The publicized $9 billion papers on the Higgs Boson are out! Both the CMS and the ATLAS collaboration at the LHC, CERN have been working against the clock for the last two months to churn out the result that the world was looking forward to – finding the Higgs Boson. Having found the Higgs Boson and announcing it on the 4th of July at Geneva, the CMS and ATLAS collaborations have now released two papers, both reporting that they have improved upon their earlier presented results.
Stating the Obvious
The 4th July conference had already stated that both the CMS and the ATLAS detectors at LHC have found the Higgs Boson, the long sought after particle responsible for endowing all massive particles with mass. The search has been on since the LHC started running more than two years ago. The long time required just goes to show the magnitude of the search – finding the Higgs Boson wasn’t easy. But make no mistake – the Higgs Boson is definitely there!
Now, these two papers, one by CMS and the other by ATLAS, do something on expected lines – they bump up the significance of the result. This simply means that they make the result more concrete.
Improving the Results
To put in the numbers, the CMS collaboration had quoted a significance of 4.9 sigma or 99.99995% surety of the presence of the Higgs at a mass of 125.3 GeV. They have just bumped up to 5.0 sigma, which means that the surety is not 99.99997% but at a mass of 125.5 GeV. The error bars stay as they are. The decay channels of highest significance are the diphoton (or the gamma-gamma) channel, where the Higgs decaying to two photons, or the ZZ channel, where the Higgs boson decays into two Z-bosons.
The ATLAS collaboration publish a more adventurous result. They have bumped up their significance from the 5.0 sigma announced on 4th July, to the 5.9 sigma! That is a huge improvement, but this also raises a few questions about the analysis of data. How is it that the ATLAS collaboration can bump up their significance so very quickly?
Both collaborations have gracefully dedicated their papers to all those who were associated with the Higgs search, but have passed away and couldn’t see the remarkable results.
All of the questions – and there are many – will be answered in an expected conference in December, when the data collected the LHC in the next three months will be analysed and presented. The LHC is set to go into a period of hibernation after that for about 14 months and expected to resume in 2014.
If you’ve been following our blog for the last few days and been interested in the science posts and those on the Higgs, you’ll know that I was very skeptical about the discovery of the Higgs Boson by the time this presentation comes about. And boy, am I proved wrong! And I am elated about it.
Let’s be honest – it isn’t the Higgs
Okay, to be honest, the exactly correct statement would be this: There is a particle, hitherto unknown, having mass between 125 and 126.5 GeV (giga electronvolt), which has been discovered with 99.999997% certainty. We still don’t know whether this is the Higgs Boson or not – it could just be another particle.
It’s not the Standard Model Higgs
In fact, to make things interesting, this is really not the Standard Model Higgs. So people claiming that this ‘completes’ the Standard Model are, at least partially, incorrect. While the Standard Model predicts – and requires – the Higgs Boson, there is nothing that we know right now that says that this discovered particle is the Higgs.
Furthermore, the Standard Model predicts a Higgs with the mass of about 140 GeV or thereabout. What we have got is something with a mass in the ballpark of 125-126 GeV. Even if this is the Higgs, this is NOT the Standard Model Higgs.
What we really have!
So what have we really got on our plates? What we do know is that there is a particle whose mass is 125 GeV or so and how it decays. We also know how much it decays through the various decay channels – the so-called ‘branching ratios’. We are yet to know the charge of this particle and its parity. We do not know whether this is a fundamental particle (like the electron, with no ‘sub-parts’) or a composite one (i.e. made up of more elementary particles like quarks).
As Fabiola Gianotti, ATLAS spokesperson, said:
We are entering an era of Higgs measurement.
That is a lot of work left. We need to figure out what we are looking at really.
Rolf Heuer, CERN Director General, said that this is like looking at someone from far away and recognizing him/her immediately to be your best friend. But you’re not quite sure. You want that person to be closer to you so that you can make sure that it is indeed your best friend and not his/her twin.
Quite true. We need to take a better look. Translated into the language of high energy physics, that means ‘We need more data’. The saga will continue till the end of this year, then there will be a break and will continue again in 2014.
So, what can it be, if it’s not the Higgs Boson? It can be one of the supersymmetric partners of some already known particle. We don’t really know anything about the energy scale at which super-symmetry sets in or when supersymmetry breaks, but there is a fair possibility that the LHC might be detecting tantalizing hints in the next three months. When it comes back in full force in 2014, running at a higher energy of 14 TeV, compared to 8 TeV currently, we will definitely rule out or embrace supersymmetry at the 5 TeV energy scale.
Don’t worry, if you don’t get this – it’s futuristic talk. We want to talk more about today’s conference and that is what we will do!
Today’s conference: what they really said!
So this particle we are seeing today – let’s just call it Particle X till CERN says that it is indeed the Higgs boson – decays via different modes. A particle decays if it is heavy and there is no law or conservation principle preventing it from decaying. And it can decay via different end products – two Z-bosons, W-bosons, photons, four leptons etc – and all of the decay channels have some probability. One can be more probable than the other, and some channels can have more background noise than others. This happens if, say, a decay product can come from more than one source.
For example, bottom quarks can be produced from a lot of different sources, like all the so-called QCD processes. This masks the signal coming from the Higgs decay. The subtraction of background often leads to subtraction of the signal itself.
In order to cut through this mess, it is imperative that one identifies ‘clean channels’. Two such channels for this particle X are lepton channels and the di-photon channels. Particle X can decay into two muons or two electrons accompanied by the respective anti-neutrinos (don’t bother about those) through the lepton channel and, as the name suggests, into two photons in the di-photon channel. And lo, the signal is the strongest in these two channels. CMS and ATLAS (the two detectors at LHC searching for the Higgs) both have been extremely diligent and successful in looking for signals in these two channels. Both have scored grand success.
Look at the green squiggle dipping down right above the black continuous squiggle. That, the legend on the left reveals, is the signal from the di-photon channel. Look how strong that signal is! This immediately suggests that the particle is a boson (otherwise, if it were a fermion, it would violate fermion number conservation) and that the particle cannot be a spin-1 particle (as the photon is a spin-1 particle and we can either have spin-0 or spin-2 for the initial particle). The Standard Model Higgs is a spin-0 object.
What about the charged lepton channel? The two lepton channel is an indirect way to infer the presence of the ZZ or the WW channel. The neutral Z-boson or charged W-bosons are formed from the decay of the Higgs boson. These then decay into muons and electrons, which are then detected. It turns out that our particle X mimics the Higgs Boson quite closely.
What does CMS say about the different channels?
For the gamma-gamma channel (another name for the diphoton channel), the surety is about 4.1 sigma for the particle X having a mass of 125 GeV.What about Z-channel? It spits out 3.2-sigma confidence level for X having a mass near 125 GeV.
Add these two in quadrature (square each, then add and then take the square root of the sum) and you get 5.2-sigma!! This is winning, as the confidence level required for announcing a discovery is 5-sigma!
More data is expected to bump up the confidence level even further. Particle X could be as certain as 7-sigma by the end of December.
It’s not worth repeating the story for ATLAS as it is very similar.
Being cautious, still
CMS hit it just right when they cautiously put this up as the defining slide, saying “We have observed a new boson with a mass of 125.3 +/- 0.6 GeV at 4.9-sigma significance”.
The conservative 4.9-sigma, instead of a two-channel combined 5.2-sigma is typical amongst high energy physicists. This takes into account other channels and the so-called ‘look-elsewhere effect’. We need not get into that for our purposes here.
Point of disagreement – a potential for trouble?
Now let’s come to the discrepancy between the two collaborations – CMS says that the boson is at 125.3 +/- 0.6 GeV, while ATLAS says that it is at 126.5 GeV. The ATLAS collaboration hasn’t put in the error bars. So what are we supposed to make of this? What about the 1 GeV discrepancy?
We don’t know right now. Rolf Heuer made light of the incident:
We have the Higgs, but which one?
Rolf Heuer’s bag of quotes doesn’t end there, and so we would like to end with one of his gems – one signifying finality:
I said we will have a discovery this year. DONE!
Done, indeed. Congrats to everyone on the CERN team and the worldwide collaborations!
CERN is all set to announce the latest in the Higgs search from the LHC. The press conference will take place in Geneva on the 4th of July, 9 AM local time. This will update the world on the ongoing search for the Higgs Boson, unfortunately dubbed the ‘God Particle’. Results from the 2012 data analyses will be presented and the path forward will also be charted out. More data will be gathered by the time LHC shuts down in December for nearly one-and-half years and we will get to know about the final fate of the Higgs Boson by December.
Interestingly, this will come on the heels of a major high energy conference, being held in Melbourne, Australia, called International Conference of High Energy Physics, 2012 (ICHEP, 2012). Australia not being a ‘member state’ of CERN’s LHC confederacy doesn’t get the honour of hosting a major Higgs update from its own soil. More here: http://techie-buzz.com/science/higgs-boson-cern-conference.html
Inside sources say that the Higgs update will announce the fact that the Higgs is almost discovered but not quite. So to clear the air first up, we ask THE question: Has the Higgs been found? The answer: NO!
No, the Higgs hasn’t been discovered. The ‘excess’ or the odd bump seems to be concentrated consistently at one only energy – 125-126 GeV. This is great news, as the LHC has gone from restricting the mass ranges for the Higgs Boson, excluding different regions with different confidence levels, to precisely pin-pointing a specific mass! That is definite indication that there is indeed some particle at that energy and it could be the Higgs.
Talking about confidence levels, the Geneva press conference is probably going to announce the fact that the Higgs excess has been located with a confidence level of about 3 to 3.5 sigma. While this is significant and worth mentioning, there is no reason to call this a discovery. A discovery requires 5-sigma confidence level. We just don’t have that much data right now to confirm a 5-sigma confidence level. Read this for more: http://techie-buzz.com/science/higgs-boson-discovery-rumors.html
A LOT of Noise!
A final word: A lot of blogs are chattering over the ‘fact’ that the Higgs boson has been discovered. At the risk of sounding utterly repetitive, we venture out “No! The Higgs Boson has not been discovered”. That will require till the end of this year. Believe whom you will.
In a few days, the floodgates will open and you’ll hear about the Higgs Boson being already found. The Holy Grail of particle physics will have been found and only CERN will need to confirm it in their press release. When CERN will deliver the promised press release, they will inevitably say that the Higgs is still far from being discovered and that they have only see a ‘statistically significant fluctuation’ about some energy range. The whole non-high energy physics world will breathe out a collective sigh and, defeated, ask ‘How much longer?’
Higgs Not Discovered!
In order to spare at least our readers from being part of this international collective gasping team, I would like to mention this: The Higgs Boson’s status on its road to being discovered hasn’t changed since the December CERN update. It hasn’t been discovered as yet!
I predict that this is the line that CERN will adopt when it gives the Higgs Boson status update during the International Conference on High Energy Physics (ICHEP) that will be held in Melbourne from the 4th of July to the 11th of July.
The Last Six Months at the LHC
But then what has changed in the last six months? Has the LHC been doing nothing?
The LHC is now operating at a new energy scale. The LHC had been colliding beams at 7 TeV energy last year, and, beginning this year, it has been colliding beams at 8 TeV energy. The good news is that they still see the 125 GeV bump in the 8 TeV data they saw in the 7 TeV data, which has been attributed to the Higgs Boson. This means that the 125 GeV bump is not some random fluctuation, but an actual particle – probably the Higgs.
Why Is It Still Not A Discovery?
However, the data collected is not enough to guarantee a discovery, not even when integrated with the 7 TeV data. The 7 TeV data had yielded a confidence level of 1.9 sigma from the CMS detector and a confidence level of 2.3 sigma from the ATLAS detector. Both numbers are far from the 5 sigma confidence level needed to guarantee a discovery. However, the coincidence of the mass range for the fluctuation in the two detectors is heartening.
As I have explained here, ‘confidence level’ is a quantitative measure which tells physicist how unlikely it is that a certain signal is a mere fluctuation. So 3 sigma means that the chances that a signal is a fluke are less than 0.13%. High Energy physics demands very high rigour at 5-sigma confidence level – that’s the doubts reducing from 0.13% to less than .00007%.
What To Expect From ICHEP
The ICHEP announcement will say that the Higgs has been seen in the same energy range – 125 to 126 GeV mass range – and that the amount of data is not enough to say that it is really there. The 8 TeV data is far too small – giving at most a 1.5 sigma confidence level and no more. Integrated with the 7 TeV data, the confidence levels for both detectors might swell up to 2.5 to 3-sigma (taking into account the look-elsewhere effect), which, though significant, is still not a discovery. Sorry for the disappointment!
The good news is that this is exactly what is to be expected. The Higgs search is expected to end by the end of this year. That is when you will REALLY get to know whether the Higgs actually exists or not.
As for ICHEP and Higgs announcements by CERN, you can rely on us for the information. We will post them as they are announced. Not before!
The most promising signatures of something beyond what we know have been coming consistently from an experiment in LHC, CERN that has received the least public attention. While the CMS and ATLAS detectors (and collaborations) at the LHC are running their proton beams day and night in search of several things, primary among them being the Higgs Boson, the other big experiment, the LHCb, has been quietly chugging along with its own set of measurements.
The latest from the LHCb detector, housed in the same compound as the CMS and ATLAS, is a result that just might signal physics from Beyond the Standard Model (BSM), fashionably titled New Physics. BSM has been a devoutly investigated area of interest for both CMS and ATLAS, but the LHCb focusses on very specific types of particles and observes their modes of decay.
The types of particles LHCb is interested in contains a very exotic type of quark – the bottom quark. Protons and neutrons don’t contain that quark; they are entirely made up of ‘up’ and ‘down’ quarks. The Standard Model accurately predicts the decay rates and lifetimes of particles and, so far, experiments and theory have always matched. The recent LHCb result, adding to a few other ‘anomalous’ results of the past, show deviation from the theoretical values. Of course, no one is jumping into the BSM bandwagon just yet, but there is clearly excitement.
The LHCb collaboration found that a specific decay – a B-meson (i.e. a particle containing the bottom quark) becoming a kaon (another short-lived ‘exotic’ particle) along with a muon-antimuon pair. Muons are like heavy electrons. The LHCb collaboration observed that there is a difference in the decay rates between a neutral B-meson going to a neutral Kaon-muon-anti-muon and a positive B-meson going to a positive Kaon-muon-antimuon. This difference – called ‘isospin asymmetry’ – is not predicted by the Standard Model and this is what is interesting.
More data is required to confirm whether this is really a BSM signal.
A full analysis of the Tevatron data collected over ten years – a mind boggling 500 trillion proton-antiproton collision events – yields a narrower range for the Higgs mass. A new statistical fluctuation, seen with a confidence level of 2.2 sigma, narrows the range of the particle causing the fluctuation to between 115 GeV to 135 GeV. A GeV is a Giga electron Volts or a billion electron volts. These are pretty strong bounds on the mass. Furthermore, the entire regions between 147 to 179 GeV can be safely eliminated. This analysis confirms what the LHC data says – the Higgs is a low mass Higgs with a mass of about 125-126 GeV and the mass range above 141 GeV is eliminated with 95% confidence.
Excluded ranges and the range to look out for
The data, collected from CDF and DZero detectors of the now-deceased Tevatron, combines well with the LHC data, specifically with that supplied by the ATLAS detector, to restrict the Higgs mass between 115 GeV and 129 GeV. This also provides more confidence to the 3.6 sigma peak announced during the 13th December 2011 CERN broadcast. Kindly check the link here for very specific details of the seminar: http://techie-buzz.com/science/higgs-boson-cern-seminar-results.html
However, this result shows that the LHC and the Tevatron results match and that’s great, but it doesn’t get us any closer to actually finding the Higgs. Of course, if the Tevatron had disagreed, then we would’ve been in serious trouble.
Two things come out of this confirmation: The Higgs is most probably a low mass Higgs, having a mass of about 125-126 GeV. This is pretty interesting in itself, since this is not just the boring Standard Model Higgs, but gives an inkling of the success of supersymmetric theories. Secondly, the “look elsewhere” effect may not be as significant as was previously thought, now that the bounds are tighter. The “look elsewhere effect” takes into account the probability of finding the Higgs at every point within a certain range and not just at a very small interval. This considerably reduces the significance of the observed bump in general. Since the “look elsewhere effect” may decrease its contribution, concentrating on local significances may be quite the right thing to do!
Of course, the game will only be decided by the LHC. We expect to have enough data to pinpoint the Higgs by the end of this year, before the LHC goes into hibernation for 15 months. The game is heating up and getting interesting. Stay tuned…
The LHC is taking a vacation right now, but it promise to return with a bang! The LHC is due to run very soon, but instead of the usual 7 TeV (1TeV = 1 Trillion electron volts) total energy, it will try and go a bit higher and reach 8 TeV. Also the luminosity (basically number of collisions per second) will increase, but the increase won’t be substantial and there are reasons for that. Physicists promise enough data to pinpoint the Higgs and to verify the tantalizing 125 GeV peak that was reported earlier(here). Furthermore, after a packed 2012 schedule, the LHC will hibernate for a longer time and will wake up in 2014. During this time, the LHC will be fitted with newer instruments.
The hardware upgrade will have to wait till end of 2012, when the LHC will shut down for an extended period of 14 months, waking up again in 2014. The hardware upgrade will allow the LHC to run at a huge energy of 14 TeV and much higher luminosity. This is crucial, since it is not only the energy, but the number of collisions that makes a lot of difference in the experimental data. More luminosity means lower uncertainty in the measured values. The current electronics won’t be able to handle the rate of data acquisition that the LHC is planning to achieve.
The LHC currently runs at 3.5 TeV per beam, giving 7 TeV on a two-beam collision. They plan to upgrade it to 4 TeV per beam, giving a total energy of 8 TeV. Each beam of protons is made up of bunches of protons, with each bunch being separated by a certain amount of time. Each bunch has a certain number of protons. The team will also look to increase the number of protons per bunch, but keep the number of bunches constant, thereby increasing the luminosity. The current bunch spacing is 50 nanoseconds. The LHC electronics is built so as to handle bunches separated by 25 ns. The LHC team might look at this small deadtime when it resumes in 2014.
All in all, the full blown search for Higgs might end soon, but the LHC is poised for more daring adventures!
The Higgs search gets hotter and hotter. Recent analysis of old data have raised the confidence level of the Higgs detection from the older value of 3.8-sigma overall to a much healthier 4.3-sigma, as indicated by the two papers sent for publication, one by CMS and the other by ATLAS. The Compact Muon Solenoid (CMS) detector group had given the confidence level of 2.5-sigma. Now, with the analysis of more data, they have pushed it up to 3.1-sigma. Remember that a 5-sigma confidence level is what you need for tagging something as a discovery – so 4.3-sigma, though exciting, is not momentous.
There – but not quite there
The results overwhelmingly predict a Higgs mass in the range of 124-126 GeV, which is exactly what scientists had reported on December 13th.
A 5-sigma means that one is 99.99997% sure, while a 4.3-sigma result means that scientists are 99.996% sure that the identified peak is the Higgs peak.
This is just an improvement over the ‘initial’ December announcements by CERN. The data is not new, since the LHC hasn’t been taking any since November, but a more thorough analysis has been done and this is what it says. I suspect that this is as far as CERN can go at the moment with the Higgs confidence levels, and they will require much more data to be completely sure.
The 3.8-sigma confidence levels shouldn’t be taken too seriously. There have been peaks of this confidence level, but they had vanished. Fortunately, this hasn’t.
A new chapter
We should have to wait another year or so before the LHC can give something definite on the Higgs search. Now that the LHC is temporarily closed down for mandatory maintenance efforts, the big bosses, meeting at Charminox, France, are discussing the energy and the luminosity it will be tuned to when it opens later this year. The scale up to 8 TeV in energy is expected, but the luminosity is not yet revealed.
The Higgs search is not yet over and is all set to go on at LHC, CERN. This is the natural consequence of the CERN official seminar.
The Higgs has been definitely observed at the energy 126 GeV at a 3.6 2.3-sigma confidence level at ATLAS, combining all decay channels!
The data presented at ATLAS, by ATLAS boss Fabiola Gianotti, is more-or-less in line with Standard Model expectations.
Result from ATLAS:
The Higgs officially lies between 114 GeV to 141 GeV. The rest of the mass range has been eliminated with 95% confidence level.Several channels like the Higgs-> WW* has been excluded.
The mass range between 113 t0 115.5 GeV has been excluded, as has been the range from 131-453 GeV, with the exception of a window from 237-251 GeV at 95% confidence.
The Higgs-> gamma-gamma is a very promising channel and this suggests the 126 GeV figure for the mass of the Higgs.This suggests the presence of a ‘low-mass’ Higgs, which is quite in line with the Standard Model. More data in 2012 will help CERN make a more definitive statement.
Bottom Line: Local Significance – 3.6-sigma; Global Significance – 2.3-sigma at 126 GeV
Result from CMS:
The CMS results ruled out a high mass Higgs, much like the ATLAS results. 270-440 GeV was excluded and the Higgs->gamma-gamma channel gave very clear results. This low mass Higgs is very consistent with the previously announced ATLAS results, which is extremely good news. There were excess events noticed between 110-130 GeV, in the tau-tau and bottom-bottom decay channels; this eliminates 134-158 GeV mass range.
A curious 4-lepton excess was noticed at 125 GeV, which is bang on target, if you take the ATLAS results (above) at face-value. This is again, very good news. The Higgs-> WW and Higgs-> ZZ excludes 129-270 GeV mass range. Multiple channel “modest excess” was noticed just below 129 GeV!
Bottom Line: Local Significance – 2.6-sigma; Global Significance – 1.9-sigma at 124 GeV
The global results take into account the so-called ‘look elsewhere’ effect, which means that it factors in the chances of observing this same local excess at any point within a certain range and also in all channels.
The CERN announcement
CERN announced today that the Higgs has been observed’, but not detected’. The subtle difference between these two words lies in mathematics. When CERN says that they have observed the Higgs, it means that they are 99.73% sure that the Higgs is there. This is, however, not enough to guarantee the tag of a discovery. For that, the confidence level has to go up to 5-sigma, which gives a 99.99994% surety. This is very important, since 3-sigma effects have been known to go away in the past.
The non-discovery of the Higgs, as yet
The only reasonable explanation for the less-than-discovery tag at the moment is because LHC still doesn’t have enough data or rather, not enough data has been crunched.
This is surely great news for the particle physics community. The Higgs may be there in this and there are strong indications from both ATLAS and CMS that it is there and this means that the Standard Model has passed its stringent test yet! However, the mass is still to be ascertained exactly. The error bars haven’t been fully established.
So, the wait continues.
The Super Symmetric Models
This mass of the Higgs Boson, if actually true, is extremely exciting. It lends credibility to the cMSSM models, which is one of the basic Super Symmetric Models. There were widespread news reports that LHC has ruled out super-symmetric models or at least the simplest ones. Not quite! The cMSSM can accommodate a Higgs of 121 GeV mass and no higher. However, a small tweaking of the parameters yield a different version of the theory, which can very well accommodate a 125 GeV Higgs.
Another revolution may be just around the corner! Watch out!
The Higgs seems to be playing the game better than ever, peeking out once in a while, but not for long enough time! Initial analysis of the data from both ATLAS and CMS indicate an excess in the gamma-gamma channel for the Higgs at about 125-126 GeV. ATLAS detects this at a high 3.5 sigma confidence level and CMS comes in at a more tentative 2 sigma confidence level. This is good enough to tag it as a proper observation. None is good enough to warrant the tag of a discoveryof the Higgs, which requires 5 sigma confidence levels.
Both ATLAS and CMS observations are in the gamma-gamma or diphoton channel.
We had told you about the Higgs search coming to an end here and also that there is a joint seminar in CERN on the 13th of this month. The announcement at this seminar is expected to be the definitive on the Higgs search. It will make or break the Higgs, and, thus, a part of the Standard Model as well.
125 GeV Higgs more interesting than a 140 GeV one!
In a way, a 125-GeV Higgs would be awesome news for physicists, since this mass would require corrections to the Standard Model, since the vacuum becomes unstable at high energies. A 140 GeV Higgs would’ve been more mundane, and would’ve been the simplest of all the scenarios.
The situation is a bit ironic really. The Tevatron had almost eliminated the Higgs gamma-gamma channel decay process. Many scientists were convinced that the gamma-gamma channel was no good, and if the Higgs is found, it will be via the ZZ or WW channels. Though this is not incorrect, the gamma-gamma channel has given one of the strongest signals of the Higgs till date and right before the big announcement at the seminar.
It is unlikely that CERN will say anything about this before the 13th December announcement. Fingers crossed!