Quark Excitement: Is there anything smaller?

The upper part of this plot shows a histogram (black dots) of ATLAS data events containing a photon and a jet, organized into bins defined by the mass of the photon-plus-jet pair. The stepped solid line represents a mathematical background function that has been fitted to the ATLAS data. If an exotic particle, having a mass in this range and decaying to a photon and a jet, were produced in the LHC, we would expect a bump to appear in the ATLAS data. Depending on the mass of the new particle and how readily it is created in the LHC, the bump could resemble one of the three indicated coloured peaks representing hypothetical excited quarks (q*) having masses of 0.5, 1.0, and 2.0 TeV. The lower part of this plot shows the statistical significance of the difference between the ATLAS data and the background function in each bin.

Mankind has forever sought to determine the most fundamental components of matter. From the atom to the nucleus to the proton and neutron, and finally to the quark, we have asked each step of the way “Is this it or is there something inside?”

ATLAS physicists have just taken another step toward tackling that very question by publishing the results of a search for new kinds of particles decaying into a jet (a spray of hadronic particles) and a photon.

The Physical Review Letters article provides the world’s best upper limits to date on the probability of producing such particles, including excited states of quarks.

recent ATLAS Blog posting explains that, as in the case of atoms, if a particle can be excited then it is necessarily composed of smaller pieces. If the LHC were able to create excited quarks, we should observe them with ATLAS as they emit photons of light and return to being regular quarks.

This plot compares how a product of quantities proportional to the number of hypothetical excited q* quarks observable in the ATLAS detector through their decays into a photon and a jet (vertical axis) varies as a function of the q* mass (horizontal axis). The black dots show the 95% credibility-level (CL) upper limits on this product measured by ATLAS in 7 TeV proton collision data. The dashed line shows the upper limits that were expected. The blue line describes how a theoretical excited-quark model predicts the product to vary with q* mass. The q* mass at which the blue curve and the ATLAS observed upper limits intersect marks the 95% CL lower limit set by ATLAS on the hypothetical excited quark mass.

Although no such excited states were found, the ATLAS study has significantly extended previous results obtained at other colliders. In fact, these measurements rule out the existence of signals ten times fainter and excited quarks 2 TeV more massive than earlier studies.

With the 2012 increase in the LHC energy to 8 TeV, and additional increases in the future, we expect to use these and other techniques to extend the reach of our understanding of matter’s most fundamental constituents.

Refer to a new ATLAS Blog article describing our technique and this new result in the search for quark substructure.
Read more: www.atlas.ch

The ATLAS detector on a smartphone

About LHsee
LHsee is an educational tool available for Android OS mobile smartphones and tablet PCs. It has been custom designed to provide an accurate and interactive visual representation of complex high-energy physics events recorded by the ATLAS detector. Features include live streaming and reconstruction of collision data from the CERN Large Hadron Collider.

The Large Hadron Collider (LHC) at CERN is one of the most inspirational science projects of our time. The machine has been designed to address fundamental questions about the nature of our universe, including the origin of mass, the difference between matter and antimatter and evidence for grand unified theories. The experiment has captured the public imagination and offers an unparalleled oppor tun ity to explain science to a wide r ange of audiences.
Even before the first operations of the LHC (in September 2008) a variety of introductory and educational resources had been produced [1]. Modern mobile smartphones (and tablet PCs) offer a new and highly capable platform for science education.
Such devices typically include 3G/WiFi internet access, dedicated graphics hardware and touchscreen control….

Read more:iopscience.iop.org

LHC reports discovery of its first new particle

The Large Hadron Collider (LHC) on the Franco-Swiss border has made its first clear observation of a new particle since opening in 2009.

Known as Chi-b (3P), it is a boson – the label given to particles that can carry the forces of nature.

The as-yet unpublished discovery isreported on the Arxiv pre-print server.

The LHC is exploring some of the fundamental questions in “big physics” by colliding proton particles together in a huge underground facility.

Detail in the sub-atomic wreckage from these impacts is expected to yield new information about the way matter is constructed.

The Chi-b (3P) is a more excited state of Chi particles already seen in previous collision experiments, explained Prof Roger Jones, who works on the Atlas detector at the LHC.

“The new particle is made up of a ‘beauty quark’ and a ‘beauty anti-quark’, which are then bound together,” he told BBC News.

“People have thought this more excited state should exist for years but nobody has managed to see it until now.

“It’s also interesting for what it tells us about the forces that hold the quark and the anti-quark together – the strong nuclear force. And that’s the same force that holds, for instance, the atomic nucleus together with its protons and the neutrons.”

The LHC is designed to fill in gaps in the Standard Model – the current framework devised to explain the interactions of sub-atomic particles – and also to look for any new physics beyond it.

In particular, it is using the collisions to try to pin down the famous Higgs particle – another boson that physicists hypothesize can explain why matter has mass.

Discoveries such as Chi-b (3P) are an important part of this quest because they add to the wider background knowledge, says Prof Jones.

“The better we understand the strong force, the more we understand a large part of the data that we see, which is quite often the background to the more exciting things we are looking for, like the Higgs.

“So, it’s helping put together that basic understanding that we have and need to do the new physics.”

Read also: A New Particle at the LHC? Yes, But… and ATLAS Discovers New Chi_b Resonance

Update on the Standard Model Higgs searches in ATLAS and CMS

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


Fabiola Gianotti (ATLAS)

We observe an excess of events arount mH~126 CeV …… (ATLAS presentation)

The ATLAS results are here

Search for the Standard Model Higgs boson in the diphoton decay channel with 4.9 fb-1 of ATLAS data at √s =7 TeV

Search for the Standard Model Higgs boson in the decay channel H → ZZ (∗) → 4ℓ with 4.8 fb−1 of pp collisions at √s = 7 TeV

Search for the Higgs boson in the H->WW(*)->lvlv decay channel in pp collisions at sqrt{s} = 7 TeV with the ATLAS detector

Combination of Higgs Boson Searches with up to 4.9 fb−1 of pp Collision
Data Taken at √s = 7 TeV with the ATLAS Experiment at the LHC

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.


Guido Tonelli (CMS)

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[1], 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 [2]) 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 [3], 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 [4] 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

The Plot Of The Week – Heavy Particle Production In ATLAS

If you work in experimental high-energy physics you soon acquire a particular sensitivity to the economical display of relevant information. Producing figures that convey the most meaning with the minimum effort is sort of an art, and it is a necessary consequence that HEP experimentalists -the smart ones- end up converging on the definition of graphs which are better than all others in this respect.

There is a deep, pragmatical reason, by the way, why economical display of information is valuable for a HEP experimentalist: we see far too many graphs every day… And most of them are routinely produced without thinking too much at their clarity. If you sit in internal meetings of a typical collider physics experiment, you most of the times find yourself staring at a powerpoint slide with a graph with no axis labels or unreadably small ones, with multiple histograms overlaid one on top of the other such that none is visible clearly, etcetera, etcetera, etcetera.

You cannot really blame the speaker -they are most of the times overloaded graduate students who produced the plot ten minutes before, with their advisors yelling at them on their back in the meantime. But it’s really a headache at times -and one cannot always rise his or her hand to ask the speaker for more explanation, so sometimes one just sits back and assumes that the information provided in the graph was not too relevant, otherwise the producer of the figure would have spent more time putting it in a more readable format. But this is a failure for both the speaker and for you.

So, well-crafted figures are valuable. Below is one, courtesy ATLAS collaboration. Of course it was not ATLAS which invented this particular graph – the plotting of different process cross sections in the same figure, as a way of both comparing them quantitatively on a log scale and comparing each with the theoretical prediction, is a rather well-established practice. Nonetheless the figure has charm. Even leaving aside the appreciation of the sound graphical choices, there is something to praise about the content, which is a five-star one. You get to see at a glance what is the rate of the different production processes of heavy objects (W and Z bosons, alone and with accompanying photons or other W and Z, and top quark pairs) at the Large Hadron Collider, as currently measured by ATLAS, and how these measurements compare with electroweak theory predictions. Marvelous, isn’t it ?

Now think at the fact that most of the 6000 physicists working in CMS and ATLAS are working to try and prove wrong the Standard Model, the theory which provides us with such a delicious understanding of subnuclear interactions, and you get the picture: physicists are never happy to fully understand something -they want to find something they do not understand! Yeah, that’s just so much more fun…

New ATLAS Limits On Higgs Mass

By Tommaso Dorigo

Much awaited, the results of searches for the Higgs boson at the Large Hadron Collider have been released by the ATLAS collaboration, and are being shown at the Lepton-Photon conference in Mumbay, India. I will provide here just the main results, with little commentary – I wish to let the cake cool down a bit before discussing the subject in detail, examining the various inputs.

So let us jump to the money plot: the combined limit produced by ATLAS by putting together information from a dozen search channels, which employ from 1.0 to 2.3 inverse femtobarns of proton-proton collision data at 7 TeV centre-of-mass energy (the range is due to some analyses taking longer to be produced than others). The relevant figure is shown below.
And a blowup of the low-mass region is in the following figure:

always, this “Brazil band” plot shows the upper limit in the Higgs boson production rate, obtained by observing that no signal stands out in the data, and thus placing a limit in the number of possible Higgs boson decay events may be still hidden there without being detected. The limit which is obtained is the black curve, and it should be compared to two different things: one is the horizontal line at 1.0, which represents the production rate that the Higgs boson should have in the Standard Model, given the particular mass hypothesized for it….. Continue reading New ATLAS Limits On Higgs Mass