Quantum Diaries

After six hours of presentations dedicated to the search for the Higgs boson at the Moriond conference, here is a summary of the many new results shown today. Both the CMS and ATLAS experiments presented their latest updates, and no matter the angle studied, the new boson is still perfectly compatible with being a Higgs boson. More will be presented next week, once further checks are completed.

The experiments are now trying to establish not only how the new boson decays but also how it is produced. This will eventually help determine if the new boson is really a Higgs boson, either the one prescribed by the Brout-Englert-Higgs mechanism or one associated with supersymmetry, or even not a Higgs boson at all. To answer this question, both teams measured several of the new boson properties, quantities like the signal strength in various production modes, the different decay channels as well as its mass, spin and parity.

Only two decay channels, namely when the boson decays into two photons or four leptons, are used to measure its mass but for all channels, one can measure the signal strength (how many events are found compared to what the Standard Model predicts) and the spin.

An unambiguous signal obtained by the CMS collaboration in the search for a Higgs boson decaying into two Z bosons, each one decaying in turns into two leptons. This is the so-called four-lepton channel. We can see the data (black dots) matching the simulation of a Higgs boson shown by the red line.

The experiments had already checked that the new boson can decay into a pair of other bosons, namely W, Z ou photons, but it had not been established for fermions, the particles of matter like quarks and leptons. This is now a done thing since CMS observes decays into two tau leptons after analyzing the whole data sample. This remains to be proven for b quarks, which might have to wait until more data become available given the high background in this channel. Across the Atlantic though, the Tevatron experiment reported today seeing it at the 3 sigma level, i.e. three times stronger than possible statistical fluctuations.

Other novelty: ATLAS presented the first limit on possible decays of the new boson into invisible particles such as dark matter. This is not expected to happen in the framework of the Standard Model and indeed, with a limit placed at 68% of the time, it is compatible with the model.

The latest signal strength and mass measurements are shown in bold types in the table below along with the older results from last December.

CMS observes a number of events slightly inferior to the expected value in the four-lepton and WW channels while ATLAS reported small excesses for the number of events observed when the new boson decayed either in two photons or four leptons. This is still statistically too weak to draw any conclusion except to notice that all values are still compatible with the Standard Model predictions, all deviations being at most 2.3sigma for ATLAS.

The signal strength for different decay channels as seen by CMS (left plot) and ATLAS (right plot). A value of 1.0 is expected if everything behaves as prescribed by the Standard Model for a Higgs boson.

It will be particularly interesting to see what CMS obtains in the two-photon channel in their next update. If any deviation gets confirmed, it will draw a lot of attention from theorists due the possible huge consequences. A significant deviation with respect to the theoretical predictions would reveal a flaw in the model and help zoom on the right solution.

It is a well-known fact that the current theory, the Standard Model, has its limits. Everyone agrees that there should be a more encompassing theory to describe phenomena like the existence of dark matter, something the Standard Model fails to explain. But what is this new theory is the big question. All attempts so far have failed to find a crack in the Standard Model. Hard to improve on an impressive  theory that can make predictions accurate up to the tenth decimal.

New mass measurements were also presented today.  No anomalies here either. Last December, with only a third out of the 2012 data sample (21fb^-1) analysed, ATLAS had obtained two different mass values for the new boson when measuring it using two different decay channels. Although an impressive series of crosschecks were performed, no experimental mistake was uncovered. The difference was ascribed to a statistical fluctuation. Today, after analyzing the whole data set, the difference is getting smaller, but so are the uncertainty margins. Nevertheless, this is probably a non-issue.

Finally, a few new spin and parity measurements were shown today, such that both experiments observe that the new boson is more compatible with a spin-parity of 0+ as expected for a Higgs boson than with any other spin-parity hypotheses. This is reinforcing the hypothesis that we are indeed dealing with a type of Higgs boson.

CMS checks to see if the new particle is more likely to have a spin-parity of 0+ (in yellow) as expected for a Higgs boson than other hypotheses (all shown in blue). The red arrow shows the value obtained for the new boson. The compatibility with each hypothesis is measured by the amount of the curve lying to the right of the arrow. There is always more yellow remaining than blue, meaning in all cases, the new boson is more likely to have spin 0+ than any other values.

While we are waiting for new results, some of which will be announced next week, you can entertain yourself by watching an animation (or another)recreating how the new boson signal appeared in ATLAS data over time. Meanwhile, as the information is trickling in, the identity of the new boson is slowly being revealed.

Pauline Gagnon

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Monday morning, at the Moriond conference, the most expected talk in the dark matter session contained unfortunately no results. Although the AMS collaboration was supposed to reveal their very first measurements, Bruna Bertucci could only present apologies to an eager audience since the approval process had not been completed in time for the conference,

The AMS or Alpha Magnetic Spectrometer is a particle detector that was installed on the International Space Station in May 2011 and has been collecting data ever since. The scientific community is now eagerly waiting to hear about their results, in the hope of getting some clues as to what makes up 24% of all content of the Universe, namely what are the mysterious particles that form dark matter.

AMS is due to release data that will compare the flux of positrons in outer space with the flux of electrons. Positrons are the antimatter counterpart of electrons. The interest all stems from the fact that a few years ago, the PAMELA collaboration observed a larger positron flux at high energy than expected. It is relatively easy to think of various sources of electrons since we live in a world made of matter. But what could be a source for antimatter? One possible  explanation is to suppose that dark matter particles are annihilating into pairs of electrons and positrons, and hence providing a source of positrons.

Another group operating a satellite-born experiment, the FERMI-LAT collaboration partially confirmed that observation but only AMS has all the capabilities to really cross-check the PAMELA results. We will have to be a bit more patient until the AMS collaboration publishes with its first results.

The increase in the positron flux with respect to the electron flux as seen by various experiments. The AMS data should bring a definitive confirmation of the excess observed at high energy.

Meanwhile, the FERMI group has work on its hands as explained by Gabrijela Zaharijas since a theorist, Christoph Weniger, analysing data collected by FERMI, detected a signal in the form of a sharp spectral line at 130 GeV – gamma rays of a specific energy – coming from a region in the galactic center.  His approach was to look in areas of the galaxy where he expected to find the most dark matter and fewest sources of gamma rays of known origin. He studied five such locations in the center of our galaxy where dark matter is known to be more concentrated. For three of these locations, he found events in excess of the known sources of gamma rays, i.e more signal than background. The signal was also very strong, four times stronger than possible statistical fluctuations of the background level, that is 4.4 sigma.

The excess of events found by Christoph Weniger in FERMI data seen above the background described by a power law spectrum.

The FERMI collaboration has since improved the data calibration and modeling of energy dispersion, which should have led to an increase in the signal strength. On the contrary, they found the signal got fainter, making them doubt is was a real effect. In fact, while checking a region containing only background (the Earth atmosphere where lots of gamma rays are produced by incoming cosmic rays), they detected a similar “signal”, although fainter at 2.3 sigma. This is not quite enough to explain the anomaly detected in the galactic center but seems to indicate some instrumental error. Further investigations are underway.

We should soon get to the bottom of this story since a new telescope, HESS-2 in Namibia will start observing the galactic center region this month. In less than 50 hours in good operating conditions, they should be able to accumulate enough data to confirm or contradict the presence of this 130 GeV signal.

Will we soon have some hints on the mysterious nature of dark matter? It is well worth a bit more patience in the hope to learn more soon.

Pauline Gagnon

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No! But some surprises might come after the Moriond conference, once theorists have time to combine and interpret the numerous improved results and newly designed analyses that will be presented over the next two weeks. New results will come not only from the Higgs boson searches but also from a plethora of new measurements. This is exactly what theorists need to put the Standard Model to the most stringent tests and find the way to a more encompassing theory. High precision measurements such as those presented by LHCb last year have a huge impact in removing some of the leeway in theoretical models.

The “Rencontres de Moriond” is the first major physics conference of the year. It will start on March 2 at an Italian ski resort. Traditionally, this is where most High Energy physics experiments present their latest results but this year, the conference might come too soon after data-taking stopped at the Large Hadron Collider (LHC), not giving enough time to the experiments to produce new results on all topics. The next updates will be prepared for the Large Hadron Collider Physics Conference in May, the European Physical Society Meeting in July and possibly for the CERN Council meeting in December to name a few.

Many people were hoping CMS and ATLAS, the two large multi-purpose experiments operating at the LHC at CERN, would finally announce that the boson discovered last year is really a Higgs boson. Unfortunately, it is still too early to say. Nevertheless, both experiments can be expected to show interesting updates on the new boson mass measurement, decay rates and spin, all of which will provide a clearer picture.

What will be of particular interest will be to see if the small deviations with respect to the Standard Model expectations observed last year by both experiments in various decay rates are going away or increasing. Both ATLAS and CMS obtained sometimes more, sometimes fewer events containing the new boson than what is expected from the Standard Model although these observations are all still consistent with the Standard Model. An excess of events in the two-photon decay rate could indicate that new particles contribute to the process, a possibility that many theorists hope would reveal the presence of supersymmetry.

A summary of all mass and decay rate measurements from ATLAS and CMS as of last December. The signal strength should be one for a Standard Model Higgs boson. The error margins are still too large everywhere to draw any conclusion.

New results will also be presented on searches for new particles such as heavier bosons or supersymmetric particles. Of course, if only one experiment observes a small deviation, the excitement will be limited until the other experiment responds. If both experiments see similar hints, it could get interesting.

Many physics topics will be covered and theorists will provide their latest models and interpretations.  So stay tune over the next two weeks, as I will be reporting all the highlights from this conference as they unfold.

Pauline Gagnon

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This time, it’s true. The Large Hadron Collider (LHC), will stop making collisions for about two years tomorrow, 14 February at 6:00 am, after postponing the date by three days to give the heavy ion community enough data. Shortly after this extension was announced,  dozens of volunteers signed up to staff the experiments and accelerators control rooms.

I was one of them, happy for the opportunity to say goodbye to the ATLAS control room where I have had lots of great moments on shifts. The ambiance is always special: this is where you meet or get to know better many collaborators who normally work outside  CERN, and often, on different continents. Many come to CERN to take their share of the operation load and participate in the data taking.

So here we are, nine people staffing the control room from as many different nationalities plus a few experts on call, coming and going during the one before last shift.

Stephanie Zimmermann, one of the two people in charge of running the detector, confides she would welcome a short break, a few months would be great. But two years will be long. But Anna Sfyrla, one of the trigger experts who has to attend the run meetings six days a week says she won’t miss those meetings and is looking forward to have a breather. Nevertheless, she will miss the fun of the control room.

One other obvious person to ask is Kerstin Lantzsch. Easy to catch her since she practically lives in the control room. She is run coordinator for the pixels, which means she is responsible for the sub-detector placed closest to the beam, the one most likely to be damaged when beams are injected inside the accelerator. She has been coming to the control room every time for the past seven months when the LHC brings fresh beams into collisions, which means a few times a day. No wonder she is looking forward to having a more normal life but nevertheless, she knows she will miss the action.

Similar thoughts for Giovanna Lehman who is one of the experts working on the central data acquisition system. This entails answering all sorts of tough questions at all times of day or night when there is a hiccup in the system that the shifter cannot handle. She looks forward to sleeping more regular hours and getting involved in the many improvements they are planning for this system.

Then some only come occasionally to the control room to take a few shifts. People like two students, Aungshuman Zaman from Stony Brook in New York and Nedaa Asbah from Université de Montréal. They will both have plenty to do. Aungshuman has his work cut out on a detector upgrade and Nedaa will write her masters thesis.

Cyril Bécot, a student at Orsay near Paris will use the time afforded by the long shutdown to complete his PhD thesis. Since he studies Higgs boson decays to two photons, he had to work under great pressure over the last six months given the high profile of the Higgs search. Far from being sad to see the beams go, he looks forward to taking his time to improve his analysis and go more in depth instead of constantly racing against the clock.

Same thing for Anna Lipniacka, a professor at Bergen University in Norway. Sure, she feels a bit sad, but just like Cyril, hopes it will give us time to think a bit more deeply on how to look into the data and develop new analysis techniques.

Mansoora Shamim agrees. She is a post-doc at University of Oregon and admits being moderately sad for the same reasons. She is happy to have more time to work on her analysis, searching for black holes. But she will miss the ambiance of the control room.

And so will I, even if it means more chances for greater discoveries later on. In the meantime, thanks to fresh snow outside, the champagne is cooling off as we plan to toast the LHC at the end of our shift.

Pauline Gagnon

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The Year of the Dragon (2012) came with a roar: a wonderful discovery and a greater understanding of how matter works. What might 2013, the Year of the Serpent, have in store for CERN? The serpent could very well represent the long and winding road of the many new upgrades ahead.

On Monday 11 February at 6 am Geneva time, the Large Hadron Collider (LHC) will stop producing collisions, marking the start of a major overhaul for all accelerators at CERN. This will be the first in a series of three long-shutdowns to allow a complete refurbishing of the main accelerator, the LHC. The goal is to be able to increase its energy from the actual 8 TeV to 13 or even 14 TeV. This means an increased reach for new particles.

This is not just to play a game of who will find the biggest particle, but rather an attempt at finding the passageway to new theories. Since energy (E) and mass (m) are two forms of the same essence, as stated by the famous equation E = mc², where c² acts as a conversion factor between the two, increasing the accelerator energy will give us the possibility to create particles more massive than we have ever been able to produce before.  It will also enhance the production rate of known particles – like the newly discovered boson – to better study them.

Finding new particles will tell us what else is out there. As it stands, the current theoretical model – the Standard Model of particle physics – only describes the tip of the iceberg, namely the matter that we are made of. But we already know that dark matter exists, even though scientists have absolutely no clue what kind of particles make up this strange type of matter. All we know is that it accounts for about 26% of matter in the Universe whereas regular matter is only worth 4%. The rest, that is 70% of the content of the Universe, is a form of energy called “dark energy”, which is even more mysterious and is responsible for the accelerated expansion of the Universe.

Far from being a time to rest, this long shutdown will be an intense period for everyone at the lab. Accelerator physicists, engineers and technicians will be working on all the needed upgrades and consolidations. For the LHC alone, this entails mostly opening up the interconnections between each of the machine’s 1,695 magnet cryostats and consolidating all of the 10,170 electrical junctions carrying current to these dipole and quadrupole magnets. And it goes without saying that 27 km of high technology don’t mind a bit of maintenance once in a while.

It is foreseen that the accelerator complex will come back to life in 2014, with the LHC becoming operational again in 2015.

Just about all experiments at CERN, not only those operating at the LHC but also the ones taking place at all the smaller accelerators, will be taking the opportunity for consolidations and upgrades.

Experimentalists will also take time to finalise their analyses, often after fully reprocessing all of the accumulated data with the latest calibration and reconstruction algorithms, ensuring that new results will keep coming out at a steady pace.

Last but not least, CERN will open its doors to the general public on Sunday 29 September. Here is your chance to see what keeps thousands of scientists very busy. Mark your calendar: this will be a day to remember.

Pauline Gagnon

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Gluinos and Higgsinos are some of the many undiscovered particles we may find at the Large Hadron Collider (LHC) if a theory called supersymmetry, also known as SUSY, turns out to be true. This theory is built on the Standard Model, the current theoretical model of particle physics.

The Standard Model relies on the Higgs boson to hold true. But even with this boson, physicists know that this model cannot be the final answer as it has a few shortcomings. For example, it fails to provide an explanation for dark matter or why the masses of fundamental particles such as electrons and muons are so different. This theory of supersymmetry is one of the most popular and most promising ways to extend the Standard Model, but it has yet to manifest itself.

SUSY is very popular since it brings lots of harmony in the world of sub-atomic particles. In the Standard Model, there are two types of particles: fermions and bosons. The fermions include quarks and leptons and are the building blocks of matter.  These particles have “spin” values of ½. The force carriers are bosons, the other family of particles. They have integer values of spin, that is, 0 or 1.

Supersymmetry would blend fermions and bosons together by associating partners to each particle: a fermion would be paired with a boson, and vice-versa. For example, each quark would come with a “squark,” the name given to the supersymmetric partners of quarks. The squarks would be bosons rather than fermions and would carry spin 0. The same thing goes for leptons. Likewise, the known bosons (gluons, Higgs, W, Z and photons) would come with fermion superpartners with half spin values. These would be the gluinos, Higgsinos, winos, zinos and photinos. A mixture of the force carrier superpartners (all except the gluinos) gives charginos and neutralinos, the latter being particles that would be the perfect candidates for dark matter.

But it is difficult to work with SUSY (nothing personal of course!). Even in its minimal version called the Minimal Supersymmetric Model or MSSM, the theory comes with 105 free parameters. This means each parameter, like the masses of all these particles, is free to take any value it likes.

Think of a parameter as a dimension. Say we need a search party to locate hikers lost anywhere in the Alps. We would have to check every 10 m or so within that huge area. So even when trying to select one single point in a two-dimensional space, the task is daunting. The exact location can be any of a multitude of points within a huge two-dimensional space. Now try to imagine the same situation with a 105-dimensional object! It becomes hopeless.

Adding some reasonable constraints helps such as saying the location can only be on firm ground and not in a lake. This is why theorists have been trying to limit the range of each parameter to reduce the space that would need to be searched to find all these new SUSY particles. A subset of the MSSM model called the Constrained MSSM (CMSSM) model was built leaving only a handful of free parameters. This was achieved by picking somehow arbitrary values for some of these parameters, often guided by taste or guesses for lack of experimental constraints. This is a bit as if in our search for the lost hikers, we decided to ignore Switzerland because we did not like cheese, instead of taking into account the hikers’ interests or habits. But despite all its shortcomings, this model is still largely used.

Since every new theory can only be valid if it can reproduce all known observations, the phenomenological MSSM or pMSSM model was developed using all sorts of measurements done over the past decades in particle physics to constrain the original set of 105 free parameters of the MSSM. With these experimental assumptions, the pMSSM model is reduced to 19 free parameters.  There is progress.

Three theorists Alex Arbey, Abdelhak Djouadi and Nazila Mahmoudi, and one experimentalist, Marco Battaglia, form one of the teams who are now going one step further. They are using all available experimental information to see which values of each parameter are still allowed for the different models. This technique requires lots of computing power to test each point of the multi-dimensional space but in the end, one can really see where supersymmetry can still hide.

This method had already revealed that very constrained and specific versions of SUSY like the CMSSM model were getting squeezed into small corners. On the other hand, the pMSSM model has been reduced considerably but still has plenty of space available. Taking into account the recent experimental constraints from the LHC and astrophysics results on dark matter searches, including the recent value obtained by LHCb on Bs mesons decaying into two muons, about 10% of the hundred million possibilities these scientists studied remain valid. And when the measurements on the Higgs-like boson mass and decay rates are taken into account, only 2.5% of the original scenarios in their study survive.

Thanks to this technique of weaving together experimental facts and theoretical knowledge, this team of scientists has been able to reduce a near infinite number of possibilities to only a few percent of what it originally was, making it easier to narrow down the search. Nevertheless, this still leaves plenty of space where one form or another of supersymmetry can exist. We might not be lucky enough to discover SUSY this year but will surely have a good chance at it once the LHC comes back to full power in 2015 after extensive work aimed at increasing the capacity of the accelerator in the coming two years.

Meanwhile, SUSY is still alive and might be kicking around in one point of its now much more confined 105-dimensional space.

Pauline Gagnon

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Gautier Crépin-Leblond, a 13-and-a-half year old high school student who came to CERN as an intern to fulfil his school internship requirement. He is studying in a French high school, Sainte-Ursule de Riedisheim. He has three days to discover what is happening at CERN. As I had the chance to supervise him for part of the afternoon, we thought of writing a blog together. His interest and determination were striking, so we took him on-board, even though CERN cannot unfortunately accommodate all requests for internships given the sheer number of applications.

What do these internships bring us, both as intern and supervisor? Gautier says, “Lots of information, both for my future career and at a scientific level”. As for myself, this internship provided the opportunity to rediscover CERN through fresh eyes and meet a highly motivated young person interested in scientific matters.

Gautier wants to be an astrophysicist and work at CERN as a theorist. Yesterday, he joined a tour organized for a Swiss writer looking for inspiration. He went first to the ATLAS control room (no problem with a visit conducted in German since he is Alsacian) then visited the Large Hadron Collider (LHC) control room. “I was surprised to see so few women there, only two for about 15 men. In school, they told us women were more qualified than men”, said Gautier. Nice to see that even young boys do notice nowadays but in fact, one usually sees more women there.

Next stop: the AD hall (Antiproton Decelerator) where several experimental teams are currently trying to produce antimatter. Gautier told me his impressions, “Rather rustic, like hay-wire setups, with aluminium foil everywhere and kilometres of wires. Very different from all the other places I visited”. Then he passed by the large hall where magnets are being tested. “Very impressive but also very noisy”, he said.

This morning, he visited the Globe of Science and Innovation, where a public exhibition is held on the world of particles. “Very nice, no long panels to read but instead they had interactive displays”.

In the afternoon, we talked about dark matter and how to detect Higgs bosons if they were to decay into a pair of particles invisible to our detectors.

Gautier got hooked to astrophysics ever since he received a book on astronomy when he was in kindergarten.  One thing leading to the next, we talked about the moon and the incredible stock of helium 3 it contains. A quick search on Wikipedia revealed that a ton of helium 3 would suffice to produce the annual Earth’s needs in energy through nuclear fusion, a process void of nuclear wastes. He was stunned to hear that my colleague, Marcelle Rey-Campagnolle had manipulated and analysed a few grams of lunar stones in Orsay brought back by the Apollo mission (me too!)

In short, we both learned new things, about each other and the moon while staying grounded, although Gautier was slightly floating, his eyes and head full of all he had seen so far.

Education is one of the missions of CERN, which is why young people of all ages come here to learn about the research being conducted in the laboratory and the various techniques used. CERN welcomes about 300 young people like Gautier every year coming from high schools for discovery internships. During the summer, more than 250 university-level students participate in the Summer Student Programme for two to three months where they attend lectures in the morning and contribute to the research programme in the afternoon.

Throughout the year, CERN also opens its doors to some 170 technical and doctoral students. This comes on top of the 3000 doctoral students working on the various LHC experiments and sent here by their home universities from 69 different countries. High school teachers also benefit from a special training program. Of course, one can come and work at CERN.

For those just curious to visit CERN, guided visits are organised for groups and individuals. In 2013, CERN will also host an Open Day during the weekend of September 28-29 where the public will be able to visit the whole site. Interested? Then mark your calendar!

Gautier Crépin-Leblond and Pauline Gagnon.

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