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Aidan Randle-Conde | Université Libre de Bruxelles | Belgium

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Higgs update (CIPANP 2012)

Now that we’re in the conference season we’re treated to the latest results from the LHC and Tevatron. For now we focus on squeezing as much as we can from the 2011 data, so it’s a great time to look at the status of the Higgs searches. We’ll see some of the 2012 results at ICHEP in July (as summer abruptly turns into winter, with ICHEP being held in Australia.) Until then we must be content with what we can see with the data up to the end of 2011.

Both CMS and ATLAS are still searching for the Higgs boson, and that means that if it exists, it must exist in the difficult low mass region. This is something that Standard Model advocates have “known” all along, since the global fit to electroweak data all point to a Higgs mass around 95GeV. The further away the mass of the Higgs is from 95GeV the more we need to explain why it has the mass that it does. The diagram below shows the electroweak fit and the right hand axis shows how many sigmas away the point is from what we expect. (I explained about sigmas in a previous post. About one third of all results are more than \(1\sigma\) away from expectation. For 2, 3, 4 and 5\(sigma\) these numbers are about 5% , 0.25%, 1 in 15,000, and 1 in 1.7 million respectively.) As we can see, moving up to about 160GeV the probability for discovering the Higgs is already as low as a few percent.

The electroweak fit (arXiv:1107.0975v1 hep-ph)

The electroweak fit ( arXiv:1107.0975v1 hep-ph)

It gets very tricky to reconcile a very high mass Higgs boson with existing constraints, so a high mass Higgs suggests physics beyond the Standard Model. The high mass region is cleaner, it’s easier to study, and it’s more exciting if there is a discovery. By contrast the lower mass region is takes much longer to see any evidence, the final states are more complicated and take more time to analyze. If we discover the Higgs bosons and only the Higgs boson then all that happens is we confirm that the Standard Model is an accurate description of reality. It looks like nature is teasing us with a low mass scenario.

Taking a look into the low mass regime (less than about 150GeV) we can see why there is such a challenge. The dominant decays of the Higgs boson are \(b\bar{b}\) quarks, \(\tau^+\tau^-\) pairs, and other quark and gluon processes. There are rarer decays too, and the most important is the \(\gamma\gamma\) final state. The branching fractions are shown in the plot below. A branching fraction is the fraction of Higgs bosons which will decay into each final state:

The Standard Model Higgs boson branching fractions (arXiv:1101.0593v3 hep-ph)

The Standard Model Higgs boson branching fractions (arXiv:1101.0593v3 hep-ph)

The analyses from ATLAS and CMS are closing in on the Standard Model Higgs boson now. The limits are a few times the Standard Model, and once the yellow and green bands (“Brazil band plots”, as one speaker called them) pass below the line \(1\times\)Standard Model we can exclude the Higgs boson. If the Higgs boson exists then one point will stay far above the \(1\times\)Standard Model line, and that’s the location of the Higgs boson. If you want a primer on how to read these plots see my previous post on the topic.

There are three main ways to produce a Higgs boson:

  • • from gluon gluon fusion, which is the dominant process. In this case we get a Higgs boson, some jets from QCD and not much else. It’s a higher statistics sample, but there is nothing remarkable about the events.
  • • with associated production, which is about a factor of ten smaller. Higgs bosons love to couple of massive vector bosons, so whenever we have a massive vector boson there’s a small but significant chance we’ll also see a Higgs boson. We can use the massive vector boson to “tag” these extraordinary events, making the search with lower statistics, but cleaner.
  • • from vector boson fusion, a weird process that has a similar rate to associated production. In this mode the quarks from the protons exchange some massive bosons, which create a Higgs, and then the protons scatter off each other, leaving two jets at shallow angles. These events can be hard to reconstruct, but they are cool to look at.

The size of the background for \(b\bar{b}\) quarks is about 50 million times larger than the Higgs processes, so any analysis using a \(b\bar{b}\) final state must be very crafty. Generally we require that the Higgs is produced in association with a massive vector boson. When this happens the two bosons usually move back to back in the lab frame, so we can look for a high momentum Higgs boson. This makes things easier for the \(b\bar{b}\) final state because the two b-jets should be on the same side of the detector, and look like a “fat” jet. Even so, there are still large backgrounds from QCD processes. Since December 2011 physicists have been busy working to get as much discrimination between the Higgs and the background processes as possible, so its no surprise that we see more use of multivariate analyses in these searches. With a more dedicated study we can split up our searches based on the final states and tailor each final state accordingly. This “divide and conquer” method has lead to improved limits. The current exclusion for \(H\to b\bar{b}\) is already a few times the Standard Model:

Limits for Higgs decaying to b quarks (B LaForge, CIPANP2012)

ATLAS limits for Higgs decaying to b quarks (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to b quarks (C Palmer, CIPANP2012)

CMS limits for Higgs decaying to b quarks (C Palmer, CIPANP2012)

For the next dominant mode, the \(\tau^+\tau^-\) final state, we have a different set of challenges. \(tau\) leptons produce neutrinos, which carry away some of the momentum, making it harder for us to reconstruct the event. To make things worse, the \(\tau\) can decay to leptons or to hadrons, so we need to split up our analyses and treat each case separately. And if that wasn’t enough, we also have a large background from decays of the Z boson, which have exactly the same final state. Given all this it’s a wonder we can use this channel at all. Unfazed by the challenges, both ATLAS and CMS have shown great improvements in this channel:

ATLAS limits for Higgs decaying to tau leptons (B LaForge, CIPANP2012)

ATLAS limits for Higgs decaying to tau leptons (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to tau leptons (C Palmer, CIPANP2012)

CMS limits for Higgs decaying to tau leptons (C Palmer, CIPANP2012)

The next dominant processes are \(c\bar{c}\) and \(gg\), which are of no use to us at all. Backgrounds from QCD processes are just too high for these modes to be useful. So that leaves the \(\gamma\gamma\) final state, and this is the cleanest mode for the lower mass scenarios. To decay \(\gamma\gamma\) the Higgs boson must go through some intermediate particles in a loop. The challenges presented by the \(\gamma\gamma\) final states are mostly associated with the detectors. How do we know when we see a photon in the detector, and not a jet? What control samples can we use to calibrate our energy scale? These are tough questions to answer, and since the backgrounds for this channel are so high we need to have confidence in our abilities to recognize and reconstruct photons. (I’m actually a bit skeptical that we have seen hints of a Higgs based on these kinds of questions. Our most sensitive channel is the one with some of the biggest questions.) Even so, the limits are looking encouraging:

ATLAS limits for Higgs decaying to photons (B LaForge, CIPANP2012)

ATLAS limits for Higgs decaying to photons (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to photons (C Palmer, CIPANP2012)

CMS limits for Higgs decaying to photons (C Palmer, CIPANP2012)

I’ve skipped the massive vector boson final states (\(ZZ^*\) and \(WW^*\)), although these are sensitive to some of the range too. As we look to lower and lower mass ranges the contributions from these final states diminish rapidly, and the kinematic constraints get worse and worse. (At high mass the Higgs boson would produce real \(WW\) and \(ZZ\) pairs, giving us fantastically clean mass peaks. At lower masses one of the bosons must be virtual, and we lose one of our most useful constraints.)

Combining the results gives better exclusions. As we can see there is not much space left for the Higgs boson!

ATLAS limits for combined Higgs channels (B LaForge, CIPANP2012)

ATLAS limits for combined Higgs channels (B LaForge, CIPANP2012)

CMS limits for combined Higgs channels (C Palmer, CIPANP2012)

CMS limits for combined Higgs channels (C Palmer, CIPANP2012)

Most people’s money is on the region 124-126GeV. All we need to do now is collect the 2012 data and see if it shows the same bump. The waiting is the hardest part.

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14 Responses to “Higgs update (CIPANP 2012)”

  1. [...] Keep reading… Share this:TwitterFacebookLike this:LikeBe the first to like this post. [...]

  2. Lucas says:

    The time has almost come! What a journey it has been and how lucky we are that we have had the benefit of the world wide web to peer into such a highly detailed and technical convergence of humanity’s knowledge and skill. It’s enthralling! :)

  3. Tristan says:

    Hi! Why does the observed confidence limit for the 2-photon channel oscillate like that?

    • Hi Tristan, good question! These are due to statistical fluctuations. The limit oscillates because on average, half the data points will be above the expectation, and half will be below it. We don’t see it so much in the case for tau tau channel, because the spectra generally have large gradients, and this means that any disagreement between the data and simulation will tend to be in one direction. The monte carlo simulation and data agree very well, but non trivial background shapes can lead to small but noticeable biases.

  4. Richard Mitnick says:

    From whence cometh 95GeV? What happened to a range of 115-129GeV, with everything else ruled out?

    • Hey Richard, good question! 95GeV comes from fits to other Standard Model measurements, such as the mass of the W boson and top quark. These parameters are affected by the Higgs mass, so we can take them all and calculate the most probable Higgs mass based on them. When we do this we get 95GeV as the most probable value. As you point out, this has now been excluded, but we should not let the exclusion affect our interpretation of the electroweak fit. If the Higgs exists at 125GeV, then that mass disagrees with the electroweak fit to the order of 1.5sigma. With more precise measurements (as D0 and CDF are now presenting) we could see this disagreement increase, and if it does then we have hints of new physics! The electroweak fit also has probabilities that take the exclusion into account, but I feel uncomfortable using them because the information about the other measurements gets lost, and because the exclusions change so rapidly that the “latest” plot is usually already out of date.

      You can find the fits with and without the LEP+Tevatron exclusions in figure 4 of this paper: http://arxiv.org/pdf/1107.0975v1 . Note that this does not include the LHC exclusions, as they change quite rapidly. By the time we see that plot we might have a Higgs!

  5. John Carter says:

    Is there any physical relevance to the width of the signal excess? For example, in the combined Higgs channels the width of the signal in excess of 2 sigma is about 3 GeV. Does this width correlate to uncertainty in its energy and hence imply a decay half-life or is this purely a statistical manifestation? Thanks

    • Hi John, thanks for the question! The width corresponds to the observed width after all the detector effects (uncertainty in energy scale, geometry etc.) If you take a look at the Higgs width plot at http://hepuser.ucsd.edu/twiki/bin/view/Main/HiggsDecay you can see that the natural width of the Higgs is a few MeV at a mass of 125GeV, and the rest of the width is due to detector effects. Eventually we will need to measure the width of the Higgs very precisely to look for new physics/confirm the Standard Model, and at the moment I have no idea how that will be achieved! We will need some clever trick to get that measurement out.

    • Jochum van der Bij says:

      The width of the signal excess is at the present determined by the uncertainty from
      the measurement. However, there is the possibility, that the Higgs signal is wide,
      contrary to the standard model width, which is a few MeV, that cannot be resolved
      at the LHC. For the physics argument see my talk at Moriond.
      There is a range of parameters where the LHC cannot distinguish between the standard model
      and the HEIDI-Higgs models. Of course if the Higgs is wide enough (O(GeV)) the LHC could
      establish the HEIDI model, but not very precisely. Otherwise one needs another machine.

    • Hi Jochum, you make a good point! In fact I had a blog post lined up about what we need to do if the Higgs exists, paying special attention to the TLC and the width of the Higgs. I was holding off posting it, so that we don’t get too many posts at once, but I suppose now is a good time to share it. Thanks for your comment!

  6. dimi says:

    Hi dear researchers!
    I am afraid question will be considered as nonsense – but anyway I will ask.
    What is about ability of LHC that majority of people are looking for?
    They are not interested in calculations and safety, I am sure.
    They are interested to get an answer – is it possible to find a way to let an entity to go “backwards in time”?. I am understand that if even it happens – nobody will know about it. Physically and from point of National Defence. Thanks for your answer in advance, dear explorers.

    • Hi Dimi, thanks for your question! The short answer is that we will probably never be able to send a large object (like an apple) back in time. On the smallest scale particles can go forwards and backwards in time, as long as they obey the uncertainty principle. The product of the energy of the particle, E, and the amount of time, t, must satisfy: Et<1e-34. The mass of an electron is 1e-30kg, so it can travel back in time for less than 1e-4s if the conditions are just right. As you can see, this means that making heavier objects travel back in time is very unlikely. There have been some papers written about time travel and computing. It could be possible to send an answer back in time, which could speed up a computation. For example see http://en.wikipedia.org/wiki/Novikov_self-consistency_principle#Time_loop_logic . I don’t think that this kind of technology will ever be possible, but it’s good to see that people are looking at it.

  7. Jaylin says:

    It’s a relief to find someone who can epxailn things so well

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