Low p_t cuts in experimental measurements

[CAF123]

Just few questions based on experimental presentation of results:

In (semi-)inclusive measurements (say, p + p -> Y + X), producing some well defined state Y and the rest, X, summed over, is there typically a minimum cut made on the p_t of the state Y? If so why?

What is the reason of presenting results differential in p_t vs. Results integrand over p_t? Is one preferential to the other?

I guess p_t cannot be made too small because of angular resolution of the detectors but some exclusive reaction final states are identified with a low p_t so I wondered what the difference is there.

 

[mfb]

On Y or its decay products, often on both. You need some pT to get the particles into the detector, and at very low p_T for combined particles you still get all sorts of backgrounds you don't want.
Differential measurements have more information than integrated measurements, but they are generally more difficult to do and require more statistics.

 

[CAF123]

Thank you.

On Y or its decay products, often on both. You need some pT to get the particles into the detector, and at very low p_T for combined particles you still get all sorts of backgrounds you don't want.

Why at larger p_t you get less background?

Differential measurements have more information than integrated measurements, but they are generally more difficult to do and require more statistics.

Is that because in differential measurements you are looking for the final state Y or its decay products in some pt (or say rapidity) bin while in integrated measurements you look for the final state with any pt?

 

[mfb]

Low pT is stuff close to the beam axis where you get fragments from the protons and other low energy QCD products. Some studies are interested in it, but most are not. Larger pT is less common and easier to describe theoretically. In addition it's using your bigger, better accelerator more.

Is that because in differential measurements you are looking for the final state Y or its decay products in some pt (or say rapidity) bin while in integrated measurements you look for the final state with any pt?

I don't know what "that" refers to.

 

[CAF123]

Thanks.

I don't know what "that" refers to.

I was referring to the part of your reply where you said ‘[differential measurements]...are generally more difficult and require more statistics’.

As far as I know (maybe I am wrong), a necessity for an Exclusive process is a low p_t for the well defined state. Is this because at higher p_t there is less chance to find regions of the detector where there is no activity? (As needed for an exclusive event)

 

[mfb]

As far as I know (maybe I am wrong), a necessity for an Exclusive process is a low p_t for the well defined state.

Huh?
p_t can be whatever it wants. The sum of p_t of all particles in the event needs to be zero. Finding all particles in a high energy proton-proton collision is rarely feasible, but the particles that are not detected have a low p_t, so the overall sum over the transverse momenta of detectable particles is usually quite small (if all particles are found and there is no neutrino).

 

[CAF123]

Thanks. Yup sorry I had meant central exclusive in the above so had in mind process of the form, p + p -> p + p + Y. As the event is exclusive, the two intact protons after collision basically go down the beam pipe and final state Y therefore must have low p_t.
But I guess generally you could have exclusive process with a state Y at larger p_t, as long as there is a large area in the detector around Y where nothing is found. I would think (?) in this case background would be a problem because Y could also belong to an inclusive event.

 

[ChrisVer]

I am not sure, but some extra points:
There are many issues arising from low-$p_{T}$ measurements, apart from the experimental points that affect the objects' reconstruction (e.g. pileup, underlying event, noise in detector and acceptance). For example, you can even have theoretical challenges for measuring/reconstructing too low-$p_{T}$ jets (which later on affects your jet energy calibration). I think you are also affected by IR divergences from QCD/soft events.

Differential measurements are especially important e.g. for verifying the SM in all phase spaces (or trying to challenge it). You might have new physics appearing in a region of the phase space for example above some momentum threshold (e.g. $p_T > 400\text{ GeV}$) and below which the new processes are suppressed.
If you looked inclusively starting from $p_T > 20\text{ GeV}$ you wouldn't see it (Except for if the new physics appeared with very large cross sections). In a similar manner that you would have never seen the Higgs boson in $\gamma\gamma$ if you had looked at the integral of $m_{\gamma\gamma}$ . You need statistics because you want your regions to be well populated for you to make the hypothesis test and not just observe statistical fluctuations.
If nothing appears, the differential measurement allows us to give constraints either to SM parameters or EFTs parameters that are not very easily constrained from the rest of the phase space.