A Quantum Diaries Survivor

“One reason we like supersymmetry is that we haven’t seen any of the particles“

Michael Weinberger (interviewed here). What can I say… “So far so good”

A new, hot-of-the-press, combination of the Higgs boson search results by the CDF and D0 collaborations at Fermilab is out. The combination is based on a score of analyses based on one to two inverse femtobarns of integrated luminosity collected by the two experiments at the Tevatron collider since the start of Run II. Both CDF and D0 already have collected almost twice that amount of data, but they are still analyzing the remainder. So let us look at the current status of Higgs boson searches with the very informative summary plot shown below.

It has become customary, in this search, to show as a function of the unknown Higgs boson mass the 95% confidence level limit on Higgs production as a limit on the ratio between excluded cross section and standard model prediction. A ratio smaller than unity means that the Higgs boson should not be there if we believe the standard model to hold; a ratio larger than unity still allows the existence of the particle at that mass value. As years go by, the exclusion curve has gone down in small steps as a result of the inclusion of more data and, crucially, continuous improvements in the analysis techniques.

In the plot, the black line shows the actual limit found at the Tevatron. You can see by yourself that the limit is at 1.1 times the standard model for Higgs mass of 160 GeV - the most sensitive point in the whole mass region. What that means is that CDF and D0 are not quite able to exclude the existence of the Higgs boson in any mass point, but they have gotten very, very close to achieve that. But the line also tells more. Indeed, the fact that the limit obtained is lower, at 160 GeV, than that expected from pseudoexperiments (the band called “expected limit”), shows that there is less data at 160 GeV than what was expected by backgrounds alone. A normal downward fluctuation, and in fact the black line is only one standard deviation below the average expectation. But that also gives one the scale of what the limit really means.

In fact, a 95% confidence level limit says very, very little. It is like describing a probability distribution by saying, “oh, well, on this side there is only 5% of it”. I explained elsewhere what is hidden behind a claim of a 95% cl exclusion, and will not repeat the whole issue here. Suffices to say that if the limit at the Tevatron is 1.1 x SM at 160 GeV, this could mean that the real SM cross section has indeed been “excluded” at xx% (with xx<95) confidence level, since the most probable value of the Higgs cross section implied by Tevatron data must be below the SM prediction to have produced a limit at the quoted value.

In other words, if the probability distribution for the Higgs cross section were all lying above 1.0xSM, and only 5% of it leaked above 1.1xSM, one would conclude it is a very, very narrow distribution - and indeed, it would be a measurement!, since such a narrow distribution would definitely not be compatible with zero cross section: the Higgs boson having a non-null cross section would mean it exists. But the above is of course not the case, and the probability distribution is very well compatible with zero - so it extends down and it is wide. How wide ? Well, if one examines the expected limit, one gets the scale of how wide the observed limit might be by examining the +- 1-sigma countours of the expectation. So, xx% is not small… Eighty percent ? I guess maybe 85. That would mean that the likelihood that the Higgs boson mass is at 160 GeV has already shrunk considerably if we consider all the available Tevatron data… Bad news for LHC experiments, whose sensitivity is highest if the Higgs has a mass of 160 GeV (but the LHC will find the Higgs anywhere, given enough statistics).

Finally, let me add to this note a word of self-praise. Two months ago I discussed the D0 limit on Higgs cross sections in the WW production mode, and I ventured to make the following prediction:

After seeing this plot, which reaches a x2.4 SM value at 160 GeV, I am starting to be very curious to see the combination with CDF results (which stood at x1.9 SM at 160 GeV already last August). I think we will have to wait for winter conferences to get that plot, but I smell a x1.1 limit at 160 GeV: not yet any mass exclusion for winter 2008 (for the latest combination, yielding x1.4 SM, see here).

My prediction was based on incomplete results from D0, and I did not have insider information on the CDF limit in the WW final state in my hands yet… A good call.

A top-notch theoretical physicist who sometimes reads this blog recently asked me how much trust could be given to the Higgs discovery projections of the Tevatron experiments, in light of the fact that current averages appear quite a bit worse.

I explained that my understanding is that one must not just look at the availability of data, but also consider the time available to make the analyses, because of the complexity of the required tools and the need of understanding the subtlest details about the detector, the backgrounds, and the algorithms. I made the example of the top quark mass, whose uncertainty decreased by a half in the five years following the end of Run I, as CDF and D0 refined their analyses. However, I could not provide quantitative information about the Higgs in addition to current limits and 2003 projections on the Tevatron reach.

I can add a bit of information here, since from a talk by Darien Wood which was given today at a P5 meeting at Fermilab I obtained a plot of more recent predictions made by the D0 experiment. It is shown below.

In the graph you see, as a function of the unknown mass of the standard model Higgs boson, the luminosity needed by each of two “D0-like” experiments to exclude at 95% C.L. (red curve) or find at three-sigma significance (blue curve) a Higgs boson by combining their results. The red curve is computed by assuming the Higgs boson is not there, while the blue curve assumes the presence of the boson. Please also note that these curves are a median of wide distributions of the luminosity required to exclude or find the Higgs: the two experiments may “get lucky” and find a 3-sigma signal with much less data than the one shown by the black curve; similarly, no guarantee is given that given a certain amount of luminosity, an exclusion or a signal will be obtained. Finally, the blue lines at 5.5 and 6.8 inverse femtobarns of analyzed data per experiment refer to what is now expected that the Tevatron will obtain by running through 2009 or through 2010.

The graph implicitly assumes that D0 and CDF will more or less obtain the same sensitivity on the Higgs with their searches; it also pictures the “state of the art” of the searches, and thus does not include the possible improvements that more time and more experience will certainly bring. Nonetheless, it is a reasonable datum with which to argue whether the Tevatron will be justified in running through 2010, as will be done in the ongoing P5 meeting at Fermilab.

If one were to assume that no improvements will be made to present-day analyses, one would conclude that by the end of 2009 the Higgs could be probably excluded up to 120 GeV and in the range 145-180 GeV. These are very conservative estimates, but bear in mind they are averages. The real result may be different. I however continue to hold that the Tevatron will do better than that…

The italian workshop on LHC physics which is taking place in Perugia offered a few enlightening lectures this morning. The high point was reached when Guido Altarelli took the stage for a 50′ talk on the status of the Standard Model.

Guido is a distinguished particle theorist who has worked in quantum chromodynamics (QCD) and standard model physics since their birth; he gave a mass of important contributions to the matter, and he is best known for one cornerstone of QCD: the famous Altarelli-Parisi equation (which some call Dokshitzer-Gribov-Lipatov-Altarelli-Parisi, or DGLAP).

That formula with many fathers describes the departure of hadron cross sections from a scaling law as a result of QCD radiation off the partons, parametrized by suitable splitting functons as a function of the energy at which hadrons are probed. It would take a long post to explain it in detail, but it would be a wonderful challenge for me to write it -and I do love these challenges. So I am making a virtual knot on my handkerchief to remind myself that I need to treat myself with it soon.

In the following I give a summary of the lecture Guido Altarelli gave this morning. I have to warn my non-physicist readers: in this instance, I was unable to describe things in a way simple enough to make it understandable to everybody… I will expiate soon with some more accessible material on the Higgs boson… Stay tuned: breaking news are coming off tomorrow!

The picture Altarelli painted was “impressionist” in his own words, because of the vastness of the topic. He started with a discussion of the status of QCD, which “surpassed QED as a prototype of a gauge theory, thanks to its richer structure”. The problem with QCD is to extract consequences and predictions from it. It is a crucial job for LHC, a prerequisite for the discoveries that the new machine will make possible. For the most part, QCD phenomena are non-perturbative in nature: they cannot be computed by making first order approximations and then perfectioning them by adding smaller and smaller correctiosn to it, because the “corrections” are too large. The main methods to circumvent this hindrance are two: to perform simulations on lattice or to use “effective lagrangians”.

Guido concentrated on a discussion of the first approach, which has continued to improve and has become a very important tool to understand QCD. In lattice quantum chromodynamics, calculations are made on a lattice of points, by discretizing spacetime. The results will depend on the lattice spacing, which can be then extrapolated to zero obtaining the “continuum limit”. Lattice QCD gave us many results and is continuing to progress, ranging from hadron spectroscopy (explaining the mass of mesons and baryons) to flavor physics (with calculations of form factors in hadron decays and studies of CKM matrix elements), to the study of phase transitions.

A sector of QCD which contains a few important open issues is the decyphering of the phase diagram when both temperature and density of a partonic gas are high. This can be studied with heavy ion beams at LHC. An evidence of deconfinement coming from lattice QCD calculations is based on the slope of the potential between two colored charges (take a pair of quarks, for instance): the potential becomes flatter as the temperature increases, until it totally flattens out and stays so as T is further increased. This critical temperature is a fundamental parameter, which depends on the number of flavors.Our current knowledge of the dynamics of these ultradense states of matter suggests that it is simply described as an ideal fluid, for which hydrodynamics gives a good description. Evidence of this, however, is still indirect, and the interest of a direct confirmation at the LHC of these effects is very high.

Coming back to “standard” particle physics, Guido noticed that as far as the perturbative regime of QCD is concerned,  the technology of calculations has now reached impressive heights, such that to obtain sufficient precision we nowadays need to rely on very high orders of perturbative expansions and complex resummations of leading logarithmic contributions to all orders of the expansions. Computations which are routine today were thought impossible only ten years ago. For instance, those on splitting functions: a computation by Moch, Vermaseren and Vogt in 2005 has used 10,000 different diagrams. Another work on hadronic inclusive decays of Z bosons has reached NNNLO level - that is contributions to the fourth order in .  The famous R-ratio has now been computed to 4-th level in alpha strong too, and with this improvement the agreement with experimental results has improved further.

In summary, QCD has become a very complete theory, which is used for precision measurements and careful comparisons with experiment. But the standard model issues which LHC will have to address are mostly in the electroweak sector. The problem of the higgs boson is central and connected to all others: the flavor sector of the SM; the hierarchy problem; the existence of new physics at the TeV scale.

On the face of the centrality of the Higgs boson in today’s physics, the list of experimental results we presently have which provide information on the Higgs boson is very short. H is light because radiative correction measurements say so. LEP II on the other hand gave a lower mass limit at 114.4 GeV. We also know pretty well that if the Higgs boson exists it is a weak isospin doublet, because otherwise the ratio between W and Z mass would not equal the cosine of the Weinberg angle, .

We also know that some Higgs mechanism must exist to break electroweak symmetry. However, the real nature of the mechanism is not known yet. A single doublet of fields ? More doublets ? Susy ones ? Is the Higgs a composite ? The LHC will answer these questions.

Altarelli then mentioned the inputs coming from low energy. Most notably, the g-2 experiments, which are sensitive to new physics, especially in the Brookhaven experiment with muons - the mass of muons being a hundred times larger, sensitivity to new physics is 10^4 times higher. Presently, there is a discrepancy between theory and experiment at the level of 3.3 standard deviations. However, the fact that the largest part of the uncertainty in the theoretical prediction comes from the evaluation of virtual hadronic contributions to calls for some caution in interpreting this result. On the other hand, some light Supersymmetry could give a signal of its presence in the anomalous magnetic moment of the muon of the right amount.

If one examines the global electroweak fits to standard model parameters, one sees that the largest discrepancy affecting the global fit probability (globally at 15% - not bad, but not great either) comes from the hadronic and leptonic determinations of . If one puts that quantity on the y axis in a plot with the Higgs mass on the abscissa, and then places the hadronic and leptonic determinations for it at the x value where theory would predict the Higgs mass to be, one is able to visualize this contrasting indication: the average of the two quantities is a political compromise, since the data do not match well with each other:

What could be wrong in the leptonic versus hadronic measurements of the Weinberg angle ? There could be new physics in the Zbb vertex, which affects the third family of fermions. The size of the new physics contribution would be of the order of 30% on the left-handed couplings: a huge effect at tree level! But modifying the standard model at tree level is incredibly hard without jeopardizing the perfect agreement of all physics measurements made so far, so one faces the challenge of inventing a new particle with which W or Z bosons might mix, with suitable quantum numbers to thwart the spoiling of past measurements.

Guido also discussed flavor physics, where no effect is seen, while measurements have reached a high level of precision. Any new physics must enter very silently in these processes. Operators describing new physics effects which correct the standard model must be induced by loops and not at tree level, and since the standard model has very strong protections (such as the GIM mechanism and similar other cancellations), the new physics effects must be small. As far as neutrinos are concerned, the indication is that these particles are very light because they are Majorana particles, and counterparts which are very massive - beyond our reach - keep them light. Because of that, double beta decay experiments are very important because they could establish the violation of lepton flavor number.

To conclude, Altarelli said that the standard model awaits LHC to see on one side the completion of the scalar sector, but much more is in principle possible. When asked by Guido Tonelli to spill the guts and declare what he expects LHC will find, he mentioned that he had previously foreseen new physics at LEP II, and he was proven wrong: so he is apparently not the right person to ask. However, he mentioned that since SUSY is the best model that theorists could conceive in the last twenty years to explain many of the existing problems, he would be happier if what were found was not SUSY, but rather something different: much of the theoretical work on SUSY has already been done. He would be much happier if some kinds of extra dimensions relevant for electroweak physics was discovered: this would necessitate a much richer theoretical overhaul of our present preconceptions. On the other hand, if nothing is found by LHC, particle physics might be at its last stop. With this gloomy remark, he left the pulpit.

Tonight and tomorrow I am in Perugia, where I am following a workshop on LHC physics for italians participating in the CMS and ATLAS experiments: about 220 physicists have crowded the venue, Hotel Gio’, where they will spend three days discussing their plans for physics measurements and searches with the data that we are all waiting LHC to deliver.

The real workshop starts tomorrow, but I spent today’s afternoon at a preliminary CMS meeting, where the analysis efforts and plans of each of the italian institutions collaborating with CMS were discussed. I thus learned that the italian community in CMS has a rather strong involvement in electroweak physics analyses, a reasonable commitment with Higgs physics, and a smaller-than-expected interest in SUSY and other exotics searches. That was not a real surprise (I sort of knew that already), but it got me thinking that I might not be alone in believing that the LHC will not discover new physics beyond the standard model.

After the meeting was over I had a chance to play a little on a wonderful baby grand Steinway & Sons piano. The dinner was in form of a buffet, and despite the awkward manner of eating while standing up, a glass in one hand and a dinner plate in the other, it was a nice occasion to chat with several colleagues with which I usually have little chance to interact. Nando, Didar, Michelangelo, Simone, Vitaliano…. However, after the food ran out, and people were still lingering around discussing in small groups, I felt the need to take a couple of steps back, to observe the merry crowd with some detachment.

I am usually a sociable person and -especially after a couple of glasses of wine (I had three tonight)- I am “quick with a joke” and a fairly good converser. Nevertheless, there are times when I feel tired, and need some times for myself, to “recharge”, so to speak. Social occasions drain me quickly.

So I watched as the after-dinner activities were being organized by the younger colleagues, who planned to walk up to the center of the town. Perugia has quite a few nice places where to spend the evening in good company, but it sits on top of a hill, and our Hotel is at its feet. I drove here with my car, but the option to pick up the car and fight my way to an improbable parking in the narrow streets of the medioeval town center did not appeal to me much. I also did not fancy much the long walk… It was 8.30 in the evening, and I called it off, repairing in my room. Maybe I am really growing old!

… Or maybe, just maybe, the last month after Christmas vacations, which saw me working 110% of my time on the analysis of Higgs production with top quark pairs which constitutes the cornerstone of Marco’s PhD thesis, is calling for some rest. Marco will deliver his thesis tomorrow morning! It was indeed a hectic month, and I am happy and relieved to be looking forward to five days of vacation in Lisbon with my family, starting this coming Saturday!

This afternoon I traveled to Bassano del Grappa, a nice little town close to the first slopes of the eastern italian Alps, to give a lecture on particle physics to students of the last year of high school. This is in the context of the Master Classes, a program to publicize particle physics among students.

I had a very nice audience of about 20 students, and I talked for a bit less than two hours on the history of particle physics, the tools, the methods of investigation, and the discovery of quarks. I will have two more hours to describe more in detail a few of the most recent and present experiments in particle physics in two weeks.

You can browse the slides of my first lecture (in italian!) here (.ppt, 4.2Mb). They are not much different from those I showed one year ago, but I did change a few details and gave more emphasis on some aspects of the quark model, especially in my speech. I spent a lot of time on the chalkboard, drawing Feynman diagrams and particle reactions. The students asked meaningful questions and I was pleased with their attention and their level of understanding. All in all, a well-spent afternoon.

Today Julien Donini, for the last four years  a member of our CDF group in Padova, left for Grenoble, where he will join the ATLAS group there. I wished him good luck with a good dose of sadness, because he will leave a void. Julien worked side by side with me on the analysis and on other CDF analyses, and it was a very fruitful collaboration. The fact that he is joining ATLAS instead than CMS is really an additional sour pill to swallow: if he had joined a group working in CMS, we could have continued to work together somehow, but this way it will be really impossible - unless we both join some averaging group!

Julien is the main author of the paper I linked in the post below. Without him, that paper would not have seen the light. I owe him for that. I think I repaid him with my friendship, and with a trick or two on data analysis. Ciao Julien, see you at CERN soon!

Hot off the press: http://xxx.lanl.gov/abs/0801.3906

(in case you did not bother to download it from my local disk, you get a second chance…)

…And finally I can announce that our long-fought struggle to publish the signal analysis (which I described in several places in the past, and anecdotically the last time)  is over! That is, if Nuclear Instruments and Methods accepts it. That granted, we have completed all the necessary steps. I feel very happy because I have worked at this topic for 12 years now.

Furthermore, it is a deep satisfaction to publish a second “first observation” of a multijet decay signal in hadron collider data, after the signal of top pair decays into six jets I co-authored with my colleagues in Padova in 1997. Yes, the signal had never showed up anywhere else than at LEP (a lepton collider, where there is no issue with the huge QCD background present in proton-antiproton collisions) until now. This must make me the World Expert on the extraction of resonances from jet final states in hadron collider experiment. Does it ? Judge for yourself. The paper is here for you to grab.

In a series of recent posts (Storm over rome, The aborted speech, Ratzinger divides, Maiani speaks)I discussed the decision of the rector of “La Sapienza”, a prestigious University in Rome, to invite pope Ratzinger for a Lectio Magistralis at the opening ceremony of the academic year, and the following events: the private letter of 67 physics professors to the rector criticizing the decision, the publication of the letter, the ensuing reaction by media, clerics, and students, and the final decision of the pope to decline the invitation “for opportunity reasons”.

The story does not end there, because some right-wing lacqueys of the clerics, always sensitive to the wind blowing from cardinals in Italy, have sensed that the events had created the occasion to exploit politically the wave of indignation in the country following a distorted reporting of the whole issue by the media. The designated victim: Luciano Maiani, a esteemed theoretical physicist, former director of CERN and INFN (the institute that pays my salary), a person with a stellar curriculum and undoubted experience and skills. The occasion: Maiani has been nominated to head the CNR, the most important research organization in Italy, and a confirmation of the appointment has to come from the Senate. The plan: use the fact that he was among the 67 who signed the letter criticizing the invitation of Ratzinger to subvert his confirmation.

I just read the transcription of the discussion which took place in the 7th commission of the italian Senate a week ago, when the due act of confirming Maiani was postponed and an interrogation of minister Mussi about the opportunity to hire Maiani as head of CNR was requested. You can find the italian version here, but I will give a few highlights. It shows something about the inner workings of italian politics and how the latter is influenced by the Church, albeit in a covert, indirect way.

On the proposal to appoint professor Maiani as President of the Consiglio Nazionale delle Ricerche

Senator Asciutti (Forza Italia) asks that minister Mussi be called in commission to confirm the appointment of professor Maiani […] following the recent facts at the university La Sapienza in Rome. In reminding that the candidate is among the signers of the letter in which a small number of academics expressed their dissent to the presence of the Pope in occasion of the inauguration of the academic year, he holds that such position is incompatible with a balanced, laic behavior, the more so since at the top of CNR is needed a person representative of all opinions. […]

Senator Pellegatta (Green party) judges as non receivable the request of senator Asciutti, since it is instrumental, demagogic, and arbitrary. She expresses dissent with the attempt to link the expression of the judgement of the parliament on the appointment, essentially based on an evaluation of titles, with opinions of the candidate freely expressed in other venues. She points out that accepting the request would mean to create a dangerous vulnus.

Senator Sterpa (Forza Italia) in consideration of the statements in the debate, declares to abandon the room as a sign of protest, judging unacceptable the accusations of intolerance.  

[…]

Senator Bianconi (Forza Italia) reminds that in the past the appointments in large public institutions have always taken place in a climate of ample agreement. […] She invites minister Mussi to confirm in the Parliament a choice operated before the contested facts took place. In the meantime, she auspicates that the same professor Maiani clarifies his position, while holding that his declarations already given are by no means appeasing.

Senator Soliani (Democratic Party) acknowledges the convergence on deferring the decision on the appointment. She also notes  that a good part of the debate could have taken place in occasion of the appointment itself. She […] proposes that the convocation of minister Mussi have as a subject the guarantees for the full exercise of freedom of opinion in universities and research institutes. […]  

 I still think this is just an occasion used by the right to show off how much they care about pluralism and freedom of expression of… the pope, and that their objections to the appointment of Maiani will die out. Nevertheless, this skirmish also showed how difficult it is in Italy to obtain an agreement on appointments: even when the convergence is ample and the candidate is outstanding, ideology wants its share and may end up driving the decision.