How do you react when one person in work doesn't say hi and pretends you're not there?

[Disappointedsofa]

I have recently started a new office job, it's a small office with 6 or 7 people working in it.
I've noticed the last few days, one woman in particular doesn't say hello or talk to me but talks to everyone one else.
It's so annoying as every one else is lovely but it can just take one person to ruin my whole day.
I've started ignoring them back and am just going to mirror their energy. But it sometimes takes more effort to consciously ignore someone than to just be natural and talk to someone.
I'm quite a friendly person that can get on with most people , but I won't tolerate rudeness from someone when they don't even acknowledge I'm there so they will get the same rudeness back from me.
I know it's not something I've done as I've only started a couple of weeks ago. I also don't want to bring the subject up as I don't want it to be a 'thing' especially as I've just started recently.
How can you just walk past someone and not even look at them but speak to everyone else?
Would you do the same and just ignore them?

I am neurodiverse and have times of SM. Putting pressure on me to speak makes it worse. We've just gone through a reorganisation and I'm in a new team that I dont know, are hugely extrovert and very comfortable with each other so I am very quiet at the moment and this has been pointed out repeatedly, even though I've explained and asked for time. We had a joint meeting with some folk that I've worked with for years this week and my new team were amazed at the fact that I was at the centre of a huge group hugging and chattering away. One of my new colleagues commented on it to an old colleague. They just went "Bob needs space and time. Don't try and push your expectations on her and talk work or her special interests rather than personal til she knows you"

Interestingly we went to the pub after work and one of the people who has pushed me most to engage on the terms of the rest of the new group (lots of small talk, personal stories etc) hung around on the edges of the group being quiet and left early because she was in a group she didn't know well, while I was out til midnight gossiping and setting the world to rights. I'm hoping she takes a lesson from this and gives me the space I need to integrate into the new team.

It’s her, it’s not you.
So, don’t make it about you - continue to say hello and be friendly to everyone including her and just let it be.
Then you will always know that you are in no way responsible.
I worked with someone similar and he was really really struggling and so I just carried on saying Hello and Goodbye and including him in everything that was going on.
People at work knew he struggled and he could only talk to certain people - coming to work for him was more than 8 hours on the job, it was extremely out of his zone and it took a lot out of him mentally to be there - but he did and I admired his strength.
What he didn’t know and what no one else really knew was that I too was struggling and one particularly bad day he was extremely on edge and it culminated in me, just saying to him ‘I know you’re struggling, I am too - how can I help you’ and I helped him clear up a mess he’d made - after that we weren’t great friends but he went out of his way to greet me and that was a big effort.
Just be kind by being yourself.

It’s a form of bullying, The intention is to make exclude you and make you uncomfortable.
I wouldn’t play her at her own game as she sounds like a bitch and could cause trouble for you.
I would casually say hi / hello when passing but make no more effort than that, show her that you are completely unbothered by her behaviour and that it hasn’t even registered with you. She might stop if she sees that her nastiness is having no effect. Focus on the team members that are pleasant, you are new there, don’t get caught up in petty office politics.

So if it was me I’d try addressing her directly.

so say morning etc to everyone else and when she ignores you say “Morning “insert name here” how are you today.

if she ignores that then it’s worth bringing it up with your manager as it will ultimately make an unfriendly work team environment x

Do as you would be done by and lead by example.
Be pleasant and friendly.
Be you.
Your new colleague might just be shy.
If they still ignore you then ask them why.
Communication is usually the way out of an awkward situation.