By Tommaso Dorigo
This week’s graph comes from a recent publication by the CMS experiment, the one I am a proud member of together with about 3000 colleagues from all over the world.
CMS (see a 3-D sketch below) is one of the two huge detectors collecting the faint signals of particles produced in the powerful 8-TeV proton-proton collisions delivered by the CERN Large Hadron Collider. The CMS experiment has recently been publishing one by one the results of many largely independent searches for Supersymmetric particles in the data collected during 2011; not surprising to sceptics like me, these results are all “negative” ones: they describe the absence of a signal, which is however a very informative datum, since it can be turned into a bound on possible models of new physics…..
Read more: www.science20.com
… at sqrt(s) = 7 TeV
A search for microscopic black holes in pp collisions at a center-of-mass energy of 7 TeV is presented. The data sample corresponds to an integrated luminosity of 4.7 inverse femtobarns recorded by the CMS experiment at the LHC in 2011. Events with large total transverse energy have been analyzed for the presence of multiple energetic jets, leptons, and photons, which are typical signals of evaporating semiclassical and quantum black holes, and string balls. Agreement with the expected standard model backgrounds, which are dominated by QCD multijet production, has been observed for various combined multiplicities of jets and other reconstructed objects in the final state. Model-independent limits are set on new physics processes producing high-multiplicity, energetic final states. In addition, new model-specific indicative limits are set excluding semiclassical and quantum black holes with masses below 3.8 to 5.3 TeV and string balls with masses below 4.6 to 4.8 TeV. The analysis has a substantially increased sensitivity compared to previous searches….
Read more: arxiv.org/pdf
SPEAKER: Eva Halkiadakis
We present an update on a number of searches for New Physics, including SUSY and Exotica, based on the recent LHC data, up to the full statistics of ~5/fb recorded by the CMS experiment in 2011. (Video in CDS: Press here)
Read more: indico.cern.ch
The main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 115.5-131 GeV by the ATLAS experiment, and 115-127 GeV by CMS
13 December 2011. In a seminar held at CERN today, the ATLAS and CMS experiments presented the status of their searches for the Standard Model Higgs boson. Their results are based on the analysis of considerably more data than those presented at the summer conferences, sufficient to make significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the elusive Higgs. The main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 116-130 GeV by the ATLAS experiment, and 115-127 GeV by CMS. Tantalising hints have been seen by both experiments in this mass region, but these are not yet strong enough to claim a discovery.
Higgs bosons, if they exist, are very short lived and can decay in many different ways. Discovery relies on observing the particles they decay into rather than the Higgs itself. Both ATLAS and CMS have analysed several decay channels, and the experiments see small excesses in the low mass region that has not yet been excluded.
Taken individually, none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row. What is interesting is that there are multiple independent measurements pointing to the region of 124 to 126 GeV. It’s far too early to say whether ATLAS and CMS have discovered the Higgs boson, but these updated results are generating a lot of interest in the particle physics community.
“We have restricted the most likely mass region for the Higgs boson to 116-130 GeV, and over the last few weeks we have started to see an intriguing excess of events in the mass range around 125 GeV,” explained ATLAS experiment spokesperson Fabiola Gianotti.”This excess may be due to a fluctuation, but it could also be something more interesting. We cannot conclude anything at this stage. We need more study and more data. Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012.”
“We cannot exclude the presence of the Standard Model Higgs between 115 and 127 GeV because of a modest excess of events in this mass region that appears, quite consistently, in five independent channels,” explained CMS experiment Spokesperson, Guido Tonelli. “The excess is most compatible with a Standard Model Higgs in the vicinity of 124 GeV and below but the statistical significance is not large enough to say anything conclusive. As of today what we see is consistent either with a background fluctuation or with the presence of the boson. Refined analyses and additional data delivered in 2012 by this magnificent machine will definitely give an answer.”
Over the coming months, both experiments will be further refining their analyses in time for the winter particle physics conferences in March. However, a definitive statement on the existence or non-existence of the Higgs will require more data, and is not likely until later in 2012.
The Standard Model is the theory that physicists use to describe the behaviour of fundamental particles and the forces that act between them. It describes the ordinary matter from which we, and everything visible in the Universe, are made extremely well. Nevertheless, the Standard Model does not describe the 96% of the Universe that is invisible. One of the main goals of the LHC research programme is to go beyond the Standard Model, and the Higgs boson could be the key.
A Standard Model Higgs boson would confirm a theory first put forward in the 1960s, but there are other possible forms the Higgs boson could take, linked to theories that go beyond the Standard Model. A Standard Model Higgs could still point the way to new physics, through subtleties in its behaviour that would only emerge after studying a large number of Higgs particle decays. A non-Standard Model Higgs, currently beyond the reach of the LHC experiments with data so far recorded, would immediately open the door to new physics, whereas the absence of a Standard Model Higgs would point strongly to new physics at the LHC’s full design energy, set to be achieved after 2014. Whether ATLAS and CMS show over the coming months that the Standard Model Higgs boson exists or not, the LHC programme is opening the way to new physics. press release:press.web.cern.ch
We observe an excess of events arount mH~126 CeV …… (ATLAS presentation)
The ATLAS results are here
The latest update of the ATLAS searches for the Standard Model Higgs boson was presented at a CERN seminar on December 13, 2011.
As stated in the CERN press release, the new ATLAS and CMS results are “sufficient to make significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the elusive Higgs.
Tantalising hints have been seen by both experiments in the same mass region, but these are not yet strong enough to claim a discovery.
“”We have restricted the most likely mass region for the Higgs boson to 115-130 GeV, and over the last few weeks we have started to see an intriguing excess of events in the mass range around 125 GeV,” explained ATLAS experiment spokesperson Fabiola Gianotti.
“This excess may be due to a fluctuation, but it could also be something more interesting.
We cannot conclude anything at this stage. We need more study and more data. Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012.
“The CMS experiment also has updated their results in this same low mass region.The Higgs boson is predicted by the Standard Model.
Via the Higgs field, it gives mass to the fundamental particles.
It is so short-lived that it decays almost instantly, and the experiment can only observe the particles that it decays into.
The Higgs boson is expected to decay in several distinct combinations of particles, and what is most intriguing about these results is that small excesses of events are seen in more than one such decay mode and in more than one experiment.
To identify and discover the Higgs Boson will take an enormous amount of data because the Higgs boson is very rarely produced. A definitive statement on the existence or non-existence of the Higgs is not likely until later in 2012.Discovery of the Higgs boson would be the first step on the path to many other new advances. press release: atlas.
The excess is most compatible with a SM Higgs in the vicinity of 124 GeV …. (CMS presentation)
The CMS results are here
Higgs to gamma gamma: 2.34 sigma bump at 123.5 GeV.
Higgs to ZZ to 4l: 2 events seen near 126 GeV (expect .5 background)
Combination: 2.4 sigma excess at 124 GeV.
The Higgs boson is the only particle predicted by the Standard Model (SM) of particle physics that has not yet been experimentally observed. Its observation would be a major step forward in our understanding of how particles acquire mass. Conversely, not finding the SM Higgs boson at the LHC would be very significant and would lead to a greater focus on alternative theories that extend beyond the Standard Model, with associated Higgs-like particles.
Today the CMS Collaboration presented their latest results in the search for the Standard Model Higgs boson, using the entire data sample of proton-proton collisions collected up to the end of 2011. These data amount to 4.7 fb-1 of integrated luminosity, meaning that CMS can study Higgs production in almost the entire mass range above the limit from CERN’s Large Electron Positron (LEP) collider of 114 GeV/c2 (or 114 GeV in natural units ) and up to 600 GeV. Our results were achieved by combining searches in a number of predicted Higgs “decays channels” including: pairs of W or Z bosons, which decay to four leptons; pairs of heavy quarks; pairs of tau leptons; and pairs of photons (Figure 1).
Our preliminary results, for several statistical confidence levels , exclude the existence of the SM Higgs boson in a wide range of possible Higgs boson masses:
127 – 600 GeV at 95% confidence level, as shown in Figure 2a; and
128 – 525 GeV at 99% confidence level.
A mass is said to be “excluded at 95% confidence level” if the Standard Model Higgs boson with that mass would yield more evidence than that observed in our data at least 95% of the time in a set of repeated experiments.
We do not exclude a SM Higgs boson with a mass between 115 GeV and 127 GeV at 95% confidence level. Compared to the SM prediction there is an excess of events in this mass region (see Figure 2b), that appears, quite consistently, in five independent channels.
With the amount of data collected so far, it is inherently difficult to distinguish between the two hypotheses of existence vs non-existence of a Higgs signal in this low mass region. The observed excess of events could be a statistical fluctuation of the known background processes, either with or without the existence of the SM Higgs boson in this mass range. The larger data samples to be collected in 2012 will reduce the statistical uncertainties, enabling us to make a clear statement on the possible existence, or not, of the SM Higgs boson in this mass region.
The excess is most compatible with an SM Higgs hypothesis in the vicinity of 124 GeV and below, but with a statistical significance of less than 2 standard deviations (2σ) from the known backgrounds, once the so-called Look-Elsewhere Effect  has been taken into account. This is well below the significance level that traditionally has been associated with excesses that stand the test of time.
If we explore the hypothesis that our observed excess could be the first hint of the presence of the SM Higgs boson, we find that the production rate (“cross section” relative to the SM, σ/σSM) for each decay channel is consistent with expectations, albeit with large uncertainties. However, the low statistical significance means that this excess can reasonably be interpreted as fluctuations of the background.
More data, to be collected in 2012, will help ascertain the origin of the excess. press release: cms.web.cern.ch
From the CMS talk at Berkeley; I’ve added the red dots and excised the low-statistics four-lepton results. Table of numbers of events at CMS in various categories. MET is a measure of whether invisible particles are present; HT is a measure of how much energy is in visible particles. No-OSSF means that if there is an electron there is no positron; if there is a muon there is no antimuon; etc. No Z means there are no lepton-antilepton pairs that came from a Z-particle’s decay; if there is no-OSSF there cannot be a Z, so that isn’t marked. “obs” means number of events observed, SM expectation means the prediction of the equations of the Standard Model of particle physics (supplemented with any detector-related effects that must be modeled carefully.) N(tau) means the number of taus and anti-taus observed in the event. A few entries where observation somewhat exceeds expectation are marked (by me) with red dots……
Read more: profmattstrassler.com
The W boson carries the weak nuclear force. At the Large Hadron Collider pairs of them may be showing the first signs of the Higgs boson. But they are a decidedly mixed blessing.
The current data from the LHC show an effect which might, or might not, be the first indication of the presence of a Higgs boson. Most of this effect is due to the number of pairs of W bosons which are being produced in proton-proton collisions.
The W boson (along with the Z boson) is responsible for carrying the weak nuclear force, just as the photon carries the electromagnetic force. It is intimately connected with the Higgs boson. In the standard theory, the Higgs gives mass to the W and Z, and so breaks a symmetry which otherwise would mean the weak force was pretty much as strong as the electromagnetic one.
If the Higgs boson has enough mass (that is, enough energy if it isn’t moving, since E = mc2), it will decay very quickly into a pair of W bosons. So if the Higgs is there, extra pairs of W bosons are one way we might first see it.
Generally this is a common way of seeing new particles. It’s called resonant production. If you can measure the energy and momentum of the decay products (two W bosons in this case), you can reconstruct the mass of the original (in this case the Higgs boson). W bosons can be produced in lots of different ways, but if one way is via the Higgs, you would expect a bump in the WW mass distribution at the mass of the Higgs….. Continue reading W and double W
A US particle machine has seen possible hints of the Higgs boson, it has emerged, after reports this week of similar glimpses at Europe’s Large Hadron Collider (LHC) laboratory.
The Higgs boson sub-atomic particle is a missing cornerstone in the accepted theory of particle physics.
Researchers have been analysing data from the Tevatron machine near Chicago.
The hints seen at the Tevatron are weaker than those reported at the LHC, but occur in the same “search region”.
Physicists have cautioned that these possible hints could disappear after further analysis.
But researchers also say when the US and European results are taken together, they start to paint an “intriguing” picture.
The results are being presented and discussed at the Europhysics conference in Grenoble, France.
The Tevatron and LHC machines work on similar basic principles, accelerating beams of particles to high energies around a tunnel before smashing them together.
These collisions can generate new particles which can then be picked up by detectors built at the points where particle beams cross over.
The LHC, which is housed in a 27km-long circular tunnel below the French-Swiss border, has two detectors looking for the Higgs: Atlas and CMS. Each is staffed by a different team of scientists.
The Tevatron has a comparable arrangement, with two detectors called DZero and CDF.
Just a quirk?
On Friday, the Atlas and CMS teams reported finding what physicists call an “excess” of interesting particle events at a mass of between 140 and 145 gigaelectronvolts (GeV).
The excess seen by the Atlas team has reached a 2.8 sigma level of certainty. A three-sigma result means there is roughly a one in 1,000 chance that the result is attributable to some statistical quirk in the data.
Now, the US DZero and CDF experiments have also seen hints of something at about 140GeV.
Professor Stefan Soldner-Rembold, spokesperson for the DZero detector team, told BBC News: “There are some intriguing things going on around a mass of 140GeV.
Professor Soldner-Rembold, from the University of Manchester in the UK, added: “There might be some picture emerging from the fog.”
The Tevatron is also seeing the same type of interesting particle events as the LHC. In these events, one elementary particle “decays”, or transforms, into another with a smaller mass.
The interesting fluctuations seen at the Tevatron and the LHC are dominated by what might be the Higgs decaying into a pair of “W boson” particles.
But the Tevatron results are currently at the one-sigma level of certainty – a lower level of statistical significance than those presented by the Atlas and CMS teams.
Five-sigma is the level of certainty generally required for a formal discovery. At this significance level there is about a one in 1,000,000 chance that a bump in the data is just a fluke.
However, says Professor Soldner-Rembold, the fact that teams working independently are now seeing similar phenomena point to an exciting possibility.
The existence of the Higgs boson was first proposed in the 1960s by Edinburgh University physicist Peter Higgs. The boson helps confer the property of mass on all other particles through their interaction with something called the Higgs field.
The efforts put into finding the boson relate to its status as the last missing piece in the the Standard Model – the most widely accepted theory of particle physics.
The Standard Model is a framework that explains how the known sub-atomic particles interact with each other. If the Higgs boson is not found, physicists would have to find some other mechanism to explain where particles get their mass.
Unusual data bumps detected by two teams at Large Hadron Collider thought to be glimpse of elusive source of particle mass
Scientists may have caught their first glimpse of the elusiveHiggs boson, or “God particle”, which is thought to give mass to the basic building blocks of nature.
Researchers at the Large Hadron Collider at Cern, the European particle physics lab near Geneva, announced the findings at a conference on Friday yesterday.
The world’s most powerful atom smasher hunts for signs of new physics by slamming subatomic particles together at nearly the speed of light in an 18-mile round tunnel beneath the French-Swiss border.
Speaking at the meeting, teams working on two of the collider’s huge detectors, Atlas and CMS, independently reported unusual bumps in their data that could be the first hints of the particle.
Physicists stressed that it was too early to know whether the signals were due to the missing particle.
Bumps that look like new discoveries can be caused by statistical fluctuations in data, flaws in computer models and other glitches, they said.
“We cannot say anything today, but clearly it’s intriguing,” Fabiola Gianotti, spokeswoman for the 3,000-strong Atlas team, said. She said the picture would become clearer as the groups gathered more data and combined results in the next few months. The view was shared by Guido Tonelli, spokesman for the CMS group, said more data was needed to understand whether the bumps were due to “statistical fluctuations or possible hints of a signal”.
The long-sought particle was first postulated in 1964 by Peter Higgs, a physicist at Edinburgh University, in a theory that described how fundamental particles gained mass from an invisible field that pervaded the cosmos.
The field has been compared to a snowfield that clings to particles and slows them down to different extents. Light particles pass through the field swiftly as if they have skis on, while heavy particles trudge through as though walking barefoot.
The boson was nicknamed the “God particle” in 1993 by the Nobel prize-winning physicist, Leon Lederman. The monicker is detested by Higgs. “I find it embarrassing because, though I’m not a believer myself, I think it is the kind of misuse of terminology which I think might offend some people,” he said.
From previous work, the Higgs boson was thought to have a mass somewhere between 114 and 185GeV (gigaelectronvolts) – one GeV is roughly equivalent to the mass of a proton, a subatomic particle found in atomic nuclei.
The Atlas team reported a Higgs-like bump in their data between 120 and 140GeV. In a later session, the CMS group announced two bumps in the same region.
Matt Strassler, a theoretical physicist at Rutgers University in New Jersey, commented on his blog: “Exciting … but far too early to be sure this is anything interesting.” He added: “This is certainly something we’ll be watching.”
Surprise LHC blip hints at Higgs – again
Particle watchers could be forgiven for feeling a little weary. An unexpected blip in the data glimpsed at the Large Hadron Collider is once again being attributed to the Higgs boson – the hypothetical particle thought to endow all others with mass.
The news raises hopes that the long-anticipated particle might finally be within reach – and is on somewhat firmer ground than a tantalising report earlier this year that turned out to be a false alarm. However, the blip – an excess of particles of a certain energy – is not yet big enough to rule out a statistical fluke that might vanish when more data is gathered.
In the wreckage of colliding protons, the ATLAS detector at the LHC, located near Geneva, Switzerland, has found an unexpected abundance of pairs of W bosons, which carry the weak nuclear force, with energies between about 120 to 140 gigaelectronvolts (GeV).
That could be due to a Higgs particle with a mass in that range decaying into pairs of W bosons (particle masses and energy are treated interchangeably because mass is readily converted into energy in particle collisions and decays).
The team also saw smaller excesses in pairs of photons and Z bosons, which carry the weak nuclear force and could be due to Higgs’ decays too.
The combined statistical significance, taking all three types of excess reported by ATLAS into account, is 2.8 sigma, slightly below the 3 sigma threshold (equivalent to a 1-in-370 chance of being due to a fluke) that a measurement must pass to count as “evidence” for something new: only 5 sigma data, equivalent to a 1-in-1.7 million chance of being due to a fluke, gains “discovery” status.
The other main detector at the LHC, called CMS, has found an excess in a similar range, between 130 and 150 GeV, reports Nature. The size of that excess is roughly 2 sigma, writes physicist Adam Falkowski on the Resonaances blog.
If all this sounds a tad familiar, rewind back to April, when four physicistsclaimed to have found hints of the Higgs in ATLAS data in a study abstract leaked online. A subsequent official analysis by the collaboration of 700 physicists who run ATLAS concluded (pdf) that result was an error. Unlike that claim, the new excesses have been vetted by the ATLAS and CMS collaborations respectively.
And what of the Tevatron accelerator in Batavia, Illinois, which has beenlocked in a race with the LHC to be the first to spot the Higgs? The Tevatron has not yet found evidence for the Higgs and is set to shut down at the end of September, but there is still time for it to weigh in on this with the data it is still collecting. Exciting times ahead, but expect more fits and starts.