Smolin interviewed by IEEE Spectrum January 7, 2007
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Stephen Cass, senior editor of IEEE Spectrum, interviewed Lee Smolin about the aftermath of his new book, “The Trouble with Physics”. You can read the interview at http://www.spectrum.ieee.org/jan07/4812 . Nothing really new, but still an interesting read.
How do I like my Mw - Mt plot January 7, 2007
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A further note about the plot I posted a few days ago, attached here again for convenience. For the series “the devil hides in the details”… I am such a pain in the neck when it comes to proper display of scientific data, I realize. But I cannot help it.
So. The more I look at this plot, the less I like it.
First of all, it is wrong. The CDF measurement is at 80413 MeV, so the star in the middle of the black line should lie at around halfway between the 80.4 mark and the next one (80.425). But it is much lower than that: if you look at it closely, it seems to lie at about 80.406 or so. The same applies if you look at the extremes of the error bar: looks like 80.455 and 80.360 or so, while you’d expect 80.461 and 80.365.
Another subtle point which disturbes me is that since the red ellipse shows indirect measurements by LEP I and SLD, and since LEP II measured the W mass but give no clue about the top mass, putting together LEP II and Tevatron determinations into one single ellipse (the blue one) is deceiving and unfair to the Tevatron.
Indeed, while the Tevatron experiments can draw an ellipse with direct determinations of both the W and top quark masses, the LEP II experiments can only draw a horizontal band in that plot.
So it would suit my taste better to have a plot with:
- the odd-shaped ellipse of indirect measurements
- a band combining all LEP II measurements of the W mass
- an ellipse for Tevatron measurements of top and W masses
- another ellipse, centered at the same top mass value as the former, with the world average of direct measurements
- a bar with the latest (not yet averaged) Run II CDF W mass result.
All labeled up properly, please.
QFT books review January 7, 2007
Posted by dorigo in Blogroll, books, internet, physics, science.add a comment
I feel compelled to post a deserved link to Fliptomato’s blog: http://fliptomato.wordpress.com/2006/12/30/from-griffiths-to-peskin-a-lit-review-for-beginners/ .
He discusses in good detail his personal views and experiences with many common textbooks about quantum field theory for beginners and advanced students. While his list is not complete (I own at least two books at entry level not mentioned there - no, I did not say I’ve read them) and his opinions might be questioned here and there, I think it is a great starting point for anybody who wants to peek into the tools of the trade.
A black hole found inside a globular cluster! January 7, 2007
Posted by dorigo in Blogroll, astronomy, internet, news, physics, science.1 comment so far
Through The Arcadian Functor and Babe in the Universe (see links in the blogroll column) I learned that a team searching for black holes through rapidly varying x-ray signals has found a intermediate-size black hole inside a globular cluster in the halo of NGC4472, a not-too-conspicuous ellyptical galaxy in the Virgo cluster (also known as M49, see picture on the right).
By the way, I saw it through my dobson telescope last summer… It is a Mv=8.5 object, and I wonder how faint is the globular they found to contain the black hole.
Anyway. According to the researchers, they were prepared for a long search, but they “found one as soon as we started the search. It was only the second globular cluster we looked at.”
Besides the wonderful and intriguing implications of this observation - black holes are not expected in the core of globular clusters if you ask the experts, although I had always thought it was a pretty obvious guess that they must be there - there remains the excitations for the next bunch of data they will produce, and a question.
Indeed, the method they adopted to find black holes was to look for rapidly varying x-ray sources associated to visible light objects (the globulars) with a extremely high spatial resolution probe, the European Space Agency’s XMM-Newton satellite (see http://www.esa.int/esaCP/SEML0QZTIVE_index_0.html). But for sure they did not specifically aim at that globular cluster to see the x-ray signal! They have sensitivity to large areas of the sky. So the claim that the globular was “the second they looked at” is a lie deceiving.
What they probably did was to observe a source of x-rays, and then match it to a list of known objects lying close by. The second in the list probably matched spatially to the x-ray signal well enough to provide certainty of an identity between light and x-ray source.
I would be happy to be mistaken… Does anybody know more about this issue ?
The new number January 7, 2007
Posted by dorigo in news, physics, science.2 comments
Ok, in the last post I had originally intended to put the new top mass measurement along with an explanation of the analysis, but I ended up describing more generally the problem of detecting all-hadronic decays of top quark pairs… So I need a new post to write about the more technical information. This one.
The new result in the all-hadronic decay mode by CDF, based on 943 pb-1 of Run II data, is:
Mt=171.1+-3.7(stat+JES)+-2.3(syst) GeV
where the first uncertainty is the combination of statistical uncertainty and jet energy scale systematics, and the second combines all remaining systematic uncertainties.
The picture on the left shows the result for the subset of selected events which contain two jets b-tagged by the vertex identification algorithm: the yellow distribution is the estimated mass shape of the top events in the sample.
The result is obtained through a technique which is becoming a standard in CDF. The technique consists in performing a kinematic fit with an additional parameter beyond the top mass: the jet energy scale, which receives a strong constraint from the W->jj resonance. The jet energy scale is a number close to 1.0 which tells us if the energy correction algorithm we apply on Monte Carlo simulated jets behaves the same way or not on real jets: if we over-correct the jets in the data, we will then have a positive bias in whatever measurements we obtain from a comparison of Monte Carlo distributions to the data. Conversely, if we over-correct the Monte Carlo, we will tend to underestimate the measurement. I discussed in detail the issue of the determination of the jet energy scale in three posts a few months ago, see:
http://dorigo.wordpress.com/2006/06/16/jet-energy-scale-for-total-beginners/
http://dorigo.wordpress.com/2006/06/19/e-scale-what-do-we-do-with-it-2/
http://dorigo.wordpress.com/2006/06/20/three-jet-energy-resolution-for-novices/
Anyway, let me discuss for a second the new result and its implications.
Overall, the total error bar is 4.3 GeV wide, and this measurement will therefore contribute appreciably to the shrinking of the Tevatron average (now sitting at Mt=171.4+-2.1 GeV).
It is to be noted, however, that the data sample on which this result is based is the same of previous measurements in the all-hadronic final state, the latest of which (based on 1020 pb-1) was given last summer at Mt=174.0+-2.2(stat)+-4.8(syst) GeV and entered in the Tevatron average quoted above, and shown in the plot on the left. So it is kind of hard to eyeball what will this measurement do to the world average, especially given the correlation of the jet energy scale systematic uncertainty with the corresponding systematics of the most precise results based on the single lepton final states. I would guess that the average will go down by a few tenths of a GeV, and the uncertainty will also shrink by maybe a tenth of a GeV.
So, what is the bottom line? That the Higgs boson is getting leaner by the day!!! Yesterday I quoted the new W mass result, which, being higher than the previous world average, drives down the Higgs mass estimate. Today, the top quark is getting lighter, also driving the Higgs down… I am quite curious to see these new results fed into the global electroweak fits!
A new precise top mass measurement with jets January 7, 2007
Posted by dorigo in news, personal, physics, science.8 comments
CDF has blessed last Thursday a new precise measurement of the top quark mass, obtained from the decay of top quark pairs to six hadronic jets. In this post I will provide some background information, while in the next one I will give the measurement and discuss more technical details and additional information about it.
The “6-jet”, or “all-hadronic” decay mode of top quark pairs arises when both W bosons emitted in the disintegration of top and antitop produce a pair of quarks (see diagram on the left). Each of the six quarks (the two additional ones are b-quarks from the top decays) fragments in a stream of hadrons, which we see in our detector as a localized energy deposit, a cluster.
The six clusters are not easy to measure well, and a lot of instrumental and physical effects make a precise reconstruction of the decay kinematics quite hard to achieve from that information alone.
Moreover, this decay mode of the top pair is made tough to observe by the huge rate of QCD processes that can mimic the same final state, but which did not involve the decay of top quarks. To give you an idea, a sample of ten thousand events with six jets is expected to contain just two or three top pair decays before a careful selection is operated.
It is because of these problems that when the top quark was first sought in CDF and D0 during Run I, people focused on the golden signatures involving one or two charged leptons: the W boson decay can indeed produce a lepton-neutrino pair - 11% of the times for each lepton species. So people was then looking for events with a high energy electron or muon, a few additional hadronic jets (from the other W and the two b quarks) and missing energy from the neutrino (the single-lepton signature) or events with two charged leptons, missing energy, and just two jets.
Indeed, the top quark was discovered in those golden signatures in 1995, but two years afterwards a paper appeared in PRL: “First observation of the all-hadronic decay of top quark pairs” (a copy is available at http://www-cdf.fnal.gov/physics/preprints/cdf4025_top_allhad_prl.ps.gz), which was in part the product of my undergraduate studies - my laurea thesis. I am proud to say that most of the technology still used today to extract top quarks from the all-hadronic mode was invented by my group in the early nineties, when people frowned at us because they thought ours was just a lost cause. But we did find a signal and measure a cross section and mass!
In Run II people have realized that with the improved b-tagging capabilities of CDF and D0, and refined selection criteria based on neural networks (but still founded on our old kinematical analysis) the all-hadronic final state can indeed contribute sizably to the overall knowledge of the top quark.
This week’s result by CDF, mainly the effort of Jim Lungu and his colleagues of the University of Florida, is only the last of a string of measurements based on that decay topology. I contributed to the first few Run II results but then left that line of research, concentrating on other issues, but I still salute with pride a new precise measurement of the top quark mass, which will contribute to the world average when added to the other determinations.
