A Quantum Diaries Survivor

Ok, I just got a postscript version of my draft done… It still misses a couple of figures and I think it has still many errors buried within the text. Plus, I need to check several numbers I inserted by heart in the text so far… Any hardware expert who can check the resolutions I quoted ?

Anyway, I will appreciate any further comments you may have. I have put a copy of the paper in http://www.pd.infn.it/~dorigo/dorigo_qchs.ps

However, for your convenience I paste here the text of the two last sections as well -the ones following the piece I already posted yesterday. If I feel you have contributed to the clarity of the text I will be glad to add your name in the Acknowledgements section…

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Searches for the Standard Model Higgs boson

The search for the Higgs boson at the Tevatron is carried out by looking for its two main
decay signatures, depending on the particle mass:
if $M_H<135$ GeV the dominant decay is $H \to b\bar{b}$, while at higher masses the
$H \to WW$ decay provides the most promising signature.Both CDF and D\O\ search their datasets for $WH$ and $ZH$ associated
production with a low mass higgs boson decaying to a pair of $b$-quark jets,
while the vector boson is tagged by the reconstruction of two charged leptons
(for $Z \to ee$ or $\to \mu \mu$), a lepton and missing transverse energy (to select
$W \to e \nu$ or $\to \mu \nu$), or missing transverse energy alone (for $Z\to \nu \nu$
decays). The two critical parameters affecting signal significance are
the efficiency of $b$-quark jet identification -performed through the reconstruction
of a secondary vertex in the jet- and the resolution of a reconstructed dijet mass peak.
The dijet mass distribution is studied in search for an excess over backgrounds, which
are mainly due to real vector boson production associated with jets
from QCD radiation and top quark production (see Fig.~\ref{f:wh}, left).

That measurement, along with selected others, has been combined into a world average
of $M_{top}=171.4\pm2.1 GeV$, which is accurate to $1.2\%$ (see Fig.~\ref{f:tevbest}, right). Before the end of Run 2 the experiments are likely to get to a 1 GeV accuracy
on the top quark mass.

Electroweak physics

In the remainder of this paper it is only possible to mention one further recent
result on high-$P_T$ physics from the Tevatron experiments, namely the recent discovery
of production of pairs of $WZ$ bosons, a rare process of high
relevance for Higgs searches. CDF obtained 16 $WZ$ candidates by an optimized search for
triplets of charged leptons and missing transverse energy in $1.1 fb^{-1}$ of
data, where only 2.7 events were expected from background processes. Figure ~\ref{f:wzmet}
displays the distribution of the missing transverse energy in the events, showing
the clear signature of $WZ$ production at large missing $E_T$. Now one more process
of that kind is still missing, associated $ZZ$ production, and CDF observed one event
with the required characteristics, it will take some more data to measure that process
as well. The event display for the $ZZ$ candidate is show in Fig.~\ref{f:wzmet} (right).

Acknowledgements

The author wishes to thank [This is where your name goes... ] for his/her editorial advice.

My former post triggered two interesting questions by Gabriel:

“a little off topic, but i would like to make a question. i want to understand whats the difference betwen a pp and a ee collision, in a specific sense. first lets forget the syncroton energy loss issue and lets assume that the CM energy of the ee is 3 times the pp (naive, for taking into acount the 3 quarks). in this case i have the same “potential” of observing new physics? put in other way, how will change the menu of produced particles?

A little bit confusing to me, still in the topic but a different thing, is why they had to make pp collision to first observe the W.”

Protons and electrons are both particles, but they are as different as whales and ducks are different animals. Just as the latter can both float, but they obey different physics laws in order to do so, so protons and electrons can both collide and produce new states of matter, but they do so in a very different way, exploiting different elementary forces.

When protons collide, as you say, it is actually their constituent quarks that do. So, dividing by three is a good first guess idea of the effective energy released in the collision. Things are more complicated, though, because each quark in the proton can carry any value of the total proton energy from zero to the maximum, and the probability to find it with a given energy fraction is given by a distribution that has to be determined by experiment.

Anyway, let’s get to the core of your first question. Proton-antiproton collisions have a different menu of possible final states that get produced with respect to electron-positron collisions. The reason is that the overwhelming majority of the collisions of protons is mediated by the strong force: one that could not care less about the fact that both quarks and electrons have electric charge. Instead, the strong force sees the colour charge of the quarks, and ignores electrons as if they were not there.

The opposite, though, is not true: the electromagnetic force responsible for electron-positron interactions also can scatter quarks off one another, or annihilate them with the production of a photon. So in a way, with a proton-antiproton collision you can do what you can do with an electron-positron collision, plus more.

Let’s say there is a Higgs boson out there. How would you discover it ? You can smash electrons and positrons, producing a ZH final state through electroweak interaction. That is what LEP II has tried to do until a few years ago. But you can also smash protons and antiprotons together, hoping for an electroweak interaction of the incoming quarks, that will yield the same final state. That is because both proton constituents and electrons “feel” the electroweak interaction.

If instead you were after production of a massive strongly-interacting electrically neutral particle, you would need strongly interacting projectiles. But only in practice. In theory, you could annihilate electron-positron pairs and sometimes the high-energy photon produced would materialize into quarks, and the latter still produce the odd particle.

So it is not so much a matter of energy, but of the relative frequency with which you can produce a certain process with a given projectile.

As for the W discovery: W’s are charged, so a neutral electron-positron initial state is not the best way to produce them. To do it, you need provide enough energy to make a W together with something else carrying away a unit of charge, and the best chance is then another W. But you need lots of energy and the process is very rare (was done in LEP II, but only 15 years after the W discovery). Instead, of course electron-positron collisions are ideal to produce Z bosons. But synchrotron radiation prevented that in 1983, so Z bosons were indeed produced in electroweak processes with protons as projectiles.

Mind you, it is quite hard to separate the very rare electroweak processes amidst the huge rate of strong interactions when you smash protons. That required quite some skills by the UA1 and Ua2 collaborations….

In a comment to my post on antiprotons in the waste bin, Markk writes:

This is interesting - in a normal run can you use the decrease in intensity of interactions to test your triggers? As the beam intensities go down the probability of a rare event being in the population decreases, but your sample size as a percentage increases. Do you see any effect in the types of events collected, or isn’t it enough difference to matter?

I thought this was a good point, there indeed are some subtleties involved in collecting data in a high-rate particle collsion experiment.  I think it is interesting to discuss it in a bit of detail here.

In CDF we collect data coming from proton-antiproton collisions that happen every time a bunch of protons intersects a bunch of antiprotons orbiting the same ring in the opposite direction. The timing is such that we get one of the 36 bunches of protons - 0.3m long - to cross one of the 36 bunches of antiprotons in the very center of the CDF detector every 396ns, for a rate of 2.5MHz.

Input rates for the data acquisition system of our experiment are thus always 2.5MHz regardless of the luminosity (i.e. how many protons or antiprotons are there in each bunch or how narrow do the bunches get squeezed in the transverse direction, increasing the likelihood of interactions), because 2.5MHz is the bunch crossing rate - and there is almost always at least one interaction in each bunch crossing until the luminosity L goes down a lot, due to the continuous small loss of particles from the beams due to interactions with residual gas in the vacuum beam pipe where they circle the Tevatron ring.

The output rate is instead always around 100 Hz, since we want to collect as much data as we can. We have a farm of commercial processors that reconstruct events and write them to tape once they are accepted by our trigger system - a complex collection of hardware modules that decides what to keep and what to discard. The speed at which we can write events to tape is 100 Hz, no more, so that limits our ability to collect the data, ultimately. But we want to use always our full power of data writing! So we keep that output rate always at the maximum value.

What is the difference, then, between high and low luminosity ?And what is this big deal about trying to reach ever higher luminosities, by collecting more and more antiprotons in our accumulation rings before injecting a store in the Tevatron ? 

Indeed, the luminosity makes a HUGE difference. It dictates what is the menu of events we collect.

To see that, I have to explain that events get collected thanks to the characteristics they display. I will make a very simplified version here of what happens in reality, to let you see this point.

A collision may yield a high-energy electron - the clean signature of W boson production, which in turn can signify that event is a top quark production event or even a Higgs production event - but it will do that very rarely. Much more frequently, it will yield a couple of low energy jets of hadrons; we still like to collect jet events, but we care much less since they are not a discovery process.

So let’s say the first process has a rate of 1Hz and the second has a rate of 10000 Hz, when the luminosity is high. What we do is we take the full 1 Hz of electrons and write them to tape, while we take 99 more Hz of jet production events, discarding the 9901 remaining Hz of those. We have, that is, prescaled the jet trigger by accepting only one jet event in a hundred.

Now, as the store continues, protons and antiprotons will decrease in the beams. At a given point, the luminosity will have halved, and so the two processes. 0.5Hz of electrons and 5000Hz of jets. What we do then is we collect the 0.5Hz of electrons, and 99.5Hz of jets, by prescaling the latter by a factor 50 this time.

The above simplified view shows that our menu of collected physics processes constantly changes during a run. There are tens of different prescales, active on some of the more than hundred different triggers. Of course, the “discovery triggers” (ones that collect potentially rare processes) never get prescaled. But they have a very low rate anyway…

For you only, here is the first part of a proceedings article I am writing for the QCHS7 conference (”Quarks confinement and the hadron structure”), which I attended last September at the Azores islands.

The article discusses standard model tests at the Tevatron, and is aply titled “Standard Model tests at the Tevatron”.

If you have a chance, proofread it and send me any errors, typos, or bad sentences…

 ——————

Abstract

The CDF and DO collaborations at the Tevatron have been producing exquisite precision measurements on high-P_T physics with their large datasets of p-antip collisions.
The Higgs boson is being sought in all available channels, and the Tevatron experiments will have a chance to discover it before LHC starts operating. The top quark is being studied in great detail, and a precision of 1.2\% in the measurement of its mass has been achieved. In this brief report an overview of the most interesting topics will be given.
 

The facilities

The Tevatron accelerator has been subjected at the turn of the millennium to a massive upgrade, with the construction of an entirely new ring, the Main Injector, and several improvements in the facility producing and storing antiprotons — the most challenging part of the whole project. The collider has recently surpassed the peak luminosity of 2.3 x 10^32 /(cm^2 s). An integrated luminosity of 2.0 /fb has been delivered to CDF and DO so far, and 5 to 8 inverse femtobarns are expected by end of 2009. Up-to-date information on the performance of the machine can be found in [1].

An overview of the CDF and D\O\ detectors for Run II at the Tevatron can be found in [2]. In what follows their most important features for high-P_T physics are briefly mentioned.

Both detectors are all-purpose, near-hermetic devices consisting of a tracker immersed in a solenoidal field and an outer shell of calorimeters and muon chambers.

In DO an excellent system of silicon microstrip detectors has been installed in Run II. Six barrels of silicon sensors organized in four concentrical layers provide coverage for central tracks, while a total of sixteen silicon disks allow reconstruction of large rapidity tracks. A similar set of seven barrels of silicon strips is organized in the core of CDF.

Outside of the silicon barrels CDF features a large gas tracking chamber, and DO has a compact scintillating fiber tracker. Both allow measurement of track momenta with better than percent accuracy for P_T<100 GeV.
Associated hits in the silicon strips allow the determination of track
impact parameter with accuracy sufficient to reconstruct B-hadron decay and enable 45% efficient tagging of b-quark originated jets with fake rates well below 1%.

Calorimeters are divided in a inner electromagnetic and an outer hadronic section. Electrons within the pseudorapidity interval |\eta|<2.0 are identified with high purity and efficiency, with a resolution of 14%/E^0.5 in CDF and 17%/E^0.5 in DO.
Hadronic jets are reconstructed with resolutions better than 100%/E^0.5.
Muon chambers cover the rapidity region |\eta|<1.5 in CDF and |\eta|<2.0 in DO.

Both detectors have a sophisticated trigger system that reduces the 2.5MHz collision rate to about 100 Hz of events written to tape. Of particular relevance for the present review is the Silicon Vertex Tracker (SVT), a device designed and built in CDF to achieve precise online tracking.
The SVT identifies track candidates by comparing hit patterns to a predetermined array of possible roads stored in associative memory banks. A linearized R-phi fit of track hits in the silicon layers provides track momentum and impact parameter with precision close to that attainable offline in less than 10 us, thanks to a highly parallelized architecture. This allows the collection of datasets based on the presence of b-quarks in the final state, enhancing the B-physics program of CDF but also providing higher efficiency for several Higgs boson signatures.

———

Ok that is it for now, a second piece is coming soon…

Yesterday the day shift at the CDF control room looked promising at the beginning, when a huge stack of antiprotons was waiting to be injected in the Tevatron ring. The more antiprotons we can get to collide, the more data we collect, and the happier everybody is…

Actually, things are not so straightforward: no matter how high is the number of particles in the beam, and the resulting collision rate, we always write to tape about 100 Hz of events. But those events we write are the more interesting the more they were selected by our trigger system.

Still confused ? Ok, imagine we have a trillion antiprotons and ten trillion protons colliding. That will cause of the order of 5 MHz of collisions in our detector. We select 100 Hz of those with a trigger system that sorts out the most promising events for data analysis. The particles in the beam go on colliding for hours, and their number decrease as they collide or interact with the residual gas in the vacuum beam pipe. As the number decreases, the collision rate will go down accordingly, say to 2.5 MHz at some point. But our data acquisition system will adjust automatically the trigger selection cuts to keep the 100 Hz output constant.

Antiprotons do not come for free. They do not exist in nature, and have to be produced by colliding a beam of protons with a thin target. One such interaction every 50,000 will produce an antiproton, which is then collected in a dedicated facility, the Antiproton Accumulator ring.

It takes hours to build a stack of antiprotons large enough to make a meaningful beam for tevatron collisions. Yesterday morning we had a pretty large stack, a total of 5 trillion antiprotons. But we wasted 4 of them when, after injecting them in the tevatron tunnel, one of the magnets of the accelerator had a quench.

Magnets are used to focus the beam and to get particles trajectories to bend in the circular tunnel. The ones at the Tevatron are superconducting ones, so they produce intense fields in a compact size and with less current expense. To keep them in the superconducting state, liquid helium is flowed through them. But sometimes, the liquid helium will have not enough pressure to take away the heat from the magnet. The temperature will rise, and the field will decrease sharply. This is what is called a quench.

A quench can be destructive for the magnet, but most of the times things are not that bad. However, at the very least a fast quench causes the beam circulating in the accelerator to be lost.

Too bad for the nice stack of antiprotons, whose life had been designed to produce exotic new particles after a few hours spent orbiting and then smashing against a proton. They did not live long enough to make a fancy Higgs boson… They ended their life annihilating against a dump.

And today, we had another quench, again just before starting collisions… Too bad! These two stores might have allowed us to collect a couple of Higgs bosons and maybe 30 top quark events…

Yesterday, to my horror, I discovered that the deadline for the submission of proceedings articles for the QCHS7 conference is November 1st. I had received the e-mail a while ago, but had not read it carefully then, and now there’s only three days left for me to write up something meaningful about my presentation there.

Of course, I do not despair. First, I am only requested a 4-pages manuscript - whew, the last one I had to write in June was 32 pages long! I would never manage 32 pages in three days, but 4 pages can be easily put together in a day or two. The challenge, though, is to write an article that is pleasant to read, interesting, and new - otherwise, just cutting and pasting from previous writeups would well do the trick.

No, I am not that kind of guy. I value well-written articles, and always do my best to produce something useful when asked. So I will have to work overtime to make the deadline.

(I know, deadlines for proceedings are never met. But I make it a point of trying to avoid making it harder than necessary for the editors, who after all do a service to us all in collecting the material and publishing it).

So, I am now writing this stuff. I will post here small sections of it, as I have become used to do. Readers of this blog are encouraged to proofread the writeup, suggest modification, point out obscure passages. You might get cited in the Acknowledgements section as Demie Cheng was last year and Riqie Arneberg was last June… 

I have always thought that there is no such thing as too much information. Sure, the more information is available, the better it has to be organized and accessible, but I do not see how it could be a hindrance to a better, more organized, efficient organization of one’s work or day-to-day activities. And yet…

Today’s technology is like a 10 year-old child who is growing too fast for his clothes. We have incredibly sophisticated devices at our service, but we are too slow to learn how to best use them to our advantage.

A small example ? Tonight daylight saving time was ceasing, allowing for one more hour of sleep. Of course I was very happy, since I am on a day shift at the control room of my experiment, CDF, as a Scientific Coordinator. I have to be there at 8AM sharp every day, for a week. Today I would be able to sleep for one extra hour - actually, only half hour, since I had agreed to come earlier than 8AM, to split the extra hour of duty with the former crew which was taking the 0AM-8AM shift.

So when I went to bed last night I set the alarm clock on my cellphone to go off at 7.15, half an hour later than usual - reasoning that the gained hour would compensate: I would then have a full 75′ to get up and arrive at Fermilab at 8.30 DST, which would be actually 7.30 on all updated watches. Wrong!

The devilish little device knew everything, and it went off at 7.15 after having updated for the change of time. So It woke me up one hour too late.  

As a result, I was still able to rush to the control room in time for not pissing off anybody, but I had to take a ridiculously fast shower, run, risk a speeding ticket or two, and most of all I had no time to buy any food for my crew (one of the most important duties of the Scientific Coordinator). Darn!

The CDF experiment distributes physics analyses in a small number of working groups, called “Physics groups”, which meet regularly every two weeks to discuss the advancement of all the studies relevant to a particular branch of hadron collider physics. At the meetings of the working groups, each run by two conveners, new physics results are discussed, scrutinized, and approved (we say “blessed”) for public consumption.

Right now, we have five main physics groups: the top, bottom, exotics, QCD, and electroweak group. Each of these spawns their own subgroups, so for instance the top group has a “top properties” subgroup and a “top mass” subgroup, the exotics group has a “Higgs” subgroup, the electroweak group has the “W/Z cross section” subgroup, etcetera. The subgroup leaders report periodically to the main group meetings.

The number and division of these physics groups has not been constant during the history of CDF. Following the varying needs of the many physics analyses, the amount of work being carried out in different studies, and the importance of selected topics, groups have merged, split, branched. For instance, I remember that in 1992, when I joined CDF as a summer student, I used to follow the meetings of the “Heavy flavor group”: it was a place where B analyses and charm physics analyses (yes we did some of that as well even back then) were discussed together with top quark searches - the top was not born yet!

The CDF heavy flavor group split into the B physics group and the Top group soon thereafter, a division that lives through to our days. Interestingly, the opposite trend happened in 2000, when the leftovers of Run I physics analyses were dealt with in little-attended meetings, and a merging was decided between the top group and the electroweak group. It made sense, since W and Z boson signatures (whose study had always happened in the electroweak group) were the basis of top physics searches too. 

Now, let’s come to the subject of this post… The Higgs boson has been searched in CDF for a long time, since Run I, and the analyses have always been discussed in the Exotics physics group, together with fancy SUSY searches, extra dimensions, technicolor and what not, but specific technical details were presented at the Higgs subgroup.

Now things are going to change. At the last executive board meeting, the CDF collaboration has decided to create a new “Higgs Discovery Group”. This group will draw a lot of resources from everybody, and is designed to catch attention, manpower, trigger bandwidth, and to produce a coherent effort from the whole collaboration, in order to do our utmost to discover the Higgs boson, if Nature (the bitch) gives us a chance to do so.

It does make a lot of sense. Indeed, CDF has already bagged the two most important results it was after in Run II: the measurement of Bs mixing, and a 1% precision in the top quark mass. Well, the latter is not in the bag yet, but there seems no possibility other than a major incident to the accelerator to take that result away from us before the end of Run II.

Instead, finding the Higgs boson in CDF (and D0: on that endeavour the two experiments have long known they have to work together, to double the statistical power of their data) would be the crowning of a really illustrious career, from 1985, when the experiment was first put together. A quarter of a century of successes, O(1000) physics papers and technical articles, the discovery of a fundamental SM particle, plus countless other composite hadrons. The measurement of SM properties with unmatched precision, the discovery of Bs mixing… What would be better to top it off with the discovery of the Higgs boson ?

The demonstration that the experiment is going to start acting coherently and speak with a common voice will come soon, when the trigger menu will be revised to allow Higgs boson collection to be unhindered by bandwidth problems - at the cost of a significant reduction of other, now less urgent, datasets. Until now, in fact, the trigger was a common resource that was split among the different endeavours by trying to make everybody happy with their own little dataset… I have always thought of our 600+ people collaboration as more like two or three different sub-collaboration, a large one being the CDF-B physics collaboration, and another large one being the CDF-top physics collaboration. Now I hope CDF will be all CDF-Higgs!

As my involvement in CDF is ramping down, I will not be able to help out much, most of my attention being driven by CMS. But I still hope to see a final success of this great experiment, despite the fact that it goes a bit against my own interests, since CMS would then be left with the mission of discover the non-existent SUSY and improve by a riduculous amount the top mass precision. It would be quite interesting to see how long the LHC would then keep running…

I was sifting through the spam comments to this blog caught by Akismet, and could not help laughing at one - which repeated itself several times in the collection of 900+ comments gathered in the last few days.

The comment body was the following:

“Hei! luogo che interessante avete fatto, ben cotto!”

 This sentence appears to be an automated translation of “Hey! What a nice site you put together, well done!”. However, the way it sounds if you read it without a preconception, is rather “Hey! Place how interesting you did, cooked perfectly!”.

For some reason I found this hilarious… I admit, not so overwhelmingly fun, but hey, I am in the CDF control room during one of the smoothest stores I’ve seen, and nothing happens… Anything distracting me from that boredom is good today!

Today I started my week of shift as a Scientific Coordinator, in the CDF control room. Lots of things to learn back again, after two years off-duty.

The CDF control room is located at the second floor of the CDF assembly building, right on top of the cavern which contains the detector. It is a room 12 meters long by 7 meters, three sides of which are lined with monitors of all kind. Most of them are computer screens where applications are running, but there is a whole variety of other devices too, with blinking lights, red-led digits, buttons to press in case of alarms, stickers warning the shift crew not to push this or that particular button unless REALLY sure, and the like.

Quite a pictoresque place. In the middle of the room is the desk of the Scientific coordinator, on whose computer runs the shift E-log, where entries are made to describe in real time and in great detail everything that is happening to the detector and the data taking.

We are currently in the middle of a store, which means we are taking good data right as I write. If you want to check what is going on, I discovered the access to the CDF shift E-log is free, no password required. So here is the link:

http://www-cdfonline.fnal.gov/cgi/elog/elog.pl

Have a look… You can even browse through the past history of CDF data taking, and you will get to know every bit of detail about the last five years of CDF operations. If you don’t get bored before, of course…