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Standard Model or Minimal SUSY ? April 6, 2014

Posted by dorigo in physics.
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(I posted the text below in my current blog, which is at this link; I post it here as well as I would like to keep this blog active by writing something here now and then – TD).

If I look back at the first times I discussed the important graph of the top quark versus W boson mass, nine years ago, I am amazed at observing how much progress we have made since then. The top quark mass in 2005 was known with 2-3 GeV precision, the W boson mass with 35 MeV precision, and we did not know where the Higgs boson was, or if there was one.

Today, the top quark mass is measured with a 770 MeV uncertainty and the W boson mass with a 15 MeV uncertainty. That alone is a reduction of a factor of 10 in the allowed phase space of those two parameters; but crucially, we no also know the Higgs mass with a 0.5% accuracy. This leaves very little space for the true parameters of the standard model. On the other hand, if the SM were to be enlarged to a minimal version of Supersymmetry, then the theory predictions would blow up considerably, as the MSSM allows much more freedom to those parameters as others (like squark masses) are varied.

The summary of the experimental situation is shown in the graph below, which Sven Heinemeyer produced today for this blog (thanks Sven). The graph summarizes calculations produced by Heinemeyer and his colleagues Hollik, Stockinger, Weiglein and Zeune. In the graph the horizontal axis shows possible values of the top quark mass, in the very restricted range allowed by the latest world’s best CMS measurement; the vertical axis shows values of the W boson mass, in an even narrower range in absolute terms, thanks to precise measurements of that quantity performed by LEP2 and the Tevatron experiments. The experimental determination of those two parameters is symbolized by a grey ellipse which encompasses 68% of their probable values.


Then if we stay within the standard model, the Higgs boson mass measurements by the CERN experiments (+-0.7 GeV) force the two parameters to be bound to lie within the very narrow red line; if instead we take the MSSM as the true underlying theory, the whole green area is possible; different points of this area correspond to different value of other parameters (here a more liberal variation of the Higgs mass is taken, to cover more possibilities). The downward arrow symbolizes that as one increases the “mass scale” of the MSSM the allowed region moves closer to the SM line.

Note that in this graph the grey ellipse and the red line are the only experimental inputs; there is no “LEP indirect” oval here, as this would be too wide for the graph. In other words, the precision electroweak information from the Z boson studies of the nineties has become largely irrelevant in this particular view (it remains a formidable input to verify the general agreement of SM and data, if one studies other parameters).

So, what should we carry home from this graph ? I believe at least two things. One, that the SM likes the W mass to be a bit lower than what is currently measured, and the top quark to be a bit higher; the tension is however only mild -we are talking about just a bit more than one standard deviation for the disagreement. Two, that the MSSM is not killed by these measurements – it would live on regardless of the precise values of W and top masses, as the breadth of the green area shows.

Oh, and a third thing – the experimental measurements of these quantities rock!

Other considerations can be made, but I will stop here for tonight. Tomorrow I will be on a train at 6 in the morning, to participate in a 2-day open discussion organized by INFN in Rome, called “WHAT NEXT”. A very interesting discussion on the long term plans of italian research based on the current status of particle physics, astrophysics, cosmology, and other fundamental investigations. I will have something to report on that later on…

Devotion to the Tevatron October 2, 2011

Posted by dorigo in physics.
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(This post first appeared on my current site, www.science20.com/quantum_diaries_survivor . Please note that I post on this site quite rarely. Please visit me on the science20 site for updates on experimental particle physics and more!).

I’m nostalgic tonight. The reason ? The Tevatron has finally stopped running, for good.

It’s strange to find out one can mourn the shutdown of a synchrotron just as the passing away of an old friend, but that’s more or less how I feel like tonight. And I am not even among the ones who can claim to have been around for the full duration of the machine’s lifetime, like Giorgio Chiarelli – as Giorgio recounted here, he was there in the CDF control room when the first proton-antiproton beams collided the first time, in 1985.

I started working in CDF in June 1992. In the course of these 19 years I have learned all I know about particle physics, and I have met a large number of extraordinary people. Not only ones from CDF: the Fermi laboratories are of course a place where you interact with the “competitors” from the other experiment, DZERO -you work elbow to elbow, go to the same parties, seminars, and events, and you share joys and frustrations as the machine which feeds data to both experiments outperforms or suffers technical stops. Plus of course technicians, machinists, administrative staff: a number of people who simply did their job there, but who all shared the pride of doing their part for the success of this remarkable human adventure.

As Gary Taubes explains in juicy details in his book “Nobel Dreams” (1987), it was Carlo Rubbia in the late seventies who first launched the idea of a proton-antiproton collider at Fermilab. Back then, he got severely beaten up by the lab director -he had a bad record of changing horses mid-race and keeping proposing new projects. But Rubbia was right: the technology was just getting mature enough for such a machine to be built. In the course of six months Rubbia learned all there was to know, and then some, about making antiprotons; and then CERN accepted his project. The W and Z bosons were discovered by the SppS experiments in 1983. By then, the Tevatron was already in place at Fermilab. Too late to challenge the discovery of the vector bosons, but timely to provide a precise measurement of their mass, and to search for the sixth quark, the top.

The history of the Tevatron and its many successes will no doubt be told by people who have participated more actively and deeply in it than myself, so there is no point for me to try and do that here tonight. I only choose to tell a personal story here. One where I take the part of the moron, incidentally, but that’s beside the point (and not that uncommon after all).

I was in charge as Scientific Coordinator in the CDF control room a few years ago, leading a crew of physicists in the task of taking data as smoothly as possible during my seven-night shift. It was not the first time on that job, but I was eager to see data coming in -you get that kind of feeling when you sit during long nights waiting for something to happen. For a few times in a row during the past nights the sequence of injection of beams in the Tevatron had been started, and then aborted, for a string of reasons which were not immediately clear, and in some cases possibly caused by human errors. In a moment of scorn, I let go with a sarcastic sentence in the CDF E-log: “Injecting protons again. Let’s see where they screw up this time”.

Now I should explain that, since English is not my native language, I have often trouble gauging how strong words are in a sentence I say or write. To me it sounded a bit like saying “let’s see where they find the trouble this time”, or not too much worse than that. But it was by far too careless.

So I was not intending to insult anybody’s competence, but the sentence was indeed inflamatory, plus of course unfit to an E-log. Worse than that, and not considered by me at the moment, the CDF E-log was readable by anybody, Main Control Room Machinists included. Heck, you too could read it in real time.

The following morning I was not even reproached too much for my stupid sentence, but from the feedback I got in a number of ways, that one time I learned quite something. I learned that out there, reading the CDF E-log, there were not just us in the control room, plus the other people on shift in the Main Control Room and in the DZERO CR, plus a handful of people on call for any problem the hardware or software could be facing. There was a whole community of colleagues who read the E-log as frequently as an addicted user reads his or her Facebook homepage. People who cared for the machine, for the data taking, for the success of the experiment. People who had devoted their life to make this as good Science as it could possible be. People who were ready to provide help to solve problems even when they were not on call, and who woke up in the middle of the night just to see whether a new store was in. All of these people had felt outraged by the lack of respect I had shown to the Tevatron machinists.

I have always said I appreciate men and women who regardless how serious their job is do not neglect to take it with a grain of irony; and yet I saw a flaw in that line of reasoning, confronted with the devotion of so many brilliant minds to the common good -the advancement of Science at the hands of the machine they had contributed building and operating for many years.

So the Tevatron has been turned off today, and many people are sad, each in their own private way. To many, the Tevatron was their life. To all, it was the machine that created the opportunity of getting together to work in a friendly, stimulating environment, growing professionally and intellectually. It was a fascinating machine, which deserves a whole chapter in a history of particle physics. 28 years old, with its glitchs and hiccups, by now old and patched up, the Tevatron was still incredibly performant until the very end. Farewell, Tevatron!

Greek Blog November 1, 2010

Posted by dorigo in Blogroll, internet, language, news, personal, physics, science.
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I had forgotten to link it from here, but the internet always provides a chance for redemption. So here I go. A couple of months ago I have opened another wordpress blog, where I write on particle physics – in Greek. This is a rather extravagant choice, and I think I need to spend a few words explaining it.

First of all, there is my love of the language, which I have been studying for two years. It is a difficult language to master, due to the interplay of several factors: the different alphabet, the enormous wealth of words, and the rather quick evolution of rules and uses. Maybe because of these challenges, I have found it quite entertaining to get on top of it.

The second reason for writing in Greek is, in fact, that I have yet a lot to learn, and I think that writing about science is a very good exercise, allowing me to find a solution to the translation problems I may encounter if I discuss about my job – physics – in that language.

The third reason is that I think there is no offer whatsoever in the web for a blog about particle physics in Greek (if you know any, let me know). So I might just try to fill that hole myself.

In short, the new blog is an experiment. I do not know, nor can predict, how long it will last; for now, if you know modern Greek please stop by. Below is a list of my recent efforts:

ICHEP blog July 12, 2010

Posted by dorigo in astronomy, Blogroll, cosmology, internet, news, physics, science.
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Just one line here to mention that since May there is a new blog out there – a temporary blog that will cover the end of July event in Paris – the International Conference on High Energy Physics -, how we get there, and the aftermath. The effort includes several well-known bloggers in high-energy physics, and is definitely worth following.

You can visit it here.

Some recent posts you might want to read March 6, 2010

Posted by dorigo in Blogroll, internet, news, physics, science.
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As the less distracted among you know, I have moved my blogging activities to scientific blogging last April. I wish to report here a list of interesting posts I have produced there in the course of the last few months (precisely, since the start of 2010). They are given in reverse chronological order and with zero commentary – come see if you are curious.

A reminder for the distracted October 5, 2009

Posted by dorigo in physics.
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This blog is now inactive since April 15th, 2009, and although I will try to keep it active, by posting links every once in a while to my most relevant articles on the Scientific Blogging site which now hosts my main activities, you should update your bookmarks if you have not done so yet. I keep getting about 500 daily hits here, mostly from google searches of the few good posts among the thousand and more that I have put together in over three years of activity.

So please visit my other blog at Scientific Blogging! You will not be disappointed.

One million hits June 29, 2009

Posted by dorigo in Blogroll, internet, news, personal, physics.
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While this site has been basically inactive for over two months, it still draws some residual traffic due to google searches and links; so the hit counter has continued to click after April 15th, although at a rate of roughly a third of what it did before.

Today’s news is that we got past the millionth click. Thanks to everybody for your interest in particle physics and in my reports. Please visit www.scientificblogging.com/quantum_diaries_survivor to keep up-to-date with particle physics!

Physics Highlights – May 2009 June 2, 2009

Posted by dorigo in news, physics, science.
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Here is a list of noteworthy pieces I published on my new blog site in May. Those of you who have not yet updated their links to point there might benefit from it…

Four things about four generations -the three families of fermions in the Standard Model could be complemented by a fourth: a recent preprint discusses the possibility.

Fermi and Hess do not confirm a dark matter signal: a discussion of recent measurements of the electron and positron cosmic ray fluxes.

Nit-picking on the Omega_b Discovery: A discussion of the significance of the signal found by DZERO, attributed to a Omega_b particle.

Nit-picking on the Omega_b Baryon -part II: A pseudoexperiments approach to the assessment of the significance of the signal found by DZERO.

The real discovery of the Omega_b released by CDF today: Announcing the observation of the Omega_b by CDF.

CDF versus DZERO: and the winner is…: A comparison of the two “discoveries” of the Omega_b particle.

The Tevatron Higgs limits strenghtened by a new theoretical study: a discussion of a new calculation of Higgs cross sections, showing an increase in the predictions with respect to numbers used by Tevatron experiments.

Citizen Randall: a report of the giving of honorary citizenship in Padova to Lisa Randall.

Hadronic Dibosons seen -next stop: the Higgs: A report of the new observation of WW/WZ/ZZ decays where one of the bosons decays to jet pairs.

Post summary – April 2009 May 1, 2009

Posted by dorigo in astronomy, Blogroll, cosmology, internet, news, personal, physics, science, social life.
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As the less distracted among you have already figured out, I have permanently moved my blogging activities to www.scientificblogging.com. The reasons for the move are explained here.

Since I know that this site continues to be visited -because the 1450 posts it contains draw traffic regardless of the inactivity- I am providing here monthly updates of the pieces I write in my new blog here. Below is a list of posts published last month at the new site.

The Large Hadron Collider is Back Together – announcing the replacement of the last LHC magnets

Hera’s Intriguing Top Candidates – a discussion of a recent search for FCNC single top production in ep collisions

Source Code for the Greedy Bump Bias – a do-it-yourself guide to study the bias of bump fitting

Bump Hunting II: the Greedy Bump Bias – the second part of the post about bump hunting, and a discussion of a nagging bias in bump fitting

Rita Levi Montalcini: 100 Years and Still Going Strong – a tribute to Rita Levi Montalcini, Nobel prize for medicine

The Subtle Art of Bump Hunting – Part I – a discussion of some subtleties in the search for new particle signals

Save Children Burnt by Caustic Soda! – an invitation to donate to Emergency!

Gates Foundation to Chat with Bloggers About World Malaria Day – announcing a teleconference with bloggers

Dark Matter: a Critical Assessment of Recent Cosmic Ray Signals – a summary of Marco Cirelli’s illuminating talk at NeuTel 2009

A Fascinating New Higgs Boson Search by the DZERO Experiment – a discussion on a search for tth events recently published by the Tevatron experiment

A Banner Worth a Thousand Words - a comment on my new banner

Confirmed for WCSJ 2009 – my first post on the new site

Testing the Bell inequality with Lambda hyperons April 14, 2009

Posted by dorigo in news, physics, science.
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This morning I came back from Easter vacations to my office and was suddenly assaulted by a pile of errands crying to be evaded, but I prevailed, and I still found some time to get fascinated by browsing through a preprint appeared a week ago on the Arxiv, 0904.1000. The paper, by Xi-Qing Hao, Hong-Wei Ke, Yi-Bing Ding, Peng-Nian Shen, and Xue-Qian Li [wow, I'm past the hard part of this post], is titled “Testing the Bell Inequality at Experiments of High Energy Physics“. Here is the abstract:

Besides using the laser beam, it is very tempting to directly testify the Bell inequality at high energy experiments where the spin correlation is exactly what the original Bell inequality investigates. In this work, we follow the proposal raised in literature and use the successive decays J/\psi \to \gamma \eta_c \to \Lambda \bar \Lambda \to p \pi^- \bar p \pi^+ to testify the Bell inequality. [...] (We) make a Monte-Carlo simulation of the processes based on the quantum field theory (QFT). Since the underlying theory is QFT, it implies that we pre-admit the validity of quantum picture. Even though the QFT is true, we need to find how big the database should be, so that we can clearly show deviations of the correlation from the Bell inequality determined by the local hidden variable theory. [...]

Testing the Bell inequality with the decay of short-lived subatomic particles sounds really cool, doesn’t it ? Or does it ? Unfortunately, my quantum mechanics is too rusty to allow me to get past a careful post which explains things tidily, in the short time left between now and a well-deserved sleep. You can read elsewhere about the Bell inequality, and how it tests whether pure quantum mechanics rules -destroying correlations between quantum systems separated by a space-like interval- or whether a local hidden variable theory holds instead: and besides, almost anybody can write a better account of that than me, so if you feel you can help, you are invited to guest-blog about it here.

Besides embarassing myself, I still wanted to mention the paper today, because the authors make a honest attempt at proposing an experiment which might actually work, and which could avoid some drawbacks of all experimental tests so far attempted, which belong to the realm of quantum optics. In their own words,

Over a half century, many experiments have been carried out [...] among them, the polarization entanglement experiments of two-photons and multi-photons attract the widest attention of the physics society. All photon experimental data indicate that the Bell inequality and its extension forms are violated, and the results are fully consistent with the prediction of QM. The consistency can reach as high as 30 standard deviations. [...] when analyzing the data, one needs to introduce additional assumptions, so that the requirement of LHVT cannot be completely satisfied. That is why as generally considered, so far, the Bell inequality has not undergone serious test yet.

Being totally ignorant of quantum optics I am willing to buy the above as true, although, being a sceptical son of a bitch, the statement makes me slightly dubious. Anyway, let me get to the point of this post.

Any respectable quantum mechanic could convince you that in order to check the Bell inequality with the decay chain mentioned above, it all boils down to measuring the correlation between the pions emitted in the decay of the Lambda particles, i.e., the polarization of the Lambda baryons: in the end, one just measures one single, clean angle \theta between the observed final state pions. The authors show that this would require about one billion decays of the J/\psi mesons produced by an electron-positron collider running at 3.09 GeV center-of-mass energy (the mass of the J/psi resonance): this is because the decay chain involving the clean \Lambda \bar \Lambda final state is rare: the branching fraction of J/\psi \to \eta_c \gamma is 0.013, the decay \eta_c \to \Lambda \bar \Lambda occurs once in a thousand cases, and finally, each Lambda hyperon has a 64% chance to yield a proton-pion final state. So, 0.013 times 0.001 times 0.64 squared makes a chance about as frequent as a Pope appointment. However, if we had such a sample, here is what we would get:

The plot shows the measured angle between the two charged pions one would obtain from 3382 pion pairs (resulting from a billion J/\psi \to \eta_c \gamma decays through double hyperon decay) compared with pure quantum mechanics predictions (the blue line) and by the Bell inequality (the area within the green lines). The simulated events are taken to follow the QM predictions, and such statistics would indeed refute the Bell inequality -although not by a huge statistical margin.

So, the one above is an interesting distribution, but if the paper was all about showing they can compute branching fractions and run a toy Monte Carlo simulation (which even I could do in the time it takes to write a lousy post), it would not be worth much. Instead, they have an improved idea, which  is to apply a suitable magnetic field and exploit the anomalous magnetic moment of the Lambda baryons to measure simultaneously their polarization along three independent axes, turning a passive measurement -one involving a check of the decay kinematics of the Lambda particles- into an active one -directly figuring out the polarization. This is a sort of double Stern-Gerlach experiment. Here I would really love to explain what a Stern-Gerlach experiment is, and even more to make sense of the above gibberish, but for today I feel really drained out, and I will just quote the authors again:

One can install two Stern-Gerlach apparatuses at two sides with flexible angles with respect to according to the electron-positron beams. The apparatus provides a non-uniform magnetic field which may decline trajectory of the neutral \Lambda (\bar \Lambda) due to its non-zero anomalous magnetic moment i.e. the force is proportional to d/n (- \mu B) where \mu is the anomalous magnetic moment of \Lambda, B is a non-uniform external magnetic field and d/n is a directional derivative. Because \Lambda is neutral, the Lorentz force does not apply, therefore one may expect to use the apparatus to directly measure the polarization [...]. But one must first identify the particle flying into the Stern-Gerlach apparatus [...]. It can be determined by its decay product [...]. Here one only needs the decay product to tag the decaying particle, but does not use it to do kinematic measurements.

I think this idea is brilliant and it might actually be turned into a technical proposal. However, the experimental problems connected to setting up such an apparatus, detecting the golden decays in a huge background of impure quantum states, and capturing Lambdas inside inhomogeneous magnetic fields, are mindboggling: no wonder the authors do not have a Monte Carlo for that. Also, it remains to be seen whether such pains are really called for. If you ask me, quantum mechanics is right, period: why bother ?

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