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 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).

Jeff is a physics professor at the University of Cassino, and a long-time colleague and friend of mine. He worked in the SLD and CDF collaborations as a particle physicist, but later moved on to study radiation damage on silicon detectors for particle and astroparticle applications.

Besides admiring him for his wicked sense of humor, which he uses to make the workplace around him always a pleasant place, I have the highest esteem of Jeff as a professor, because he is quite skilled in explaining physics concepts in simple terms. He always looks for the most intuitive way to understand things, as you might appreciate in the contribution he offers below.

The following describes a very elegant and simple derivation of the relativistic formula for the addition of velocities, .

It is due to David Mermin. I fell in love with it and have been telling it for the past four years now to the students of my general physics course. The students are first year telecommunications and electrical engineering students. Before sitting in on my course all of them have heard about Einstein and most of them heard the expression “the velocity of light is constant”. I do not have the time to discuss special relativity in detail. My course is quite traditional. I discuss reference frames, inertial frames, Galilean transformations and covariance of Newton’s laws. I then point out that when describing mechanical waves the frame that is stationary respect to the medium is a special reference! In particular the wave motion can be made to disappear by moving respect to the medium with a velocity equal to that of the wave. It is clear at this point that the constancy of the velocity of light cannot be understood by assuming Newton’s laws and then modeling light as a mechanical wave in a medium (the ether). I then restate the constancy of the velocity of light and begin Mermin’s derivation.

The derivation uses:

• only one reference frame (no use of Lorentz transformations),
• simple kinematics (always good to brush up on),
• the constancy of the velocity of light (something that every telecommunications and electrical engineering student should know),
• the idea that some things are invariant; i.e. while many quantities are relative, observers will agree on some absolutes.

Consider a train of length L moving along the x-axis at a constant velocity v respect to an inertial frame of reference (the observer watching the events unfold). At the trailing end of the train a loaded gun is aimed in the forward direction and fired at time : the bullet and flash of light emerge and travel in the forward direction with different speeds: w the velocity of the bullet, c the velocity of light. A mirror at the front end of the train reflects the light back towards the advancing bullet. Let f be the fraction of the length of train that the reflected light travels before meeting up with the bullet. The constancy of light (Einstein’s dictum) tells us that the velocity of light in the forward direction is equal to the velocity of light in the backward direction; i.e. .

The space-time plot looks like this:

Let be the time for the light flash to reach the forward-going mirror and be the time the reflected light needs to return from the mirror and meet up with the forward-moving bullet. Simple kinematics allows us to label the space-time plot:

Simple algebra:

It is important to note that the expression for f we just obtained is valid if the velocity of light in the forward and backward direction are equal. Note:

• A classical pre-Einstein physicist would say this expression is valid only if the observer is stationary respect to the ether frame.
• On the other hand Einstein says that any inertial observer would use the same velocity of light; i.e. Einstein tells us that this expression is valid for any observer (generic inertial frame).

Following Einstein we consider a particular observer (frame), one that is moving along with the train. For this observer the velocity of the train is . For clarity let us use the symbol u to indicate the velocity of the bullet with respect to this observer; i.e. with respect to the train.

Suppose the train has 10 windows and the reflected light and the bullet meet up at the third window from the front (). It is important to realize that all observers will agree on the value of f. The fraction f is an invariant!

The constancy of the velocity of light allows us to impose the invariance of f the following way:

Q.E.D. !

I have been invited by David Orban, a friend and fellow blogger, to speak on the divulgation of Science next May 17th at the Scuola Politecnica di Design in Milano, at a meeting called SCI(bzaar)NET. The event, organized by Gianandrea Giacoma, is described in its web site as (my translation)

“Subjects active in the net meet in a new way to ponder on the challenges that Internet poses to scientific divulgation, production of knowledge, and Open Culture in the academic world.”

The meeting will have three main threads:

1. The hunger of scientific outreach: scientific research and the fast technological evolution are increasingly becoming, as is evident to all, among the main factors of change in the world and in our daily life. For these reasons a growing number of people, fascinated and awed, feel the need to understand and make their own opinion on the matter.
2. Production of knowledge: if internet is historically connected to the academic world, on the other hand one cannot claim that the majority of researchers as indivudials and the italian University institutions have adopted these new instruments for a more advanced presence online and a more effective handling of knowledge, students, researchers, and professors.
3. Open Culture: the growing impact of legal, economical, organizational and cultural scenarios of a diffusion of Open Culture in Universities under the pressure of internet.

I will contribute with a video, because I unfortunately cannot be there in person… On the following morning I am leaving to New Mexico for PPC 2008;. I am planning to post the video here, with a transcription (the language of the meeting is Italian…). The subject of my talk will be “Fare divulgazione scientifica con un blog: opportunita’ e limiti” (doing scientific outreach with a blog: opportunities and limits).

UPDATE - the name of this post has been modified according to the request of G.Giacoma on 4/23, reflecting the final name of the event.

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.

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.

“At least 99% of the 10^500 possible vacua are complete garbage and can be ruled out easily. Thus, the regions of the landscape for which realistic vacua may arise is limited.”

Eric (string theory enthusiast)