LHC results for dark matter from ATLAS and CMS

The ATLAS and CMS DM searches covered a huge range of final states during the first data-taking run of the LHC looking for signs of WIMP production. Although observation is consistent with SM background expectation, stringent limits have been set on different benchmark models, emphasising the complementarity of collider searches and direct detection searches. Collider searches can powerfully constraint the low DM-mass region where the direct detection experiments suffer a lack of sensitivity.
However the current benchmark models employed to describe the DM-SM interaction suffer of validity limitations in the high-energy regime. Thus a different choice will be performed for Run-II making use of simplified models which explicitly define the mediator particle, providing a more fair description of the interaction itself, and overcoming Effective Field Theory approach limitations.
Read more at http://arxiv.org/pdf/1510.01516v1.pdf


Expected sensitivity studies for gluino and squark searches using the early LHC 13 TeV

… Run-2 dataset with the ATLAS experiment

The current searches in the LHC Run-1 dataset have yielded sensitivity to TeV scale gluinos, as well as to third generation squarks in the hundreds of GeV mass range. The discovery reach in Run-2 is expected to be greatly enhanced due to the large increase in the LHC centre-of-mass collision energy from 8 TeV to 13 TeV. This document presents sensitivity studies for gluino pair production and bottom squark pair production with a full simulation of the ATLAS detector at a centre-of-mass energy of 13 TeV. Results are shown for an integrated luminosity of 1, 2, 5 and 10 fb−1
Read more at https://cds.cern.ch/record/2002608/files/ATL-PHYS-PUB-2015-005.pdf


Mysteries of matter at the LHC

Two years ago, the Higgs Boson was discovered by the ATLAS and CMS experiments. But how precisely does it fill its role as the last missing piece in the Standard Model of particle physics?
The Large Hadron Collider will restart in 2015 with almost double the collision energy to test just that. But even then, this theory only accounts for 5% of the Universe, and does not include gravity.Can the LHC shed light on the origin of dark matter? Why is gravity so much weaker than the other forces? Dr Pippa Wells explains how the LHC will explore these mysteries of matter.
Pippa Wells was the Inner Detector System Project Leader on the ATLAS Experiment at CERN. ATLAS is one of two general-purpose detectors at the Large Hadron Collider (LHC). It investigates a wide range of physics, from the search for the Higgs boson to extra dimensions and particles that could make up dark matter.

ATLAS Experiment: Search for dark matter candidates

Search for dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detector 
A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at sqrt(s)=7 TeV with the ATLAS experiment at the Large Hadron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 inverse femtobarn. No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles. http://arxiv.org/abs/1210.4491

Read more publications of the ATLAS collaboration at: twiki.cern.ch

New SUSY Limits From ATLAS

By Tommaso Dorigo
A new ATLAS search for supersymmetric signatures in 2011 LHC data has appeared last week in the arxiv. The result ? No hint of a signal, not even for ready money.

So if you are on a hurry, you can just have a glance at the graph below, which summarizes the measurement in terms of excluded regions of a slice of the complicated parameter space of SUSY theories…..

Read more: www.science20.com

ATLAS: 5.9 Sigma For A 126 GeV Higgs !

By Tommaso Dorigo
ATLAS has just released a note which summarizes the searches for the standard model Higgs boson in 7-TeV and 8-TeV data. Since July 4th the main improvement is the addition of the WW channel, which had not been shown back then. With it, the combined local significance of the 126 GeV Higgs boson excess in the WW, ZZ, and γγ channels grows to 5.9 standard deviations. In the words of a Facebook friend who’s in ATLAS: “if this is not a discovery, I don’t know what is”.

So, due congratulations to my ATLAS colleagues for this new important document. The paper is indeed full of detail about the searches, answering many of the questions that the format of the July 4th event did not allow to be asked.

The most important measurements, those of mass and cross sections, are summarized in the figure on the right, which shows the 1-sigma contours for the different final states: the WW measurement is the one which extends the most in the horizontal axis, because of the large indetermination in the mass due to the escaping neutrino pair.

The three measurements are consistent with each other, and the global signal strength is measured at 1.4+-0.3 times the standard model predictions for a 126 GeV Higgs boson…..
Read more: www.science20.com

Quark Excitement: Is there anything smaller?

The upper part of this plot shows a histogram (black dots) of ATLAS data events containing a photon and a jet, organized into bins defined by the mass of the photon-plus-jet pair. The stepped solid line represents a mathematical background function that has been fitted to the ATLAS data. If an exotic particle, having a mass in this range and decaying to a photon and a jet, were produced in the LHC, we would expect a bump to appear in the ATLAS data. Depending on the mass of the new particle and how readily it is created in the LHC, the bump could resemble one of the three indicated coloured peaks representing hypothetical excited quarks (q*) having masses of 0.5, 1.0, and 2.0 TeV. The lower part of this plot shows the statistical significance of the difference between the ATLAS data and the background function in each bin.

Mankind has forever sought to determine the most fundamental components of matter. From the atom to the nucleus to the proton and neutron, and finally to the quark, we have asked each step of the way “Is this it or is there something inside?”

ATLAS physicists have just taken another step toward tackling that very question by publishing the results of a search for new kinds of particles decaying into a jet (a spray of hadronic particles) and a photon.

The Physical Review Letters article provides the world’s best upper limits to date on the probability of producing such particles, including excited states of quarks.

recent ATLAS Blog posting explains that, as in the case of atoms, if a particle can be excited then it is necessarily composed of smaller pieces. If the LHC were able to create excited quarks, we should observe them with ATLAS as they emit photons of light and return to being regular quarks.

This plot compares how a product of quantities proportional to the number of hypothetical excited q* quarks observable in the ATLAS detector through their decays into a photon and a jet (vertical axis) varies as a function of the q* mass (horizontal axis). The black dots show the 95% credibility-level (CL) upper limits on this product measured by ATLAS in 7 TeV proton collision data. The dashed line shows the upper limits that were expected. The blue line describes how a theoretical excited-quark model predicts the product to vary with q* mass. The q* mass at which the blue curve and the ATLAS observed upper limits intersect marks the 95% CL lower limit set by ATLAS on the hypothetical excited quark mass.

Although no such excited states were found, the ATLAS study has significantly extended previous results obtained at other colliders. In fact, these measurements rule out the existence of signals ten times fainter and excited quarks 2 TeV more massive than earlier studies.

With the 2012 increase in the LHC energy to 8 TeV, and additional increases in the future, we expect to use these and other techniques to extend the reach of our understanding of matter’s most fundamental constituents.

Refer to a new ATLAS Blog article describing our technique and this new result in the search for quark substructure.
Read more: www.atlas.ch