A Quantum Diaries Survivor

During these days of holidays, I post little - also due to the lousy internet connection I have from the Alps. But I discovered I am getting lots of hits from people looking for “ch**d p**n”, “n*de ch**d pics” and the like. The reason, apparently, is that I have recently published a post with that title.

Well, I cannot of course prevent perverts from visiting this site, but I can certainly advise them to get an appointment with a psychiatrist. They might discover that even if they are ugly, or deluded, or have a micropenis, or are seventy and counting, they can still get themselves to enjoy consensual sex with other adults.

People, please, get a life. I know most of you mean no harm, and I know one cannot really decide what gets one a stiffy. But those of you that would not harm a kid should understand that looking for that kind of material on the web is by itself an action that foments criminality against children.

When I came back from Geneva three weeks ago I carried with me three decks of CERN playing cards I intended to offer as small presents to friends of my son Filippo, for Christmas. It was a good idea: the cards were appreciated by the recipients of the present. One of them is Sebastiano, who came to join us with his family - Ivan and Barbara - in our house in Padola: we are spending our days together, hiking on the snow during the day and playing in the evenings.

The cards are very nice: each of them is decorated with a picture of an experiment, a part of a detector, or some scientific information about particle physics or the big bang. A few discuss a scientific discovery made at the CERN laboratory, and some more describe shortly the standard model of particle physics or the subatomic particles. All in all, a very good idea by CERN, through which the world of research in particle physics can reach unsuspecting customers.

Today, however, as we played with the CERN deck a game of scopone scientifico - a game in which some cards have a special value - I could not help noticing a rather odd fact. You could imagine either that experiments, projects, theory facts would be dispersed on the cards without a logical arrangement or correspondence between the value of the card (in most games such as Poker), or that the correspondence were totally random: after all, how could one rank in importance the big bang, the fermions, or the LHC cavities ?

Instead, I was on for a striking discovery. There was indeed some scheme, and I could have learned it by reading on the box: clubs are about the formation of the universe, diamonds about the history of CERN, hearts and spades about the LHC projects and associated experiments. That is fine of course, but then I got suspicious and looked at the details - details are always obscene.

I could thus discover that the aces of hearts and spades were assigned respectively to “The ALICE detector for LHC” and “The ATLAS detector for LHC”. What was, then, the rank of the card titled “the CMS detector for LHC” ? A mere 6 of spades!!

Of course, one could claim it was mere chance that CMS was relegated to a disposable lowly card, while aces were assigned to the two jewels of detectors, ALICE and ATLAS. However, I think one could at least just as well read in it a higher attention of CERN for the two A experiments.

So: does CERN really love CMS as much as it loves ATLAS ?

I will be mostly offline in the next couple of weeks: I am leaving to Padola, where I will spend my days skiing and hiking. Plus, of course, at least a few of the nights watching the sky from a dark site with my dobson telescope, which I am enhancing with a few new gadgets: a green laser pointer, a H-beta filter, an O-III filter, and a Nagler 16mm eyepiece.

I have seen the Horsehead nebula a couple of times with a UHC filter, but the H-beta is the best way to make it appear, suppressing the light of anything else - so I am looking forward to a dark night for it. And the O-III filter allows to see interesting features in many planetary nebulae and other fuzzies; the Veil nebula is the one which benefits the most from it, but it is stuff for next Summer.

Above you can see the view a photographic session with excellent optics can provide of the horsehead nebula. However, what one usually sees with a amateur-size telescope is closer to the picture on the left. Still, an exciting and rare sight! 

The sky will not allow deep-sky observations for a while, however: at least until new years’ eve, because of the gibbous moon. During these first few days Mars will be my main target, although I do not expect good seeing conditions for high-resolution observations. The atmosphere above mountains is usually very disturbed, and sub-arcsecond seeing is normally hard to get.

Technically, I have a wireless connection in Padola. But I have experienced difficulties in the past, and I expect I will not be able to blog regularly… So this blog is switching to a default level of minimum activity for a while. In all cases, happy new year! I wish 2008 will provide us with the first 14-TeV collisions in this side of our galaxy!

In a recent post I mentioned my full immersion in a search for the signal of Higgs radiation off a top quark pair in the 14-TeV proton-proton collisions produced in the CMS detector by the LHC. The process is   - the Higgs is therefore sought in its decay to a pair of b-quarks.

The ttH signal is really tough to extract because of its tiny cross-section - 0.67 picobarns, not including the branching probability of . Compared to inclusive top pair production (650 pb) and W+>=4jets production ( about 300 pb) it might not look like a forbidding task, but generic QCD events with many jets in the final state (hundreds of nanobarns) remain a horrible background if one does not have powerful handles to kill it.

Normally, one tags at least one top quark decay by looking for a high energy lepton (electron or muon) produced in the chain or . QCD events then mostly disappear (only a relic of semileptonic b-quark decays to high-energy leptons may be an issue, as well as fake leptons mimicked by hadrons) and one only needs to fight the other electroweak signatures of high energy leptons.  

In the analysis Marco (the PhD student I am tutoring and who is about to graduate), Mia (my freshman PhD student) and myself have chosen, the top pair is instead tagged by looking for a neutrino. Neutrinos can only be seen by “not seeing” the energy they carry away: by pretending that the momentum of all observed bodies transverse to the beam direction balances, one infers whether a neutrino (one or more, that is) has carried away a part of it.

The philosophy is thus to neglect leptons (which other analyses may tag), and concentrate on a multijet plus missing energy signature. To reduce the QCD background, the missing energy is required to be significant: the missing Et measurement divided by its estimated error has to be larger than three units. Then, five or more jets are required to be contained in the central part of the detector and have a large transvers energy (30 or more GeV). After a trigger simulation and those cuts, the signal to background is below one part in a hundred thousand!

Fortunately, the top and higgs produce b-quarks, which in turn yield jets with tracks pointing back away from the primary interaction vertex, because they are produced by B mesons which have traveled away from it. By requiring four B-tags the signal gets reduced quite a bit, but the QCD background gets hit hard! The same does the W+jets electroweak background and other less probable processes. One then has to reckon mostly with inclusive top pair production, when two additional b-tagged jets are the result of QCD radiation of a gluon which materialized into two well-separated, energetic jets. The latter is called an irreducible background: its only difference from signal is that the two b-jets have an arbitrary invariant mass, while the Higgs has a well-defined one.

To discriminate the remaining QCD background and inclusive top production from the ttH signal, one relies on kinematic variables defined with the quadrimomenta of the jets, the information on which ones are b-tagged, and the missing energy magnitude and direction. Here is a incomplete list of possible kinematic characteristics:

• the total invariant mass of the first 6 jets in the event;
• same, for all jets (often there are as many as 10);
• the sum of jets transverse energy and missing transverse energy (a quantity called Ht)
• the mass of the two highest Et b-jets in the event
• the mass of the two b-jets whose mass is closest to 120 GeV (the mass considered in our monte carlo simulation - we are targeting the search to a 120 GeV Higgs)
• the mass of the triplet of jets containing exactly one b-tagged jet whose mass is closest to 172 GeV (there are many combinations, so this number always comes close to 170ish)
• the mass of the pair of untagged jets in the triplet with mass closest to 172 GeV (this should be a W boson decay and should thus cluster at about 80 GeV)
• the angle between missing energy and closest jet in the transverse plane
• the angle between missing energy and leading jet in the transverse plane
• the angle between the leading two jets in the transverse plane
• the centrality of the leading 6 jets, defined as the sum of their transverse energy divided by their total invariant mass
• the mass of all jets not b-tagged

And so on.

The study entails constructing histograms of these variables at different levels of selection, adding expected background histograms together, comparing the result to the histogram resulting from the signal simulation, and deciding whether the variable is discriminating the two or not. Once the best variables are spotted, a global relative likelihood can be constructed, and with it a sensitivity study can be performed.

To give you a view of the problem we are facing, I provide two graphs below (good lord, I hope my CMS colleagues will not be so picky as to have something to complain about my posting unnamed histograms with no y axis labels!). The first one shows the four main processes contributing to our sample after the request of five jets, two at least of them containing a b-tag. In red the QCD background stands out high over top pair production (green), electroweak W+jets (in cyan) and ttH (in blue). The black points show the sum of all processes. Mind you, this is a logarithmic plot! The x axis is one particularly discriminating kinematical variable.

In the second plot you can see what the additional requirements of missing transverse energy significance above 3.0 and two additional b-tags do for us: the QCD background is strongly reduced, and the main background has become the inclusive top production.

Ok, after this example let’s talk business. 

At this stage we are still finalizing the list of best kinematical characteristics capable of discriminating signal from backgrounds. Do you wish to contribute ? Define a variable which is meaningful enough to trigger my interest, and propose it. I will compute it with my code for each of the simulations, and come up with some handwaving information - a single measure of the discriminating power of the variable - describing how good it is. I may be able to show histograms too, by suppressing some information - remember, the simulation goes through CMS private code and so I cannot divulgate any result which is not explicitly approved by my collaboration. I can, though, provide generator-level information for the variables we end up discussing. In the end, if the variable you propose has some discriminating power, I will use it in the analysis.

You are supposed to use the following:

1. px,py,pz,E of jets (there are at least 5 in the event, and I consider at most 8 - the most energetic); you can also choose to work with the alternative set of variables Et, , and of each jet.
2. Ex, Ey components of the missing transverse energy; the arctangent of Ey/Ex is of course the phi angle in the transverse plane; also given is the missing Et significance, defined as missing Et divided by its error;
3. B-tag flag of each jet from 1 to 8 (mind you, the selection requires 4 b-tags, but it is quite unlikely that more than 4 are tagged). This is a true/false bit per each jet: if you are looking for the Higgs, you may want to study the mass of two b-tagged jets!
4. From the above information of course one can define invariant masses of jets, sums, triplet masses, four-body masses, angles between combined quadrimomenta, in the lab frame or in the center-of-mass frame of any sub-parton system. You name them!

Ok, if you are not into particle physics and you are still reading, you deserve some more information.

• What is likely to discriminate the production of high-mass objects from QCD multi-jet production is, for instance, the mass of the objects, or the centrality of the produced jets.
• Missing Et flags a neutrino, but in QCD events it is the result of a jet which was badly measured - so the angle in the transverse plane (remember, the missing energy vector is undefined in the z axis because the total momentum of the projectiles participating in the collision along the beam axis is unknown) between a jet and missing Et may be an indication, but in a multijet environment this is not so easy.
• a ttH event with missing Et contains also a lepton, but we miss that information. If the lepton was an electron or a tau, it usually has been mistaken for a jet; if it was a muon, it got lost and may have resulted in additional missing Et. Because of these characteristics, a ttH event of the kind we are after has nominally six partons in the final state plus a neutrino and a undetected lepton. Four of these six partons are produced by b-jets.
• The inclusive top pair background has produced missing Et, four partons, a charged lepton which went undetected, and may have other jets from radiation. These additional jets may be less central than the others.
• Since we have a missing neutrino - of which we do not know the z-component of its momentum- and also a missing or however unidentified charged lepton, there is no chance we can reconstruct the full event kinematics. However, one might rely on techniques which have been refined for the top quark mass measurement in the dilepton final state, where again two neutrinos are missing, a circumstance not too different from the one we are facing here.

The above, I believe, is sufficient information to start thinking… A good idea might be worthy of a mention in our future analysis note on the search, so take this as a chance to see your name printed on a scientific document!

Just a couple of pictures of the night before Christmas and the morning after the kids have opened their presents:

…And merry christmas to you all! 

Just a few days ago, a colleague’s blog has been re-activated. Michael Schmitt, professor of Physics at Northwestern University and a member of the CDF and CMS collaborations, is back in blogging mood after 18 months of absence from the blogosphere. A very nice little present under the christmas tree! 

I have been blogging for three years now, and I enjoy it a lot - I find it stimulating to share my views and sometimes have to defend them, as well as to distribute information related to my job to peers, and of course to make my little bit of outreach and explain particle physics to laymen. I spend blogging overall more than one hour a day, which is an outrageously large amount of time, considering I could be accused of stealing it from my job and from my family. On the other hand, the truth is I am stealing this time from other lazy occupations I used to entertain myself with: chess, or bridge, or other internet activities of lesser value.

During these three intense years I have often found out that the urge of posting something interesting (at least to me) every day or so supersedes other pastimes and obligations. I almost always manage to find the time for a post. Almost.

In the last couple of weeks I have started a full immersion in a difficult analysis for the CMS experiment: attempting the extraction of a signal of associated production of a top quark pair and a Higgs boson, with top quarks decaying into hadrons and missing energy, and the Higgs decaying to a pair of b-quark jets. The analysis of course is based on simulated events, but it is a very important first step into understanding the extractability of the signal from a final state nobody has cared to study before. So, the issue is stimulating. Actually, the complexity of the task -the signal is buried in a background at least 10,000 times larger- makes it even more exciting.

So, during these last few weeks I have found out that when I am on to something like finding a signal or coding a smart algorithm for the search, I forget everything else, including my blog! I think this is a very healthy reaction which shows I have not spent all my cartridges yet as a physicist. Of course, I have had other exciting studies to delve into during the past three years, but I admit this last one is really absorbing all my intellectual energy. Right now is a Sunday evening, I am home, with my family, the christmas tree is a few feet from me, and here I am, finally finding ten minutes to write on my blog while a root macro is running five million events to produce a relative likelihood distribution…

It would be nice if I could discuss the analysis in more detail, but I have to go by the rules: even if the data I am analyzing is simulated, it still refers to the CMS detector and so I cannot divulgate anything of what I am finding. I can only say that there is good reason why associated top and Higgs production was taken off the list of golden signatures at the LHC (and the recent CMS Physics TDR in fact does not even bother discussing it): the signal is indeed darn tough to find… But I have my own aces up my sleeve, and I am confident I can prove that something could in principle be squeezed out of the very uncommon “jets + missing Et” top signature - the one that allowed me and my PhD student, Giorgio Cortiana, to publish the third-best top pair cross section measurement, three years ago.

As awful as the congressional budget plan for 2008 looks, we have to live with it now. Fermilab is assessing the damage, after the news that of the expected $378M it will only get about $320M, and it is clear that Fermilab director Pier Oddone will close the lines of research that were nullified by the US congress.

Despite cutting on NOVA and ILC, the rest of the research activities at FNAL are facing a reduction of about $15M. In this situation the focus in 2008 and 2009 will be on running the experiments that have been providing the gold and diamonds of the physics mine; of course, nobody expects that 2009 will be better than 2008, although I know a lot of letters are being sent to representatives while I rather choose to take an observer’s attitude and write these lines. 

The Tevatron will continue providing data to CDF and D0 mostly unscathed, and so will the existing neutrino program, although in a situation of general suffering. In particular, FNAL staff will see salaries cut by 10%, and some 200 layoffs in 2008. Under such circumstances, it is a tough call to say what will happen.

Some believe that the option of running the Tevatron through 2010 is now stronger, because the more research lines are dried the more water flows to those that survive. However, it would be very silly to feel relieved. It is clear that the US congress does not believe pure research in subatomic physics something useful for the Nation. Let’s face it: we lived through sixty years of good funding on the standing wave of nuclear weapons research, but we do not seem to manage to fool anybody anymore: no more deadly tools from muons and neutrinos. So, no dough. Sad, but true. I only hope that Europe will be smarter and that particle research at CERN will continue as strong as it has been recently shown to be. 

UPDATE: a CDF colleague and now chair of the Fermilab UEC asked to distribute the information on how to send a letter to your representative. 

This just in!

ONU said yes to the proposal of a moratorium on death penalty. 104 votes in favor, 54 contrary, and 29 abstained. The vote has been cast less than one hour ago. It is a victory for all who fought for this result, and in part for Italy who brought the proposal forward.

It will be a better christmas. Let’s just avoid thinking at the other atrocities happening daily everywhere in our poor planet… A little, but important good news, and a step for mankind out of the black pit of bestiality.

“Mozart is free to create things others cannot imagine,
because he is bound by principles others cannot see.”

Margaret Boden

(and thanks to Tony for mentioning it.)