杂记

一个人想法的多少难道是随着他的知识增长成U型变化趋势的吗？以前只知道皮毛的时候想法很多很大胆，跟民科一样，现在目不暇接地学了一堆东东却似乎什么都不敢想了，因为每一个想法似乎不是有人做过就是马上会被否定掉，经历了太多这样的情况就开始变得麻木了。真是有些担心，不知道学通之后真正有用的想法能不能多起来。就算做model builder, 我的目标也是总工程师。

今年的诺贝尔奖跟我正在关心的东西关系不小，如果有时间我会自己总结一个review,关于对称性自发破缺和费米子混合问题。

美国的高能实验快进入冬天了。美国政府的科研经费申请政策太弱智了，一个大型项目每年都要重新审批，砸了无数钱建到一半都能扔下，不想出钱又狂浪费钱，目光短浅的严重表现。现在好了，金融危机一闹彻底没戏了。

LHC最早在明年4月就能出第一批数据了，而额外维的引力子貌似是需要数据最少的。RS首当其冲啊，要死的话就是死得最早的。这样也好，省得我在上面白白浪费青春。

LHC Physics Mini Workshop

LHC is coming soon, every one in particle physics is excited. And you can smell in the air how crazily LHC-driven they are. The mini workshop of LHC physics was held at UMD, for model builders to discuss the phenomenology of some model which are expected to be tested on LHC.

Quirks:
Markus Luty and Roni Harnik
Both of them talked about “quirk”(sounds like the bird who calls “quark, quark, quark” catches a cold). Quirk is the simplest extension of SM gauge symmetry. It’s a QCD inspired SU(N) field, with it’s strong coupling scale . So if you stretch a quirk-antiquirk pair, the energy reserved in a unit length is , unlike QCD, it is less than the energy required for producing another quirk-antiquirk pair, so the string will never broken. It can be stretched to macro scale, depending on the cutoff. Because of the string, this pair can oscillate like a spring. This gives the quirk very rich phenomenology. The two talks focused on the detection signal of quirks, they argued about different life time of the string before it annihilates and the stuff shaken off during the oscillation.
Markus’ quirks also carry QCD colors. So they will be surrounded by quarks and gluons to form a so called “brown muck”. When the string is oscillating, soft pions are the dominating shaken off particles. They lifetime of the string is estimated by using WKB approximation of the wave function of a quantum oscillator, and then considering the energy and angular momentum change by shaking off pions
Roni actually studied squirks, which are superpartners of quirks. They are uncolored under QCD. And in his “folded SUSY” model, squarks and quirks are orbifolded out by symmetry, leaving quark and uncolored squirks to be superpartners. This scenario can be realized in extra dimension. Since the squirks are uncolored, the dominating shaken off stuff are soft photons and glueballs(I do not understand why there are still gluons even though they are not colored.). And the life time is longer since the energy taken away by photons are much lower.

Loopy fermion mass:
Patrick Fox
This is most precise “posdiction” of fermion mass I have ever seen, which is too good to be true. But it’s interesting and maybe useful to my project. In this model, it assumes that only top quark (or one quark which we call “top”) gets mass at tree level, and all the other uptype fermions get their mass at loop level.
The conventional yukawa term is forbidden by a new U(1) symmetry of Higgs, so another U(1) charged scalar field is introduced to form a 5 dimensional operator, whose UV completion is the propagator of a massive U(1) charged fermion . The UV completion allows only one type of quark coupled to and , so after this U(1) symmetry is broken by vev of , only one type of quark obtain mass. But my question is, is there any symmetry to forbid other types of fermions in this coupling?
The next work is more generic. By introducing a QCD and EW charged scalar field r and a set of coupling constants, the following fermions get mass at 1,2,3,4,5 loops level respectively: . The generated mass are very close to the real values, by varying the couplings only between 0.3 and 3. This is the most beautiful part of the model.
The down type masses are much more messier. They have to introduce several weird fields and coupling terms, but the couplings are still order one. And the CKM matrix is in the right form. Neutrino mass is not explained in this model. But it can be done via seesaw mechanism.
The most intriguing LHC signals will be those colored and charged scalar fields. The constrain of available data is , which gives no hope to see them at LHC. But the constrain for a scalar field for downtype mass is .

LHC – The 0th Year

End June:
Machine cold
Expt beampipe bakeout finished

Mid-July:
Experiments configured ready for beam
Patrol of tunnel and caverns
Controlled access only

End-July:
First injection
Commissioning with beam, including at some point
few hours collisions at 900 GeV cm

End Sept:
Collisions at 10 TeV..2008 physics run.

End Nov??
Winter shutdown: commission to 14 TeV.

Dark Energy – Ten Years

There is a symposium held by STScI to review the ten years’ progress in cosmology after the discovery of accelerating expansion of the universe. I missed some very important talks on Monday, not only because one of the speakers was Witten but also it was the only series of physical topics (rather than observational ones) of dark energy in these 4-day talks. I only attended several short talks(mostly 15 minutes each). Some topics for record:

1 First Results from the WiggleZ Galaxy Redshift Survey
Chris Blake (Swinburne University of Technology)
The WiggleZ project at the Anglo-Australian Telescope is a large-scale redshift survey of UV-selected emission-line galaxies. The survey is mapping a co-moving volume of approximately 1 Gpc^3 at a significantly higher redshift (0.5 < z < 1.0) than has been previously achieved by projects such as the 2dFGRS and SDSS. The main science goal is to use baryon acoustic oscillations in the galaxy clustering pattern as a standard ruler to measure the cosmic distance scale and expansion rate to z=1 and hence perform a robust test of the cosmological constant model. The survey is approximately 50% complete and is scheduled to finish in 2009. I will introduce the project and present initial results on the clustering, environments and luminosity function of high-redshift star-forming galaxies. I will also discuss forecasts for testing dark energy models with WiggleZ in the context of current and future cosmological datasets.

2 The Dark Energy Indicator: A Measure of Deviations of w from -1″
Ruth Daly (Penn State University)
The dark energy indicator provides a tool to measure deviations of the equation of state of dark energy from -1 over the redshift range from zero to one. The indicator is model-independent, and will be shown for the most recently available supernova and radio galaxy data sets. The preliminary results are consistent with a constant equation of state w of -1 from a redshift of zero to about one.
Note: This is interesting and may also be useful. The usual way we constrain the cosmological parameters, eg. , , is model dependent, ie., take a particular model with undetermined parameter and calculate the expected observational curve and fit with the data. This talk provided a way to draw the parameters directly from data. The only assumption is RW metric and GR. From the general Friedman equation, we can express the coordinate distance and it’s first and second derivatives (wrt z) with those parameters. By analyzing the distance-redshift curve directly, we can fit the parameters. In order to determine w, which can vary with z, we need to construct another parameter called “dark energy indicator” s, which is 0 for w=-1. The fitted s with supernovae data is 0 for z<1, but when z goes close to 1, there is a bump. It’s still unclear whether this bump is caused by systematics or it has any physical meaning.

3 Uncorrelated Estimates of Dark Energy Equation of State
Asantha Cooray (UC Irvine)
I will give a talk on some of the recent work we have done on how to extract and establish equation of state with supernovae and other cosmological data.
Note: There are three subtopics. I can only remember two. One is about the measurement of spectrum by putting many many filters for each wavelength. Another one is that type Ia supernovae actually have two subtypes with different light curved, ie., the distribution of time difference of maximum and some certain fraction of luminosity has two peaks. And these two subtypes’ population change with redshift differently, one increases with z, but the other one decreases. So the precision of measuring distance by using the time difference (cosmological time dilution) will be reduced by a factor of 2-3. Adam Riess complained soon. He said by investigating low z supernovae, we can get enough information to distinguish these two types. Well, he is the quasi-Nobel on supernovae, no one doubt he can do that.

4 Inflation and Dark Energy: Is There a Connection?
Scott Watson (University of Michigan)
We now have convincing evidence that both today and in the very early universe, the cosmic expansion went through a period of acceleration. A natural question arises: Are these periods of acceleration connected? I will briefly review past attempts to address this question, as well as more recent attempts motivated by the string landscape. In particular, I will discuss a crucial theoretical difficulty that arises in constructing such models, due to the vast range of energy and length scales involved. I will also discuss the possible experimental signatures that may arise if such models can be realized.
Note: If the acceleration today is due to the same mechanism as inflation, w should not be a constant, because the effective w of inflation is changing in order to end inflation at some time. Theoretically, the scalar field may not be elementary dynamical field. One model is ‘cascade universe’ with a stair of vacuums, and more generally they are inspired by string landscape. They difficulty of these models: I can’t remember. Think about these possible difficulties: no slow roll condition, no fluctuation seeds, unnatural(well, this is the weakest one), etc.

概率问题

一个运动员在药检时结果呈阳性（即怀疑服用违禁药品）。药品检验器材的厂方提供的信息是：在阳性结果里有1%的失误率，在阴性结果里有20%的失误率。在 已经受检的20000名运动员中，有300人结果呈阳性。请问这位运动员确实服用了违禁药品的几率是多大？（提示：不是99%，我也搞不懂为什么）。

Update:

这是一个条件概率问题。关键在如何理解厂方信息。所谓“阳性结果里有1%的失误率”是什么意思？是不是说一个人如果被检验成阳性，他吃药的几率就是99%？从字面理解好像是这样，但仔细想想，这个数据不是厂方可以提供的，因为要想得到这个数据，我们需要一堆事先并不知道是否吃药的样本人群，让他们去测验，拿到结果后再跟他们是否吃药的真实情况比较，但是否吃药显然跟整个人群的吃药率有关，因此这个量不是能够独立测量的。但另一个量是只跟仪器有关，跟人群吃药率无关的，那就是一个事先确定吃药或没吃药的人的检查结果，厂方正是用这个方法来定义和测量仪器灵敏度的。用数学语言说，如果用P(A|B)表示条件B成立的情况下A的概率，那么厂方提供的是P(阳性|未吃药）=1%，P(阴性|吃药)=20%，而我们想要求的是P(吃药|阳性）。最后答案是32%。

New physics seen?

The UTfit group claimed that they find the ‘first evidence’ of new physics in transition.By combining analysis of all the experiments on mixing, the CP violation phase is 3 deviated from both SM and MFV(minimum flavor violation). Sounds thrilling, but we still need to wait and see.

Nima’s Talk

Nima Arkani-Hamed may be the most famous and interesting name in the new generation of physicists. I attended his talk today in UMD, and his words are so impressive that even though I can’t understand the details, I feel deeply interested and attracted.

His title is ‘quantum gravity: possible and impossible’. In all, his purpose is to convince us that because of gravity, IR and UV are not totally decoupled as we thought before, gravity can contribute to high dimensional operators in EFT, and that not all IR effective field theory have a consistent quantum gravity theory as its UV completion, no matter it is string theory or not. The most important criteria he uses is the Weak Gravity Conjecture, which says that gravity must be the weakest long range (U(1)) force, and an effective theory should break down even below plank scale, at , where g is the gauge coupling, otherwise this system will decay into a black hole. A sharper form of WGC is that any matter must have . So if an IR theory violate WGC, it can never find a consistent quantum gravity theory as its mother theory.

So what EFT may be on the ‘dead’ list? One most important example is the inflation models which produce gravity wave, such as chaotic inflation. If gravity wave is generated, we require that the scalar field vev must be greater than plank scale. Naively, this can be realized in a 5 dimensional model with a very small 5D gauge coupling and it seems very natural. However, no one can find it in string theory, no matter what form they tried, there is always something, such as diliton, coming out to destroy it. Is there any general reason why they failed? The answer may be yes: it violates week gravity conjecture. There is another model named ‘N-flation’, with N species of scalar field triggering inflation. It is also problematic because it violates species bound which comes from constrain of entropy of a black hole. Moreover, if considering loop correlation of , we have which also leads to violation of WGC for large N. So Nima boldly guess that we can not see gravity wave in CMB, otherwise, it will be a big crisis.

Another interesting model he talked about was Euclidean warm hole. It was proved by Coleman that warm hole theory can be a local theory with global symmetry breaking, which can be realized with axion coupling with gravity. However, as chaotic inflation, we can not find it in string theory. And the obstacle is also WGC. To realize a warm hole, we just need to find a geodesic longer than , which means we have a energy scale higher than plank scale and the corresponding U(1) gauge coupling is weaker than gravity. Further, he said that if we realize it in a AdS/CFT world, it will violate unitarity of CFT, which I do not understand.

Nima also talked about some other models, checking them with WGC. Especially, he mentioned RS. He said we can not ignore the compacted in when we go down from string theory. Otherwise if we take a brane near the UV side with a string on it, the corresponding particle(gauge boson) will have mass which can be larger than . We need the extra-extra dimensions to ‘dilute’ the mass of the particle. I do not understand how either, too ’string’.

Nima is really a good speaker. He makes each point very clear, and goes through the details very smoothly. Speaks fast, writes faster.

Also Sprach David Gross

David Gross was invited to our department for 3 days. During one public lecture, one colloquium and some informal chat, there are something he said which may be interesting to hear. One thing to remind you, Gross is a very conservative reductionist, so you may not like some of his words. Welcome to leave your comments. I just quote them (not word by word) in a random order:

Things I learned these days

• Neother Theorem is the Lagrangian form of Ehrenfest Theorem which is in Hamiltonian form. There are two correspondences (not very exact) :

1. The Lagrangian is invariant under some symmetry corresponds to the Hamiltonian commutes with some operator (symmetry group’s generator)
2. The Neother charge is a time constant corresponds to the expectation value of the symmetry operator is a time constant, actually, the Neother charge is just the expectation value of the generator.

• From Kaplan’s Colloquium (11.29):

1. EW theory must break down because of 4W interaction, just like 4 fermion interaction must break down, which is caused by violating unitarity. And the solution is similar, 4 fermion need a gauge boson to change 4 vertex to 3 vertex, and 4W need a Higgs (or other new particle?) to change 4 vertex to 3 vertex.
2. MSSM has more than 100 parameters! ( I heard this for the first time)
3. Higgs mechnism can be traced back to Schwinger, who proved that massive gauge bosons do not necessarily violate gauge symmetry, by introducing a scalar field, but did not mention symmetry breaking. And there were other guys’ work following this, showing symmetry breaking, until Higgs pointed out the existence of a scalar particle following the referee(highly suspected to be Schwinger)’s suggestion. So came the name ‘Higgs particle’.

• From Tom’s seminar (12.3)

SUSY can be broken explicitly at UV (elementary sector) but emerges accidentally at IR (composite sector). The symmetries at lower energy are more than that at higher energy, which seems blizzard, but not. From 5D view, IR have more gauge symmetry and so more degrees of freedom just because UV and IR are two vacuums separated by the bulk (or domain wall); from 4D view, the different symmetries become global symmetry so it will not take more degrees of freedom, and the low energy global symmetry is always broken at high energy. (Because of gravity)

A New Group Blog

We started a new group blog, discussing all the things we care about in physics.

SuperExtra

开博致辞：

学好物理需要什么？废寝忘食地看书还是埋头刻苦的计算？这当然是必要的，但是还不够。我们需要交流，活跃的交流，广泛的交流，不但要听，还要说，不 但要学，还要争。不过很遗憾，这样的交流氛围在国内外似乎都不够，特别是对于刚进入这个领域的我们而言。于是，在跟 Umbriel 达成强烈共鸣之后，我们决定用blog搭起桥梁，让远隔千里的我们能够有一个平台进行有效的学术讨论。

其实，利用网络的学术交流已经实践很久了。繁星客栈和格志便是论坛和blog的优秀典范。然而，论坛在国内的生存空间已经越来越小，繁星客栈和其他 很多学术论坛都已经因为屏蔽而不得不暂时关闭；而格志又因为作者太多领域太泛而降低了交流的效率。因此，我们这个blog不准备搞得很大，仅仅定位为理论 粒子物理和宇宙学及相关话题的讨论平台，参与的作者都是这个领域的研究生。为了提高交流的效率，我对作者们提一些建议：

1. 博文的内容主要为理论粒子物理和宇宙学，当然也欢迎数学、实验及其它相关话题。

2. 博文的形式可以很广泛，比如学习心得，科研介绍，疑难提问，文章评论，报告总结，实验谣言，历史八卦……总之要 keep going, 让我们的氛围活跃起来。

3. 既然这是讨论的平台，就不要当成个人空间。为了提高讨论效率，写出来的东西要尽量提供足够的知识背景，让大家看懂，因为虽然是同专业，但做的东西也不完全一样。当然也不用太科普，尽量学术化一些。

4. 博文语言中英文不限，不建议使用其他语言（包括火星文）。72pines支持输入（尽管没有wordpress自己的版本好看), 格式：[tex ]latex code[/tex] （不含空格）。
我希望这个blog能够结合个人日志和BBS的优点，对大家的学习研究有所帮助。

最后喉一声：让我们团结起来，为中国的物理事业奋斗终生！