Archive for January, 2009
The mass of the proton is 0.938 GeV, and the mass of a single charm quark is something like 1.3 GeV, so the answer would appear to be: obviously not. In the wonderful world of quantum mechanics, and perturbative QCD, the answer is not so obvious.
The question is normally phrased: Is there an intrinsic component of charm in the nucleon? Recently the D0 Collaboration posted a measurement which provides a partial answer. The paper is Measurement of photon+b+X and photon+c+X cross sections… (arXiv:0901.0739). They used advanced data analysis tools to identify photons and jets containing heavy flavor, and then measured differential cross sections using 1 fb-1 of Run II Tevatron data. These measurements allows a test of a calculation by Tzvetalina Stavreva and Jeﬀ Owens based on “normal” PDFs (parton distribution functions) and on two alternative sets with intrinsic charm components.
To make this measurement, the authors needed a sample of events with a single, well-identified high-pT photon, and a bottom or charm jet. They needed the rapidity and transverse momentum of both. They present their results as a function of the photon pT, for the cases when the photon and the jet are on the same end of the detector or on opposite ends (as defined by a plane perpendicular to the beam and passing through the event vertex). For each pT bin, they must ascertain the number of b-jet, c-jet, and light quark jets, and they do this by fitting three templates to the distribution of an artificial neural network variable. A nice example is shown below.
This fit is repeated for several pT bins, twice, depending on the relative sign of the photon and jet rapidity:
The measured cross section confirms the preliminary calculations of Stavreva and Owens for photon+b-jets, but for charm jets the agreement is less good. This is easy to see from the plots of the ratio of the measured cross sections to the prediction:
In particular, the data show a charm contribution much larger than predicted at high jet pT (bottom two plots). In the figure, the red dash-dotted lines show the level of enhancement predicted when a PDF with intrinsic charm is used in the calculation. The intrinsic charm does indeed predict larger cross sections at high pT, as one would expect, but not large enough.
This measurement presents an interesting puzzle for jet production in hadron colliders. Can one say that this confirms the existence of an intrinsic charm component to the nucleon? I would not say that this one measurement suffices, but it is suggestive. In any case, more measurements would be welcome.
So maybe the particle physics community could set up something like that for the elementary particles, including intermediate vector bosons and the Higgs boson. Maybe we could have well-known, interesting people record short explanations and stories about each of the particles, for example:
- bottom quark: Leon Lederman
- charm quark: 50% Burt Richter and 50% Sam Ting
- W boson: Carlo Rubbia
- Z boson: Steven Weinberg
- gluon: Sau Lan Wu
- top quark: a carefully chosen panel from D0 and CDF…
- Higgs boson: Peter Higgs, of course!
- others, who?
I’ll bet that most of these people would be happy to support an uncomplicated educational exercise like this one.
Alternatively, the videos could feature distinguished young people at early stages of their careers, such as those who have recently won national or international awards. It would be interesting to view the videos twenty or thirty years hence… Or perhaps all of the videos could be recorded by women, in an effort to foster the participation of young women in the sciences.
We could also have a kind of “side bar” with discussions of crucial phenomena, such as neutrino oscillations, CP violations, spontaneous symmetry breaking, confinement, partonic structure of nucleons, jets and fragmentation, electric dipole moments of pointlike particles, muon anomalous magnetic moment, etc. Clearly there are plenty of topics even if we stay strictly within the Standard Model and eschew all extensions.
The public is very interested in what we do, and that interest will only increase in the coming years, with the turn-on of the LHC, the results of Higgs searches from the Tevatron, and the strengthening of ties between particle physics and astrophysics including cosmology. Imagine how well brief, entertaining, uncomplicated videos would serve their interest..
The International Astronomical Union and UNESCO have declared 2009 the International Year of Astronomy, or IYA2009 for short. There is an excellent web site, www.astronomy2009.org, with all kinds of interesting information and links to other excellent web sites.
These activities seem to be very well prepared. For example, the stated goals for IYA2009 are:
- Increase scientific awareness
- Promote widespread access to new knowledge and observing experiences.
- Empower astronomical communities in developing countries.
- Support and improve formal and informal education.
- Provide a modern image of science and scientists.
- Facilitate new networks and strengthen existing ones.
- Improve the gender-balanced representation of scientists at all levels and promote greater involvement by underrepresented minorities in scientific and engineering careers.
- Facilitate the preservation and protection of the world’s cultural and natural heritage of dark skies in places such as urban oases, national parks and astronomical sites.
(Personally, I particularly like goals 1-4.)
Some of the projects that caught my eye are:
- The Galileoscope, which seeks to connect amateur astronomers with the general public in the hopes of having 10 million people looking at the sky;
- The Cosmic Diary, a blog written by a couple dozen young people from all around the world (Serbia, Sudan, Spain, South Africa, etc.)
- The Dark Skies Awareness Project, which seeks to preserve and protect dark skies in appropriate places, such as national parks and astronomical sites
- Universe Awareness Program for young children, which offers an array of educational and entertaining materials on their web site
- From Earth to the Universe, and odd name for an effort to bring stunning astronomical images to the general public.
This is exciting, impressive and very encouraging in light of our hopes that the public will come to support science more strongly in the future!
Yesterday I wrote about the CDF di-jet mass spectrum and suggested that the measurement could be used to improve PDF’s. Ken Hatakeyama (CDF/Rockefeller) pointed out that a recent detailed CDF measurement of jet production will be used to improve PDF’s. The relevant paper is Measurement of the Inclusive Jet Cross Section (Phys. Rev. D78, 052006, 2008). CDF report measurements of differential cross sections as a function of jet ET for five ranges of rapidity.
The paper provides very clear explanations of the basics of jet measurements: Section III discusses jet reconstruction algorithms, pointing out the differences between cone-based and kT-type algorithms. Section V covers jet energy corrections, which, as I pointed out in the earlier post, are crucial for this kind of analysis. If you want a readable account of those corrections, see this section.
The measured cross sections are slightly lower than predicted. A quantitative test of the standard model predictions (NLO pQCD) was performed based on a chi-squared which incorporated systematic uncertainties and their correlations (Section VIII). The probability of the chi-squared is 4% which does not rule out the standard model. It does suggest that these data might help adjust the PDF’s, a point made in Section VIII. The forward (high rapidity) regions are particularly useful in this regard, especially in connection to the gluon distribution function at high x.
The CDF public web page for this measurement is here.
D-zero published similar measurements somewhat earlier: Measurement of the inclusive jet cross-section (Phys.Rev.Lett. 101, 062001, 2008) and they observed similar trends at high rapidity:
The D-zero web page for this analysis is here.
The CDF Collaboration recently published a very nice paper: Search for New Particles Decaying to di-Jets. The authors have taken the most basic kind of event available in a hadron collider, events with two energetic jets, and made a precise measurement of the di-jet mass spectrum, looking for a deviation that might be a signal for physics beyond the standard model. A brief summary is available from the corresponding CDF public web page.
No evidence for new physics was found.
Nonetheless, the paper is very interesting for several reasons. First, the data set is not small, corresponding to more than 1.1 fb-1, and the capabilities of the CDF detector have been pushed to their ultimate level, as far as jet physics is concerned. The previous run of the Tevatron, called Run I, collected only 10% of this luminosity, yet was able to find the top quark. Both CDF and D0 have about 5 fb-1 on tape, ready to be analyzed, and some analyses have been shown which use a large fraction of that amount. Some years ago, pessimists thought the Tevatron would not deliver more than 4 fb-1 before being shut down – we will certainly have 6 fb-1 and hopefully more like 8 fb-1 per experiment, at which point Higgs searches will become extremely fruitful.
Collider detectors like CDF measure the kinematics of individual tracks well, and electrons, photons and muons are also very clear and well measured. Jets, however, are bundles of hadrons which cannot be fully reconstructed as tracks (about a third of them are neutral) and which do not leave the kinds of showers in calorimeters that allow precise energy measurements. For various technical reasons, the calorimeters themselves do not deliver a nice, linear signal, so a whole tier of corrections is required before measurements of jet energies – and di-jet masses – are accurate. Many scientists at CDF contributed to the testing and refinement of these corrections, including some of the main authors of this paper. The control of the jet energy scale and the removal of measurement bias is central to this measurement, and the success of this analysis is proof of the mastery of these corrections by the relevant people within the CDF Collaboration. Let me point out that the di-jet mass spectrum presented in this paper extends over 1 TeV in energy, and the rate falls by eight orders of magnitude.
Second, the methodology of this kind of analysis – including the search for “bumps” and “shoulders” in the di-jet mass spectrum – is improved incrementally in this paper. In that sense, this paper and others like it serve as a “how-to” example for searches to be carried out in the future.
Third, the measurements are so accurate that the experimental errors (mainly coming from those jet energy corrections) are not large compared to the uncertainties coming from the parton distribution functions (PDF’s for short). The distribution of quark and gluon energies in the incoming proton and anti-proton beams is needed for predicting the di-jet mass distribution, yet we cannot calculate those parton (quark and gluon) energies without reference to many other empirical measurements from the past. The PDF’s amount to an intelligent parametrization of the momentum fraction of the partons based on these earlier measurements, so the uncertainties from the measurements as well as the arbitrariness of the parametrization unavoidably lead to an uncertainty in those parton energy distributions. One hopes that this measurement, and other similar measurements from both Tevatron experiments, can be folded back into the PDF’s to reduce these kinds of uncertainties.
Finally, the CMS and ATLAS collaborations will make this same measurement at the LHC as soon as they get good collision data. Since the center-of-mass energy will be much higher than at the Tevatron (the Tevatron is slightly less than 2 TeV, and the LHC will run, hopefully, at 10 TeV this year, and 14 TeV in 2010), high di-jet masses can be reached with a lot less luminosity – just a small fraction of 1 fb-1. The control of the jet energy measurement, however, will take a lot of work and effort, and cannot be expected in the first weeks of LHC collision data. That said, both collaborations have detailed and tested procedures in place which will deliver the whole set of necessary corrections. A public document explains the CMS plan for jet energy corrections, for example. The craft involved in this kind of search will be inherited from the Tevatron experiments – indeed, some of the authors of the CDF paper are members of CMS…
ZapperZ (blog: Physics and Physicists) provides the perfect antidote to the very negative comments written by the public in response to article about budget challenges to physics departments. His latest post today provides a link to a New York Times article Eleven Questions for Obama’s Science Team. The questions are really good! Number 3 (Can Science Get Respect?), Number 7 (Will You Make Science Cool?) and Number 9 (Boosting U.S. Brainpower) are particularly germane, and heartening.
What a nice first day of 2009!