Latest Results of Higgs Search Presented Jointly By ATLAS and CMS, LHC, CERN at Lepton Photon ’11, Mumbai

The latest results on the Higgs search are out. Results were presented separately by ATLAS and CMS detectors of LHC, CERN today(i.e. 22st August, 2011) at the Lepton-Photon Conference, 2011. In this semi-technical article, we present the most important results in a systematic form. The verdict is, however, out the Higgs hasn’t been found as yet.

Check out our first (non-technical) post on this discovery here. A countdown to the Lepton Photon Conference itself is here.

Higgs Production and Decay channels

There are a few things that should be kept in mind right throughout the article. The Higgs boson is primarily produced by interaction of two gluons. (A gluon is what keeps protons and neutrons in an atomic nucleus together.) This is called gluon-gluon production of the Higgs boson.

Next, the Higgs, being highly massive (i.e. having a high mass) decays into lighter particles. This is what massive particles always do they decay into lighter particles. The only thing is that different particles decay at different rates. Heavier particles will decay much faster than comparatively lighter particles.

Higgs event

The Higgs can decay into a number of lighter products. Each of these products leaves a distinctive signature on the detectors and the different modes of decay are called different decay channels’. The Higgs primarily has a gamma-gamma (Higgs decaying into two gamma ray photons.) channel, a WW and a ZZ channel. These are the main channels of interest. The gamma-gamma channel will be the preferred channel if the Higgs is a comparatively light particle about 100 GeV in mass. If the Higgs decays by producing two Z-bosons (the ZZ channel) or two W-bosons (WW channel) then its mass is above 130 GeV.   In other words, the gamma-gamma channel fixes the upper limit of the Higgs mass at 130 GeV, while the WW and ZZ channels fix the lower energy bound at 130 GeV.

Now, here is the interesting part. The WW or ZZ bosons are themselves quite heavy and decay into a number of products. These decay channels produce characteristic detection patterns in the detectors. Comparing the observed rate of decay into these channels with that of the expected value, the data is reconstructed to see if this indeed was a Higgs event.

Now for more technical details

ATLAS Results

The ATLAS detector found no significant excess in the gamma-gamma channel. The bottom-bottombar (b-bbar) channel (this is what the WW bosons break down into bottom and anti-bottom quarks) gave big excess of Higgs event above the theoretically expected Standard Model(SM) production rates. Even though the excess was nearly 10 times the SM predictions, the sensitivity needs to be improved. Furthermore, Tevatron has a much greater say in the b-bbar channel than the LHC, given that it has recorded much higher number of events and has a higher luminosity at that energy range. The tau-tau (tau is a lepton, an electron like particle) channel gave a 4 to 5 times excess.

The ATLAS detector at LHC, CERN

Overall, there was no significant excess in any of the channels to warrant a discovery. There was no significant excess number of events noticed for the Higgs in the mass range of 110 GeV to 160 GeV. This mass range is tentatively excluded with 95% confidence level. However, at 99% confidence level, there is a window about 142 GeV, which can be a possible detection window. Further experiments will probe this window more thoroughly.

CMS results

CMS detected no excess in the gamma-gamma channel. A slight excess was noticed in the tau-tau channel and this is expected to be an important channel for further investigation, owing to the fact that data reconstruction from this channel points to a Higgs mass of about 140 GeV.

Excess of events in the WW going to lepton-lepton channel suggests a mass range of 130 GeV to 200 GeV. Three pairs of events have been notices at three mass ranges 122, 142 and 165 GeV for the ZZ channel. Only the 142 GeV event is consistent with Standard Model predictions. Happily, this is the very window that wasn’t excluded earlier with 99% confidence level.

Out of theoretically expected mass range exclusion of 145 to 440 GeV, three ranges have been excluded 145 to 216 GeV, 226 to 288 GeV and 310 to 400 GeV. Anything above 400 GeV is unlikely and the crucial 130 to 145 GeV window is still open. These mass ranges have been excluded with 98% confidence level.

Higgs search continues with full force. LHC will provide a lot more data samples in the coming months and this might ultimately lead us to achieve the Holy Grail of Particle Physics.

Countdown to Lepton-Photon Conference, 2011: ATLAS To Make Major Announcement on Higgs Search

Some big news is just around the corner. The ATLAS collaboration at LHC, CERN is all set to announce the status of the Higgs Boson search at the giant collider in the upcoming week at the Lepton-Photon Conference 2011, being held in Mumbai from 22-27 August, 2011. The announcement is one of particular importance since it is rumored to be the definitive one in the quest for the Higgs Boson. Whether the Standard Model of Physics, one of the most beautiful and successful edifices of physics ever constructed, will stand or need revision will hinge crucially on this one announcement.

The Lepton Photon 2011 Logo

The Lepton-Photon Conference, 2011

The Conference The XXV International Symposium on Lepton-Photon Interactions at High Energy will take place in Tata Institute of Fundamental Research, Mumbai, India and will attract prominent personalities from the world of high-energy physics. The coming week is expected to be a hectic one for both students and physicists at the Institute, with the who’s who of particle physics presenting and discussing current progress, while also charting the road ahead. Preparations are on at full swing within the Institute premises.

We at Techie-Buzz will be covering the huge scientific event from Ground Zero and presenting all the major announcements in real time from it.  You might want to bookmark the website and visit it frequently or subscribe to our newsletter, if you aren’t already on the subscription list.

Some exciting developments precede the event

The watch-word is Higgs’ for everyone and with certain encouraging signs noticed in the last few months, everyone is excited. Particularly stunning are the two results graphed below. Explanations follow the graphs.

The CMS (above) and ATLAS (below) Results for Higgs event, July, 2011

Look at the two graphs (don’t get scared!). The thick black line in each graph represents the Higgs signals. The dashed line represents the predicted Higgs production rate by Standard Model Calculations. A proper signature is said to be found when the observed signal overtakes the predicted signal. Look at the region marked, just between 130 to 150 MeV, where the production rate far exceeds the predicted rate. This coincides with the predicted mass range for the Higgs. This in itself proves nothing, as this might be due to something completely different. What is exciting is the fact that this weaksignal is being noticed in both the LHC detectors, ALICE and CMS. Concurrent results have a better chance of surviving thorough data analysis.

For clarity let me reiterate the two important takeaway points: First, both detectors, ATLAS and CMS, agree on the Higgs signature. Second, the signals have been noticed in the theoretically expected mass range (about 130-150 GeV).

The results are now quoted at a 95% confidence level (or 2 sigma) and do not warrant the label of a discovery’. For that, you’ll require 99.997% confidence (or 5 sigma) from both detectors. We might be onto that.

At the risk of being repetitive, let me again emphasize that the announcement at the Conference in the coming week will nearly finalise the fate of the search for the Higgs Boson. If not found, it may be the beginning of new physics.

Hope to see you here through next week.

Update: The CERN Announcement on the ATLAS and CMS results on the Higgs Search is here. Check it out, its big news.

LHC At Home: Now, You Can Help CERN Find The Higgs Boson Sitting At Home

The greatest science project ever designed by man is now calling out to you, dear average Joe or hotshot scientist, for helping it find the elusive Higgs Boson. CERN has launched an extended version of its LHC@home campaign, naming it unimaginatively as LHC@home 2.0, in which CERN wants you to share a part of your computer’s processing power to do science.

The Colossal Collider Comes Computer Hunting

The Large Hadron Collider (LHC) has been actively looking for the Higgs Boson particle, constantly eliminating mass ranges and probing higher and higher energies. Tantalizing signs have been seen, only to be later refuted by CERN itself. The Higgs particle, dubbed The God Particle’ by the popular media, is so far living up to the given divine billing. The Higgs is the ultimate piece of the puzzle of the Standard Model, with all the other particles discovered. No one said that finding the final piece would be easy.

The ALICE Detector at LHC

LHC@home 2.0 is a volunteer computing platform. It aims to use a part of the computing power of your computer, so that CERN can simulate more data. This is the best implementation of the notion of GRID computing, in which computers around the world, linked to a network, can donate a part of the processor’s facilities, which would have otherwise remained unused anyway. The result is a massive increase in processing speed for the central computing facility.

Simulations: Why So Serious?

The most important aspect of a collider experiment, other than building the machine itself, is the collision simulation. Simulations are a vital part because solving multi-particle dynamics is a stressful, often impossible, job. Several particles interacting with several other particles through different interactions at relativistic energies can give physicists nightmares. The way out is to prepare plausible models for the collision and then use computers to simulate the result, should such a collision take place. Important results are noted from the simulation data, like tell-tale signs of new particles, decay channels and sensitive hidden parameters. After documenting the actual collision, data is compared, especially the most conspicuous simulation results.

Simulated Decay of the Higgs

Reconstruction helps refine the model and unexpected bumps occasionally produce excitement. These bumps can be due to a number of causes, but careful analysis helps scientists rule out experimental causes or error. If the bump survives, it’s a new discovery. So far, no Higgs bump has survived.

… And You Can Join In!

CERN gives a detailed instruction manual to anyone interested to join here. Currently, the LHC@home 2.0 is in its test phase and is testing a program Test4Theory@Home.

Learn more about the CERN projects and how you can help here.

Here’s your dream come true: Have a virtual atom smasher at your home, revealing the greatest mysteries of the Universe.

News from LHC, CERN: CMS Results Rule Out Large Mass Range for Higgs Boson Particle

Last year’s run of the LHC has set a cutoff for the expected mass of the Higgs boson. This important result came out of the recently concluded Europhysics Conference 2011.

CMS has excluded the mass ranges of 149-206 GeV and 300-440 GeV for the Higgs with a confidence level of 95%. It has also excluded the masses from 145-480 GeV with a lower confidence level of 90%. This excludes a huge part of the expected mass range, and has gotten particle physicists both excited and demoralised.

Read about the Quark Conference 2011 here, rumour of a Higgs being detected at ATLAS, LHC, CERN here and that news being rejected here. We covered a very recent discovery by Fermilab  here.

The Higgs Boson, also called ‘God Particle’ is the only particle predicted by the Standard Model but has not been detected in any collider. It has been assigned the function of endowing all particles in the Universe with mass. Any particle interacting with the Higgs field, mediated by the Higgs boson, is said to have mass. The boson is named after physicist Peter Higgs, the person who came up with the idea. Unfortunately, the Standard Model says little more than the lower limit of the mass of the Higgs Boson. CERN has given itself till 2012 for proving the existence of the Higgs.

Higgs Event

Further news is that is the results of the 2010 and 2011 runs are interpreted in the light of a Standard Model having four generation of fermions (SM4), instead of just three, scientists put a 95% confidence level on the exclusion of the Higgs boson in the mass range of 120-600 GeV.

There are also many particle physics theories not involving any Higgs boson called Higgless theories and they may now come to the forefront, if the Higgs remains elusive.

CMS is carrying on the search using decays of different particles. It is trying to produce the Higgs using two photons, two tau leptons, two W bosons and two Z bosons.

New Particle Discovered At Fermilab; Existence Confirmed

This time it appears to be genuine. Fermilab has indeed detected a new particle the Xi-sub-b baryon.

A few days ago Fermilab got the entire physics community excited by announcing that it was almost sure of a discovery of a new particle the W/Z-bar boson. We covered the sensational news here. It turns out that it was not the case.

The CDF detector, where the new particle was detected

The particle

A baryon is a particle (technically, a bound state’) of three quarks. A proton is an example and is made up of two up and one down quark. Apart from these two types, there are four more types of quarks top(some people call it truth’), bottom (some prefer beauty’), strange and charm. The top quark is the most massive, followed by bottom, charm and strange in that order. The Xi-sub-b baryon has an up quark, a strange quark and a bottom quark. The discovery was made yesterday and reported today in Fermilab’s press release.

The standard model list for the quarks and all other fundamental particles.

The Xi-sub-b baryon is extremely unstable, being able to travel just a fraction of a millimetre before disintegrating into lighter particles. The particle was detected in the CDF detector.

Confidence levels

There have been 25 isolated detections, or events, that confirm the existence of the new particle. As we mentioned in earlier posts, the criteria for a discovery is confidence level’. We need a minimum of five-sigma confidence level, or a confidence of more than 99.997% that a certain detection is genuine. Here, Fermilab, after data analysis, puts the confidence level at seven sigma, much higher than the threshold!

The details of the discovery have been sent to Physical Review Letters (PRL) for publication.