PLEASE don’t call it “the God particle”
Let me make a plea to all science journalist out there:
Please don’t call it “the God particle”!
That name was invented by Leon Lederman, very much tongue-in-cheek, back in 1993 when he published a rather good popular science book. Leon is an nobel prize winner and devoted much of his life to improving math and science education in the US. His talks were clever and witty and this “God Particle” terminology is meant to be full of irony.
To be clear: the Higgs boson has nothing to do with divinity – neither does any other particle of the standard model and whatever lies beyond. No one I know believes that the Higgs boson has any direct impact on theology or religion, and in fact, we all hate the term as being irritating at best and embarrassing at worst.
from Claire Evans’ blog
When journalists employ this term, they deviate from good science reporting toward sensationalism. They know that segments of the general population will be drawn into the article, most likely with less than positive attitudes toward the term.
So don’t do it! Stick with “the Higgs boson” or some term that physicists use, please.
For thoughtful commentary on this same issue, see the blog post from Claire Evans, from which the image above comes.
Tevatron Higgs Results – Evidence?
The bottom line is:
- 2.5 standard deviations for all channels
- 2.9 standard deviations for bb channel alone
These are global p-values. The local p-values are 3.0 s.d. and 3.2 s.d. respectively. Below are my notes and comments from the presentation today.
Eric James and Wade Fisher, CDF and D0 speakers, respectively
The Tevatron Higgs group (CDF+D0) just presented their update – and very nearly final – results for the search for the Higgs boson. Eric James gave the first half (CDF results) and Wade Fisher gave the second half (D0 results and Tevatron combination). Although the Tevatron has stopped taking data, and we have seen impressive results from the Tevatron last winter based on essentially all of their data, this update is important because the D0 analyses have been improved significantly, thereby improving the sensitivity of the Tevatron Higgs search. (Recall that they had an excess at about the 2.5σ level last winter.) The Tevatron data set corresponds to 10 fb-1 recorded data per experiment.
Eric started by reminding us of the impact of indirect constraints on the Higgs boson, valid within the standard model (SM). At present those indirect constraints on MH are consistent with the range not already excluded by LEP, the Tevatron and the LHC. One should not forget that the precision measurements of the top and W masses play a key role in those indirect constraints. Wade stated that the Tevatron searches took as their goal the ability to exclude a SM Higgs boson across the full range favored by the precision electroweak data: 100 – 150 GeV, roughly. They very nearly achieved this goal.
CDF and D0 both search for Higgs decays to γγ, WW, ZZ and bb, but the most sensitive channel by far is pp→VH with V→L or ν and H→bb. The sensitivity of each channel is a strong function of the Higgs mass, MH, but for the interesting region (120 < MH < 140 GeV), the γγ, WW and ZZ channels are still several times the SM signal because the production cross sections are really very small. The bb channel, on the other hand, has a sensitivity near 2×SM or even better. As Eric pointed out, this is actually better than what CMS and ATLAS have achieved so far in this channel.
Eric and Wade, and the entire Higgs search teams that they represent, recognize this channel (“Vbb”, or pp→VH + H→bb) as the main opportunity for the Tevatron to make a major contribution to understanding the Higgs, if there indeed is a Higgs with a mass near 125 GeV. (LHC will make a statement about that on Wednesday.) One should not underestimate what these teams can do, as Eric nicely illustrated with a plot of sensitivity versus integrated luminosity. To be plain, the Tevatron Higgs searches today exceed even the most optimistic projections from five years ago. Inspiring. Eric also showed that the significance of any excess around 125 GeV at the Tevatron in the Vbb channel is quite comparable to what ATLAS and CMS can achieve with other channels.
Wade explained succinctly that the D0 Higgs searches have all improved due to technical improvements and the addition of a little more data. The improvements are on the oder of 10% per channel, and some small improvements are still expected over the summer. One the main technical improvements comes from splitting backgrounds in any given channel into categories which are then suppressed individually. This bolsters the S/B ratio and was also done by CDF (and the LHC experiments).
Both CDF and D0 see a fairly broad excess in the MH range 120 – 140 GeV. The characteristics of this excess are very similar for CDF and D0, appearing most clearly in the Vbb channel, with only weak signals in γγ and ZZ, and none in WW. (Recall that the LHC results hint at an enhancement in γγ and a slight deficit in ZZ and WW, assuming SM signal strengths.) Here is the comparison of the exclusion limits from CDF (left) and D0 (right):
exclusion limits from CDF and D0
Naturally, since they are so similar, the combined exclusion limit will be about the same, only sharper in its features:
combined CDF and D0 exclusion
Since the observed limit curve lies well above the expected one, we know that we have an excess. This excess comes mainly in bb. Wade showed the following plot to quantify the excess: 
This plot shows the signal strength μ (σ×BF normalized to the SM calculation) for the three main channels. Clearly the WW channel indicates no signal, while the γγ has low precision (though it does not favor zero). The interesting bit is the bb channel, showing a signal strength of approximately μbb=2.0±0.7 – according to my ability to read the graph… The plot on the right shows the probability density for the bb channel; the most probable value is indeed μbb=2 and μbb=0 is unlikely, according to this picture.
The Tevatron Higgs group likes to show a likelihood ratio as a function of mass, and indeed the plot is informative:
LLR versus MH
The likelihood ratio (LLR) shows the separation of the signal hypothesis from the null hypothesis. For a given MH, there will be two Gaussians showing the possible outcomes if a signal is present or absent. If these Gaussians are well separated, then one has very good sensitivity to the signal. The data then pick one Gaussian or the other. Looking at the plot of LLR vs. MH, the dotted black line shows the expectation if the null hypothesis is correct, while the red dotted line shows the expectation of a signal is present. The data from the Tevatron, represented by the solid black line, twists and turns as a function of MH due to statistical fluctuations in the data. But one can clearly see a preference for the signal hypothesis for MH in the 115 to 135 GeV range, and a preference for the null hypothesis elsewhere. The plot on the right shows the ideal case if a Higgs signal is present with MH = 125 GeV and with a signal strength μ=1.5. The value μ=1.5 comes from the statistical analysis of the Tevatron data; basically the Vbb `signal’ is a bit stronger than one would expect with for the SM Higgs boson; I eye-ball it to be μ=1.4±0.6.
This result is impressive and exciting, but Wade and the Tevatron Higgs community made a rather cautious, sober statement: The significance of the excess is still under 3σ, so we are not making any announcements today. Still, the Tevatron results are getting interesting. I’ll say!
In my opinion, this result from the Tevatron is extremely important. While it does not constitute discovery of a Higgs boson, or even `evidence’ in a technical sense, it does illuminate the issue of a new state if indeed the LHC experiments have independent evidence for something at 125 GeV. And it cannot be over-emphasized that this result is completely independent of what goes on at the LHC: the beam energies and particle types are different, the final state is different, and the analysis teams are different.
If there is a new particle, some sort of Higgs boson, at 125 GeV, then I certainly believe it decays to bb with a fairly large branching fraction.
So the Tevatron did not `scoop’ the LHC but it will play an important part in the coming months and years to elucidate whatever Nature will give us.
PS: sorry for the poor quality of the images, this is the best I could get while sharing the live broadcast with more than 1200 other people. For more information, see the Fermilab press release.
PPS: Of course many excellent blogs have already written about the Tevatron results. I recommend that you visit viXra log, Tommaso Dorigo and Resonaances and keep an eye on Matt Strassler and Not Even Wrong, among others…
CMS puts new constraints on Dark Matter
CMS recently released a paper on the search for monojets (arXiv:1206.2664 25-June-2012). Normally one thinks of monojets (events with one energetic jet and large missing energy or “MET'') in the context of supersymmetry or maybe large extra dimensions. But this paper follows a treatment first published by CDF (arXiv:1106.4775 23-June-2011) in which the monojet search is reinterpreted as a limit on the production of dark matter particles in hadron colliders. Credit for this idea should probably go to Beltran, Hooper, Kolb, Krusberg, and Tait (arXiv:1002.4137 22-Feb-2010) among others.
Setting aside supersymmetry, one can expect dark matter particles to be produced in pairs at the LHC or the Tevatron if they have weak-scale couplings to standard model particles. Crudely put, producing DM particles would be a lot like producing a pair of heavy neutrinos (with minor modifications of the DM particles are not fermions; CMS assumes they are spin-1/2 fermions). It makes sense to calculate production cross sections in a generic framework – perhaps even taking just a contact interaction to couple a pair of DM particles to matter. In any case, the DM particle pair may sometimes recoil against an energetic jet, and since they themselves leave the apparatus undetected, a nice monojet signature arises.
The kicker is that the acceptance and efficiency for a DM signal hardly depends on the mass of the DM particle, after one has asked for an energetic jet and large MET. This leads to interesting bounds for very light DM particles at masses inaccessible to most direct-detection DM searches.
Events were selected in a straight-forward way: ask for a good-quality jet consistent with the primary vertex. The jet should have pT > 110 GeV and |η| < 2.4. The MET should be at least 250 GeV. Any second jet should not be back-to-back with the leading jet. Events with leptons or isolated, energetic tracks are vetoed.
The contamination is mainly Z→νν, as one would guess. It is monitored and estimated using Z→μμ events: one predicts 5106±271 events. There is also significant contamination from W+jet events; again this contamination is estimated by finding W→μν events: one predicts 2632±237 events. Much smaller contaminations come from top and Z→LL events. The total number of events expected from SM sources is 7842±367, and CMS observes 7584 events. No signs of DM particle production. Systematic uncertainties are at the 20% level.
The non-observation of monojets implies upper limits on the contact interactions (or whatever theoretical framework you want to use), that can be translated into upper limits for DM-nucleon scattering. Here is the result:
limits on dark matter – nucleus scattering inferred from the CMS monojet search
Similar results were obtained by CDF, as shown in the plot. Also, the CMS results on photon+MET give results not quite as stringent as those obtained from the monojet search.
I like this result because it nicely illustrates the synergy between collider and direct-detection searches for dark matter particles. What we would really love to see, of course, is a signal in both. Alas, that has not yet happened…
Independence-Day Higgs Seminar
The CERN management have decided to convene a special seminar on July 4th at 9am Geneva time (2am in Chicago) at which the results from CMS and ATLAS will be presented. According to the official announcement, the main auditorium will be reserved for CERN personnel, so reporters and visitors will have to view the proceedings via live feed into another large hall at CERN. Of course there will also be a live broadcast that you might be able to access from the announcement page.
The timing of the seminar has nothing to do with U.S. history, of course – it is dictated by the beginning of the ICHEP Meeting in Melbourne, Australia (4-11 July). Nonetheless, American particle physicists might, in the end, have a good extra reason to celebrate this holiday. We will see…
Do you like to spread rumors?
Everyone is excited about the coming ICHEP conference and what will be shown by CMS and ATLAS concerning the search for the standard model Higgs boson. It is hard to be patient, and the urge to indulge one’s curiosity and to speculate without bound is hard to control.
One must ask, though: is it really OK to spread rumors?
The main rumor-monger these days is Peter Woit at Not Even Wrong — a formerly excellent blog that in recent months sometimes seems more devoted to scandal and gossip than to education. You can read there that ATLAS and CMS are seeing “about 4 sigma” in the H→γγ channel. Peter’s statements were quickly echoed over on viXra log and even Tommaso Dorigo posted an elliptical entry on Quantum Diaries. I’m sure other blogs are posting rumors – I am too lazy to go look for them now.
As a member of the CMS Collaboration, I know precisely what we have. But my loyalty remains with my collaboration, especially the people who are working right now to carry out the analysis and verify the results, as well as to the people at the top who have to chart strategy and make difficult decisions. A little splash in a blog is not worth the bother it would cause all these people.
I don’t mean to sound sanctimonious. I am just stating who I am and what choice I have made. Other people obviously make other choices, for all sorts of reasons. The education of the interested public is essential and I applaud those who do it well. The desire to get off of high horses and to step out of ivory towers is good and I value that, too.
But where does one draw the line? At what point does a blogger make a transition from expressing his/her enthusiasm, and explaining something to the general reader, toward increasing ratings and readership of his/her blog? Maybe I am too straight-laced, hailing from New England, while others prefer to hang loose. I guess the only thing to say is, as one says in this part of the world : vive la différence!
The Flame Challenge
Last week I was reading Science and I came across a touching editorial by Alan Alda – the actor.
His concern is a basic one that nearly all of us share: how do we connect in a fundamentally human way to non-scientists, especially children, about science?
If a child asks you: What is a candle flame? You can’t tell her “oxidation” or “self-sustaining combustion” because those words mean nothing to her, they drain the question-answer experience of warmth, and ultimately discourage further such questions.
So, how would you answer the child in a way that avoids those pitfalls?
This is the question posed by Alda, and in fact the starting point of a serious exercise to connect scientists with schoolchildren in a way that promotes the beauty of Nature and an interest in science. There is an outreach program and a contest and an opportunity to make suggestions and provide feedback. Go to http://www.flamechallenge.org/ to get involved.
Any ideas?
ICARUS refutes OPERA
(Somehow there is a good pun lurking there, but at the tail end of a bout with the flu, I’m not finding it…)
Anyway, this plot says it all:
TOF of neutrinos compared to the TOF for light
I’ve taken this image from a new paper posted by the ICARUS Collaboration (arXiv:1203.3433).
The ICARUS experiment sees the same neutrino beam that OPERA sees, and it is located in the same Gran Sasso underground laboratory. Their experimental technique is based on an ultra-pure liquid Argon chamber that produces exquisite images of neutrino interactions, such as this one:
charged-current interaction from the ICARUS experiment
The neutrino enters from the right and interacts after traveling a few cm into the liquid Argon volume. An energetic muon is produced that flies forward to the left and exits the volume; other charged particles coming from the collision with an Argon nucleus move slower and are visible as stubby tracks coming from a common vertex. There is no doubt that this is a good neutrino interaction, and its position in the detector is very clear.
The timing capabilities of the ICARUS detector sound very impressive. The photomultiplier tubes can collect the earliest signals from the interaction and establish the time with precision in the ns range, including the inevitable systematic uncertainties coming from channel-to-channel variations, etc. Given the precision of the timing and the position measurement, the time of the neutrino arrival at, say, the wall 11 m in front of the detector can be established to better than 10 ns.
Recall that the OPERA anomaly corresponds to 60 ns.
The challenge is in making an absolute TOF measurement. The issues and challenge are essentially the same as for OPERA, and indeed ICARUS based the absolute time reference on the system set up by OPERA. They conducted a distance measurement again based on GPS that is accurate at the cm level. Although there is no independent, non-GPS cross check of the absolute time stamp, one presumes that the accuracy is as stated: much better than 10 ns. And in any case, if there were a systematic problem with the absolute time standard, it should be the same for ICARUS and OPERA (well, at least in some cases…).
CERN provided a special neutrino beam last fall in which the neutrino bunches were 3 ns wide rather than 10 us. Concerns about the treatment of pulse shapes, variations in horn currents and efficiencies across a 10 us pulse, possible technical failures of the equipment that record the pulse shapes all are swept away. The much narrower pulse leads to a much cleaner statistical analysis; the 3 ns spread is essentially negligible for the purposes of testing a 60 ns offset. This beam is ideal and indeed OPERA already reported some results earlier on.
ICARUS recorded 7 events consistent with the known neutrino flux. Three of these have interaction points inside the Argon (the event picture above is one of these three), and the other four have interaction points inside the rock upstream of the detector; muons produced in the interaction passed through the Argon allowing an accurate time measurement. The three contained events have δTOF values of 0, -5 and +3 ns. As seen in the plot above, the set of seven events is nicely centered on zero with absolutely no indication of a 60 ns offset. The ICARUS Collaboration quantify their result as δTOF = 0.3 ± 4.0 ± 9.0 ns.
This measurement, if correct, completely refutes the OPERA result. I expect the community will scrutinize the ICARUS measurement, which is reported in far less detail than the OPERA one. Given the doubts associated with cable connections in the OPERA detector, however, it is nearly impossible to believe that neutrinos indeed travel faster than the speed of light…
