The size of the proton matters in the field of the ultra-small and it seems that no one can agree on the correct value. The answer was long believed to be well-known, but the puzzle seems to be back to haunt the physics community. The proton seems to have suddenly shrunk in size.
How do we look?
The radius of the proton is found out by shooting high energy electrons at it and then finding how it forms a bound state. It’s very much like forming an atom, except that this atom is much smaller than the normal atoms which make up matter. Energetic electrons fired at protons often get bound to the proton, and form a hydrogen-like object. However, since the electron has a lot more energy than the ordinary hydrogen atom electron, it is attached much closer to the proton than the normal hydrogen electron. As a result, the proton can no longer be treated as a point particle, but its spatial extent become important.
So we can form a bound state and then measure the minute transition between energy levels and these now have an imprint of the proton magnetic moment and the proton radius. And thus, the proton radius can be determined. For a long time, physicists were safe in their determination of the proton radius and their value was 0.8768 femtometers (a femtometer is a millionth of a billionth of a meter, or a meter divided by 10^15). Case closed, right? Wrong…
A new experimental result threatens to blow this question of the radius wide open again. The muon is a close cousin of the electron. It has a negative charge and behave very much like the electron in a magnetic field, except that it is 200 times heavier than an electron. Recent experiments shoot these heavy electrons – or muons – at protons and these now form a bound state. The higher mass of the muon (by a factor of 200) means that at same energies, the muon is much closer to the proton (by a factor of 800 million). It can ‘see’ the proton much better and measure the radius to greater accuracy.
However, this has produced a shocking reduction in the accepted value – 0.84087 femtometers – a reduction of 4%. That is huge, well above the experimental uncertainties.
So, what’s going on?
Physicists are not very sure what’s going on. Why should the muon behave any differently from the electron? Is the muon, being closer to the proton experiencing some short range force, other than the usual long ranged electromagnetic and the short ranged weak force, that we just don’t know about? Is a new force of nature at work here? Is there new physics, something beyond the Standard Model of particle physics?
The muon measurements were made by a group of scientists at the Max Planck Institute of Quantum Optics, led by Randolf Pohl. Of course, the crudest explanation to all of this is that the experimentalists simply bungled and got the value wrong. No one’s ruling that explanation out right now, but other avenues are also being explored.
Muon scattering experiments like MuSE will only be ready in a few years, so this debate will continue for some time. When size does matter, we just don’t want it to change.
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.
In the world of high-powered lasers, a new kid is on the block. And it’s damn powerful! The Berkeley Lab Laser Accelerator (BELLA) has developed a laser which can deliver a huge 1 petawatt of power in a single pulse which lasts for 40 femtoseconds. It can do this once every second, making it a one-pulse-per-second laser.
Now, what do those terms with the funny units mean?
A petawatt is a million billion watts – a quadrillion watts, in everyday language. And a femtosecond is a quadrillionth of a second – one part of 1015 parts in a second! No other laser in the world has this high a peak power and still function at that high a pulse rate (1 pulse per second). This is now officially a world record – this is the world’s most powerful laser.
Building an accelerator
Enter the Laser and Optical Accelerator Systems Integrated Studies (LOASIS) program and there is where BELLA really scores. The really powerful laser is a prototype for the ultimate laser that will be built to accelerate particles for an accelerator. This accelerator will use the laser’s energy to speed up electrons and protons.
Conventional particle accelerators, the LHC included, uses electric fields to accelerate the charged particles, while magnetic fields bend them. The laser uses its intense heat to generate plasma. This plasma has a lot of free electrons swimming around and they can absorb energy and be accelerated to very high speeds. The trick is to make all of these electrons accelerate in phase.
Says Wim Leemans of Berkeley Labs Accelerator and Fusion Research Division (AFRD), the dept. responsible for the construction and maintenance of the LOASIS:
BELLA will be an exceptional tool for advancing the physics of laser and matter interactions. The laser’s peak power will give us access to new regimes, such as developing compact particle accelerators for high-energy physics, and tabletop free electron lasers for investigating materials and biological systems.
BELLA laser will be used to build the world’s first plasma accelerator, which will be able to accelerate electrons to 10 GeV. This is about a thousandths of what the LHC produced, but it’s still very high energy achieved on a device a millionth of the size of the LHC. This successful preliminary test of the BELLA laser was part of the LOASIS test which is due to start this autumn.
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.
The latest status of the Higgs Boson search at the LHC will be announced on the 4th of July. The conference will be held in Geneva. Incidentally, the International Conference on High Energy Physics (ICHEP), 2012, will also commence on the same date, but in faraway Melbourne.
The Higgs announcement
The announcement is expected to a big one – especially with the predicted discovery of the Higgs by the end of the year. The status of the Higgs will not be changed to ‘discovered’, but we will get to know how far we have actually reached.
We have already told you why you shouldn’t believe the rumors going around about the Higgs being discovered (LINK). It hasn’t been discovered as yet!
Just to sum up that post in the link, we predict that the CERN conference will announce that the Higgs bump in 8 TeV data matches with the bump in the 7 TeV data. Better make your way to that post!
CERN and murky money matters
CERN’s problem with making the official announcement from ICHEP in Melbourne is that Australia is not a ‘member state’ of CERN. Why make an official announcement in a country that doesn’t foot the bill for running of the LHC? So Geneva it is! And July 4th.
The CERN conference is scheduled for 0900 hours Geneva local time, which is GMT + 2 hours (i.e. it is two hours ahead of GMT) currently, due to daylight savings.
Do stay tuned to our coverage of the announcement.
Experiments may just be showing some chinks in the massive armour of the Standard Model of particle physics. The Stanford Linear Accelerator (SLAC) experiment BaBar has just produced data that contradicts the Standard Model, but the confidence level is still not high enough to warrant the tag of a discovery. So the Standard Model stays where it is, at least for the moment. Physicists are waiting for a confirmation of the same anomalous effect from the Belle experiment.
The Decay Deal
So this is what’s happening. A particle called the B-bar meson (i.e. a bound state of a quark and an anti-quark in which the antiquark is a anti-bottom or b-bar) decays into a D-meson (i.e. a meson containing a down quark) and a tau anti-neutrino and a tau lepton. The problem is not in the decay products but in the rate of decay. The Standard Model predicts very definite rates for particle decays, but the observed decay rate is higher than what the Standard Model predicts. This excess is what has got physicists interested.
The confidence level of the observation is about 3.4 sigma, which is not enough to claim a discovery, which requires 5 sigma. Confidence level is a quantitative measure of how solid a set of data is. The higher the confidence level, the lesser the chances that the result is a mere fluctuation.
There is work to be done, though. Seasoned particle physicists are wary. BaBar spokesperson, Michael Roney, professor at University of Victoria, Canada is visibly enthusiastic:
The excess over the Standard Model prediction is exciting
but then puts up a cautious front with:
But before we can claim an actual discovery, other experiments have to replicate it and rule out the possibility this isn’t just an unlikely statistical fluctuation.
And that’s exactly how these things work.
The LHC Angle to “New Physics”
Incidentally, this one is not coming from the LHC at CERN. LHC has been hard at work and the data it is churning out is killing off one ‘New Physics’ or Beyond Standard Model theory after the other, confirming the Standard Model even further. The greatest confirmation of the accuracy of the Standard Model will come from the Higgs discovery, which is where the irony lies. The irony is that, if the Higgs is discovered at 125-126 GeV range, and this is the likeliest scenario, physics beyond the Standard Model is a definite requirement.
As I mentioned, the wait now is for another experiment to repeat these results. Says Roney:
If they do, the combined significance could be compelling enough to suggest how we can finally move beyond the Standard Model.
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!
Shortage of funds has hit the scientific laboratories badly. This is quite evident from the attitude Fermilab has towards the deceased Tevatron. Fermilab is planning to recycle many parts of the once-biggest particle collider for other experiments. It’s to save cost, they clarify.
Of course, there is nothing wrong in that – in fact, this is a good practice. However, given the amount of history the Tevatron has, many people are frowning. The ex-spokesperson for the CDF detector at Tevatron, Rob Roser says:
Some parts are worth pennies, but in this budgetary climate, even pennies are worth saving
The Tevatron was the biggest beast in the particle physics world till the Large Hadron Collider (LHC) came onto the scene. It has fulfilled all of the expectations and has done more. It discovered the top quark, accurately measured the mass of the W and Z bosons and was instrumental in the Higgs search, especially in the low mass range. The Tevatron probed the Higgs decaying to two photon channel and, now it seems that this is the most promising channel.
However, now the collider parts are being utilized for some other experiments. Demands are being met for photomultiplier tubes (PMT’s). These are used to catch light as particles deposit energy while travelling through the detector material.
Looking into the Future
There are other things planned in the near future. Fermilab is all set for lepton colliders, which will collide particles like electrons and positrons, or muons-antimuons. The muons can change to electrons and this process will be studied in greater detail by the new lepton collider. This process should answer certain questions about the electroweak force and put strong bounds on the different constants in the electroweak theory, especially the magnetic moment of the muon. This is the so-called ‘g-2’ (g minus two) experiment. The muonic magnetic moment, supposed to be just 2, is actually a bit more. The difference between the electronic and muonic magnetic moments is due to the difference in masses. The electron to muon process should involve hadronic processes as well and this new experiment could yield very strong bounds on these hadronic processes. The hadronic processes from leptonic processes can indicate supersymmetry and, thus, can tell tales about Physics beyond the Standard Model.
There are also many long baseline Neutrino experiments planned. Fermilab’s own MINOS experiment has to be upgraded and the data made more precise.
Even in death, the Tevatron is fuelling research, this time donating parts of itself for future experiments. Scavenging may be a strong term to use for Fermilab and what it is doing to the Tevatron, but there is no doubt that the desecration of the giant will disappoint a few.