN would have to voluntarily submit to the court’s jurisdiction, he said, adding that he and Mr. Sancho could have sued in France or Switzerland, but to save expenses they had added CERN to the docket here. He claimed that a restraining order on Fermilab and the Energy Department, which helps to supply and maintain the accelerator’s massive superconducting magnets, would shut down the project anyway.

James Gillies, head of communications at CERN, said the laboratory as of yet had no comment on the suit. “It’s hard to see how a district court in Hawaii has jurisdiction over an intergovernmental organization in Europe,” Mr. Gillies said.

“There is nothing new to suggest that the L.H.C. is unsafe,” he said, adding that its safety had been confirmed by two reports, with a third on the way, and would be the subject of a discussion during an open house at the lab on April 6.

“Scientifically, we’re not hiding away,” he said.

But Mr. Wagner is not mollified. “They’ve got a lot of propaganda saying it’s safe,” he said in an interview, “but basically it’s propaganda.”

In an e-mail message, Mr. Wagner called the CERN safety review “fundamentally flawed” and said it had been initiated too late. The review process violates the European Commission’s standards for adhering to the “Precautionary Principle,” he wrote, “and has not been done by ‘arms length’ scientists.”

Physicists in and out of CERN say a variety of studies, including an official CERN report in 2003, have concluded there is no problem. But just to be sure, last year the anonymous Safety Assessment Group was set up to do the review again.

“The possibility that a black hole eats up the Earth is too serious a threat to leave it as a matter of argument among crackpots,” said Michelangelo Mangano, a CERN theorist who said he was part of the group. The others prefer to remain anonymous, Mr. Mangano said, for various reasons. Their report was due in January.

This is not the first time around for Mr. Wagner. He filed similar suits in 1999 and 2000 to prevent the Brookhaven National Laboratory from operating the Relativistic Heavy Ion Collider. That suit was dismissed in 2001. The collider, which smashes together gold ions in the hopes of creating what is called a “quark-gluon plasma,” has been operating without incident since 2000.

Mr. Wagner, who lives on the Big Island of Hawaii, studied physics and did cosmic ray research at the University of California, Berkeley, and received a doctorate in law from what is now known as the University of Northern California in Sacramento. He subsequently worked as a radiation safety officer for the Veterans Administration.

Mr. Sancho, who describes himself as an author and researcher on time theory, lives in Spain, probably in Barcelona, Mr. Wagner said.

Doomsday fears have a long, if not distinguished, pedigree in the history of physics. At Los Alamos before the first nuclear bomb was tested, Emil Konopinski was given the job of calculating whether or not the explosion would set the atmosphere on fire.

The Large Hadron Collider is designed to fire up protons to energies of seven trillion electron volts before banging them together. Nothing, indeed, will happen in the CERN collider that does not happen 100,000 times a day from cosmic rays in the atmosphere, said Nima Arkani-Hamed, a particle theorist at the Institute for Advanced Study in Princeton.

What is different, physicists admit, is that the fragments from cosmic rays will go shooting harmlessly through the Earth at nearly the speed of light, but anything created when the beams meet head-on in the collider will be born at rest relative to the laboratory and so will stick around and thus could create havoc.

The new worries are about black holes, which, according to some variants of string theory, could appear at the collider. That possibility, though a long shot, has been widely ballyhooed in many papers and popular articles in the last few years, but would they be dangerous?

According to a paper by the cosmologist Stephen Hawking in 1974, they would rapidly evaporate in a poof of radiation and elementary particles, and thus pose no threat. No one, though, has seen a black hole evaporate.

As a result, Mr. Wagner and Mr. Sancho contend in their complaint, black holes could really be stable, and a micro black hole created by the collider could grow, eventually swallowing the Earth.

But William Unruh, of the University of British Columbia, whose paper exploring the limits of Dr. Hawking’s radiation process was referenced on Mr. Wagner’s Web site, said they had missed his point. “Maybe physics really is so weird as to not have black holes evaporate,” he said. “But it would really, really have to be weird.”

Lisa Randall, a Harvard physicist whose work helped fuel the speculation about black holes at the collider, pointed out in a paper last year that black holes would probably not be produced at the collider after all, although other effects of so-called quantum gravity might appear.

As part of the safety assessment report, Dr. Mangano and Steve Giddings of the University of California, Santa Barbara, have been working intensely for the last few months on a paper exploring all the possibilities of these fearsome black holes. They think there are no problems but are reluctant to talk about their findings until they have been peer reviewed, Dr. Mangano said.

Dr. Arkani-Hamed said concerning worries about the death of the Earth or universe, “Neither has any merit.” He pointed out that because of the dice-throwing nature of quantum physics, there was some probability of almost anything happening. There is some minuscule probability, he said, “the Large Hadron Collider might make dragons that might eat us up.”

I met Mangano in Perugia at the end of January, and we indeed discussed the issue of black holes at the LHC in that occasion. I only remember Michelangelo mentioning that some evidence against the danger of LHC creating harmful effects came from the existence of neutron stars. I however respect his wish to wait for a review of his report…

Ever since the Tevatron Run II experiments have started to produce results of the search for a Standard Model Higgs boson, we have had a chance to compare observed and expected limits, and indulge in vacuous but entertaining speculations.

Now, however, the datasets amassed by CDF and D0 start to be large enough that a Higgs signal could -with a good dose of luck- start to cause observable effects. In other words, the game of limit-setting has been the default for seven years, but at some point one has to decide that rather than setting a limit one can start to quote a significance. When a significance is a number significantly smaller than two or three, the exercise is of dubious utility, but still, it gives some indication. If experiments are confident in their evaluation of systematic uncertainties, there really is nothing wrong in estimating the significance of a departure from background of the accumulated data in some Higgs-sensitive final state.

Of course, there is a boundary. Experimentalists in search for a new particle usually have to follow some kind of moral prescription. One usually starts trying to establish evidence for a particle, and if the data do not grant any, then one sets a limit. It is the case, for instance, of the search for a t’ quark which I described a few days ago.

In the case of Higgs searches, however, the fact that the expected signal is very, very small has changed that. Rather than first seeking evidence and then setting limits, experimentalists in CDF and D0 have so far relied on the expected reach -obtained through pseudo-experiments based on the analysis strategy and the dataset size- to decide what direction to take: determining a significance or quoting a limit. And the latter has always been the outcome.

As I mentioned above, despite the insufficient sensitivity, there is nothing wrong with estimating a significance. Even better than a significance, however, is a quantity cooked up by statisticians, which is the ratio between the likelihood that the data is signal+background-like and the likelihood that it is only background-like. Such a quantity has recently been computed by Wade Fisher - one of the experimentalists in D0 in charge of computing combined results of Higgs searches.

Below you can see what Fisher finds, in a very informative plot. The curves you should concentrate your attention on are the red and black hatched lines describing the relative likelihood of signal+background and background alone, plus the black line which represent the value assumed by the data, as a function of the Higgs mass (green and yellow areas represent the and range of variation of the background-alone hypothesis).

Three things to take home from this plot:

1. the signal+background and background alone curves already are quite separated at 160 GeV: a signal there would start to have an impact in CDF and D0 data - at about 1.5 standard deviations, as you can gather by observing that the red hatched line lies in the middle of the yellow band ( to ). That fact can be deduced also by the limit set by the Tevatron experiments at 95% CL limit on the 160 GeV Higgs cross section (<1.09 times the SM value): in practice, they have already “excluded” a 160 GeV Higgs at 85 to 90% CL (but these are my own guesstimates, so please do not trust them too much: they are not official). It was possible to reach the <1.09 point because of a negative fluctuation of backgrounds, as also shown by the full black line.
2. At lower masses, the two hatched curves are much closer together: there, the sensitivity is still wanting.
3. Between 115 and 130 GeV, however, the data is marginally more in line with the hypothesis that both signal and background are contributing. In other words, the 1.7-sigma significance obtained by LEP II experiments for a 115 GeV Higgs boson is starting to receive a small confirmation by the Tevatron data. Quite insufficient to get excited yet, but you know how these things work: discoveries may come in the matter of a night, or take months to consolidate.

It remains to say that despite having half a foot in CDF and a foot and a half in CMS, I am still rooting for CDF to discover the Higgs! As I mentioned in an interview three years ago (see video here), it would be fantastic if we found the Higgs in CDF. Go Tevatron!

The italian alpine club CAI has just released the results of a re-analysis of the history of the italian ascent to Mount Godwin-Austen, the 8611-meter peak in Karakorum called K-2, the second-highest peak in the world. And it is a wholesome rehabilitation of the role and testimony of Walter Bonatti (right), a member of the expedition and arguably the strongest climber of the time.

In the vigil of the final push to the summit of K2, at camp 8 on July 29th, 1954 Lacedelli and Compagnoni, together with Bonatti, had discussed the plan of assault and had decided where to place the tents of the ninth camp: just above 8000 meters. Despite his excellent physical form Bonatti had been given orders from base camp by Ardito Desio, the chief of the expedition, to leave the glory of the final ascent to his two colleagues, and he had accepted the agenda. Together with a sherpa he was to bring two air cylinders (19 kg each) to camp 9 for Lacedelli and Compagnoni, who would be waiting for them.

As he arrived to the agreed point on the evening of July 30th, Bonatti however found no camp, since Compagnoni had insisted with Lacedelli to place it higher - arguably because he feared Bonatti would join them in the ascent if given a chance. Bonatti and the sherpa Mahdi were thus prevented to reach camp 9 and they had to bivouac with no shelter at an altitude of over 8000 meters, risking their life (and Mahdi losing all the toes of his feet). They were the first men to ever survive such an ordeal.

On July 31st Compagnoni and Lacedelli retrieved the cylinders where Bonatti had left them, and ascended to the summit, conquering K2 for the first time. They later claimed they had found the air cylinders almost empty, and blamed Bonatti for using the air to survive during that night. This, along with other allegations moved to the unguilty Bonatti,  caused a huge controversy that lasted for decades. The official version for a long time favored the reconstruction of the events given by Compagnoni and Lacedelli. 

The committee appointed by CAI has re-examined the whole story and has concluded that Bonatti had not lied: he did not use the oxygen which Lacedelli and Compagnoni later breathed in their final, 12-hour climb: this is probably the last word in the long-standing issue. A picture taken at the summit, showing the two climbers still with their masks on, is probably the clearest evidence.

I could not resist playing the vacuous game of putting my opinions on a two-axis plane, offered by kataweb. Below you can see the result.

Unsurprisingly, I am close to Veltroni and Bertinotti, and very far away from Berlusconi. Duh!

Bee and Stefan at backreaction have just published a very nice post listing 10 (well, 11) fairly well-known physics effects, from the photoelectric effect to the MSW effect. The descriptions are concise and matter-of-fact, with good references for further reading. Definitely a pleasant grab-and-go read.

An “effect”, a reader has argued in the comments thread of the post, should be something experimentally observed, and then (or contextually) explained by a theory. Instead, there are things such as Hawking radiation -the thermal emission from black holes- which is dubbed “Hawking effect” despite having never been observed (a fact that is likely to continue in the future).

Rather than discussing the largely nominalistic issue of what is in earnest an “effect” (which we can do at Bee’s and Stefan’s blog), I would like to elaborate here on the observation that science has become increasingly bold in the XXth century. Perhaps the cause of this effect is our getting accustomed to the routine exploration of realms to which our senses have utterly no access: very few XXIst century men and women would be willing to negate the existence of atoms, although we have never seen one (waiving a few electronic microscope pictures which arguably do not show anything directly either). Can we do the same with cosmic strings ? Surely not, we need proof. But what is an acceptable proof, these days ? If we abandon some of the foundations of scientific investigation - reproducibility, falsifiability, direct testing - through confidence in our means, strength of our prejudices, and mastery in the practice of our science, aren’t we becoming sorcerers ?

Once the foundations are gone, Science is in danger. We are approaching the dangerous terrain where to progress one is required faith. Faith in a theory, faith in conjectures, faith in methodologies. I see this trend quite clearly in particle physics, my research field. On one side, we have started during the last twenty years to peruse multivariate methods such as Neural Networks, which indeed work wonders but contain a good measure of magic within; as an example, CDF and D0 both have evidence in their Run II datasets of single top production  -a process which the standard model predicts with great accuracy, and thus must exist- with neural networks playing a major role. On the other side, theorists get enamoured of concepts such as fine tuning, renormalizability, unitarity - things that strictly speaking are not physical but mathematical arguments - to justify assumptions and build or kill theories.  

I know I am being a bit provocative here. But I have a point: I do see a trend. Humanity faces big challenges in the future, challenges for its own existence. Is global warming a prejudice or a scientific fact ? If we cannot even all agree on something like that, we are bound to extinction. And maybe there is some logic in it.

UPDATE: I hate when this happens. After writing something that I think has some originality, or at least some personality, I stumble in another recent post discussing quite similar matters in more length, more depth, and in front of a larger audience. It happened before, and it is awkward: I have to swear I did not read the other piece first… In any case, Sean’s piece is worth a look (I learned about it in a post at NEW - so I have to thank Peter for keeping us all updated on most of what is worth being aware of from around the web).

Ever since quark and lepton families were first discerned in the reality of subnuclear physics, the question of how many such generations of matter particles exist  has been in the mind of particle physicists.

Fermion “families”, or generations, are composed each by three pairs of quarks and a pair of leptons. It might be argued that the first generation alone - a up and a down quark for each of the three colors, an electron, and its neutrino - would be all that is needed to make stars burn, planets form, and coke taste better than pepsi.  But Nature (the bitch, not the magazine) appears to have decided otherwise, to the bewilderment of all of us - primus inter pares Isidor Isaac Rabi, who famously said of the freshly discovered muon: “Who ordered that ?”.

Three generations 

Three generations of quarks were recognized as the minimum in 1971 by Kobayashi and Maskawa to accommodate a complex phase in the quark mixing matrix responsible for weak interactions, and thus justify the 1964 observation of CP violation by Christensen, Cronin, Fitch and Turlay. Indeed, the discoveries of the charm quark in 1974, the tau lepton in 1975, the bottom quark in 1977, the top quark in 1994, and the tau neutrino in 2000 filled up a tidy three-generation scheme which is pleasing although mysterious. Why not a fourth generation ? Why on earth not a fifth ?

In the mind of particle physicists, three is a very round number: three is the number of color charges a quark may have; and electrons have an electric charge that is three times larger than that of down quarks. But while the two above “coincidences” are in fact deeply intertwined, the presence of additional fermion generations would make no apparent damage to the overall structure of the Standard Model.

However, there are experimental hints that point to three generations. The one which is most unequivocal  comes from the Large Electron-Positron (LEP) ring at CERN, the machine which had to be decommissioned in 2000 to leave room for the Large Hadron Collider, due to start slamming protons against protons at 14 TeV this fall. What did LEP find ? Aleph, Delphi, L3, and Opal - the four experiments housed along the LEP beam - detected millions of Z bosons produced in 91 GeV electron-positron collisions in the nineties. By an extremely precise measurement of the Z cross section as a function of beam energy - what is called the Z lineshape - they could determine a parameter of the Z boson called its natural width: the inverse of its lifetime. The Z boson decays to fermion-antifermion pairs, and its lifetime would be significantly shorter than it is if a fourth species of neutrino existed. The LEP experiments were thus able to determine that only three neutrinos exist with a mass smaller than half the Z mass. Below you can see the Z lineshape and the three different curves one would observe if the number of light neutrinos were two, three, or four.

Caveats, assumptions, approximations…. Can we ever get rid of them ? Indeed, strictly speaking the LEP result says nothing on the number of generations! It just says that only the three lightest neutrinos have a mass smaller than 45 GeV. Given that in the nineties we had no experimental clue that neutrinos do have a non-zero mass, the LEP result seemed back then a quite compelling argument: if all neutrinos are massless, then there are three of them… But since we proved that they are massive, the LEP constraint on the possible theories of Nature has started to look quite soft. 

LEP, however, did much more than measure the Z lineshape. A large number of standard model parameters have been determined with great precision. Together with external information from other experiments -most notably those coming from the precise measurement of W and top quark masses- these parameters allow to place further constraints on the number and characteristics of supernumerary fermion generations. From the Particle Data Group (PDG) database one gathers that any additional quark or lepton doublet is ruled out if the difference in mass between up- and down-type fermions is larger than 85 GeV; and on the other hand, the analysis of so-called oblique parameters S,T,U allows to determine that no additional generations of fermions which are degenerate in mass exist.

Implications on Higgs phenomenology

It is important to stress that the above constraints are valid in the context of the Standard Model. More complicated frameworks might allow more generations! The Standard Model is a theory: a remarkably successful one indeed, but we have to keep in mind that it might one day be proven wrong… For that reason, direct experimental searches for new quarks and leptons are not a vacuous pastime. 

I will come back to direct searches at the end of this post, to describe the new result by CDF on the existence of a fourth-generation t’ quark. Suffices here to say that collider data are so far unable to exclude the existence of additional fermions. We can thus dream on for a moment, and discuss one further characteristics of a world with more quarks and leptons: their effect on the Higgs boson.

At a hadron collider, the majority of Higgs bosons are produced through a mechanism called gluon fusion: the two colliding hadrons emit an energetic gluon each, these “fuse” together, and a Higgs boson is generated. The fusion involves something called a fermion loop, as in the picture on the right: Higgs bosons do not “couple” to gluons, in fact, but rather to fermions, with a strength proportional to the square of the fermion mass. So gluons create a fermion loop, and the loop materializes a Higgs boson. 

To the fermion loop any existing fermion with mass smaller than contributes proportionally to its mass; but a fermion with a mass larger than half the Higgs boson mass will contribute a fixed, non-zero amount. Quite a remarkable fact: you could make this new hypothetical fermion as heavy as you want, and it would still influence the rate of production of Higgs bosons at hadron colliders!

In fact, the enhancement in production rate of Higgs bosons granted by a fourth heavy generation of fermions has already allowed the Tevatron experiments to exclude a Standard Model-like Higgs with mass close to 160 GeV in this four-generation scenario. As you might know, in fact, that mass region is the one where CDF and D0 are most sensitive to Higgs production: they are by now close to exclude a 160 GeV Higgs even in the regular, three-generation case.

An increase in Higgs  rate is always good news, but there is a catch - and a very important one! In fact, the fermion loop described above does not only occur at the production vertex: it influences also the Higgs decay modes. If a fourth generation of fermions makes the loop more probable, thus enhancing direct production through gluon fusion, it also makes the decay to two gluons more probable!

A decay to a gluon pair is horrible news! The two gluons would be utterly indistinguishable from QCD backgrounds at a hadron collider. Let us look at a typical scenario for the decay rates. In the plot below (taken from hep-ph/0706.3718) you can observe that a gluon decay enhancement would negatively affect the rate of observable decay signatures at low mass - to b-quark pairs, and also to pairs of photons. And the more high-mass generations you add, the worse things get.

The plot shows, as a function of Higgs mass, the fraction of decays to the possible final states. The purple curve shows the large fraction of virtually invisible  decays enhanced by the presence of a fourth-generation of quarks. Note that the gamma-gamma final state (in black) -the one on which LHC relies the most for a discovery of a light Higgs boson - is quite a bit less than one in a thousand in this scenario. 

What conclusions should we draw from the above picture ? I think just a simple lesson: when you buy a box of cereals you are allowed to read the ingredients - they are printed, albeit in small fonts, on the side of the box. Instead, when you examine a graph illustrating a particle physics result which claims to exclude the existence of a particle, you do not get the same treatment. A number of assumptions are implied and, if you are lucky, they will be listed in the accompanying paper, but they will not fit in the graph label nor in the caption. So, the next time you look at a Higgs mass exclusion plot, keep it in mind!

Direct searches for a fourth generation t’ quark

Finally, let me discuss the new CDF limit on the existence of heavy fourth-generation quarks. As I said above, direct search limits are not compelling on such animals. In the past CDF found a lower mass limit at 199 GeV on a heavy b’ quark decaying to a Z boson, , using the fact that one would then observe a Z boson produced away from the interaction vertex. Other searches set less stringent limits. 199 GeV is heavy stuff if compared to the mass of a lightweight up quark (about 0.003 GeV), but not terribly so if compared with the next-of-kin top quark (172 GeV). So, there is room for improvement here!

And improvement it is. With 2.3 inverse femtobarns of Tevatron Run II proton-antiproton collisions, CDF -through the sapient hands of John Conway, Robin Erbacher, and their collaborators at UCDavis- has produced a new search for fourth-generation quarks. The search is in this case focused on top-like t’ quarks, which are assumed to decay 100% of the times in a W boson and a light quark: that is, one assumes that , where b’ is the isodoublet partner of the sought t’. If the t’ is lighter than the sum of W and b’ mass, it will decay to a W and a down-type quark belonging to the first three generations: d,s, or b. Further assumptions (see, I stick to the cereal box rule) include a regular pair production of t’ and anti-t’ quarks via QCD processes. Also, the search focuses on t’ masses larger than 172 GeV.

The mass of the hypothetical t’ quark is reconstructed in events with a lepton plus jets topology in much the same way as is done for the mass measurement of the top quark. The analysis then uses the reconstructed mass along with the variable, which is defined as the sum of transverse energies of all final state objects in the event: jets, missing Et, and the triggering charged lepton. A two-dimensional fit on these two variables is performed on the data, interpreting them as the sum of all standard model backgrounds (dominated by regular top-antitop production and W+jets production) and a possible t’ signal. The fit is performed for different masses of the t’ quark, using a Monte Carlo simulation of the expected shape of the mass and distributions of the signal. 

The figure above shows the mass distribution of the data, overlaid with the backgrounds as interpreted by the fit: in blue top quark production, in green W+jets, in grey residual QCD backgrounds, and in yellow the t’ signal allowed by the fit assuming a t’ mass of 280 GeV.

The procedure discussed above extracts an upper limit on the abundance of signal allowed by the fit as a function of the unknown t’ mass. Such a curve can be translated, accounting for data luminosity and selection efficiency, into a limit on the production cross section of the new quark pair. Since the latter is well predicted by quantum chromodynamics as a function of t’ mass, the limit translates into a lower limit on the t’ mass. In the figure below you can indeed see, as a function of t’ mass, the cross section limit (red line) and the theoretical prediction (purple line) crossing at 284 GeV, which is the new lower limit on t’ mass.

Inquisitive minds will have by now started wondering about the “bump” in the mass distribution at about 400 GeV. Indeed, with some fantasy one could see the presence of a signal there. The significance of an excess in the tails of the and mass distributions has been estimated by the authors, and is less than two standard deviations: no reason to get excited about it… Yet.

In any case, a look at the high-mass events is common practice. Here is the event display of one of them: in green the high-Pt muon, in red the missing transverse energy vector, and in grey tracks belonging to jets (seen in the calorimeter as blue/red energy depositions). The reconstructed t’ mass of this event is 45o GeV!

More event displays and other information can be found in the
public web page of the t’ search analysis.

Headlines around the world today announced the conversion to catholic creed of ex-muslim Magdi Allam, vice-director of the italian newspaper “Il Corriere della Sera”. Magdi lives in Italy under continuous watch and armed escort due to the several fatwas (death sentences) issued against him by religious leaders because of his articles, where he often expressed a deep criticism of islamic fundamentalism and of the violent nature of islam.

I believe his conversion to christianity -which is, in Magdi’s own words, “the arrival point of a gradual and deep interior meditation”- is indeed interesting and remarkable in a 56-years-old, cultured individual. I however think the real news is the fact that his conversion was so widely publicized, and the fact that the sacraments of Baptism, Confirmation, and Eucharist were given to Magdi in the spotlights of Easter’s vigil yesterday by none less than Pope Benedict XVI.

Ratzinger’s explicit act is a sort of challenge to islam. Because the catholic church has always tried to handle the conversion of muslims to christianity discreetly, in the knowledge of the risks involved and the wish to avoid a direct confrontation with islamic leaders. In his letter to Il Corriere Magdi explains:

“His Holiness launched an explicit and revolutionary message to a Church that so far has been even too cautious in the conversion of muslims, abstaining from proselitism in countries with an islamic majority and being silent on the reality of conversions in christian countries. For fear. The fear of being unable to protect the converts from their death penalty for apostasy and for fear of retaliation against christians living in islamic countries.”

I wonder whether this kind of putting out fire with gasoline is the right thing to do, in a world increasingly polarized by a clash between catholic and islamic countries. News of clerics stabbed to death in countries with a strong islamic presence, in the meantime, do not make it to the front page any more. If we agree that the West is to speak to the moderate ear of islamic countries in an attempt at damping conflicts, rather than sending bombers and army divisions to the Middle East, we cannot cheer to the choice of Ratzinger. Pope Wojtila would have avoided the provocation.

I may look more anti-patriotic than I really am but sorry, I can’t help it - down with Alitalia! The airline, which is supported by the italian finances (Italy owns 49% of the company’s shares) and has lived of subsidies for years, is probably at the end of its tether. And I rejoice.

Alitalia is unable to stand on its feet - it has demonstrated that quite clearly during the last years, with huge losses in their balances and plummeting shares - and it is finally on sale. The last offer by Air France is humiliating - a hundred something Alitalia shares in exchange for each one of the french company - but it pictures well the rotten state of the deficitary italian company, and it runs the risk of being accepted, in the absence of any other meaningful offer. The only alternative for Alitalia is simply going broke, since the european union has already warned they will not allow further economical aids by the italian government.

On the other hand, we are assisting these days to the strumental use of the bad situation of Alitalia by Berlusconi, who claims he can guarantee a better, all-italian bid which includes his sons as financers. Just amazing: in a month, italians will have to decide whether to elect Berlusconi as the next premier, and he is offering to buy Alitalia! The conflict of interest of a tycoon who owns three television networks and newspapers is not enough: he now wants to buy the country’s airline, probably reasoning that he can then manouver from the government seat into increasing the company’s profits. In the five years as a premier (2001-2006), Berlusconi’s wealth increased threefold. Guess why. And the guy is doubly smart: he knows italians who love their country also hate to see the selling of Alitalia to France, and so his offer is going to win him more votes at the elections of April 13th.

Anyway, why am I happy about this rotten situation ? Well, if you flew Alitalia enough in the past, and had a chance to compare the service it provides to that offered by other major european airlines, chances are you will agree with me: it sucks. I have only found such a nasty mix of bitchy hostesses and stewards, bad service, and crappy planes in Air India, another company in my black list. I guess any frequent traveler has his or her own antipathies, and I invite you to write below your own experience. I can only tell you what was the last time I flew Alitalia, and what happened.

In May 2000 I had a post-doc position with Harvard and was based at Fermilab. I had to travel to Elba for a conference, “Frontier Detectors for Frontier Physics”, where I would present a poster on the muon system upgrade of the CDF II experiment (the paper, later published to NIM, is available at this link [coming shortly]). I reluctantly bought an Alitalia ticket to go from Chicago to Malpensa and from there to Pisa -Alitalia was not my best choice, but the ticket was the cheapest. The flight was not bad, but as I got into Pisa I waited for my suitcase for a full hour, and only then was notified the suitcase had been left in Malpensa “because it did not fit in the plane”. It was a small suitcase, certainly smaller than the ATR700 with which I had arrived in Pisa, but I did not object. I was told the suitcase would arrive with the afternoon flight and they would take care of bringing it to the Elba island.

Three days later, the suitcase was not there yet. I had been told I could buy some clothing, whose price would be refunded if I kept the receipts. I spent hours on the phone with Malpensa offices: the suitcase was nowhere to be found. Eventually, I found some charitable soul in a private office there, and the person went himself to check some racks where lost luggages had been placed. I got it the next day, and proceeded to put together a letter where I listed the items I had bought: a swimsuit, a pair of trousers, a t-shirt, some slips. I think it was about 150$ worth of goods. I of course could not include my telephone bills for the calls to Malpensa, which probably amounted to a third of that, nor other expenses I had ran into because of the lack of my suitcase. Then, I waited.

Three months later, I finally received a letter from Alitalia. It said they were sorry to be unable to process my request, because my letter -which duly included the ticket information and stubs, the receipts, and everything else- lacked a piece of paper they had attached to the lost luggage (and I never found). A lame excuse!

To summarize: they do not notify me that they voluntarily neglect to load my suitcase in Malpensa. Then they fail to let me know. I lose time in Pisa looking for it. Then they make false promises about the delivery. They are not helpful on the phone. It is only through my endless calls that the suitcase is found. And in the end, they refuse a minimal refund! 

I never flew Alitalia after that incident. And I am considering not flying Air France next - these kinds of cancers tend to spread.

I am subscribed to several discussion groups in the Internet, and my mail box is usually stuffed with messages I do not need. However, just every so often a message worth reading - and passing on - appears. It is the case of the following, which I am glad to paste below. It is a message from George Gliba (gliba@milkyway.gsfc.nasa.gov)

Fellow Observers,

So, were you watching Bootes last night ? If you saw a star lit up and then fade away, you might as well send a note to George… Chances are you would be contributing to our still sketchy knowledge on these fantastically energetic explosions.