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CERN | Geneva | Switzerland

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Where do we stand on the Higgs boson search?

One of the most sought after particles in our field, the Higgs boson, is playing hard to catch. It might be that it does not even exist. All we physicists know is that something new is required by the theory. It might be the Higgs boson: that’d be the simplest solution, or we need to exclude its existence and move on to explore the next set of possibilities.

We have a theory called the Standard Model of particle physics that has withstood decades of experimental scrutiny without showing any cracks. The Standard Model tells us how many events containing a Higgs boson we should see if it exists but it does not predict its exact mass, making it much harder to find.

Each event is just a snapshot of what happens when protons moving at near the speed of light collide in the Large Hadron Collider (LHC). The Standard Model also tells us how many other types of events collected in our detectors could mimic the signature of a Higgs boson decay. These events are called the background. We design our searches specifically to select the desired signal events while minimizing the background. In the end, we work out how many signal events from Higgs boson decays and how many events from other processes we should retain given a specific set of selection criteria. Then we compare these estimates with what is collected with our detectors to see if Higgs bosons were present or not.

Imagine that all selected events were like the contents of a small lake. If a hidden fish creates a disturbance underneath, we will see a wave on a calm water surface. But of course, if there is some wind, ripples would appear, making it harder to spot the wave caused by a fish. The presence of a Higgs boson would do just that: appear like a wave on top of the calm water. As with the wind, the background creates small ripples one could easily mistake for a signal. The background can also fluctuate following statistical laws, like a random wind. In our case, having more data is equivalent to having more fish in the same spot, making their presence easier to detect.

To see if the Higgs boson or any new particle exists, we need to collect as many events as possible. Until then, it is pure guesswork since statistical fluctuations can easily fool us.

This is what happened this summer, when the first Higgs results were presented in July. We only had about one inverse femtobarn of data available (those are just the units we use to measure the data sample size). Some tantalizing ripples appeared as if we were seeing something. A month later, the CMS and ATLAS experiments each had two inverse femtobarns of data analyzed. The initial hint had completely disappeared, making statistical fluctuations once more the bane of a particle physicist’s existence. In the calm lake analogy, the first ripples we had seen were not caused by a real source like a hidden fish but simply by small variations at the water surface.

Now after much effort, the first combination of these August results was made public last Friday.  This is equivalent to one experiment having four inverse femtobarn of data, four times more than in July. This time, a wide mass range is excluded, namely between 141 and 476 GeV at the 95% confidence level. This means there is less and less space where the Higgs might still be hiding. In fact, it is now limited to be between 114 and 141 GeV.

This low mass range is where it was most expected, based on various theoretical hints and experimental factors. But this is also the range where it is most difficult to see, meaning more data is needed to see a real wave above all the small ripples.

This year, the ATLAS and CMS experiments each collected five inverse femtobarns of data. People are now bending over backwards to analyze these data and present new results at the scheduled meeting of the CERN Council planned at CERN in mid-December . Let’s hope both teams will manage and that some interesting signal will emerge. Combining this data will take a few more months and is expected in March.

What’s for sure, if the LHC, and the CMS and ATLAS experiments continue to perform as they did this year, we will have a final answer on finding the Higgs boson or excluding it definitively by the end of next year.

Let’s keep our fingers crossed…

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

The combined Higgs boson search result for the ATLAS and CMS experiments using 2 inverse femtobarns of data. The dotted line represents what one would expect with this much data based on theoretical predictions and statistical laws. The green and yellow bands indicate the error margin around this prediction, respectively with 68% and 95% chances of being correct, if all sources of experimental and theoretical errors have properly been accounted for. The black points are the experimental results. The horizontal axis gives the possible Higgs mass value on a logarithmic scale. Every time the data (black line) falls below the horizontal red line means we exclude a Higgs boson with this possible mass. The region above 476 GeV is still allowed but disfavored by theoretical considerations, meaning all eyes and efforts are now on the low mass region. In fact, below 141 GeV, what we observe experimentally is slightly more events than expected, that is, the black line goes above the yellow band. The bigger the deviation, the more likely we are to find the Higgs boson in that area.

 

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25 Responses to “Where do we stand on the Higgs boson search?”

  1. [...] “Where do we stand on the Higgs boson search?“, Quantum Diaries, Nov. 23, 2011 [...]

  2. Filip says:

    Higgs will be found at 137 GeV ;)

  3. Emanuel Hoogeveen says:

    One thing about the plot above confuses me: at ~250GeV the experimental results seem to more-or-less match the cross-section that the SM predicts for a Higgs of this mass (if I’m reading the plot right). In contrast, in the 114-141 range, the cross-section is higher than would be expected. So why is ~250GeV excluded, but 114-141GeV not?

    • Dragos says:

      Hi, they have drawn a dual plot there, in the sense that:
      1) the black line should be above the red line, and
      2) “The bigger the deviation, the more likely we are to find the Higgs boson in that area.”, so it’s the opposite way around.

    • I think it is just an artifact of reading the graph which is in log-log.

    • Moreover, dots have certainly been dropped out the plot for it to be more readable

    • anna v says:

      Emanuel Hoogeveen
      The answer is that if you look at the plot carefully you see that it is not a mass plot. It is called a “brazil band plot”. The y axis is a ratio of sigma/sigma-standard-model at a given assumed higgs mass in the x axis. If we only had the standard model and a high statistics experiment y=1 except where the standard model is exceeded. All the data enter in every x value. So the real information is about exclusion of areas where there is not enough statistics to exceed by two sigma the standard model expectation. 250 data crossection/SM is below 1 so it is excluded in the 95% limit.

    • Pauline Gagnon says:

      Hello Emanuel,

      as Anna explained, the vertical axis gives the ratio between the number of events observed and the number of events predicted by the standard model. It is expressed in term of “cross-section”, which is just how likely it is to produce a Higgs boson in a proton-proton collision. So if the number of events we observed falls below the horizontal line at 1, this means the number of events we found is less than what was predicted by the Standard Model. Therefore, at this specific mass point, like 250 GeV in your example, we see less events than what was expected. Hence, a Higgs boson with a mass of 250 GeV is excluded.

      Any mass point where the black curve is still above this horizontal line at 1 has not been excluded. So if the Higgs exists, it can only be between 114-141 GeV (below 114 was excluded by the LEP experiments at CERN by 2000 or so) or above 476 GeV.

      I hope this helps. It’s a complex plot, no doubt about that!

      Pauline

    • Niko says:

      Thanks for explaining.
      allthough I’m not a math or physics person, I can follow the agumenents posted here quit well although I don’t understand the them fully. I believe it’s one of the most exciting things to follow at the moment. I hope you will find it (or not) because both are exciting!!

      I like to ask some questions which should be a piece of cake for you all:

      Could the mass effect not be a derived effect of a combination of 2 or 3 quarks of which one is unknown. So that only the combination gives the mass effect?

      Does this mass has anything to do with gravitational force. I Know there is interaction, but does mass generate the gravitational force?

  4. karen serbert says:

    If this were quantum physics,,, surely this higgs particle will only appear if it wants to…. I’m not a scientist, nor have any understanding of such things…. Well, you know what they say about little fish… entice them to the surface with the food they like….. :)

    • Pauline Gagnon says:

      Hello Karen,

      I don’t think the Higgs boson has a will of its own… but if it exists, it has a mass. We need to see an excess of events at that specific mass. Indeed, if we know what it “likes”, it’d be easier to find it!

      Pauline

  5. Mike says:

    Analysts should say all: “They can not observe a Higgs within this energy region by 5% confidence level given by the data taken so far
    and the given theory”. To rule it out by 95% is a hard statement just
    after the first ramp up of a new collider and new detectors. I learned
    that some 100 inverse femtobarn are needed to be really convinced in
    every reaction and possible channel – say in 3-5 years from now. The Higgs “has to hide” like a ripple on the wavy water as it is its nature and very hard to see…
    I guess you have to wait a bit more and not to hurry in profound
    statements concerning rules of nature too early, to go back to the working places and learn to understand the “ripples” where it hides.

    • Pauline Gagnon says:

      Hello Mike,

      you are right: understanding the “ripples” like in my analogy is a crucial point. This is where we spend most of our time, making sure we understand the background in the smallest details. True, we have new detectors but since we have already been able to reproduce know phenomena, we know how well they work and what kind of precision we can expect. All this is taken into account.

      One way we do that is by looking at very well known particles, like Z bosons or the J/Psi particle. We can reproduce their know mass with great accuracy. This tells us we already know our detectors much better than we even expected. One reason is that due to delays with the start of the LHC, we had plenty of time to collect data coming from cosmic rays, which allowed us to do very good calibration of our detectors and lots of debugging, before we had the first collision data. This had never been the case for other experiments in the past. But we know our detectors much better than we anticipated. Things are working remarkably well.

      We should know much more about the Higgs by this time next year.

      Pauline

  6. Frank says:

    You state: “The Higgs boson: that’d be the simplest solution …”

    Actually, if you read Sidney Coleman’s old book, the simplest solution would be electroweak symmetry breaking to discrete subgroups of the. EW gauge group, We would need one subgroup for each lepton family and one for each quark family. Consequently, the continuous EW gauge group is an approximation to Nature’s real choices, sort of acting as a ‘covering group’. One would then predict a 4th quark family but only 3 lepton families.

  7. Tanya Small says:

    Go back to the old website design, blogs-lhc. This stinks.

  8. Dan Visser says:

    My new dark energy force formula (the-latest-specific-version) and a new time (‘t’)in a new cosmological hypothesis, the Double Torus, theoretically prove why and how neutrinos could go faster than light in vacuum. I published a ‘paper’ posted in the viXra-archive about this issue. It discloses 62.8 nanoseconds time-gain in flight-path of neutrinos (compared to the lightspeed in vacuum) from CERN to Gran Sasso within the experimental statistical and systhematic limits published in the CERN-paper(see my paper-reference [1]). But more remarkable is that the ‘neutrino-time-gain’ is independent of any trajectory ! It is dependent of two neutrinos being in the same ‘duo-state of energy’ (like superconductivy in electromagnetism). This ‘duo-state of energy’ enables them to pass the boundaries of Relativity. My theoretical research discloses the discovery that Einstein’s Relativity (Gravity- and Big Bang-framework) is being part of a Double Torus Cosmology, instead of a Big Bang cosmology.
    Kind Regards, Dan Visser, Almere, the Netherlands.
    [1]: http://vixra.org/abs/1110.0030
    All rights on this comment are strictly reserved to Dan Visser 2011. For more information: Find my website and or contact me by phone or email.

  9. dequantization says:

    since few days my comments do not get posted?

    • Quantum Diaries says:

      Sorry about that dequantization — Your comment had inadvertently been flagged as spam. It should be visible now.

  10. dequantization says:

    hello all:

    Consider this: would it not be possible that if some objects like the analogies given by other commentors to murky H2O or ripple and wavy effect preventing special fish from being observed, or caught in a timely manner, that if we try to remove this murkiness or rippling, we could be on the superhighway to detecting the higgs if it exists.

    According to above commentors, possibly the extremely ultra energy density collisions are what actually prevent higgs from being created / deployed at those nano or femtosecs of existence. That in fact, our detection devices fail to observe these stream higgs because of their ultra-short lifespan; smaller than a 1 X 10-33sec (don’t ask how I got that!) which is certainly undetectable by any means within the super LHC detector experiments or any other device that could detect lifespans in those non-earthly devices.

    So, the problem of finding the higgs is indeed a two-fold problem:

    - 1st fold is due to 1 X 10-33sec lifetime span undetectable by any device available to date, and our technologies far lag behind for such situations.

    - 2nd fold is the activation state, creation state, appearance state transformation_to_existence state… are not available under our ultra-high electromagnetic, magnetic, or electric fields. At our 7TeV collision energy density, instead of creating the higgs boson, the possibility of any states like fragmention, defragmentation, fluid-plasma-surge, transformation… is highly likely to surmount the higgs creation, generation, or deployment. So, we destroy without knowing trying to think the BIG BANG. That is, our experiments in fact kill it, and some other superthinking has to be deployed to detect within current CERN or any other lab means.

    thank you for thinking with me a little further and deeper.
    fluidic

    • Marcel van Velzen says:

      Hello dequantization,

      You are right, the Higgs but also the W and Z have extremely short lifetimes and can only be detected indirectly by their decay products. But the same is true for a ball. If you look at a moving ball, in some kind of sport, you only see the reflection of light by the ball not the ball itself. Nevertheless the concept of a ball is useful and also agrees with other ways of detecting the ball, for example by holding it. In the same way physics is a complicated set of constructs that can only really be understood by studying physics where you will get convinced that these concepts make sense (and also will understand their limitations).
      The Higgs and W and Z fields are concepts in a mathematical description of nature. These fields can have excitations that can be considered as particles. These particles decay and the way these particles decay can be calculated and compared with experiment. For the W and Z this turns out to be correct for the Higgs we will see. However, because these bosons have such short lifetimes you are right that they are extremely abstract objects but then so is a ball :-)

  11. Ruben Rydberg says:

    If the gravitational field not can be unified with the other
    fundamental “forces”, then perhaps you have to look at the
    Higgs-boson in an other way.For example the Higgs could be made of a mixture of the other fundamental field particles.
    Closed together in some form of a cluster with a specifik
    interaction with the other fields and particles around.

  12. GG says:

    It is more than obvious that SM Higgs will be discovered… I suspect it was already found at LEP but.. politics :)

  13. Careina Yard says:

    To label this new found particle as ‘God’s Particle’ seems rather extreme, however significant it may be in the understanding of quantum physics. As a laymen, I am trying to understand how the determination of this particles mass would affect what we know of atoms? What will it reveal? This field of science is extremely interesting to me. If someone could explain the significance of determining if this particle gives all other particles their mass, it would be greatly appreciated.

    Furthermore, what ever happened to the study regarding the newly found particles ‘neutrinos’ which have been found to travel faster then the speed of light?

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