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A hint of something new in “W+dijets” at CDF

Update: for those who read this in time, there will be a seminar on this result broadcast online through the Fermilab webpage. (The talk will be at the level of physicists working in the field.) This result was also mentioned in the NY Times today.

Even though its running days are numbered, the Tevatron reminds us that it can still muster up an interesting hint of new physics. This is a quick post on a brand new result from the CDF collaboration, “Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at Sqrt(s) = 1.96 TeV” [arXiv:1104.0699]. Just to whet your appetite, here’s one of the plots that we’d like to understand (hot off the press!):

Unpacking the title

Before diving in, let’s first understand what the title means. It’s somewhat cumbersome, but that’s because it encodes a lot of good physics. It turns out that it’s easier to start at the end of the title and work our way backwards.

• “…in p-pbar Collisions at Sqrt(s) = 1.96 TeV
This is telling us about the general Tevatron experiment: they collide a proton (p) with an anti-proton (p-bar) at an energy of about 2 TeV. A TeV is roughly the kinetic energy of a flying mosquito. In Feynman diagram calculations it’s often useful to use the parameter s, which is the square of the energy, rather than the energy itself, hence the weird “Sqrt(s) = 1.96 TeV.” For comparison, recall that the LHC is now operating at 7 TeV and will eventually go up to 14 TeV.
• “…in association with a W boson …”
This should sound a lot like the “associated production” diagram that we drew for the Higgs boson in a previous post:Reading from left to right, we have a quark and an antiquark producing a gauge boson (in this case the W boson) along with a Higgs. I’ll tell you right now: the CDF result does not appear to be the Higgs, but if there is a new particle responsible for it, it is produced from the same diagram (with the dashed line representing the new particle). [Correction, 6 Apr: a few people have correctly pointed out that the new state could also come from a “t-channel” diagram with an intermediate fermion. (I’ll leave the actual diagram as an exercise for those who have been reading my Feynman diagram posts).] By the way, the W decays before it reaches the detector. Two signals are W?e? or W???, which appear in the detector as a charged lepton and “missing energy” (the neutrino).
• “… Jet Pairs Produced
We also know that the Higgs decays into other stuff before it reaches our detectors. Thus we have to tell our detectors to keep an eye out not for the exotic particle, but for the ordinary stuff that it decays into. If it decays into quarks or gluons, then we know that we end up with jets. Thus this paper is looking for a W boson (which itself becomes a lepton + neutrino/missing energy) and a pair of jets. This is the criterion for picking out ‘interesting’ events relevant for this analysis.
• Invariant mass distribution of…
Now that we’ve picked out interesting events, we want to plot them in a way that can tell us if there’s a new particle hidden in the data. The easiest way to do this is to go “bump hunting,” which is what we discussed for the detection of the Z boson. The key idea is this: if we sum the energy and momentum of the two jets, we should get the mass of the intermediate particle that produced them (if they were produced by the same particle). This sum is called the invariant mass, and by plotting the number of interesting events based on the invariant mass, we can look for bumps that are characteristic of new particles.

Phew—that was quite a lot packed into a title. But now we’ve established most of the physics to understand the plot and see what people are [cautiously] excited about!

Results

Here’s the plot, once again:

On the horizontal axis is the invariant mass of the two jets, which is roughly the sum of the jet energies. The vertical axis is the number of events in the data set with the given invariant mass. If there were a particle which produced the two jets, then there should be a bump in the number of events with an invariant mass around the mass of the new particle.

So… what’s all of that colorful mess in the plot? It’s the thing that makes experiments hard: background. We have identified a particular experimental signal (2 jets, a lepton, and missing energy) which could come from a new particle. What we still have to account for are “boring” processes which could lead to the same signal. By “boring” I mean Standard Model processes that we already understand. Here’s the inventory from the plot above:

• The red contribution to the histogram are events where a pair of Ws or a WZ pair are produced. The second W (or the Z) then decays into a pair of jets.
• The big green contribution comes from various processes where a single W is produced and the two jets are separately produced independently of the W.
• The white (with a pink border) sliver are events where a top-anti-top pair are produced. These tops each immediately decay into b (or anti-b) and a W boson. One of these W bosons decays to lepton + neutrino and is tagged by the experiment, while the other one might decay into jets. Now we have four jets (2 from the b quarks, two from the decay of a W), but it is possible for two of those jets to get ‘lost’ because they don’t fulfill the detector criteria for identifying jets. (This is a notoriously subtle thing.) There is also a contribution from the production of a single top.
• The blue sliver is the production of a single Z boson with two jets. The Z decays into two leptons One of the leptons can be ‘hidden’ because the particular search only looks at particles which are fairly perpendicular to the beam direction, or misidentified as a jet.
• Finally, the shaded sliver is QCD background: these are gluon mediated processes that can give two jets and a lepton.

Once we take all of these background effects into account—and this is a very nontrivial thing to do—we can subtract these from the actual number of observed events per invariant mass bin. This “background subtracted data” is plotted below:

Now things look rather interesting. First note that not all of the background is subtracted: they leave in the WW and WZ background because these will also produce a characteristic bump (red line) because the 2 jets come from a single particle (indeed the bump is around the W and Z mass). The other backgrounds have broad, smooth profiles and can be reliably subtracted—bumps are harder to subtract so we keep them in. (Update: I’m told that this may also be partly included for comparison reasons: the W/Z bump is really well understood, so it helps to be able to use it as a measuring stick.)

What’s particularly neat, however, is that there seems to be a second bump with a peak right around 150 GeV. This is what is shown in the blue line. The significance of this bump is around 3.2 standard deviations, which roughly means that we can be 99.7% sure that this is not a statistical fluctuation.

It’s not the (standard) Higgs

If this bump really does come from a particle with mass around 150 GeV, then the first thing one might think is that this is the first hint of the Higgs boson. Indeed, we even showed above that the production of the Higgs includes diagrams that would give this particular signal when the Higgs decays into two quarks. However, one very, very interesting part of the analysis is that it does not seem like this bump could come from the standard Higgs boson!

The reason is simple: we understand the standard Higgs well enough to know that if it had a mass of 150 GeV, then we would expect an effect (a bump) that would be about three hundred times smaller. In the parlance of the field, the observed bump corresponds to a particle with a 4 picobarn dijet cross section, while a 150 GeV Higgs is expected to have a 12 femtobarn dijet cross section.

Further, CDF has already done a closely related analysis: WH ?l? b b-bar. This is basically the same analysis as the present paper, except that they are able to identify jets that come from b-quarks (this is called b-tagging). The analysis with b-tagged jets showed that there was no significant excess in the range 100 — 150 GeV.

What this means is that if this bump is indeed coming from a new particle, then it must not be a particle which decays into b-quarks, at least not very often. We know, however, that the standard Higgs does decay into b-quarks, so this hypothetical new particle could not be the usual Higgs.

This is actually much more interesting, since this could either suggest a non-standard Higgs sector or it could be a sign of completely different new physics.

I should note that there is one sentence whose significance is a little unclear to me:

We compare the fraction of events with at least one b-jet in the excess region (120-160 GeV) to that in the sideband regions (100-120 and 160-180 GeV) and find them to be compatible with each other.

Basically, they look at how many b-quark jets were in the bump versus those that weren’t in the bump, and they find that the number is roughly the same. This seems to imply that whatever is causing the bump is not decaying into b quarks, but I’m not an expert on this and might be misreading it.

Where to go from here

Don’t get too excited, though. Nobody is breaking out champagne bottles yet. Three standard deviation effects have been known to come and go—i.e. it is possible that it is just an unlucky statistical/systematic fluctuation. For example, it might be a mis-modeling of the background that had to be subtracted. (The three standard deviation significance assumes that one “knows how to estimate what one doesn’t know,” as one person explained it to me.) All the same, I expect that there will be plenty of model-building papers by eager theorists in the next few weeks. [By the way, CDF has known about this effect for some time; the current excitement comes from breaking the 3 standard deviation significance and their subsequent publication of the result.]

There are a few things to look out for:

1. More data! We measure data in “inverse femtobarns” (1/fb). The current paper is based on the analysis of 4.3/fb. My [outsider’s] understanding is that CDF should have around 10/fb by the end of the year, so the collaboration should be able to say something with more significance if this is a real effect.
2. What about D0? Fermilab’s other collaboration should be able to corroborate (or refute) this effect.
3. I do not believe that the LHC has enough data to say much about this at the moment, though I understand that we could be looking at 1/fb of data by summer time, and maybe a few inverse femtobarns through 2012. If the signal is real, there might be some hope to see the effect before the long shutdown at the end of 2012.

There are a lot of people who are cautiously optimistic about this. It’s almost certain that many theorists will jump on this to see if their favorite models can be tweaked to give a 150 GeV particle decaying to jets (but visible in the b-jet analysis), and that’s part of the fun. I look forward to seeing how things develop (and perhaps jumping in if the opportunity presents itself)!

Acknowledgements: I would like to thank my experimental colleagues, SP and DP for many helpful conversations. Any errors in this post are purely due to my own misunderstanding. I challenged some hep-ex grad students to foosball to try to squeeze info out of them before the paper was published… but I lost and they didn’t spill any beans. [They’re also strictly prohibited from such gossip… especially to theorists.]