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Difference between missing momentum and missing energy

(2011-08-14 19:55:57)
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杂谈

分类: 知识
resource net: http://atlas-service-enews.web.cern.ch/atlas-service-enews/2010/features_10/features_ptmiss.php

E
Tmiss is a key final state signature. The ATLAS calorimeters play a vital role in the ETmiss measurement and have been designed to provide an excellent resolution at the LHC energy scale. However, several effects could either fake large ETmiss or degrade its resolution. Among them the presence of dead or noisy read-out channels, dead detector regions, poorly estimated inactive material, escaping particles or mis-measurement of their energies. This could make the estimation and the rejection of the background from physics processes with no real intrinsic transverse missing energy more challenging as the measured ETmiss would not reflect the one originating from the non-interacting particles.

To better evaluate and check the quality of the missing transverse energy measured by the calorimeters, one needs an independent measurement of a similar quantity using another sub-detector. The high performance ATLAS Inner Detector, with its three components: the pixel detector, the silicon micro-strip detector (SCT) and the Transition Radiation Tracker (TRT), offers such a possibility as it reconstructs the tracks of charged particles produced in the collision allowing the measurement of their momentum. We have used this feature to measure a new quantity, the missing transverse momentum
pTmiss , which is strongly correlated to the missing transverse energy in a proton-proton collision with non interacting particles such as neutrinos or neutralinos.
The absence of such correlation in events with either fake or poorly measured
ETmiss will allow rejecting them.
However, its performances are mainly limited by the Inner Detector smaller geometrical coverage and its inability to measure neutral particles momentum. The presence of direct photons or neutral pions would leads to a larger pTmiss than ETmiss for the same event.

总结: ETmiss 是在热量计处测量的,容易受很多因素的影响,pTmiss是在内部探测器上测量的,可以重建带电粒子的轨迹,测量它们的动量,但是内部测量器覆盖面有限。对于有光子和中性pion的过程,pTmiss 比 ETmiss大,因为pTmiss把中性粒子当做missing粒子了。

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