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Some posts you might have missed in 2008 – part II January 6, 2009

Posted by dorigo in physics, science.
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Here is the second part of the list of useful physics posts I published on this site in 2008. As noted yesterday when I published the list for the first six months of 2008, this list does not include guest posts nor conference reports, which may be valuable but belong to a different place (and are linked from permanent pages above). In reverse chronological order:

December 29: a report on the first measurement of exclusive production of charmonium states in hadron-hadron collisions, by CDF.

December 19: a detailed description of the effects of parton distribution functions on the production of Z bosons at the LHC, and how these effects determine the observed mass of the produced Z bosons. On the same topic, there is a maybe simpler post from November 25th.

December 8: description of a new technique to measure the top quark mass in dileptonic decays by CDF.

November 28: a report on the measurement of extremely rare decays of B hadrons, and their implications.

November 19, November 20, November 20 again , November 21, and November 21 again: a five-post saga on the disagreement between Lubos Motl and yours truly on a detail on the multi-muon analysis by CDF, which becomes a endless diatriba since Lubos won’t listen to my attempts at making his brain work, and insists on his mistake. This leads to a back-and-forth between our blogs and a surprising happy ending when Motl finally apologizes for his mistake. Stuff for expert lubologists, but I could not help adding the above links to this summary. Beware, most of the fun is in the comments threads!

November 8, November 8 again, and November 12: a three-part discussion of the details in the surprising new measurement of anomalous multi-muon production published by CDF (whose summary is here). Warning: I intend to continue this series as I find the time, to complete the detailed description of this potentially groundbreaking study.

October 24: the analysis by which D0 extracts evidence for diboson production using the dilepton plus dijet final state, a difficult, background-ridden signature. The same search, performed by CDF, is reported in detail in a post published on October 13.

September 23: a description of an automated global search for new physics in CDF data, and its intriguing results.

September 19: the discovery of the \Omega_b baryon, an important find by the D0 experiment.

August 27: a report on the D0 measurement of the polarization of Upsilon mesons -states made up by a b \bar b pair- and its relevance for our understanding of QCD.

August 21: a detailed discussion of the ingredients necessary to measure with the utmost precision the mass of the W boson at the Tevatron.

August 8: the new CDF measurement of the lifetime of the \Lambda_b baryon, which had previously been in disagreement with theory.

August 7: a discussion of the new cross-section limits on Higgs boson production, and the first exclusion of the 170 GeV mass, by the two Tevatron experiments.

July 18: a search for narrow resonances decaying to muon pairs in CDF data excludes the tentative signal seen by CDF in Run I.

July 10: An important measurement by CDF on the correlated production of pairs of b-quark jets. This measurement is a cornerstone of the observation of anomalous multi-muon events that CDF published at the end of October 2008 (see above).

July 8: a report of a new technique to measure the top quark mass which is very important for the LHC, and the results obtained on CDF data. For a similar technique of relevance to LHC, also check this other CDF measurement.

Some posts you might have missed in 2008 January 5, 2009

Posted by dorigo in cosmology, personal, physics, science.
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To start 2009 with a tidy desk, I wish to put some order in the posts about particle physics I wrote in 2008. By collecting a few links here, I save from oblivion the most meaningful of them -or at least I make them just a bit more accessible. In due time, I will update the “physics made easy” page, but that is work for another free day.

The list below collects in reverse chronological order the posts from the first six months of 2008; tomorrow I will complete the list with the second half of the year. The list does not include guest posts nor conference reports, which may be valuable but belong to a different list (and are linked from permanent pages above).

June 17: A description of a general search performed by CDF for events featuring photons and missing transverse energy along with b-quark jets – a signature which may arise from new physics processes.

June 6: This post reports on the observation of the decay of J/Psi mesons to three photons, a rare and beautiful signature found by CLEO-c.

June 4 and June 5 offer a riddle from a simple measurement of the muon lifetime. Readers are given a description of the experimental apparatus, and they have to figure out what they should expect as the result of the experiment.

May 29: A detailed discussion of the search performed by CDF for a MSSM Higgs boson in the two-tau-lepton decay. Since this final state provided a 2.1-sigma excess in 2007, the topic deserved a careful look, which is provided in the post.

May 20: Commented slides of my talk at PPC 2008, on new results from the CDF experiment.

May 17: A description of the search for dimuon decays of the B mesons in CDF, which provides exclusion limits for a chunk of SUSY parameter space.

May 02 : A description of the search for Higgs bosons in the 4-jet final state, which is dear to me because I worked at that signature in the past.

Apr 29: This post describes the method I am working on to correct the measurement of charged track momenta by the CMS detector.

Apr 23, Apr 28, and May 6: This is a lengthy but simple, general discussion of dark matter searches with hadron colliders, based on a seminar I gave to undergraduate students in Padova. In three parts.

Apr 6 and Apr 11: a detailed two-part description of the detectors of electromagnetic and hadronic showers, and the related physics.

Apr 05: a general discussion of the detectors for LHC and the reasons they are built the way they are.

Mar 29: A discussion of the recent Tevatron results on Higgs boson searches, with some considerations on the chances for the consistence of a light Higgs boson with the available data.

Mar 25: A detailed discussion on the possibility that more than three families of elementary fermions exist, and a description of the latest search by CDF for a fourth-generation quark.

Mar 17: A discussion of the excess of events featuring leptons of the same electric charge, seen by CDF and evidenced by a global search for new physics. Can be read alone or in combination with the former post on the same subject.

Mar 10: This is a discussion of the many measurements obtained by CDF and D0 on the top-quark mass, and their combination, which involves a few subtleties.

Mar 5: This is a discussion of the CDMS dark matter search results, and the implications for Supersymmetry and its parameter space.

Feb 19: This is a divulgative description of the ways by which the proton structure can be studied in hadron collisions, studying the parton distribution functions and how these affect the scattering measurements in proton-antiproton collisions.

Feb 13: A discussion of luminosity, cross sections, and rate of collisions at the LHC, with some easy calculations of the rate of multiple hard interactions.

Jan 31: A summary of the enlightening review talk on the standard model that Guido Altarelli gave in Perugia at a meeting of the italian LHC community.

Jan 13: commented slides of the paper seminar gave by Julien Donini on the measurement of the b-jet energy scale and the p \bar p \to Z X \to b \bar b X cross section, the latter measured for the first time ever at a hadron machine. This is the culmination of a twelve-year effort by me and my group.

Jan 4: An account of the CDF search for Randall-Sundrum gravitons in the ZZ \to eeee final state.

Scientific wishes for 2009 December 31, 2008

Posted by dorigo in astronomy, Blogroll, cosmology, personal, physics, science.
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I wish 2009 will bring an answer to a few important questions:

  • Can LHC run ?
  • Can LHC run at 14 TeV ?
  • Will I get tenure ?
  • Are multi-muons a background ?
  • Are the Pamela/ATIC signals a prologue of a new scientific revolution ?
  • Will England allow a NZ scientist to work on Category Theory on its soil ?
  • Is the Standard Model still alive and kicking in the face of several recent attempts at its demise ?

I believe the answer to all the above questions is yes. However, I am by no means sure all of them will be answered next year.

Where the heart beats October 16, 2008

Posted by dorigo in personal, physics, science.
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The picture above made the headlines today in repubblica.it, one of the news sites I read most often. It shows a little mouse, originally intended as a meal supplied to a viper, managing to invert the food chain, killing the monster. I am stating the obvious when I say we usually rejoice when we see something like that happening: we always root for the weak against the strong, especially if we feel weak ourselves -and don’t we all, in some respect ?

The picture had me thinking about the competition between the Tevatron and LHC particle accelerators. Since the LHC has not produced a single proton-proton collision in the core of the CMS and Atlas detectors yet, while the Tevatron has supplied CDF and D0 with an enormous amount of collisions which are fruiting scores of groundbreaking physical results, the two machines might be argued to not be competing yet. But that would be a myopic assessment, on several levels.

  • There is competition for the funding of experiments. Lab directors, experiment heads, and faculty members are very sensitive to this issue of course, since it affects their chances of leadership and power. Funds to particle physics experiments are getting cut these days, and the many experiments have to fight against each other to keep their budget plans intact. The LHC has been competing for funding with the rest of the HEP facilities since the start of its construction, or arguably even before then.
  • Then there is of course the fight for a place under the media spotlights. It is a level of competition tightly connected with the funding one, but it has some additional branches, since the media attention can be an important fuel to boost the career of scientists. The recent media hype for the startup of LHC on September 10 was a masterful organization by CERN general director Aymar, and the following incident with the magnets in sector 34 was troublesome to CERN as much for its media impact as it was for the lab schedule. Of course, at the Tevatron and elsewhere many met the global interest for LHC in the former occasion with ill-concealed jealousy, and the latter with more evident satisfaction.
  • A third level of competition, in turn somewhat connected to the second, has its ground on the scientific conferences around the world which are now scheduled every second week. Presenting scientific results at conferences is a very important item in the construction of a strong curriculum, and some talks increase the prestige of the speakers. The Tevatron has had a large share of talks allotted at all the major conferences in the recent past, due to the mass of new results it produced; but Atlas and CMS have started being allotted several talks already, which will be used to discuss “Monte Carlo analyses” rather than results on real data. Still, this causes a compression of the benefits of the Fermilab scientists.
  • And then there is the fight for the Higgs. The Tevatron experiments are still caressing hopes to find the Higgs boson before LHC does, and are thus squeezing their brains to improve the already excellent analyses they have been producing. This year, for the first time, a direct limit on the Higgs boson mass has been set by CDF and D0 in a joint effort. Although the limit is not stringent yet (a single mass value has been excluded, 170 GeV), the two experiments are working to wipe off the board the whole high-mass region, where the LHC would have no trouble in finding a significant signal with one year worth of data. If the Higgs is proven to be lighter than 130 GeV or so, the fight between the two sides of the Atlantic is promised to become red-hot in the next two-three years, depending on whether the Tevatron gets funded to run for the fiscal year 2010.
  • Finally, there might be new physics out there, and it might still be at reach of the Tevatron. The six-months delay of the LHC data taking is making this race even more interesting, especially since CDF and D0 are reaching a level of sensitivity in the SUSY parameter space that might enable them to discover new physics before LHC.

So, as you see, there is competition in HEP between LHC and the Tevatron even if the former has not begun taking data yet. The mouse of the picture above, in my mind, is the Tevatron biting the LHC’s throat, turning the tables when everybody expects the latter to entertain itself with a quiet meal. This is still possible. If CDF or D0 discovered SUSY, such an event would be a defeat of incredible proportions for CERN, echoing the disasters of the sixties and the seventies, when the US were banqueting lavishly with new discoveries, barely leaving bread crumbs for Europe.

And where do you stand“, you might well ask, since I am working for both CMS at CERN and CDF at the Tevatron ? Well, I am slightly embarassed to answer, but I must say that until CDF closes down, my heart beats for it. The first love is the one you never forget, and to that piece of rethorics today I can add the image of the little mouse. Curious feeling: I am spending 80% of my research time doing my best to help make CMS a success, but I still caress some hope that it will, at least to some extent, fail to the hands of my former love. If you think that is despicable, please consider: what is important is not who gets a Nobel prize here. It is a win-win situation for whom, like me and most of you, only cares for the advancement of science. CMS, Atlas, CDF, D0: who cares ? All I care is to find out the truth!

Another pro-LHC top mass measurement October 3, 2008

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A few months ago I reported here on a CDF technique to measure the mass of the top quark without relying on hadronic jets, whose energy measurement is plagued by many systematic uncertainties. Techniques not relying on the calorimetric measurement of jet energy deposits are quite important for the CMS and ATLAS experiments at the LHC, since the jet energy scale will be very difficult to determine with sufficient accuracy there.

A large statistics of top pair production events is warranted by the 14 TeV proton-proton collisions of the LHC: with respect to the Tevatron, a factor of 100 increase in cross section will be due to the x7 increase in energy, and will be compounded by a factor of 10 increase in instantaneous luminosity.  The rate of production of events of any kind is given by the master formula

\large N = \sigma L,

where N is the rate, in Hertz, of events produced, \sigma is the cross section responsible for the production, and L is the instantaneous luminosity.

The above means that LHC, after the initial warm-up phase (when energy will be 10 TeV and luminosity will be low), will be producing top quarks at a rate a thousand times higher than what the Tevatron is doing now. This huge statistics will thus allow CMS and ATLAS to extract precise determinations of the top mass from otherwise statistics-limited methods, provided these come with light-weight systematic uncertainties.

And the Tevatron is providing another one proof-of-principle. A new result by CDF uses events selected with a single-lepton topology: one of the top quarks decays to three hadronic jets, the other decays to a jet plus a lepton – neutrino pair. The lepton (an electron or a muon, with a transverse energy larger than 20 GeV) triggers the event collection with high efficiency, and its presence reduces backgrounds quite effectively. After some additional, now standard, selection cuts the data sample contains a large fraction of real top-pair decays. At this point, we note that two of the hadronic jets produced by the fragmentation of quarks emitted in the top decay process are in truth due to b-quarks: each top quark almost always produces a b-quark in its decay, in fact.

Just as the top quark may yield a lepton in its decay (see graph on the right, which describes also decays only yielding quarks), through the chain t \to W b \to l \nu b (l stands for the lepton), the b-quark may also decay “semi-leptonically”, as this particular chain is called; in the case of the b-quark, the chain is b \to W^* c \to l \nu c, where the superscript asterisk on the W stresses the fact that this particle is virtual, having much less energy than its rest mass.

Because the mass of the b-quark is light with respect to that of the heavy top, all bodies produced in its decay remain within the jet: the b-quark is emitted from the top decay with a large momentum, and the same momentum is imparted to the b daughters, which conserve the original quark direction. The jet will therefore often contain an identified electron or muon. The new technique focuses on these additional, “soft” leptons produced within the hadronic jet, by noting that the invariant mass of the combination between the soft lepton and the primary lepton which triggered the event -the one directly coming from the top quark decay chain- is a kinematical quantity strongly correlated with the mass of the decayed top quark. By measuring the former, one gets information on the latter!

Above, the average soft lepton-trigger lepton mass is shown as a function of the top mass, as predicted by a Monte Carlo simulation of top decays with different input mass values. The correlation is linear and well-behaved.

Alice Bridgeman, Lucio Cerrito, Ulysses Grundler, and Xiaojian Zhang, a group of physicists from University of Illinois led by Tony Liss (left in the picture; also shown Cerrito, second from left, and Grundler, last on the right), a veteran in CDF with two decades of experience in top quark physics, exploited the above feature with a sample of 2 inverse femtobarns of proton-antiproton collisions. 240 candidate top-pair events, containing about 130 real top-pair decays, were selected to contain a single-lepton topology and a soft-lepton in one of the jets. The invariant mass distribution of the trigger lepton-soft lepton pair resulting from the above selection is shown in the picture below.

The black points describe the dilepton mass in the data, the cyan histogram is the background contribution, and the blue line shows the likelihood fit. The inset shows the likelihood values as a function of the unknown top mass value. The minimum is found at M_t = 181.3 \pm 12.4 \pm 3.5 GeV/c^2: the first uncertainty is statistical, and it shows that the method is not useful at the Tevatron -where CDF and D0 have already measured the top mass with an uncertainty of less than two GeV. However, the systematic uncertainty is much smaller, and it can still be reduced by more accurate studies. Most importantly, systematic effects due to the measurement of the jets in the calorimeter are totally avoided by this technique.

More information on this particular analysis is available in the public page of the measurement.

6 billion euros a year September 30, 2008

Posted by dorigo in news, science.
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That is the amount of money, according to a report heard on the radio today, that italians spend yearly to consult magicians, astrologers and other assorted crooks. This, according to the investigation, is money spent across the board -all geographical parts of Italy contribute evenly, and people with different levels of instructions are affected in a similar way, from the tycoon who needs to take a financial decision to the underdog who wants to get his loved one to fall for him.

Six billion euros a year is a gigantic amount of money. Just imagine: if one year those people decided to spend their money on something less idiotic, we could build another LHC with the money they instead choose to throw out of the window, fostering a crew of parasites of this sick society based on superstition rather than reason. Of course, when people argue that building the LHC was “a waste of taxpayers’ money”, their mind is not crossed by the thought that it would be just so much better to strip those taxpayers of a bit more cash, to build a few more scientific experiments, given the way they instead use it.

An agorà of education and scientific communication ? September 23, 2008

Posted by dorigo in internet, news, personal, physics, politics, science.
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These are strange days. Information runs around the world so fast, one can be at the center of the action and still learn details of what is really going on from the other side of an ocean. In fact, CERN appears a bit up-tight about the latest events in sector 34 of the LHC tunnel. People ask me questions in the comments thread of posts here, but I have less information to provide than others; and I am happy it is so, since my blog is targeted as a possible source of leaks, and I decided I want this to be a place where people get educated about science, and not about scientific rows. And if I play fair, maybe I am allowed to survive here, and maybe one day I will stop being threatened every other day, in the name of protecting internal information of the experiments I am part of.

Of course, I still assert my complete disagreement at a way to conduct scientific experiments paid with your tax money which resembles the management of the Pentagon rather than an agorà of education, research, and scientific communication.

So, by all means, if you have information you want to share, anonymously or not, you are welcome to comment, but please, do not ask me for any.

Google LHC search results September 10, 2008

Posted by dorigo in internet, personal.
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Despite the fact that this blog received several “heavy” links from high-traffic sites (NYT, NEW, Cosmic Variance, plus a dozen lighter ones) between yesterday and today, a good portion of the thousands of readers who crowded this site landed here somewhat misguided by google, which is not too smart in its searches, and it reports some relatively older posts of mine at the very top of its list for the following:

plus a few more combinations, including CMS control room. I apologize to internauts who arrived here (or rather, to the outdated posts linked by google) and found nothing of what they were searching for – the fault is with Google, not with me!

The Say of the Day September 10, 2008

Posted by dorigo in games, humor, internet, news, physics, science.
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Well, with things moving as fast as today – and with thousands of people around the world frantically clicking links, looking at video feeds, trying to make things make sense to them… Well, for today and maybe tomorrow I feel that “the Say of the Week” traditional post here becomes a daily thing. So here it is, your Say of the Day, nicely brought to you by Jester, talking about the LHC beams circulating smoothly in his quite appreciated live blogging today:

“Robert Aymar said that it’s working smooth as a roulette. I hope he didn’t mean Russian roulette.”

Interviewed for Nature (the magazine…) September 10, 2008

Posted by dorigo in internet, news, personal, physics, science.
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Yesterday I had lunch at the Meyrinoise (courtesy Nature) with Geoff Brumfiel, a reporter from Nature (the magazine, not the bitch) who came to CERN to witness the big media event of today. We talked about several things, but in the end what was left to discuss for the podcast we recorded was the least interesting thing of all -the fact that we are not going to disappear in a black hole after LHC will eventually start colliding beams operation (which, for the absent-minded among you, hasn’t happened yet -only a beam at a time has been circulated in the machine so far).

In any case, you can hear some more interesting interviews along with mine at the Nature site, specifically here.

UPDATE: fixed the link to the main page of Nature news. There, you find my pic (not a good one actually) linking to the interviews.

UPDATE: Hmm, if Nature (the magazine AND a little bit of a bitch today) keeps changing the address of pages I link, I am going to download the darn site here and stop worrying. Anyway, here is the updated link to the LHC special.