Recent Results Of CMS

Two days ago I discussed at ICFP 2012 the most recent results of the CMS experiment at the CERN Large Hadron Collider. In the allotted time of my talk I could only cover few analyses, and I obviously chose some of the most interesting ones, so that was already a summary. Here I am bringing the information collapse one step further, by giving a itemized summary of some of the points I made, just in case you are interested. If you want to, you can also download the original slides of my talk from here (but be careful, it’s a 8Mb file).

– The LHC has yielded over 5 inverse femtobarns of proton-proton collisions to CMS to analyze in 2011, and these data have been used for dozens of new results. Now we have on tape another 5/fb of data from the 2012 run, but these have not been looked at yet (results will be ready in a few days).

– We can broadly divide 2011 results into three areas: searches for Higgs bosons, Standard Model measurements, and new physics searches.

– CMS searched for the Higgs boson in eight independent final states, further divided in over forty categories. The combined results of these searches say that the particle must be lighter than 127 GeV (and heavier than 115 according to LEP II), or heavier than 600 GeV. We know the latter is not an option as far as the Standard Model is concerned, because it would now be utterly inconsistent with other electroweak measurements. So we might argue that if the SM Higgs exists, we already know its mass to better than 10% accuracy.

– CMS finds a signal with a local significance of 3.1 standard deviations at 124 GeV. If this excess is due to the Higgs boson, it is likely that the new data, once analyzed, will produce additional evidence which can be considered conclusive proof for the particle’s existence.

– A new baryon, the Ξb*, has been observed in its fully exclusive cascade decay into J/ψ, proton, and pions (with intermediate Ξb and Λ states). Its mass is just short of 6 GeV (see picture on the right, showing the peak in the distribution of Q-value of the two-body decay Ξb*–>Ξb π).

– Rare decays of the Bs meson have been searched, and a tight limit on the Bs->μμ decay has been obtained by combining CMS results with LHCb and ATLAS ones. New physics models are strongly constrained by this limit because many realizations of NP would yield enhancements in the branching ratio for the dimuon decay mode.

– CMS now measures the top quark mass and cross section in a number of different techniques. The precision on the top mass is reaching the Tevatron average (1.25 GeV total error now). A new era of precision top physics measurements has started, with e.g. limits on Flavour-changing neutral current top decays constrained at the 0.34% level, and top-antitop mass difference measured to within 0.5 GeV (of course it is zero!).

– A large number of interesting searches for new physics returned null results. Supersymmetry has been investigates in dozens of possible signatures, with no positive result.

Below is my conclusions slide:

Read more: www.science20.com

No SUSY In New CMS Search


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

Search for microscopic black holes in pp collisions …

… at sqrt(s) = 7 TeV


CMS Collaboration
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

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

ATLAS

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.

CMS

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

Something Curious at the Large Hadron Collider

Matt Strassler
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

W and double W

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.

Diagram showing a Higgs boson decaying to a W+ and a W-, which then decay to leptons. This might be happening.

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