There is palpable tension at CERN for sure people might not sleep till Tuesday, the 13th. The premier particle physics laboratory is all set to hold a special seminar that day, possibly to announce latest in the search for the Higgs boson. The search is nearing an end and this is expected to be a landmark announcement, possibly one that can either confirm or rule out the Higgs boson.
The 13th December joint announcement might be the last one on the Higgs. We are hoping for either a confirmation or nullification.
We had covered a seminar given jointly by the ATLAS and CMS collaborations last month here and we had told you that the search was nearing an end.
The latest news yet!
The mass ranges for finding the Higgs has been narrowed down to just 30 GeV between 114 to 144 GeV, as per the simplest version of the Standard Model. The finding of the Higgs boson is a crucial step, as it would be the final confirmation of the tremendous success of the Standard Model.
The Standard Model
The Standard Model of particle physics is a framework based on very fundamental principles of physics. It describes the interaction between different particles and the three forces electromagnetism, weak and strong. Although there are a number of freely adjustable parameters, the Standard Model has been the most successful theory of physics ever. The progress of physics in this direction has been extremely rapid, especially in the 1950’s to the 1990’s. Each of the particles predicted by the Standard Model has been detected, giving both theorists and experimentalists enormous confidence that this is the correct model. Theory has always been immediately confirmed by experiments, and they have invariably confirmed the Standard Model predictions! All that remains is finding the Higgs and this one last piece is playing hard-to-get.
Many physicists are tensed about the prospect of there being no Higgs! Many others, among them notably Nobel Prize winner Steven Weinberg, feels that it would be exciting if the Standard Model Higgs is not found! There are many alternative models and these would then gain center stage.
All in all, it is safe to say that this one announcement may be the fork in the path for physics going forward. Higgs or no Higgs – that is the question. The answer comes on the 13th!
The Higgs Boson may have finally have been caught or, may be, not! Only a clutch of scientists with direct access to latest LHC data knows whether the Higgs has been found or not! Whatever the result be, one thing is for sure the Higgs hunt is nearly over. CERN researchers have restricted the Higgs mass to a window of only 30 GeV, taking into results from the Large Electron Positron (LEP) collider, the Tevatron and, of course, from the LHC itself.
The Higgs, if present in Nature, has got extremely little energy space to hide in. At a conference in Paris, held today (18th November), ATLAS and CMS researchers got together and erased out a HUGE range for the possible mass of the Higgs. A large swathe from 141 to 476 GeV was wiped out in one fell swoop. Says Guido Tonelli, the spokesman for CMS
We’ll know the outcome within weeks.
This is surely going to increase the pulse rate of any particle physicist in the world.
What happens if the Higgs is not found? A lot of problems for the Standard Model. The Higgs boson is the simplest way to generate masses for fermions (like electrons and protons) and bosons (like W and Z bosons). There are other possibilities, but this one Higgs model is the simplest and most beautiful of all the possible models. However, as Feynman would say, if theory disagrees with experiment, then it’s wrong and it doesn’t matter how beautiful the theory might be.
For long, has the Higgs mass been pinned at about 140 GeV. There is still a strong possibility that the Higgs, if found, will be of this mass. We may be on the brink of history.
The tantalizing possibility of new physics may just be around the corner. The LHCb preliminary results surely hint towards that possibility with the first ever detection of CP violation in the charm quark sector. We reported this big news here and in this editorial piece, we intend to elaborate on what the results mean or might imply in layman’s terms.
We will follow the following sequential treatment of the entire subject:
What is CP symmetry and what does its violation mean?
What is baryon asymmetry and what does CP violation have anything to do with this?
What are the generations of quarks?
What decay process are we looking at?
What about the Standard Model? What does this predict?
What are the experimental results and how might we interpret them?
If you think you know any of the sections, you might skip it. Let’s begin our journey.
1. CP Violation
There are certain symmetries that exist in Nature. Many of the symmetries are continuous symmetries, like the rotational symmetry for a sphere. No matter how small an angle of rotation you give to the sphere, it will still look the same. This is not true for an equilateral triangle, whose rotation angle has to be 600 in order for it to look the same. The first one is a continuous symmetry and the latter a discrete one.
Having known what symmetry means, we can look for symmetries in a quantity called the Lagrangian. A Lagrangian reflects all the possible dynamics of a system, (which are manifested through its derivatives). Symmetries of the Lagrangian can be both continuous and discrete. In the Lagrangian for the electromagnetic field, apart from a lot of continuous symmetries, there is also the symmetry of charges. Namely, if you replace all charges with their opposite (i.e. positive charges with negative and vice-versa), the Lagrangian will still be the same. CP (Charge-Parity) symmetry means that whatever operation you perform, if you replace the particles with the anti-particles (charge’) and then switched their positions or reflect them (parity), then no experiment will be able to tell the difference.
CP violation refers to the breaking of this symmetry. Some experiments can differentiate between the above mentioned configurations and, thus, CP is violated. Most notable violation of CP symmetry is given by the weak interaction. This violation is explicitly put in the Lagrangian, which is otherwise CP invariant.
2. Baryon Asymmetry and CP violation
We see that the Universe, as we know it today, is made up of matter and not anti-matter. If there is nothing to differentiate between matter and anti-matter (the labels of particle and anti-particle are human constructs and nothing physically differentiates them), we couldn’t possibly have had more matter than anti-matter. One of the unsolved mysteries is then this: Why is there so much more matter than anti-matter in the Universe. This is known as Baryon Asymmetry puzzle’.
One of the theoretical ways to resolve this is to look for CP violation (see previous section) signatures. CP violating processes can produce more matter particles and hinder the production of anti-matter particles, treating them on unequal footing as explained above. Even though there are models without CP violation, which predict the Baryon Asymmetry, none of them is as beautiful as the Standard Model with the CP violation plugged in. For this to work for every particle, the Baryon number conservation has to be perturbatively broken. In the Standard Model framework, this is not possible. The mechanism for CP violation generating excess baryons is not understood as of now.
3. Generations of Quarks
There are three generations of quarks in the Standard Model. Later generations of quarks are heavier than earlier generations. The three generations are given below.
Most matter is made up of just up and down quarks (the lightest of the lot), given that the proton and neutron are made up of these quarks. The charm quark is a second generation quark and is quite heavy. The heaviest is the top quark, which is so heavy that it cannot exist long enough to form a bound state. We can only identify the top quark by its decay signature.
For our current purposes, only the first two generations of quarks are important. The charm quark, being heavy can decay into strange, anti-strange and up quarks or into down, anti-down and up quarks. The up and down, being the lightest of the lot, doesn’t decay into anything. We shall find out the effect of this decay in the next section.
It was just yesterday, yet we have come so far! The first proton beams at a respectable 7 TeV energy was started only on 30th March, 2010. It has been hardly a year and a half and so much has already been achieved. This was the basic message sent out by CERN speakers Frederick Bordry and Rolf Heuer, also the Director of CERN at the Lepton Photon Conference, 2011, being held at Tata Institute of Fundamental Research, Mumbai, India.
There are a lot of projects on the horizon, both short time and long time. Obviously, the long term projects are ambitious and a bit ambiguous as of now. However, as Heuer said, they are practical. We should not be afraid that it is not easy, he said.
Among the many new developments happening or proposed at LHC is the development of magnets that can generate extremely high magnetic fields, called high-field magnets. These will be required to increase the energy of a beam, without lengthening the collider tunnel. Prof. Michael Peskin of SLAC, who was in the audience, asked if this is a dream or a programamidst chuckles, to which Bordry replied that it was certainly a realistic program.
The LHC is expected to have a long shutdown period from 2013 to mid 2014 for repairs and maintenance work.
New physics and monster accelerators
A number of new projects are upcoming, even though they haven’t been officially sanctioned. Yesterday’ it was the synergy of the Tevatron, HERA and SLAC that led to the discovery of the Standard Model, said Heuer, adding that the LHC results will guide the way at the energy frontier.
About the Higgs search, Heuer said that while finding the Higgs will be a discovery, not finding the Higgs and ruling it out will also be a major discovery. People should not say that these scientists are searching for nothing, he quipped. Not finding the Higgs will be a major result, since it will completely destroy the Standard Model, allowing other models of physics to come into the limelight.
Among a plethora of futuristic plans announced, the most spectacular was the announcement of a hadron-lepton collider the LHeC. The LHC is a hadron-hadron collider. It can collide protons together or lead/silver nuclei etc. A hadron-lepton collider will be able to collide a proton, and say an electron. The energy per beam of the LHeC will be 16.5 TeV, combining to give a massive 33 TeV in total. The LHeC design is on my desk right now, but I shouldn’t be mentioning that here, he remarked drawing loud laughter from the global audience. As far as LHC physics is concerned, he said that 2012 will be a decisive year. The TeV results will either lead to the discovery of new particles and some new physics will be known or it will be a reformulation of the physics we already know. Both will be progressive steps for particle physics.
Heuer spoke at length on the building of the linear accelerators International Linear Collider (ILC) and the Compact LInear Collider (CLIC). Today, we need to keep our choices openwas Heuer’s advice.
On the question of collaboration, Heuer said that CERN was throwing its doors open to non-European countries. The E’ in CERN is going from European’ to Everybody’. We’re not changing our name, however, said Heuer.
Exciting times in particle physics beckon us! As usual this sentiment was put emphatically in Heuer’s own words -We are just beginning to explore 95% of the universe.
I’ll let the scientist in Heuer have the final word on this report. When asked if he’ll be bothered if the next big accelerator is located in the US, instead of at CERN, Heuer put it beautifully, I don’t care where the collider is! I only care about the science coming out of it.
The scientific enterprise is a greater binding factor than anything else. It’s a silent messenger of world peace, uniting the world in the pursuit of truth and never advertising that facet.
If a high energy collision means fast billiard balls colliding or a train wreck, this article is tailor made for you. As the world, or at least the 0.042% of it who are interested in particle physics, gets their opinions ready about the yet unsuccessful Higgs search at the Large Hadron Collider (LHC), CERN, we thought it might be a good idea to explain what physicists mean by what they say. However, we don’t want to explain why they speak that way.
Here we will list out a few things you’ve heard and are likely to hear in the coming days, along with what they might possibly mean.
Units and Particles:
In particle physics, people prefer to use units of energy, instead of that of mass to measure mass. The idea is a good one, since we can use Einstein’s relation E=mc2 directly. The unit of energy is an electron volt or eV. If you force an electron to go through one volt, then it has the energy of 1 eV. It’s too small to use, so people use 1Mega eV (MeV) 1 million eV, or 1Giga eV (GeV) 1 billion eV. If that is not large enough, the LHC demands that we use 1 Tera eV (TeV) or 1 million million eV. The LHC is currently operating at 7 TeV.
The more important point to note about energy is what is considered big! How much really is 1 TeV of energy in human scales? It’s much less than the kinetic energy of a house-fly! So why is it called high-energy physics? It’s because this energy is carried by particles that are really really small! If a proton having 3 TeV of energy is scaled up to our scales, then we would each have energy exceeding that of a Supersonic jet plane. There’s a large amount of energy packed into a small volume.
About particles, there are just two types Fermions and Bosons. Know that fermions don’t like hanging out together, while bosons have no such ego issues. You can stuff a lot of bosons at one place, while that is impossible in case of fermions. Leptons are one kind of fermions, which are electron-like. Photons are bosons. (As to why this happens, it’s related to the Pauli Exclusion Principle, a fundamental result from quantum mechanics, responsible for single-handedly creating all of chemistry! Fermions follow Pauli Principle, bosons do not).
This is probably the most important phenomenon in this story, so pay attention. This is the deal. Every modern physical theory has some symmetry associated with it. For example, if you made all the positive charges in the Universe negative and the negative charges positive, the Universe will still look exactly like it does. Here’s the crucial part with every (continuous) symmetry, is associated a quantity that is conserved, or doesn’t change value. Example: Take time translation. If time were to flow backwards, at the microscopic scale, we would notice nothing. This symmetry leads to conservation of energy. Symmetry breaking refers to the fact that under certain circumstances, a physical theory loses the symmetry it started out with. We can then distinguish between different states. We can choose.
A Useful Analogy
The best analogy I’ve heard asks you to picture a round table with 10 people sitting equally spaced from each other. There is one glass of water kept identically placed between each two adjacent persons, making it 10 glasses in total. Assuming that the glasses are all identical, one can go and reach out for either one there is no preferred choice as yet. (Situation 1 in the graphic below.) But, say someone does pick up a glass say the glass placed to his/her right.(Situation 2 in the graphic below.) Then, everybody will HAVE to pick the glass to his/her right (assuming that no one wants chaos!). Now, you can differentiate between this system and another such system in which someone went for the glass on the left. Symmetry breaking has allowed us to identify a parameter, which had earlier left the system invariant. The word spontaneous’ is clear from this context. The perturbation has to come from within the system and everyone will choose a glass. The symmetry has been spontaneously broken!
In physics too, there arises situations in which the ground state (or state of lowest energy) of a theory is invariant under certain symmetries. This symmetry can, however, be broken and spontaneously, too if the system interacts with a field. It can be shown that this leads to two distinguishable states one having mass. Physicist believe that this is how mass is generated. The interaction field is named Higgs field’ and the force carrying boson associated with the field (as with any force field) is the Higgs Boson.
We could like to conclude this brief glossary of explanations here. Know that these explanations are highly simplified and the whole picture is much richer in beauty and technical details than this. Unfortunately, I won’t be able to communicate that beauty to a person, not having enough background in physics (being a major in physics is a must) and maths. If you already have a background, you’ve probably already seen a bit of the beauty. Go in search of more.
The world is not only queerer than we suppose, remember; it is much queerer than we can possibly suppose. In the words of the immortal Richard Feynman, I think Nature’s imagination is so much greater than Man’s, She’s never going to let us relax.
Links to related articles:
The Higgs search being unsuccessful so far, ATLAS and CMS collaborations of CERN jointly announce. Initial report here.
The figures and numbers (statistics) from the CERN announcement here.
Last year’s run of the LHC has set a cutoff for the expected mass of the Higgs boson. This important result came out of the recently concluded Europhysics Conference 2011.
CMS has excluded the mass ranges of 149-206 GeV and 300-440 GeV for the Higgs with a confidence level of 95%. It has also excluded the masses from 145-480 GeV with a lower confidence level of 90%. This excludes a huge part of the expected mass range, and has gotten particle physicists both excited and demoralised.
Read about the Quark Conference 2011 here, rumour of a Higgs being detected at ATLAS, LHC, CERN here and that news being rejected here. We covered a very recent discovery by Fermilab here.
The Higgs Boson, also called ‘God Particle’ is the only particle predicted by the Standard Model but has not been detected in any collider. It has been assigned the function of endowing all particles in the Universe with mass. Any particle interacting with the Higgs field, mediated by the Higgs boson, is said to have mass. The boson is named after physicist Peter Higgs, the person who came up with the idea. Unfortunately, the Standard Model says little more than the lower limit of the mass of the Higgs Boson. CERN has given itself till 2012 for proving the existence of the Higgs.
Further news is that is the results of the 2010 and 2011 runs are interpreted in the light of a Standard Model having four generation of fermions (SM4), instead of just three, scientists put a 95% confidence level on the exclusion of the Higgs boson in the mass range of 120-600 GeV.
There are also many particle physics theories not involving any Higgs boson called Higgless theories and they may now come to the forefront, if the Higgs remains elusive.
CMS is carrying on the search using decays of different particles. It is trying to produce the Higgs using two photons, two tau leptons, two W bosons and two Z bosons.
This time it appears to be genuine. Fermilab has indeed detected a new particle the Xi-sub-b baryon.
A few days ago Fermilab got the entire physics community excited by announcing that it was almost sure of a discovery of a new particle the W/Z-bar boson. We covered the sensational news here. It turns out that it was not the case.
A baryon is a particle (technically, a bound state’) of three quarks. A proton is an example and is made up of two up and one down quark. Apart from these two types, there are four more types of quarks top(some people call it truth’), bottom (some prefer beauty’), strange and charm. The top quark is the most massive, followed by bottom, charm and strange in that order. The Xi-sub-b baryon has an up quark, a strange quark and a bottom quark. The discovery was made yesterday and reported today in Fermilab’s press release.
The Xi-sub-b baryon is extremely unstable, being able to travel just a fraction of a millimetre before disintegrating into lighter particles. The particle was detected in the CDF detector.
There have been 25 isolated detections, or events, that confirm the existence of the new particle. As we mentioned in earlier posts, the criteria for a discovery is confidence level’. We need a minimum of five-sigma confidence level, or a confidence of more than 99.997% that a certain detection is genuine. Here, Fermilab, after data analysis, puts the confidence level at seven sigma, much higher than the threshold!
The details of the discovery have been sent to Physical Review Letters (PRL) for publication.
The very first ALICE center in India was setup in the Variable Energy Cyclotron Center, Kolkata and was inaugurated on 16th June, 2011. CERN hopes that this will be a small step in improving the capability of their Tier-II and Tier-III grid computing facilities and will go a long way in analysis of the huge amounts of data generated by LHC and, in particular, the ALICE detector. The 16th of June happens to be VECC’s foundation day.
Hopes and aspirations for the ALICE Indian Center
VECC for its part hopes to be a more integral part of the global collaboration at the LHC, CERN. It further hopes that this will give students valuable exposure in handling data and in communicating with physics groups around the world. VECC was already equipped with a 80 TB data storage facility receiving data from CERN, which is a part of the grid computing facility.
The Alice Control Room (ACR), part of the whole facility at VECC, will have good conferencing facilities, making it easier for research scholars and faculties alike to have more involved interactions with other groups around the globe and also directly with people working at CERN.
The ACR will be managed by staff, all of whom will be compelled to serve shift duties. CERN and VECC both hope that the establishment of the ACR will help in making plans in the coming future. The ACR currently monitors two workstations for Detector Control System and Data Acquisition.
The inauguration was attended by the who’s who of the Indian physics community and by members from CERN. Among the prominent personalities present were Prof. Srikumar Banerjee, the Chairman of the Atomic Energy Commission (AEC) and Secretary of the Department of Atomic Energy (DAE), Govt. Of India, Prof. Rakesh Bhandari, current Director of VECC and Dr. Bikash Sinha, the former director of Saha Institute of Nuclear Physics (SINP) and the current Homi Bhabha Professor, DAE. Paolo Giubellino and Jurgen Schukraft represented CERN from Geneva with their virtual presence via Skype. They briefly reiterated the need for the center in India and outlined the plans for the ALICE collaboration.
Gone are the days when geniuses would come up with theories working in isolation behind closed doors. This is the age of information and knowing the ultimate answer in Physics will take the whole world working together. This is another small step in the long journey.
It is disappointing news for the particle physics community coming out of Fermilab, we’re afraid. Fermilab has confirmed that the earlier bump seen in the data, presumed to be a new particle, is not significant enough to be considered a detection. We broke the news in emphatic fashion of a new particle discovered in Fermilab in an earlier post.
Story So Far
We had reported that there was a bump found at about 145 GeV with a 5 GeV spread. Data acquired from proton-antiproton collisions with semi-leptonic dijet emissions showed a peak at 145 GeV. The curve was Gaussian in nature with a spread of 5 GeV on either side of the peak. Initial analysis showed that the curve had a three-sigma confidence level (More on confidence levels later). There was thus a strong possibility that a new particle was on the way, since no boson is known having a mass of 145 GeV. The new particle was named as the Z’ or the W’ (Z-primed or W-primed) boson. The Standard Model, wildly successful in particle physics, did not predict this and to fit this in would have required a serious rethinking of known physics. Physicists were naturally excited.
This detection was made at Fermilab at their CDF detector.
Fermilab was the biggest particle accelerator till the Large Hadron Collider came onto the scene. It has been a major progressive force for particle physics over the last three decades, also serving to etch the American superiority in the particle physics arena. It is however expected to be closed down forever late this year. Data recovered from it over the years is still being analysed, and as such will continue for the next five years. One of the two detectors at Fermilab the CDF had detected the anomalous bump of our present interest.
So What’s Wrong?
There are two problems with the CDF data it cannot be corroborated and it falls outside the required confidence levels.
The other detector at Fermilab, named DZero, repeated the experiment, but failed to come up with any conclusive evidence of detection. The negative DZero result would definitely cast shadows over the CDF discovery. Scientists are now baffled as to how the two detectors extremely alike could give such widely varying results under the same experimental conditions. However this is a very good safeguard.
Problem 2: Remember that earlier we had said something about a three-sigma confidence level? It means that the data is reliable and the chances of it being wrong are one-in-a-thousand (99.9% accurate). Confidence levels measure reliability of data. For a discovery to be accepted by the scientific community, the event must have at least a five-sigma confidence level or higher, which means that doubts must reduce to less than one-in-a-million. The problem with the current bump is that it lies just below the five-sigma confidence level.
Take a look at the above graph. Never mind the mathematics and abstruse symbols. Know that the horizontal axis represents the mass of the particles and the vertical axis represents the number of particles detected. At the 145-150 GeV range (point 145 GeV on the horizontal axis), you’d have expected a curve if the previous CDF results were replicated. This is marked with the dotted curve. There is nothing there as far as DZero is concerned. The red regions represent detections and these are in complete agreement with the Standard Model. There is no anomaly to be seen anywhere.
On both counts, the bump is rejected as a new discovery.
What changes then?
Practically nothing changes. The 145 GeV particle, if discovered, would have been interesting, as the Standard Model doesn’t predict it. Further, it could have provided a mechanism for particles acquiring mass without the need of the Higgs boson (essentially becoming the new God particle’). With it being ruled out, the Standard Model stands as it is with the Higgs mechanism being the most favoured mechanism for mass generation.
The discovery would have been exciting, but the field’s exciting even without it. After all, science is like this. DZero spokesperson Stefan Soldner-Rembold put it approproiately in a Fermilab press conference:
This is exactly how science works. Independent verification of any new observation is the key principle of scientific research.
The results of the three experiments conducted by the LHC till the end of last year, during which lead ions were collided, was released by CERN yesterday, i.e. 23rd May, at the Quark Conference 2011, and there is plenty to be really excited about. In the last two weeks of 2010, LHC switched from colliding protons to colliding beams of lead ions. (For news of recent records of beam intensity and beam bunches from LHC, CERN, see here)
Peter Steinberg of Brookhaven, co-convener of the collaboration said:
The first LHC heavy-ion run was a great success for ATLAS.
Why Lead Ions?
Lead ions being heavier carry a lot more energy than protons and thus their collisions produce new heavier and exotic particles and new forms of matter, not seen in proton-proton collisions. Lead ions are produced by stripping lead atoms of all their electrons. In these collisions, scientists have detected traces of Quark Gluon Plasma, the stuff that was present in the primordial Universe.
The early universe, a few hundred microseconds old, was a soup of extreme energy density, in which quarks, gluons and leptons moved around. Quarks and gluons were believed to form a plasma like substance, called ‘Quark Gluon Plasma’. In colliding lead ions, scientists hope to recreate the plasma and study the very early universe.
What is QGP?
QGP can be theoretically made in the laboratory by heating matter upto 2×1012 K, which is roughly a hundred thousand (100,000) times hotter than the Sun’s core temperature and about a billion (1,000,000,000) times hotter than the surface of the Sun. This can only be achieved by colliding heavy ions at ultra-relativistic speeds(speeds close to that of light, or more technically, having a high Lorentz factor). Lead and gold ions have been selected by LHC. Most collisions don’t happen head on, but a few do. If enough of these head-on collisions are produced at a certain small region, they may form the exotic matter QGP, which has densities higher than those found on neutron stars. (Just a layman’s comparison: One teaspoonful of neutron star matter outweighs all the cars of the Earth put together!!) By studying the flow of this material, scientists can fit data into theory and also verify theoretical predictions.
Detectors and Achievements
QGP was first produced by the Relativistic Heavy Ion Collider (RHIC), Brookhaven, demonstrating for the first time that such dense exotic forms of matter can indeed be produced in a laboratory. ALICE, a detector at LHC, specifically designed and calibrated for very high energy collisions and for studying Quark Gluon Plasma, has confirmed the fact and has been able to study the properties of the plasma. It has verified theoretical predictions than QGP is an ideal fluid.
CMS, another detector in LHC, designed as a general purpose detector for high energy particles, have also detected the production of the W and Z boson, both critical components in the electroweak theory. CMS has also detected a marked suppression of the weakly bound states of the bottom (or, as some people prefer, beauty) quark, which scientists believe has great importance in further study of QGP.
ATLAS, one of the general purpose detectors for high energy particles alongwith CMS, has studied the macroscopic properties of the plasma, like the number and density of charged particles emerging out of it. It has also mapped out the energy and matter density, elaborating on the collision mechanisms and transport properties of the plasma.
As CERN’s director, General Rolf Heuer says:
These results from the LHC lead ion programme are already starting to bring new understanding of the primordial universe. The subtleties they are already seeing are very impressive.
LHC is now setting a benchmark in high energy physics, going where no collider has dared to go before. It would thus be appropriate to close with the following words from CMS spokesperson Guido Tonelli:
We are entering a new era of high precision studies of strongly interacting matter at the highest energies ever. By deploying the full potential of the CMS detector we are producing unambiguous signatures of this new state of matter and unravelling many of its properties.
An exciting future in high energy physics awaits us.
News of new records seem to be tumbling out of CERN in the past two days, with both the number of bunches per beam and the beam luminosity in the Large Hadron Collider (LHC) hitting a record high. Though this is still a long way away from the theoretical maximum predicted by the engineers for LHC, this is a significant step.
In this post, we discuss about the records only. In a subsequent post, we shall discuss what LHC has managed to uncover about quarks the basic building block for all almost matter you see around you.
First: bunches per beam
Yesterday, i.e. on 22nd May, 2011, CERN announced that the LHC had crossed the mark of 900 bunches per beam, creating a new record for itself with 912 bunches. This betters the previous count of 480 bunches, which was achieved only a month back. But exactly what is a bunch?
What is a bunch?
The particles in a beam in LHC is not a continuous stream, as you’d have thought. They are broken down into small collections each containing the same number of particles. These collections are called ‘bunches’. So each beam is made up of a number of bunches, each bunch being made up of a huge number of particles. Bunches arrive with a certain time delay. The present time delay between two bunches is 50 nanoseconds, while the theoretical upper limit (the one permitted by the detector) is 25 nanoseconds. Also, the number of bunches in a beam, currently 912, will also increase. The theoretical upper limit is 2808 bunches per beam.
Upgrading the LHC to a higher number of bunches per beam is a delicate process, since increase in the number of bunches dramatically increases the beam energy. Engineers have to be sure that not only is the LHC under no danger from the increased energy, the detectors function properly and the calibration remains true.
Take a bunch of protons. This may consist of a hundred billion protons (that’s one followed by 11 zeros), each separated by a distance much greater than the theoretical radius of protons. When such beams from opposite sides collide, the number of collisions is really small owing to the huge separations (the beams pass through each other mostly as ghosts). It’s the relatively small number of collisions that happen that are important.
Next, luminosity of a beam:
The next milestone achieved by the LHC early today was the attainment of a huge luminosity of 1033. But what does this really mean? Roughly, this corresponds to the collision rate, but it is not exactly so! It refers to the probability of collisions rather than the rate of actual collisions happening. The difference is a subtle and important one. Physicists call this the ‘cross-section’ of the process. Think of cross-section as an area (in fact, this is where the name comes from). The bigger the cross-section, the bigger the probability of a collisions happening. (Think of a door and a small slit in a postbox. If you randomly throw small rocks at each one from a good distance, the probability of hitting the door is much higher than that for hitting the mailbox slit. The door is said to have a higher ‘cross-section’.)
Each process has its own cross-section. For example, the cross-section for the production of a muon from an electron-positron collision is much higher that the same for the production of a Higgs from a proton-antiproton collision. Multiplying the cross-section with the luminosity gives the number of events to expect (call it, ‘expectation’) at that luminosity. (This is indeed different from cross-section, if you’re wondering. Cross-section is an inherent quantity, fixed for a certain process. Expectation depends on the number of particles in the beam being fired.)
Most protons in the LHC beams, as mentioned above, don’t collide at all and some collide only at glancing angles. The head-on collisions are the ones that produce the most exotic phenomenon (utilizing their full energy in the collision), and this is what interests scientists the most.
The highest luminosity, set last year, was 1032, with the theoretical highest being 1034. Currently, there is predicted about a 100 million collisions.
The LHC is expected to operate at 3.5 TeV this entire year set to hit 7 TeV in the next couple of years. Exciting physics is round the corner. Watch this space for more…
The mystery of the Higgs boson, the so called God Particle, will be solved by 2012. This claim came from CERN a couple of days back. CERN spokesperson, Fabiola Gianotti, said:
By the end of 2012 we will either discover the Standard Model Higgs Boson, if it exists, or we will rule it out
Make or (almost) break:
This has caused quite a bit of excitement in the physics quarters, with people betting both ways, but leaning towards a successful search. The Higgs boson is supposed to endow all particles in the universe with mass. The mass of a particle depends on the strength of interaction with the Higgs field. Whereas a photon (particle of light) doesn’t interact with the field at all, an electron neutrino (a very light particle) interacts very weakly, while a proton (comparatively heavy) interacts much more strongly. The mechanism by which an interacting particle gets mass is called the Higgs mechanism, after the British physicist Peter Higgs, who first proposed it. Since then, the Higgs has been hot property in the particle physics arena, and has evoked strong interest outside it too. Probably, it is the most popular boson after the photon. The Higgs fits in nicely with the present Standard Model. The only problem is that it hasn’t been detected as yet.
If the Higgs boson were found, it would put an everlasting stamp of approval on the Standard Model. However, an unsuccessful search will not invalidate the Standard Model, as some people believe. There are many less known mechanisms not involving the Higgs, which describes how particles get mass.
The LHC was built with a primary aim of detecting the Higgs, especially with the Tevatron at Fermilab, the biggest collider before the LHC, shutting down in 2011. The enthusiasm to get at the Higgs is palpable within the science community and the desperation can be felt by the leakage of a memo at CERN a few days back. ATLAS, CERN has however invalidated the memo, which stated that a Higgs signature has been found.
The CMS Detector at the LHC, CERN
Smashing protons (and heavier nuclei) travelling at velocities close to that of light, scientists hope to recreate the conditions that supposedly existed right after the Big Bang. The LHC recently crossed the 7 TeV energy limit and also set a new record for beam intensity. This is, however, just half of its maximum capability. Scientists are busy acquiring and sifting through data of the order of terabytes, looking for a sign of the elusive Higgs and any of its heavier super-symmetric partners that the Standard Model predicts will have to exist, if the Higgs exists.
There will be an answer. Whether that answer will please Standard Model physicists is another question, and we will have to wait till 2012 for that.
The confirmation of the negative is finally here. A report has been released from ATLAS, CERN, which, after extensive analysis, has put to rest all rumors about any detection of the Higgs Boson the God Particle, which is supposed to endow all particles in the Universe with mass. We covered the news of the rumor here, urging readers to take the rumor with a healthy pinch of salt. The aforementioned report, surely enough and as we predicted, hasn’t found any Higgs.
The bottom line: The Higgs remains as elusive as ever, and physicists remain hopeful.
The news of this negative report was greeted with a little disappointment and lot of relief from the physics community, the latter emotion stemming from the fact that the Higgs was never expected at the position it was reported to have been found by the leaked memo. Many physicists were also appalled by the way a confidential memo was leaked a violation of both the ethical code of conduct and a personal breach of innate scientific spirit.
The present report (about the di-photon mass spectrum) from ATLAS has a lot more data points than the memo, and the data has been thoroughly analyzed. Find the report here. The key diagram in the report is given below.
Forget all the complications and the different plots. Note the monotonic step-like appearance of the plot. If the Higgs were detected, you’d find bumps in the graph (or at least one bump) localized at particular energies. If the memo were really right about the detection, there would have been a bump at 115 GeV (on the yellow band above the number 115′ marked on the horizontal axis), as the memo had reported the detection at this temperature. No irregularity is found, as is plainly visible to the naked eye.
There’s no real ambiguity anymore. At the risk of sounding repetitive, let’s say it once more: The Higgs has not been found.
If the Standard Model is correct, however, you should expect news of detection soon enough, especially when higher energy ranges are being probed. Watch this space…
Some people might liken it to the situation described in Dan Brown’s novel Angels and Demons. CERN scientists trapped dangerously slippery anti-matter for a record 1000 seconds (or 17 minutes), a huge achievement in terms of anti-matter confinement.
This remarkable feat was achieved by scientists working on the Anti-hydrogen Laser Physics Apparatus (ALPHA). 1000 seconds is a huge improvement (almost 4 orders of magnitude greater) than the previous best, of 172 millisecond (i.e. 0.172 s). This new trapping ability will allow physicists to design new experiments that take a closer look at the properties of anti-matter. Theoretically, anti-hydrogen should occupy the same quantum energy levels as normal hydrogen, but this has never been theoretically confirmed. Also this will settle many long standing problems like Baryon Asymmetry and give insights into CP violation, or even CPT violation. There is also hope of cooling these anti-atoms to temperatures low enough so that gravitational effects become significant.
Confining neutral (or even charged) particles cannot be done using electric fields. Magnetic fields are used. A Penning trap is used to produce and hold the anti-hydrogen produced. These are then held in a magnetic field with a three-dimensional minimum. (See figure)
It is clear that deeper the well (see the second figure from the top), lesser is the probability of the atoms escaping. The ALPHA collaboration has gone up to 309 atoms from a measly 38 that it started off with. Most anti-hydrogen atoms were in their ground state as expected.
An exciting question is whether anti-matter is attracted or repelled by gravity i.e. whether it should fall up or down. (By all means, physicists expect it to fall down! Nature is not that asymmetric. However, enough anti-matter has not been observed to fall down or up.)
Exciting news has been tumbling out of the Relativistic Heavy Ion Collider (RHIC), since today morning. Scientists have found the distinct signature of an anti-helium nucleus, the heaviest anti-matter particle detected till date. They can also figure out the production rates and compare them to theoretical values, verifying known calculations. This is big news!
The STAR collaboration at the RHIC, Brookhaven National Laboratories, smashed together extremely fast moving gold nuclei, producing conditions similar to that of the hot, early Universe. Out of these billions of collisions, trillions of charged particles and anti-particles are produced. The huge data sets are sifted through to identify the details of the particles and anti-particles produced. Generally, anti-matter are stable for long enough to be detected. They eventually collide with matter on the outer margins of the detector and get annihilated.
Sifting through this particular dataset, the STAR team found at least 18-20 distinct signatures’ of anti-helium(IV) nuclei. This is a bound state of two anti-protons and two anti-neutrons, having an overall double negative charge (just opposite to the helium(IV) nucleus, which is made up of two protons and two neutrons with an overall double positive charge). The data clearly shows the anti-He-3 (bound state 2 anti-protons and one anti-neutron) and anti-He-4 (bound state of two anti-protons and two anti-neutrons) peaks.
The exciting part
The exciting part is that the rate can also be measured. This rate is then compared with theoretical values. Scientists are ecstatic with the present data as the rates match theoretical predictions extremely well. This also augers well for an experimental project called Alpha Magnetic Spectrometer (AMS), which will be sent to the International Space Station by early May this year. AMS is designed to search for anti-matter in space. This experiment by the STAR team will set the expected rates and provide a good calibration rate for comparisons for AMS. If there is anti-matter concentrated somewhere in the Universe, AMS will catch it. This will go a long way in explaining the asymmetry in the matter-antimatter production rates. (If anti-matter is produced at the same rate as matter, as predicted in theory and observed in the laboratories, why are we surrounded by only matter and not anti-matter?)
The STAR collaboration is jubilant about the discovery and reckons that this will be the heaviest anti-particle detected for quite some time. The next heavy nucleus of anti-lithium is 2.25 times heavier and a trillion times rarer, at least theoretically. Finding such a particle is beyond today’s technology.