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！

latex test

WordPress supports input both in the posts and the comments.

To include code in your post, use the following.

$ latex your-latex-code-here$
(without the space after the first $)

So, for example,

$ latex i\hbar\frac{\partial}{\partial t}\left|\Psi(t)\right>=H\left|\Psi(t)\right>$

produces

This output cannot be seen even in ‘Visual’ mode until you publish the post.

Some lessons

It’s not easy to start research. The first step is to get use to your advisor and learn many many lessons.

My advisor asks me to calculate a flavor model in the simplest case. It seems that what I am calculating always goes away from what he is thinking, and when I come back to look for his help after wasting a lot of time, he seems already know the answer to the next step. It’s so clear to him what will happen next, but I have to spend many time and energy on it before I see the simple point. I believe he would not even bother to talk with me on these problems if he was not training me. This is the distance between a sophisticated professor and a beginner, he have much more intuition, both mathematically and physically, that is, he can easily guess the result without any complicated calculation. I can’t, I lack the intuition and spent much time on meaningless calculations, I used too many parameters which I do not know how to simplify and have to use mathematica, and of course, no meaningful things came out. He told me not to use mathematica at this time, approximation is much more powerful than exact calculation for the first step of a model. He is right, I should think much before I calculate. I did find a much simpler way to calculate after all, although the setup was too simple to be right.

I find myself doing something I disgusted in my college – model building. I don’t want to become a professional model builder, although my advisor is, and he is really a successful one. In fact he gave me two directions to choose in the beginning, one is more formalistic, something on RS and string theory. I considered that one and gave up, although it’s somewhat closer to me early dream, because he is not a string theorist and string is not a good start point for me. I chose model building for a good chance to learn LHC related phenomenology and more about SM and beyond, but my long goal is not sticked on it. So I should keep in mind: I am not building model, but doing physics; I am not playing toys, but playing with the real world. I should try my best to find the more fundamental physical meaning for what I am doing. I think my advisor is trying to do this. He is not a pure model builder, at least, he is more like an engineer than a worker if we compare model building to house building. But now, I have to start as a worker – do what he tells me to do with the tools he provide to me. I have to make a change, as someone said, good graduate students can lead their advisor in the end.

XD ppt

This is my presentation in the 2nd year seminar, a brief introduction to extra dimensions, which lasted 25 minutes. See here.

What a Sadness

I just learned the news from Motl’s blog, that Julius Wess died on Aug 8th, just several days after SUSY 07. He even gave a colloquium on that conference, and we were joking that Bagger just went there to see this old freind. Because of his contribution to Supersymmetry, Wess would be sure to ge the Nobel Prize if LHC found SUSY particles. Now he died too suddenly, and too early, to witness his beautiful theory to be tested.

Well, maybe now he can go to ask the God directly, and find the answer even earlier than us. But will the God give him any rewards if he was right?

大牛现身

今天一打开arXiv，就发现 ‘t Hooft 同学挂了两篇帖子。我还以为是他新发paper了，原来都是报告发言稿。其中一篇The Grand View of Physics，短短6页，从Salam的高度和胆量谈开去，为的是再次阐明自己对以量子力学为基础的从量子场论到超弦的一系列理论的怀疑。他说建立在Hilbert空间线性变换上的量子力学很可能是不完备的，我们仍然需要努力寻找量子力学“背后”（尽管很多人认为量子力学没有“背后”）的解释以发现可能的全新物理。不知道他是否真的接爱因斯坦的班，是个彻底的决定论者。不过退回到量子力学重新寻找全新的道路是个极其艰巨的任务，恐怕也只有’t Hooft 这种功成名就的老前辈才敢玩。无论如何，’t Hooft这种牛人存在的价值就是让人大开眼界。还有一篇比较长，Emergent Quantum Mechanics and Emergent Symmetries. 只看了abstract, 把量子统计涌现性（根源是决定论的）跟规范对称性和广义坐标变换等价性联系起来，很难说有多大理论价值，但貌似很有意思，什么时候有空再看看。