W boson candidates in 7 TeV Collision Data

First W boson candidates recorded by ATLAS in 2010.

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ATLAS Experiment Reports Its First Physics Results from the LHC

March 2010

The first physics results from the ATLAS Experiment with proton-proton collisions at an energy of 0.9 TeV in late 2009 have now been accepted for publication in the journal Physics Letters B.
Almost 20 years since the inception of the ATLAS detector, the labors of 3000 scientists along with large numbers of engineers and other support staff have found fruition in the earliest results. These results demonstrate that this mammoth detector (seven stories tall and twice as long) works almost flawlessly. It is a great achievement that a 7000 tonne detector – with 3000 km of cables and close to 100 million channels to be read out – has successfully measured particle tracks with a precision of 0.001 centimeters so quickly.
In later stages, ATLAS will focus on rare events and filter out less interesting events. This first paper reports on results with the “trigger” set to accept almost all events (this setting is called “minimum bias”). The paper reports results for the number of charged particles per collision and its dependence on several variables such as the particles’ momentum perpendicular to the beam.
The results from over 300,000 proton-proton collisions were compared with sophisticated computer simulations and with results from other experiments at the same collision energy. These comparisons demonstrate that the ATLAS detector performs remarkably well even at these early stages of the research. The inner detector of ATLAS (which follows the tracks of the charged particles emerging from the collisions) was key to the measurements. Data from the inner detector matched the simulations excellently.
Tom LeCompte, the Physics Coordinator for ATLAS, commented that: “It's particularly gratifying that our result be published now, on the eve of the LHC beginning its multi-year program of the highest energy collisions in the world. Teamwork was essential in being able to produce a physics result this quickly, especially with a brand new detector as complex as ATLAS.”
This first ATLAS paper with physics results represents a major milestone for the experiment at the Large Hadron Collider. It demonstrates the enormous potential for making major discoveries in the years ahead.
ATLAS spokesperson, Fabiola Gianotti, noted that "This first paper is very special for all of us, as it marks the beginning of a very exciting era of physics results and hopefully great discoveries. I am particularly delighted by the fact that the analysis described in this paper was mainly done by students and young post-docs. We are all very proud of these achievements."

M. Barnett

Puzzling antimatter

For many years, the absence of antimatter in the Universe has tantalised particle physicists and cosmologists: while the Big Bang should have created equal amounts of matter and antimatter, we do not observe any primordial antimatter today. Where has it gone? The LHC experiments have the potential to unveil natural processes that could hold the key to solving this paradox.

Every time that matter is created from pure energy, equal amounts of particles and antiparticles are generated. Conversely, when matter and antimatter meet, they annihilate and produce light. Antimatter is produced routinely when cosmic rays hit the Earth's atmosphere, and the annihilations of matter and antimatter are observed during physics experiments in particle accelerators.

If the Universe contained antimatter regions, we would be able to observe intense fluxes of photons at the boundaries of the matter/antimatter regions. “Experiments measuring the diffuse gamma-ray background in the Universe would be able to observe these light emissions”, confirms Antonio Riotto of CERN's Theory group. “In the absence of such evidence, we can conclude that matter domains are at least the size of the entire visible Universe”, he adds.

What caused the disappearance of antimatter in favour of matter? “In 1967, the Russian physicist Andrej Sakharov pointed out that forces discriminating between matter and antimatter, called “CP-violating” effects, could have modified the initial matter-antimatter symmetry when deviations from the thermal equilibrium of the Universe occured”, says Antonio Riotto. In the cold Universe today, we can only observe very rare CP-violating effects in which Nature prefers the creation of matter over antimatter. Following their discovery in the decays of K-mesons containing strange quarks, they have now also been observed in the decays of B mesons, which contain bottom quarks.

Today, scientists think that the early Universe might have gone through a transition phase in which the thermodynamic equilibrium was broken, when the density of the Universe was very high and the average temperature was one billion or more times that inside the Sun. "Some physicists think that this might have happened through the formation of ‘bubbles’ which have progressively expanded, thus ‘imposing’ their new equilibrium on the whole pre-existent Universe", explains Antonio Riotto. Whatever the real dynamics of this phase actually were, the important thing is that one particle of matter in every 10 billion survived, while all the others annihilated with the corresponding antiparticles.

How can the LHC help to solve the mystery? By studying rare decays, experiments can bring us more accurate information about phenomena related to CP-violation involving both known and new particles, such as mesons containing both bottom and strange quarks. Moreover, if new supersymmetric particles are discovered at the LHC, some of the possible scenarios leading to a non-equilibrium phase could find experimental support. "If the LHC finds a Higgs boson with a mass less than about 130 GeV, and if this discovery comes with the detection of a light supersymmetric particle called ‘stop‘, this could be the experimental proof that the non-equilibrium phase happened through the formation of bubbles", concludes Antonio Riotto.

In any case, since the disappearance of primordial antimatter cannot be explained by the current Standard Model theory, it is clear that we have to look for something new. Scientists are exploring different avenues but, given the fact that what we observe represents only about 4% of the total energy and matter that the Universe is made of, one can guess that part of the key to solving the antimatter mystery could be held in the yet unknown part of the Universe. With its very high discovery potential, the LHC will certainly help shed light on the whole issue.

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The LHC is not alone in the search for the solution to the antimatter mystery. BaBar at SLAC in the US and BELLE at KEK in Japan have measured decays of B-mesons in detail , and the Tevatron experiments CDF and D0 are also exploring CP-violation effects. Later this year, the AMS (Alpha Magnetic Spectrometer) experiment will be docked to the International Space Station (ISS) and will start looking for evidence of antimatter particles resulting from the decay of dark matter.

 francesco.poppi

Course on the Physics of Accelerators

http://accelerator.ipm.ac.ir/doc/HelmutWiedemann.pdf

Helmut Wiedemann

Stanford University, USA

April 26 - May 19, 2010

سخنراني دكتر خلخالي در مورد رفتار فضا در هندسه ناجابه‌جايي

http://math.ipm.ac.ir/Monthly_Colloquium/Khalkhali.pdf

جلسه‌ي پنجم هندسه ذرات و كيهان‌شناسي

اين جلسه به دليل وقت كم جلسه‌ي قبلي و عدم استقبال دوستان عزيز، به قسمت دم نرسيد كه به جبران آن موضوعات جلسه‌ي پنجم در تاريخ 11 ارديبهشت به آدرس و ساعت مذكو ردر پوستر (پست‌هاي را نگاه كنيد) به قرار زير است:

بخش اول: 

تعريف منيفلد

نكات و مثالهايي در اين زمينه

بخش دوم: 

تقارن گسسته و اعداد كوانتمي و نقض
CP
با سپاس
فرهاد ذكاوت

جلسه‌ي چهارم هندسه، نسبيت و ذرات

اين جلسه ساعت 16.50 در كلاس 107 دانشكده‌ي شيمي در همان طبقه‌ي دوم روبروي فيزيك برگزار مي‌شود.
موضوعات:
section one:
linear algebra
Group theory
tensors
......................
section 2:
Diffrentiable Manifolds and tensonrs:
Manifolds
One-forms
Fiber Bundles