A Search for Collinear Muons

The D0 Collaboration recently posted a brief paper describing a search for Higgs bosons in the NMSSM (arXiv:0905.3381). The model supposes that the scalar Higgs boson, h, decays predominantly to a pair of very light pseudo-scalar bosons, a. If the a bosons are very light, then they may be expected to decay predominantly to a pair of muons or tau leptons. So the signature would include events with four muons (very distinctive!) or two muons and the decay products of two tau leptons (also distinctive).

I like the idea of finding a light particle produced in high-energy collisions through its decay to muons (or photons or electrons) since it helps underscore the fact that hadron collider experiments are also a good place to look for very rare processes, not just heavy particles. I wrote earlier this year about looking for a bosons at a hadron collider and it is very nice to see this analysis by D0.

The physicists who conducted this analysis were faced with at least one interesting challenge: reconstructing two high-energy muons which come close together in space. (The muons are close together because they come, hypothetically, from a fairly light particle which itself comes from the decay of a fairly heavy particle. So the muons have a large Lorentz boost in the laboratory frame.) Most muon detectors are designed to register a single muon well, or perhaps two muons that land far apart. Their granularity is poor, compared to tracking devices or even the calorimetry. There can be problems with producing a valid muon trigger, and also with reconstructing the muon tracks themselves, even offline. Finally, one has to be careful when demanding that the muons be isolated, since they are not isolated, strictly speaking.

For the four-muon channel, the D0 physicists approach this challenge by asking only that one muon out of each pair be reconstructed, and then they pair each of two reconstructed muons with a “companion” track, meant to be the muons that were not reconstructed successfully. This may sound like it should lead to a large background, but remarkably, it does not, thanks to the isolation criteria they applied. Only two events are selected in over 4 fb^-1, consistent with expected backgrounds, and neither of these has more than two muons.

The reconstruction of nearly-collinear tau decays is much harder. In fact, the D0 group did not try to reconstruct the tau’s directly, but rather leaned on the fairly large missing energy coming from the neutrinos emitted in the tau decays. (For a nice simulated event, see the analysis web page.) With two muons, significant missing energy, and then evidence of tau decays and vetos against jets, again the expected background is quite small. The two muons from the a decay (remember, for this channel one a decays to a muon pair, and the other to a tau pair) can be used to reconstruct the mass of the a boson. So the natural strategy is to look for a bump in the di-muon spectrum for this sample of events.

Here is the D0 result:

di-muon spectrum for candidate events with two muons and two tau leptons

The eye catches the “peak” at 4 GeV, but of course this peak is not statistically significant. It is nice that the D0 physicists have produced a smooth background prediction from data (blue dashed histogram) – the selected events appear quite consistent with that. Narrow peaks are also drawn on the graph, indicating hypothetical signals, as black curves and black histograms. These are meant for illustration, to show the narrowness of the peak that might have been reconstructed
with the D0 detector, if a real signal had been present.

The D0 paper contains some limits placed on this production channel, which require a few not unreasonable assumptions. I’m not so interested in those limits, which generally are higher than the theory predicts. What pleases me about this piece of work is the ability of the D0 detector and event reconstruction to look for such events.

100 Hours of Astronomy – Historic Telescope at Northwestern

Earlier this year I made a brief post drawing attention to the International Year of Astronomy, i.e., this year. Risa Wechsler at Cosmic Variance also made a post on 29-March, and I am sure there are many others.

The idea is to build a sense of community among amateur astronomers and to promote an appreciation of science by ordinary citizens.

Northwestern University, and in particular, the Department of Physics and Astronomy, is hosting a special Skygazing Event at Dearborn Observatory located on the sometimes verdant campus in Evanston, Illinois.

On Saturday evening, April 4th, visitors will be able to gaze through the telescope (pictured above) at the moon and Saturn, as well as connect in live webcast to similar activities around the world. Astronomers from NWU will be on hand for discussions and to answer questions. Details are given in a press release.

Light Neutralinos and Charginos are Expected!

One of my favorite papers of the last year is a very well-done scan of the phenomenological-MSSM (pMSSM) by Berger, Gainer, Hewett and Rizzo: arXiv:0812.0980 which has the wonderful title, Supersymmetry Without Prejudice. The authors applied an absolute minimum of theoretical constraints, and a maximum of experimental ones, and they took real care to do it right. (OK, you do have to accept the premise that the MSSM is the right theory of nature, but many people are prepared to do that for the sake of discussion, i.e., as an hypothetical.)

So what are the consequences?

A follow-up paper just appeared, arXiv:0903.4409, which elucidates the consequences of the first paper for the properties of dark matter particles. The authors (R.C. Cotta, J.S. Gainer, J.L. Hewett and T.G. Rizzo) find that the lightest neutralino and chargino states should be rather light, in the range 100 to 400 GeV, and this conclusion does not depend crucially on the way they sample parameter space. That’s the good news. The bad news is that the mass difference typically is small – less than 10 GeV in 80% of the cases generated by the scan. Interestingly, searches for quasi-stable, heavy charged particles at the Tevatron have had an important impact here – something that the LHC experimenters should (and will) notice.

M(NLSP) - M(LSP) versus M(LSP)

Another interesting point is that the majority of the models surviving current experimental bounds are inconsistent with mSUGRA.

The lightest neutralino provides the best candidate for dark matter, but as it turns out, only a small fraction of the models predict a relic density high enough for this one particle to explain what is observed. Typical models predict a relic density ten times smaller than what is observed. So while it is true that Supersymmetry provides a good candidate for dark matter particles, obtaining a high enough relic density does not happen naturally. It might have been interesting to see whether the models which do match the WMAP relic density are different from the bulk of the models (sorry for the pun), but the authors do not furnish that information, unfortunately.

The authors checked predictions for direct detection of dark matter and found a huge range of predicted cross sections extending some eight orders of magnitude below the current bounds from XENON10 and CDMS. Most of the models, however, could be tested if the sensitivity of these experiments were improved by a factor of a thousand. (Again the question: is there anything distinctive about the models which are near the current XENON and CDMS sensitivities?) The authors also compared predictions to the PAMELA data; large boost factors are needed and the shape does not, to my eye, agree with the published data.

Bottom line: SUGRA-lovers and tri-lepton devotees, beware!

Workshop in Torino shows CMS is ready!

illuminated building in Torino, Italy

This week CMS held a three-day workshop dedicated to studies of detector performance using real cosmic ray data (so-called CRAFT = “cosmic rays at four Tesla”). For me, the workshop was a real pleasure. First, it was great to get away from rainy, chilly Geneva and walk around the warm, sunny streets of Torino.

Secondly, and more importantly, the presentations were excellent, and the content was impressive. We talked about tracking, muon reconstruction, triggering and calorimetry, as well as certain “physics analyses” associated with cosmic rays. These topics are predictable, but the level of the work was much higher than anyone would have expected. It is clear that CMS physicists are in the mind set to do top-level physics analysis. The attitude of individuals talking about calibration, efficiency, alignment was very serious. These people are aiming for the highest levels of detector performance, and they will get to the bottom of existing detector problems. And of course such problems exist – the point is to find them now and solve them before collision data arrive.

Kudos to the organizers of the workshop, members of the CMS group at Torino. They did a great job setting up and running the conference – everything was smooth and well-organized. There were no technical problems, and the refreshments were excellent!

Does the Proton Contain Charm Quarks?!?

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-p_T 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 p_T, 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 p_T 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.

fit three templates to observed distribution of gamma artificial network output variable

This fit is repeated for several p_T bins, twice, depending on the relative sign of the photon and jet rapidity:

measured cross sections compared to theoretical calculation

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:

ratio of measured cross sections to the predictions

In particular, the data show a charm contribution much larger than predicted at high jet p_T (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 p_T, 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.

Proposal: A Video “Periodic Table” of Elementary Particles

Yesterday, ZapperZ (Physics and Physicists) posted a note about a Period Table of Videos. Check it out – it is a lot of fun!

elementary particles in the standard model

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..

International Year of Astronomy: IYA2009

IYA2009

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:

1. Increase scientific awareness
2. Promote widespread access to new knowledge and observing experiences.
3. Empower astronomical communities in developing countries.
4. Support and improve formal and informal education.
5. Provide a modern image of science and scientists.
6. Facilitate new networks and strengthen existing ones.
7. Improve the gender-balanced representation of scientists at all levels and promote greater involvement by underrepresented minorities in scientific and engineering careers.
8. 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.

There is even a calender of astronomical observations, listing for example the Quadrantid Meteor Shower that Tommaso Dorigo just wrote about.

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!

A couple enjoys watching the night sky

Update – Jet Measurements to be used in PDF fits

ratio of measured cross section to NLO pQCD prediction, in the high-rapidity region

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 E_T 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:

ratio of the D0 measured cross section to NLO pQCD prediction, in the high-rapidity region

The D-zero paper, however, does not describe any discrepancies, instead stating that in all rapidity (y) regions, the predictions agree well with the data. D-zero and CDF use different PDF’s for their analyses (D-zero uses CTEQ6.5 while CDF uses CTEQ6.1M).

The D-zero web page for this analysis is here.

New Particles Decaying to di-Jets

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.

CDF di-jet mass spectrum

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.

ratio of measured spectrum to predicted spectrum. Shows sensitivity to PDF's.

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…

Antidote for “No Sympathy for Physics Departments”

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!