Particle physics is the study of the fundamental forces of nature as they are embodied in the interactions of elementary and composite particles at high energies and short time and distance scales.

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What allows the modified Urca process to work at lower density than direct Urca in neutron star cooling?

The dominant method of neutron star cooling is neutrino emission. There are two regimes usually presented, the "direct Urca" and "modified Urca" processes, each of which are sequences of neutron decay ...
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2answers
244 views

Where to find cross section data for e- + p -> p + e-?

Where to find cross section data for e- + p -> p + e-? PDG's cross section data listing does not include it.
13
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5answers
820 views

Speed of neutrinos

Everyone knows it is close to $c$, but how close? What are the recent results?
6
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2answers
475 views

What is the percentage of useful energy do we get from matter-antimatter annihilation?

This is a theoretical question since we haven't made enough antimatter to try it in reality of course. But I am asking about the physics part in this. Also, by "useful energy" I mean the energy we ...
4
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2answers
376 views

More on matter and anti-matter

1.) Does every particle that has rest mass also have an anti-particle with which it would annihilate? 2.) Does annihilation only occur between like particles? For example what happens if a ...
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2answers
7k views

What's the difference between inclusive and exclusive decays?

For example, why is the semileptonic $B$ decay $B \to X\ell\nu$ inclusive? I can't find any definition of these frequently used terms, strange.
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4answers
2k views

What is your simplest explanation of the string theory?

How would you explain string theory to non physicists such as myself? I'm specially interested on how plausible is it and what is needed to successfully prove it?
15
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2answers
3k views

Particle physics plots

I'm having a hard time understanding what some of the plots that are presented by ATLAS/CMS actually show. See for example: http://resonaances.blogspot.com/2011/07/higgs-wont-come-out-of-closet.html ...
16
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1answer
5k views

Are all electrons identical?

Why should two sub-atomic (or elementary particle) - say electrons need to have identical static properties - identical mass, identical charge? Why can't they differ between each other by a very ...
9
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2answers
991 views

How are Monte Carlo simulations used in experimental high energy physics?

How are Monte Carlo simulations used in experimental high energy physics? Particularly in studying detectors limitations (efficiencies?) and data analysis. I will appreciate giving a simple example ...
4
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2answers
766 views

Why do physicists think that the electron is an elementary particle?

When we first discovered the proton and neutron, I'm sure scientists didn't think that it was made up of quark arrangements, but then we figured they could be and experiments proved that they were. ...
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3answers
4k views

Is it pions or gluons that mediate the strong force between nucleons?

From my recent experience teaching high school students I've found that they are taught that the strong force between nucleons is mediated by virtual-pion exchange, whereas between quarks it's gluons. ...
13
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3answers
479 views

Why are all force particles bosons?

All of the force-particles in the standard model are bosons, now my question is pretty short, namely: Why are all force particles bosons? This can't be a coincidence.
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5answers
564 views

Matter-Antimatter Asymmetry in Experiments?

As I hope is obvious to everyone reading this, the universe contains more matter than antimatter, presumably because of some slight asymmetry in the amounts of the two generated during the Big Bang. ...
8
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2answers
967 views

How can neutrinos oscillate though the lepton flavors have differing masses?

Since the total mass-energy for the neutrino presumably does not change when a neutrino changes lepton flavor, though the mass is different, what compensates for the gain or loss of mass? Does the ...
5
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3answers
510 views

Cosmic ray hazards

The Pierre Auger Observatory site mentions the detection of a 3E20 eV (48 J) cosmic ray whose energy, well above the GZK cutoff, was based on an analysis of its atmospheric shower. This was equivalent ...
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1answer
599 views

Neutrino Oscillations and Conservation of Momentum

I would like to better understand how neutrino oscillations are consistent with conservation of momentum because I'm encountering some conceptual difficulties when thinking about it. I do have a ...
9
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1answer
596 views

Universality in Weak Interactions

I'm currently preparing for an examination of course in introductory (experimental) particle physics. One topic that we covered and that I'm currently revising is the universality in weak ...
8
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1answer
291 views

Baryogenesis only at the Planck scale, or none at all?

I can think of three general ways of explaining why the universe contains more matter than antimatter: (1) Near the Planck time, the universe had zero baryon asymmetry, but at some later time, ...
6
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1answer
580 views

Is there a concise-but-thorough statement of the Standard Model?

I’m a grad student in high-energy physics. I’m familiar enough with the Standard Model, but I’ve always wondered whether there existed a canonical statement of, effectively, “what we talk about when ...
4
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1answer
1k views

Does photon have size measurement because of its particle nature

Does photon have size measurement because of its particle nature like electron's 3.86*10^-13m etc..
3
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1answer
526 views

What is “charge discreteness”?

I assume it is some kind of quantity. Google only made things more confusing. I get that it has something to do with circuits. I also get what a discrete charge is. In fact, I thought charges ...
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2answers
295 views

Why are atoms particles?

The Oxford English Dictionary definition of particle is as follows: "A component of the physical world smaller than the atom." I read an article in NewScientist and it said "...all particles from ...
8
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2answers
711 views

How do you find spin of a particle from experimental data?

So I was wondering, with all this Higgs talk going on, they just detected a particle with a mass of 125 GeV (CMS) or 126.5 GeV (ATLAS). But they still don't know what it is, since there is tons of ...
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1answer
4k views

Why isn't Hydrogen's electron pulled into the nucleus? [duplicate]

Possible Duplicate: Why do electrons occupy the space around nuclei, and not collide with them? Why don’t electrons crash into the nuclei they “orbit”? From what I learned in chemistry, ...
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3answers
2k views

What are the details around the origin of the string theory?

It is well-known even among the lay public (thanks to popular books) that string theory first arose in the field of strong interactions where certain scattering amplitudes had properties that could be ...
24
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1answer
748 views

Status of experimental searches for tachyons?

Now that the dust has settled on the 2011 superluminal neutrino debacle at OPERA, I'm interested in understanding the current status of experimental searches for neutrinos. Although the OPERA claim ...
20
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3answers
6k views

Why you need a graviton when you have the higgs boson?

Since I studied General Relativity I had this question running on my mind. As I see it (just taking lectures of Quantum Field Theory right now) "Why you need a gauge boson for gravity when the higgs ...
11
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1answer
878 views

How to determine the mass of a quark?

As far as I know quarks are never found in isolation, so how can we determine their rest mass?
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2answers
635 views

Are there massless bosons at scales above electroweak scale?

Spontaneous electroweak symmetry breaking (i.e. $SU(2)\times U(1)\to U(1)_{em}$ ) is at scale about 100 Gev. So, for Higgs mechanism, gauge bosons $Z$ & $W$ have masses about 100 GeV. But before ...
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3answers
2k views

Can a third type of electrical charge exist?

Upon reading my book on physics, it mentions that there are only two discovered types of electric charges. I wonder if there could be a third type of elusive charge, and what type of effects could it ...
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2answers
2k views

What is the definition of colour (the quantum state)?

I heard somewhere that quarks have a property called 'colour' - what does this mean?
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5answers
1k views

Intrinsic structure of electron

The electron contains finite negative charge.The same charges repel each other.What makes electron stable and why does it not burst? Is it a law of nature that the electron charge is the smallest ...
25
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4answers
1k views

“Slightly off-shell”?

I'm not new to QFT, yet there are some matters which are quite puzzling to me. I often come across the statement that real particles (the ones we actually measure in experiments, not virtual ones) are ...
15
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2answers
2k views

Please explain the physics of a Cloud Chamber

A friend of mine was telling me about building a cloud chamber while he was in graduate school. As I understand it, this allows you to "see" interactions caused by high energy particles going through ...
11
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2answers
414 views

Black hole “no hair” theorem

The "no hair" theorem (or conjecture), suggests that black holes can be entirely described by their mass, angular momentum and charge. All other details of the BH formation are lost. Is there a ...
8
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4answers
6k views

Why can't a single photon produce an electron-positron pair?

In reading through old course material, I found the assignment (my translation): Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta. ...
5
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2answers
679 views

Why absence of electron is called hole?

I am having hard time in understanding the concept of holes: If there is no electron than how can it be a hole? For a moment lets assume absence of electron is termed as hole but how can this absent ...
5
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4answers
1k views

If the electron is point like, then what is the significance of the classical radius of the electron?

What is the physical meaning/significance of the classical radius of the electron if we know from experiments that the electron is point like? Is there similarly a classical radius of the photon? The ...
12
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2answers
404 views

Is there any theory for origination of charge?

We have a theory of a Higgs field that describes how a particle gets mass. Since mass and charge both are intrinsic properties of a particle, is there any similar theory for how particles get electric ...
8
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2answers
363 views

How come the universe is made of matter and not antimatter?

Antimatter is like matter on opposite day: it has the same properties as the stuff that makes up planets, stars and galaxies, but one vital piece is different—its charge. The universe supposedly ...
7
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2answers
6k views

What is p_T? (transverse momentum?)

I've been looking at a few papers in experimental physics (from the ATLAS collaboration, for example) and I've often run across phrases such as "high-p_T electron." What exactly is p_T? Is it simply ...
6
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3answers
2k views

Why does electron-positron annihilation prefer to emit photons?

If gravitons are massless, and neutrinos nearly so, why aren't pairs of either of them normally expected products of electron-positron annihilations? Are they possible but simply unlikely, or is there ...
4
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3answers
1k views

What if we could give photons some mass?

I was reading an article and these paragraphs got me wondering... Before I list the replies, here is some background. The Higgs mechanism describes an invisible field that, it is argued, split one ...
3
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1answer
870 views

Are There Strings that aren't Chew-ish?

String theory is made from Chew-ish strings, strings which follow Geoffrey Chew's S-matrix principle. These strings have the property that all their scattering is via string exchange, so that the ...
2
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2answers
327 views

How the nucleon structure has been identified experimentally?

It is known that nucleons (proton, neutron) are composed of partons (quarks, etc.). How was this identified experimentally? In particular, how it has been identified that nucleons comprise of more ...
9
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3answers
465 views

“Weakness” of gravitational force

I often hear that gravitational force is much "weaker" than electroweak and strong forces. But how can you compare the strength of interactions without the parameters like mass, charge on which it ...
7
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4answers
922 views

Why are neutrino oscillations considered to be “beyond the Standard Model”?

Is this just a historical artifact - that the particle physics community decided at some point to call all of the pre-oscillation physics by the name the "Standard Model"? The reason I ask is because ...
3
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3answers
282 views

Can a single particle create a black hole?

Let us suppose a particle with so much energy $ E= h \frac{c}{\lambda} $ so $ \lambda $ is smaller than Planck's length ? Would it be possible? I mean if the particle has so much energy then its mass ...
2
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1answer
809 views

Why some particles interact with the Higgs field and others don't?

Why some particles interact with the Higgs field and others don't? Higgs doesn't explain that much: why some particles have mass and others don't? is like why some particles interact with the Higgs ...