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  arXiv:1410.7061 [pdf, ps, other]

Title: Once more on the W-loop contribution to the Higgs decay into two photons
Comments: 8 pages with one figure
Subjects: High Energy Physics - Phenomenology (hep-ph)

The imaginary part of the Feynman amplitude of the $W$-loop contribution to the Higgs decay into two gammas (viewed as a function of the square of the off shell Higgs momentum) is finite and unambiguous. It is presented as the product of an invariant amplitude ${\cal A}$ times a bilinear in the components of the (on shell) photon momenta factor which takes the Ward identity into account. The unsubtracted dispersion integral of ${\cal A}$ is convergent and reproduces the amplitude computed by R. Gastmans, S.L. Wu and T.T. Wu [GWW1,GWW]. In particular, the decoupling theorem, criticized as a unjustified assumption in a subsequent paper [SVVZ12], is obtained as a corollary. The result makes the discrepancy between the observed decay rate of the Higgs into two photons and the Standard Model prediction larger than currently believed and hence points to a possible sign of new physics.

+ نوشته شده در  سه شنبه ششم آبان 1393ساعت 10:9  توسط یاسمن فرزان 

arXiv:1410.6168 (cross-list from astro-ph.HE) [pdf, other]
Discovery of a New Galactic Center Excess Consistent with Upscattered Starlight
Comments: 5 pages, 3 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph)

We present a new extended gamma ray excess toward the Galactic Center that traces the 3.4 micron infrared emission morphology. Combined with its measured spectrum, this new extended source is consistent with inverse Compton emission from a high-energy electron-positron population with energies up to about 10 GeV. Previously detected emissions tracing the 20 cm radio, interpreted as bremsstrahlung radiation, and the Galactic Center Extended emission tracing a spherical distribution and peaking at 2 GeV, are also detected. We show that the inverse Compton and bremsstrahlung emissions are likely due to the same source of electrons and positrons. All three extended emissions may be explained within the framework of a model where the dark matter annihilates to leptons or a model with unresolved millisecond pulsars in the Galactic Center.

+ نوشته شده در  شنبه سوم آبان 1393ساعت 11:38  توسط یاسمن فرزان 

با سلام
یکصد و نوزدهمین نشست باشگاه فیزیک انجمن فیزیک ایران، ساعت 17 روز دوشنبه 5 آبان‌ماه 1393 در سالن آمفی تئاتر دانشکده فیزیک دانشگاه تهران (انتهای خیابان کارگرشمالی) برگزار خواهد شد.
در آغاز آقای دکتر علیرضا بهرامپور از دانشگاه صنعتی شریف، درباره «جایزه نوبل برای نور آبی» سخن خواهند گفت. سپس آقای دکتر خسرو حسنی از دانشگاه تهران پرسش ماه را مطرح خواهند کر� � و آقای ایمان مهیایه از دانشگاه صنعتی شریف جدیدترین اخبار فیزیک در ماه گذشته را به آگاهی حاضران خواهند رساند.
سپاسگزار خواهیم شد پوستر باشگاه را در محل تحصیل خود برای آگاهی دوستانتان نصب کنید.
http://www.psi.ir/html/acts/bashgah/poster.asp?s=119&city=tehran


با احترام
انجمن فیزیک ایران

+ نوشته شده در  پنجشنبه یکم آبان 1393ساعت 8:59  توسط یاسمن فرزان 

Dear Colleagues,

The Ohio State University Departments of Astronomy and Physics invite applications for Postdoctoral Fellows in the Center for Cosmology and AstroParticle Physics (CCAPP).  We would greatly appreciate your bringing the ad below to the attention of suitable candidates.

Best regards,
John

-- 
------------------------------------------------------------------------------------------------------
John Beacom
Professor and Director of CCAPP (http://ccapp.osu.edu)
Department of Physics and Department of Astronomy
Ohio State University
email: beacom.7@osu.edu, phone: (614) 247 - 8102
Dept. of Physics, OSU, 191 W. Woodruff Ave., Columbus, OH 43210
------------------------------------------------------------------------------------------------------



Postdoctoral Fellows in the Ohio State Center for Cosmology and AstroParticle Physics (CCAPP)

A successful candidate should be able to carry out a vigorous independent research program in theoretical, observational, or experimental cosmology, astroparticle physics, or astrophysics, interacting with researchers in CCAPP and other institutions while utilizing extensive CCAPP resources and infrastructure.  We have recently welcomed three new faculty: Linda Carpenter, Chris Hirata, and Annika Peter.  Two more new faculty will be arriving soon: Adam Leroy and Laura Lopez.

 
CCAPP has strong efforts in theoretical work, including on dark energy, large-scale structure, Galactic dynamics and substructure, cosmic microwave background anisotropies, gravitational lensing, dark matter, cosmic rays, gamma rays, neutrinos, nucleosynthesis, stellar physics, supernovae, gamma-ray bursts, and active galactic nuclei.

 

CCAPP's multi-messenger research initiatives include institutional affiliations with the Dark Energy Survey (DES), Sloan Digital Sky Survey (SDSS-III & IV ), Fermi Gamma-ray Space Telescope, Pierre Auger Observatory, ANtarctic Impulsive Transient Antenna (ANITA), Askaryan Radio Array (ARA), and IceCube.  CCAPP also has efforts in the time-domain astronomy of stars, quasars, and supernovae, including using the Large Binocular Telescope and the All Sky Automated Survey for Supernovae (ASAS-SN). 

 

Appointments will be for Regular (3 year) or Long-Term (5 year) Fellowships.  In addition to competitive salaries, these positions come with substantial independent research budgets.  Applicants will be considered for both appointment levels as well as for other postdoctoral positions.  Women and members of other underrepresented groups are especially encouraged to apply.
 
Applications will be reviewed after November 15, 2014; we strongly recommend that all materials arrive by this date, though later submissions will be considered.


See http://ccapp.osu.edu/jobs.html for details.

+ نوشته شده در  جمعه یازدهم مهر 1393ساعت 11:19  توسط یاسمن فرزان 

مطلب زیر را از اینجا برداشتم.

 

Dark matter or pulsars? AMS hints it's neither

.

 

Yesterday AMS-02 updated their measurement of cosmic-ray positron and electron fluxes. The newly published data extend to positron energies 500 GeV, compared to 350 GeV in the previous release. The central value of the positron fraction in the highest energy bin is one third of the error bar lower than the central value of the next-to-highestbin.  This allows the collaboration to conclude that the positron fraction has a maximum and starts to decrease at high energies :]  The sloppy presentation and unnecessary hype obscures the fact that AMS actually found something non-trivial.  Namely, it is interesting that the positron fraction, after a sharp rise between 10 and 200 GeV, seems to plateau at higher energies at the value around 15%.  This sort of behavior, although not expected by popular models of cosmic ray propagation, was actually predicted a few years ago, well before AMS was launched.  

Before I get to the point, let's have a brief summary. In 2008 the PAMELA experiment observed a steep rise of the cosmic ray positron fraction between 10 and 100 GeV. Positrons are routinely produced by scattering of high energy cosmic rays (secondary production), but the rise was not predicted by models of cosmic ray propagations. This prompted speculations of another (primary) source of positrons: from pulsars, supernovae or other astrophysical objects, to  dark matter annihilation. The dark matter explanation is unlikely for many reasons. On the theoretical side, the large annihilation cross section required is difficult to achieve, and it is difficult to produce a large flux of positrons without producing an excess of antiprotons at the same time. When theoretical obstacles are overcome by skillful model building, constraints from gamma ray and radio observations disfavor the relevant parameter space. Even if these constraints are dismissed due to large astrophysical uncertainties, the models poorly fit the shape the electron and positron spectrum observed by PAMELA, AMS, and FERMI (see the addendum of this paper for a recent discussion).
Pulsars, on the other hand, are a plausible but handwaving explanation: we know they are all around and we know they produce electron-positron pairs in the magnetosphere, but we cannot calculate the spectrum from first principles.

But maybe primary positron sources are not needed at all? The old paper by Katz et al. proposes a different approach. Rather than starting with a particular propagation model, it assumes the high-energy positrons observed by PAMELA are secondary, and attempts to deduce from the data the parameters controlling the propagation of cosmic rays. The logic is based on two premises. Firstly, while production of cosmic rays in our galaxy contains many unknowns, the production of different particles is strongly correlated, with the relative ratios depending on nuclear cross sections that are measurable in laboratories. Secondly, different particles propagate in the magnetic field of the galaxy in the same way, depending only on their rigidity (momentum divided by charge). Thus, from an observed flux of one particle, one can predict the production rate of other particles. This approach is quite successful in predicting the cosmic antiproton flux based on the observed boron flux. For positrons, the story is more complicated because of large energy losses (cooling) due to synchrotron and inverse-Compton processes. However, in this case one can make the  exercise of computing the positron flux assuming no losses at all. The result correspond to roughly 20% positron fraction above 100 GeV. Since in the real world cooling can only suppress the positron flux, the value computed assuming no cooling represents an upper bound on the positron fraction.

Now, at lower energies, the observed positron flux is a factor of a few below the upper bound. This is already intriguing, as hypothetical primary positrons could in principle have an arbitrary flux,  orders of magnitude larger or smaller than this upper bound. The rise observed by PAMELA can be interpreted that the suppression due to cooling decreases as positron energy increases. This is not implausible: the suppression depends on the interplay of the cooling time and mean propagation time of positrons, both of which are unknown functions of energy. Once the cooling time exceeds the propagation time the suppression factor is completely gone. In such a case the positron fraction should saturate the upper limit. This is what seems to be happening at the energies 200-500 GeV probed by AMS, as can be seen in the plot. Already the previous AMS data were consistent with this picture, and the latest update only strengthens it.

So, it may be that the mystery of cosmic ray positrons has a simple down-to-galactic-disc explanation. If further observations show the positron flux climbing  above the upper limit or dropping suddenly, then the secondary production hypothesis would be invalidated. But, for the moment, the AMS data seems to be consistent with no primary sources, just assuming that the cooling time of positrons is shorter than predicted by the state-of-the-art propagation models. So, instead of dark matter, AMS might have discovered models of cosmic-ray propagation need a fix. That's less spectacular, but still worthwhile.

Thanks to Kfir for the plot and explanations.

پی نوشت:

در مورداین مقاله که با لحن تایید آمیزی در نوشته ی بالا از آن یاد شده می خواهم خاطره ای بگویم. چند سال پیش  یک همایش در آی-سی-تی-پی در همین زمینه هابرگزار شد. در آن نویسندگان این مقاله کسی دعوت نشده بود و به این مقاله و دیدگاه محافظه کارانه ی آن هم اشاره ای نشد. من در بین جلسات با یکی از متخصصین درجه یک دنیا که در این زمینه کار می کنه در مورد مقاله صحبت کردم و او گفت عدم قطعیت هایی که در آن مقاله برای انتشار ذرات باردار در کهکشان در نظر گرفته شده زیادی گل وگشاد هست. اما ظاهرا نتایج جدید در جهت تایید آن هست. البته تا تایید قطعی راه زیاد هست. صبر کنیم ببینیم داده های بیشتر چی می گن!

پی نوشت دوم: از اصطلاح a simple down-to-galactic-disc explanation

نویسنده خوشم اومد. از down-to-earthکه به معنای افتاده و خاکی خودمون هست تقلید کرده. جستر در ساختن این اصطلاح  ذوق به خرج داده!

+ نوشته شده در  شنبه بیست و نهم شهریور 1393ساعت 12:34  توسط یاسمن فرزان 



با سلام
به آگاهی می‌رساند یکصدوهجدهمین نشست باشگاه فیزیک تهران ساعت ۱۷ روز دوشنبه 7 مهر‌ماه 1393 در سالن آمفی‌تئاتر دانشکده فیزیک دانشگاه تهران (انتهای خیابان کارگرشمالی) برگزار خواهد شد.
در باشگاه فیزیک این ماه آقای دکتر حمیدرضا سپنجی از دانشگاه شهید بهشتی درباره «آیا ابعاد جهان بیش از آن است که می‌بینیم» خواهند گفت.
ساعت 18:20 آقای دکتر خسرو حسنی از دان شگاه تهران پرسش ماه را مطرح خواهند کرد و پس از آن آقای ایمان مهیایه از دانشگاه صنعتی شریف اخبار مهم چند هفته گذشته را به آگاهی حاضران خواهند ‌رساند.
یادآوری می‌شود که مخاطبان باشگاه علاقه‌مندان به فیزیک هستند و از شما درخواست می‌شود که با فرستادن این نامه به دوستان خود یا چاپ و نصب پوستر باشگاه در محل کار خود دیگر علاقه‌مندان فیزیک را آگاه کنید.

با احترام
انجمن فیزیک ایران

+ نوشته شده در  شنبه بیست و نهم شهریور 1393ساعت 11:52  توسط یاسمن فرزان 

یادآوری می کنم که هفته بعد یکشنبه کارگاهی در مورد ماده تاریک و لپتوجنسیس داریم. ثبت نام و هزینه ثبت نام هم نداره. هرکسی خواست تشریف بیاره.

اطلاعات بیشتر در این سایت موجود است.

+ نوشته شده در  سه شنبه بیست و پنجم شهریور 1393ساعت 16:4  توسط یاسمن فرزان 

  arXiv:1409.3889 [pdf, ps, other]
Implications of a nonvanishing $Z γγ$ vertex on the $H\to γγγ$ decay
Comments: 4 pages, 1 figure
Subjects: High Energy Physics - Phenomenology (hep-ph)

The $Z\to \gamma \gamma$ and $H\to \gamma \gamma \gamma$ decays are strictly forbidden in the Standard Model, but they can be induced by theories that violate Lorentz symmetry or the CPT theorem. By assuming that a nonvanishing $Z\gamma \gamma$ vertex is induced in some context of new physics, and by analyzing the reaction $H\to \gamma Z^*\to \gamma \gamma \gamma$ in the $Z$ resonance, we obtain an estimation for the branching ratio of the $H\to \gamma \gamma \gamma$ decay. Specifically, it is found that $BR(H\to \gamma \gamma \gamma)\lesssim BR(H\to \gamma Z)BR_{Exp}(Z\to \gamma \gamma)$, which is of the order of $10^{-7}$.

 

 

The decay of the Z gauge boson into a pair of photons
or gluons is forbidden in standard field theories due to angular
momentum conservation and Bose statistics. This
fact, which is known as the Landau-Yang theorem [1], applies
to any transition of a massive vector boson into two
massless vector particles

+ نوشته شده در  سه شنبه بیست و پنجم شهریور 1393ساعت 10:1  توسط یاسمن فرزان 


The subatomic physics group at the University of Bergen (Norway) has an opening for a PhD student starting in early 2015.  The complete announcement and the online application can be found at
http://www.jobbnorge.no/en/available-jobs/job/105349/research-fellow-phd-candidate-in-theoretical-particle-physics

+ نوشته شده در  جمعه چهاردهم شهریور 1393ساعت 22:51  توسط یاسمن فرزان 

تدریس خصوصی و گروهی زبان تخصصی فیزیک توسط fluent speaker:

اگر در خواندن مقالات یا کتاب های علمی مشکل دارید یا میخواهید مطلبی را به انگلیسی آکادمیک برگردانید میتوانید از دوره های کمکی ما استفاده کنید. برای دریافت فایل نمونه و نحوه ی صحبت مدرس و اطلاع از کلاس ها به آدرس  y.moghadamnia@gmail.com  ایمیل بزنید یا با شماره ی 09333817136 (مقدم نیا) تماس بگیرید.

 یادآوری: کلاس ها به صورت اینترنتی نیز برگزار میشود که درآن صورت، از تخفیف برخوردار خواهید شد.

یادآوری دوم: برای اطلاع از دیگر دوره های تدریس (غیر از زبان تخصصی فیزیک) در ایمیل خود این مورد  را ذکر کنید.

+ نوشته شده در  چهارشنبه پنجم شهریور 1393ساعت 17:13  توسط یاسمن فرزان 

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