Dijet mass (TeV) v. no. of events. Source: ATLAS/CERN
Looks intimidating, doesn’t it? It’s also very interesting because it contains an important result acquired at the Large Hadron Collider (LHC) this year, a result that could disappoint many physicists.
The LHC reopened earlier this year after receiving multiple performance-boosting upgrades over the 18 months before. In its new avatar, the particle-smasher explores nature’s fundamental constituents at the highest energies yet, almost twice as high as they were in its first run. By Albert Einstein’s mass-energy equivalence (E = mc2), the proton’s mass corresponds to an energy of almost 1 GeV (giga-electron-volt). The LHC’s beam energy to compare was 3,500 GeV and is now 6,500 GeV.
At the start of December, it concluded data-taking for 2015. That data is being steadily processed, interpreted and published by the multiple topical collaborations working on the LHC. Two collaborations in particular, ATLAS and CMS, were responsible for plots like the one shown above.
This is CMS’s plot showing the same result:
Source: CMS/CERN
When protons are smashed together at the LHC, a host of particles erupt and fly off in different directions, showing up as streaks in the detectors. These streaks are called jets. The plots above look particularly at pairs of particles called quarks, anti-quarks or gluons that are produced in the proton-proton collisions (they’re in fact the smaller particles that make up protons).
The sequence of black dots in the ATLAS plot shows the number of jets (i.e. pairs of particles) observed at different energies. The red line shows the predicted number of events. They both match, which is good… to some extent.
One of the biggest, and certainly among the most annoying, problems in particle physics right now is that the prevailing theory that explains it all is unsatisfactory – mostly because it has some really clunky explanations for some things. The theory is called the Standard Model and physicists would like to see it disproved, broken in some way.
In fact, those physicists will have gone to work today to be proved wrong – and be sad at the end of the day if they weren’t.
Maintenance work underway at the CMS detector, the largest of the five that straddle the LHC. Credit: CERN
The annoying problem at its heart
The LHC chips in providing two kinds of opportunities: extremely sensitive particle-detectors that can provide precise measurements of fleeting readings, and extremely high collision energies so physicists can explore how some particles behave in thousands of scenarios in search of a surprising result.
So, the plots above show three things. First, the predicted event-count and the observed event-count are a match, which is disappointing. Second, the biggest deviation from the predicted count is highlighted in the ATLAS plot (look at the red columns at the bottom between the two blue lines). It’s small, corresponding to two standard deviations (symbol: σ) from the normal. Physicists need at least three standard deviations (3σ) from the normal for license to be excited.
But this is the most important result (an extension to the first): The predicted event-count and the observed event-count are a match across 6,000 GeV. In other words: physicists are seeing no cause for joy, and all cause for revalidating a section of the Standard Model, in a wide swath of scenarios.
The section in particular is called quantum chromodynamics (QCD), which deals with how quarks, antiquarks and gluons interact with each other. As theoretical physicist Matt Strassler explains on his blog,
… from the point of view of the highest energies available [at the LHC], all particles in the Standard Model have almost negligible rest masses. QCD itself is associated with the rest mass scale of the proton, with mass-energy of about 1 GeV, again essentially zero from the TeV point of view. And the structure of the proton is simple and smooth. So QCD’s prediction is this: the physics we are currently probing is essential scale-invariant.
Scale-invariance is the idea that two particles will interact the same way no matter how energetic they are. To be sure, the ATLAS/CMS results suggest QCD is scale-invariant in the 0-6,000 GeV range. There’s a long way to go – in terms of energy levels and future opportunities.
Something in the valley
The folks analysing the data are helped along by previous results at the LHC as well. For example, with the collision energy having been ramped up, one would expect to see particles of higher energies manifesting in the data. However, the heavier the particle, the wider the bump in the plot and more the focusing that’ll be necessary to really tease out the peak. This is one of the plots that led to the discovery of the Higgs boson:
Source: ATLAS/CERN
That bump between 125 and 130 GeV is what was found to be the Higgs, and you can see it’s more of a smear than a spike. For heavier particles, that smear’s going to be wider with longer tails on the site. So any particle that weighs a lot – a few thousand GeV – and is expected to be found at the LHC would have a tail showing in the lower energy LHC data. But no such tails have been found, ruling out heavier stuff.
And because many replacement theories for the Standard Model involve the discovery of new particles, analysts will tend to focus on particles that could weigh less than about 2,000 GeV.
In fact that’s what’s riveted the particle physics community at the moment: rumours of a possible new particle in the range 1,900-2,000 GeV. A paper uploaded to the arXiv preprint server on December 10 shows a combination of ATLAS and CMS data logged in 2012, and highlights a deviation from the normal that physicists haven’t been able to explain using information they already have. This is the relevant plot:
Source: arXiv:1512.03371v1
The one on the middle and right are particularly relevant. They each show the probability of the occurrence of an event (observed as a bump in the data, not shown here) of some heavier mass of energy decaying into two different final states: of W and Z bosons (WZ), and of two Z bosons (ZZ). Bosons make a type of fundamental particle and carry forces.
The middle chart implies that the mysterious event is at least 1,000-times less likelier to occur than normally and the one on the left implies the event is at least 10,000-times less likelier to occur than normally. And both readings are at more than 3σ significance, so people are excited.
The authors of the paper write: “Out of all benchmark models considered, the combination favours the hypothesis of a [particle or its excitations] with mass 1.9-2.0 [thousands of GeV] … as long as the resonance does not decay exclusively to WW final states.”
But as physicist Tommaso Dorigo points out, these blips could also be a fluctuation in the data, which does happen.
Although the fact that the two experiments see the same effect … is suggestive, that’s no cigar yet. For CMS and ATLAS have studied dozens of different mass distributions, and a bump could have appeared in a thousand places. I believe the bump is just a fluctuation – the best fluctuation we have in CERN data so far, but still a fluke.
There’s a seminar due to happen today at the LHC Physics Centre at CERN where data from the upgraded run is due to be presented. If something really did happen in those ‘valleys’, which were filtered out of a collision energy of 8,000 GeV (basically twice the beam energy, where each beam is a train of protons), then those events would’ve happened in larger quantities during the upgraded run and so been more visible. The results will be presented at 1930 IST. Watch this space.
Featured image: Inside one of the control centres of the collaborations working on the LHC at CERN. Each collaboration handles an experiment, or detector, stationed around the LHC tunnel. Credit: CERN.
During a lecture in 2012, G. Rajasekaran, professor emeritus at the Institute for Mathematical Sciences, Chennai, said that the future of high-energy physics lay with engineers being able to design smaller particle accelerators. The theories of particle physics have for long been exploring energy levels that we might never be able to reach with accelerators built on Earth. At the same time, it will still be on physicists to reach the energies that we can reach but in ways that are cheaper, more efficient, and smaller – because reach them we will have to if our theories must be tested. According to Rajasekaran, the answer is, or will soon be, the tabletop particle accelerator.
In the last decade, tabletop accelerators have inched closer to commercial viability because of a method called plasma wakefield acceleration. Recently, a peer-reviewed experiment detailing the effects of this method was performed at the University of Maryland (UMD) and the results published in the journal Physical Review Letters. A team-member said in a statement: “We have accelerated high-charge electron beams to more than 10 million electron volts using only millijoules of laser pulse energy. This is the energy consumed by a typical household lightbulb in one-thousandth of a second.” Ten MeV pales in comparison to what the world’s most powerful particle accelerator, the Large Hadron Collider (LHC), achieves – a dozen million MeV – but what the UMD researchers have built doesn’t intend to compete against the LHC but against the room-sized accelerators typically used for medical imaging.
In particle accelerator like the LHC or the Stanford linac, a string of radiofrequency (RF) cavities are used to accelerate charged particles around a ring. Energy is delivered to the particles using powerful electromagnetic fields via the cavities, which switch polarity at 400 MHz – that’s switching at 400 million times a second. The particles’ arrival at the cavities are timed accordingly. Over the course of 15 minutes, the particle bunches are accelerated from 450 GeV to 4 TeV (the beam energy before the LHC was upgraded over 2014), with the bunches going 11,000 times around the ring per second. As the RF cavities switch faster and are ramped up in energy, the particles swing faster and faster around – until computers bring two such beams into each other’s paths at a designated point inside the ring and BANG.
A wakefield accelerator also has an electromagnetic field that delivers the energy, but instead of ramping and switching over time, it delivers the energy in one big tug.
First, scientists create a plasma, a fluidic state of matter consisting of free-floating ions (positively charged) and electrons (negatively charged). Then, the scientists shoot two bunches of electrons separated by 15-20 micrometers (millionths of a metre). As the leading bunch moves into the plasma, it pushes away the plasma’s electrons and so creates a distinct electric field around itself called the wakefield. The wakefield envelopes the trailing bunch of electrons as well, and exerts two forces on them: one along the direction of the leading bunch, which accelerates the trailing bunch, and one in the transverse direction, which either makes the bunch more or less focused. And as the two bunches shoot through the plasma, the leading bunch transfers its energy to the trailing bunch via the linear component of the wakefield, and the trailing bunch accelerates.
A plasma wakefield accelerator scores over a bigger machine in two key ways:
The wakefield is a very efficient energy transfer medium (but not as much as natural media), i.e. transformer. Experiments at the Stanford Linear Accelerator Centre (SLAC) have recorded 30% efficiency, which is considered high.
Wakefield accelerators have been able to push the energy gained per unit distance travelled by the particle to 100 GV/m (an electric potential of 1 GV/m corresponds to an energy gain of 1 GeV/c2 for one electron over 1 metre). Assuming a realistic peak accelerating gradient of 100 MV/m, a similar gain (of 100 GeV) at the SLAC would have taken over a kilometre.
There are many ways to push these limits – but it is historically almost imperative that we do. Could the leap in accelerating gradient by a factor of 100 to 1,000 break the slope of the Livingston plot?
Could the leap in accelerating gradient from RF cavities to plasma wakefield accelerators break the Livingston plot? Source: AIP
In the UMD experiment, scientists shot a laser pulse into a hydrogen plasma. The photons in the laser then induced the wakefield that trailing electrons surfed and were accelerated through. To reduce the amount of energy transferred by the laser to generate the same wakefield, they made the plasma denser instead to capitalise on an effect called self-focusing.
A laser’s electromagnetic field, as it travels through the plasma, makes electrons near it wiggle back and forth as the field’s waves pass through. The more intense waves near the pulse’s centre make the electrons around it wiggle harder. Since Einstein’s theory of relativity requires objects moving faster to weigh more, the harder-wiggling electrons become heavier, slow down and then settle down, creating a focused beam of electrons along the laser pulse. The denser the plasma, the stronger the self-focusing – a principle that can compensate for weaker laser pulses to sustain a wakefield of the same strength if the pulses were stronger but the plasma less dense.
The UMD team increased the hydrogen gas density, of which the plasma is made, by some 20x and found that electrons could be accelerated by 2-12 MeV using 10-50 millijoule laser pulses. Additionally, the scientists also found that at high densities, the amplitude of the plasma wave propagated by the laser pulse increases to the point where it traps some electrons from the plasma and continuously accelerates them to relativistic energies. This obviates the need for trailing electrons to be injected separately and increases the efficiency of acceleration.
But as with all accelerators, there are limitations. Two specific to the UMD experiment are:
If the plasma density goes beyond a critical threshold (1.19 x 1020 electrons/cm3) and if the laser pulse is too powerful (>50 mJ), the electrons are accelerated more by the direct shot than by the plasma wakefield. These numbers define an upper limit to the advantage of relativistic self-focusing.
The accelerated electrons slowly drift apart (in the UMD case, to at most 250 milliradians) and so require separate structures to keep their beam focused – especially if they will be used for biomedical purposes. (In 2014, physicists from the Lawrence Berkeley National Lab resolved this problem by using a 9-cm long capillary waveguide through which the plasma was channelled.)
There is another way lasers can be used to build an accelerator. In 2013, physicists from Stanford University devised a small glass channel 0.075-0.1 micrometers wide, and etched with nanoscale ridges on the floor. When they shined infrared light with wavelength of twice the channel’s height across it, the eM field of the light wiggled the electrons back and forth – but the ridges on the floor were cut such that electrons passing over the crests would accelerate more than they would decelerate when passing over the troughs. Like this, they achieved an energy gain gradient of 300 MeV/m. This way, the accelerator is only a few millimetres long and devoid of any plasma, which is difficult to handle.
At the same time, this method shares a shortcoming with the (non-laser driven) plasma wakefield accelerator: both require the electrons to be pre-accelerated before injection, which means room-sized pre-accelerators are still in the picture.
Physical size is an important aspect of particle accelerators because, the way we’re building them, the higher-energy ones are massive. The LHC currently accelerates particles to 13 TeV (1 TeV = 1 million MeV) in a 27-km long underground tunnel running beneath the shared borders of France and Switzerland. The planned Circular Electron-Positron Collider in China envisages a 100-TeV accelerator around a 54.7-km long ring (Both the LHC and the CEPC involve pre-accelerators that are quite big – but not as much as the final-stage ring). The International Linear Collider will comprise a straight tube, instead of a ring, over 30 km long to achieve accelerations of 500 GeV to 1 TeV. In contrast, Georg Korn suggested in APS Physics in December 2014 that a hundred 10-GeV electron acceleration modules could be lined up facing against a hundred 10-GeV positron acceleration modules to have a collider that can compete with the ILC but from atop a table.
In all these cases, the net energy gain per distance travelled (by the accelerated particle) was low compared to the gain in wakefield accelerators: 250 MV/m versus 10-100 GV/m. This is the physical difference that translates to a great reduction in cost (from billions of dollars to thousands), which in turn stands to make particle accelerators accessible to a wider range of people. As of 2014, there were at least 30,000 particle accelerators around the world – up from 26,000 in 2010 according to a Physics Today census. More importantly, the latter estimated that almost half the accelerators were being used for medical imaging and research, such as in radiotherapy, while the really high-energy devices (>1 GeV) used for physics research numbered a little over 100.
These are encouraging numbers for India, which imports 75% of its medical imaging equipment for more than Rs.30,000 crore a year (2015). These are also encouraging numbers for developing nations in general that want to get in on experimental high-energy physics, innovations in which power a variety of applications, ranging from cleaning coal to detecting WMDs, not to mention expand their medical imaging capabilities as well.
It finally happened! The particle-smasher known as the Large Hadron Collider is back online after more than two years, during which its various components were upgraded to make it even meaner. A team of scientists and engineers gathered at the collider’s control room at CERN over the weekend – giving up Easter celebrations at home – to revive the giant machine so it could resume feeding its four detectors with high-energy collisions of protons.
Before the particles enter the LHC itself, they are pre-accelerated to 450 GeV by the Super Proton Synchrotron. At 11.53 am (CET), the first beam of pre-accelerated protons was injected into the LHC at Point 2 (see image), starting a clockwise journey. By 11.59 am, it’d been reported crossing Point 3, and at 12.01 pm, it was past Point 5. The anxiety in the control room was palpable when an update was posted in the live-blog: “The LHC operators watching the screen now in anticipation for Beam 1 through sector 5-6”.
Beam 1 going from Point 2 to Point 3 during the second run of the Large Hadron Collider’s first day in action. Credit: CERN
Finally, at 12.12 pm, the beam had crossed Point 6. By 12.27, it had gone a full-circle around the LHC’s particles pipeline, signalling that the pathways were defect-free and ready for use. Already, as and when the beam snaked through a detector without glitches, some protons were smashed into static targets producing a so-called splash of particles like sparks, and groups of scientists erupted in cheers.
Both Rolf-Dieter Heuer, the CERN Director-General, and Frederick Bordry, Director for Accelerators and Technology, were present in the control room. Earlier in the day, Heuer had announced that another beam of protons – going anti-clockwise – had passed through the LHC pipe without any problems, providing the preliminary announcement that all was well with the experiment. In fact, CERN’s scientists were originally supposed to have run these beam-checks a week ago, when an electrical glitch spotted at the last minute thwarted them.
In its new avatar, the LHC sports almost double the energy it ran at, before it shut down for upgrades in early-2013, as well as more sensitive collision detectors and fresh safety systems. For the details of the upgrades, read this. For an ‘abridged’ version of the upgrades together with what new physics experiments the new LHC will focus on, read this. Finally, here’s to another great year for high-energy physics!
The world’s single largest science experiment will restart on March 23 after a two-year break. Scientists and administrators at the European Organization for Nuclear Research – known by its French acronym CERN – have announced the status of the agency’s upgrades on its Large Hadron Collider (LHC) and its readiness for a new phase of experiments running from now until 2018.
Before the experiment was shut down in late 2013, the LHC became famous for helping discover the elusive Higgs boson, a fundamental (that is, indivisible) particle that gives other fundamental particles their mass through a complicated mechanism. The find earned two of the physicists who thought up the mechanism in 1964, Peter Higgs and Francois Englert, a Nobel Prize in that year.
Though the LHC had fulfilled one of its more significant goals by finding the Higgs boson, its purpose is far from complete. In its new avatar, the machine boasts of the energy and technical agility necessary to answer questions that current theories of physics are struggling to make sense of.
As Alice Bean, a particle physicist who has worked with the LHC, said, “A whole new energy region will be waiting for us to discover something.”
The finding of the Higgs boson laid to rest speculations of whether such a particle existed and what its properties could be, and validated the currently reigning set of theories that describe how various fundamental particles interact. This is called the Standard Model, and it has been successful in predicting the dynamics of those interactions.
From the what to the why
But having assimilated all this knowledge, what physicists don’t know, but desperately want to, is why those particles’ properties have the values they do. They have realized the implications are numerous and profound: ranging from the possible existence of more fundamental particles we are yet to encounter to the nature of the substance known as dark matter, which makes up a great proportion of matter in the universe while we know next to nothing about it. These mysteries were first conceived to plug gaps in the Standard Model but they have only been widening since.
With an experiment now able to better test theories, physicists have started investigating these gaps. For the LHC, the implication is that in its second edition it will not be looking for something as much as helping scientists decide where to look to start with.
As Tara Shears, a particle physicist at the University of Liverpool, toldNature, “In the first run we had a very strong theoretical steer to look for the Higgs boson. This time we don’t have any signposts that are quite so clear.”
Higher energy, luminosity
The upgrades to the LHC that would unlock new experimental possibilities were evident in early 2012.
The machine works by using powerful electric currents and magnetic fields to accelerate two trains, or beams, of protons in opposite directions, within a ring 27 km long, to almost the speed of light and then colliding them head-on. The result is a particulate fireworks of such high energy that the most rare, short-lived particles are brought into existence before they promptly devolve into lighter, more common particles. Particle detectors straddling the LHC at four points on the ring record these collisions and their effects for study.
So, to boost its performance, upgrades to the LHC were of two kinds: increasing the collision energy inside the ring and increasing the detectors’ abilities to track more numerous and more powerful collisions.
The collision energy has been nearly doubled in its second life, from 7-8 TeV to 13-14 TeV. The frequency of collisions has also been doubled from one set every 50 nanoseconds (billionth of a second) to one every 25 nanoseconds. Steve Myers, CERN’s director for accelerators and technology, had said in December 2012, “More intense beams mean more collisions and a better chance of observing rare phenomena.”
The detectors have received new sensors, neutron shields to protect from radiation damage, cooling systems and superconducting cables. An improved fail-safe system has also been installed to forestall accidents like the one in 2008, when failing to cool a magnet led to a shut-down for eight months.
In all, the upgrades cost approximately $149 million, and will increase CERN’s electricity bill by 20% to $65 million. A “massive debugging exercise” was conducted last week to ensure all of it clicked together.
Going ahead, these new specifications will be leveraged to tackle some of the more outstanding issues in fundamental physics.
CERN listed a few–presumably primary–focus areas. They include investigating if the Higgs boson could betray the existence of undiscovered particles, the particles dark matter could be made of, why the universe today has much more matter than antimatter, and if gravity is so much weaker than other forces because it is leaking into other dimensions.
Stride forward in three frontiers
Physicists are also hopeful for the prospects of discovering a class of particles called supersymmetric partners. The theory that predicts their existence is called supersymmetry. It builds on some of the conclusions of the Standard Model, and offers predictions that plug its holes as well with such mathematical elegance that it has many of the world’s leading physicists enamored. These predictions involve the existence of new particles called partners.
In a neat infographic by Elizabeth Gibney in Nature, she explains that the partner that will be easiest to detect will be the ‘stop squark’ as it is the lightest and can show itself in lower energy collisions.
In all, the LHC’s new avatar marks a big stride forward not just in the energy frontier but also in the intensity and cosmic frontiers. With its ability to produce and track more collisions per second as well as chart the least explored territories of the ancient cosmos, it’d be foolish to think this gigantic machine’s domain is confined to particle physics and couldn’t extend to fuel cells, medical diagnostics or achieving systems-reliability in IT.
Here’s a fitting video released by CERN to mark this momentous occasion in the history of high-energy physics.
Featured image: A view of the LHC. Credit: CERN
Update: After engineers spotted a short-circuit glitch in a cooled part of the LHC on March 21, its restart was postponed from March 23 by a few weeks. However, CERN has assured that its a fully understood problem and that it won’t detract from the experiment’s goals for the year.
Through an extraordinary routine, the most powerful machine built by humankind is slowly but surely gearing up for its relaunch in March 2015. The Large Hadron Collider (LHC), straddling the national borders of France and Switzerland, will reawaken after two years of upgrades and fixes to smash protons at nearly twice the energy it did during its first run that ended in March 2012. Here are 10 things to look out for: five upgrades and five possible exciting discoveries.
Technical advancements
Higher collision energy – In its previous run, each beam of protons destined for collision with other beams was accelerated to 3.5-4 TeV. By May 2015, each beam will be accelerated to 6.5-7 TeV. By doubling the collision energy, scientists hope to be able to observe higher-energy phenomena, such as heavier, more elusive particles.
Higher collision frequency – Each beam has bunches of protons that are collided with other oncoming bunches at a fixed frequency. During the previous run, this frequency was once every 50 nanoseconds. In the new run, this will be doubled to once every 25 nanoseconds. With more collisions happening per unit time, rarer phenomena will happen more frequently and become easier to spot.
Higher instantaneous luminosity – This is the detectability of particles per second. It will be increased by 10 times, to 1 × 1034 per cm2 per second. By 2022, engineers will aim to increase it to 7.73 × 1034 per cm2 per second.
New pixel sensors – An extra layer of pixel sensors, to handle the higher luminosity regime, will be added around the beam pipe within the ATLAS and CMS detectors. While the CMS was built with higher luminosities in mind, ATLAS wasn’t, and its pixel sensors are expected to wear out within a year. As an intermediate solution, a temporary layer of sensors will be added to last until 2018.
New neutron shields – Because of the doubled collision energy and frequency, instruments could be damaged by high-energy neutrons flying out of the beam pipe. To prevent this, advanced neutron shields will be screwed on around the pipe.
Research advancements
Dark matter – The LHC is adept at finding particles both fundamental and composite previously unseen before. One area of physics desperately looking for a particle of its own is dark matter. It’s only natural for both quests to converge at the collider. A leader candidate particle for dark matter is the WIMP: weakly-interacting massive particle. If the LHC finds it, or finds something like it, it could be the next big thing after the Higgs boson, perhaps bigger.
Dark energy – The universe is expanding at an accelerating pace. There is a uniform field of energy pervading it throughout that is causing this expansion, called the dark energy field. The source of dark energy’s potential is the vacuum of space, where extremely short-lived particles continuously pop in and out of existence. But to drive the expansion of the entire universe, the vacuum’s potential should be 10120 times what observations show it to be. At the LHC, the study of fundamental particles could drive better understanding of what the vacuum actually holds and where dark energy’s potential comes from.
Supersymmetry – The Standard Model of particle physics defines humankind’s understanding of the behavior of all known fundamental particles. However, some of their properties are puzzling. For example, some natural forces are too strong for no known reason; some particles are too light. For this, physicists have a theory of particulate interactions called supersymmetry, SUSY for short. And SUSY predicts the existence of some particles that don’t exist in the Model yet, called supersymmetric partners. These are heavy particles that could show themselves in the LHC’s new higher-energy regime. Like with the dark matter WIMPs, finding a SUSY particle could by a Nobel Prize-winner.
Higgs boson – One particle that’s too light in the Standard Model is the Higgs boson. As a result, physicists think it might not be the only Higgs boson out there. Perhaps there are others with the same properties but weigh lesser or more.
Antimatter reactions – Among the class of particles called mesons, one – designated B0 – holds the clue to answering a question that has astrophysicists stymied for decades: Why does the universe have more matter than antimatter if, when it first came into existence, there were equal amounts of both? An older result from the LHC shows the B0 meson decays into more matter particles than antimatter ones. Probing further about why this is so will be another prominent quest of the LHC’s.
Bonus: Extra dimensions – Many alternate theories of fundamental particles require the existence of extra dimensions. The way to look for them is to create extremely high energies and then look for particles that might pop into one of the three dimensions we occupy from another that we don’t.
The CERN Council has elected a new Director-General to succeed the incumbent Rolf-Dieter Heuer. Fabiola Gianotti, who served as the ATLAS collaboration’s spokesperson from 2009 to 2013 – a period that included the discovery of the long-sought Higgs boson by the ATLAS and CMS experiments – will be the first woman to hold the position. Her mandate begins from January 2016.
A CERN press release announcing the appointment said the “Council converged rapidly in favor of Dr. Gianotti”, implying it was a quick and unanimous decision.
The Large Hadron Collider (LHC), the mammoth particle smasher that produces the collisions that ATLAS, CMS and two other similar collaborations study, is set to restart in January 2015 after a series of upgrades to increase its energy and luminosity. And so Dr. Gianotti’s term will coincide with a distinct phase of science, this one eager for evidence to help answer deeper questions in particle physics – such as the Higgs boson’s mass, the strong force’s strength and dark matter.
Dr. Gianotti will succeed 15 men who, as Director Generals, have been responsible for not simply coordinating the scientific efforts stemming from CERN but also guiding research priorities and practices. They have effectively set the various agendas that the world’s preeminent nuclear physics lab has chosen to pursue since its establishment in 1945.
In fact, the title of ‘spokesperson’, which Dr. Gianotti held for the ATLAS collaboration for four years until 2013, is itself deceptively uncomplicated. The spokesperson not only speaks for the collaboration but is also the effective project manager who plays an important role when decisions are made about what measurements to focus on and what questions to answer. When on July 4, 2012, the discovery of a Higgs-boson-like particle was announced, results from the ATLAS particle-detector – and therefore Dr. Gianotti’s affable leadership – were instrumental in getting that far, and in getting Peter Higgs and Francois Englert their 2013 Nobel Prize in physics.
Earlier this year, she had likened her job to “a great scientific adventure”, and but “also a great human adventure”, to CNN. To guide the aspirations and creativity of 3,000 engineers and physicists without attenuation1 of productivity or will must have indeed been so.
That she will be the first woman to become the DG of CERN can’t escape attention either, especially at a time when women’s participation in STEM research seems to be on the decline and sexism in science is being recognized as a prevalent issue. Dr. Gianotti will no doubt make a strong role model for a field that is only 25% women. There will also be much to learn from her past, from the time she chose to become a physicist after learning about Albert Einstein’s idea of quantum mechanics to explain the photoelectric effect. She joined CERN while working toward her PhD from the University of Milan. She was 25, it was 1987 and the W/Z bosons had just been discovered at the facility’s UA1 and UA2 collaborations. Dr. Gianotti would join the latter.
It was an exciting time to be a physicist as well as exacting. Planning for the LHC would begin in that decade and launch one of the world’s largest scientific collaborations with it. The success of a scientist would start to demand not just research excellence but also a flair for public relations, bureaucratic diplomacy and the acuity necessary to manage public funds in the billions from different countries. Dr. Gianotti would go on to wear all these hats even as she started work in calorimetry at the LHC in 1990, on the ATLAS detector in 1992, and on the search for supersymmetric (‘new physics’) particles in 1996.
Her admiration for the humanities has been known to play its part in shaping her thoughts about the universe at its most granular. She has a professional music diploma from the Milan Conservatory and often unwinds at the end of a long day with a session on the piano. Her fifth-floor home in Geneva sometimes affords her a view of Mont Blanc, and she often enjoys long walks in the mountains. In the same interview, given to Financial Times in 2013, she adds,
There are many links between physics and art. For me, physics and nature have very nice foundations from an aesthetic point of view, and at the same time art is based on physics and mathematical principle. If you build a nice building, you have to build it with some criteria because otherwise it collapses.2
Her success in leading the ATLAS collaboration, and becoming the veritable face of the hunt for the Higgs boson, have catapulted her to being the next DG of CERN. At the same time, it must feel reassuring3 that as physicists embark on a new era of research that requires just as much ingenuity in formulating new ideas as in testing them, an era “where logic based on past theories does not guide us”4, Fabiola Gianotti’s research excellence, administrative astuteness and creative intuition is now there to guide them.
Good luck, Dr. Gianotti!
1Recommended read: Who really found the Higgs boson? The real genius in the Nobel Prize-winning discovery is not who you think it is.Nautilus, Issue 18.
2I must mention that it’s weird that someone which such strong aesthetic foundations used Comic Sans MS as the font of choice for her presentation at the CERN seminar in 2012 that announced the discovery of a Higgs-like-boson. It was probably the beginning of Comic Sans’s comeback.
3Though I am no physicist.
4In the words of Academy Award-winning film editor Walter S. Murch.
In a paper published in Physical Review Letters on July 17, 2014, a team of American researchers reported the most precisely measured value yet of the mass of the top quark, the heaviest fundamental particle. Its mass is so high that can exist only in very high energy environments – such as inside powerful particle colliders or in the very-early universe – and not anywhere else.
For this, the American team’s efforts to measure its mass come across as needlessly painstaking. However, there’s an important reason to get as close to the exact value as possible.
That reason is 2012’s possibly most famous discovery. It was drinks-all-round for the particle physics community when the Higgs boson was discovered by the ATLAS and CMS experiments on the Large Hadron Collider (LHC). While the elation lasted awhile, there were already serious questions being asked about some of the boson’s properties. For one, it was much lighter than is anticipated by some promising areas of theoretical particle physics. Proponents of an idea called naturalness pegged it to be 19 orders of magnitude higher!
Because the Higgs boson is the particulate residue of an omnipresent energy field called the Higgs field, the boson’s mass has implications for how the universe should be. Being much lighter, physicists couldn’t explain why the boson didn’t predicate a universe the size of a football – while their calculations did.
In the second week of September 2014, Stephen Hawking said the Higgs boson will cause the end of the universe as we know it. Because it was Hawking who said and because his statement contained the clause “end of the universe”, the media hype was ridiculous yet to be expected. What he actually meant was that the ‘unnatural’ Higgs mass had placed the universe in a difficult position.
The universe would ideally love to be in its lowest energy state, like you do when you’ve just collapsed into a beanbag with beer, popcorn and Netflix. However, the mass of the Higgs has trapped it on a chair instead. While the universe would still like to be in the lower-energy beanbag, it’s reluctant to get up from the higher-energy yet still comfortable chair.
Someday, according to Hawking, the universe might increase in energy (get out of the chair) and then collapsed into its lowest energy state (the beanbag). And that day is trillions of years away.
What does the mass of the top quark have to do with all this? Quite a bit, it turns out. Fundamental particles like the top quark possess their mass in the form of potential energy. They acquire this energy when they move through the Higgs field, which is spread throughout the universe. Some particles acquire more energy than others. How much energy is acquired depends on two parameters: the strength of the Higgs field (which is constant), and the particle’s Higgs charge.
The Higgs charge determines how strongly a particle engages with the Higgs field. It’s the highest for the top quark, which is why it’s also the heaviest fundamental particle. More relevant for our discussion, this unique connection between the top quark and the Higgs boson is also what makes the top quark an important focus of studies.
Getting the mass of the top quark just right is important to better determining its Higgs charge, ergo the extent of its coupling with the Higgs boson, ergo better determining the properties of the Higgs boson. Small deviations in the value of the top quark’s mass could spell drastic changes in when or how our universe will switch from the chair to the beanbag.
If it does, all our natural laws would change. Life would become impossible.
The American team that made the measurements of the top quark used values obtained from the D0 experiment on the Tevatron particle collider, at the Fermi National Accelerator Laboratory. The Tevatron was shut in 2011, so their measurements are the collider’s last words on top quark mass: 174.98 ± 0.76 GeV/c2 (the Higgs boson weighs around 126 GeV/c2; a gold atom, considered pretty heavy, weighs around 210 GeV/c2). This is a precision of better than 0.5%, the finest yet. This value is likely to be updated once the LHC restarts early next year.
Two years ago, physicists working on the Large Hadron Collider first announced the discovery of a Higgs boson-like particle, setting the high-energy physics community atwitter. And it was only a couple weeks ago that physicists also announced that the particle was definitely the one predicted by the sturdy Standard Model of particle physics, the theory that governs the Higgs boson’s properties and behavior.
But new results from the ongoing International Conference on High Energy Physics in Valencia, Spain, could add a twist to this plot. Physicists announced that they had evidence – albeit not strong enough – that the Higgs boson was showing signs of disobeying the model.
Members of the ATLAS and CMS collaborations, who work with the detectors of that name, said they had results showing the Higgs boson was decaying into a pair of particles called W bosons at a rate some 20% higher than predicted by the Standard Model. This non-compliance will be a breath of fresh air for physicists who have been faithful to a potent but as yet unobserved theory of new physics called supersymmetry, in short and fondly SUSY.
The W boson mediates the decay of radioactive substances in nature. At sufficiently high energies, such as produced inside the Large Hadron Collider (LHC), these bosons are produced by a multitude of particle interactions. Since their discovery in 1983, they have been widely studied. In these circumstances, announcing signs of SUSY through Higgs decays into WW pairs provides little room for uncertainties.
SUSY predicts that for every fermion, or matter particle, of the Standard Model there is a partner particle that is a boson called a sfermion. Conversely, for every boson, or force particle, of the Standard Model there is a partner particle that is a fermion called a bosino. Physicists who believe SUSY is a plausible theory use these extra particles to solve problems that the Standard Model can’t. One of them is that of dark matter; another is to explain why the Higgs boson weighs much lighter than it should.
Jong Soo Kim et al have described how the anomalous decay rates could be explained using a simple version of SUSY in a pre-print paper uploaded to arXiv on June 27. The paper is playfully titled ‘Stop that ambulance! New physics at the LHC?‘. The ‘Stop’ is a reference to the name of the suppersymmetric partner of the top quark. The authors describe how a combination of supersymmetric particles including the stop boson could explain the new results with only a 1-in-370 chance of error. Even though this means physicists have a confidence of 99.7% in the results, it’s still not high to claim evidence. When the LHC comes online in 2015, physicists will be eager to put these results to the test.
The paper’s title might also refer to a comment that physicist Chris Parkes, spokesperson for the UK participation in the LHCB experiment at the LHC, made to the BBC during the Hadron Collider Physics Symposium in Kyoto, Japan, in November 2012. Results had been announced of the B_s meson decaying into lighter particles at a rate predicted exactly by the Standard Model, nudging SUSY further toward impossibility. Parkes had said, “Supersymmetry may not be dead but these latest results have certainly put it into hospital.”
CERN has announced the restart schedule of its flagship science “project”, the Large Hadron Collider, that will see the giant machine return online in early 2015. I’d written about the upgrades that could be expected shortly before it shut down in 2012. They range from new pixel sensors and safety systems to facilities that will double the collider’s energy and the detectors’ eyes for tracking collisions. Here’s a little timeline I made with Timeline.js, check it out.
(It’s at times like this that I really wish WP.com would let bloggers embed iframes in posts.)
