Archive for May, 2006

ATLAS observes cosmic-ray events underground!

Very good news – according to Travis Stewart the ATLAS tile calorimeter has recorded clean cosmic-ray events in the underground cavern. Here is the picture he posted:

Please visit his blog in order to see more information about this.While people are not interested in using ATLAS to do comic-ray physics, this accomplishment is important because it shows an expected signal successfully recorded and understood – the hits are in the right place, there are no garbage hits, the event has the right features, etc. Progress over the next year or two will consist of moving from one modest success (like this one) to the next one, which will be slightly less trivial. So, after a year of comic rays, we will hope to count the number of tracks in minimum-bias events…

(A disclosure: while I am happy about this success, it also makes me slightly nervous and envious, since I am a member of the competing CMS collaboration…)

May 30, 2006 at 6:40 pm 7 comments

predictions from Lattice QCD

Georg from Life on the Lattice recommended a couple of papers which show the success of LQCD.

He is right! The post- and pre-dictions of real experimentally accessible quantities are confirmed now at the couple percent level – something which is not generally appreciated in the HEP community.

For example, in hep-lat/0304004 show that computations with staggered quarks correctly reproduce the known values for nine diverse and independent non-perturbative quantities, after the quarks masses and lattice spacing (equivalent to the strong coupling constant) have been tuned. As pointed out in the paper, this result is important not only for the impressive level of agreement for those nine quantities, but also for the way LQCD physics is inextricably linked to B-physics. To put it bluntly, we have a confirmed theory here, not just a clevel phenomenological model. (Not that clever phenomenology is bad – it does help us to think about the physics, at least at an early stage of the game.)

Even more impressive are the successes discussed briefly in hep-lat/0509169. These are true predictions – LQCD was used to obtain precise values for three quantities which were first published and then subsequently confirmed by actual measurements in real experiments. These quantities are, specifically:

  1. the q2-dependence of the form-factor in semileptonic D-decays
  2. the decay constants for D+ and Ds mesons
  3. the mass of the Bc-meson

I consider Fig.1 in Kronfeld et al. (hep-lat/0509169) to be especially striking.This is a big deal for particle physics. There are many places in which fundamental quantities such as elements of the CKM matrix cannot be extracted reliably from measurements due to poorly known “phenomenological” constants. For example, we would like to extract precise values for third-generation CKM matrix elements from the recent measurements of Bs-mixing (D0: hep-ex/0603029 and CDF: FERMILAB-CONF/06-076-e) and lattice will play a key role. It’s time to start paying closer attention to LQCD, at least for me!

Thanks, Georg, for the info!!

May 30, 2006 at 1:23 pm 8 comments

mono-jets!

It would be great to observe a set of mono-jet events at the Tevatron!

Recently the CDF Collaboration published an account of their search for mono-jet events in hep-ex/0605101. (Note: As a member of the CDF Collaboration, my name appears on this paper, but I had little to do with this analysis. I'm not posting comments on my own work today!) It is a well-written paper on a well-motivated search for new physics.

Here are the basics: You trigger on events with at least one energetic jet, and then tighten the jet criteria in your offline analysis to be sure your trigger efficiency is easy to understand. In this case, the jet must have ET>150 GeV, fall in the central calorimeter, and contain at least a couple of charged tracks. Next, demand some missing energy, at least 120 GeV, which would be rare in ordinary multi-jet events. Now you have a set of "interesting" events.

This event sample will contain some events from Standard Model processes. For example, a Z-boson decaying to neutrinos produced together with an energetic jet (a gluon radiated in the initial state) will amount to a perfect mono-jet event. A W-boson decaying leptonically with the charged lepton missed by the reconstruction amounts to the same thing. There will also be some messier backgrounds coming from cosmic rays and from muons flying along the beam direction and passing through the calorimeters. The authors of this analysis demonstrated clearly that they understand the SM production of W and Z-boson production in association with an energetic jet. They also dicuss a clear and straight forward method for estimating the contribution from di-jet events in which one jet is lost or badly mis-measured. The dominant contribution to their sample comes from a Z-boson and a jet.

The agreement of the data wth their prediction from SM and instrumental backgrounds is extremely good: expect 265±30 and observe 263. The shape of their missing energy distribution also matches the prediction beautifully: CDF MET distribution in the mono-jet sample
Given no signs of an anomalous contribution to this event sample, the authors derive an upper limit of 67 signal events at 95%CL (systematic uncertainties play a major role here) which they use to place constraints on a popular scenario for extra-dimensional phenomenology. The constraint is somewhat weaker than a direct search for deviant graviational forces when assuming a small number of extra dimensions, but is quite a bit stronger when one assumes a larger number of extra dimensions.

For more details, and to find out who did this very nice analysis, go to the CDF public web page for this particular analysis.

So, is that it? No extra dimensions and we need to add more data or simply wait for the LHC?

Is this just a big yawn?

Certainly not! First, the analysis is very well done. The cuts are uncomplicated, the sample is quite clean and the estimate of its composition is robust. We should hope that the equivalent analysis at the LHC will share these qualities. In fact, I expect this kind of search at the LHC to be quite hard, since the SM sources of a jet and missing energy will be much larger (for example, from di-boson and top quark production), and the resolution on the missing energy will be difficult to understand due to the underlying event and multiple interactions in each beam crossing. There may even be some confusion from multiple new physics channels contributing to this particular channel, though it would be perverse to complain about that… I hope that people who plan to explore this channel at the LHC have read carefully the corresponding studies with Tevatron data.

Second, the authors chose to focus on models with extra dimensions, but there are other models which might produce a signal in this channel. It might have been interesting to consider an invisible Higgs, not because this is popular or likely, but rather to see how well suited this kind of analysis might be. One might even expect some non-negligible acceptance for certain SUSY channels, such as a pair of squarks decaying to q+chi01. The authors did not explore such channels in detail because the paper would have become too heavy, and in any case a real attempt to constrain, say, R-parity conserving SUSY would involve many channels.

Third, this is an important analysis to do. We must examine events with large missing energy because we hope that new physics will turn up there. This is a benchmark search along with several others without which the collider program would not be a complete success. We don't know which channel will reveal new physics (and it may be an odd, peculiar, obscure channel) so we must look in as many places as possible. The only requirement is to do a good job, as has been done here.

Finally, I would like to ask why this analysis and others like it do not receive much attention from the community. We can read in a dozen places that the LHC will surely find new physics at the TeV scale, and it will likely show up in a missing-energy channel. So why aren't people waiting in anticipation for precisely these results from the Tevatron? It seems that people have written off the Tevatron because they would rather wait for the LHC. And worse, in some quarters it seems that people feel that the LHC will hold no major surprises, so we should go ahead and start building the ILC so that we can really start to answer the big questions of particle physics.

I wish there were a way to restore a more general and more genuine interest in the kind of work exemplified by this particular analysis. It is true that we have all been waiting many years for signs of new physics. But since none of turned up yet, I think we need to look all the more intensively and creatively at the data we already have…

May 28, 2006 at 6:45 pm 2 comments

When is a b-jet not a b-jet?

Tommaso recently wrote an excellent account of how to tag jets which contain a b-quark. As he explains, one uses the precise tracking made possible by silicon detector technology to look for sets of tracks which do not point back to the point where the proton and anti-proton collided. This technique is very well established with already two or three variations, and everyone in collider experiments knows something about it.

Another interesting and older technique to pick out jets which may contain a b-quark is to exploit the tendency for b-hadrons to decay semi-leptonically. A jet with a relatively energetic electron or muon inside is a good candidate for a b-quark jet.

Suppose you are making a measurement, of, say, a top-quark cross-section, and you need to understand quantitatively how well your silicon-tracker is helping you to tag b-quark jets. There will be two questions: (1) how often do I get a b-quark jet that is there? and, (2) how often do I get a jet which is not a b-jet (and may very well be a c-jet)?

One approach will be to find jets which are tagged by the silicon method, and then see what fraction of those are also tagged by the lepton method (and vice-versa of course). If you know what’s going on, then you can predict these fractions. Observing the values that you predict confirms your understanding, and one could then assume that your measurement is valid.

But suppose observation does not bear out your expectations! Usually that means you don’t understand something at a technical level, and you can feel frustrated. But there is also the possibility that some of your jets contain something new – perhaps even evidence for new physics! That would be truly wonderful!!

This is not just a series of what-if’s. This happened with CDF data taken in Run I and the team who came across this discrepancy tried hard to investigate the peculiar jets – which they called superjets – to decide whether new physics was present or not. Over the objections of many of their colleagues, they published their results which you can read at hep-ex/0109012.

I find this result remarkable not for the saga, on which I will not comment, but rather because it raises the very real possibility that new physics could arise almost anywhere, if one is alert and willing to keep an open mind. People sometimes pay too much attention to theoretical models and then overlook or neglect what might be there, right in front of them. What they regard as a “control region” or “cross-check” might in fact be the first glimpse of something exciting.

There is a corollary, too. If an anomaly like this is observed in one data set, then it is extremely important to check in another data set for confirmation. In fact, the CDF Collaboration did check their new data and compared the fraction of silicon-tagged jets with a muon to their expectation, and found good agreement. You can read that paper in hep-ex/0512065. Their conclusion is that there is no confirmation of a signal for new physics, in the new data.

It is sad that new physics is not there. But it is good that people did the first analysis, and that they did the second one, too. Now let’s see what the next alert person finds, and hope that it is genuine.

May 26, 2006 at 3:02 pm 1 comment

Hello, World!

Here begins my Collider Blog.

The Tevatron at Fermilab is currently the world’s highest energy accelerator. Very high energy protons and anti-protons collide at the centers of the CDF and D0 detectors, which record the traces left by the particles produced from the energy of those collisions. Sometimes rare processes occur which lead to interesting events; by identifying and studying these events we learn about those processes and, ultimately, about basic particle physics.

The LHC (Large Hadron Collider) at CERN will supercede the Tevatron in a couple of years. It will have an energy that is seven times higher than that of the Tevatron, which allows rare processes to be studied more easily. There are two detectors here, too, called CMS and ATLAS.

Accelerator-based particle physics clearly is at a cross road, and much has been written about the promise of the LHC just as impressive results are produced with the Tevatron data. The start-up of the LHC is bound to be frustrating as well as exciting, yet I personally believe that much is still to come from the Tevatron program. It seems like a good time to observe the field, especially the science that emerges over the next few years, and perhaps make some notes and comment on what is happening.

My goal is to discuss the some of the research done at these two hadron colliders in a serious way. As an experimenter I will try to bring out what I think is interesting, exciting or beautiful about analyses from CDF, D0, CMS and ATLAS (I am a member of CDF and CMS, by the way), and hopefully complement some of the excellent theory-oriented blogs, and blogs by experimenters working at other facilities.

May 23, 2006 at 9:16 am 5 comments


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