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CERN | Geneva | Switzerland

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Grey matter confronted to dark matter

Thursday, April 4th, 2013

After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron.  To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars.  How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.

Addendum:

Here are two plots to show how different the positron fraction spectrum (i.e. the curve showing the fraction of positrons as a function of energy) would differ at high energy (the rightmost part of the plot) if the positrons come from the sum of all pulsars around or if it comes from dark matter annihilation. Note they are not on the same scale and difficult to compare, but they still give some idea. It will be easier once theorists update their plots with the new AMS-02 data points on them and of course, once AMS-02 releases further information at high energy.

This is one theoretical prediction of what the positron fraction spectrum should look like if the positrons come from dark matter particles like neutralinos (represented by the symbol χ). Two curves are shown, depending on the hypothetical mass of the neutralino (mχ) at 400 GeV or 800 GeV. In each case, the maximum energy the positrons can get is roughly equal to the the mass of the neutralino, such that the curve ends close to the neutralino mass. Note the logarithmic scale on both axes.

Here is the expected spectrum if the positrons come from the sum of all pulsars. Three hypotheses were shown but only the middle one seemed to fit the PAMELA experimental results. The important feature is that this curve comes down smoothly, and not sharply at neutralino mass as with the dark matter hypothesis. Again, this curve only represents one theoretical prediction as done by Dan Hooper and his colleagues. The data point in red are from the PAMELA experiment and stop around 100 GeV. The hope is that AMS-02 will be able to provide accurate measurements at higher energies, up to several hundred GeV.

Pauline Gagnon

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How to attract, hire and retain more women in science

Wednesday, April 3rd, 2013

On March 27, three young women from CERN participated via a video link in the UN Economic and Social Council “Youth Forum”, delivering a series of recommendations to improve the situation for women in science. During this all-day event held in New York, young people were invited to contribute ideas on how to improve our world, no less.

ECOSOC is still seeking input from young people ahead of its 1 July meeting where governments will meet in Geneva to address the important topics of Science, Technology, Innovation and Culture. They will adopt a Ministerial Declaration for scaling up actions in this field.

At the start of the meeting, the United Nations Secretary General, Mr Ban Ki-moon asked the young audience if the UN was doing enough for youth. A resounding “No” came back from the audience but he got the opposite answer when he said “Could the UN do more for the world’s youth?”

This ECOSOC meeting provided CERN with its first opportunity to engage directly with a UN organization since it was granted Observer status at the United Nations General Assembly last December.

Three graduate students currently based at CERN were speaking during the “Women in Science” session on behalf of a larger group of young women scientists who had gathered to draft a series of recommendations aiming at improving the situation of women in science.

Kate Pachal, a young Canadian woman currently enrolled in a PhD program at Oxford, discussed what could be done to attract more women into science. Her three points were:

 

  • Fight gender stereotypes at all levels. Improve the representation of women in textbooks, including in the phrasing of problems; Use gender-neutral language when referring to scientists; Increase the visibility of women scientists in the general culture by providing more female contacts for the media.
  • Help young people build a strong “physics identity”: Students who do not feel good at maths or science do not pursue a career in it. Encouragements from peers, teachers and family help young girls believe in their own ability. Classroom activities such as having discussions on cutting-edge physics topics, being encouraged to ask questions or teaching peers all contribute to build a strong  “physics identity”. Having discussions on why fewer women are in science also helps young women see the problem does not come from them but has social roots.
  • Provide role models and mentors for young women. Do it at all stages. Hold career fairs to reinforce girls’ self-esteem and provide a context where they can discuss with other girls facing similar challenges. Provide places where young women can talk with peers and find support.

Sarah seif el Nasr, an Egyptian-Canadian doctoral student at CERN, delivered three recommendations to hire more women in physics and science in general:

  • Implement anonymous job application processes. The applicant’s gender should be hidden during the job application process to avoid gender bias since a study revealed that both men and women discriminate against women. The number of female musicians tripled at five major orchestras once job applicants performed behind a curtain.
  • Implement equitable parental leaves. Both men and women should be given parental leaves and men strongly encouraged to take them. Young women of child-bearing age would then be less likely to be disfavored in hiring if both parents had to share the weight more equally. Shared or split positions would also allow both parents to participate equally in child responsibilities.
  • Add spousal considerations to hiring processes. Institutions should recognize the existence of the dual-career situation and choose to deal with it since half the women with a PhD in physics have a spouse with similar education level (as opposed to only 20% for men). Institutions should take action before beginning a search to provide assistance for spouses and consider split/shared positions. This would help young women find positions without taxing their relationships.

 

Finally, Barbara Millan Mejias, a Venezuelan graduate student at University of Zurich, explained what can be done to retain women in science:

  • Provide mentors for young women starting their careers. The mentor should be different from their boss or supervisor and have proper institutional support. The mentor could for example make sure the young woman progresses properly, that she is given adequate funding and support, that she gets to attend meetings and give talks at various conferences. The mentor should be able to advise the young women on academic and professional issues.

 

  • Have broad discussions about gender issues at large scientific meetings. Men are often unaware of the situation faced by women in science and lack opportunities to discuss this situation, even though they are most often open to it. Men often unconsciously discriminate against women. Education would improve the situation.
  • Hold scientific meetings for women where young women could see how valuable women’s work is, find positive reinforcement, get to talk with peers and get support. This would also provide a place for discussions on issues facing young women as well as opportunities to share experiences and support each other.
  • Implement equitable parental leaves. This point is crucial not only at hiring time but also to retain young women in science.

Let’s hope the voice of these young women will be heard and that laboratories like CERN and universities will make all possible efforts to implement these recommendations.

Pauline Gagnon

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Win your own Higgs boson

Monday, April 1st, 2013

In an unprecedented gesture in the history of particle physics, Sergio Bertolucci, Director of Research, announced this morning that CERN is going to do something unusual: give away fundamental particles.

“Given the interest manifested over the past years by the general public for the Higgs boson search, we felt that we had to give some back as a token of appreciation”, said Dr Bertolucci. “As CERN, we have always believed in sharing the results of our research, and the time has come to make that tangible. This is our way of saying thanks for the incredible level of enthusiasm that has greeted this discovery”. The new particle’s discovery was announced at a special seminar on 4 July last year.

Both the ATLAS and CMS experiments have generously accepted to donate some of their precious Higgs bosons. Particles such as Higgs bosons are created from the energy released in proton-proton collisions in the Large Hadron Collider (LHC). However, Higgs bosons are extremely rare, being created only once out of one million million such collisions.

“We hope the lucky few who will receive a Higgs boson will cherish them as much as we do”, said Dr Bertolucci.

Each boson will come with a complete set of instructions on how to properly care for it. To enter this lottery, please send an e-mail to [email protected]. A Higgs boson will be sent to the ten lucky winners chosen randomly from all requests received within 24 hours of publishing this post.

Pauline Gagnon

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Science, engine of innovation

Friday, March 22nd, 2013

Unlike most businesses and organisations, CERN and its experiments operate on a completely different basis. All the experiments are conducted in a collaborative manner where every one has a lot of liberty in defining her or his role. There is no rigid top-down decision-making process. Each group and each individual has to find a way to contribute, balancing the needs of the experiment with the skills and interests at hand. Such a collaborative model leaves plenty of room for initiatives, creativity and innovation.

Innovation is what we are good at even though we never know in advance what might become useful at some point. Spin-offs are just incidentals to the scientific process. Take the World Wide Web: it was developed at CERN out of the need to provide a communication means for scientists working on different continents. The scientific process forces us to constantly push the limits ever further.

It is impossible to predict what will find some application, but it is easy to bet on scientific research. Science is the engine of innovation. And the business world is taking notice.

Unbeknown to most physicists working on these large collaborations, such collaborative models are now drawing a lot of attention from management and business scholars. So much so that the Strategic Management Society, a non-profit organisation for management scholars and academics, held a special meeting at CERN to take a closer look. They wanted to see how we operate under this strange, seemingly utopian, form of management.

Given the complexity of the tasks we are facing, collaboration is the only way to proceed. No single individual or even team could have designed any of the Large Hadron Collider (LHC) detectors, let alone build them. It took the unbridled creativity of thousands of people to succeed.

Usually in statistical studies, businesses collect data, look for the strongest trends and ignore the “outliers”, that is, the data points sitting far from the average. But neglecting unusual behaviour may lead to missing out on interesting ideas, away from the pack. This is precisely what is catching the attention of the members of the Strategic Management Society who are looking for new ideas from non-textbook organisations.

The meeting brought 300 business and management scholars to CERN on March 21 for a sold-out conference. All of them were treated to a visit of the ATLAS detector, 100 m underground.

I asked a group of participants what drew them to CERN. “Innovation!” said one, explaining that the business world is good at repeating and reproducing known processes but often fails to innovate. Many echoed him while another said he was interested in Technology Transfer. All agreed that the opportunity to visit CERN after all the recent media coverage was an added bonus. As one of the many guides for the day, it was a pleasure to take such keen observers around, before they headed off for day two of the conference, in the more familiar surroundings of Lausanne’s International Institute for Management Development, IMD.

Pauline Gagnon

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It looks very much like we have “a” Higgs boson

Saturday, March 16th, 2013

No more Higgs-like, Higgs-ish or even Higgsy boson. The CMS and ATLAS collaborations, the two large experiments operating at the Large Hadron Collider (LHC) at CERN, have now gathered sufficient evidence to say that the new boson discovered last summer is almost certainly “a” Higgs boson. Note that we are going to call it “a” Higgs boson and not “the” Higgs boson since we still need more data to determine what type of Higgs boson we have found. But all the analysis conducted so far strongly indicates that we are indeed dealing with a type of Higgs boson.

The Standard Model predicts there should be only one Higgs boson and so far, our Higgs boson is compatible with being the Standard Model Higgs boson. But it could still be one of the five types of Higgs bosons postulated by supersymmetry, a theory that would build on the Standard Model and complete it in a way that it would not only be able to explain the world made of matter that we know, but also provide a possible explanation for something still completely unknown called dark matter.

Both ATLAS and CMS checked not only the mass but also the couplings of the new boson. In all cases where the experiments have sensitivity, the couplings are consistent with the Standard Model. But the truth may lie in the tiniest detail. Take for example the signal strength, a quantity that measures how many signal events are found in different decay channels compared with the numbers expected from the Standard Model. The Standard Model boson would then come out with signal strength of one in all decay channels. But if other, yet undiscovered particles exist, then they would provide more options in the ways the Higgs boson could decay, and we should start seeing more signal events or if additional Higgs bosons exist we might see less signal strength in some channels.

Among new results shown this week at the Moriond QCD conference, CMS reported updated results for a Higgs decaying into two photons and ATLAS had an update on the Higgs decaying to a pair of W bosons.  CMS presented their main result and a result from a cross-check analysis using a different analysis approach. The two results, of 0.78±0.27 for the main analysis and 1.11±0.31 for the cross-check, are consistent within uncertainties.  ATLAS measured a signal strength of 1.0±0.3 in the WW channel and, 1.30±0.21 for all channels combined.  These results are so far in reasonable agreement with a value of one predicted by the Standard Model. Values different from one can come from statistical fluctuations as well as from new physics as mentioned earlier. Only more data and more study will allow us to tell.

The latest results presented at Moriond mark an important step forward in the Higgs analysis, but also serve as a reminder that we still have a long way to go. It looks very much as though we have “a” Higgs boson, the question now is what kind?

Pauline Gagnon

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The Standard Model passes with flying colors

Thursday, March 7th, 2013

After hearing about huge quantity and variety of new results all week at the Moriond conference, we were finally treated this morning to the bigger picture. Roman Kogler, an experimentalist speaking on behalf of the Gfitter group (a small group of experimentalists and theorists) showed how the Standard Model gives a coherent picture of particle physics as we know it today.

The equations of the Standard Model contain several closely correlated parameters. The Gfitter team collected the best theoretical calculations performed by theorists to-date and injected in the equations the various parameters determined experimentally. These are quantities such as the masses of various particles (top quark, W and Z bosons masses) and several couplings (parameters related to how often a particle will decay into lighter ones).

This technique is called an “electroweak fit” as it refers to making a global fit to all the parameters of the electroweak theory. Such fits were used to predict the mass of the top quark before it was even discovered at Fermilab in 1995 as shown on the plot below.

The predictions made on the mass of the top quark using a simultaneous fit to all the parameters of the electroweak theory. The blue band represents the predictions obtained form this fit whereas the black points show the experimental value measured by the Tevatron experiments from Fermilab.

The agreement between the prediction and the actual measurement, as seen on the above plot for the mass of the top quark, is remarkable. As the measurements of several parameters used in the fit became more precise over the years, so did the predicted value coming out of the fit to eventually agree very nicely.

One new and essential parameter of the electroweak theory is the mass of the Higgs boson. For many years, one important outcome of these global fitting procedures was a prediction of what this mass could be, guiding experimentalists in their search.

We can now play two different tricks: use all other measured parameters and predict the mass of the Higgs boson. Or we can assume the new boson discovered last summer is the Higgs boson, and use its mass to check the model for self-consistency. We want to see if everything fits nicely together or if the model starts to burst at the seams.

Both answers are shown on the following plot. The point where the wide-open grey curve touches the horizontal axis provides the most probable Higgs boson mass value given all the other constraints imposed on the Standard Model by all the measurements injected into the equations. The width of this curve gives the uncertainty on the mass prediction. The answer is 94 plus 25 or minus 22 GeV, in agreement (within 1.3 times the uncertainty on the measurement or 1.3 sigma) with the mass of the new boson which is now roughly 125.7 ± 0.6 GeV. Adding 1.3 times the uncertainty of the fit (25 GeV) to the value found (95 GeV) bridges the gap between the predicted and measured values.  So the fit makes a prediction consistent with for the mass of the new boson.

The second curve, the narrow blue curve, shows the prediction of the fit if you inject the experimental value of the new boson mass into the fit. If the theory has internal consistency, the returned value from the fit should agree with the injected value for the Higgs boson mass. And it does, with a much reduced uncertainty margin, falling pretty much on top of the injected value.

This means that the electroweak part of the Standard Model has great self-consistency. As it stands, there is only a 7% chance it could be in worse agreement. The difference comes mostly from two parameters used in the fit, namely the mass of the W boson and the so-called left-right asymmetry parameter measured in Z boson decays to b quarks.

By further reducing the uncertainties on the input parameters used for the fit, we will eventually see if the Standard Model gets in trouble. But as it stands, all is good although it has less and less leeway. This means such fits may eventually reveal flaws in the model.

Pauline Gagnon

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Latest news on the Higgs boson

Wednesday, March 6th, 2013

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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Dark matter retains all its mystery

Tuesday, March 5th, 2013

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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Is the big Higgs news for next week?

Friday, March 1st, 2013

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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Doomed Universe?

Wednesday, February 27th, 2013

Gian Giudice is a rather smiling and relaxed person for someone who has just shown the Universe might be doomed. This rather shaking discovery did not induce any lack of sleep to this CERN theorist whom I met yesterday. He and his colleagues showed in their latest calculations that if the Standard Model holds beyond all what we have seen so far, the Higgs field will change its value and all matter as we know it will simply cease to exist.

But rest assured, nothing is due to happen for roughly another 10100 years, that is 1 followed-by-100-zeros years. As Gian put it, we should not stop paying our taxes. Given that the Universe is only about 13.77 billion years old, it still gives us plenty of time. One billion is “only” nine zero, a very small number in comparison with the time estimated for this change to happen.

What he and his colleagues found is that we live in a Universe having parameters sitting just on the edge. Their calculations established that the stability of our Universe depends on the specific values assumed by various entities such as the masses of some fundamental particles. Assuming the new boson found last July is the Higgs boson and has a mass of 126 GeV, and injecting the known value of the top quark mass (roughly 173 ± 1 GeV), implies the Universe sits in a meta-stable region. This means the Universe is doomed to undergo some sort of “phase transition” at some distant time in the future.

The left plot, extracted from their paper, shows three types of regions depending on the value of these two masses: the red ones indicate that the Universe would have been unstable and would not have formed. The green region corresponds to a set of values leading to eternal stability, where the Higgs field would remain unchanged forever. The yellow region describes a meta-stable region. The right plot shows that, with the assumed mass values, we fall in the meta-stable region, where eventually the Higgs field value will change, leading to a complete collapse of all atoms.

The Higgs field is a physical entity, just like a magnetic field around a magnet. And the Higgs boson is simply an excitation of this field, just like a wave is an excitation of the surface of the ocean.

This change of the Higgs field value would be just a phase transition similar to what happens when a liquid starts to boil. Bubbles form and eventually, the liquid evaporates and disappears. Since the value of the Higgs field has a direct impact on the mass of quarks and electrons, it also determines the size of atoms. If the field value changes sufficiently, the atoms equilibrium is at risk and all matter could collapse.

What is puzzling Gian Giudice the most is why are these parameters such as to put us right on the edge between the meta-stable and stable region. Why has Nature chosen such unlikely values out of all possibilities? Could it be that all values are possible and we simply happen to live in a Universe having these specific ones? This would then mean there would be zillions of other Universes out there, each one having its own set of parameters, some of them being completely unstable and undergoing rapid phase transitions, others simply never being born. Our Universe would be part of a multiverse.

Much food for thoughts! The easiest way out is still for the Large Hadron Collider (LHC) to lead to the discoveries of new particles, revealing that the Standard Model does not provide the full picture. This in turn would mean all these calculations would just be good for the garbage, as Gian Giudice is the first to point out laughingly.

Pauline Gagnon

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