# Soft gluons and colinear gluons

• varphi42
In summary, soft and collinear gluons refer to gluons with low momentum and those produced near the beam direction, respectively. Gluons can be both soft and collinear, and they are mostly emitted in this way according to the probability distribution.
varphi42
Is there a difference between soft and colinear gluons?

Are soft gluons the ones with small energy, or with small PT with respect to the qqbar axe (in this case soft and colinear would have the same meaning)?

Usually soft gluons are those with low momentum, i.e. the less energetic ones, while collinear gluons are those produced near the beam direction, i.e. with low PT.
Obviously a gluon can be both soft and collinear. In fact, one can show that gluons are mostly emitted soft AND collinear as the probability of emitting a gluon from a quark is given by:

$$dP_{q\rightarrow qg}\propto \frac{dk}{k}\frac{dk_\perp^2}{k_\perp^2}$$

and so the probability is strongly centered around k≈0 (soft) and PT≈0 (collinear).

Ok thank you!

## 1. What are soft and colinear gluons?

Soft gluons are gluons with low energy or momentum, typically in the infrared region. Colinear gluons are gluons that are emitted in the same direction as a high-energy quark or gluon, but with a much lower energy or momentum.

## 2. What is the role of soft and colinear gluons in QCD calculations?

Soft and colinear gluons play a crucial role in QCD calculations as they are responsible for the majority of the radiation and scattering processes in high-energy particle collisions. They also play a significant role in the development of parton shower algorithms used in simulations of particle collisions.

## 3. How are soft and colinear gluons treated in QCD calculations?

In QCD calculations, soft and colinear gluons are usually treated using factorization techniques. This involves separating the calculation into two parts: the hard process, which involves high-energy interactions, and the soft and colinear radiation, which is treated separately using effective theories such as soft-collinear effective theory (SCET) or parton distribution functions (PDFs).

## 4. Can soft and colinear gluons be experimentally observed?

Due to their low energy or momentum, soft and colinear gluons cannot be directly observed in experiments. However, their effects can be indirectly observed through their contributions to the final state particles in high-energy collisions.

## 5. How do soft and colinear gluons affect the accuracy of QCD calculations?

The inclusion of soft and colinear gluons in QCD calculations is crucial for achieving accurate results. Neglecting their contributions can lead to large discrepancies between theoretical predictions and experimental data. Therefore, it is essential to accurately model and include these gluons in QCD calculations.

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