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Michael Schmitt | USLHC | USA

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Tevatron Experiments reach the stars

Thursday, September 29th, 2011

Yesterday we at Northwestern enjoyed a site visit by the DOE. The point of a site visit is to allow the DOE representative to assess, first hand, what the researchers are actually doing. Many senior physicists in HEP can write wonderful prose extolling the achievements of their groups, but a face-to-face meeting stretching over several hours allows the DOE representative to probe and check.

Anyway, one of the pleasures of the site visit is hearing what other researchers in your own institution have accomplished. There was one very brief presentation concerning the D0 experiment at the Tevatron, during which the speaker trotted out some of the nicest D0 results in electroweak and top physics. Since I am working hard in electroweak physics in CMS, and used to work in electroweak physics in CDF, I tend to view such presentations with a very critical eye. But indeed, setting aside any quibbles about systematic uncertainties and acceptance corrections, the achievements of the D0 Collaboration – and of the CDF Collaboration – are astounding considering the starting point back in the 1980s when I was still a graduate student. As Ken Bloom nicely explained, the physics results produced by D0 and CDF over a decade of 2 TeV running are far beyond anyone’s expectations, back in the 1980s. For example, the possibility of measuring of the top quark with an error better than 1.5 GeV was ridiculed at the start of Run II – yet look how well the Tevatron experiments have done and how important this result is for particle physics. Look at the measurement of the W mass, of Bs mixing and heavy flavor spectroscopy, of a wide range of QCD tests and studies of weak boson production, etc. etc. These results are like stars in the constellation of collider physics.

Two or three generations of Tevatron experimenters achieved what no one would have expected – projections for such measurements would have been considered pipe dreams, pie in the sky, or fantasy. Yet they did it, providing an excellent starting point for the LHC.

During the dinner that concludes the DOE site visit, we discussed prospects for measuring longitudinal WW scattering, which is intimately related to the existence and properties of a Higgs boson. The common wisdom is that it is very difficult and perhaps impossible to measure the cross section accurately. However, one or the theorists argued that, given time and data, experimenters always achieve more than one ever expects. I think he is right – and the record of the Tevatron proves it.


Neutrinos and Quantum Gravity

Thursday, September 22nd, 2011

Kathy Copic wrote a nice article about the exciting rumors of superluminal neutrinos observed by the OPERA Collaboration. See her article in QD here.

Other great articles include the ones from Adrian Cho at ScienceMag.org, at vixVra log and from Matt Strassler’s blog.

Apparently an important theoretical impetus for the OPERA measurements comes from a 2008 paper by John Ellis, Nicholas Harries, Anselmo Meregaglis, Andre Rubbi and Alexander S. Sakharov, Probes of Lorentz Violation in Neutrino Propagation. The authors point out that neutrinos are special among the elementary particles, not simply because their masses are extremely small and the interact via the weak force only, but also because they are candidates for violating relativity at the quantum level. Photons, electrons and protons are known to obey special relativity (think of the LEP and SLD electron machines, or the Tevatron and LHC proton colliders). Neutrinos, however, oscillate due their mass and the fact that neutrinos are electrically neutral, and this makes the key difference. In their scenario, the violation of special relativity would be greater when the neutrino energy is greater.

As Kathy pointed out, the timing ability of the experiment and the time structure of the neutrino beam is crucial for the measurement, and the Ellis et al. paper discusses the important points as well as outlining a possible analysis. This might turn out to be a prototype for the actual OPERA analysis we expect to hear at the CERN seminar on 23-Sept. Key ideas are to use the time structure of the beam and the spread in energy and to look at the edges of the distribution of event times. See, for example, this figure which represents the standard model expectation and an example of evidence for neutrinos which come 100 ns too early:

Figure from the Ellis et al. paper

(Keep in mind that the rumors say that the OPERA results show an offset of 60 ns.)

Variations of this analysis are discussed including changes to the time structure of the beam, which could greatly boost the sensitivity of the OPERA experiment. I do not know whether these changes were implemented (I would assume not).

As to quantum gravity, that lies well beyond what I can explain. As Jonathan Asaadi says, let’s see now what OPERA shows at the seminar!


Whoa — slow down!

Friday, September 16th, 2011

Everyone is excited about the prospect of settling the question of the Higgs boson – does it exist, or doesn’t it?

Of course, we are talking about the Standard Model Higgs Bosons. Higgs bosons in other models such as Supersymmetry have different properties (decay modes & production cross sections) so the negative results of searches from CMS and ATLAS do not necessarily pertain to them. (And keep in mind that no theorist accepts the Standard Model as correct, so focussing exclusively on the SM Higgs boson seems not very wise…)

That said, why do we – the particle physics community – set a deadline of the “end of the year” for coming up with a definitive statement about the Higgs boson? Hopes of finding it have been alive for decades, so surely we do not have to rush through the last months and weeks of the analysis of brand new data. The gravity of the Higgs question requires that no mistakes be made, and that the results of the analyses are definitive. We shouldn’t skip any cross checks or leave auxiliary methods out of the game plan in the interest of getting an answer faster.

If today there is any particle physics analysis that requires the highest standards of our field, then this crucial and fundamental test of the Standard Model (even as a mere effective theory) is it.

My hope is that our eagerness and excitement to have the answer to the Higgs question is tempered by our desire to get the right answer, one that will not require revision a few months after we have communicated our conclusions to the world…


One Inverse Femtobarn!

Wednesday, June 15th, 2011

A milestone was passed since yesterday: the LHC has delivered 1.0 fb-1 integrated over exactly three months! Here is the canonical plot from CMS (source):

CMS luminosity

integrated luminosity versus time for the CMS detector

As you can plainly see, the LHC had delivered 1.00 fb-1 as of yesterday evening, with 92% of it successfully recorded by the CMS detector. You can also see this information displayed at the LHC Programme Coordination Home Page.

I find it awesome and sobering that so much data have been collected in such a short time. Remember that the LHC experiments produced an explosion of high-quality physics analyses based on only 0.04 fb-1 – and now there is twenty-five times more data.

The consequences for all measurements and studies of standard model processes is huge. One can make truly precise measurements of the production of W and Z bosons, and top quarks for example. The kinematic ranges covered by samples of multi-jet events will be huge compared to the past, allowing better tests of pQCD calculations.

Of course, the consequences for searches beyond the standard model are even more profound. The 2010 searches supersymmetry turned up no evidence, thereby significantly extending the regions of disallowed parameter space, for example. The expectations for 25 times more data are very exciting, clearly. The same can be said of the searches for new gauge bosons, or extra dimensions, or other candidate theories.

Probably the most anticipated results center on the standard model Higgs boson. The educated guesses from last year indicate that wide ranges of Higgs mass will be definitively probed this year, and 1 fb-1 is already a big step in that direction.

I am thrilled to see so much good data coming in, and I am happy for the whole LHC community. This will be a crucial year for our field, so let’s see what Nature has in store for us!


A time when the Tevatron and the LHC Overlap

Thursday, June 9th, 2011

Today I was reading a superb paper by the D0 Collaboration on the Measurements of inclusive W+jets production rates (arXiv:1106.1457, 7-June-2011) when I was struck by a sentence in the introduction:

Therefore, it is crucial to make precision measurements of W+Jets production at the Fermilab Tevatron Colllider and the CERN Large Hadron Collider…

At this point in time, the particle physics community is blessed by results from two complementary colliders: ppbar at 2 TeV and pp at 7 TeV. So if you want to look at some important process from two angles, so to speak, now is the time to do it.

It is well known that the analyses done by the LHC collaborations follow the lead, at least in part, of prior analyses done at the Tevatron. It is also known, perhaps less well, that the performance of the LHC detectors is so superb that some major analysis problems at the Tevatron turn out not to be so horrible at the LHC. Some obvious examples include the calibration of the calorimeters, the tails of the missing energy distribution, and the hermeticity of the detectors (i.e., how few “holes” they have in their coverage).

But if we set aside the question of evolution and competition, a third point emerges which the authors of the D0 paper plainly see: due to the differences in the machines, the same process when studied at both the Tevatron and the LHC is richer and can teach us more. I am wondering whether there might be some opportunities that we might stand in danger of missing.

Take a simple example: the cross section for the production of W and Z bosons. This is a bread-and-butter measurement, and experimenters at CDF, D0, CMS, ATLAS and LHCb know well how to make an accurate measurement. This is a real art which is now in the Nth generation, and the methods are very refined.

The point is: the same refined measurements performed at the Tevatron and at the LHC tell us somewhat different things about the beam particle (protons), especially about the momentum distributions of quarks and anti-quarks inside. At the Tevatron, most Ws and Zs are produced by the annihilation of a quark in the proton and an anti-quark in the anti-proton, and in fact these are “valance” (anti-)quarks which carry a significant fraction of the beam particle’s energy. At the LHC, by contrast, there are no anti-protons, so the anti-quark must come from the “sea” quarks (in contradistinction to the valence quarks) which we know will carry only a small fraction of the beam particle’s energy. This means that the two measurements of the same process (W and Z production) probe different parts of the proton, and that can be quite interesting.

Another example is the sample of top – anti-top quark pairs produced. There is an intriguing result from CDF showing that the forward-backward asymmetry of the top and the anti-top quark is much larger in magnitude than expected. At the LHC, top quarks are produced copiously, but through a different mechanism, so it is not obvious that the LHC experiments can make an immediate contribution to the question of what might be going on with the ttbar asymmetry.

The Tevatron experiments CDF and D0 have an impressive history of achievement across decades of high-energy collisions. The LHC experiments CMS, ATLAS and LHCb have practically exploded with new physics results starting already at quite a high level of sophistication. The overlap of the two sets of physics results should be viewed in stereo and one should perhaps look for points at which measurements do not quite seem to “jive”.


How should we check the CDF Anomaly?

Thursday, April 7th, 2011

There has been lots of buzz about the CDF anomaly, and many blog entries and discussion.   For example, there is Flip’s excellent discussion here, as well as entries at many other physics blogs.

Everyone understands, of course, that the next step is for CMS and ATLAS to check their data and see whether they might also observe a hint of a signal.   While the LHC luminosity is about 100 times smaller than what CDF used, the detectors are better and the cross sections are larger, so maybe they can see something.  It’s worth looking, right?

So, if you are are physicist with LHC data on your hard drive, how do you begin?  Instinct says: just implement the CDF cuts and run on the data.  If those cuts produce a signal at CDF, they should produce one at CMS or ATLAS, too, right?

Well, maybe.  But that attitude might be too simplistic.   If you look at the CDF plot below, you’ll see two peaks.   The point is not:  CDF looked at the di-jet mass (MJJ) spectrum and saw a bump.  The point is:  CDF established a signal for a predictable standard model (SM) process, namely, ppbar→W+V, where  V is W and Z together, and next to this SM signal, saw an extra bump.  The fact that they see W+V at the expected rate provides some strong proof that their analysis is valid. If they could not manage to see W+V, then I would be reluctant to take their analysis seriously.  And although they did not couch their discussion this way, for me an important fact is that the anomalous bump is roughly half the size of the SM W+V one.

So, in my opinion, any experiment that wants to check whether the CDF anomaly is present in their data must show that they at least see the W+V signal, first.  This is more important than blindly implementing the CDF cuts.   Show the benchmark, and then we will see whether there is an additional peak, roughly half the size, about 60 GeV higher in mass.

CDF anomaly

CDF MJJ distribution